., e, . viz/“i“ /”:Iiu“f’-:*’.~"‘”“"" ,2 ,e ,v , A! :1? 1"" /'V/.) L f "1) v. D‘Spuéeochemistry and Petrology of the Alkalie Igneous Complex at Magnet Cove, Arkansas Prepared in cooperation with the Defense Minerals Procurement Agency XXL/f. r" , w I} f M- /:g " ,Ha ‘ I x ._ ”f" ’/ ,‘ ‘~ / 15 “f,” / ("K 9‘ ‘ ‘N Q \ ‘// ‘M" .» HR .1 ,v n .. A 54.1 ‘7 III I!!! \ , IENCFE. \\ (b ;/ LIBRAL \. 5;“: g}: Geochemistry and Petrology of the Alkalie Igneous Complex at Magnet Cove, Arkansas By R. L. ERICKSON and L. V. BLADE GEOLOGICAL SURVEY PfiRZOFESSIONéAL PAPER 425 Prepared in cooperation with the Defense Minerals Procurement Agency UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director The U.S. Geological Survey Library has cataloged this publication as follows: Erickson, Ralph Leroy, 1923— Geochemistry and petrology of the alkalic igneous complex at Magnet Cove, Arkansas, by R. L. Erickson and L. V. Blade. Washington, US. Govt. Print. Off., 1963. v, 95 p. maps (part fold, 1 001., in pocket) diagrs., tables. 29 cm. (U.S. Geological Survey. ProfeSSional paper 425) Prepared in cooperation with the Defense Minerals Procurement Agency. Bibliography: p. 90—91. (Continued on next card) Erickson, Ralph Leroy, 1923-— Geochemistry and petrology of the alkalic igneous complex at Magnet Cove, Arkansas, 1963. (Card 2) 1. Geochemistry—Arkansasr—Magnet Cove. 2. Petrology—Arkan- sas—Magnet Cove. 3. Rocks, Igneous. 4. Rocks—Analysis. I. Blade, Lawrence Vernon, 1917— joint author. II. US Defense Minerals Procurement Agency. III. Title: The alkalic igneous com- plex at Magnet Cove, Arkansas. (Series) For sale by the Superintendent of Documents, US. Government Printing Office Washington 25, DC. 62575 Pt v. 4 Q 52 U14 ‘EARTH CONTENTS SClENCES LIBRARY Page Abstract ___________________________________________ 1 Dikes—Continued Introduction _______________________________________ 1 Dikes within the complex—Continued Location and surface features ____________________ 2 Aplite ------------------------------------- Previous work __________________________________ 2 Eudialyte—nepheline syenite pegmatite _________ Scope and methods of work ______________________ 2 Garnet fourchite ---------------------------- Acknowledgments _______________________________ 4 Dikes outside the complex _______________________ Veins __________________________________________ G ner 1 e010 ____________________________________ 5 e Aagegof thiyrocks ________________________________ 5 Contact zone _______________________________________ Metamorphosed Arkansas novaculite ______________ Structure """"""""""""""""""""""" 6 Metamorphosed Stanley shale ______________________ Igneous complex ———————————————————————————————————— 6 Geochemistry ______________________________________ Outer ring _____________________________________ 7 Rocks _________________________________________ Sphene-nepheline syenite ———————————————————— 7 Variation diagrams ______________________________ Garnet-pseudoleucite syenite _________________ 10 Minerals _______________________________________ ’ Miscellaneous syenites _______________________ 15 Feldspar ___________________________________ Feldspathoidal leucosyenite ______________ 15 Nepheline __________________________________ Sphene—cancrinite syenite ________________ 16 Zeolite ____________________________________ Sphene-garnet—nepheline syenite __________ 16 Pyroxene __________________________________ J acupirangite and sphene pyroxenite __________ 17 Garnet ____________________________________ Garnet-biotite melteigite _____________________ 21 Biotite ____________________________________ Intermediate ring _______________________________ 23 Apatite ____________________________________ Altered phonolite ___________________________ 23 Magnetite _________________________________ Undivided trachyte-phonolite ________________ 25 Perovskite _________________________________ Inner core ______________________________________ 27 Sphene ____________________________________ Ijolite _____________________________________ 28 Pyrite _____________________________________ Fine-grained ijolite __________________________ 30 Calcite ____________________________________ Carbonatite -------------------------------- 34 Geochemistry of niobium ________________________ Lime-silicate rock ___________________________ 39 Rutile and brookite _________________________ Dikes _____________________________________________ 39 Carbonatite ________________________________ Dikes within the complex ________________________ 39 Igneous rocks and minerals __________________ Tinguaite __________________________________ 39 Radioactivity ______________________________________ Sodalite trachyte ___________________________ 42 Age relations of the rocks ____________________________ Analcime-olivine melagabbro _________________ 42 Origin _____________________________________________ Nepheline syenite pegmatite _________________ 44 Economic geology ___________________________________ Trachyte porphyry __________________________ 46 References cited ____________________________________ Miscellaneous trachytes _____________________ 47 Index _____________________________________________ ILLUSTRATIONS [Plates are in pocket] PLATE 1. Map of bedrock geology of Magnet Cove igneous area, Hot Spring County, Ark. 2. Map showing relation of igneous rocks of the Gulf Coastal region to the Ouachita geosyncline. 3. Map showing relation of the igneous complex to total intensity aeromagnetic contours in the Magnet Cove area. FIGURE 1. Index map and generalized geology of the Magnet Cove area _____________________________________________ 2. Isogonic chart of Magnet Cove _______________________________________________________________________ 3. Map showing distribution of jacupirangite inferred from high magnetometer readings _______________________ 4. Map showing location of igneous dikes in relation to the Magnet Cove intrusive complex, Garland and Hot Spring Counties, Ark ____________________________________________ . Ternary diagram showing igneous rocks of the complex plotted against atomic proportions of the major elements- . Ternary diagram showing igneous rocks of the complex plotted against atomic proportions of Ca—Na+K—Si___ . Ternary diagram showing igneous rocks of the complex plotted against atomic proportions of Na—K—Ca _______ . Ternary diagram showing igneous rocks of the complex plotted against atomic proportions of Fe—Mg-Na+K__ m 203 Page IV CONTENTS Page FIGURE 9. Lime variation diagram _____________________________________________________________________________ 68 10. Variation diagram showing composition of material (A) subtracted from an olivine basalt to produce magma of the weighted-average composition of the Magnet Cove complex ________________________________________ 86 11. Variation diagram showing composition of rocks and residual magmas for the main periods of intrusion at Magnet Cove ____________________________________________________________________________________________ 88 TABLES Page TABLE 1. Spectrographic sensitivities of the elements __________________________________________________________ 7 2. Analyses, norm, and modes of sphene-nepheline syenite _______________________________________________ 9 3. Spectrographic analyses of mineral separates of sphene-nepheline syenite ________________________________ 10 4. Analyses, norms, and modes of garnet-pseudoleucite syenite ___________________________________________ 12 5. Analyses, norm, and modes of garnet-nepheline syenite _______________________________________________ 13 6. Spectrographic analyses of mineral separates of garnet-pseudoleucite syenite _____________________________ 14 7. Spectrographic analyses of mineral separates of garnet-nepheline syenite ________________________________ 15 .8. Analyses, norm, and mode of feldspathoidal leucosyenite ______________________________________________ 16 9. Analyses, norm, and modes of jacupirangite _________________________________________________________ 18 10. Analyses, norms, and modes of sphene pyroxenite ____________________________________________________ 19 11. Spectrographic analyses of mineral separates of jacupirangite __________________________________________ 20 12. Spectrographic analyses‘of mineral separates of sphene pyroxenite ______________________________________ 21 13. Analyses, norm, and mode of garnet-biotite melteigite ________________________________________________ 22 14. Spectrographic analyses of mineral separates of garnet-biotite melteigite ________________________________ 22 15. Analyses and norms of altered phonolite ____________________________________________________________ 24 16. Analyses, norms, and mode of the undivided trachyte-phonolite ________________________________________ 27 17. Analyses, norms, and modes of biotite-garnet ijolite and garnet ijolite __________________________________ 29 18. Average and type urtite, ijolite, and melteigite ______________________________________________________ 30 19. Spectrographic analyses of mineral separates of biotite-garnet ijolite ____________________________________ 31 20. Spectrographic analyses of mineral separates of garnet ijolite __________________________________________ 32 21. Analyses, norms, and mode of fine-grained ijolite _____________________________________________________ 33 22. Descriptive logs of core holes ______________________________________________________________________ 34 23. Analyses of carbonatite ___________________________________________________________________________ 35 24. Analyses of saprolite of carbonatite ________________________________________________________________ 36 25. Spectrographic analyses of mineral separates from carbonatite _________________________________________ 37 26. Composition of kimzeyite _________________________________________________________________________ 38 27. Spectrographic analyses of mineral separates of lime—silicate rock _______________________________________ 40 28. Analyses and norm of tinguaite ____________________________________________________________________ 41 ‘29. Analyses and norm of sodalite trachyte ____________________________________________________________ 42 30. Analyses, norm, and mode of analcime-olivine melagabbro ____________________________________________ 43 31. Spectrographic analyses of mineral separates of analcime-olivine melagabbro ____________________________ 44 32. Analyses and norm of nepheline syenite pegmatite ___________________________________________________ 45 33. Analyses and norm of trachyte porphyry ____________________________________________________________ 46 34. Analyses, norm, and mode of eudialyte-nepheline syenite pegmatite ____________________________________ 48 35. Chemical analysis of tobermorite from eudialyte-nepheline syenite pegmatite ____________________________ 48 36. Analyses of garnet fourchite _______________________________________________________________________ 49 .37. Description of dike rocks that occur outside the complex ______________________________________________ 51 38. Analyses of igneous dikes outside the complex _______________________________________________________ 53 39. Range of Nb, TiOz, V, Y, and La content in veins of the major deposits ________________________________ 54 40, 41. Analyses of veins ____________________________________________________________________________ 55, 56 42. Analyses of apatite—pyrite vein ____________________________________________________________________ 57 43. Spectrographic analyses of minerals from veins _______________________________________________________ 57 44. Analyses of metamorphosed sediments of the contact zone ____________________________________________ 58 45. Analyses of sedimentary rocks in the Magnet Cove area ______________________________________________ 59 46. Computed values for Magnet Cove rocks and the average igneous rock _________________________________ 60 47. Analyses of igneous rocks from Magnet Cove ________________________________________________________ 60 48. Lime-alkali sums of igneous rocks compared to Magnet Cove rocks _____________________________________ 62 49. Average minor element content of minerals at Magnet Cove ___________________________________________ 66 50. Chemical analyses of sodic orthoclase ______________________________________________________________ 69 51. Spectrographic analyses for minor elements in feldspar ________________________________________________ 69 52. Analyses of nepheline _____________________________________________________________________________ 7O 53. Spectrographic analyses for minor elements in nepheline ______________________________________________ 70 TABLE 54. Analysis of zeolite from ijolite-__- 55. Spectrographic analyses of zeolites CONTENTS 56. Analyses of pyroxene _______________________________ 57. Spectrographic analyses for minor elements in pyroxene _______________________________________________ 58. Analysis of dark—brown garnet from biotite-garnet ijolite ______________________________________________ 59. Spectrographic analyses for minor elements in g arnet _________________________________________________ 60. Analyses of biotite _______________________________________________________________________________ 61~70. Spectrographic analyses. Minor elements in biotite _______________________________________________________________________ Apatite _______________________________________________________________________________________ Magnetite ____________________________________________________________________________________ Perovskite ____________________________________________________________________________________ Rutile ____________________________________________________________ Brookite ______________________________________________________________________________________ Paramorphs of rutile after brookite _______________________________________________________________ 71. Summary of analytical data from the principal titanium deposits at Magnet Cove, Ark ___________________ GEOCHEMISTRY AND PETROLOGY OF THE ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARKANSAS By R. L. ERICKSON and L. V. BLADE ABSTRACT The Magnet Cove alkalic igneous complex together with the surrounding contact zone has been mapped on a scale of 1 : 6,000. The complex, about 4.6 square miles in area, is composed of a series of ring dikes post-Mississippian in age that were intruded into faulted and folded Paleozoic sedimentary rocks of the Ouachita geosyncline. The Paleozoic sedimentary rocks in the area are predomin- antly shale, sandstone, and novaculite, but include minor amounts of conglomerate and limestone. The rocks range in age from Ordovician to Mississippian. The contact zone ranges in width from 1,000 to 2,500 feet and is composed of Missouri Mountain shale of Silurian age, Arkansas novaculite of Devonian and Mississippian age, and Stanley shale of Mis- sissippian age. Deformation of the sedimentary rocks by the intrusion has been slight. The shales have been changed to spotted argillite, hornfels, and gneiss. The quartz of the nova- culite has been recrystallized and near the intrusion appears as a friable sandstone. Except for a slight recrystallization of the quartz, the sandstones have not been much affected. The ring dike igneous complex has a core of ijolite and car- bonatite, an intermediate ring of trachyte and phonolite, an outer ring of nepheline syenites, and two masses of jacupirangite, one on the west edge of the complex and the other on the north- east edge. Smaller dikes of tinguaite, trachyte porphyry, nepheline syenite, miscellaneous trachytes, pegmatite, aplite, gabbro, fourchite, and carbonatite, and a variety of veins are widespread. Mineralogically, the igneous rocks of the complex can be divided into two groups, those rocks containing feldspar and those virtually free of feldspar. The igneous rocks con- taining feldspar include: phonolite, various trachytes, tinguaite, various syenites (some pegmatitic), alkalic gabbro, and aplites. The igneous rocks that are generally free of feldspar include: jacupirangite, sphene pyroxenite, ijolite, melteigite, fourchite, and carbonatite. The carbonatite occurs in irregularly shaped bodies in the central part of the complex. Aggregates of apatite, magnetite, pyrite, monticellite, perovskite, and kimzeyite (zirconium gar- net) are scattered through a groundmass of coarse calcite. Part of the carbonatite has weathered to porous rock composed of residual apatite, magnetite, and perovskite in a matrix of secondary apatite. In contact with the ijolite, the carbonatite fluids were reactive and altered the minerals of the ijolite. Dikes outside the complex are grouped in the following cate- gories: pegmatite, aplite, syenite, trachyte porphyry, trachyte, tinguaite, andesite, diorite, monzonite, and lamprophyre. Veins of various types are found both within and outside the complex: quartz—brookite-rutile veins, most common in the recrystallized novaculite on the east edge of the complex; feld- spar-carbonate veins, most common in the northern two-thirds of the complex; and feldspar, quartz-feldspar, and fluorite veins. Late quartz veins are associated with the carbonatite. Molyb- denite and apatite veins are found in the carbonatite and ijolite. Earthy monazite, apparently a weathering product of an apatite- pyrite vein, was found in the central part of the complex. Paramorphs of rutile after brookite are found as float throughout the complex but are most abundant in the northern two-thirds of the complex. Rocks were analyzed chemically and spectrographically; some of the minerals separated from the analyzed rocks were chemi- cally analyzed, and all were spectrographically analyzed. Chemically the igneous rocks are high volatile, high lime, alkalic, and subsilicic; they include intrusive carbonatite masses as well as the iron, titanium, zirconium, and phosphate minerals that characterize similar alkalic rocks throughout the world. High niobium substitution in titanium minerals and rare- earth substitution in apatite and perovskite are also typical. Compared to average igneous rocks, the Magnet Cove rocks are low in Si02, MgO, Cr, Ni, Co, Cu, U, and Th and high in A1203, F8203, F60, MDO, CaO, B30, N320, K20, H20+, T102, 002, P205, Cl, F, S, Be, Sr, Sc, Y, La, Zr, V, Nb, Ga, and prob- ably S03. The oxide contents of the rocks were plotted against CaO in variation diagrams. Significant quantities of niobium in the igneous rocks and in the associated titanium deposits are concentrated in rutile, brookite, perovskite, sphene, garnet, aegirine, and hornblende. The niobium content ranges from 0.11 to 6.6 percent in rutile, 0.5 to 3.2 percent in brookite, 0.37 to 4.6 percent in paramorphs of rutile after brookite, 0.2 to 9.0 percent in perovskite, 0.1 to 0.7 percent in sphene, 0 to 0.1 percent in garnet, 0 to 0.03 percent in aegirine, and 0.007 to 0.01 percent in hornblende. The rocks of the complex were intruded during separate but closely related periods; however, the true age sequence is not clear because rock outcrops are relatively scarce and deeply weathered. The available field evidence and the writers’ prejudices after 3 years of working in the area suggest the fol- lowing sequence from oldest to youngest: 1. phonolite and trachyte; 2. jacupirangite; 3. alkalic syenites; 4. ijolite; 5. car- bonatite, dike rocks, and veins. The writers belive that the Magnet Cove complex of alkalic igneous rocks was derived by differentiation and fractional crystallization of a residual magma of a mafic phonolite com- position rich in alkali, lime, and volatile constituents. The high concentration of volatiles is belived to be of great impor- tance in the development of the many varieties of unusual rock types. This residual magma is believed to have been derived by fractional 'crystallization from a regional undersaturated olivine basalt magma. INTRODUCTION Recent studies on alkalic rock complexes, like that at Magnet Cove, have revealed substantial resources in such commodities as the rare earths, barite, niobium (columbium), uranium, phosphate, and agricultural lime. 1 2 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. In the past, the Magnet Cove Titanium Corp. deposit has been worked for rutile; the Kimzey cal- cite quarry for agricultural lime; and the Kimzey magnetite pit for magnetite. Niobium, or columbium as it is called in commerce, is known to occur in the rutile and brookite of the veins and in the perovskite of the carbonatite. Its dis- tribution in the other rocks and minerals of the Magnet Cove igneous complex, however, has not been known. LOCATION AND SURFACE FEATURES The Magnet Cove alkalic igneous complex and its surrounding contact zone occupy about 8 square miles in the northeastern part of Hot Spring County, Ark. This heart-shaped complex is easily accessible by U.S. Highway 270, which crosses the middle of the area in a generally east-west direction. Hot Springs is about 12 miles west and Malvern is about 7 miles southeast of the area; both are on U.S. Highway 270 (fig. 1). The Magnet Cove complex lies at the east end of the Mazarn basin of the Ouachita Mountain physiographic province. The complex is bounded on the south by the Trap Mountains and on the north and east by the Zigzag Mountains (fig. 4). Altitudes range from slightly less than 340 feet in the central basin to slightly more than 600 feet on the surrounding ridges. Most of the central basin and other low—lying areas are farmland or pasture. Parts of these areas are covered with alluvium and the remaining parts are saprolite from which most of the hard rock float has been removed. Most of the ridges are covered with timber or thick brush; brambles and thorny trees are always present. Pine trees are abundant in areas of sedimentary rock, and deciduous trees in areas of igneous rock. Hard rock exposures are rare. In the central basin they are confined to one quarry, a few open pits, and a few outcrops, mostly in stream bottoms. They are more common on the ridges than in the basin, and are most abundant in valley floors and walls where streams cut the ridges. Rock float is abundant on ridges, although in some areas much of it has been removed by the early settlers to clear fields and to build stone fences. The climate is warm and humid. PREVIOUS WORK From 1806, when Macrery (1806) first mentioned minerals from the Magnet Cove area, to 1891 when Williams published his classic study of the area, many papers were published on the mineralogy of the region. Williams (1891) summarized this earlier work, made a detailed petrologic study of the igneous rocks, and published the first detailed map of the area. A few years later, Washington (1900) published a revised version of Williams’ map. In a later paper, Washing- ton (1901) presented additional chemical analyses and redefined several of the rock names. The most com- plete description of the sedimentary rocks of the region was published by Purdue and Miser in 1923. Landes (1931) presented a list of most of the minerals present, discussed the structure of the igneous rocks, and offered a theory of origin for the calcite. Parks and Branner (1932) published a map that showed the separate sedimentary rock units in contact with the Magnet Cove complex. From 1892 to 1952 individual minerals from Magnet Cove Were discussed in several papers. The minerals and authors include: natrolite, Melville (1892); ana— tase, Penfield (1894); monticellite, Penfield and Forbes (1896), Kouvo (1952); magnetite, Harrington (1907); catapleiite, Foshag (1923); astrophyllite, Gossner and Reindl (1934); sodalite, Glass (1937), Miser and Glass (1941); taenolite, Miser and Stevens (1938); carbonate— apatite, McConnell and Gruner (1940); molybdenite, Sleight (1941); hackmanite, Miser and Glass (1941); schorlomite, McConnell (1942); and perovskite, Mur- doch (1951). From 1938 to 1954 many papers were published on the individual rutile and brookite deposits. The several authors include: Miser and Stevens (1938); Ross (1938, 1941 Ross and Hendricks, 1945); Spencer (1946); Holbrook (1947, 1948); Reed (1949a and b); Kinney (1949); Fryklund (1949); Fryklund and H01- brook (1950); and Fryklund, Harner, and Kaiser (1954). SCOPE AND METHODS OF WORK This project was designed to make a detailed petro- logic and geochemical study of the igneous rocks at Magnet Cove, Ark., in order to improve understanding of the origin and geochemical environment of niobium and associated elements. The bedrock geology was mapped at a scale of 1:6,000 (pl. 1) in order to gain further knowledge of the petrology, intrusive history, and structure of the igneous complex and to determine the relation of the complex to the surrounding sedi- mentary rocks. Erickson and Blade mapped the igneous complex during the spring and fall of 1953 and 1954. Blade added details and mapped the contact zone during 1955 and 1956. Mapping was on an enlarged topographic base taken from the Malvern quadrangle, and compiled by the U.S. Geological Survey. The alidade—planetable method was used in the cleared areas, and the pace and compass method was used in the brush-covered areas. Williams’ isogonic chart (1891, pl. 13), trans- ferred to a modern base and corrected for annual R.18W I \ \\ \\\\\\‘ \\E \ \\ \\\\\ 1‘1“" INTRODUCTION R. 17 W \ \ ./ .. a“. / 99‘ sm/ 0° " “IIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIIII \\\§\\\\\\\\\\\\\\ \\\\\\\\\\\ IIIIIIIIIIIIIIIIIIIIIIfiI‘IIIIIIII‘ImmIIIIIIII‘III‘IIIIIIIIIIIIIIIIIIIIIIII \\\\\\\\\\\\\\\\\\\\\\\\\\\\\ “‘IIIIIII’IIjj ' ... IIIIIIIIIIIIIIIIIIIIIIIIIII “‘\ iiiii \E X}mmfllfllfllfllmIIIIIIIII I After 8. C. Parks and G. C. Branner, 1932 EXPLANATION WiIcox and Midway groups V\I 7 I\VI\ Igneous rocks Stanley shale \\ Hot Springs sandstone IIIIIIIIIIIIIIII Arkansas novaculite AND DEVONIAN MISSISSIPPIAN CRETACEOUS(?) TERTIARY MISSISSIPPIAN F—w—J CARBONIFEROUS Dashed where approximately located Axis of anticline K- Axis of syncIine O 1 | I AREA OF THIS REPORT 3 MILES 41 FIGURE L—Index map and generalized geology of the Magnet Cove area, Hot Spring County, Ark. 659634—62——2 4 ALKALIC IGNEOUS COMIPLEX AT MAGNET COVE, ARK. R.18 S. R. l7 W. Base from US. Geological Survey map of Malvern quadrangle, Arkansas, 1949. lsogonic lines adapted from J. F. Williams, (189], pl. 13) 0 EXPLANATION / Lines of equal magnetic declination 5000 FEET FIGURE 2.—Isogonlc chart of Magnet Cove. change (fig. 2), proved indispensable in the pace and compass traversing. The chart was unreliable in the area of the Kimzey magnetite pit (pl. 1) probably because much of the magnetite has been mined since the chart was made. Most of the mapping was based on the distribution of rock float, as outcrops are scarce. In areas where float was absent, saprolite was used either directly or by comparison of the heavy minerals with panned concentrates from saprolites of known origin. Samples were obtained by drilling through overburden with a power auger to the saprolite zone. In the Cove Creek valley flat just north of US Highway 270 and east of the Kimzey calcite quarry (pl. 1) several auger holes were attempted but the gravel of the valley fill was too coarse to allow penetration. An unpublished magnetom- eter survey made for the US. Bureau of Mines by the Heiland Research Corp. in 1942 proved useful in two areas in outlining bodies of jacupirangite (fig. 3; pl. 1). All the rocks were examined in thin section and analyzed spectrographically. Standard chemical anal- yses were made of the major rocks. Minerals were separated from the coarser grained major rocks for quantitative spectrographic analysis and a few for standard chemical analysis. Heavy liquids, the Franz Isodynamic Separator, and hand picking were the methods used in making the separation. Eighty- six rutile and brookite samples were selected for spectrographic analysis. Chemical, spectrographic, and X-ray analyses were made in both the Wash- ington and Denver laboratories of the US. Geological Survey. ACKNOWLEDGMENTS Several US. Geological Survey geologists have con- tributed to the Magnet Cove study through field conferences, discussions, and laboratory investigations. W. R. Griffitts spent 6 weeks in the field when the project was initiated in February 1953, and made many helpful suggestions during the subsequent field work and laboratory work. Walter Danilchik mapped the sedimentary rocks adjacent to the complex (pl. 1), as a part of more extensive work in surrounding areas. GENERAL We are indebted to N. F. Williams, Director of the Arkansas Geological and Conservation Commission, and his staff for their generous cooperation in all phases of this work. The power auger and certain other equipment were provided by the State. W. A. Keith, lessee of much of the carbonatite area, allowed us to log the core from four holes drilled in the carbona- tite. The cooperation of J. W. Kimzey and other residents of the Magnet Cove area is gratefully acknowledged. This investigation, as part of the niobium program of the US. Geological Survey, was supported largely by funds from the Defense Minerals Procurement Agency. GENERAL GEOLOGY The Magnet Cove igneous complex, about 4.6 square miles in area, is but one of many silica-poor igneous complexes that intrude the folded and faulted Paleozoic sedimentary rocks of the Ouachita geosyncline (pl. 2). The undersaturated alkaline igneous rocks studied in this investigation occur at the east end of a belt of dikes and other small intrusive bodies that extend westward to Hot Springs and intrude Ordovician to Mississippian sedimentary rocks in the east—west trending Mazarn synclinorium. This synclinorium is bounded by northeastward—trending folds of the Zigzag Mountains on the north and the east-west trending Trap Mountain anticlinorium on the south. This abrupt change in strike of the major structural elements probably influenced the localization of the igneous rocks. The intruded sedimentary rocks are overturned and thrust to the north and northwest and include rocks ranging in age from Ordovician to Mississippian. These rocks are tabulated below from information given by Walter Danilchik (written communication, 1955): Age Formation Description Thickness (feet) Stanley shale .......... Shalle, sanglstone and con- i3, 500 . g omera e. Mississ1pplan """ Hot Springs sand- Sandstone, conglomerate, 0—200 stone. and shale. Mississippian Arkansas novaculite._ Novacnlite, calcareous no— 100-800 and Devonian. vaculite, shale, sand- stone and conglomerate. Missouri Mountain Shale, sandstone, quartz- 50—100 Silurian __________ shale. ite and conglomerate. Blaylock sandstone- __ Sandstone and shale _______ 0—550 Polk Creek shale ______ Shale,t sandstone, and 25-200 c er . . Bigfork chert __________ Chert, shale, sandstone, 700 Ordowcian. ' ""' and limestone. Womble shale _________ Shale, sandstone, lime- 250—900 5 one. Tertiary deposits of the Gulf Coastal Plain occur to the east and south (fig. 1). Stanley shale is in contact with the complex for about three-fourths of its perimeter; Arkansas novaculite, GEOLOGY 5 and a little Missouri Mountain shale are in contact with the eastern part of the complex. These rocks have been altered by the Magnet Cove intrusive mass for distances up to 2,500 feet from the contact. Igneous rocks of varied composition are found as dikes and sills in the contact zone and in the Paleozoic sediments to the west. These rocks include andesite and various types of pegmatite, aplite, syenite, trachyte porphyry, trachyte, tinguaite, and lamprophyre. Quartz-brookite—rutile veins are concentrated in the recrystallized novaculite of the contact zone. Feld— spar-carbonate veins are concentrated in the northern two-thirds of the complex. Quartz veins are closely associated with the carbonatite. Feldspar, apatite, and molybdenite veins are found both within and outside the complex. AGE OF THE BOOKS The maximum possible age for the igneous rocks of the complex is Mississippian because the youngest sedimentary rock intruded by them is the Stanley shale. The alluvium in some of the stream valleys is probably Quaternary. Inasmuch as this leaves a large gap, we must look outside the area for a reasonable age. Lead-alpha age determinations were made by Thomas W. Stern (written communication, 1958) on zircon separated from a feldspathoidal syenite peg- matite dike that occurs about 500 feet west of the igneous complex in the N% sec. 24 (L—287, pl. 1). The zircon gave ages of 178 and 184 million years (Triassic). Unfortunately, zircon was not detected in any of the more than 300 thin sections 0" igneous rocks from within the complex. Thus the usefulness of this age determination in relation to the main igneous complex is questionable. Bramlett‘e (1936) found the nepheline syenite of the bauxite area of Arkansas to be younger than the metamorphosed Paleozoic sedi- mentary rocks and older than the Midway group of Paleocene age. Miser (1912) found the peridotite near Murfreesboro, Ark. , to be of early Late Cretaceous age. Ross, Miser, and Stephenson (1929) found phonolitic rocks similar in composition to those in the Magnet Cove complex as water—laid volcanic deposits in the lower Upper Cretaceous in southwestern Arkansas, southeastern Oklahoma, and northeastern Texas. The evidence indicates that the igneous rocks at Magnet Cove are Mesozoic in age; igneous activity may have extended from Triassic to Cretaceous time. The igneous rocks are shown as Cretaceous(?) in age on the geologic map (pl. 1). A more positive dating of these rocks may be achieved by K/Ar or Rb/Sr age determinations on biotite. 6 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. STRUCTURE The Magnet Cove igneous area is an elliptically shaped ring dike complex with a small central core that may be a stock or an undecapitated ring dike. The complex has been emplaced in a tightly folded, west- ward-plunging group of Paleozoic anticlines and synclines that appear to be subsidiary features on a major southwest plunging anticlinal nose. In general terms the complex can be divided into three parts: the central part of the area, a topographic basin, is occupied by mineralogic and textural varieties of ijolite and carbonatite. Fine~grained rocks of phonolitic composition as well as calcareous intrusive breccias surround the ijolotic core and form the second part of the complex. The third part of the complex is the outside ring of topographic ridges composed of several types of feldspathoidal syenites. As the contacts were mapped predominantly from float, the attitudes of the contacts are not accurately known. However, a study of the outcrop trends in relation to topography suggests that most contacts are vertical or dip steeply away from the center of the complex. Plate 3 shows the relationship of the igneous complex to total intensity aeromagnetic contours in the Magnet Cove area. L. C. Pakiser, Jr., of the U.S. Geological Survey (oral communication, 1959) made the following observations from inspection of this figure. Comparison of the magnetic field over Magnet Cove with prismatic models in a similar magnetic latitude (Vacquier and others, 1951, p. 123) and of roughly comparable horizontal dimensions indicates that the magnetic body is vertical or near vertical and extends to a depth many times greater than its horizontal dimensions. The peak of the magnetic high lies near the southern edge as would be expected for a vertical body; and the absence of flanking magnetic lows corresponds to what would be expected from a body of large vertical extent. The two magnetic highs shown in the northern part of the map reflect bodies of jacupirangite, a magnetite- rich pyroxenite. The gross structural elements shown on plates 1 and 3 indicate that the intrusive complex is nearly vertical and that the thrust planes and overturned folds in the Paleozoic sediments dip south. These south-dipping zones of weakness may have tapped a deep-seated magma reservoir. Note also (fig, 1) the abrupt change in regional trend of the major structures from northeast to east—west. It seems very probable that a major east-west fault zone may occur in the area of the change in structural trends. Miser (1934) noted that “the Ouachita belt in which the igneous rocks reached the surface apparently contained zones of structural weakness in the deformed Paleozoic rocks where the molten and fragmental material could move upward most readily.” The Magnet Cove igneous area is interpreted to be a ring-dike complex. This conclusion is based upon the arcuate dikelike relation of the exposed rock bodies, and their similarity to such ring complexes in other parts of the world. It is interesting to note that in many localities of the world, strongly alkaline under- saturated rocks occur in continental volcanic areas as ring dikes and cone-sheet complexes. The importance of this association will be discussed in the section on origin of the igneous rocks. The most widely accepted theory for the formation of ring dikes requires a period of cauldron subsidence into a relatively steep-sided magma reservior. Howel Williams (1941) attributes subsidence to lack of support of the roof caused by removal of material from the magma chamber along vertical or steep outward dipping fractures. While the central block subsides, magma rises from the reservoir into the circular fracture that bounds the sinking block. These fractures may extend to the surface and give 'rise to explosive volcanic activity or they might be exposed only through deep erosion. It is the writers’ belief that the intermediate ring of fine-grained rocks and breccias of phonolitic com- position represents the earliest period of igneous activity in the area. The abundance of volatile constituents, particularly COZ, and the presence of miarolitic cavities and amygdaloidal textures in these fine-grained rocks strongly suggest a period of explosive volcanic activity at Magnet Cove. The abundance of volcanic tuffs of phonolitic composition in nearby Cretaceous rocks supports this conclusion. This phase of activity was attended and immediately followed by cauldron subsi- dence or sinking of the central block. Erosion to the present level has removed all traces of the volcanic superstructure. It may be postulated that the upper part of the sinking block was composed of volcanic tufl’ and agglomerates which had accumulated around the volcano. IGNEOUS COMPLEX The feldspathoidal igneous complex may be con- veniently subdivided for purposes of rock description as outlined in the previous section: a core of ijolite, an intermediate ring of trachyte and phonolite, and an outer ring of nepheline syenite. Two masses of jacupirangite, one on the west edge of the complex and the other on the northeast edge are described with the outer ring. The minor rock units are described last. Most of the mapped units are arcuate bodies, but one of them, garnet-pseudoleucite syenite, forms a nearly complete ring. ‘ Topographically, syenite and trachyte form the ridges, and ijolite, melteigite, and jacupirangite form the basins. Carbonatite appears as low hills in the ijolite. IGNEOUS Mineralogically, the rocks can be divided into two groups—those containing feldspar (syenite group) and those virtually free of feldspar (ijolite group and carbon- atite). Those containing feldspar form the intermedi— ate and outer rings and have been classified in accordance with Wahlstrom (1947). They include altered phono- lite, undivided trachyte, sodalite trachyte, trachyte porphyry, tinguaite (dike phonolite), feldspathoidal leucosyenite, garnet—pseudoleucite syenite, garnet-neph— eline syenite, sphene—nepheline syenite, sphene-garnet— nepheline syenite, sphene-cancrinite syenite, analcime- olivine melagabbro, nepheline syenite pegmatite, eudialyte—nepheline syenite pegmatite, and aplite. Rocks that are practically free of feldspar occupy the central basin area (jacupirangite excepted) and have been classified after Johannsen (1938). They are chiefly nepheline pyroxene rocks and may be subdivided into four groups on the basis of percen- tage of their feldspathoids: urtite, more than 70 percent nepheline or its alteration products; ijolite, nepheline between 50 and 70 percent; melteigite, nepheline between 5 and 50 percent; jacupirangite—magnetite pyroxenite with less than 5 percent nepheline. In the following pages, the rocks are described in order of their position on the geologic map (pl. 1): outer ring, intermediate ring, inner core, and miscel— laneous dike rocks. The location and field number of described or analyzed samples are plotted on plate 1. Spectrographic sensitivities of the elements for this investigation are shown in table 1. Tin, unless other- wise noted, is not reliable in the Spectrographic analyses of the mineral separates because metallic tin was used to stabilize the methylene iodide used in separating the minerals. OUTER RING The outer ring of igneous rocks occurs as topographic ridges and is composed chiefly of mineralogic and tex- tural varieties of feldspathoidal syenites. Two masses of jacupirangite and one mass of melteigite occur in the outer ring, and although they have no mineralogic or chemical similarity to the syenites, they will be included in the description of outer ring rocks. Five rock groups are described: sphene—nepheline syenite, garnet-pseudoleucite syenite, miscellaneous syenites, jacupirangite, and garnet—biotite melteigite. The location of analyzed and described rocks is shown on the geologic map (pl. 1). SPHENE-NEPHELINE SYENITE msmmurxon AND DESCRIPTION Sphene-nepheline syenite, about 7 percent of the exposed igneous rocks of the complex, is best exposed in the road cut in the NEM sec. 25 and SE% sec. 13. The top of the ridge consists of coarse-grained sphene-nephe- COMPLEX 7 TABLE 1.-—Spectrographic sensitivities of the elements Spectral limits when the standard exposure is made for the ultraviolet region Weight Weight Element percent Element percent .001 a special exposure is made for the visible region CS ___________________________ 0. 005 Li ____________________________ 0. 0001 F ____________________________ . 1 Na ___________________________ . 0001 K ............................ . 0005 Rb ........................... . 001 line syenite; downslope to the west the rock is medium grained, and downslope to the east it is fine grained. This arcuate-shaped body of rock is about 7,500 feet long and ranges in width from a few feet to about 1,300 feet. The western contact is with the Stanley shale, and in sec. 25 it surrounds an older body of jacupi— rangite. The eastern contact is largely with younger(?) pseudoleucite syenite and the ends of the body pinch out against the garnet-pseudoleucite syenite-Stanley shale contact (pl. 1). The joint system follows the elongate trend of the body and dips both east and west. A smaller body of sphene-nepheline syenite is exposed along Cove Creek in the SE% sec. 19. The only other known outcrop of this rock is in the SEM sec. 21 where it makes a small isolated knob in the surrounding sedimen- tary rocks. Called fine-grained eleolite syenite by Williams (1891) and covite by Washington (1901), the rock is typically a fine- to medium-grained light-gray to gray phanerite that weathers to a reddish-brown or mottled (reddish- brown, tan, and brownish—green) saprolite. Feldspar, pyroxene, nepheline, and sphene are readily visible. Al- though the mineralogy of the textural varieties is almost identical, the contacts are sharp and suggest separate intrusions. In general the fine—grained varieties cut coarser grained syenite. 8 ALKALIC IGNEOUS COLIPLEX AT MAGNET COVE, ARK. In thin section the phanerites have a hypautomorphic granular texture. Minerals in the rock include (in approximate order of abundance): subhedral laths of kaolinized sodic orthoclase (some perthite) with minor sericite alteration; anhedral nepheline partly altered to cancrinite ; corroded and zoned crystals of neutral or very pale reddish-brown diopside-hedenbergite rimmed with green aegirine-diopside partly altered to green and bluish—green hornblende and green or brown biotite. The accessory minerals include euhedral sphene, apatite, magnetite, sodalite or analcime, and pale-brown garnet; plagioclase (andesine) is found in the coarser grained phases. In contrast to the garnet-nepheline syenite, sphene but not garnet is macroscopic in this rock. The analyzed sample (MC—1, table 2) is a light—gray medium-grained syenite. In thin section the texture is hypautomorphic granular. The chief minerals are sodic orthoclase (53 percent), nepheline (25 percent), and pyroxene (9 percent). Although the sodic orthoclase tends to be elongated (average about 2 mm), the sides of the laths are generally not well developed. Carlsbad twinning is present though not common and all the laths are moderately kaolinized. N epheline occurs as unusually fresh blocky subhedral grains averaging 1mm across. Rows of colorless inclu- sions are common. Although sodic orthoclase and nepheline have mutual boundary textures, the nepheline is better crystallized, and in places appears to be older than the orthoclase. Three ages of pyroxene minerals are present as shown in a few crystals which have three zones: an inner core of colorless diopside—hedenbergite rimmed by pur- ple titanaugite which in turn is rimmed . by green aegirine—diopside. The diopside-hedenbergite occurs as well-formed crystals but their sodic rims are irregular and in some cases, altered to sodic amphibole. The average length of the crystals is 0.8 mm. The late pyroxene crystals are small and have formed directly as titanaugite or aegirine-diopside. Hornblende occurs as lath-shaped or anhedral grains (up to 1 mm in length) that are clearly younger than the pyroxene. Pleochroism ranges from pale greenish brown to deep bluish green. The extinction angle ranges from 20° to 40° and at the position of maximum extinction the mineral has an anomalous deep copper color. Sphene, as euhedral crystals up to 1 mm across, is the most abundant accessory mineral. Sodalite occurs as blocky subhedral and anhedral grains and appears to be a late stage magmatic or deuteric mineral. Other accessory minerals are apatite, magnetite, and pyrite. The coarse-grained syenite on top of the ridge in sec. 13 contains more amphibole and less nepheline; andesine (AboAm) occurs as early formed laths included in larger sodic orthoclase grains. In some thin sections the andesine is mantled with sodic orthoclase. One other local variety of the coarse—grained nepheline syenite occurs on the small knob in the SE% sec. 21 Where the rock contains about 20 percent andesine and about 50 percent pyroxene so that the rock is closer to a camptonite. In the fine-grained sphene—nepheline syenite only a few pyroxene grains show original diopside-hedenbergite and titanaugite cores; most of the grains are completely altered to sodic varieties. The groundmass is an equi— granular mosaic of nepheline (20 percent) and sodic orthoclase (50 percent) in which larger anhedral to euhedral grains of mafic minerals (25 percent) are present. The dike of sphene-nepheline syenite (L—286) that cuts jacupirangite in the northern part of sec. 24 is also fine grained. The texture in thin section is trachytic and the rock is composed of subhedral sodic orthoclase (some perthite)—about 74 percent; anhedral plagioclase (trace); anhedral nepheline partly altered to cancrinite (about 8 percent); anhedral sodalite or analcime (late) (about 2 percent); and anhedral to subhedral dark brownish—green hornblende, needles of green aegirine and colorless acmite, sphene, apatite, magnetite, and green biotite (about 15 percent). The presence of plagioclase in the coarse-grained variety and its virtual absence in the finer grained nepheline syenite suggest that part of the magma cooled slowly so that plagioclase crystals formed with the early pyroxene crystals. The late minerals followed an orderly sequence giving amphibole and sodic ortho- clase. In the younger, finer grained rocks, which cut medium— and coarse-grained bodies, the magma‘not only cooled quickly but was probably somewhat de- ficient in lime so that plagioclase crystals did not form and most of the calcium went into pyroxene. The amphibole also tended to form chiefly as alteration rims on pyroxene crystals rather than as separate grains. Panned concentrates of saprolite of sphene-nepheline syenite contain magnetite, clay aggregates, sphene, apatite, and mica. Rutile in the panned concentrates indicates weathered rutile veinlets not apparent when the sample was taken. A. J. Gude 3d, (written com- munication, 1956) reported major kaolinite and trace illite in an X-ray analysis of the clay and silt. parts of the saprolite. . cumsmy Chemical and spectrographic analyses, norms and modes of the analyzed rocks, table 2, show a typical nepheline syenite composition; SiOz is low and A1203 and the alkalies are high. The nepheline is potassic, orthoclase is very sodic, and K20 is about the same in both minerals. This suggests considerable solid solu- IGNEOUS COMPLEX 9 TABLE 2,—Analyses, norm, and modes of sphene-nepheline syem‘te, in percent Sample No. Sample No. Salnliple Sample No. 0. MC—l MC—l—ll MC-l—Q MC—1—10 MC-l L—27 L—24 MC—l MC—l L-24 L-27 Standard chemical analyses 1 Spectrographic analyses 3 Norm Mode l 43.27 46. 2 57 9 0. 0004 0.0002 0.0002 S0131; orthoclase (perth- 53 58 72 1 e . 8.10 32.0 23.1 n.d. .3 2 Nepheline (cancrinite)-_ 25 5 15 5. 02 n.d. n.d. n.d. . 07 03 Andesine __________________________ 15 __________ 9. 54 n.d. n.d. .065 .01 01 Sodalite and analcime 2 2 3 . 76 n.d. n.d. . 40 . 2 1 Diopside—hedenbergit 9 8. 27 n.d. n.d. 0 Aeglrine—diopside _ 18.14 n.d. n.d. .0005 .001 . 003 Hornblende __ 6 n. n.d. n.d. .0036 0 .003 Sphene_-__ 3 1. 91 14. 2 9. 0 . 024 . 009 . 03 Apatite. _ _ __________ 20 10 .96 6. 1 7. 0 . 0003 0 . 0009 lVlagnetite. _ _ _ 2 . 03 n.d. n.d. n.d. 1 Pyrite—pyrrhotite. - __________ n.d. n.d. n.d. 040 .04 04 Biotite __________ __ 2. 80 n.d. n.d. 18 .02 04 Garnet ____________________________ n.d. n.d. n.d. 0087 02 05 n.d. n.d. n.d. 013 002 001 , , n.d. n.d. n.d. 0006 0 Colorimetric analyses 4 mg. 11.3. 11.3. 13d 4 08 <10 04 n. . n. . n. . n s . n.d. n.d. n.d. 0012 .002 001 AS """""""""""" 0; 3332 018332 <°' 80032 Ignition loss. n.d. n.d. 1.0 2. 2 0012 .001 002 001 n d ‘ n d 0021 . 002 005 ' 010 (“'0‘ (“'0‘ Total ______ 100.30 98 80 99. 5 99. 2 0022 .002 004 ‘ ' ' Less 0 _____ . 0 .001 005 Radiometric analyses 5 eU ...................... 0. 004 0. 001 <0. 001 Low-level chemical manly-es 9 n.d. n.d. n.d. n.d. 1 Analyses: MC—l by L. M. Kehl; MC—l-ll by L. N. Tarrant; MC—1-9 and MC- MC—l. Sphene-nepheline syenite, SE34 sec. 13. 1—10 by P. L. D. Elmore and K. E. White. MC—l—Q. Nepheline separate from MC—l. 7' Looked (spectrographically) for but not found: Ce, Hf, Th, P, Ta, W. Re, Ru, MC—1—10. Sodic orthoclase separate from MC—l. Rh, Pd. 0s,Ir, Pt. Ag. Au, Zn. Cd.Hg,In. Tl. Ge, Sn, As, Sb. Bi, and Te. Analy— MC—1—11. Pyroxene-amphibole composite from MC—l. ses: MC—l by H. J. Rose; L—27 and L—24 by Sol Berman. L—24. Sphene-nepheline syenite, NE% sec. 24. Mode on fresh rock, analyses on 8 Modes by L. V. Blade. saprolite. ‘ Analyses by H. E. Crowe and A. P. Marranzino. 1r27. Sphene-nepheline syenite, NEV; sec. 24. Mode on fresh rock, analyses on 5 Analyses by B. A. McCall. saprolite. 0 Analyses by J. C. Antweiler. n.d.=not determined. tion of kalisilite in nepheline and albite in orthoclase Discrepancies in the results on like mineral phases (some of the modal sodic orthoclase is perthitic). such as nepheline, sodic orthoclase, and diopside-heden- Excess N320 occurs in aegirine—diopside as indicated bergite may be due to several factors which include: by chemical analysis of the pyroxene-amphibole nonuniform distribution of trace elements within the composite. minerals, different methods of separation, and different The abundant chlorine makes sodalite the logical analysts. Any or all of these factors may be operative choice for the bulk of the colorless low-index isotropic here, since the like phases were separated at different mineral in the rock. No fluorite was detected in thin times and analyzed by different analysts. section so the fluorine must be found in apatite, biotite Of all the trace elements reported in table 3, only Ba and perhaps minor amounts in hornblende (although and Ga are higher in the felsic minerals than in mafic 110116 was detected in the composite) and sphene. The minerals. Strontium occurs in felsic minerals at about titanium occurs in sphene, magnetite, and pyroxene. the same level of concentration as Ba, but is three to Spectrographic analyses show f" higher barium content four times more abundant in the mafic minerals. Ba+2 than the chemical analyses. D1str1bution of barium in (134 A) is a large cation and tends to concentrate in the mmeral separates (table 3) suggests that the spec- . . +1 trographic analyses for barium in the rock are probably the late fractions of the magma, and may replace K. high. (1.33 A) in the sodic orthoclase as long as the charge is balanced by Al+3 in tetrahedral coordination. The Trace elements in the rock that are equal to or _ +2_ . _ ' +2 exceed 0.01 percent and in order of abundance include: ‘ Slze 0f the ST “”1 (1-13 A) 13 s1m11ar to Ca (0'99 A) Sr, Zr, L9,, V, Cr, and Nb. The distribution of the and substitutes for Ca in the pyroxene and apatite. trace elements is shown by the spectrographic analyses Following are some further observations on the trace- of minerals separated from the analyzed rock (table 3). element distribution in sphene nepheline syenite. 10 TABLE 3.——Spectrographic analyses, ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. in percent, of mineral separates of sphene-nepheline syenite L—24—1 M C—l—c M C—l-b M C—l—6 M C—1—5 M C—1—8 M C—1—9 M C—1—2 M C—1—10 M C~l—1 0. 001 0 0.001 0 0. 0006 0. 0003 0 0. 0001 0 0.0001 .2 .03 >10 >10 >10 .04 .02 .06 .02 n.d. >10 >10 n.d. n. . n.d. .44 n.d. .28 ______________ .01 .3 .4 .05 .2 .3 .12 .2 .14 .3 .0006 .l .01 0 .006 .02 .2 .3 .3 .4 . 0002 . 006 . 01 . 004 . 0006 0 0. 0 0 . 04 . 02 0 . 008 . 008 0 0 0 0 . 02 . 04 0 . 02 . 04 0 0 0 0 n.d. . 0008 . 001 n.d. n.d. n.d. 0 n.d. 0 n.d. 2. 2 1—10 1—10 1. 8 1. 5 1. 3 . 06 03 02 . 03 Zr-- . 006 . 05 . 05 . 007 . 04 . 08 .002 0 . 003 0 V.- .5 .04 .02 06 .08 .1 .001 .01 .001 .01 N ._. 0 .7 0 . 01 0 0 CL--. . 009 0002 . 08 1 . 0009 . 0007 0 . 0007 0 0005 M0.-. 0 . 0006 0 0 0 0 0 0 0 Mn.-. . 5 . 001 . 08 . 08 . 7 .9 . 001 . 006 .003 . 01 Fe-... >10 .4 1—10 >10 >10 >10 .40 .5 .24 .3 Co . 008 . 0008 . 004 . 003 . 002 . 003 0 0 0 0 Ni . 007 . 01 . 01 . 01 . 002 . 003 0 0 0 0 Cu- . 01 . 005 . 001 . 002 . 002 .002 . 0001 . 001 . 002 . 001 Zn._ . 1 0 0 0 0 0 0 0 0 Ga 005 0 . 002 . 0007 . 003 . 004 . 011 . 004 . 005 . 002 Sn n.d. n.d. . 005 .006 . 007 0 0 0 0 Pb. 0 . 005 0 0 0 0 002 0 . 002 . 006 Specific gravity ......... 3. 35d: 05 3. 4:]: 1 3. 24:|:. 05 .............. 2. 653:. 05 .............. 2. 55:l:. 02 Indices of refraction l a _____ _- _ =1. 697:]; 005 B = 1. 708:1; 005 7 .-__ ............. _ =1. 7303:. 005 1 Determined by D. J. Jameson. n.d. =not determined. Looked for but not found: B, Th, P, Ta, W, U, Re, Pd, Ir, Pt, Ag, Au, Cd, Hg, In, Ge, As, Sb, Bi. EXPLANATION or SAMPLES Analyst Sample X-ray X-ray Spectrographic L-24—1 ............. Magnetite ................. n.d. H. J. Rose. MC—l—c. Sphene .................... n.d. J. D. Fletcher. M C—l—b _ Diopside-hedenbergite. - _ . n.d. Do. MC—1—6 ..... do ..................... Monoclinic pyroxene prob. intermediate be- F. A. Hildebrand ..... H. J. Rose. tween diopside and augite. MC—l—5... Aegirinedio side _____ o ..... do . Do. MC—l—8. _ Green hornb ende .-_ Amphibole probably hornblende ..................... do ................. Do. MC—1—9 ............ Nepheline ....... _ n.d. Harry Bastron. MC—1—2 ..... do Nepheline and minor analcimefi’) ............... F. A. Hildebrand ..... H. J. Rose. MC—1—10 ___________ Sodic orthoclase-. - n.d. Harry Bastron. MC~1—1 ................. do ..................... n.d. H. J. Rose. 1. Zirconium occurs in amphibole (0.08 percent), pyroxene 0.007~ 0.05 percent) and sphene (0.05 percent). 2. Most of the lanthanum is in early formed sphene and in the late forming mafics, aegirine-diopside and hornblende. 3. Vanadium is concentrated chiefly in magnetite (0.5 percent) and also occurs in the other mafic minerals in concentration ranging from 0.02 percent to 0.1 percent. Late formed pyroxene contains more vanadium (0.08—0.1 percent) than early more titaniferous pyroxene (0.02—0.06 percent). . Most of the chromium is in early pyroxene. . Niobium is concentrated almost exclusively in sphene. was detected in hornblende. 6. Most of the copper, cobalt, and zinc are in magnetite, whereas most of the nickel is in sphene and early pyroxene. Scandium occurs in pyroxene and amphibole and is most abun- dant in sodic pyroxene (0.01 percent). By comparing the trace—element content of the sap- rolite, L—24, with that of the fresh rock, MC—l, we can get some idea of the efl’ects of weathering on the trace elements. Ti, Nb, Sc, Yb, and Pb are enriched in the saprolite, whereas Sr, Mn, and Or are decreased. Re- 011% Some sistant sphene probably is responsible for the concen— tration of Yb, Nb, Pb, and part of the Ti. Resistant magnetite probably is responsible for concentration of part of the Ti. GARNET-PSEUDOLEUCITE SYENITE DISTRIBUTION AND DESCRIPTION The garnet—pseudoleucite syenite, the most abun— dant rock type, represents about 21 percent of the igneous rock exposed in the complex. It forms a nearly complete ring ranging from a few to nearly 2,000 feet in width (pl. 1). Good exposures include: The Diamond Jo quarry in the western part of sec. 29, the creekbeds of secs. 29 and 30, the creekbed in the northwestern part of sec. 18, and the highway cut in the northeastern part of sec. 24. The latter exposure is saprolite but the porphyritic texture of the original rock is preserved and a few re- sidual boulders with a fresh rock core can be found. IGNEOUS COMPLEX 11 Several mineralogic and textural varieties of this rock can be found but the typical fresh rock is light gray, medium grained, and composed of pseudoleucite, feld- spar, black titanium garnet, pyroxene, and nepheline. Nepheline is much more abundant than in the sphene- nepheline syenite. Weathered surfaces are rough due to the protuberance of sodic orthoclase laths. The white pseudoleucite crystals are the most conspicuous criteria for defining this rock type but they are not al— ways present, Whereas the consistent occurrence of black garnet and absence of visible sphene in all var- ieties are diagnostic. Black titanium garnet is abun— dant in ijolite also, but the ijolite contains no feldspar. Inclusions in the rock are abundant and include met- amorphosed sediments and fine— to coarse-grained ijo- lite and melteigite fragments. These inclusions up to 10 feet across are most easily seen and studied in the north face of the Diamond J o quarry but they are also found in the float throughout the mapped unit. Miarolitic cavities up to 3 inches across are common in the rock in the Diamond J0 quarry. The minerals that partly fill these cavities are generally automorphic and include: tabular white orthoclase, needles of green aegirine, needles of colorless pseudowavellite, and short prisms of colorless apophyllite. Color variations in the fresh rock include light gray, light greenish gray, and gray. The saprolite is gen- erally some shade of brown but is sometimes gray and if the original rock was porphyritic the residual pheno- crysts of pseudoleucite stand out as white spots. A coarser grained more slowly cooled phase of the garnet—pseudoleucite syenite was mapped separately as garnet nepheline syenite and comprises about 3.5 percent of the exposed igneous rocks. The best ex- posures of this rock are in the NEM sec. 18 where it occupies the top of the ridge and is surrounded by the more typical garnet—pseudoleucite syenite. Earlier workers mapped this rock as part of the sphene- nepheline syenite, but the presence of macroscopic garnet, absence of macroscopic sphene and gradational contact with garnet pseudoleucite syenite mark it as a coarser grained phase of garnet pseudoleucite syenite. The early formed leucite crystals must have reacted with the magma and were completely eliminated. In the more rapidly cooled medium-grained rock, the early leucite crystals were not resorbed but adjusted to stability conditions by exsolution of fine-grained masses of nepheline and orthoclase but preserving the outward crystal form of leucite——hence, pseudoleucite. The chemical analyses of the two garnet syenites are very similar and they differ from the sphene-bearing nephe— line syenite. A well-developed major joint system strikes parallel to the ring-dike outcrop pattern. The major set dips about 55° SE; the minor set dips northwest. A second minor system strikes at right angles to the major system and dips range from 75° to vertical. In most thin sections, nepheline occurs in two, gener- ations: as unusually clear, blocky subhedral to euhedral crystals or phenocrysts up to 10 mm across that appear to be earlier than the sodic orthoclase, and as late, potash-rich, anhedral equigranular grains in the ground- mass. Phenocrystic nepheline poikilitically includes aegirine laths. The borders of the crystals are com- monly altered to analcime, sodalite group, cancrinite and calcite. Nepheline is more abundant in this rock than in the sphene-bearing nepheline syenite and ranges from 20 to 60 percent of the rock. In the coarse— grained variety, nepheline comprises only 15 to 25 percent of the rock and is probably younger than the feldspar. Sodic orthoclase as elongate tabular crystals as much as 5 mm long comprises 5 to 40 percent of the pseudo- leucite phase and 40 to 60 percent of the coarse—grained phase. In some rocks the potash feldspar is micro- perthite; in others, anorthoclase. In one thin section anorthoclase forms large optically continuous areas that poikilitically include all other minerals. In a few samples potash feldspar occurs as phenocrysts up to 4 mm long. Early diopside—hedenbergite, late aegirine-augite and aegirine are the chief pyroxene minerals. In some areas the diopside-hedenbergite is tinted purple due to the presence of titanium in the structure. The relative proportions of the pyroxenes vary widely but together they average 15 percent of the rock. The diopside- hedenbergite occurs as colorless ravaged euhedral crys- tals that are rimmed with green aegirine—diopside. Aegirine occurs as very late formed small euhedral crystals. Rarely pyroxene is altered to green biotite; no amphibole has been observed in the 18 to 20 thin sections studied. A black titanium garnet that ranges in composition from titanium-bearing andradite to melanite is one of the most diagnostic minerals of the garnet—pseudo- leucite syenite. The proportion of garnet ranges from an accessory mineral to a major constituent status. In thin section the garnet is a deep brown, late—formed mineral, commonly subhedral, and poiki- litically includes early—formed minerals. Pale-yellow, highly birefringent alteration masses of the garnet may be sphene formed by release of iron from the garnet leaving calcium, titanium, and silica, the components of sphene. Perhaps some of the released iron goes into deuteric green biotite. White pseudoleucite crystals ranging from V; to 2 inches across are also diagnostic of the garnet-pseudo- leucite syenite, but their proportion in the rock is 12 extremely variable ranging from 0 to 60 percent. On Weathered surfaces they stand out in positive relief and almost perfect crystals may be picked up in grus accumulations. Pseudoleucite is rarely present in the coarse-grained phase of the rock which suggests com- plete reaction of early leucite crystals with magma. The pseudoleucite crystals are composed of fine— grained masses of orthoclase and nepheline that have the trapezohedron form of the original leucite crystals. One thin section of pseudoleucite showed a myrmekitic- like intergrowth of sodic orthoclase and nepheline. Commonly the nepheline is altered to cancrinite. A brief description of each of the analyzed samples follows: M C—1 11 (NE}.{ sec. 18).—Gray, porphyritic, medium- grained garnet-pseudoleucite syenite (table 4). Phenocrysts of pseudoleucite up to 7 mm, and black garnet as much as 6 mm make up about 3 percent of the rock. In thin section the min- erals are subhedral to anhedral nepheline partly altered to analcime, sodalite group, and cancrinite; subhedral to anhedral sodic-orthoclase partially altered to analcime, sodalite group, ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. and cancrinite (some of the feldspar crystals poikilitically include nepheline, diopside-hedenbergite, garnet, sphene, aegirine and magnetite); subhedral to anhedral diopside-heden- bergite rimmed with aegirine-diopside and the whole partly altered to biotite and magnetite; euhedral to anhedral aegirine partly altered to biotite; anhedral garnets, some of which are zoned from brown in the center to colorless on the borders and partly altered to sphene; anhedral sphene; anhedral perovskite; and anhedral fluorite generally intergrown with sphene. No apatite was detected in thin section but the P205 in the analysis indicated some was probably present in the rock. M C—1 21 (west edge, sec. 29) .—Gray porphyritic medium-grained garnet—pseudoleucite syenite (table 4). In thin section the rock is composed of subhedral nepheline (early) and anhedral nephe- line (late) both partly altered to analcime, sodalite-group min- erals, and cancrinite; subhedral to anhedral sodic orthoclase partly altered to analcime,sodalite-group minerals, and cancri- nite; euhedral to anhedral diopside-hedenbergite, rimmed with aegirine-diopside and aegirine and partly altered to biotite and magnetite; euhedral to anhedral aegirine—diopside rimmed with aegirine and partly altered to biotite and magnetite; euhedral to anhedral aegirine partly altered to biotite and magnetite; euhedral to anhedral garnet some of which is zoned, brown in the center to colorless borders, and partly altered to sphene; TABLE 4.—Analyses, norms, and modes of garnet-pseudoleucite syenite, in percent Sample No. Sample No. Sample No. f Sample No. MC—111 MO—121 MC—llllMC-lzll L—2l L—50 MC—lll!MC—121( MO—lll ' MC—121 L—21 L—50 MC—64 Standard chemical analyses 1 Spectrugraphic analyses 3 Norms Modes 3 $102 ...... 49. 43 47. 31 0. 0007 0. 0004 0. 0005 0. 0005 or _______ 36. 70 38. 36 Orthoclase ________ 30 25 35 35 A120; _____ 20. 17 20. 10 n.d. n.d. .7 . 2 ab _______ 11.00 Nepheli1)1e (can- 40 40 41 41 crinite . Fean _____ 4. 00 3. 57 n.d. n.d. >5 .02 an _______ 6. 39 6. 39 Analcime (or 3 1 3 3 sodalite group). FeO ______ 1. 71 2. 62 . 014 . 044 . 2 . 05 _________________________ Monoclinic pyroxene: MnO...-. . 32 .30 . 26 . 48 .3 .3 29. 82 Diopside-heden— 5 2 1 2 bergite. MgO _____ . 64 .89 0 0 .002 . 001 Aegirine—diop- side. 12 16 7 9 030--...- 5.08 6. 67 .0003 0003 0 0 Aegirine-_....-.. BaO.-.... . 27 .36 . 0024 0038 .003 0 2. 54 Bigtite (magnet- 4 3 6 7 1 e . 8. 30 7. 98 .027 . 018 006 03 Garnet (sphene)... 5 11 6 2 6. 19 6. 50 n.d. n.d. 6. 24 Apatite ___________ . 10 . 10 .0002 . 0002 .0008 0 7. 31 Calcite ____________ l. 73 1. 42 n.d. n.d. 1 . 5 5. 10 Clinozoisite _______ . 63 . 88 . 022 . 021 . 03 . 03 Fluorite..- 1 2 1 1 . 96 1. 07 . 046 . 045 . 06 03 1. 67 Magnetite. 13 . 21 . 0096 .0074 . 03 03 . 34 ' 13 . 01 . 0034 . 0005 . 0004 . 001 . 39 08 . 04 26 . 22 . 5 1 . 12 . 20 19 n.d. n.d. 4 4 . 03 06 . 0008 . 0009 . 0008 . 0008 Colorimetric analyses 4 0 . 0006 0 0 Total.-- 100.10 100. 28 . 0010 .0008 . 02 .0003 <0. 001 0.003 <0. 001 0.002 n.d. Less 0... . 12 . 12 .0027 . 0027 .003 .003 .0001 <. 0001 .0001 . 0002 n.d. .001 <. 001 n.d. n.d. n.d. 99. 98 100. 16 0 0 0 .001 .015 .012 .015 .013 n.d. ’ Radiometric analyses 5 eU ................ I 0.003 0.002 <0. 001 0.003 Chemical analyses 0 0.00020 0.00020 n.d. n.d. 0.00017 n.d. . 00055 n.d. n.d. . 00069 1 Standard chemical analyses. MC—lll and MC—121 by L. M. Kehl; MC—lll—l by L. N. Tarrant. 2 Spectrographic analyses. MC—lll and MC—121 by H. J. Rose; L—21 and L—50 by Sol Berman. 3 Modes by L. V. Blade. 4 Colorimetric analyses by H. E. Crowe and A. P. Marranzino. l Radiometric analyses by B. A. McCall. 0 Low-level chemical analyses of uranium and thorium by J. C. Antweiler. n.d.—not determined. Looked for (spectrographically) but not found: Hf. Th. P. Ta, Mo, W, U, Re, Ru. Rh. Pd, Os, Ir, Pt,Ag. Au. Zn, Cd,Hg,In. Tl. Ge, Sn. As. Sb. Bi, Te. M C—111. Porphyritic garnet pseudoleucite syenite, NEV; sec. 18. MC—121. Porphyritic garnet pseudoleucite syenite, SW34 sec. 29. MC—lll-l. Pyroxene composite from MC—lll. L—21. Porphyritic garnet pseudoleucite syenite, NEM sec. 24. Mode of fresh rock, analyses of saprolite. L—50. Porphyritic garnet pseudoleucite syenite, NEM sec. 24. Mode of fresh rock, analyses of saprolite. 0—64. Porphyritic garnet pseudoleucite syenite, N EM sec. 18. IGNEOUS COMPLEX TABLE 5.—Analyses, norm, and modes 13 of garnet-nephelt’ne syem‘te, in percent Sample No. Sample No. Saigmle Sample No. 0. _________._— ______—___—. MC—112 ‘ MC—112-6 MC—ll2 ‘ L—54 L—84 MC—112 MC—112 \ L—54 L-84 Standard chemical analyses 1 ~ Spectrographic analyses 1 Norm Modes 3 0. 0007 0. 0003 0.0003 or _________ 40. 03 Sodic orthoclase _____________ 27 42 36 n.d. . 1 .2 ab....__._- 7. 34 Nepheline (cancrinlte and 43 42 33 zeolite). n.d. .04 .07 5. 56 Sodalite and (or) analcime... 1 2 5 .054 .1 . 05 24. 99 Diopslde-hedenbergite ______ 2 33 5 . 5 .12 Aegirine—diopside ____________ }16 11 }19 0 .001 2. 65 0003 0 6 . 002 0 4. 47 .024 0 0 5. 68 n.d. .3 .3 4.18 03 . 02 03 1. 28 1 . 052 . 009 05 1. 52 .0099 . 02 04 . 34 .002 .0005 001 . 62 . 0008 001 28 . 2 2 n.d. 1 4 . 0008 .0005 001 0 . 0007 0008 .0009 .0009 .001 0.001 .0002 n.d. . 0033 . 002 . 003 . 013 . 01 . 001 Radiometric analyses 5 eU __________________________ 0.005 ‘ 0.006 \ 0.005 Chemical analyses 0 U ___________________________ 0.00027 n.d. n.d. Th __________________________ . 00045 mi. n.d. 1 Standard chemical analyses. MC—112 by L. M. Kehl and MC-112-6 by L. N. Tarrant. 3 Spectrographic analyses. MC—112 by H. J. Rose, L-54, L-84 by Sol Berman. 3 Modes by L. V. Blade. 4 Colorimetric analyses. MC—112, L—54, L—84 by H. E. Crowc and A. P. Marten- zinc. 5 Radiometric analyses by B. A. McCall. 0 Low-level chemical analyses of uranium and thorium by J . C. Antweiler. n.d.——not determined. and euhedral apatite. No fluorite was detected in thin section but it is probably present in the sample. L—21 (NE1/4 sec. 24).—Light—gray medium—grained garnet— pseudoleucite syenite (table 4). The fresh rock in thin section is composed of subhedral to anhedral sodic orthoclase partly altered to analcime or sodalite group, cancrinite, and calcite; nepheline as corroded crystals (earlier than the feldspar) partly altered to analcime or sodalite group, cancrinite and calcite; corroded crystals of very pale brown diopside-hedenbergite rim— med with green aegirine—diopside and aegirine; subhedral to anhedral green aegirine-diopside rimmed with green aegirine and partly altered to green biotite and magnetite; euhedral to anhed- ral crystals of green aegirine some partly altered to biotite and magnetite; euhedral to anhedral zoned garnet with brown centers (melanite) and colorless rims (andradite) partly altered to sphene, green biotite, magnetite, and calcite; subhedral crystals of magnetite some rimmed With biotite; and euhedral apatite. Material in the concentrate of the panned saprolite occurs in about the following order of abundance: feldspar, nepheline (some pink), garnet, pale-brown mica, green pyroxene, magnetite, pyrite, and apatite. A spectrographic analysis of the saprolite and the mode of the fresh rock are shown in table 4. L-50 (N E1 /4 sec. 24).—Light gray, porphyritic, medium- grained garnet-pseudoleucite syenite (table 4). Phenocrysts of pseudoleucite as much as 10 mm, sodic orthoclase 10 mm long, nepheline 10 mm, black garnet 5 mm, respectively, and agglo- Looked for (spectrographically) but not found: Ce, Yb. Hf, Th. P. Ta, W, Re, Ru. Rh, Pd. Os. Ir, Pt, Ag, Au. Zn. Cd. Hg. In. Ge. Sn, As. Sb. Bi, Te. MC—112. Garnet-nepheline syenite, NWM sec. 17. MC—112-6. Pyroxene composite from MC—llz. L—54. 1C}tarnet-nepheline syenite, NWVr sec. 17. Mode on fresh rock, analyses on sapro l e. L—84. Garnet-nepheline syenite, NE% sec. 20. Mode on fresh rock, analyses on saprolite. merates of fine—grained pyroxene and sodic orthoclase make up about 5 percent of the fresh rock. In thin section the minerals are: subhedral nepheline earlier than the feldspar and anhedral nepheline later than the feldspar both partly altered to analcime or sodalite group and cancrinite; subhedral to anhedral sodic 0r- thoclase partly altered to analcime or sodalite group and can- crinite; subhedral diopside—hedenbergite altered on the borders to aegirine diopside; euhedral to anhedral aegirine-diopside partly altered to biotite and magnetite; anhedral brown— to brownish- yellow zoned garnet partly altered to sphene and fluorite; and euhedral apatite as inclusions in the diopside-hedenbergite. Ma- terial in the concentrate of the panned saprolite occurs in about the following order of abundance: feldspar, nepheline, pale- brown mica, garnet, pyroxene, pyrite, apatite, and magnetite. From an X-ray analysis of the clay- and silt-size part of the sap- rolite sample, A. J. Gude 3d, (written communication, 1956) reported major kaolinite and minor illite. A spectrographic analysis of the saprolite and the mode of the fresh rock are given in table 4. CHEMISTRY Chemical and spectrographic analyses, norms, and modes of garnet—pseudoleucite syenite and garnet-neph- eline syenite (tables 4 and 5) show that these rocks are typical nepheline syenites but distinctly different from the previously described sphene-nepheline syenite, par- l4 ticularly in silica and in volatile content. Silica is lower, but the alkali is the same resulting in a much higher nepheline content. 002 is uniformly high and occurs in cancrinite; the high CI (in sodalite) character- istic of sphene-nepheline syenite is negligible in these rocks. The TiOz content is lower and is chiefly in gar— net, whereas the higher TiOz content of sphene-nephe- line syenite occurs in sphene. The amount of modal nepheline is much greater than normative nepheline. However, the nepheline defi- ciency in the norm can be ascribed to the use of much of the sodium in normative albite and some of the potas- sium in normative orthoclase. The excess silica would combine with normative anorthite, wollastonite, ilmen- ite and part of the iron oxides to make the modal pyrox- enes and titanium garnet which comprise 22 to 29 per— cent of the rock. The high Fe203 and N azO in the analyzed pyroxene composite reflect the aegirine-diopside and aegirine al— TABLE 6.~Spectrographic analyses, ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. teration of early diopside hedenbergite; the K20 is prob- ably in minor biotite alteration products. Trace elements equal to or greater than 0.01 percent include, in order of abundance, Ba, Sr, V, La, Zr, and Nb. The distribution of most of the trace elements within these rocks is shown in the spectrographic analyses of minerals separated from the analyzed rocks (tables 6 and 7). 1. Most of the manganese in the rock is in magnetite, sodic pyroxene, and garnet. . Most of the nickel is in garnet. . Vanadium has its highest concentration in magnetite, garnet, and perovskite but is also present in the pyroxenes, parti- cularly sodic pyroxene. . Yttrium is concentrated in perovskite and garnet with lesser amounts in the diopside hedenbergite. Lanthanum, how- ever, is not present in the pyroxenes, but occurs in the garnet and perovskite in concentrations up to 0.06 percent. The high lanthanum content of the rocks (0.018 to 0.027 percent) suggests that not all of the lanthanum is accounted for by the minerals so far analyzed in this study. 0019 yh in percent, of mineral separates of garnet-pseudoleucite syenite ’ MC—121—1 MC—121»c MC—121—b I MC—111—c MC—111—d MC—111—10 MC—llI—a MC—121—a MC—111—9 MC—lll—b MC—111—11 0 0 0 0 0 0 0 0 0 0. 001 0.00008 3 .02 1 1 >10 >10 >10 >10 1 .01 n.d. >10 >10 10 >10 n.d. >10 >10 n.d. >10 n.d. .0009 .3 .2 .7 1 .2 .4 .4 .3 .6 .02 .005 .002 .02 .05 1 .003 .009 .02 . 002 .08 .003 0 0 . 0004 . 001 . 002 . 0003 0 0 .1 .1 .05 .06 .006 .02 .02 0 0 0 0 .05 .01 . 02 .06 0 0 0 0 n d n. n.d. n.d. . 2 n.d .0008 .0005 n.d. 0 n.d. 3. 4 >10 1—10 1—10 1—10 1. 5 1—10 1—10 . 5 . 4 . 008 0 0 .2 .08 .2 .04 .06 .002 .01 .02 .02 .5 .1 .3 .2 .2 .1 .04 .03 .2 .06 .007 0 .3 . 009 . 02 . 1 0 0 0 0 0 . 004 . 0008 . 0002 . 0002 . 0002 . 0007 . 003 . 003 . 0007 . 0002 . 0007 1 .04 .2 .3 . .09 .1 .06 .7 .5 .001 >10 >10 >10 >10 1-10 >10 1—10 1—10 >10 >10 03 . 008 . 003 . 001 . 003 0 . 0008 . 003 . 002 . 0006 . 001 0 . 007 0 .004 .001 .004 .002 0 .003 0 0 .002 .009 .002 . 002 .003 .003 .002 .001 .003 .001 . 0006 0 0 0 0 0 0 0 0 0 .0001 0 . 2 0 0 0 0 0 0 0 0 0 0 . 003 0 . 0002 0 . 003 . 0009 . 002 . 002 . 001 . 001 003 n.d. n.d. n.d. n.d. 0 n.d. n.d. .008 n.d. 0 Specific gravity __________________________________________________________________________ 3. 355:. 05 3. 55¢. 05 2. 253:. 05 Indices of refraction 1 1. 7253:. 005 1. 7123:. 005 1. 737:|:. 005 _ 1. 7383:. 005 1. 7503:. 005 1. 785:1: 005 l Determined by D. J. Jameson. n.d.—not determined. Looked for but not found: B, Th, P, Ta, Mo, W, U, Re, Rh, Pd, Ir, Pt, Au, Cd, Hg, In, Ge, Pb, As, Sb, Bi. EXPLANATION or SAMPLES Analyst Sample X—ray Analyst _ a M M C—121—1 __________ H. J. Rose, Jr _______ Magnetite _____________________________ MO—lZl—c - J. D. Fletcher..- Perovskite. - MO—121—b _ _____ do ___________ Garnet (mel . MC—lll—c . _ -_do.. - _____ do _________________ ___ MC—111—d . _____ do ___________ . Garnet (pale-brown an -__ Andradite _______________________________________________ W. F. Outerbddge. M C—111—10 _ . H. J. Rose, Jr... _ Diopside-hedeubergite Monoclinic pyroxene, probably augite and minor biotite.. F. A. Hildebrand. MC—lll—a J. D. Fletcher." _ _____ (10..-. MC—121— _ _ ..... do ........... _ _____ do_.__ __ MO—lll—Q. H. J. Rose, Jr _____________ Aegirine-diopside Monoclinic pyroxene, probably intermediate between Do. diopside and augite. MC—lll—b _____ ___ J. D. Fletcher ............. Aegirine _______________________________ MC—lll—ll _________ H. J. Rose, Jr _____________ Analcime _____________________________ Analcime _______________________________________________ Do. IGNEOUS TABLE 7.—-Spectrographic analyses of mineral COMPLEX separates of garnet-nepheline syem'te, in percent MC—llZa MC—112b MC-112-1 MC—112—2 MC—112—3 MC—112—4 MC—112—5 Be ______________________ 0 0 0 0. 0001 0. 0001 0 0. 0001 Mg _____________________ . 4 >10 >10 >10 >10 .02 .04 Ca ______________________ > 10 >10 n.d. n.d. n. n.d. n.d. Sr ______________________ . 3 . 4 . 1 . 4 . 4 . 5 . 1 Ba ______________________ . 04 . 008 . 001 . 002 . 001 9. 5 . 009 Sc ______________________ 0 . 002 . 003 _ 0 0 0 0 Y _______________________ . 02 . 03 . 008 . 004 . 004 0 0 La ______________________ 0 0 0 0009 . 0009 0 0 Yb ______________________ n.d. . 001 n.d. n.d. n.d. n.d. n.d. Ti ______________________ 1—10 1—10 2. 4 . 5 . 6 . 03 . 01 Zr ______________________ . 04 . 05 . 03 . 02 . 05 . 04 . 01 V _______________________ .2 .03 .03 .2 .2 .01 .01 Nb _____________________ . 06 0 0 0 0 0 0 Cr ______________________ . 0001 . 0002 . 0007 . 0007 . 007 . 0007 . 002 Mn _____________________ . 18 . 06 . 04 1. 1 1.0 .001 . 005 Fe ______________________ >10 1—10 >10 >10 >10 . 9 . 2 Co ______________________ . 0005 . 003 . 002 . 001 0 0 0 Ni ______________________ . 0006 0 0 . 003 . 004 0 0 Cu ______________________ . 002 . 002 . 0009 . 002 . 001 . 0008 . 0008 Ag ______________________ 0 0 0 . 0003 . 0004 0 0 Ga ______________________ . 0002 . 004 . 001 . 002 . 001 . 003 . 008 Sn ...................... n.d. n.d. . 002 . 004 . 002 0 0 Specific gravity ___________________________________ 3. 35:I:. 05 3. 35 i. 05 3. 55 :|:. 05 2. 75:5; 05 2. 3 Indices of refraction 1 a _______________________________________________ 1. 713:1;0. 005 1. 693:}:0. 005 B _______________________________________________ 1. 721:|: . 005 1. 719i . 005 y _______________________________________________ 1. 745i .005 1. 779:1: .005 1 Determined by D. J. Jameson. n.d.=not determined. Looked for but not found: B, Hf, Th, P, Ta, Mo, W, U, Re, Pd, Ir, Pt, Au, Zn, Cd, Hg, In, Ge, Pb, As, Sb, Bi. EXPLANATION or SAMPLES Analyst Sample X-ray X—ray Spectrographic MC—112a _____ Dark garnet __________________________________________________________________________ J. D. Fletcher. MC—l 12b _____ Diopside—hedenbergite __________________________________________________________________ Do. MC—112—1____ _____ do ___________________ Monoclinic pyroxene probably augite ______ F. A. Hildebrand-___ H. J. Rose. MC—l 12—2- ___ Aegirine __________________ Monoclinic pyroxene probably aegirine and _____ do _____________ Do. minor biotite. MC—112—3____ ________________________ Monoclinic pyroxene probably aegirine ____ _____ do _____________ Do. MC—112—4_-__ Barian feldspar ________________________________________________________________________ Do. MC—112—5- --_ Zeolite ___________________ Natrolite and minor sodalite ___________________ do _____________ Do. 5. Zirconium occurs chiefly in the garnet and diopside-heden— bergite, with lesser amounts in sodic pyroxene and felsic minerals. 6. Niobium is concentrated in perovskite and garnet. 7. Strontium is concentrated in the mafic minerals. The quan- tity of barium found in the analyzed minerals is insufiicient to account for the barium content of the rock. Most of the barium probably is present in feldspar with lesser amounts in the garnet. The high barium content of the barian feldspar of the garnet nepheline syenite is particu- larly noteworthy. 8. Gallium is not concentrated to any great extent in any mineral but is highest in the zeolite. MISCELLANEOUS SY'ENITES Scattered about the complex are gray medium-grained syenites that are not readily classified with any of the foregoing syenites nor with each other. These rocks are chemically similar to the previously described rocks, but small mineralogic difierences in isolated bodies permit differentiation of these units on the geologic map. rarnsmrnormr. LEUCOSYENITE A unit mapped as feldspathoidal leucosyenite occurs in the southeastern of the complex and represents about 0.7 percent of the exposed part of the igneous complex (pl. 1). Good exposures are rare. The rock is a light-gray fine— to medium-grained phanerite that ranges in composition from alkalic to nepheline syenite. In thin section the texture is hypau- tomorphic-trachitoid. Felsic minerals include sodic 16 orthoclase, analcime or sodalite, perthite, plagioclase, and nepheline. Other minerals (usually not abundant) include aegirine-diopside, biotite, muscovite, horn- blende, colorless to brown garnet, apatite, sphene, fluorite, magnetite-ilmenite, pyrite, and pyrrhotite. Alterations include sodic orthoclase to kaolinite, seri- cite, or cancrinite; nepheline to cancrinite, and pyroxene to biotite. Analcime or sodalite and calcite (Where present) are late. Some of the variants are briefly described below: MC—146.—Light-gray, medium-grained feldspathoidal leuco- syenite, southwestern part of sec. 21. In thin section the rock has a hypautomorphic-trachitoid texture and is composed of analcime or sodalite, about 2 percent; nepheline partially altered to cancrinite, about 4 percent; plagioclase probably albite, about 2 percent; kaolinized sodic orthoclase and perthite, about 82 percent; and neutral diopside-hedenbergite almost completely altered to brownish-green biotite, sphene (some euhedral), and pyrrhotite, about 10 percent. M 0—933.—Light—gray fine—grained nepheline syenite; south- eastern part of sec. 20. The rock has a hypautomorphic-trachi- toid texture and contains analcime or sodalite, about 2 percent; nepheline partly altered to cancrinite, about 15 percent; sodic orthoclase partly altered to cancrinite, about 71 percent; and aegirine-diopside, fluorite, pyrrhotite, and clinozoisite(?), about 12 percent. M C—934.~—Light-gray medium-grained feldspathoidal leuco- syenite, same locality as MC—233. The rock has a hypautomor- phic-trachitoid texture and contains analcime or sodalite, about 4 percent; nepheline partly altered to cancrinite, about 6 percent; kaolinized sodic orthoclase and perthite partly altered to sericite, about 85 percent; and sphene, brown to colorless garnet, brown- ish-green biotite, brownish-green hornblende, and pyrrhotite, about 5 percent. L—325.—Light—gray, medium-grained alkalic syenite, same locality as MC—233 and MC—234. The rock has a hypautomor- phic trachitoid texture and contains late calcite associated With garnet, analcime, kaolinized sodic orthoclase partly altered to sericite, yellowish-green biotite, muscovite, colorless garnet inter- grown with sphene, apatite, magnetite-ilmenite, pyrite, and pyrrhotite. Chemical and spectrographic analyses, the norm, and the mode of feldspathoidal leucosyenite (table 8) are distinctly different from the previously described nepheline syenites. The K20 is higher than NazO—a rarity in Magnet Cove rocks. This relationship is brought out in the mode by the high content of sodic orthoclase. N epheline (21.3 percent) appears in the norm but not in the mode because sericite alteration and biotite and muscovite (low-silica minerals) occur in the mode whereas this silica deficiency shows in the norm as nepheline. The FeO/Fe203 ratio of the chem- ical analysis is not reliable because a soluble sulfide, pyrrhotite, is present in the mode. The isotropic min— eral in the mode is interpreted to be analcime, since 803 and 01 are low in the analysis. Trace elements equal to or greater than 0.01 and in order of abundance include Sr, Ba, Zr, V, Nb, and La. ALKALIC IGNEOUS COL/IPLEX AT MAGNET COVE, ARK. TABLE 8,—Analyses, norm, and mode of feldspathoidal leuco- syenite, in percent Chemical Spectrographic Norm Mode 8102 _______ 54. 67 0.0002 Soda orthoclase 82 (sericite) . 20. 83 n.d. Analclme 3 . 20 n.d. Calcite__ 1 3. 69 . 6 4 . 32 . 2 _ 1 . 20 . 009 Biotite __________ 2 2. 59 n.d. Muscovite ______ 4 . 20 .03 Magnetite‘ ilmenite 5. 27 n.d. . 47 Pytrirtlte-pyrrho- 3 e 8. 43 .03 2. 76 Apatite (tin) _ - .. . 09 . 03 1. 40 1. 60 n.d. . 35 n.d. . 60 . 001 . 03 . 004 . 01 . 002 . 01 . 23 1. 48 Total. _ __ 100. 80 Less 0..- . 84 99. 96 n.d.=not determined. Looked for (spectrographically) but not found: 'B, Se. Y, Ce. Yb, Hf. Th, Ta, Cr. Mo. W, U, Re, Co. Ni. Ru, Rh. Pd, Os.Ir. Pt. Ag. Au. Zn. Cd. Hg. In. Ti. Ge, Sn. As. Sb. Bi. Te. 11—325, Alkallc syenite, SE14 sec. 20. Standard chemical analysis by M. K. Balazs. Spectrographic analysis by Sol Berman. Mode by L. V. Blade. SPHERE-CANCRINITE SYENITE A sphene—cancrinite syenite, at least 0.04 percent of the exposed igneous rocks of the complex, is found in the northeastern part of sec. 17 as a small dike in jacupi- rangite and in the eastern half of sec. 24 as small irregular shaped bodies. The rock is a gray- to dark-gray fine— to medium- grained phanerite. In thin section the texture is holo— crystalline hypautomorphic-granular and the minerals include: anhedral sodic orthoclase partly altered to cancrinite, calcite, and analcime or sodalite, 50 to 70 percent; anhedral cancrinite (late interstitial and alter— ation product), 10 to 25 percent; and corroded diopside- hedenbergite partly to completely altered to brown and green biotite and leucoxene. Other minerals include sphene, pyrite, pyrrhotite, apatite, and fluorite, 20 to 25 percent. spnmamnnm-nnrnumn syrmrn Sphene—garnet-nepheline syenite, less than 0.1 percent of the exposed igneous rocks of the complex, occurs as small dikes in jacupirangite or sphene pyroxenite. The samples taken for thin section all came from sec. 17. The rock is a light- gray to gray, fine, to medium- grained phanerite. In thin section the texture is holo— crystalline hypautomorphic-granular and the minerals include: anhedral sodic orthoclase partly altered to calcite, cancrinite, and sodalite or analcime, 30 to 80 percent; anhedral to euhedral nepheline partly altered to calcite, cancrinite, and sodalite or analcine, 12 to 40 percent; and anhedral to euhedral green aegirine. Accessory minerals include sphene, anhedral zoned IGNEOUS COMPLEX (brown to colorless) garnet, biotite (reddish—brown and green), apatite, magnetite, pyrite, pyrrhotite. A few minor syenite units are mapped as miscella- neous syenites. Rocks from two of these bodies are described below: M 0—154.—-—Altered, gray medium-grained alkalic syenite (NEV; sec. 17). This rock apparently intrudes jacupirangite and sphene pyroxenite. In thin section the texture is holo- crystalline hypautomorphic trachitoid, and the minerals include: sodic orthoclase partly altered to sericite, cancrinite, and calcite, about 92 percent; and colorless to brown garnet, brown and green biotite, and pyrite, about 8 percent. M 0—212.——Altered, gray medium-grained alkalic syenite (NWV; sec. 19). This rock occurs as a mappable body in contact with several rock types. However, all contacts are float contacts so relative ages are unknown. In thin section the texture is holocrystalline hypautomorphic-trachitoid, and the minerals in- clude: sodic orthoclase and some perthite partly altered to cancrinite, calcite, and analcirne or sodalite, about 80 percent; and green biotite, apatite, yellowish-brown acm1te, green aegir- ine, pyrite, and magnetite-ilmenite, about 20 percent. JACUPmANGITE AND SPHENE PYROXENITE DISTRIBUTION AND DESCRIPTION J acupirangite, amagnetite pyroxenite that may con- tain afew percent of feldspathoids, constitutes about 10 percent of the exposed igneous rocks of the complex. It crops out at two localities in Magnet Cove and forms magnetic highs which are helpful in delimiting the boundaries of the unit on the map (pl. 1). The largest body of jacupirangite occurs in sec. 17 and ranges in composition from sphene pyroxenite (by reaction with novaculite along the contact) to mel- teigite. Swarms of light—colored, vertical dikes of nepheline syenite, ijolite, and melteigite ranging from less than 1 inch to 3 feet in width cut the jacupirangite and stand out in bold relief on weathered surfaces. The second and smaller body of jacupirangite makes the magnetic high and topographic low on the west side of Magnet Cove in sees. 24 and 13. No fresh rock is exposed but banks of highly magnetic saprolite cut by resistant dikes and stringers of nepheline syenite indicate the character of the rock. One residual knot of moderately fresh rock in the saprolite was composed almost entirely of pyroxene and magnetite. The limits of this rock mass were determined from magnetometer readings (fig. 3) and from panned concentrates of saprolite. Called eleolite mica syenite (cove type) by Williams (1891) and jacupirangite by Washington (1901), the rock is typically a dark-gray fine- to medium-grained phanerite that weathers to a dark-brown or mottled reddish-brown and olive-green saprolite. Pyroxene is the chief constituent and always comprises more than 50 percent of the rock. Magnetite is abundant. Apa- tite, biotite, sphene, garnet, and perovskite are always ————i 0 .o / 3233 3151 / °6319 °2965 5915° 128600 // o O D 02511 °2225 1493 0 106904, «19.758 712,397 5920 o 02983 2803 1048 // / 3158 o 01037//-d o / “39 f“ 0 /11,327 06m \6 // 5497/ / 4‘7, °1286 °2500 / 10,179 0 \‘Zfi / /2‘s703 / 5540 {0° 0 / . / / 09949- ('9 2675 0604 o / l ”9195 21.288 / o / / 5537 O A K °17.954 / 6207 o / 2739 / 9763 o . 5032 18'3“ ($0339 / -'?' a: 1 / é‘ / ‘1" ° °6875 / a“ ° 11335 3 00 22.012/ 0 / 6" 3000/ ' 13' 93 1: o 4479 //\ o I ‘3 21.637 5841 a o \ I 1335" 17°615 : o o 5032 ) . ' 0 may 6769 o / 9505‘ 0 O Q K ”4841 3531} \ } 10.106 13,979 20,359 06631 p 0 \ / ° 14915 4951 \ \/ 9079 020.916 24%7901 o (6)626 0 ° ' / G552 °5872 o 4179\ 6508 o “”788 / 5167 \ 4710 o /o \ 14.411 °17541 11,622 (€847 05449 0 \05806 o \o °5979 1822 I 21,454 \ 21,230 €1,283 0 I / 0,3379 24,637 0 l 05926 / 17,774! 0 ° ° 5510 4610 ( 11.516 021886 / $833 / (132,520 0 \ cism / o o 5502 i ~_,/ 8076 7283 ‘ ° °6053 O 5257 I 08733 08160 5359 I ° ° °6511 o 8269 6562 I 7092 o \ ° 0 o o 5195 D 7150 6850 5905 6004 \ 6700 07919 %209 N °5515 \ \ 0 11000 2000 FEET FIGURE 3.—Map showing distribution of jacupirangitejnferred from high magne- tometer readings, in gammas. After magnetometer survey map made by Heiland Research Corp. for U.S. Bureau of Mines (written communication, 1942). present, sometimes in proportions greater than 10 per- cent. Zeolite formed from the alteration of nepheline, calcite, and cancrinite is common. Other accessory minerals include pyrite and pyrrhotite. The pyroxene crystals (salite) up to 10 mm long appear to have formed an early crystal mush. Many of the grains have a light—green to colorless core that is rimmed by pink-tinted titanaugite; other grains have ravaged edges marked by formation of sodic pyroxene or biotite. Inclusions of magnetite, perovskite, and apatite are common. Magnetite—ilmenite in grains as much as 6 mm across comprise about 2 to 25 percent of the rock. Some of the grains are rimmed with perovskite and have apatite h 18 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. inclusions. In some thin sections this assemblage plus brown to colorless garnet, aegirine—diopside, sphene, pyrite, biotite, and apatite occur in veinlets cutting the rock and appears to be a late introduction. Small rods of ilmenite (?) occur as minor inclusions in the pyroxene. The panned concentrate of saprolite of jacupirangite contains magnetite, pyroxene, sphene, apatite, perov- skite and (or) garnet, mica, and clay aggregates. From an X-ray analysis of the clay— and silt-size portion of the saprolite, A. J. Gude 3d (written communication, 1956) reported major chlorite, minor montmorillonite, and a trace of kaolinite (‘3). Two noteworthy varieties of the jacupirangite are produced by reaction with sedimentary rocks. A small body of sphene pyroxenite in the northeastern part of the complex has formed from the reaction of jacupi- rangite and Arkansas novaculite. Exposures are poor and the contact with novaculite is covered with colluvium. The color of the rock in hand specimen depends on the amount of sphene present, but this fine- to medium- grained phanerite is typically dark gray with reddish- gray and greenish—gray variations. In thin section the texture is hypautomorphic—granular, and mineralogi- cally there are two types represented by MC—151 and MC—172 described below: M C’-—151.—Sphene pyroxenite composed of faintly pleochroic (pale yelloWish—brown to pale bluish-green) diopside-hedenberg— ite, about 67 percent; anhedral to euhedral sphene, about 8 percent; apatite, about 4 percent; anhedral brownish—violet perovskite rimmed with sphene, about 1 percent; pyrite, about 1 percent; and a late interstitial mixture of birefringent zeolite and analcime or sodalite group minerals which replaces pyroxene in part, about 19 percent. MC—172.—Quartz sphene pyroxenite composed of very pale green diopside rimmed with green aegirine-diopside; euhedral and anhedral sphene; apatite; pyrite; pyrrhotite. Orthoclase, perthite, plagioclase probably albite, quartz, calcite, and myrme- kitic intergrowths of quartz and orthoclase occur as late intro- duced minerals in the interstices between pyroxene grains. The lack of perovskite suggests that silica from the novaculite com- bined with titanium and iron to form silicate (sphene) rather than the oxide (perovskite). At the contact with Stanley shale, the rock is a meladiorite consisting of pyroxene (50 percent), plagio— clase (30 percent), biotite (5 percent), with accessory magnetite, sphene, and apatite. The plagioclase, oligoclase—andesine (Ab7An3), occurs as large, late- forming, optically continuous areas that poikilitically include pyroxene crystals. It seems probable that TABLE 9.——Analyses, norm, and modes of jacupinangite, in percent K MO—173 L—81—2 (1) MC—173 L—81 ‘ L—124 MC—173 I ' M C—173 , L—81 { L-124 fix“ Chemical analyses 2 Spectrographic analyses 3 Norm Modes 4 0. 0001 .2 .05 . 006 Less 0... 16. 96 Diopside-hedenberg- 78 78 its (salite). 2. 62 agnetite-ilmenite____ } 5 10 6. 53 Pyrite-pyrrhotite ______ . 14 Perovskite ______ . 3 5 37. 15 Sphene ________ _ 2 1. 54 Apatite _____ _ 6 5 7. 40 Biotite. _ _ l 2 11. 14 Calcite“ _ 1. 28 Zeolite ________________ 4 7. 60 Analeime and (or) sodalite group. 5. 04 Garnet ________________ l . 72 . 20 Colorimetric analyses 5 0.001 0.001 0.001 . 0001 . 0001 . 0001 . 013 . 013 . 015 Radiometric analyses 0 eU .................... 0. 001 0. 001 0. 001 Chemical analyses 7 ________ 0 00049 n.d. n.d. __________ 00339 n.d n.d. 1 Type jacupirangite from Brazil, Washington (1901). TI Stantdard chemical analyses. MC—173 by S. M. Berthold; L—81—2 by L. N arran . Ft 3 Spectrographic analyses. MC-l73 by I. D. Fletcher; L—Sl and L—124 by Sol erman. 4 Modes by L. V. Blade. I Colorimetric analyses by H. E. Crowe and A. P. Marranzino. rRadiometric analyses by B. A. McCall. ' Low-level chemical analyses for uranium and thorium by J. C. r.Antweile n.d.—not determined. Looked for (spectrograghically)but not found: Hf. Th. Ta. W. U, Re. Ru. Rh. Pd. Os. Ir. Pt. Ag, Pb. Au. Zn. Cd. Hg, In, Tl, Ge, As. Sb. Bl, Sn. MCI—173. Jacuplrangite, NEM sec. 17. L—Sl. J acupirangite, near center of sec. 17. saprolite. L—81«2. Apatite from L—81. L—124. Saprolite of jacuplrangite, N EM sec. 17. Mode on fresh rock, analyses on 'IGNEOUS COMPLEX sufficient silica was added from the shale to make plagioclase; sodic orthoclase would not be developed since the potassium content of the original magma was very low and is used in biotite. CHEMISTRY Chemical and spectrographic analyses, norms, and modes of jacupirangite and sphene pyroexnite are given in tables 9 and 10. The presence of an acid-soluble sulfide, pyrrhotite, renders the FeO/Fe203 ratio of the analyses unreliable. The analyzed rock (table 9) is similar to the type jacupirangite from Brazil, but is lower in SiOz, total iron oxides, and MgO and higher in A1203, N a20, K20, and P205. Felsic minerals occur in the norm calculation but not in the mode. The CaO and A1203 of normative an- orthite is in modal pyroxene; K20 of normative leucite is in biotite; N agO of normative nepheline is in zeolite and analcime. It is noteworthy that although jacupirangite has the highest content of magnesia (7.77 percent) of all rock types in the complex, the pyroxene (salite), which makes up 78 percent of the rock, does not have the highest content of magnesia of all pyroxenes (table 56). This is partly explained by the high magnesia content of 19 more than 10 percent in early crystallized magnetite (magnesioferrite) and partly by the abundance of pyroxene in jacupirangite. The sphene pyroxenite has a higher SiOz and lower A1203 and total iron content than the jacupirangite and reflects the reaction of typical jacupirangite with novaculite. Trace elements that exceed or are equal to 0.01 per~ cent include Sr, Ba, V, Zr, La, Cu, and Y. Distribu- tion of the trace elements Within the rocks is shown by the analyses of minerals separated from the analyzed rocks (tables 11 and 12). 1. Strontium occurs mainly in apatite (>1 percent), perovskite, sphene, and pyroxene. 2. The bulk of the zirconium is in sphene (0.1 to 0.19 percent) With lesser amounts in apatite and pyroxene. 3. Vanadium occurs in all mineral separates but is particularly enriched in magnetite (0.2 percent), pyroxene, and apatite. 4. Niobium is concentrated in perovskite (0.3 percent) and sphene (0.1 to 0.15 percent). 5. Lanthanum and yttrium are concentrated chiefly in perovs- kite, sphene, and apatite. 6. Copper was detected in all separates but is most abundant in pyrite, perovskite, magnetite, and apatite. 7. Cobalt and nickel are concentrated chiefly in pyrite with lesser amounts in magnetite. 8. Molybdenum was detected only in zeolites. TABLE 10.—Analyses, norms, and modes of sphene pyroxenite, in percent Sample No. Sample No. Sample No. Sample No. MC—l72 L—326 MC—172 L—326 MC—172 L—326 MC-172 L—326 Standard chemical Spectrographic analyses 1 Norms Modes 3 analyses 1 49.10 0 0.0001 7. 38 5. 94 Diopside—hedenbergite and aegi- 73 85 rine—diopside. 3.11 6.12 3. 34 Sphene ____________________________ 20 8 3. 21 9. 96 1. 57 Apatite ___________ 1 3 6. 52 . 5 . 4 3. 23 7.85 Pyrite-pyrrhotite.. 1 1 .30 .05 .001 39. 99 47. 74 Orthoclase ______ 7. 35 13. 57 13. 92 Perthite 19. 63 . 002 .003 3. 02 1. 62 Albite_. 5 3 008 .004 9. 88 9. 73 Quartz. 1. 60 02 .009 4.03 3.36 Calcite. 1.04 n.d. 0 .07 .16 . 18 0006 .0005 1. 35 l. 96 .26 7.0 n.d. .70 2. 00 5. 21 .02 .03 .32 n.d. 1. 68 058 .3 As ................................ n.d. .59 008 .03 Sb _____ n.d. . 08 0004 . 002 Zn ________________________________ n.d. . 16 003 . 001 . 55 2 n.d Radiometric analyses 5 Total ________ 100. 89 Less 0......_ .36 005 004 eU ________________________________ <0. 001 0.001 100. 53 .003 .001 0 0. 03 Chemical analyses 0 0 002 0 U __________________________________ 0.00020 n.d. Th ________________________________ .00048 n.d. BIIStandard chemical analyses. MC—172 by S. M. Berthold and L—326 by M. K. a MS. I Spectrographic analyses. MC—172 by I. D. Fletcher and L—326 by Sol Batman. 3 Modes: MC—172, R. L. Erickson; L—326, L. V. Blade. 4 Colorimetric analyses by H. E. Crowe and A. P. Marranzino. lRadiometric analyses. MC—172 by B. A. McCall and L~326 by D. L. Schafer. ° Low-level chemical analyses of uranium and thorium by J. C. Antweiler. N.d.=not determined. Looked for (spectrographically) but not found: Hf, Th, Ta. W. U, Re, Ru, Rh, Pd. Os, Ir, Pt. Ag. Au, Zn, Cd, Hg. In. Ti, As, Bi. Sn. MC—172. Sphene pyroxenite, NEV; sec. 17. L—326. Sphene pyroxenite, NW% sec. 16. ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. 20 .mmom H. .m ................ od .......... mIHwLH .2333 .83 £29qu .Q A. ............ ed ..... e93 otdfiddom-wfidomaosfi ................ od .......... 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No. moo. 8. dd 3 A dd dd mo. dd .3 . dd dd 3 A S A --------- mg 38 .o go .o 88 .o o o o o o o «.89 .o 88 .o o o o c o o o .......... em :2 mini bImB mnmfi $52 7»? mum? mum? «1%; 3me mIHwLH TELH 102 «Aw-A TSLH '02 102 '02 :1wa :02 '02 wIHmLH IDS S-ELH I02 :02 RTE-A ICE duggséasufi. Re 83:33 3.83:: “8 .38wa g £3323. uEdEdEBKmI. H H 8:38 IGNEOUS COMPLEX 21 TABLE 12.—Spectrographic analyses, in percent, of mineral separates of sphene pyroxenite MC—172-5 MC-176 M0—177 MC—172—4 MC—172-3 MC—172—1 MO—172-2 Be ______________________ 0 0 0 0 0. 0002 0 0 Mg _____________________ n.d. . 00x . 00x n.d. n.d. n.d. n.d. Sr ______________________ .002 .x .x .05 .4 .07 .003 Ba ______________________ . 004 . 00x . 00X . 002 . 02 . 09 . 01 So ______________________ 0 0 0 . 0004 . 002 0 0 Y _______________________ 0 . 0X . 0x . 037 . 001 0 0 La ______________________ 0 . 0X . x . 02 0 0 . 005 Yb ______________________ 0 . 00x . 00X 0 0 0 0 Ti ______________________ . 2 >10 >10 >10 .2 .03 . 02 Zr ______________________ 0 . 0x . 0x . 14 0 0 0 V _______________________ 0 . 29 . 12 15 . 2 . 001 0 Nb _____________________ 0 . 22 . 20 12 0 O 0 Cr ______________________ 0 . 000x . 000x . 0004 0 0 Mn _____________________ . 02 . 0x . 0x . 01 2 . 003 . 002 Fe ______________________ n.d. 2. 5 2. 0 n.d. n.d. n.d. n.d. Co ______________________ . 22 0 0 . 007 0 0 Ni ______________________ . 12 0 0 . 006 . 004 0 0 Cu ______________________ .09 . 00x . 00x . 01 . 01 . 002 . 001 Ga ______________________ 0 0 O 0 0 . 002 0 Sn ______________________ 0 . 00x . 00x 0 0 0 0 Pb ______________________ . 007 0 0 0 0 0 0 Specific gravity ______________________________________________ 3. 4i. 1 3. i. 3 2. 55 :I:. 05 2. 65:1; 05 Indices of refraction l = 1. 693 :l:. 005 = 1. 699 i. 005 = 1. 712 i. 005 I Determined by D. J. Jameson. n.d. = not determined. x§='found but quantity not determined. Looked for but not found: B, Ce, Th, P, Ta, Mo, W, Re, Pd, Ir, Pt, Ag, A11, Zn, Cd, Hg, In, Ge, As, Sb, Bi. EXPLANATION OF SAMPLES Analyst Sample X-ray X-ray Spectrographic MC—172—5 ______ Pyrite __________ H. J. Rose. MC—176 ________ Sphene _________ J. D. Fletcher. MC—177 _____________ do _________ Do. MC—172—4 ___________ do _________ H. J. Rose. MC—l72—3 ______ Diopside _______ Monoclinic pyroxene probably diopside _____ F. A. Hildebrand ________ Do. MC—172—1 ______ Orthoclase ______ Feldspar probably microcline or sanidine- _ _ _____ do _________________ Do. MC—172—2 ______ Quartz _________ Do. 9. The greatest concentration of beryllium (0.005 percent) is in a thomsonite-gonnardite type of zeolite. 10. Most of the scandium is in pyroxene. Comparison of the content of trace—elements of the saprolites with the content of the fresh rock indicates that Nb, Sc, and Or are concentrated during weathering and Be and Sr are lost. Resistant magnetite probably accounts for most of the concentration of chromium and a part of the scandium, and resistant perovskite, sphene, and garnet probably account for most of the concentra- tion of niobium and a part of the scandium. GARNET-BIOTITE MELTEIGITE DISTRIBUTION AND DESCRIPTION A small area of melteigite, about 0.4 percent of the exposed igneous rocks of the complex, was mapped in the northeastern part of sec. 18 and the northwestern part of sec. 17 (pl. 1). Good exposures are rare and most of the area was delimited by means of float and saprolite. The rock contains rounded phenocrysts of brown biotite set in a dark—gray groundmass. Occasionally irregular inclusions of ijolite are found. In thin section the fine- to medium-grained groundmass has a holo- crystalline xenomorphic—granular texture and is com- posed of very pale brown diopside-hedenbergite rimmed with pale-green diopside and partly altered to brown biotite; pale-green diopside, pale-brown to greenish- brown biotite, brownish-violet perovskite rimmed with sphene and some rimmed with brown garnet; brown to colorless garnet some intergrown with sphene; apatite; magnetite-ilmenite; pyrrhotite; and nepheline partly 22 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. TABLE 13.—Analyses, norm, and mode of garnet—biotite meltetgtie, in percent Chemical Spectrographic Colorimetric Norm Mode ! MC—113 MC-113 L—86 MC-113 L—86 MC—113 M C—113 40. 08 0. 0004 0. 0001 As ________ 0. 002 0. 003 4. 73 Nepheline _________ 21 ll. 74 . 044 . 08 Sb ________ . 0001 . 0003 16. 13 Sodalite group- _ ______ 5 7. 19 . 18 . 2 W ________ <. 001 n.d. 17. 32 Orthoclase-nepheline intergrowth 6 4. 76 . 0009 . 002 Zn ________ .018 .050 . 28 Diopside-hedenbergite __________ - 30 . 32 . 0022 . 004 Pb ________ <. 0005 n.d. 34. 99 Biotite____ 9 6. 72 037 . 02 . 42 Garnet“ . 22 15. 69 0 . 0003 6. 19 Sphene. 5 . 14 062 . 02 eU ’ _______ . 002 . 001 3. 94 Apatite- . 5 3. 89 096 . 05 4. 48 Pyrrhotite. . ______ } 1 3. 47 . 014 . 03 6. 54 lVIagnetite ________ . 06 . 018 . 002 2. 02 Calcite ________________________________ . 5 . 87 0 . 001 U 3 ________ . 00019 n.d. . 31 3. 44 . 14 . 4 Th ‘ _______ . 00055 n.d. 1. 20 . 18 . 0045 . 005 . 50 .87 .0065 .001 . 13 . 024 .004 .04 .003 .003 . 24 . 65 100. 48 . 44 100. 04 1 Mode by D. J. Jameson. 2 Radiometric analyses. 3 Chemical analyses. n.d.=not determined. Looked for (spectrographieally) but not found: B. Ce. Hf. Th, P, Ta. W. U, Re, Rh, Pd. Os, Ir, Pt. Ag. Au, Zn, Cd, Hg. In. T1, Ge, Sn. Pb, As, Sb. Bi, To. Standard chemical analyses by L. M. Kehl. altered to cancrinite, sodalite group minerals or analcime, and calcite. A calcite veinlet and an intergrowth of sodic orthoclase and nepheline were found in the analyzed rock. C HEMISTRY Chemical and spectrographic analyses, norm, and mode of the garnet-biotite melteigite, table 13, empha- size the feldspathoidal character of this rock type and its close association to the ijolite group. Thenardite in the norm probably appears in the mode as a member of the sodalite group. Fluorine in the analysis is in biotite and apatite. Compared with N ockolds’ average melteigite (table 18), the analyzed rock is lower in Si02, CaO, NazO, 002, and P205; and higher in F6303, MgO, K20, and Ti02. Trace elements in the rock that are equal to or exceed 0. 01 percent include Mn, Sr, Ba, La, Zr, V, Nb, Cr, Ni, and Cu. Spectrographic analyses of minerals separated from the analyzed rock show the distribution of the trace elements (table 14). 1. Strontium occurs chiefly in apatite and pyroxene. 2. Barium occurs in biotite (0. 4 percent) probably as potassium replacement. The bulk of the barium, however, is probably in felsic minerals, none of which could be separated for analysis. 3. Lanthanum and yttrium are chiefly in perovskite and apatite with lesser amounts in garnet. 4. Zirconium is concentrated in garnet and sphene. 5. Most of the vanadium is in magnetite, garnet, and sphene. 6. Niobium is concentrated in perovskite and sphene. 7. Chromium and copper are detected in all analyzed minerals but chromium is concentrated chiefly in magnetite and bio- tite and copper in magnetite and apatite. 8. Most of the nickel occurs in biotite (0. 02 percent). Spectrographic analyses. MC—113 by H. J. Rose; L—86 by Sol Berman. Colorimetric analyses by H. E. Crowe and A. P. Marranzino. Radiometric analyses by B. A. McCall. Low-level chemical analyses for uranium and thorium by J. C. Antweiler. M C—113. Garnet-biotite melteigite, N WV; sec. 17. L—86. Saprolite of garnetbiotite melteigite, NWM sec. 17. TABLE 14,—Spectrographic analyses of mineral separates of garnet—biotite melteigite, in percent MC— MC— MC— MC— MC— MC- MC— MC— 113~1 113—d 113—2 113—3 113-a 113—c 113—e 113—b 0 0 0.0007 0 .04 1—10 10 1—10 1. .4 .9 .2 .08 .03 .01 .4 .002 0 0 0 . 0005 . 006 . 001 . 0008 .03 0 0 0 .2 0 0 0 .0007 0 0 0 .05 1—10 .6 1—10 .02 .002 .002 04 01 .01 01 .02 0 .0001 .02 .003 .06 .008 .05 .09 . . 07 1—10 1—10 1—10 0 .002 .003 .004 0 .004 .006 .02 .02 .006 003 .003 0 .003 .002 n.d. n.d. n.d. n.d. 0 0 0 n.d. =not determined. Looked for but not found: Ce, Th, Ta, Mo, W, U, Re, Rh, Pd, Ir, Pt, Ag, Au, Cd, Hg, In, Ge, Pb, As, Sb, Bi. EYPLANATION 0F SAMPLES Analyst Sample X»ray X-ray Spectrographic MC—113-1._._ Magnetite _______ H. J. Rose. MC—113—d.__ Perovskite ______ Dysanalyte W. F. Outer- J. D. Fletcher. (columbian bridge. perovskite) MC—113—2..-_ Dark-colored H. J. Rose. garnet. MC-113—3._.. Sphene __________ Do. MC—ll3—a--- Apatite J. D. Fletcher MC—113~c_.._ Diopside- Monoclinic W. F. Outer— Do. hedenbergite. pyroxene. bridge. MC—ll3—e..._ Diopside ________ Diopside— ..... do ......... Do. sphene. MC—ll3—b--- Biotite __________ Do. IGNEOUS Spectrographic analysis of the saprolite, L—86, indicates that Sr, Sc, Y, Yb, Nb, and Mn are concen- trated and Be, Zr, V. Cr, Ni, and Cu are lost during weathering. Resistant perovskite accounts for part of the concen— tration of Sr, Y, and Nb; resistant garnet accounts for part of the concentration of So, Y, and Mn ; resistant apatite accounts for part of the concentration of Sr and Nb. INTERMEDIATE RING The intermediate ring of igneous rocks is composed chiefly of fine—grained rocks and breccias of trachytic and phonolitic composition, which the writers believe represents the earliest phase of intrusive activity at Magnet Cove. Although the complex has been deeply eroded, the fineness of grain, abundance of breccia, and high volatile content of these rocks suggest that the intermediate fracture ring was the original channelway to the surface and supplied explosive volcanic material to the surrounding area. Rutile—bearing veins and irregular masses of meta- morphosed sedimentary rocks are most abundant in this unit. The rocks described in this section include altered phonolite and undivided trachyte-phonolite. ALTERED PHONOLITE DISTRIBUTION AND DESCRIPTION The altered phonolite unit, about 7 percent of the exposed igneous complex, is a dark—gray aphanite that weathers to a mottled greenish—gray and reddish- brown saprolite with a fine-grained texture. The unit is most abundant in the western half of the intermediate ring. Good exposures of the altered phonolite occur in the western part of sec. 18 where the contact with the fine—grained chilled border phase of younger garnet pseudoleucite syenite is exposed in the stream that cuts through the ridge. In thin section the rock is porphyritic with a trachytic or xenomorphic—granular groundmass and ranges in composition from alkalic trachyte to nepheline trachyte or phonolite. The chief minerals are pyroxene pheno- crysts up to 2 mm long that are altered to green biotite, calcite, and magnetite; hexagonal outlines of former nepheline phenocrysts replaced by calcite, biotite, and fine—grained anhedral nepheline; and pyrite cubes in a fine-grained groundmass of calcite, nepheline, ortho- clase, and green biotite shreds. Apatite prisms up to 1 mm long appear as phenocrysts. Sodalite, fluorite, and sphene are accessory. Both pyrite and calcite are particularly abundant in and near the breccia. One of the most interesting features of this rock is the uniform abundance of calcite (15 to 20 percent) 23 COMPLEX thrOughout the mapped phonolite bodies. It occurs chiefly as small grains, 0.02 mm, disseminated in the groundmass, and sometimes as coarse-grained anhedral patches that suggest recrystallization of the calcite. It is clearly a late—forming mineral because together with biotite and magnetite, it replaces early formed pyroxene and nepheline crystals. In many samples of the phonolite, fine-grained green biotite (0.015 mm) comprises more than half the rock; it ranges from 20 to 60 percent. Biotite, like calcite, is a deuteric mineral formed at the expense of early- formed mafic minerals which were the source of magne— sium and iron. Fresh pyroxene is very rare in the phonolite; a few remnants of diopside—hedenbergite mantled with aegirine-augite are present in some thin sections. Lath-shaped relics of former phenocrysts up to 2 mm long are outlined by rims of fine granular magnetite. The interior of the grains consists of fine-grained calcite, biotite, and magnetite. Hexagonal aggregates of calcite, biotite, and fine granular nepheline(?), outlined by fine granular mag- netite indicate the presence of former euhedral nephe— line phenocrysts. For the most part, the felsic min— erals occur with biotite and calcite in the groundmass as a fine—grained composite and are difficult to iden— tify. Some of the rock was treated with dilute HCl which produced abundant silica gel indicative of feldspathoids. Euhedral pyrite in crystals up to 0.5 mm across is a late—forming mineral and poikilitically includes parts of the groundmass. Locally pyrite exceeds 10 percent of the rock. Locally the rock is a breccia containing fragments that include pieces of metamorphosed sedimentary rocks, altered pyroxene—rich rocks, and fragments simi- lar to the interstitial altered phonolite. In some places phonolite fragments are predominant, and the brec- ciated character of the rock is not readily apparent except on weathered surfaces. Good exposures of brec- cia in the southwestern part of sec. 19 on the west side of Cove Creek contain abundant angular fragments averaging 1 inch in diameter that are composed of the same material as the groundmass. This suggests that the rock is an intrusive breccia formed by repeated pulses of phonolitic magma into previously partly cooled rock in the conduit of a volcano. One inclusion of light-colored syenite about 4 inches across indicates that at least some of the syenites are older than the altered phonolite. Upstream the breccia is very coarse, contains boulders as much as 3 feet across, and is cut by a pseudoleucite tinguaite dike. The predominant pieces are metamorphosed shale and novaculite. The included igneous types are chiefly pyroxene-rich por- 24 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. TABLE 15.--Analyses and norms of altered phonolite, in percent MC—114 MC—229 MC—114 MC—229 MC—114a MC—114b M C—114c MC—114 MC—229 Chemical analyses 1 Spectrographic analyses 1 Norm 39. 77 0. 0005 O 0 0. 002 0.001 1. 53 15. 76 n.d. n.d. .03 .8 1.0 16. 68 21.13 1. 53 ad. n.d. . 2 1—10 1—10 32. 49 16. 77 7.85 . 071 . 08 .0003 . 3 . 4 4. 45 .31 .29 .1 .07 .1 .2 11.64 19.60 2. 40 0 0 0 0 0 . 70 1. 17 8. 88 . 0005 0 0 0002 . 43 . 99 . 16 0030 008 0 0 02 . 42 7. 32 032 . 02 0 01 02 3. 86 3. 63 0002 . 0004 0 0 n.d. 7. 98 4. 58 . 07 11.11. 1. 8 . 06 1—10 1—10 1. 86 2. 09 1. 25 . 056 . 04 . 01 . 03 . 01 4. 26 5. 32 2. 76 052 . 06 0 . 07 . 03 1. 34 1. 68 4. 46 015 . 01 0 . 02 . 05 .39 . 62 . 68 0007 . 001 0 . 004 . 0005 1. 68 4. 58 . 58 0 . 003 0 0 O 17.40 10. 20 . 71 . 19 . l . 02 . 4 . 2 . 39 0021 . 002 . 02 . 01 . 009 2. 46 . 0009 0 . 002 . 02 . 002 100. 97 . 0074 . 005 . 006 . 03 . 01 .94 . 0036 . 002 . 0003 . 001 . 001 100.03 Colorimetric analyses 3 0.004 . 0002 . 001 d . 018 Radiometric analyses 4 eU _______________________________ 0. 003 0. 005 Chemical analyses 5 U ________________________________ 0.00033 0. 00040 Th ............................... . 00043 . 00110 1 Standard chemical analysis. MC—114 by L. M. Kehl and MC—229 by L. N. Tarrant. ? Spectrographic analyses. MO—114 by H. J. Rose; MC—114a, b, c, by J. D. Fletcher; and MC—229 by Harry Bastron. _ 3 Colorimetric analyses by H. E. Crowe and A. P. Marranzmo. 4 Radiometric analyses by B. A. McCall. 5 Low-level uranium and thorium analyses by J. C. Antweiler. n.d.=not determined; phyritic basic rocks. The coarse breccia at this locality may be a contact breccia not related to the intrusive breccia described above. Following is a description of the analyzed samples (table 15): MC—114.—A1tered phonolite. In thin section the rock has a microporphyritic texture. Lath-shaped relics of pyroxene(?) crystals up to 2 mm long are outlined by rims of fine—grained magnetite surrounding an aggregate of fine—grained green bio— tite, calcite, magnetite, and leucoxene. The fine—grained Xeno- morphic granular groundmass consists of green biotite, pyrite, magnetite or ilmenite, and sodic orthoclase and nepheline partly altered to calcite, cancrinite, and a sodalite group mineral. Euhedral apatite and late cubes of pyrite are scattered through- out the groundmass. Visual estimates of mineral percentages are: sodic orthoclase, nepheline, cancrinite, sodalite, 30; cal- cite, 15; biotite, 44; magnetite—ilmenite, 6; pyrite, 3 ; apatite, 2; leucoxene, trace. M 0—229.—Altered phonolite breccia. The breccia fragments average less than 1 inch in diameter and are composed of the same material as the groundmass, altered feldspathoidal trachyte. In the trachytic groundmass, euhedral apatite, anhedral sphene, Looked for (spectrographically) but not found: Li, Cs. Ce. Hf. Th. P, Ta, W, U, Re. Rn, Rh, Pd, Os, Ir. Pt. Au. Zn, Cd, Hg. In. T1. Ge. Sn. As. Sb. Bi. Te. MC—114. Altered phoneme, Wlé sec. 18. MC—229. Altered phonolite, N EM see. 29. MC-114a. Pyrite separate from MC—114. MC-114b. Magnetic (hand magnet) fraction of heavy concentrate. MC—114c. N onmagnetic fraction of heavy concentrate. anhedral magnetite, and anhedral green biotite are disseminated in sodic orthoclase, sodalite group minerals, plagioclase, and nepheline all partially replaced by calcite, sodalite group min- erals, and cancrinite. Euhedral and anhedral pyrite is late. A Visually estimated mode follows: calcite, 10; sodic orthoclase, plagioclase and nepheline, 55; sodalite group minerals, 5; biotite, 20; pyrite and magnetite, 5; apatite and sphene, 5. CHEMISTRY Chemical and spectrographic analyses and norms of this rock type are given in table 15. The chemical anal- yses most closely resemble that of ijolite, particu- larly in the K20—Na20 ratio and silica content. Silica. is much too low for a normal phonolite and it seems doubtful that it would escape during deuteric alter- ation in a quick-cooling rock. However, if the analy- sis is recalculated omitting the 002 the silica content would more closely approximate that expectable in a phonolite. There is little agreement between the chemical anal— yses and the estimated mode. There is not suffi- IGNEOUS cient K20 and MgO to make a rock containing more than 40 percent common biotite. This suggests that the green mica is a biotite of unusual composition in which sodium probably substitutes for potassium and ferrous iron for magnesium. If so, normative corun- dum, olivine, and a portion of albite and nepheline appear in the mode as biotite. Thus the salic portions of the norm are reduced to a more reasonable agree- ment with the mode. The fluorine is probably also in the modal biotite. The isotropic mineral of the mode is interpreted from the high Cl and 803 in the analysis to be a member of the sodalite group. In general, the rock was too fine grained to separate the various minerals but a pyrite separate was hand picked for spectrographic analysis (table 15). Iron, cobalt, nickel, and lead are concentrated in the pyrite. Two separates of that portion of the rock greater in specific gravity than 3.3 were made with the hand mag- net. The most magnetic fraction is dominantly mag- netite—ilmentite, and the other fraction is a mixture of pyrite, pyroxene, and leucoxene (table 15). Trace elements in the whole rock that exceed or are equal to 0.01 percent include Sr, B, La, Ti, Zr, V, Nb, and Mn. Of these Sr, Ti, V, Nb, and Mn seem to be concentrated in the heavy fraction of the rock. ORIGIN The uniform distribution and abundance of calcite in the altered phonolite suggest that the alteration of this rock is not due to local hydrothermal activity but is a product of deuteric reactions. The magma, heav- ily charged with 002, water, Cl, and F, cooled quickly so that all the gases did not escape but reacted with the early formed mafic minerals to form calcite, biotite, and magnetite, but still preserving the crystal outline of the mafic minerals. The feldspar and to some extent, the nepheline, appear to have been less subject to deu— teric alteration. This interpretation agrees with our concept of volcanic activity at Magnet Cove. Thus, this rock represents the portion of phonolitic magma trapped in parts of the ring conduit during the waning stages of volcanic eruption. Local hydrothermal activity associated with the em— placement of feldspar-Garbonate—rutile veins in the frac— tured and brecciated phonolite introduced additional carbonate to this rock unit, particularly in sec. 18. UNDIVIDED TRACHYTE-PHONOLITE DISTRIBUTION AND DESCRIPTION The undivided trachyte-phonolite unit, about 20 per- cent of the exposed igneous complex, forms the horse- shoe—shaped ridge that separates the garnet ijolite from the garnet-pseudoleucite syenite in the eastern half of the complex. These are the rocks that Williams (1891) mapped as metamorphosed sandstones and shales. 25 COMPLEX The trachytes—phonolites are all holocrystalline aphanites that range in color from light gray to greenish gray and dark gray and include alkalic trachyte, calci- alkalic trachyte, and nepheline trachyte or phonolite. Porphyritic, microporphyritic, trachytic, subtrachytic, xenomorphic granular, and amygdaloidal textures are represented. They weather to a fine-grained gray to brown saprolite. Locally the rocks are brecciated and contain abundant calcite and pyrite. A coarser grained foliated rock that occurs in contact with garnet ijolite has been mapped separately as banded trachyte. Mineralogically the rocks contain such felsic minerals as sodic orthoclase, plagioclase (albite or andesine), nepheline, analcime, and sodalite group minerals. Mafic minerals are diopside-hedenbergite, biotite, garnet, hornblende, aegirine, and aegirine—diopside. Sphene, magnetite—ilmenite, pyrite, pyrrhotite, apatite, and fluorite are accessory minerals. There are no extensive exposures of the trachyte—phonolite but fresh rock is readily available from the abundant float and intermittent outcrops. Abundant breccia float is found just south and east of the center of sec. 19. The angular breccia fragments average less than 1 inch in diameter, and are similar in composition and grain size to the groundmass trachyte except for a higher percentage of dark minerals. A few very dense and light-colored fragments may be pieces of sedimentary rocks. In the southwestern part of sec. 19, a breccia is well exposed on the east bank of Cove Creek. Angular fragments of metamorphosed sediments up to several feet across and smaller, less abundant fragments of fine- to coarse-grained pyroxene-rich rocks are found in a matrix of altered melaphonolite. The undivided trachyte—phonolite grades into a banded trachyte that becomes progressively more foliated and coarser grained toward the garnet ijolite contact (pl. 1). The nearly vertical foliation banding trends parallel to the garnet ijolite contact and the ring structure of the trachyte. In thin section the foliation consists of layers of anhedral equidimensional crystals of light- and dark-colored minerals alternating with discontinuous layers of predominantly dark—colored minerals. The layers bend around the inclusions of dark-colored pyroxene-rich rocks that are locally present in the foliated zone. Some of the variants of the undivided trachyte are described below: M C—138 (N W1/4 sec. 19).——Altered phonolite with amygda- loidal texture. The groundmass is composed of plagioclase, sodic—orthoclase, and nepheline all partly replaced by zeolites (about 50 percent of the rock); ragged brownish-green horn- blende; anhedral green biotite; apatite; sphene; pyrite; and magnetite rimmed with sphene. The edge of the amygdules is usually rimmed with pale—brown to colorless garnet (andradite?) 26 and the centers contain calcite, colorless diopside, pyrite, zeolite, garnet and wollastonite. MC—170 (NE1/4 sec. 90).—Phonolite (table 16). The texture is xenomorphic granular. About 80 percent of the minerals in the rock are felsic and include nepheline, minor albite, and sodic orthoclase slightly altered to sericite. The remaining minerals are brownish-green hornblende (about 15 percent) and the accessory minerals—sphene, brown garnet, pyrite, pyrrhotite, apatite, colorless fluorite and rare calcite. M 0—227 (SWM sec. 20).—Altered alkalic trachyte (table 16). In thin section aggregates of pale-brown biotite and anhedral magnetite appear to have replaced early phenocrysts of pyrox- ene(?). These together with the disseminated darker minerals, pale-brown biotite, magnetite, sphene, apatite, pyrite, and green hornblende occur in a subtrachytic groundmass of sodic ortho- clase and albite. Melilite (‘1), colorless to brown garnet, colorless diopside, and calcite appear to have been introduced as small ir- regular veinlets. An estimated mode of the rock shows the fol- lowing proportions: 55 percent sodic orthoclase and albite, 30 percent biotite and the remaining minerals in accessory amounts. L—129 (N EM sec. 20).——Saprolite of calci—alkalic trachyte (table 16). In thin sections of hard rock, corroded phenocrysts of anorthoclase or sanidine are scattered through a subtrachytic groundmass of sodic orthoclase, andesine, greenish—brown biotite, magnetite, pyrite, and pyrrhotite. There is a minor sericite alteration of the anorthoclase and sodic orthoclase. Material in the concentrate panned from the saprolite occurs in the follow- ing order of abundance: clay aggregates, feldspar, and magnetite. From an X—ray analysis of the clay minerals in the clay- and silt-size portion of the saprolite, A. J. Gude 3d (written commu- nication, 1956) reported major illite and minor montmorillonite. L—16‘2 (N EM sec. 24).—Microporphyritic alkalic trachyte. Corroded phenocrysts of diopside-hedenbergite and magnetite with sphene are set in a subtrachytic groundmass of sodic ortho- clase that forms subhedral laths and equidimensional anhedra. The sodic orthoclase is partly altered to deuteric analcime(?), cancrinite, and calcite. The diopside-hedenbergite is altered to aegirine-diopside and brown biotite. Sphene forms a reaction border on the magnetite. Pyrrhotite is rare. 11—1520, (central part of sec. 19) .—-—Phonolite. The rock has a xenomorphic granular texture, and consists of about 15 percent nepheline and analcime or sodalite group, and about 77 percent sodic orthoclase. The remainder consists of brownish—green biotite and less abundant pyrite, magnetite, and sphene. L—85 (N WM sec. 20).—Saprolite of banded alkalic syenite (table 16). In thin section the rock appears as an equigranular mosaic of orthoclase (about 68 percent), plagioclase (about 5 percent), sodalite or analcime (about 3 percent), and dark-colored minerals (about 25 percent) including olive—green hornblende, green aegirine—diopside and accessory apatite, sphene, magnetite, and pyrite. Some of the orthoclase and hornblende crystals con— tain poikilitic inclusions of other minerals. Magnetite crystals are commonly rimmed with sphene. Material in the panned concentrate occurs in about the following order of abundance: clay aggregates, orthoclase, weathered pyrite, magnetite, sphene, and apatite. From an X—ray analysis for the clay minerals on the clay- and silt-size portion of the saprolite, A. J. Gude 3d (written communication, 1956) reported major kaolinite and minor montmorillonite. M 0—1 71 (N WM sec. 20) .—Banded nepheline syenite (table 16). In thin section a few ravaged and cloudy phenocrysts of ortho- clase remain in an equigranular mosaic of orthoclase (clear), nepheline, cancrinite, sodalite, green hornblende, green aegirine- diopside, sphene, apatite, colorless to pale-green biotite, pyrrho- tite, and zoisite(?). ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. L—159 (SWM sec. 20).——Dark-gray, very fine-grained foliated melaphonolite. The texture is xenomorphic granular with the dark-colored minerals in slightly smaller equant crystals than the light-colored minerals. The minerals include orthoclase, albite, nepheline partly altered to cancrinite, neutral to very pale green diopside, and the accessories—sphene, pale-brown biotite, pyrrhotite, and apatite. L—101 (central part of sec. 19).——Calci-alkalic trachyte. The texture in most of the thin sections is xenomorphic equigranular but in some subhedral orthoclase laths lie with the long dimen- sion roughly parallel to the foliation. Estimated mineral per- centages are: orthoclase, 58; andesine, 20; perthite, 2; and the dark minerals (about 20 percent) which include green hornblende and accessory amounts of sphene, apatite, biotite and magnetite. In general, the contact between banded trachyte and garnet ijolite is poorly exposed. However, in the cen— tral part of sec. 19 (11—153, pl. 1) a fair exposure of garnet ijolite near the contact has a crude foliation caused by an imperfect segregation of light— and dark- colored minerals. In thin section the rock shows a xenomorphic granular texture and is composed of nepheline (about 40 percent), a micrographic inter— growth of orthoclase and nepheline (about 40 percent), zoned brown to colorless garnet, pale-green diopside partly altered to pale—red and pale—green biotite, and accessory apatite, pyrite, magnetite, and sphene. The ratio of orthoclase to nepheline in the intergrowth is about 2 to 1. This is probably a contaminated garnet ijolite. Sample L—161, from a poor exposure in central part of sec. 20, appears to be the same rock in the same relationship to the contact as that described above. In thin section, however, there is only about 8 percent orthoclase and no intergrowth with nepheline. This rock is even closer to the feldspar—free garnet ijolite. The interpretation is that younger garnet ijolite near the contact has reacted with the more silica—rich trachyte producing some orthoclase in the garnet ijolite. CHEMISTRY The chemical and spectrographic analysis and norms, table 16, 0f the undivided trachyte—phonolite and banded trachyte show the phonolitic composition of these rocks; however, CaO and total Fe are higher and silica is lower than in average phonolite. The analyses also show a wide composition range for these rocks, particularly in CaO and alkali. High Cl and 803 in the banded trachyte (MC—171) suggests a member of the sodalite group in the mode. Fluorine occurs in the apatite and biotite. In the altered alkalic trachyte, MC—227, not enough K20 was reported to make the abundant orthoclase and biotite, and the amount of MgO reported is also insufficient to make a rock containing 30 percent of normal biotite. Sodium and perhaps some calcium probably substitute for potassium and ferrous iron for magnesium. IGNEOUS COMPLEX 27 TAB LE 1 6.—Analyses , norms, and mode of the undivided trachyte-phonolite, in percent M0- M0— M0— M0— M0— L— MC-171 MC— L—85 M0— M0— M0— M0— 170 227 171 (1) 170 227 122 171—1 170 227 171 171 Standard chemical analyses 2 Spectrographic analyses 3 Norms Mode 4 52. 60 50. 48 49. 62 56. 90 Be. - . - 0. 002 0 0. 0002 0. 001 0 0. 0009 O _____ . 61 Orthoclase ______ 37 22. 18 20. 98 19. 13 20. 17 Mg- - - . 2 ILd. . 1 1—10 Ind. . 4 or _____ 43. 92 20. 02 28. 36 Nephellne _______ 21 . 29 2. 29 1. 04 2. 26 Ca.-_- >10 n.d. . 2 >10 n.d. . 5 ab--- - 12. 58 34. 58 14. 15 Cancrinite ______ 1 3. 83 5. 16 5. 59 1. 85 Sr ..... . 5 . 08 . 07 . 5 . 05 . 08 an_--- 13. 62 18. 63 8.06 Sodalite group- _ 6 . 27 . 29 . 23 .19 Ba.-_- . 2 . 1 . 2 . 2 . 002 . 3 ne--.-. 18. 74 8. 52 22. 72 Aegirine— 21 diopside. 0 . 004 . 002 hl _____ . 35 Homblende _____ 5 0 0 .0003 . 0004 . 002 th--..- . 14 Biotite_.-- 1 .007 0 . 007 .04 .002 no _____ . 11 Sphene. 1 . 02 . 009 . 02 . 04 0 di _____ 1. 43 15. 52 Apatite- . _ 1 . 0002 0 . 0004 n.d. . 0005 01 _____ 3. 07 5. 66 Py'rrhotite- - - 6 1.1 .2 1.9 >10 1 mt.--_ .46 3. 25 1.39 .04 .05 .03 .4 .05 il-----_ 1.37 4. 26 3.80 .02 .008 .029 .2 .04 31).--- .34 1.01 1.01 .01 .01 .02 .3 .02 if _____ .78 .62 .39 . 001 . 002 . 001 . 0008 . 002 pr----. 1. 95 . 69 2. 91 0 0 . 0007 . 001 cc ..... . 70 .1 .09 .2 .07 .2 wo_--_ .58 n.d. 3.0 1—10 n.d. 5 . 001 . 002 . 002 0 . 002 0 . 001 . 001 . 006 . 0009 Less 0... 0008 0 .001 .003 .02 .002 . 002 . 004 . 001 0 . 005 99. 80 99. 96 100. 00 0 0 0 0 . 003 n.d. . 002 0 . 001 0 0 . 001 Colorimetric analyses 5 As- - __ <0. 001 0. 001 <0. 001 <0. 001 Sb. _ _ - . 0001 . 0002 . 0003 . 0002 Zn___- .01 .01 <.002 .013 Radiometric analyses 5 eU- - .- 0.006 0.003 0. 005 0.004 0.002 Chemical analyses 7 U ..... 0. 00043 0. 00025 0. 00029 Th.-. . 00101 . 00069 . 00053 1 Average phonolite (Nockolds, 1954). n.d.—not determined. 1 Standard chemical analysis. MC—170, MC—171 by S. M. Berthold; MC-227 by Looked for (spectrographically) but not found: Li, Cs, Ce, Hf , Th. P . Ta, W, U , L. N. Tarrant. 3 Spectrographlc analyses. Harry Bastron; L-122, L~85 4 Mode by D. J. Jameson. 5 Colorimetric analyses by H. E. Crowe and A. P. Marranzino. ° Radiometric analyses by B. A. McCall. 7 Low-level uranium and thorium chemical analyses by J. C. Antweiler. MC-170, MO-171 by J. D. Fletcher; MO—227 by by Sol Berman; MC—171-1 by H. J. Rose. Compared with the average of the analyses of the igneous rocks in the Magnet Cove complex (table 46) the trachytes are lower in MgO, total iron, MnO, CaO, 002, P205, 803, and higher in SiOz and A1203. Trace elements that exceed 0.01 percent include Sr, Ba, La, Zr, V, and Nb. Spectrographic analysis of sphene from the banded trachyte show that Zr, Nb, V, and Cu, are concentrated in this mineral. omenv As mentioned previously these rocks were mapped by Williams (1891) as metamorphosed sandstones and shales and his interpretation has been followed by later workers in the area. However, the field and laboratory evidence indicates an igneous origin for these rocks. Following are the observations that led us to this conclusion: 1. Trachytic and phonolitic composition, mineralogically and chemically. 659634—62————3 Re, Ru, Rh, Pd, Os, Ir, Pt. Ag, Au, Zn, Cd, Hg.In, MC—170. Phonolite, NEV; sec. 20. MC—171. Banded nepheline syenite, NWM sec. 20. MO—171—1. Sphene from MO-17l. L-85. Saprolite of banded alkalic syenite, NWV4 sec. 20. L—122. Saprolite of calci-alkalic trachyte, NEVi sec. 20. Tl, Ge, As. Sb, Bi, Te. 2. Lack of any residual sedimentary structures or bedding throughout the entire width of exposure—up to 2,500 feet. Paleozoic units cannot be traced through this unit. 3. Trace element distribution is about the same as in other igneous rocks of the area. 4. Local development of breccia with trachyte matrix. 5. Projection of the sedimentary section from outside the complex would put novaculite in position as one of the units to be metamorphosed. It is not reasonable that metamorphism of novaculite would produce a subsilicic rock. 6. Arcuate shape of the rock body, discordant to the folds in the sedimentary rocks but similar to those of other intrusive bodies. INNER CORE The inner core rocks occupy a topographic basin and consist chiefly of mineralogic and textural varieties of ijolite, carbonatite, and lime—silicate rocks. 28 IJOLITE DISTRIBUTION AND DESCRIPTION Ijolite, a feldspar—free rock composed chiefly of nepheline, pyroxene, titanium garnet, biotite and magnetite, occupies a topographic low in the central part of the complex and has been divided—~on the basis of grain size, and the proportion of biotite—into two map units, biotite—garnet ijolite and garnet ijolite. Biotite-garnet ijolite, about 7 percent of the exposed igneous rocks of the complex, occupies the central part of the basin and is well exposed in the creekbed in front of the Magnet Cove church on US. Highway 270. It is surrounded by garnet ijolite and the contact appears to be gradational. Colluvium and alluvium are thick over parts of the area particularly at the lower altitudes and outcrops of fresh rock are very scarce. The samples for chemical analyses were taken from resistant boulders in saprolite. Biotite-garnet ijolite ranges from a medium grained to a very coarse grained phanerite (pegmatitic). Masses of nepheline, garnet, and magnetite as much as 4 inches across and mica plates that reach 6 inches across occur as local segregation pods and stringers. The color of the fresh rock is a mottled mixture of the White and pink of nepheline, white of zeolites, brown and black of garnet, green of diopside, green and black of biotite, and some yellow of pyrite. The unit shows increasing intensity of alteration on ap- proaching the lime silicate mass exposed in the Kimzey magnetite pit in the N WM sec. 20. Garnet ijolite ranges in composition from a dark—gray fine—grained phanerite to a meuium-grained mottled phanerite. Both rock types weather to a saprolite mottled in gray, brown, pink, and black. Although the grain size and relative proportions of the minerals of the ijolitic rocks vary widely (urtite to melteigite), the absence of feldspar and abundance of black titanium garnet is diagnostic. In thin section these rocks have a holocrystalline hypautomorphic- granular texture. The chief minerals are nepheline, black titanium garnet, pyroxene, and biotite partly altered to phlogopite. Apatite, pyrrhotite, perovskite, sphene, and primary calcite are accessory minerals. Thomsonite is abundant as an alteration product of nepheline particularly around the periphery of the mag- netite pit area in sec. 20, and on Cove Creek where the ijolite is in contact with carbonatite. Cancrinite and calcite are other common alteration products; late- forming biotite is sometimes developed at the expense of pyroxene. The largest single crystals and masses of nepheline are found in the ijolite and range from less than 1 mm to several inches in diameter. The color ranges from greasy grayish white to brown to deep pink. Com- ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. monly the large crystals have only a residual core of nepheline and are replaced by white birefringent zeolite, calcite, cancrinite, idocrase, colorless garnet, pectolite, and tremolite. N epheline (10 to 95 percent of the rock) is older than garnet or biotite but probably later than pyroxene. A light—green diopsidic pyroxene with high birefring- ence is generally the earliest formed mineral and comprises from less than 5 to more than 50 percent of the rock. Many of the grains have a mottled appear- ance under crossed nicols, caused by patches of slightly greener pyroxene that go to extinction at a different position from the almost colorless material. The green patches appear to be early and residual, have slightly lower birefringence and are most abundant in the in— terior of the grains. N o pyroxene crystals larger than 1 inch in diameter have been observed whereas some crystals of garnet, mica, and nepheline are several inches in diameter. Garnet is a late—forming mineral that occurs both as irregular anhedral masses and as euhedral crystals. Poikilitic inclusions of altered nepheline and pyroxene are common. The color of the garnet in thin section ranges from light brown to very dark brown which probably is a reflection of the titanium content. In some thin sections of the biotite-garnet ijolite, the garnet contains residual cores of purple perovskite. Biotite, probably closer to phlogopite, is pleochroic in shades of light green. A few residual brown patches suggest that original dark-brown biotite has been altered to a pale phlogopite mica. The mica, like garnet, is a late-forming mineral and commonly occurs as mica- rich stringers or dikelets in the ijolite. cnzmsrn Chemical and spectrographic analyses, norms, and modes of garnet ijolite and biotite-garnet ijolite, table 17 are similar to N ockolds’ average ijolite (table 18). The only appreciable consistent difference is the lower N ago and higher CaO in the Magnet Cove rocks. The Magnet Cove ijolites are characterized by high CaO, high alkali and low silica. Individual rocks show appreciable differences in Si02, Fe203, P205, and TiOz which are a reflection of the relative proportions of the chief constituent minerals. The high H20+ reflects zeolite alteration of nepheline. A member of the sodalite group in the mode is de- duced from the sulfur trioxide in the analyses; fluorine in the analysis is probably in biotite. Trace elements in the ijolites that are equal to or exceed 0.01 percent include: Sr, Ba, Zr, and V. 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TABLE 18.—Average and type urtite, ijolite, and melteigite Chemical analyses (percent) Modes (percent 2 3 4 5 6 4 5 6 S102 _______________ 42. 59 42. 58 41. 90 45. 43 42. 79 40. 64 Nepheline __________ 85. 72 49. 41 20. 86 A1203 ______________ 27. 42 18. 46 12. 20 28. 77 19. 89 10. 58 Cancrinite __________ 4. 83 Fe203 ______________ 2. 49 4. 01 6. 41 3. 10 4. 39 4. 18 Muscovite __________ 1. 23 FeO _______________ 1. 89 4. 19 4. 32 . 40 2. 33 4. 18 Apatite ____________ 1. 97 4. 02 4. 53 M110 ______________ . 09 . 20 . 22 . 41 . 28 Biotite _____________ 6. 16 MgO ______________ . 69 3. 22 5. 45 . 22 1. 87 6. 47 Pyroxene ___________ 11. 96 41. 86 47. 14 CaO _______________ 4 38 11 38 16. 60 1. 86 11. 76 19. 91 Garnet _____________ 2. 15 4. 43 B20 _______________ . 11 Sphene _____________ 1. 67 4. 23 NazO ______________ 14. 12 9 55 5. 10 16. 16 9. 31 4. 75 Microlith ___________ . 86 K20 _______________ 3. 82 2 55 2. 66 3. 38 1. 67 1. 86 Magnetite __________ . 61 20 _______________ . 99 . 14 Calcite _____________ 5. 50 H20+ _____________ . 42 55 . 87 . 27 Ti02 ______________ . 35 1. 41 2. 21 1. 10 2. 24 002 _______________ 1. 30 . 38 . 82 2. 08 P205 _______________ . 44 1 52 1. 24 1. 70 1. 91 Cl ________________ . 03 F _________________ . 12 S _________________ . 05 ZI'02 ______________ . 10 Total ________________________________ 99. 32 99. 81 99. 90 1. N ockolds’ average urtite (1954). 2. Nockolds' average ijolite (1954). 3. N ockolds’ average melteigite (1954). 1. Strontium is concentrated in apatite, calcite, and pyroxene. 2. Beryllium and molybdenum are concentrated almost exclu— sively in zeolite (up to 0. 02 percent Be and 0. 003 percent Mo). . Lanthanum and yttrium are in perovskite and apatite. . Scandium occurs chiefly in diopside. . Zirconium is concentrated in sphene (0. 6 percent) and dark garnet. 6. Vanadium occurs chiefly in apatite. 7. Niobium is concentrated in perovskite and sphene with lesser amounts in dark garnet. 8. Cobalt, nickel, and copper occur chiefly in pyrite and to a lesser extent, in garnet. Copper also occurs in concentra- tion up to 0. 01 percent in perovskite. 9. Gallium occurs chiefly in the felsic minerals but is also detected in biotite and garnet. enthw garnet, diopside, sphene, and Spectrographic analyses of the saprolites show the following effects of weathering on the trace elements: a loss of Be and Sr; and a gain of Ba, Sc,Y, Zr,Nb, Cr, Ni, and Cu. Resistant perovskite accounts for a part of the concentration of Sc, Y, Nb, and Cu; resistant garnet accounts for the concentration of Zr and a part of the Sc, Y, Nb, Cr, Ni, and Cu; resistant biotite accounts for a part of the concentration of Ba, resistant apatite accounts for a part of the concentration of Y and Cr; resistant magnetite probably accounts for a part of the Cr, Ni, and Cu. FINE- GRAIN ED IJOLITE DISTRIB UTION AND DESCRIPTION Fine-grained ijolite, about 5 percent of the exposed igneous rocks of the complex, is found as a topographic 4. Type urtite—Lujavr—Urt, Kola Peninsula—Ramsay,‘1896,fromJohannsen (1938) 5. Type ijolite—Iiwaara, Finland—Ramsay and Bcrgheli, 1891, from J ohannsen (1938). 6. Type melteigite—Fen District, Norway—Brogger, 1921, from Johannsen (1938). low in the west—central part of the complex. Fresh rock exposures are rare, but saprolite is abundant. The concealed contact between altered phonolite and fine- grained ijolite was delimited by samples of saprolite taken with a power auger, but the alluvium in Cove Creek valley, which conceals the contact between fine- grained ijolite and garnet ijolite, could not be penetrated With the auger. The rock ranges in composition from melteigite to ijolite; in grain size from a dense aphanite to medium- grained phanerite; and in color from gray and greenish gray to dark gray. It Weathers to a saprolite colored various shades of brown and gray and often mottled in these colors. The dense aphanite is easily confused with the dark trachytes except that when the garnet content of this ijolite is high the rock has a distinctive resinous luster. The chief minerals are pyroxene, biotite, nepheline, and garnet. The textures include porphyritic, glomero-porphyritic, xenomorphic—granular, micro—porphyritic, amygdaloidal, and fine—grained bree- cia; all are holocrystalline. Some of the variants are described below: M 0—21 7 (N W% N W% sec. 19) .——A dark-gray aphanite with a few fragments of grayish-green altered alkalic trachyte. The aphanitic groundmass has a xenomorphic-granular texture and is composed of calcite, sodalite group, and nepheline partly altered to cancrinite—about 15 percent; pale brownish-green biotite, brown garnet (melanite), pale—green diopside, sphene, apatite, magnetite-ilmenite, and pyrite—about 85 percent. The altered alkalic trachyte fragments are composed of about 68 percent sodic orthoclase; 2 percent calcite and analcime and (or) sodalite group; and 30 percent brown biotite, sphene, apatite, and pyrite. IGNEOUS COMPLEX 31 TABLE 19.—Spectrographic analyses of mineral separates of biotite—garnet ijolite, in percent [Spectrographic analyses by H. J. Rose] L—133—1 MC-216—10 MC-216—7 MC—216—S MC—216-9 MC—216—5 MC-216—4 MC—216—6 MC-216—3 MC-216—1 MC-216—2 0 0 0 0 0. 0003 0 0 0. 0005 0. 0005 0. 0006 n.d. n.d. n.d. n.d. n.d. n.d. .01 n.d. .3 .3 .2 .05 .05 .01 .5 .006 .84 .4 .01 .06 .002 0 0 . 002 .003 . 1 . 02 . 06 .001 .001 n.d. 0 0 0 0 .002 .002 . 001 . 01 . 02 n.d. n.d. n.d. n.d. n.d n.d. .04 n d >10 >10 .0004 .0009 .0009 0 0 0 0 0 0 .02 .03 .03 0 0 .002 .02 0 0 0 .1 0 0 0 0 0 .04 0 0 0 >10 >10 >10 .6 .05 1.1 .006 01 .009 .008 0 . 1 . 1 0 0 0 0 0 0 0 0 0 0 0 0 0 >10 0 0 0 .04 .2 .2 .09 .03 .009 1 03 .005 .008 .9 .03 .03 0 0 0 0 0 0 0 0 0 0 .0003 0 0009 . 0004 0 0 0 0 0 0 0 0 0 . 0004 . 002 .003 04 .005 .3 .2 .3 .3 .01 .007 .01 .005 n d n.d. n.d n d n.d. n.d. .04 n.d. . 3 . 3 .002 .002 .000 . 05 0 0 0 0 0 . 009 . 009 . 005 0 0 0 0 0 0 01 . 003 . 01 . 0005 . 0002 . 001 . 0002 . 0004 . 0002 . 0006 .001 . 0009 . 001 . 0003 .003 0 . 003 . 002 .002 003 003 .003 002 .1 0 .004 0 0 0 Specific gravity ___________________ 4:. 1 3. 5:1:. 1 3. 8i. 1 3. 8:1: 1 3. 1:|:. 2 ............ 3.13:. 2 2. 653:. 05 2. 253:. 05 2. 35¢. 05 Indices of refraction 1 «1:1. 677:. 005 ________________________ c=1. 5393:. 002 B=L 682d: 005 ________________________ w=1. 5455:. 002 7:1. 6993:. 005 _ 1 Determined by D. J. Jameson. n.d.=not determined. Looked for but not found: Ce, Yb, Th, Ta, W, Re, Pd, Ir, Pt, Ag, Au, Zn, Cd, Hg, In, Ge, Pb, As, Sb, Bi. EXPLANATION or SAMPLES Sample X-ray Analyst Ir133-1 _______ Perovskite ...... MC-216—10..- ._.. MC—216—7. . __ - MC—216—8 _________ do Andrtaditfifi) slightly larger F. A. Hildebrand. uni ce . MC—216—9..._ Yellow garnet..- Andradite ................... Do. MC—216—5.___ Diopside ........ MC—216—4..-_ Phlogopite ...... MC—216—6-.__ Apatite... MC—216—3-.-- Nepheline _. MC-216—1-.-. Zeolite ________ Natrollte .................... Do. MC-216—2 ________ do ______ fin __ Do. L-327 (S W% sec. 17.)—Porphyritic micromelteigite composed of corroded phenocrysts of very pale reddish-brown diopside- hedenbergite rimmed with pale-green diopside and partly altered to pale greenish—brown biotite, calcite, sphene, and brown garnet; pale-green diopside; greenish-brown biotite; anhedral to euhedral brown to colorless garnet; sphene; apatite; pyrrhotite; late anhedral nepheline; late anhedral member of the sodalite group; and late calcite. L—3 (S WV; sec. 18) .——Gray, fine-grained ijolite with xenomor- phic-granular texture and composed of pale greenish-brown biotite, brown to colorless garnet, green aegirine, sphene, apatite, pyrite—about 30 percent; and zeolite(?) which poikilitically includes all of the above minerals—about 70 percent. The min- eral tentatively identified as a zeolite is colorless, uniaxial positive, birefringence about 0.008, and index less than 1.537. It probably is an alteration product of nepheline. From an X-ray analysis of the silt- and clay-size portion of the saprolite, A. J. Gude 3d (written communication, 1956) reported major montmorillonite and minor chlorite. L—19 (N W%, sec. 1.9) .-—Greenish-gray ijolite with xenomorphic— granular texture and composed of pale—green diopside, brown to colorless garnet, pale greenish-brown biotite, sphene, apatite, pyrite and magnetite—about 45 percent; an interstitial sodalite group mineral or analcime and late calcite—about 55 percent. L—92 (SWV; sec. 17).——Dark—gray ijolite with xenomorphic— granular texture composed of sphene, brown to colorless garnet, pale greenish—brown biotite, pale-green diopside, euhedral apatite, and pyrrhotite—about 42 percent; interstitial nepheline and sodalite group mineral or analcime—about 52 percent; and late calcite—about 6 percent. L—157’ (SEVr sec. 19) .—-—This dark-gray aphanite has a. fine- grained amygdaloidal texture. The amygdules (up to 1 mm in diam) are irregular in shape and are intermittently connected to form chains that are crudely parallel and give a lineated appear- ance to the rock. In thin section the amygdules are filled by anhedral calcite and rimmed with brown to colorless garnet. Minerals in the rock are pale-bluish-green hornblende, brown to colorless garnet, brown biotite, sphene, colorless to pale-green diopside, apatite, pyrite, pyrrhotite, and magnetite—about 55 percent; and nepheline partly altered to calcite and analcime or sodalite group mineral—about 45 percent. Along Cove Creek north of the bridge on US. 270, the fine-grained ijolite has been altered by the later ALKALIC IGNEOUS COMIPLEX AT MAGNET COVE, ARK. 32 .on ........... . ....... c6 .......... zsAEafiouénooN ............... 66---: ------nAALA £233: .Q .A .............. 8---- ....... :ALA .35 6Q --------------- o 6.:- ------ unwmALH 5:30AM .9 .n - ---------------- o6 ----- 36;:AAom.3A:omEoAAB --------------- 8:.-- ------- «AA-A 62326 32.32: 60 ------------------ 66:--- ------------------- o6 ------------------ .66 ----------- WANNA-A .onA ------- 6:8no6om .4 .-A 65:23 2::852 ............... 66--.: ------ AINALA 69 c6 ----- £3562 --3=8N ------ ArmaA-A .oQ ............ 38:65 ------- Tang .8 .----6:Eno6om .4. ..A - Ezénoz 66 ----------- $2.: .on .............. Seam --:--wAALA .oQ ........... 0:22:02 ....... EALA .oQ ....... u 25% Eng ------ SAALA 6Q --------------- cu ----- -ln-InAAA-A .onA --------------- co ----- ---.o|~.ALH .an . ...... an ......... 925m in ...... 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A . no. no. A. 8. 6.: 6.: A. A. mo. 8. 6.: 6 : 6.: 6.: 6.: 6.: 6.: 6.: 6.: moo. woo. woo. 8. «co. 8. mo. vo. 8. No. N. . a. 6. S. m. m. a. mo. oooo. mooo. moo . Aoo . o o o o o o o o o o o o o o o ooooo . o o o «ooo. «ooo . o o o o mooo. mooo . Aoo . mooo. o o o moo. o o o o o o o o o o o o o o o m. o S. 8. o A8. A8. woo. boo. mo. No. moo. moo. A. A A. no. mo. N. v. m. m m. o A1: A2: o o o o o o oAA 6.: o o o o o o o o o o o o o o o o a o o o o mo. o. S. N m. o voo- ooo. woo. woo. ooo. So. poo. moo. moo. o S. mooo. «.28. no. oAA NA SA SA woo. o o o o o 6.: o o o Nooo. 6.: o o 6.: 6.: 6.: Ar: o 6.: o o o o o o moo . moo. no. Ao. o o o moo. S. o o o o o o o o o o o o woo . Ao. o o o Aoo. ooo. moo. mo . mo. o o o o o Aoo o o o o o o ow Aoo . Ao . 38. o mooo. moo. o 6.: 6.: S A A: A 6.: 6.: 8. mo. no. 6.: 6.: 6.: 6.: 6.: 6.: 6.: 6.: 6.: 6.: o woo. woo. ooo. o moo. So. So. o o o o o o o o o o o co. m. S. Ac. 3. 3. A. A. ooo. mo. woo. woo. moo. moo. o o woo. woo. woo. o. nIA Ao. woo. N. N. «o. 6m. 6w. nnA m. . v. n. o. oo. mo. 8. o 8. E. A. m. 6.: 6.: mo. no. 8. moo. 6.: 6.: 6.: 6.: 6.: 6.: 6.: 6.: 6.: 3 .o No .o oooo .o mooo .o mooo .o mooo .o mooo .o o o o mooo .o mooo .o oooo .o nooo .o o o o o o onuALH nanLH NIMNALH AloNALA EALH TBA-A nINALA TomAkH olmmALH AAALH oInALH hummAkA oImNALH nInALA wInAIA oAnhALA olnALH mummAIA AAINALA olmmALA 33:3” 3 .8636.» owgoo k: «So-3&6” 636:6»: x: 333:». Soagoguomkmldm H.549 'IGNEOUS COMPLEX 33 TABLE 21.~Analyses, norms, and mode of fine-grained ijolite, in percent MC—217 L—327 MC—2l7 L—327 11-3 11-19 . L—92 L—157 13/1170; MCI-217 L—327 11-327 Standard chemical analysesx Spectrographic analyses 1 Norms Mode I 36. 89 0. 000 0. 0006 0. 0004 0.0002 0. 0003 Nepheline _____________ 14 13. 28 n.d. 2 2 3 . 9 n.d. Analcime or socialite 7 group. 7. 58 n.d. >10 >5 >5 . l n.d. Diopslde-hedenbergite 40 3. 71 . 9 . 03 . 02 . 06 . 02 . 08 Biotite ________________ 14 .33 .2 .4 .2 .2 .3 .03 GarueL.-. __ 4 4. 22 0 .001 0 0 . 003 0 Sphene._-_ .- 6 14. SO . 002 . 003 . 002 . 003 . 002 . 0009 Apatite ..... 3 . 18 . 003 . 01 . 005 . 006 . 004 . 01 Pyt'rhotlte. - 6 5. 29 . 009 . 03 . 006 . 01 . 02 0 Calcite ________________ 6 4. 62 0 0 0 . 06 n.d. . 14 . 0003 . 0007 . 001 . 0005 . 0005 n.d. 1. 11 n.d. 2 2 4. 8 3. 32 . 04 . 03 . 04 . 04 . 04 . 06 1.54 .1 .06 .08 .03 .04 .1 .99 .02 .02 .03 .02 .009 0 . 80 . 003 . 005 . 002 . 002 . 004 . 002 . 41 . 0008 0 0 0 . 40 n.d. 3 . 4 . 2 . 07 . 2 . 95 n.d. >5 >10 n.d. Total" ___ 100. 56 . 002 .004 . 002 . 001 . 003 . 0008 Less 0.--- . 50 .0007 .002 .001 .001 .0007 .005 100. 06 . 01 . 01 . 007 . 01 . 004 . 006 . 003 . 003 . 003 . 002 . 004 . 001 0 0 0 0 0 . 001 0 . 001 0 . 004 . 001 0 Colorimetric analyses 4 AS ...... <0. 001 n.d. 0. 003 0. 002 0. 001 n.d. Sb..--__ .0001 n.d. .0004 .0004 .0002 n.d. Zn ...... . 015 n.d. . 025 . 015 . 010 n.d. Radiometric analyses 6 eU ...... 0. 003 0. 002 0. 003 <0. 001 0. 002 <0. 001 Chemical analyses 6 U _______ 0.00041 n.d. n.d. n.d. n.d. n.d. Th ...... . 00060 n.d. n.d. n.d. n.d. n.d. BilStandard chemical analyses. MC—217 by L. N. Tarrant and L—327 by M. K. a azs. 2 Spectrographic analyses. MC-217 by J. D. Fletcher; L—32'7, L—3, L—19, L—92, L—157 by Sol Barman; MC—217-1 by H. J. Rose. 3 Mode by L. V. Blade. 4 Colorimetric analyses by H. E. Crowe and A. P. Marranzino. 5 Radiometric analyses. MC-217, L-3, L-19, L—92. L—157 by B. by J. P. Schafer. 0 Low-level chemical analyses for uranium and thorium by J. C. Antwellet. n.d.=not determined. A. McCall; L—327 carbonatite. Nepheline is replaced by zeolite (probably thomsonite), brown garnet is partly altered to colorless garnet and idocrase, biotite and diopside are bleached, and a little tremolite (‘3) has formed. Abundant pale— green biotite has formed along calcite veinlets in the ijolite. CHEMISTRY The analyzed rocks (table 21) have a melteigite com- position and are very similar to Nockolds’ average melteigite (table 18); 8102, MgO, CaO, and P205 are lower, total iron oxides and N320 are about the same and A1203, MnO, K20, Ti02, and C02 are higher. K20 and TiOz are higher in these rocks than in the coarser grained ijolite. H20+ is low, reflecting less zeolitic al- teration in fine—grained rocks than in the more easily attacked coarse-grained ijolite. Looked for (spectrographically) but not found: Hf. Th, P. Ta, W, U. Re, Ru, Rh , Pd, 0s, Ir. Pt. Ag. Au. Zn. Cd, Hg. In. T1. Ge. As. Sb. Bi. Te. MC—217. Micromelteigite breccla, NWM sec. 19. L—327. Porphyritic micromelteiglte, SWV; sec. 17. MC-217—1. Garnet (melanite) from MC—217. L-3. Saprolite of fine-grained ijolite. NW54 sec. 19. L—19. Saprollte of fine-grained ijolite. NWV; sec. 19. L—92. Saprolite of fine-grained ijolite. SW34 sec. 17. L—157. Saprolite of micromelteigite, SE34 sec. 19. From the abundant Cl and 803 in the analyses, the isotropic low index mineral in the mode is inferred to be some member of the sodalite group. Apatite and bio- tite probably account for the abundant fluorine. Trace elements in the rock that exceed or are equal to 0.01 percent include: Sr, Ba, La, Ti, Zr, V, Nb, and Mn. Spectrographic analysis of a dark—colored garnet sepa- rated from MC—217 (table 21) show that Ti, Zr, V, Ni, and Y are concentrated in the garnet. Spectrographic analyses of saprolite compared with those of the fresh rock indicate that La, Y, Yb, and Pb are slightly concentrated and that Sr is lost during weathering. Resistant garnet would account for the concentration of Y and resistant sphene for the concen- tration of La. The concentration of Yb and Pb is not explainable on the available evidence. 34 CARBON ATITE DISTRIBUTION AND DESCRIPTION Masses of coarse-grained calcite (carbonatite) that contain unusual accessory minerals including niobium— bearing perovskite and zirconium garnet (kimzeyite) are one of the most interesting rock types that occur at Magnet Cove. The carbonatite, about 1.8 percent of the exposed igneous rocks of the complex, is divided into two map units-carbonatite and residual phos— phate. Residual phosphate is characterized by abun- dant float of a porous rock composed of apatite, mag- netite, and perovskite in matrix of secondary apatite, whereas the carbonatite is characterized by calcite out— crops or residual apatite, magnetite, and perovskite in the saprolite. Core drilling by W. A. Keith established the presence of carbonatite under the areas mapped as residual phosphate. Logs and locations of the core holes are given in table 22. The carbonatite mapped in secs. 18, 19, and 20, represents the mappable bodies of this rock type but small stringers of calcite in the surrounding ijolite and reports of calcite found in sev— eral wells and drill holes in the immediate vicinity sug— gest that concealed bodies of carbonatite may exist. Although the carbonatite is more resistant than the surrounding ijolite, good exposures are rare; the best occur along Cove Creek and in the Kimzey calcite quarry in the northwestern part of sec. 19 (Fryklund, Harner, and Kaiser, 1954). TABLE 22.—Descriptive logs of core holes [All holes were drilled by W. A. Keith, lessee of much of the carbonatite area, with 8. Cooper rig using a wire—line core barrel. The cores were logged by L. V. Blade. Except where noted all descriptions are of recovered core; minerals are listed in de- creasing order of abundance] Hole [4-130 Location: N Wl/4 sec. 19. Date drilled: August 23—28, 1954. Depth (feet) Description 0—37 __________ Cuttings taken at surface, Ijolite saprolite, apatite, clay. avernous section, no samples taken. ably carbonatite. uggy carbonatite, coarse-grained calcite (75 percent), apatite, clay, pyrite, rutile, mag- netite partly altered to hematite, fine-grain- ed calcite, perovskite, mica, wavellite. Vuggy carbonatite, coarse-grained calcite (80 percent), apatite, clay, magnetite, partly altered to hematite, pyrite, rutile, wavellite, chalcedony ( ?). uggy carbonatite, coarse-grained calcite (80 percent), apatite, clay, pyrite, magnetite partly altered to hematite and limonite, rutile, biotite, perovskite, pale yellow mica, wavellite. uttings, no core. (90. percent), 10, 12, and 20 ft. weathered mica, garnet, 37—72 _________ C Prob- 72—74 _________ V 74—78 _________ 84—94 _________ V 94—111 ________ C Carbonatite, coarse calcite apatite, pyrite, magnetite, rutlle. 0 sample. Driller’s log shows that hard rock (probably ijolite) was cut at 113 ft. Cuttings, 60 to 90 percent ijolite, 40 to 10 percent carbonatite. Ijolite cut by calcite veins. Cuttings. Calcite. 111—115 _______ N 115—129 _______ 129439 _______ 139—145}£ ______ ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. TABLE 22.——Dz'scriptive logs of core holes—Continued Hole IrlSO—Continued Depth (feet) Description hwy—146%- _ _ ._ Ijolite and calcite with abundant biotite along the contact between the two. 146}é—152 ______ Ijolite with small stringers of calcite. 152—155 _______ Ijolite. 155—156.7 ...... Ijolite with pyrite-rich stringers at base of core. Hole L—132 Location: NWV; sec. 20 Date drilled: August 30-September 17, 1954 Depth (feet) Description 0—22 __________ Gossan composed of limonite, and minor hema- tite, mica, pyrite, apatite and magnetite. 22—28 _________ Transition zone composed of partly weathered apatite-pyrite vein of grayish-green color and limonite. 28—44 _________ Partly weathered grayish-green apatite—pyrite vein. 44—52 _________ White clay. Minerals include pyrite, apatite, rutile, perovskite(?). Probably a rutile vein. 52—90 _________ Partly weathered grayish—green, apatite-pyrite vein, and magnetite. 90—96 _________ Apatite-magnetite rock. Magnetite up to 25 percent of rock. 96—106 ________ Apatite-green biotite-magnetite rock. Apatite 85 to 90 percent. 106—116 _______ Transition zone, apatite, biotite, magnetite, calcite. Apatite 85 to 90 percent. 116—139 _______ Carbonatite cuttings, calcite 75 percent, apa- tite, biotite, magnetite. Vuggy carbonatite, coarse—grained calcite 75 to 95 percent; apatite, biotite, magnetite and pyrite. 221—226 _______ Upper 0.9 ft carbonatite, coarse—grained yellow calcite (90 percent), apatite, pyrite, biotite. Lower 1.2 ft vuggy carbonatite, coarse- and medium-grained calcite (60 percent), pyrite, montmorillonite(?), clay, apatite, biotite, minor garnet, rutile. 226—236 _______ Soft interval, 227 to 230. Carbonatite, coarse- and medium-grained calcite (some White, black, and yellow, 90 percent), apatite, py- rite, biotite, minor magnetite, pyroxene. Upper half: carbonatite, coarse- and medium- grained calcite (60 percent), pale-brown mica (phlogopite(?)), biotite, pyrite, apatite. Lower half: carbonatite, coarse— and medium- grained calcite (90 percent), pyrite, biotite, minor pale brown phlogopite(?), apatite. 242—244 _______ N 0 sample. 244—254 ....... Carbonatite, coarse-grained calcite (75 per- cent), fine-grained calcite, biotite, phlogo- pite(?), pyrite, minor apatite, garnet. 254—254. 7 _____ Carbonatite, coarse-grained calcite (80 per- cent), apatite, biotite, minor monticellite, pyroxene, magnetite. Carbonatite, coarse-grained calcite (60 per- cent), monticellite, magnetite, apatite, minor lemon yellow pyroxene, perovskite. Carbonatite, coarse—grained calcite (80 to 95 percent), monticellite, apatite, pyroxene, magnetite, biotite. 266—289_-_____ Carbonatite, coarse-grained calcite (55 to 95 percent), apatite, biotite, magnetite, pyrite, pyroxene, amphibole. Carbonatite, coarse-grained calcite (90 per- cent), apatite, cryptocrystalline green ma- terial, monticellite, magnetite, biotite, py- roxene, cryptocrystalline red material. Carbonatite, coarse- and medium-grained cal— cite (60 percent), light-green cryptocrys- talline material (a mixture of calcite and probably amphibole), apatite, biotite, mag- netite, minor green amphibole, monticellite, pyrite, fine-grained purple calcite. 139—221 _______ 236—242 _______ 254. 7—256 _____ 256—266 _______ 289—294 _______ 294~299 _______ IGNEOUS COMPLEX TABLE 22,—Descm'ptive logs of core holes—Continued Hole Irl32—Continued Depth (feet) Description 299—311 _______ Carbonatite, coarse- and medium—grained cal- cite (80 percent), fine-grained green material, apatite, pyrite, biotite, magnetite, monti— cellite, amphibole, pyroxene, perovskite. Carbonatite, coarse— and medium-grained cal— cite (70 to 95 percent), magnetite, apatite, biotite, monticellite, fine—grained brown ma- terial, fine-grained green material, pyrox- ene, fine—grained red material, pyrite, gar- 311—417 _______ net(?), and fine-grained white material. Perovskite detected in interval from 311— 321 ft. Hole 14-135 Location: Perovskite'Hill NW1/4 sec. 19. Date drilled: September 26-30, 1954. Depth (feet) Description 1—7 ___________ Mixture of alluvium and carbonatite saprolite. Alluvium in the slush pit 2 to 4 ft thick. 7—8 ___________ Magnetite (50 percent), coarse calcite, apatite, perovskite. 8—19 __________ Carbonatite, calcite (90 percent), magnetite, apatite, perovskite, pyrite. 35 TABLE 22.—Descripttve logs of core holes—Continued Hole L-l35—Continued Depth (feet) Description 19—135 ________ Very poor core recovery. Carbonatite—coarse- grained calcite (50 to 95 percent), apatite. monticellite, magnetite (some altered to hematite), fine-grained calcite (red, brown, and green), perovskite, amphibole(?), pyrite, chalcedony( ), blue clay, rutile. Hole L—134 Location: NE1/4 sec. 19. Date drilled: September 19-25, 1954. Depth (feet) Description 0—10 __________ Mixture of carbonatite saprolite and alluvium. In the slush pit 10 feet north of the hole, alluvium is 2 to 3 ft thick lying on carbona- tite saprolite. 10—106 ________ Carbonatite, calcite ranges from 90 to 95 per- cent, apatite, biotite, magnetite, limonite, hematite, manganese oxides(?), pyrite. Mi- nor rutile detected in following intervals: 39—71 and 84—101 ft. The rock is composed chiefly of white to light—gray medium- to coarse-grained calcite with scattered irreg- TABLE 23.——Analyses of carbonatite, in percent Colorimetric L-304 L—166—2 L—304 L—12 L—166—a L—117 L—135 L—170—a L—170—b L—131—a L—131—b L—131—c L—ll7 Colorimetric Spectrographic analyses analyses 0 0. 002 0. 0004 0. 0005 0 0 0. 0008 0. 0005 {70. 0002 0. 0004 .2 3 .5 .5 1 .3 .03 .03 . .0004 . 5 . 9 . 9 . 9 . 9 . 9 . 2 . 2 . 02 .2 .009 .3 .2 .02 .009 .2 .2 . 001 . 001 . 001 0 . 001 . 001 . 001 . 004 . 002 . 007 . 007 . 007 . 007 . 002 . 002 . 01 . 009 . 04 . 03 . 05 . 04 . 01 . 01 .06 .04 .08 .08 .2 .2 .02 .08 .3 .04 .3 .09 .5 .5 .02 .02 . 002 . 0004 . 003 . 003 . 002 . 002 . 001 . 0007 . 08 . 6 . 2 . 6 1 1 . 1 . 07 .002 .03 .006 .02 .2 .2 .003 .002 . x . x xx x x xx xx xx xx .03 .004 .05 .06 .1 .1 .06 .04 .05 .06 .02 .01 .2 .4 .02 0 . 0007 . 0004 . 002 . 002 . 0008 . 001 . 001 . 0008 0 . 003 0 . 004 0 . 007 . 009 Q . 5 . 1 . 8 2 . 5 . 09 . 5 . 6 2 2 4 3 >5 2 1 0 . 002 . 003 . 001 . 003 . 002 0 0 0 ........ 0 0 . 002 . 001 . 003 . 004 . 003 . 003 . 0006 ________ .004 .03 .07 .01 .001 . 001 .01 .01 .004 0 . 002 . 001 . 002 . 002 0 0 0 0 0 0 . 008 . 003 0 0 0 0 0 Radiometric analyses eU ______________________________________________ <0. 001 <0. 001 <0. 001 0. 004 0. 001 0. 004 0. 007 <0. 001 0. 002 <0. 001 \ Chemical analysis U _______________________________________________ <0.001 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. | n.d.=not determined. Looked for (spectrographically) but not found: Hf. Th, Ta. W, U. Re. Ru. Rh. Pd. Os, Ir. Pt. Ag. Au, Zn, Cd. Hg. In. T1. Ge. Sn, As. Sb. Bi. Te. Standard chemical analyses. L—304 by M. K. Balazs; L—166—2 by L.N.Tarrant. Spectrographic analyses by Sol Berman. Radiometric analyses. L—304, by D. L. Schafer; all others by B. A. McCall. Chemical analysis for uranium by J. P. Schuch. Colorimetric analyses by H. E. Crowe and A. P. Marranzlno. L—304. Carbonatite, NWM sec. 19. 11-12. Black carbonatite, SW34 sec. 18. 659634—62—4 L—166~a. Altered inclusion in carbonatite, NWV; sec. 19. L—117. Carbonatite saprolite, NWM sec. 19. L—135. Carbonatite saprolite, NWM sec. 19. L—170—a. Carbonatite saprolite minus clay, NWM sec. 19. L—170-b. Carbonatite saprolite minus clay, and hand magnetic fraction, NWM sec. 19. L—131~a. NWM sec. 19. L—131—b. NWM sec. 19. L—131—c. NWM sec. 19. L—166—2. Residual and secondary apatite, Residual and secondary apatite, Residual and secondary apatite, Apatlte from carbonatito. 36 ular areas enriched in apatite, brown monticellite, mag- netite, black perovskite, black zirconium garnet (kim- zeyite), green biotite, and pyrite. Minute irregularly shaped black inclusions agglomerated and disseminated in the calcite have produced a black medium-grained variant found in contact with the analcime—olivine melagabbro in the southern part of sec. 18. Inclusions of ijolite a few inches to more than 50 feet across occur in the carbonatite. The larger blocks in the quarry have reaction rims of varying thickness. Fryklund, Harner, and Kaiser (1954) have described a typical rim composed of three zones: 1. A narrow outer zone, usually less than 1 inch wide, of magnetite and pyrrhotite lying in a medium—grained calcite matrix. 2. An intermediate zone rich in green biotite. Some biotite grains may reach 1 centimeter in diameter. 3. A fine-grained idocrase-rich inner zone of variable width, in some cases including the whole block. A few large idocrase crystals, reaching 3 to 4 centimeters in length, are present in these zones. ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. The alteration of the ijolite inclusions is similar to the alteration along the contact of the carbonatite and ijolite and involved thorough replacement of nepheline by zeolite (probably thomsonite), partial to complete alteration of brown garnet to colorless garnet and ido- crase, bleaching of the biotite, and the formation of tremolite(?) and pectolite(?). Weathering of the carbonatite yields two products— a black or brown saprolite and the porous rock com— posed of both residual and secondary apatite. The latter rock was called tufa by Williams (1891), but Fryklund, Harner, and Kaiser (1954) suggested it was a product of weathering, and we concur. Near the Kimzey calcite quarry the secondary apatite can be seen on the surface of the fresh rock. Several apatite samples were analyzed (tables 23 and 24). The samples are described below: L—181, 131a, 131b, and 131c.—These samples are composed of residual apatite, magnetite, and perovskite in a matrix of sec- TABLE 24.~Analyses of saprolite of carbonatite, in percent L—117 L—117—a L—117—b L—131 L—135 L—l35—a L—l35—b l L—l70—a L—170-b Standard phosphate analyses [Analysts G. Edgington and E. Y. Campbell] 11.4 1.1 0.8 2.1 15.2 3. 7 3.0 2. 7 2.7 8. 0 3. 3 1.0 l. 4 8. 8 4. 0 3. 4 12. 2 1. 8 24. 9 47. 9 51. 5 50. 4 16. 1 40. 3 41. 7 43. 9 48. 5 19. 7 37. 2 40. 4 36. 8 14. 7 31. 6 32. 2 29. 2 31. 6 .31 .43 .35 1.9 .93 1.5 1.5 .26 .27 25. 3 9. 1 4. 8 1.7 31. 9 11.3 10. 9 17. 4 14. 6 Radiometric analyses [Analyst. B. A. McCall] O. 007 ’ 0. 006 I 0. 005 0. 002 0. 003 0. 004 0. 004 0. 006 0. 007 Chemical analyses [Analysts. A. Sweeney and Wendell Tucker] 0.003 I 0.005 t 0. 004 0.002 0.001 0. 002 0.005 0.005 0.007 Semiquantitative spectrographic analyses [Analyst. K. E. Valentine] Sample (percent) L—117 L—117—a L-117—b 12.-131 11-135 L-135-a L—135-b L—170-a L—170—b Ca, P _________ Ca, P ......... Ca, P ......... g9, P. 1 e, g Si, Al, Mg, Fe. Si, Al K, Fe_- Ti, Fe, Al _____ Si, Al, Fe Mn, K, Sr . Mn, K,La, Sr- Mg, 1', La--_. Mg, Sr,La,Si.- Sr, La, Mg La, Ce, Y, Li. Ce Na,Y,Ti, Mn, Ti, Ce, Zr, Na, Ce, Ce, V, Y, Zr, 9 N , Na, Mn, A1, Na, Mg 121 Na, Y, Li Mn Mn, Na , a 0.0501 ................ Nd, V, Y, LL. Nd, Li ........ MNHVLF’ N21}, Ti, Li, V, Ba, V, Na _____ Ba, V, Nd-_-. Nd, Ba, V---- Nd, V, Y----- Nd, Ba 0.01-0.05 ............... Cu, B, Pr ..... B, Br, Ba, K, Ti, Pi‘, K, Ba. Ce, La, Y, B, Nd, Pr, Yb... Pr, Yb ________ Pr ____________ Nb, Li, K, Nb, Li, K, Pr Tl, Pb, Cu Nd Ba, Pr 0.005-0.01 .............. Zr, Nb, Yb, Nb, Gd, Dy, Pb, Gd, Yb, Cu ............ B, Ga, Dy, B, Gd, Er, B, Er, Gd, ............. a, Dy Yb E, Cu, Dy Cu, Pb Dy, Pb Dy, Yb 1- 0001-0005 ............. Er, Ni, Sc ..... . Ni Zr Nb, Ni, Zr, Mo, Zr, Er, Ni, Sc, Sc, Nb, Zr Cu, Nb, Pb, Sn,Yb, Ni,Sc- Dy, Yb, Ni, 80 do, so Nb, Yb Mo, Nb Ni, Cu, 0 Mo, Sc, N1, Or, Be ........ Cr ____________ Cu, Cr ........ Cu, Cr ------------ Ag..-_.__-__-_ Be ............ Be ............ L-117. Carbonatite saprolite, N WM sec. 19. L—135—a. Carbonatite saprolite minus clay, NWX sec. 19. L—117-a. Carbonatite saprolite minus clay, N WM sec. 19. L~135—b. Carbonatite saprolite minus clay and hand magnetic fraction, NWM sec. L—117-b. Carbonatite saprohte minus clay and hand magnetic fraction, N WM sec. 19. ‘ xiii-$1 Residual and secondary apat'te wa s 19 11:38:? (éarlgonagtée samince mums ciay’ NE]? stic' 19' a n- u NW . 1 cc. . . at o e ' ' ' s . L—135. Carbonatite saprolite, N W% sec. 19. 4' 19. na sapro l 9 minus 0 ay an an magne 0 so on % ec IGNEOUS ondary apatite. The secondary apatite occurs in cryptocrystal— line granular masses and as radiating fibers in colloform crusts. L—117 and L—185.—Saprolite. From X-ray analyses of the clay- and silt-size portion of these saprolites, A. J. Gude 3d, (written communication, 1956) reported major montmorillonite and minor kaolinite in L—l 17 and major kaolinite, and traces of montmorillonite and chlorite in L—135. L—117a, 135a, and 170a.—Panned concentrates of the sapro- lite. Of the saprolite, L—117-a represents about 40 percent and L—135a represents about 26 percent. L—117b, 135b, and 170b.——Panned concentrates with the hand magnetic fraction (mostly magnetite) removed. Visual estimates of the composition of these fractions are as follows: L—117b (39 percent of the saprolite) apatite 90 percent, and the remainder—— iron-stained clay aggregates, weathered pyrite, rutile, perovskite, and anatase. L—135b (25.5 percent of the saprolite) apatite 70 percent, and the remainder—iron-stained clay aggregates, weath- ered pyrite, rutile, perovskite and anatase. L—170b apatite 90 percent, and the remainder—perovskite, anatase, kimzeyite, and mica. CHEMISTRY Chemical and spectrographic analyses of a composite of the carbonatite in the quarry, table 23, show that the carbonatite is a relatively pure calcite rock. Phos- 37 COMPLEX phate, silica, and magnesia are the only other oxides greater than 1 percent. The magnesia occurs chiefly in magnetite (magnesioferrite) and monticellite; phos- phate occurs in light-green acicular crystals of apatite. The early crystallization of these minerals as well as perovskite and kimzeyite provided for the capture of most of the trace elements and explains the high purity of the late crystallizing calcite. Trace elements in the rock that are equal to or exceed 0.01 percent and in order of abundance include: Sr, Mn, Ba, Ti, and V. The difficulty of getting a representative sample of the carbonatite is illustrated by the content of niobium in the carbonatite. Fryk- lund, Harner, and Kaiser (1954) out 21 channel samples of the carbonatite for Nb, TiO2, V205, Y, and La analysis. Niobium was detected in only 6 of the samples, but a simple average yields a value of 0.01 percent niobium for the carbonatite. From the standard phosphate analyses (table 24) of the concentrates of the saprolite, it is obvious that the grade is adequate for commercial phosphate. How— TABLE 25.—-Spectrographic analyses of mineral separates from carbonatite, in percent L—166-1 L—166—2 Ir166—3 L—166—4 L—166—5 L—166—6 L—135-a L—129—c L—l29-d Be ___________________ 0. 003 O 0 0 0 0. 001 0 0 Mg __________________ >10 . 03 . 1 >10 >10 . 5 n.d. . 0X . 0x Ca ___________________ n.d. n.d. n.d. n.d. n.d. n.d. n.d. >10 >10 Sr ___________________ .1 .1 .5 .008 0 .01 .02 x .x Ba ___________________ . 002 . 002 . 02 0 0 . 002 . 005 . 0X . 0x Sc ___________________ . 0004 . 0006 0 0 . 001 . 02 . 001 . 00x . 00x .004 .02 0 .02 0 .1 .19 .0x .0x . 04 . 1 O 0 0 . 04 . 2 x x . 5 . 005 . 005 . 01 1. 0 3. 2 >10 n.d. n.d. . 06 0 0 0 . 03 >10 . 63 0 0 0 >10 0 0 0 0 2. 0 0 0 . 04 . 2 . 03 . 006 . 07 . 01 . 92 . 025 . 025 .02 0 0 0 .03 .2 6.8 8.2 9.2 . 002 . 0007 . 003 . 002 . 0007 . 001 . 0002 . 000x . 000x . 02 . 002 . 01 1. 2 2. 7 .05 . 13 .0x .0x 0 0 0 . 004 . 01 0 0 0 0 0 . 002 0 0 0 0 . 006 . 00x . 00x . 0009 . 0004 . 0006 . 0009 . 001 . 0006 . 018 . 00x . 00x . 004 0 0 0 . 009 . 008 0 0 0 0 0 0 0 0 . 02 0 0 0 0 O 0 0 O 0 . 004 0 0 n.d.—not determined. Looked for but not iound: B, Yb, Hf, Th, Ta, Mo, W, U, Re, Rh, Pd, Ir, Pt, Ag, Au, Zn, Cd, Hg, In, T1, Ge, As, Sb, Bi. EXPLANATION OF SAMPLES Sample X-ray X—ray anaIYst Spectrographic analyst L—166—1 __________ Idocrase ____________ H J. Rose L—166—2 __________ Apatite ____________ D0. L—166—3 __________ Calcite _____________ Do. L-166—4 __________ Monticellite ________ Do. L—166—5 __________ Magnetite __________ Do. Ir166-6 __________ Kimzeyite __________ Do. L-135—a __________ Anatase ____________ Anatase and small amount of F. A. Hildebrand ________ Do. a atite. L—129—c __________ Perovskite __________ p J. D. Fletcher. L—-129—d __________ Perovskite __________ Do. 38 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. ever, more drilling and sampling must be done to determine the size of the deposit. Several minerals Were separated from the carbonatite in the quarry for spectrographic analyses (table 25). The anatase and perovskite are float crystals found south of the quarry on Perovskite Hill. The fine- grained pale—brown anatase occurs as an alteration product of perOVSkite With the replacement varying from partial to complete. Anatase in this rock has been observed only in the saprolite and thus it may be a product of weathering as suggested by Williams (1891). The zirconium garnet is a new mineral, and has been named kimzeyite (Milton and Blade, 1958). In- dividual crystals as much as 2 mm across are black and where euhedral have the crystal habit of garnet, the dodecahedron in combination with the trapezo- hedron. X-ray work by J. M. Axelrod and F. A. Hildebrand indicates that the mineral is isotypic with garnet; the cell edge (a0)=12.46A. In thin section, it is isotropic, light brown, and has an index of refrac- tion near 1.95. The computed oxide percentages from spectrographic analyses by Harry Bastron (table 26) show relatively high Nb, Sc, and Sn content. The total is low; however, P205, H20, F, S, and 002 were not tested. Further work on this mineral is in progress by Charles Milton. TABLE 26.—Composition of kimzeyite [Data from Milton and Blade (1958)] Computed as oxide Element Amount (percent) Oxide Percent Si ___________________ 10 SiOz _____________ 21. 4 Al ___________________ 6 A1203 ____________ 11. 4 Ca __________________ 12 CaO _____________ 16. 8 Fe __________________ 11. 5 Fe203 ____________ 16. 45 Ti ___________________ 3. 5 TiOz _____________ 5. 8 Zr ___________________ 15 Zr02 _____________ 20. 25 Nb __________________ . 5 Nb205 ____________ . 72 Mg __________________ . 3 MgO _____________ . 5 Mn _________________ . 1 MnO ____________ . 13 Sn __________________ . 07 SnOz _____________ . 09 SC ___________________ . 06 80203 ____________ . 09 Cu, Ba, Sr ___________ Tr. Total ____________________________________ 94 Looked for but found: Ag, Au, Hg, Ru, Rh, Pd, Ce, Ir, Ge, Pb, As, Sb, Pt, Mo, W Re, Bi, Zn, Cd, Te, In, Co, Ni, Ga, Cr, V, Y, La, Hf, Th, Ta, Be, Li, Na, K, B. The distribution of the trace elements within the carbonatite is shown by the spectrographic analyses of the separated minerals. 1. The calcite is relatively pure; Sr is the most abundant trace element (0.5 percent) followed by V, Ba, and Mn, in order of abundance. 2. The high magnesium content of magnetite indicates that it is more properly called magnesioferrite. 3. Strontium occurs in calcite, perovskite, apatite and idocrase. 4. Scandium is in kimzeyite. 5. Lanthanum is particularly concentrated in perovskite (greater than 1 percent) and to a lesser extent in anatase and apatite. 6. Zirconium occurs as a major constituent in kimzeyite and is also found in anatase (0.63 percent) with lesser amounts in idocrase. 7. Vanadium is in anatase (0.92 percent), apatite and magnetite. 8. Niobium is highly concentrated in perovskite and anatase (up to 9.2 percent). 9. Cobalt and gallium occur in magnetite; gallium is in kimzeyite. 10. Nickel and copper are most abundant in anatase. 11. Yttrium is most abundant in anatase and kimzeyite. The trace-element content of the carbonatite sapro— lite ShOWS that Be, Sr, Ba, Sc, Y, La, Ce, Yb, Ti, Zr, P, V, Nb, Cr, Mn, Co, Ni, Cu, Ga, and Pb, are concentrated during weathering. Resistant minerals account for most of the concentration of these elements particu- larly because the easily weathered calcite is poor in trace elements; 012er The origin of similar carbonate bodies from other localities has been ascribed to sedimentary limestone xenoliths, hydrothermal replacement bodies, and mag- matic intrusions. Reexamination of these various modes of origin as well as detailed studies of alkalic rock complexes have been stimulated recently by the economic potential of carbonatite deposits for com- modities such as niobium, rare earths, apatite, uranium, and agricultural lime. Pecora (1956) reviewed the carbonatite problem and listed 32 worldwide occurrences which appear to have originated “from a hot carbonatic fluid geneti- cally derived through some magmatic process.” He concludes from his review that ”carbonatites were deposited by carbonatic solutions having a wide range of temperature, pressure, and concentration and derived from alkalic magmas during the process of silicate crystallization.” And further that “a car- bonate magma in the normal sense is less likely to exist than carbonate—rich solutions which at elevated temperature and pressure can have a higher concen- tration of dissolved ingredients than normally believed for hydrothermal solutions.” The field observation and laboratory work on the Magnet Cove carbonatite indicate that it is an in— trusive mass and probably was deposited from late- stage heavily gas charged, magmatic solutions. Some of the observations are listed below: 1. Virtual absence of limestone in the known Paleozoic section of this region of Arkansas. 2. Abundance of inclusions of syem'tic and possibly ijolitic type rocks in the carbonate body. These inclusions range in size from a few inches to blocks 7 to 8 feet long and DIKES have a strong reaction zone characterized by an outer rim of pyrrhotite, magnetite, green biotite, and idocrase. Many of the inclusions are completely altered to mica, zoisite(?), and an impure aggregate that may be zeolitic alteration of nepheline. The inclusions attest to the introduced, mobile, and reactive character of the carbonate and indicate that the carbonate body is younger than most of the igneous rocks and probably represents a late stage in the development of the Magnet Cove complex. 3. Absence of any structure or layering in the coarsely crys- talline mass of calcite. 4. Abundance of carbonate as a major constituent of the feldspar- carbonate pyrite-rutile veins and dikes that cut the igneous rocks and are so abundant in the Magnet Cove area. 5. Abundance of coarsely crystalline calcite dikes that cut ijolite in the basin of the Cove area. 6. The abundance of rare earths, niobium, titanium, and zir— conium in the silicate minerals in the carbonatite that could not be supplied by a normal sedimentary limestone, nor by reaction with the included blocks of igneous rock. These elements must have been a part of the carbonate mass as it moved into its present position. 7. The presence of primary calcite in coarse—grained ijolite rocks that occupy most of the Cove basin. Fine-grained calcite is a major constituent of the altered phonolite and phonolite breccia which we believe is produced by reaction of trapped CO; with early-formed minerals in quick- cooling magma. 8. Calcite—pyroxene-nepheline pegmatite also testify to primary nature of calcite. These observations indicate that the carbonatite bodies at Magnet Cove are not sedimentary limestone xeno- liths. The mobile introduced nature of the carbonate has been firmly established but whether this intro- duction is hydrothermal or magmatic is not so easily resolved. However, primary calcite in many of the rocks even to the extent of being a major constituent of the altered phonolite and one of the pegmatites suggests that the carbonate is a late stage differentiate—pegma— titelike—-on the borderline between magmatic and hy- drothermal, greatly enriched in 002, Ti, Nb, and rare earths. The origin of the carbonatite is further dis- cussed in the section on origin of the rocks. LIME-SILICATE ROCK A complex body of lime-silicate rock, about 0.4 per- cent of the exposed igneous rocks in the complex, occurs in the northwestern part of sec. 20. Like the surround- ing ijolite it is deeply weathered, and therefore age re- lations with the ijolite are not determinable. Small areas of fresh rock are exposed in a pit opened to mine residual magnetite. The bulk of the rock is a fine- to medium-grained intergrowth of anhedral idocrase and colorless diopside with a few euhedral crystal of apatite. Abundant miarolitic cavities are lined with euhedral crystals of diopside, aegirine, apatite, idocrase, and andradite. A few large crystals of melilite, some more than 18 inches across, are partly replaced by a fine- to coarse-grained 39 mixture of idocrase, diopside, tremolite, pale brownish- green biotite, perovskite, brown to colorless garnet, calcite, and zeolite. Irregular but rounded masses of magnetite, up to 1 foot across, are found, some alined like beads on a string in the idocrase—diopside matrix. Pieces of fine-grained zeolite with miarolitic cavities containing euhedral crystals of natrolite were found on the waste—rock dump but none were seen in place. The melilite with its alteration products looks similar to the unoompahgrite of the Iron Hill complex in Colorado. It seems most probable that the rock type is a basic crystal differentiate of magnetite, melilite, and minor feldspathoid from the ijolite magma and that its contact with the surrounding ijolite is gradational. Late intro- duction of the high-volatile carbonatite fluids probably caused the intense alteration of melilite. Carbonatite occurs within 500 feet to the northwest of the lime- silicate body. Spectrographic analyses of several handpicked min- erals, table 27, show that: 1. Magnetite is both titaniferous and magnesian probably close to magnesioferrite. The trace elements in this magnetite in order of abundance are: Mn, V, Co, Zr, Zn, Ga, and Sc. 2. The three analyzed garnets show the direct relationship be- tween dark color and high titanium content. V, Zr, Nb, and Y are concentrated only in the dark garnet. 3. Aegirine captures the greatest amount and variety of trace elements of the analyzed minerals. In order of abundance these include: Zr, Sr, Nb, Mn, V, Y, Ba, Cu, Be, Co, Sc, Ga, Cr, Ag. Concentration ranges from 0.2 percent Zr to 0.0003 percent Ag. . The bulk of the Sc is in colorless diopside (0.01 percent). . Sr is particularly high in apatite, idocrase, and melilite; barium is in biotite. onus DIKES Although the entire Magnet Cove igneous area is a dike complex, the rocks described in this section are clearly late stage, of minor areal extent, and show cross- cutting relationships to the major ring dike rock units. These dike rocks will be described in two sections: dikes within the complex and dikes outside the complex. DIKES WITHIN THE COMPLEX Dike rocks within the complex include in approximate order of abundance: tinguaite, analcime olivine mela- gabbro, nepheline syenite pegmatite, trachyte porphyry, aplite, eudialyte nepheline pegmatite, and garnet fourchite. TINGUAITE DISTRIBUTION AND DESCRIPTION Tinguaite (dike phonolite), the most abundant dike rock exclusive of ring dikes in the Magnet Cove igneous complex, occurs as narrow, fine-grained, green to black dikes and irregular or oval-shaped bodies that cut all other igneous rock types. Some of the tinguaites are porphyritic and contain pseudoleucite and (or) nephe— 40 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. TABLE 27.——Spectrographt'c analyses, in percent, of mineral separates of lime-silicate rock L—168—2 L—168-5 L—168—6 L—168-11 L—168—7 L—168—13 L—168—14 L—168—10 L—168—8 L—168—4 L—168—12 L—168—3 L—168—1 D1684 0 0 0 0. 0004 0. 0004 0. 0009 0. 004 0. 002 0. 004 0 0 0 0. 002 0. 0009 >1 .4 .2 .09 1 >1 1 >1 >1 1 .1 .02 >1 .06 0 .002 .002 .005 .1 .1 .04 1.2 .6 .005 1 .2 .6 .007 0 .002 0 .002 .01 .002 . 007 .04 . 004 1. 2 .005 . 02 . 01 . 007 . 004 .006 0 0 0 . 01 . 001 0 0 . 004 0 0 0 0 0 .02 0 0 0 0 .01 0 0 0 .01 O 0 0 0 0 0 0 0 0 0 0 0 0 .01 0 0 0 3.6 4.2 .4 .03 .01 .03 .004 .02 .04 .6 .004 .002 .02 .005 .02 .2 0 0 0 .006 .2 0 O .008 .004 0 0 0 0 0 0 0 0 . 9 0 0 0 0 >10 0 0 0 .2 .3 .2 .006 .005 .005 .02 .005 .02 .02 .1 .009 .01 .004 0 . 04 0 0 0 0 .03 0 0 0 0 0 0 0 .0009 .001 . 0009 .0009 . 002 .0009 . 0006 .0009 . 0006 . 0006 . 0009 .003 . 001 . 0006 .8 .2 04 02 .03 .06 .02 .02 .01 .1 .002 .01 .08 .001 >10 >10 >10 >10 1. 5 . 4 >10 3. 2 4. 4 4 . 01 .001 2. 4 . 08 .02 0 0 0 . 003 .003 . 003 0 . 001 0 0 002 0 .002 .002 .002 0 0 0 0 0 0 0 0 0 0 . 001 . 001 . 0008 . 001 . 0006 . 0006 . 005 . 002 . 0008 . 002 . 0006 . 0008 . 001 . 0007 0 0 0 0 0 . 0003 0 0 . 0001 0 0 0 . 0001 . 01 0 0 0 0 0 0 0 0 0 0 0 .004 .001 .004 0 0 0 .001 .003 .002 .003 0 0 .001 .002 Indices of refraction l 1. 67l:l:0. 005 1. 662d:0. 005 1. 679i .005 1. 669:]: . 005 1. 707:}: .005 1. 693d: . 005 l Determinations by D. J. Jameson. Looked for but not found: Ce, Hf, Th, ’I‘a, Mo, W, U, Re, Pd, Ir, Pt,EAu, Cd, Hg, In, Ge, Sn, Pb, As, Sb, Bl. Spectrographlc analyses by H. .1. Rose. X—ray analyses by F. A. Hildebrand. EXPLANATION or SAMPLES Sample X—ray analyses L—168—2 _____ Megnetite L—168—5 _____ Dark garnet L—168—6 ..... Brownish-yellow garnet L—168—11__.- Pale greenish-yellow andradlte- . Garnlet .ttype mineral unit cell- me am e. L—168—7 ..... Pale-green dlopside L—168—13_ _._ Colorless diopside L—l68—14- -_. Aegirine .................... Monoelinic pyroxene probably aegirme. L—168—10_-.- Idocrase-diopside mixture _______ Idocrase plus moderate amount of monoclinic pyroxene. Idocrase _________________________ Idocrase. Brown and green biotite Apatite Calcite Melilite Natrolite line phenocrysts. Many of the nonporphyritic ting- uaite outcrops are easily mistaken for fine—grained metamorphic rocks, but the green color and uniform texture are characteristic. 7 The pseudoleucite tinguaite porphyry, the most striking of the tinguaitic rocks, is best exposed about 1,000 feet south of US. Highway 270 in sec. 21. White pseudoleucite phenocrysts up to 1 inch across are abun- dant in a fine-grained green matrix. In thin section, the texture of the rock ranges from tinguaitic to pilotaxitic. The fine-grained, holocrys— talline groundmass consists of a mesh of rod-shaped sodic pyroxene microlites with equigranular nepheline and sodic orthoclase in the interstices (tinguaitic tex- ture). In some of the rocks the sodic orthoclase occurs as a felt of lath-shaped microlites in flow lines (pilotaxitic texture). ' ‘ ‘ The proportion of phenocrysts in the tinguaites ranges from 0 to 50 percent; Pselidbleucite phenocrysts are most abundant; nepheline and pyroxene phenocrysts are common. The pseudoleucite crystals are hexag- onal in part and composed of clear fresh nepheline and dirty, partly kaolinized sodic orthoclase. Some of the nepheline is altered to cancrinite. Sodic pyroxene rods tend to concentrate around the edges of the pseudoleu- cite crystals. Some of the larger, almost euhedral early formed pyroxene crystals are zoned and have a core which is near normal augite. Fluorite, sphene, magne- tite, apatite, sodalite and plagioclase are accessory minerals. Calcite, cancrinite, and kaolinite are altera- tion products. Brief descriptions of some of the variants are given below: M 0—152.—Grayish»green tinguaite porphyry cuts sphene pyroxenite and the contact zone sediments in the NE% sec. 17 and the NW% sec. 16. Phenocrysts of sodic orthoelase up to 3 inches long and nepheline up to 3 mm across make up about 25 [percent of the rock. In thin section the minerals are: sodic orthoclase as phenocrysts and groundmass anhedra plus minor BIKES perthite and albite—about 45 percent, nepheline as phenocrysts and groundmass anhedra both altered to cancrinite and sodalite (?)-——about 20 percent, and green aegirine as stubby phenocrysts and groundmass needles—about 35 percent. L—23.——Grayish-green porphyritic tinguaite cuts fine-grained ijolite, analcime-olivine melagabbro, garnet fourchite, and altered phonolite in the SW% sec. 18. Phenocrysts of nepheline up to 2 mm across and pseudoleucite of about the same size make up about 10 percent of the rock. In thin section the minerals are: sodic orthoclase as phenocrysts and minute laths—about 40 percent, nepheline as phenocrysts partly to completely altered to sodalite(?) and minor cancrinite—about 20 percent, green aegi~ rine as phenocrysts and minute needles plus accessory anhedral magnetite—about 40 percent. The saprolite of this rock was analyzed spectrographically (table 28). Material in the panned concentrate occurs in the following order of abundance: clay aggregates, feldspar, weathered pyrite, magnetite, and a trace of rutile. The presence of rutile and perhaps pyrite indicates contamination by weathered rutile veinlets not apparent when the sample was taken. From an X-ray analysis of the clay- 41 and silt-size portion of this sample, A. J. Gude 3d (written communication, 1956) reported halloysite(?) and chlorite(?). M C—1 15.——Greenish- gray porphyritic tinguaite dike cuts gar- net-pseudoleucite syenite in the NE% sec. 24. Phenocrysts of pseudoleucite up to 5 rock. In crysts composed of kaoli nepheline, crysts partially altered to sod mm across make up about 5 percent of the thin section the minerals are: pseudoleucite pheno- nized orthoclase and partly altered kaolinized orthoclase phenocrysts, nepheline pheno- alite(?) and cancrinite—about 65 percent; green aegirine in elongated prisms—about 35 percent; and accessory anhedral fluorite. CHEMISTRY Chemical and spectrographic analyses and norm, table 28, show the highly alkaline character of the tinguaite and its similarity to Daly’s average tinguaite. The total alkali content is greater (16.83 percent) than in any of the other analyze d rocks from Magnet Cove. Sodalite and fluorite were not seen in the rock, but the TABLE 28,—Analyses and norm of tinguaite, in percent M 0—115 (1) H M (3-115 M 0—1158. M C-115b M C—115c L—23 MC—152—1 M 0-115 Standard chemical analyses 1 Spectrographic analyses 1 Norm 52. 85 0. 0005 0. 0007 0. 002 0. 001 0. 0005 0 47. 20 19. 44 n.d. . 07 .2 . . 2 . 004 1. 05 4. 82 n.d. . 3 1-10 1—10 . 08 n.d. 29. 25 .57 .066 .3 .7 .8 .005 .4 .94 .37 .48 .07 .4 .08 .3 .2 .32 . 13 0 0 0 . 002 n.d. 12. 47 2. 53 0 0 0 0 . 001 0 2. 13 . 35 0 0 0 0 . 004 0 3. 13 8. 87 . 021 0 04 . 02 . 07 0 . 70 7. 96 n.d. n.d. n d n.d 0 n.d. . 61 . 07 0 0 0 0 . 0007 n.d. .80 .78 n.d. .06 .5 .6 3 .01 . 33 . 027 . 01 009 . 02 07 . l3 . 014 .007 01 . 07 . 02 007 . 04 . 022 0 05 . 03 . 03 . 03 . 0007 . 05 0002 00008 . 002 . 0006 . 60 n.d. 002 . 001 . 02 . 42 . 23 . 8 . 4 1 . 2 . 002 . 04 n.d. >10 1—10 >10 3 3 100. 30 0 . 004 0 0 0 . 4 0 . 03 0 0 . 003 0 99. 94 .0006 .01 .008 . 002 .003 . 0009 , . 0045 . 0008 . 003 . 0008 . 004 . 004 . 0072 0 . 02 . 04 . 005 0 0 0 0 0 0 . 0001 « Colorimetric analyses 4 As ________________ <0. 001 0.001 <. 001 . 0002 . 004 n.d. . 25 - - - . 15 Radiometric analyses 5 all ________________ 0. 006 .................................... 0. 004 Chemical analyses 0, U _____________ o. 00b43 n.d. Th ________________ . 00054 n.d. 1 Average tinguaite (Daly, 1933). . 9 Quantitative chemical analyses by L. M. Kehl. ‘ 8 Spectrographic analyses: MC—ll5, MC-152—1 by H. J. Rose; MC-115a, 115—1), 115—0 by J. D. Fletcher; L—23 by Sol Barman. 4 Colorimetric analyses by H. E. Crowe and A. P. Marranzino. 5 Radiometric analyses by B. A. McCall. 0 Low-level uranium and thorium chemical analyses by J. C. Antweiler. n.d.=not determined. Looked for (spectrograéihlcallygut not found: Hf, Th, P, Ta, W, U, Re, Rh, Pd, Os, Ir, Pt, An, Zn, d, Hg, , Tl, Ge, Sn, As. Sb, Bi, Te. MC—115. Tinguaite, NE1/4 sec. 24. 5 E MC-115a. Magnetic (hand magnet) fraction of heavy concentrate. .MC-ll5b. Nonmagnetic fraction of heavy concentrate. MC—ll5c. Aegirine separate from MC-115. L—23. Saprolite from tinguaite, MC—152—1. sec. 16. SW34 sec. 18 Sodic orthoclase separate from.tinguaite porphyry, MO—152, NWM 42 high chlorine and fluorine content suggests that they could be found. Compared with the average of analyses of the rocks in the complex (table 46) the tinguaite is lower in MgO, FeO, CaO, H2O+, Ti02, 002, P205, 803, S, and higher in Si02, A1203, Fe203, MnO, BaO, N a20, K20, Cl, and F. BaO is unusually high and suggests that the feldspar is barian orthoclase. Trace elements equal to or exceeding 0.01 percent include Sr, Ba, La, Zr, V, Nb, and Zn. Pb approaches this value (0.007 percent) and Be is unusually high (0.0005 percent). Sr, La, Ti, V, Mn, Nb, and Pb are concentrated in the heavier fraction of the rock. Mn, Ti, Sr, V, Pb, Nb, and Be are concentrated in aegirine. The high Mo content (0.02 percent) of sodic ortho— clase deserves special attention and will be discussed further in the section on geochemistry. SODALITE TRACHYTE Sodalite trachyte, about 3.5 percent of the exposed igneous rocks, is widely distributed in the complex, but the bulk of these rocks is found in the garnet-pseu— doleucite syenite in the southern part of the area. The trachytes are all .holocrystalline, greenish-gray to dark-gray aphanites. Light—gray to greenish-gray and dark—gray breccia is closely associated with the trachyte. A good exposure of sodalite trachyte occurs on the south bank of Stone Quarry Creek in the northeastern ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. part of sec. 29. Irregular patches of aegirine (20 per— cent), pleochroic in green and brownish yellow, are scat- tered through a groundmass of sodic orthoclase (70 percent) and sodalite (10 percent). Anhedral sphene is an accessory mineral. Microscopic veinlets of can- crinite, sodic orthoclase, plagioclase, and calcite cut the rock. The chemical analysis of sodalite trachyte (table 29) shows a phonolitic composition and is almost identical with the analysis of tinguaite (table 28). The isotropic mineral in the mode is assumed to be sodalite because of the abundant chlorine in the rock. Trace elements equal to or greater than 0.01 percent include Sr, Ba, La, Zr, Nb, and V. ANALCIME-OLIVINE MELAGABBRO DISTRIBUTION AND DESCRIPTION Analcime-olivine melagabbro, about 0.7 percent of the exposed igneous rocks of the complex, forms an elon- gated dikelike mass about one-half mile long and up to 500 feet wide that cuts fine-grained ijolite in the SW1 /4 sec. 18. A much smaller dike but with similar orienta~ tion is exposed in the Magnet Cove Titanium Corp. pit. The rock is easily weathered and good exposures are rare. Williams (1891) and Fryklund and Holbrook (1950) called the rock a monchiquite. One sample was found which could be called a monchiquite but abundant lab- TABLE 29.——Analyses and norm of sodalite trachyte, in percent [Sample MC—228, sodalite trachyte, N El/4 sec. 29] Chemical 1 Spectrographic Radiometric Chemical 53. 61 Ga ________ 0. 00066 Standard chemical analyses by L. N. Tarrant. Bpectrographic analyses by Harry Bastron. Radiometric analyses by B. A. McCall. Low-level chemical analyses for uranium and thorium by J. C. Antweller. Looked for (spectrographically) but not found: Li, Cs, Ce, Th, P, Ta, W, U, Re, Ru, Rh, Pd, Pt, Ag, Au, Zn, Cd, Hg, In, Tl, Ge, Sn, Pb, As, Sb, Bi, Te. DIKES radorite in most of the thin sections makes gabbro a better name. The rock is dark gray, fine grained to medium grained, and weathers to mottled (brown and green) saprolite. Altered olivine appears as minute light—colored spots against a dark background. In thin section the texture is holocrystalline hypautomorphic-granular. Subhedral pyroxene up to 3 mm long with pale-lav— ender tints, probably titanaugite, is the most abun- ant mineral-40 to 60 percent of the rock. Colorless olivine, near forsterite, makes up as much as 20 per- cent and is remarkably fresh in the fine-grained rocks but almost completely altered to yellow-green and green serpentine and calcite in the coarser grained rocks. Labradonite (Am-,2) occurs as anhedral grains, up to 2 mm long, in the interstices between pyroxene and oli- vine crystals and comprises 5 to 20 percent of the rock. Late-forming orange—brown hornblende, near lampro- bolite (5 to 10 percent) occurs as anhedral grains less than 1 mm long. Apatite, as crystals up to 2 mm long, and deuteric analcime occur in quantities approaching 5 percent. Magnetite and pyrite are accessory minerals. Panned concentrates of two samples, L—22 and L—95, of saprolite show the following minerals in order of abundance: L—22, apatite, rutile and brookite, clay TABLE 30.—-Analyses, norm, an ————i 43 aggregates, mica, and magnetite-ilmenite; L—95, mag- netite, clay aggregates, weathered pyrite, pyroxene, apatite, rutile and brookite, mica, ilmenite, and gar- net. Rutile and brookite in the concentrates indicate contamination by rutile and brookite veins and garnet indicates contamination by garnet fourchite or ijolite. C HEMISTBY Chemical and spectrographic analyses, norm, and mode of analcime-olivine melagabbro, L—lla, are given in table 30. Discrepancies between the mode and the norm indicate that the one thin section taken of the coarse-grained rock is not representative. Normative albite is present in the mode in analcime and labradorite. Much of normative ilmenite, magne- tite and a part of anorthite is probably in modal olivine, titanaugite, and hornblende. Trace elements in the rock that exceed or are equal to 0.01 percent include: Sr, Ba, Cr, V, and Ni. Cu and Sc approach 0.01 percent. Spectrographic analyses of minerals separated from the analyzed rock (table 31) indicate the distribution of trace elements Within the rock. Of the two pyroxenes analyzed, sample L—11a—2a is slightly more magnetic than L—lla—la. (1 mode of analcime-olivine melagabbro L—ua ‘ L—lla L—22 L—95 L—lla L—22 L—95 L-na Smnmggemiml Spectrographic analyses 1 Colorimetric analyses 3 Norm SiOz ______ 34. 51 Be _____ 0 0. 0005 0. 0003 As _____ <0. 001 n.d. <0. 001 or ______________ 3. 89 Alg03 _____ 10. 24 Mg____ n.d. 5 1 Sb _____ . 0001 n.d. . 0001 ab _____________ 13. 62 F620;; _____ 5. 58 Ca _____ n.d. >5 3 Zn _____ . 010 n.d. . 150 an _____________ 18. 63 Ill/€00 ..... 8. $3 1831‘ ...... . 2 . é . (2)04 d1 ______________ 6. 36 n _____ . a _____ . . . h _____________ 17. 71 MgO _____ 8. 26 B ______ 0 . 003 . 001 Radhmemmlym ‘ 01y. _____________ 2. 09 gag ______ 15. 62 §c ..... 88;) . 88: . 339 mt _____________ 8. 12 a ______ . 1 ______ . . . _ _ , 11 ______________ 9. 27 Nazo _____ 1. 62 La _____ .005 .02 .1 3U" ' 0 001 0 004 0 901 ap _____________ 6. 38 20 ______ . 70 Ce _____ n.d. 0 09 pr ______________ . 48 zO—____ . 51 Yb _____ . 0002 . 001 002 Chemical analyses ‘ cc ______________ 12. 20- ¥“8+"" 1'06 5i """ 0%?" 4 03 4 03 i 2 _____ 88 r _____ . . . 00z ______ 5. 36 P ______ n.d. 0 0 $15,} ----- °~ 883g; ”-3- 11.3. Mode . P205 _____ 2. 72 v ______ . 04 .09 .01 ----- ' n~ - n- - SO; ______ . 03 Nb____ . 002 . 02 . 02 Cl ________ . 02 Cr _____ . 05 . 02 . 07 Analcime _______ 3 F ________ . 18 Mo- _ _ _ 0 0 . 001 Labradorite _____ 18 S ________ . 26 Mn-__- 1 . 6 . 4 Olivine (ser— 5 Fe _____ n.d. > 5 > 5 pentine) . Total- _ 100. 39 Co _____ . 004 . 003 . 009 Augite ___________ 49 Less 0. __ . 2 Ni _____ . 02 . 0009 . 03 Hornblende _____ 3 Cu _____ . 008 . 01 . 01 Biotite _________ l 100. 18 Ga _____ . 002 . 002 . 003 Apatite _________ 4 Pb _____ 0 0 01 Magnetite— 11 Zn _____ 0 0 05 ilmenite. Calcite _________ 3 Zeolite __________ 3 1 Standard chemical analyses by L. M. Kehl. ' S ctrographlc analyses. L—lla by J'. D. Fletcher; 11-22, L—95 by Sol Berman. * olorlmetric analyses by H. E. Crowe and A. P. Marranzlno. ‘ Radiometric analysis by B. A. McCall. I Low-level chemical malyses for uranium and thorium by J. C. Antweller. | Mode by L. V. Bled a. n.d.:not determined. Looked for (spectrographically) but not found: Hf. Th. Ta. W. U. Re, Ru, Rh, Pd. Os, Ir. Pt. Ag, Au. Cd. 111. T1. Ge. Sn. Pd, As. Sb. Bl. Te. L-lla. Analcime-ollvine melagabbro, SW34 sec. 18. L—22. Saprolite of analcime—olivine melagabbro, SW34 sec. 18. 11-95. Saprollte oi analcime-olivine melagabbro, SW54 sec. 18. 44 p—A Strontium is most abundant in apatite and labradorite and to a lesser extent in pyroxene and hornblende. Scandium and chromium are concentrated in the pyroxene. Vanadium is concentrated in pyroxene and ilmenite. Zirconium occurs chiefly in hornblende. Cobalt, nickel, and copper are concentrated in ilmenite. Niobium occurs only in the hornblende. A surprisingly high amount of silver (0.004 percent) was de— tected in one of the pyroxenes. None of the analyzed minerals contained as much gallium as indicated for the rock. It is probably concentrated in unanalyzed zeolite. $991599.” 9° TABLE 31,—Spectrogmphz'c analyses of mineral separates of analcime-olim'ne melagabbro, in percent L—lla—4 L—lla— L—lla Irlla— L—lla—l L—lla—2 L—lla—3 1a 2a 3a 11 d n.d 0 1 0. 05 0.03 0 3 0. 22 l— —5 . 1 01 .09 01.5 .3 01 .001 0 0 003 .009 03 0 0 002 .01 009 0 0 n.d .0008 0 0 2. 4 6.8 .008 .1 .05 .009 .02 0 .004 1 . 02 . 02 0 0 0 .007 0 0 0 05 . 002 0 . 0002 0 3 . 1 . 04 .003 .002 n d n.d. 2 .2 .06 002 .004 0 0 0 008 007 0 0 0 004 004 .0008 . 009 . 001 .004 0 0 0 0 . 0005 0008 0 . 0003 . 0007 .005 .03 .002 0 0 Specific gravity ....... 3.1:l:.2 3.13:.2 ’ 3.1:1:.2 J 3. 1:1:.2 ’ 2. 75:1; 15 I 2.25:1:.05 n.d:=not determined. Looked for but not found: Be, B Ce, Th, P, Ta, Mo, W, U, Re, Pd, Ir, Pt, Au, Zn, Cd, Hg, In, Ge, Pb, As, Sb, Bi. EXPLANATION OF SAMPLES X-ray X-ray analyst Spectrographic analyst L—11a—4__ Ilmenite ______ Ilmenite..___.___. F. A. Hildebrand. H. J. Rose L—lla—la. Augite ________ Monoclinic pyrox- _____ do _____________ Do. ene probably augite. L—11a—2a. _____ do _________ _..._do __________________ do _____________ Do. L—lla—3a. Hornblende--. Amphibole _____ do _____________ Do. probably horn- blende. L—11a—1__ Apatite J. D. Fletcher L—lrla—2.. .Labradorite Do. L—lla—3.. Analcime _____ Analcime .............. do _____________ Do. NEPHELINE SYENITE PEGMATITE DISTRIBUTION AND DESCRIPTION N epheline syenite pegmatite, about 0.2 percent of the exposed igneous rocks of the complex, occurs in small irregular bodies, particularly along the outer con- ,tact of the complex with sedimentary rock. Good ex- posures are found all along the south edge of the garnet- pseudoleucite syenite in secs. 28, 29, and 30; in the eastern part of sec. 24; and along the north edge of the garnet-pseudoleucite syenite in sec. 18. Williams (1891), and later geologists who adapted his rock names, named this rock Diamond J0 syenite and included other medium- and fine-grained’syenites in this type, How- ever, the chemistry and mineralogy of the pegmatite set ALKALIC IGNEOUS CONIPLEX AT MAGNET COVE, ARK. it apart from any other. Most of the rocks previously mapped as Diamond Jo syenite in the northern two- thirds of the area are a coarse-gramed chemical equiv- alent of the garnet—pseudoleucite syenite. The best exposure of nepheline syenite pegmatite is in the quarry at the west edge of sec. 29 where it is in contact with garnet—pseudoleucite syenite. The con- tact strikes east-west across the quarry parallel to the north face. Light—colored minerals, chiefly barian sodic ortho- clase with minor nepheline and cancrinite, make up 70 to 90 percent of the pegmatite. Black titanium garnet and zoned pyroxene are the chief mafic minerals and tend to be segregated in patches. Pyrite, calcite, and magnetite are accessory constituents. Blue sodalite and purple fluorite occur as thin skins, one-eighth inch thick, on vertical joint planes. ‘ Barian sodic orthoclase, the most abundant mineral in the rock (70—80 percent), occurs as very light gray, lustrous, tabular crystals as much as 30 mm long. A fine network of kaolinite formed along cleavage planes gives the crystals the appearance of perthite. In thin section, these crystals have a faint suggestiOn of micro- cline structure and a moderately low 2V. Nepheline, 10 to 15 percent of the rock, occurs as greasy gray grains in the interstices between feldspar crystals. Much of the nepheline is altered to cancrinite. Sodalite is common on joint surfaces. The pyroxene, chiefly green aegirine-diopside and aegirine—acmite, occurs as elongate prisms up to 15 mm in length that have low birefringence and a reaction rim which is com- posed of fine—grained green biotite and granular mag— netite. In the southern part of sec. 13 on the west border of the complex (MC—198), the pegmatite is light gray and has a holocrystalline xenomorphic-granular texture. The minerals in thin section are: nepheline partly altered to cancrinite, sericite, analcime and (or) sodalite group minerals——about 20 percent; sodic orthoclase plus microperthite——about 75 percent; and brown and green biotite, colorless to purple fluorite some intergrown with sphene, pyrite, and magnetite—about 5 percent. A panned concentrate of the saprolite taken adjacent to the analyzed sample (MC—120) contains the follow- ing minerals in order of abundance: feldspar, clay aggre- gates, nepheline, pyroxene, hematite, magnetite, and mica. From an X-ray analysis of clay- and silt-size portion of the saprolite, A. J. Gude 3d (written communication, 1956) reported major kaolinite and traces of mont- morillonite and chlorite. CHEMISTRY Chemical and spectrographicj analyses and norms of nepheline syenite pegmatite, table 32,.shothhat chemi- BIKES 45 cal composition is very similar to the analyzed tinguaite, sis appear as normative sodium carbonate and halite, table 28. It contains more K20 than Na20; BaO con- and are present in the mode in cancrinite and sodalite. tent is unusually high (1.5 percent) ;_ CaO, P205, and Trace elements in the rock that equal or exceed 0.01 TiOz contents are low. V and Zr are low in comparison percent are Sr, Ba, La, Zr, and V. Niobium contents to the other analyzed rocks;Co,Ni, Y, and Yb were not of 0.01 percent were detected in 2 of the analyzed detected; Sr is high. samples. Comparison of the norm and mode indicates that Spectrographic analyses of minerals separated from part of the sodium in normative albite is probably in the analyzed rock (table 33) show the distribution of the barian sodic orthoclase and part in sodic pyroxene. trace elements within the rocks. Nb, V, and Zr occur BaO which is in the modal sodic orthoclase was com— chiefly in titanium garnet; part of the Nb is in aegirine. bined with CaO in calculating normative anorthite and The iron content of the sodic pyroxene suggests that it wollastonite. High 002, 803, and Cl in the analy- is probably closer to acmite than aegirine. La, Cr, Mo, TABLE 32.—Analyses and norm of nepheline syem‘te pegmatite, in percent M C~120 M (3-120 MC-198 ‘ 11-154 L—155 M C—120—b M C—lZO—a M C—l20—c M C-120—d M C—120 Standard chemical analysis 1 Spectrographic analyses Norm 0. 0002 0. 0001 0 0 0. 0007 0 01' _____________ 1 .09 . 2 1-10 1-10 .01 3b-. ___- 2 . 1 >10 > 10 > 10 . 13 . 6 . 08 4 . 6 . 61 . 4 . 5 02 5 . 06 2. 4 0 . 003 0 0 0 0 0 0 . 0002 0 0 0 0 O 0 0 0 0 . 02 0 0 0 0 0 0 0 0 0 0 0 . 3 . 2 1-10 . 2 . 7 . 01 02 . 01 . 08 01 . 02 001 04 . 02 . 2 . 04 . 05 0 01 . 005 . 04 0 . 02 0 0 . 002 0 0001 . 00008 0 0 0 0 0 0 2 l . 4 2 005 3 >10 >10 >10 26 0 0 001 002 0 0005 . 0008 0 0 . 0004 0001 001 . 0009 . 001 . 0001 002 003 002 0 . 001 003 0 001 0 0 0 0 Colorimetric analyses 1 Fees? 9999? PP??? 9999? as??? as??? Radiometric analyses 3 eU ............ ‘ 0. 004 ‘ 0.005 ‘ 0.006 0. 006 Chemical analyses ‘ U ............. 0.00010 0. 00164 n.d. n.d. Th ___________ . 00044 . 00097 n.d. n.d. 1 Standard chemical analysis MC-120 by L. M. Kehl. - n.d.=not determined. 2 Colorimetric analyses by H. E. Crowe and A. P. Marranzino. Looked for (spectrographlcally) but not found: L1. Cs, 0e, Hf. Th. P, Ta. Cr, W.U, I Radiometric analyses by B. A. McCall. Re, Ru. Rh, Pd. Os. Ir. Pt, Ag. Au. Zn, Cd. Hg, In. T1. Ge. Sn. As. Sb. Bi. Te. 0 Low-level chemical analyses for uranium and thorium by J. C.Antweller. EXPLANATION OF SAMPLES X-ray X-ray analyst Spectrographic analyst MC—120 Nepheline syenite pegmatite--.- H. J. Rose. MC-19 _--- o ______________________ _. Sol Barman. L—154- _-_.do ............... - Do. L—155 Saprolite of MC-120._ _ MC—fzo-b. __ Dark garnet .......... MC—120—a____ Aegirine-dlopside__ Acmlte.. _- W. F. Outerbridge.--_ MC-120-o._.. A d egirine ........................................ o. MC-l2o—d. _- Sodic orthoclase... . ..-_ Harry Bastron. Do. J. D. Fletcher. Do. 46 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. TABLE 33.—Analyses and norm of trachyte porphyry, in percent Chemical analyses I Spectrographic analyses Radiometric analyses 1 I Chemical analyses 14—324 ' 14—324 L—324 MC-193 ‘ L-324 MC—193 Sl02 ______________ 53. 89 Be _________ 0. 0002 eU _____ 0. 004 n.d. U- _ _ _ __-n.d. 0. 00069 M203 _____________ 21. 63 Sr _________ . 2 Th_____-n.d. . 00136 Fe203 _____________ 1. 74 Ba _________ 2 e0 ______________ 1. 82 Y _________ . 001 MnO _____________ . 15 La _________ . 007 MgO _____________ . 43 Yb ________ . 0002 CaO ______________ 3. 08 Ti _________ n.d. BaO ______________ . 08 Zr _________ 03 NaZO _____________ 6. 62 P __________ n.d. 20 ______________ 7. 80 V __________ O2 20—- ____________ . 10 Nb ________ . 009 H20+ ____________ 1. 09 Mn ________ n.d. TiOz ______________ . 60 Fe _________ n.d. C02 ______________ . 26 Cu _________ . 001 P205 ______________ . 08 Ga ________ . 002 803 ______________ . 07 Pb _________ 002 C1 ________________ . 09 F ________________ . 32 S ________________ . 35 Total _______ 100. 20 Less 0 ______ . 33 99. 87 n.d.=not determined. Looked for (spectrographically) but not found: B, Sc. Ce Th, Ta, Cr, Mo, W, U, 1Igt_e.’1(‘30, NI Ru, Rh. Pd. 0 , Ir, Pt, Ag. Au. Zn. Cd, Hg, In. Tl. Ge. Sn, As. Sb, 1, e. Standard chemical analysis by M. K. Balazs. and Ga are not concentrated in any of the minerals analyzed. These elements can perhaps be accounted for by the unanalyzed minerals: biotite, magnetite, calcite, nepheline, cancrinite, and sodalite group min- erals. TRACHYTE PORPHYRY DISTRIBUTION AND DESCRIPTION Narrow dikes and small irregular-shaped bodies of porphyritic rocks with sodic orthoclase phenocrysts in a light- to dark-gray trachytic or tinguaitic ground- mass are scattered throughout the complex. Sodic orthoclase phenocrysts comprise 0 to 40 percent of the rock and are in a groundmass of equi— granular nepheline and sodic amphibole and pyroxene. The accessory minerals are sphene, apatite, magnetite, pyrrhotite, pyrite, and fluorite. Some of the variants are briefly described below: MC—6‘1.-—In the northeastern part of sec. 18 a small irregular- shaped body of gray prophyritic alkalic trachyte cuts pseudo- leucite syenite. Corroded phenocrysts of perthite and sodic orthoclase, and rare corroded phenocrysts of plagioclase make up 5 percent of the rock. Aggregates of these feldspar crystals up to 5 mm across occur with a purple mineral that makes up about 2 percent of the rock. The anhedral purple mineral, probably a member of the sodalite group, is isotropic and color- less to reddish violet with the color irregularly distributed. Groundmass sodic orthoclase as narrow laths—all partly altered Spectrographic analysis by Sol Berman. Radiometric analysis by B. A. McCall. Low-level chemical analysis for uranium and thorium by J. C. Antweiler. L—324. Trachyte porphyry, SM: sec. 19. MCI—193. Trachyte prophy-ry, SEM sec. 19. to sericite—comprises about 78 percent of the rock. The remaining 15 percent consists of anhedral sodalite(?) diopside~ hedenbergite rimmed with aegirine-diopside with minor calcite alteration, aegirine—diopside, green biotite, sphene, colorless garnet, and magnetite. MC—193.—A narrow dike of light-gray phonolite porphyry cuts undivided trachyte and banded trachyte in the south- eastern part of sec. 19. Phenocrysts of tabular sodic orthoclase and perthite up to 10 mm long make up about 20 percent of the rock. The groundmass is composed of sodic orthoclase, about 55 percent of the rock; plagioclase probably oligoclase, about 5 percent; nepheline and analcime or sodalite both with cancrinite alteration, about 10 percent; and late anhedral green hornblende, sphene, apatite, pyrite, pyrrhotite(?), and mag- netite, about 10 percent. L—97.—A narrow dike of dark-gray alkalic trachyte porphyry cuts sphene-nepheline syenite and undivided trachyte breccia in the south-central part of sec. 19. In the hand specimen, phenocrysts of tabular sodic orthoclase up to 8 mm long make up about 20 percent of the rock. In thin section, corroded phenocrysts of sodic orthoclase and perthite plus groundmass sodic orthoclase, all with sericite alteration, comprise about 83 percent; phenocrysts of andesine, about 2 percent; and brownish-green biotite, pyrite, pyrrhotite(?), and magnetite, about 15 percent. L—324—In the south-central part of sec. 19, the undivided trachyte is cut by a light-gray phonolite porphyry. In the hand specimen, phenocrysts of tabular sodic orthoclase up to 8 mm long make up about 20 percent of the rock. In thin section, corroded phenocrysts of sodic orthoclase and perthite, both kaolinized, and groundmass laths of sodic orthoclase partially DIKES altered to sericite, about 70 percent; anhedral nepheline partly altered to cancrinite, about 10 percent; and green hornblende, pale—brown biotite, colorless to brown garnet, colorless mica (uniaxial negative), apatite, pyrrhotite, fluorite, and rare mag- netite—about 20 percent. CHEMISTRY Chemical and spectrographic analyses and norm, of trachyte porphyry, table 33, show the similarity of this rock to tinguaite and nepheline syenite pegmatite. The most distinctive characteristic of all these dike rocks is that K20 is higher than Na20. The presence of pyrrhotite in the mode renders the FeO/Fezoa ratio unreliable. Trace elements equal to or greater than 0.01 percent include Sr, Ba, Zr, V, and Nb. MISCELLANEOUS TRACHYTES Fine—grained dikes generally too small to be mappable and not readily classified With the major types are found in various parts of the complex. A few of these are described below: L—25.——In the northeastern part of sec. 24 on the north side of U.S. Highway 270, sphene-nepheline syenite is cut by a light- gray phonolite. The rock has a hypautomorphic granular tex- ture and is composed of analcime or sodalite, about 2 percent; nepheline (clear), about 25 percent; sodic orthoclase and minor perthite both with slight sericite alteration, about 65 percent; green aegirine-diopside, pale-brown to colorless garnet, sphene, magnetite, and rare pyrrhotite, 8 percent. MC—27.—About 100 feet west of L—25, the sphene-nepheline syenite is cut by gray phonolite. In thin section corroded phenocrysts of green aegirine-diopside and subhedral olive-green hornblende crystals are set in an equigranular groundmass of nepheline, sodic orthoclase and sodalite or analcime. Apatite, sphene, and anhedral pale—brown garnet are the accessories. MC—147.——In the southwestern part of sec. 21, feldspathoidal leucosyenite is cut by a dark—gray porphyritic phonolite com- posed of phenocrysts and anhedra of nepheline partly altered to cancrinite, analcime, or sodalite, and calcite, about 20 percent; anhedral plagioclase, probably albite, about 1 percent; anhedral sodic orthoclase partly altered to cancrinite and calcite, about 39 percent; corroded phenocrysts of diopside-hedenbergite partly altered to aegirine, needles and anhedral grains of aegirine and acmite, brown to colorless garnet intergrown with sphene, and yellowish—green biotite, about 40 percent. M C—155.—-In the northeast part of sec. 17, the jacupirangite is cut by a dark-gray feldspathoidal trachyte. Phenocrysts of pyroxene up to 2 mm long make up about 5 percent of the rock. In thin section the groundmass is trachytic and is composed of analcime or sodalite, about 10 percent; subhedral laths of sodic orthoclase with some calcite alteration, about 60 percent; zoned hornblende (brown centers rimmed with green) partly altered to green biotite; very pale brown phenocrysts of diopside—heden— bergite(?) partly altered to green biotite; apatite; sphene partially intergrown with colorless garnet; magnetite; pyrite, about 30 percent. 47 L—317.——Another sample, taken of the same mapped unit and in the vicinity of MC-155, is somewhat different from MC—155. This dark-gray feldspathoidal melatrachyte, is porphyritic with macroscopic phenocrysts of pyroxene forming about 10 percent of the rock. In thin section the minerals are: analcime or sodalite, about 5 percent; sodic orthoclase and perthite partially altered to zeolite and calcite, about 35 percent; and phenocrysts of very pale brown diopside-hedenbergite partially altered to green biotite, green biotite, sphene, apatite, magnetite, pyrite, and brown biotite, about 60 percent. An inclusion of novaculite in the rock has a reaction rim of plagioclase and aegirine. APLITE Scattered about the complex, as occasional pieces of light-gray to gray aphanite or fine-grained phanerite float, are the aplites. Usually the float is not abundant so the bedrock units must be small. They all are holocrystalline with a Xenomorphic granular texture and mineralogically they are variable. Some of the variants are described below: M 0—40 (S Wl/i sec. 18) .——Fe1dspathoida1 syenite aplite composed of sodic orthoclase (about 80 percent) deuterically altered to calcite and analcime and (or) sodalite group (about 10 percent) and acmite, green biotite, fluorite, apatite, magnetite, and pyrite (about 10 percent). M C~44 (SEX sec. 13) .——-Nepheline syenite aplite composed of nepheline partly altered to analcime and/or sodalite group, about 15 percent; sodic orthoclase and perthite, about 80 percent; and aegirine magnetite and pyrrhotite (?), about 5 percent. L—30 (SWM sec. 18) .—A1kalic syenite aplite composed of sodic orthoclase and perthite partly altered to sericite (about 80 percent) and colorless to purple fluorite, green biotite, musco- vite, ilmenite partly altered to leucoxene, and pyrite partially weathered to hematite, about 20 percent. L—318 (N E%. sec. 17) .—A1kalic syenite aplite composed of sodic orthoclase and perthite (about 80 percent) and pale—green diop- side-hedenbergite, sphene, magnetite, and pyrite—about 20 percent. EUDIALYTE-NEPHELINE SYENITE PEGMATITE DISTRIBUTION AND DESCRIPTION Eudialyte-nepheline syenite pegmatite comprises less than 0.1 percent of the exposed igneous rocks of the complex and occurs in two localities—both in the north- ern part of sec. 19. A poorly exposed pegmatite with very coarse aegirine cuts fine-grained ijolite in the southeastern part of sec. 18 on Cove Creek. No eu- dialyte has been observed in this pegmatite but for mapping purposes it has been lumped with eudialyte- nepheline syenite pegmatite. The eudialyte—bearing pegmatite varies in texture from a fine-grained to very coarse grained phanerite. The coarse-grained parts are well known for beautiful specimens of aegirine crystals up to 6 inches long, and ruby-colored eudialyte crystals up to 1 inch across. Williams (1891) has described in detail the mineralogy of the coarse-grained part. He mentions garnet, il- 48 menite, magnetite, nepheline, orthoclase, thomsonite, and wollastonite, and describes aegirine, astrophyllite, brucite, eucolite, eudialyte, manganopectolite, micro- cline, natrolite, and sphene. In thin section the fine— to medium-grained parts of the dikes have a hypautomorphic-granular texture and are composed of aegirine; anhedral sodic orthoclase and (or) microcline partly altered to cancrinite, calcite, pectolite, zeolite, and sodalite group' nepheline partly ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. the N 8.20 in normative nepheline occurs as aegirine in the mode. Material in the pegmatite that looked like chert was identified from an X-ray analysis by F. A. Hildebrand as tobermorite, a hydrated lime silicate. Chemical analysis of tobermorite is given in table 35. TABLE 35.—Chemical analysis of tobermom‘te from eudz'alyte- nepheline syenite pegmatz‘te [Sample L—167—7. Standard chemical analysis by L. N. Tarrant] . _ . _ _ _ _ Constituents Percent altered to cancrinite and usually p01k111tically Included SiOz ___________________________ 45. 41 in the feldspar; euhedral to anhedral sphene some par— fiézga -------------------------- 4- 3:75 . . . 2 3 —————————————————————————— - tially altered to leucoxene; apatlte; magnetite; euhedral eo ___________________________ _oo eudialyte; pyrite; pyrrhotite. $28 -------------------------- 0‘ 05 Ca0_‘:_—:::::::::::IIZIIII 35. 75 our. my ms Na.o __________________________ . 32 Chemical and spectrographic analyses, norm, and 3528 --------------------------- 3- 3% . . . 2 -' ————————————————————————— . mode of the fine-grained part, table 34, are very sumlar H20+ _________________________ 9. 48 to normal nepheline syenite except that K20 is un- $182 -------------------------- 0- 01 . 2 5 —————————————————————————— usually high. 01 ____________________________ 0 Halite and thenardite of the norm appear in the F ----------------------------- 0 mode as a member of the sodalite group. Much of Total ____________________ 99. 88 TABLE 34.-Analyses, norm, and mode of eudz'alyte—nepheline syenite pegmatt’te, in percent Chemical 1 Spectrographic 3 Radiometric 3 Chemical 4 Norm Mode 5 L—l67—l L—l67—2 L—167—3 L—167—4 L-167—5 L-167—6 L—167—7 L—303 L—303 L-303 11-303 0 0 0 0 0 0. 002 0. 002 eU-.- 0. 006 U._.. 0. 002 Nepheline ....... 5 . 05 >10 . 02 . 006 . 03 . 02 . 009 Sodic orthoclase. 47 Sodallte group. - l . 8 04 . 01 . 0009 . 2 . 05 . 01 Zeolite (bire- fringent). 15 .2 0 .07 .06 .01 .02 .04 Cancrim‘te ______ .0004 0 0 0 0 Calcite _____ . 009 0 0 . 003 0 0 Aegirine__ 27 . 04 O 0 . 02 0 . 02 Sphene.-- . 4 n.d. n.d. n.d. n.d. n.d. n.d. n.d. Apatite_. . l . .4 5 .006 .006 .04 ' .02 .003 Magnetlte ....... Tr. >10 07 0 0 0 0 .01 .l .01 .009 .02 .006 .009 .09 0 0 0 0 . 002 . 001 . 001 . 0009 . 001 . 002 . 002 n.d n.d. n.d. n.d. n.d. n.d. n.d. .48 .48 .007 0 .5 .005 .05 2. 2 >10 . 4 . . 2 . 02 . 002 0 0 0 0 0 0 Total _____ 100. 18 011...- . 002 002 . 0007 . 001 . 0009 . 001 . 0009 0008 Less 0... .10 G8.-. .003 .004 .004 0 .002 100.03 811.... 0 .09 0 0 0 0 0 0 P -_-- 0 .008 0 0 0 . 003 0 0 1 Standard chemical analyses by M. K. Balazs. I Spectrographic analysis: L—303 by Sol Berman; all other samples by H. J'. Rose. Radiometric analyses by D. L. Schafer. I Chemical analysis for uranium by .T. P. Schuch. 5 Mode by L. V. Blade. 0=looked for but not found. n.d.=not determined. Looked for (spectrographically) but not found: Ce, Hf. Th. P , Ta, W, U. Re, Ni. Pt. Ir. Pd, Ag. Au, Zn. Cd . .Hg, In, TI. Ge, As. Sb, Bi, Te. B EXPLANATION OF SAMPLES Sample X-ray Analyst L—167—1 _______ Eudialyte.- L—167—2_ - Aegirine-__. L—167~3_ _ Microcline. MicrocIine __________ F. A. Hildebrand. L—167—4_ _ Microcline..- ..._ o _______________ Do. L—167—5. _ Pectoh’te _____ L-167—6- ._ Natrolite _____ __ L—167—7 ....... Tobermcrite __________ Tobermorite ........ Do. L—303. Eudialyte-nepheline syenite pegmatite, N W% sec. 19. L—167—1. Eudialyte from coarse-grained part of pegmatite, N E% sec. 19. L—167—2. Aegirine from coarse-grained part of pegmatite, N E14, sec. 19. . Microcline from coar . Tobermorite from se-grained part of . Microcline from coarse-grained part of pegmatite, NE sec. 19. pegmatite, NE 4, sec. 19. Pectolite from coarse—grained part of pegmatite, N E% sec. 19. . Natrolite from coarse-grained part of pegmatite, coarse-grained part 0! pegmatite, N E% sec. 19. NE% sec. 19. DIKES Compared with the average of analyses of igneous rocks in the complex (table 46), the analyzed rock is lower in MgO, F8203, FeO, MnO, CaO, BaO, TiOz, C02, P205, Cl, F, S, and higher in Si02, A1203, NaZO, K20, 803. Trace elements in the rock that are equal to or exceed 0.01 percent include: Sr, Ba, La, Zr, V, Nb, and Mn. Spectrographic analyses of several minerals hand picked from the coarse to very coarse pegmatite, table 34, show that Sr, Ba, Sn, Pb, Nb, and La are concen- trated in eudialyte. La also occurs in pectolite and tobermorite; V and Zr occur chiefly in aegirine; Ga is in microcline. GARNET FOURCHITE Garnet fourchite occurs in the southwestern part of sec. 18, where the saprolite and float pattern suggest a small dike that cuts fine—grained ijolite and analcime olivine melagabbro. The rock is a dark-gray porphyritic aphanite in which phenocrysts of pyroxene comprise 20 to 40 percent of the rock. It weathers to a light greenish-brown saprolite with black spots. In thin section the augite phenocrysts are a very pale brown in color and appear zoned only under crossed nicols. They are set in a xenomorphic—granular groundmass of brown to colorless garnet—about 18 percent; magnetite-ilmenite rimmed with sphene—about 23 percent, analcime(?)—about 8 percent, nepheline partially altered to calcite—about 5 percent, and a trace of pale brownish—green biotite. Material in the panned concentrate of the saprolite occurs in about the following order of abundance: garnet, magnetite, pyroxene, rutile, and brookite. From an X—ray analysis of the silt— and clay-size portion of the saprolite, A. J. Gude 3d (written com- munication, 1956) reported major montmorillonite and chlorite. Spectrographic analyses of the fresh rock and saprolite, table 36, indicate that Cu, Co, Ni, and rare earths are unusually high in this dike rock. The repeat determinations show excellent agreement within the limit of spectrographic sensitivities for the elements detected. BIKES OUTSIDE THE COMPLEX Igneous dikes, ranging in width from less than 6 inches to at least 25 feet and in length from less than 6 feet to at least 4,500 feet, are abundant in the broad valley to the west of the complex (fig. 4), and they are if 49 TABLE 36.——Analyses of garnet fourchite, in percent ‘ L—94a L—94a L—94b L—94b Spectrographic analyses 1 0. 0001 0. 0002 0. 0003 0. 0001 4 ________ 2 3 >5 >5 >5 . 5 . 04 04 . 1 . 3 . 3 . 001 0 . 002 . 004 . 004 . 006 . 005 006 01 . 03 . 03 05 Ce ____________________ <. 1 09 0 Yb ____________________ 0 O . 0005 . 0009 Ti _____________________ 2 3 2 >5 Zr _____________________ . 02 . 03 03 . 03 V _____________________ . 02 . 04 02 05 Nb ____________________ . 01 . 01 01 01 Cr ____________________ . 007 . 02 007 01 Mn ___________________ . 2 . 4 . 2 . 5 Fe ____________________ 3 >5 4 >5 Co ____________________ 002 . 004 . 002 . 005 Ni ____________________ . 002 . 004 . 003 004 Cu ____________________ 02 . 02 02 03 Zn ____________________ 0 . 04 0 0 Ga. ____________________ . 002 . 003 . 001 . 002 Colorimetric analyses 9 As ____________________ n.d. ________ 0. 001 ________ Sb ____________________ n.d. ________ . 0002 ________ Zn ____________________ n.d. ________ . 015 ________ Radiometric analyses ' eU _____________________ 0. 002 _______ 0. 002 ________ 1 Spectrographic analyses by Sol Berman. 2 Colorimetric analyses by H. E. Crowe and A. P. Marranzino. 3 Radiometric analyses by B. A. McCall. L-94a, Garnet fourchite, SWM; sec. 18. L—94b, Saprolite of garnet fourchite, SWM sec. 18. n.d.=not determined. Looked for but not found: Hf, Th, P, Ta, W, Mo, U, Re, Rh, Pd, Os, Ir, Pt, Ag, Au, Cd, Hg, In, T1, Ge, Sn, Pb, As, Sb, Bi, Te. particularly abundant in the contact zone (pl. 1). Commonly, the dikes are revealed by float pieces and they rarely are exposed across their full width. The abundance of weathered dikes exposed in the road out along the south side of the complex indicates that dikes probably are much more common than indicated by the float. Mineralogically, they are variable. Some can be grouped with the types within the complex but most cannot be so grouped. The rock types include pegma- tite, aplite, syenite, trachyte porphyry, trachyte, tinguaite, andesite, diorite, monzonite, and lamprophyre and are described in table 37. .MSV £03380 unwam com ES "3336 £2958 9:355 960 “onwas 2.: 8 non—£2 5356 38:3 .8 .5382 3:323 anemia. auburn _IIJJJIJJ hum“. 000.2 000m 0 v N m o H m v F . .mwowm mz__ 0H Hoo .o o o o o Hoo . o Hoo . o mooo wooo . o o Hoo . Nooo . o o o o ............................... 22 mo .Io wo No . No o oooo . o wooo . o o wooo . o o o oooo . o Hoo . o o ................................ .5 No .Iooo . Ho Ho . Ho Ho . No . Ho . No . No No No . Ho Ho . No Ho . ooo . woo . No Ho . ............................... 22 oo (Noo . mo wo wo wo . wo wo wo. no . woo oo . moo . moo oo mo . woo moo oo Noo ................................. > N .INo . No no no no . wo mo oo . oo . No oo . no mo no No . no N oo . No ................................ .HN wnwo . N m w m m N m H H m mo H H o . N N 5 oo ................................ HrH. Nooo: . o o Nooo . Nooo . 88 o mooo . Hooo . o mooo . o o wooo Nooo . wooo mooo Nooo o ............................... 9% nova H.V H V H.V H.V o H v H.V H.V no o No. we a No. me o o o ................................ oo wo ..o No . mo wo . no No . no wo . No . No No no . no No no No No . o Ho ................................ NA Ho .Io woo moo poo . Ho ooo . moo Ho ooo . o Noo o o woo woo Noo moo . woo o ................................. My woo .Io woo woo . moo . wooo Noo . oooo oooo Nooo . o oooo . o o mooo Hoo o oooo . o o ................................ ow .Io Noo Hoo . woo . Hoo Hoo . o o Noo . So So . Ho . woo Hoo o Ho No . moo . ooo . ................................. m ”Ego. oo oo. H. H H. H N H. H H oo. No. H. oo. H no. N. poo. ................................ am ouooo. N N. N. o m m o o m N wo. mo. m. N. wo ooo. H. mo. ................................ .Hm nANo. 0A nA “A nA ”A X WA DA H mA No. 5. mA w 8 o. 8. w. ........................ . ...... do Two. M x N n N x M M H. x wo. S. n N mo. m. o. H. ............................... m2 mooo .HTHooo . 88 .o Nooo .o Nooo .o Nooo .o mooo .o Nooo .o Nooo .o mooo .o wooo .o mooo .o wooo .o mooo .o mooo .o wooo .o wooo .o wooo .o wooo .o mooo o ................................ mm Haaniwm How .ambaufi womb—Ea 92.358825 omnam NNNIH HomkH ENLH ooNLH owNLH ENLH mmNLH NmNLH owhHuH wkaH NNNLH HNNRH meLH SH.OH>H MNNKH 9TH mHmLH mkaH $338 2: 93230 935% 382:: Re .2523 3 .wwmooogwylwm ":de 54 The spectrographic analyses of dike rocks (table 38) show relatively uniform high concentrations of Sr, Ti, Zr, Nb, and rare earths and emphasize the genetic re- lationship of these dike rocks to the Magnet Cove in- trusive complex. The concentrations of individual minor elements are related to rock types. Sr, Sc, Ti, Cr, Co, Ni, Cu, and Zn are most abundant in the basic lamprophyre dikes; B and Pb are most abundant in the felsic dike rocks. Particularly notable enrichments are 0.02 percent B and 0.2 percent Zr in an alkalic syenite aplite (L—36) ; 0.08 percent Cr and 0.02 percent Ni in a biotite monchiquite (L—227); 0.08 percent Zn in a gabbro lamprophyre (L—276); 0.02 percent Pb in a hornblende feldspathoidal trachyte porphyry. VEIN S Significant deposits of rutile, brookite, and molyb- denite occur in veins within the igneous complex and the contact zone. Fryklund and Holbrook (1950) made a detailed study of the veins of the Magnet Cove Titanium Corporation deposit, the Hardy-Walsh brookite deposit, and the Christy brookite deposit. Holbrook (1948) and Fryk— lund and Holbrook (1950) have described the veins in the Mo-Ti Corporation molybdenum-titanium prospect. In the Magnet Cove Titanium Corp. open pit in secs. 17 and 18, Fryklund and Holbrook (1950) described six major vein types and several minor types that occur as small veinlets. The six major vein types are: sugary-textured 'albite-dolomite microcline-calcite, al- bite-ankerite, coarse—grained albite-perthite carbonate, calcite-rutile, and coarse-grained calcite which may be equivalent to the carbonatite. The minor types include: dolomite, dolomite-rutile, dolomite-pyrite, dolomite-rutile—pyrite, dolomite-rutile—pyrite-green bio- tite, dolomite—green biotite, fluorite, and pyrite. The Hardy-Walsh and Christy deposits are in re- crystallized novaculite of the contact zone. Fryklund and Holbrook state that the veins in these deposits are composed of recrystallized quartz (clear and smoky) and introduced rutile, brookite, taeniolite, pyrite(?), and clay minerals, kaolinite and nontronite(?). A late quartz—goethite vein cuts residual ore in the Hardy- Walsh deposit. The range of Nb, T102, Y, V, and La, in samples from the Magnet Cove Titanium Corp, Christy Brookite, and Hardy—Walsh Brookite deposits is shown in table 39. Spectrographic analyses of rutile—bearing veins outside of the major deposits are shown in tables 40 and 41. Molybdenum-bearing veins cut jacupirangite in the Mo-Ti prospect in sec. 17; they Were first described by Sleight (1941). Holbrook (1948) and Fryklund and Holbrook (1950), who described the veins in detail, ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. found that molybdenite mineralization followed the introduction of feldspar veins composed of microcline (about 80 percent), albite, apatite, brookite, rutile, py- rite, and sphene in varying amounts. Molybdenite also was noted in the Magnet Cove Ti- tanium Corp. deposit by Fryklund and Holbrook (1950) and in the carbonatite at the Kimzey calcite quarry by Fryklund, Harner, and Kaiser (1954). Spectrographic analyses of a molybdenite-colorless mica veinlet in car- bonatite (L—171) and material dredged from a stock pond in the western part of section 20 (L—150), com- posed of colorless mica—about 75 percent, molyb- denite——about 15 percent, and apatite, hematite, limo- nite, and pyrite—about 10 percent, are given in table 40. Other veins containing dominant quartz, quartz- feldspar, quartz—brookite~rutile, feldspar, feldspar-car- bonate, or apatite were mapped and are shown on the geologic map, plate 1. Concentrations of float crystals of rutile, brookite, and paramorphs of rutile after brook- ite also were noted on the geologic map. TABLE 39.—Range, in percent, of Nb, TiOz, V, Y, and La content in veins of the major deposits [From Fryklund, Hamer, and Kaiser (1954)] Deposit Nb I TiO: ' V Y La Magnet Cove Titanium 0—. 12 0. 25—15. 8 0. 013-. 47 0-. 31 0—. 51 orp. Christy Brookite ____________ O—. 45 2. 0—26. 3 . 08—1. 0 0-. 14 __________ Hardy-Walsh Brookite ...... 0—. 46 .082-37. . 019—. 34 0—. 034 0—. 076 Fluorite and sodalite coat the joints in the pegmatite at the Diamond Jo quarry, and Fryklund and Holbrook (1950) report small fluorite veinlets in the Magnet Cove Titanium Corporation deposit. A vein composed of colorless and purple fluorite, anhedral quartz, chalced- ony, zeolite(?), and witherite(?) was found in the area known as the V—intrusive in sec. 19, T. 3 S., R. 18 W., west of the Magnet Cove complex (fig. 4). Apatite veins composed of occasional clusters of bio— tite and of garnet in a matrix of subhedral apatite, cut the biotite—garnet ijolite in the N W1 /4 sec. 20. Fresh material from a bulldozer cut is composed of apatite, pyrite, and zeolite(?) in varying proportions. The fresh rock is partly fine grained, L—116b, and partly medium grained L—llfic. It weathers to a very fine grained mass of iron oxide rich in secondary apatite, and occasional crystals of rare earth phosphates(?), L—116 and L—116e. L—116f is composed of resistant apatite and fine- grained limonite and hematite from partly weathered pyrite. Spectrographic analyses of these samples are given in table 42. A very irregular vein, 2 to 8 inches wide, of earthy, very fine grained pale greenish-yellow material occurs within the extremely weathered part of the apatite- DIKES pyrite vein. As determined by X-ray the material is a mixture of monazite and a plumbogummite group min- eral. The monazite is characterized according to Murata and others (1957) by a high La+Ce+Pr index, 87, and little or no thorium (no thorium was detected in the analysis, table 42). From the following evidence it appears that the mon- azite is a product of the weathering of the apatite- pyrite vein: 1. No monazite was detected in the fresh rock. 2. The monazite is earthy and no remains of original crystals were detected in the smaller vein. 3. The smaller vein was observed only in the weathered portion of the apatite-pyrite vein. 4. Lack of detectable thorium in both the monazite and apatite— pyrite vein. analyses of veins (tables 40, 41, (table 43) indicate that the trace The spectrographic 42) and vein minerals 55 elements have a wide range of concentration but Nb, M0, Ce, Pb, Ga, and U are more consistently enriched in the vein rock than in the major rock types. Other elements such as Co, Y, Zn, Zr, and V, show erratic, high concentrations in individual veins. The niobium is concentrated in rutile and brookite; molybdenum is in molybdenite and feldspar; cerium is in apatite; lead and gallium are in feldspar. The high- est uranium (0.058 percent) is in a monazite—plum- bogummite group(?) vein; lesser concentrations (0.01 to .018 percent) occur in the clay material from brookite veins. A lithium content of 0.03 percent was detected in the three veins analyzed for lithium. It probably oc- curs in a micaceous clay mineral. Similar concentra- tions of lithium can be anticipated for most of the other veins. TABLE 40.—-Analyses, in percent, of veins 11—150 L—171 W-19 L—31 L—60a L—74 L—76 L—78 L—112 L—278 L—310 11—3108 L—339a Ir339b Range Semiqunntitative spectrographic analyses [Analyst, H. J. Rose] n.d. n.d. n.d. 0.03 n.d. n.d. n.d n.d n.d. ______________ 0. 0001 0. 0003 0. 0003 0. 0003 0. 0003 0. 0003 0. 001 0- . 0003 . 3 . 3 . 03 . l 03 . 03 . 03 1 . 01 . 01—10 . 01 . 03 . 003 . 03 . 003 1. 0 . 003 1 . 03 003—10 . 03 . 1 . 003 . 003 001 . 03 . 003 01 1 . 001— . 3 . . 3 . 03 1 . 003 . 03 . 03 . 01 l 003— . 3 . 003 0 0 . 001 . 003 0- . 003 0003 0 . 0003 . 0003 . 0003 . 003 . 001 . 003 . 003 0- . 003 003 0 . 003 0 . 01 03 . 01 . 03 . 01 0- . 03 01 .03 .01 0 1 .03 .003 .1 0— .1 0 0 0 0 3 . 1 . 1 0— 3 .0 .1 .1 .3 10 1 .3 3.0 3.0 03—10 1 .03 .01 .01 .1 .003 0 .01 .003 0— 0 0 0 0 3. 0 0 . 3 0— 3 01 . 003 . 3 . 03 . 3 . 003 . 003 . 3 . 1 003~ 3 .03 .03 .03 0 .3 .1 .03 .1 .1 0- 3 . . 0003 . 003 . 001 . 03 . 001 . 001 . 01 . 003 0003— . 03 . 003 . 003 0 O 0 0 0 . 003 . 01 0- . .01 .03 .3 .03 .1 .1 .03 .003 .003 003- .3 3 3 10 . 3 3 3 3 . 3 . 3—10 . 0003 0 . 001 0 . 001 0 0 . 001 003 0- . 003 . 003 0 0 0 0 0 0 . 003 . 003 . 001 . 0003 . 0003 . 0003 . 01 . 003 . 001 . 003 . 003 . 0003- . 03 0 0 0 0 0 0 . 0001 0- . 0003 0 0 . 03 0 0 0 0 0 0 0- . 03 . 003 . 003 . 003 . 003 0 0 0 . 003 . 003 0— . 003 0 003 . 003 0 . 01 . 01 . 03 . 03 . 01 0- . 03 n.d. n.d. n.d. . 1 n.d. n.d. n.d. n.d. n.d. Radiometric analyses (Analyst, B. A. McCall] eU ________________________ 0. 005 0. 002 0. 004 0. 008 0. 007 0. 010 0. 005 0. 006 0. 008 0. 001 0. 010 0. 004 0. 020 0. 012 Chemical analyses (Analyst, Roosevelt Moore] U _________________________ 0. 007 0. 001 0. 003 0. 004 0. 005 0. 001 0. 003 0. 004 0. 003 0. 001 0. 008 0. 002 0. 018 0. 010 n.d.:not determined. Looked for (spectrographically) but not found: Hf. Th. Ta. W. U. Re. Ru, Rh, Pd. 0s,1r. Pt, Au. Cd. Hg. In, Tl. Ge, Sn, As. Sb. Bi. Te. L—150. Weathered molybdenite, mica-bearing vein, NWVi sec. 20. L—171. Molybdenite-bearing vein in carbonatite, NW% sec. 19. W-19. Weathered rutiie-bearing vein, NEM sec. 19. L—31. Weathered rutile-bearlng vein, SW34 sec. 18. 1.4503. Weathered rutile-zlroon-bearing vein. NWIA sec. 17. L—74. Rutile-bearing vein, SE54 sec. 17. L—76. Weathered pyrite-bearing vein, SEV; sec. 17. L—78. Weathered rutile-bearing vein, SEV4 sec. 17. L—112. Weathered rutile-bearing vein, SW34 sec. 21. L—278. Weathered brookite-zircon-bearing vein. SW14 sec. 7. L—310. Weathered rutile-apatite-bearing vein, NEM sec. 19. Ir3108. Weathered rutile-chalcedony-beariilrlg vein, NEH. see. 19. L—3398. Brooklte and gray clay from vein, W54 sec. 8. L—339b. Purple clay from vein, NWV; sec. 8. 56 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. TABLE 41.-——Analyses, in percent, of veins [Spectrographic analyses by Sol Berman. Colorimetric analyses by H. E. Crowe and A. P. Marranzino] L—62 L—115 L—118 Mag. M.A. L—233 Range MC—266 MC—225 MC-218 Spectrographic analyses Colorimetric analyses 0. 0002 0. 0006 0. 0005 0. 0004 0. 0001—0. 0006 Mn ________ 0. 050 0. 3 0. 050 . 2 2 . 009 . 06 . 009—2 As _________ . 003 . 002 . 001 .2 .5 .02 . 2 . 02—)5 Cu ________ <. 002 <. 002 . 005 . 001 . 05 . 001 . 003 . 001—. 6 Sb _________ . 0001 . 0004 . 0002 . 02 . 1 . 003 . 06 . 003—. 1 Zn _________ . 04 . 013 . 005 . 001 0 . 005 . 003 0—. 005 Nb ________ . 02 . 045 . 014 . 001 . 007 0 0 0—. 007 . 001 . 02 0 0 0—. 02 0 . 04 0 0 0—. 04 0 . 09 0 0 0—. 2 0 . 002 0 0 0—. 002 . 4 . 7 . 05 . 05 . 05—. 9 . 006 . 006 . 004 . 01 . 006—. 02 0 . x 0 0 O—x. . 004 . 03 . 002 . 002 . 002—. 05 . 01 . 02 0 . 005 0~. 03 . 004 . 004 . 0003 . 0003 . 0003—. 004 . 04 . 02 0 0 0—. 04 . 2 . 5 . 03 . 1 . 03—. 5 >5 >5 . 9 1 . 9—>5 . 002 . 002 0 0 0—. 005 . 002 . 002 0 0 0—. 002 . 007 . 01 . 0002 . 0004 0002— 01 . 04 0 0 0 -. 04 . 002 . 004 . 003 . 003 001— 004 . 02 . 02 . 004 . 007 —. 02 Radiometric analyses [Analyst, B. A. McCall] eU ____________ I 0. 006 l 0. 002 l 0. 014 I 0. 003 0. 003 Chemical analyses [Analyst, Roosevelt Moore] U _____________ I n.d. l n.d. I n.d. ’ 0. 0011 l 0. 0011 ' n.d.=n0t determined. x=found, quantity not determined. Looked for (spectrographically) but not found: Hf. Th. ’I‘a. W. U, Re, Ru. Rh. Pd, Os, Ir, Pt, Ag, Au. Cd, Hg. In, Tl. Ge. Sn. As. Sb. Bl, Te. L—62. Weathered rutile-bearing vein, central part of see. 17. L-115. Weathered rutile—bearing vein, SE54 sec. 17. CONTACT ZONE A zone of metamorphosed sediments 1,000 to 2,500 feet in Width and about 4.6 square miles in area sur- rounds the igneous complex. The formations in con- tact With the complex at the present erosion surface are Missouri Mountain shale, Arkansas novaculite, and Stanley shale that have been altered to gneiss, hornfels, argillite, and quartzite. Since most of the metamor- phic zone is more resistant than the adjacent sediments it appears as a ridge on the north, west, and south sides of the complex. As the Missouri Mountain shale occurs along only about 500 feet of the contact with the complex, the zone of metamorphosed sediments consists very largely of the Stanley shale and Arkansas novaculite. L—118. Weathered rutiIe-bearing vein, east-central sec. 17. Mag. M.A. Quartz-bearing vein, NM sec. 20. L—233. Quartz-bearing vein, N W34 sec. 28. MCI—226. Weathered rutile-bearlng vein, NWM sec. 17. MC—225. Weathered rutile—bearing vein, N E34 sec. 18. MC—218. Weathered rutile-beating vein, NEK sec. 17. METAMORPHOSED ARKANSAS NOVACULI'I'E Metamorphism of Arkansas novaculite has resulted in recrystallization of the quartz from a chert (grain size, 0.002 to 0.025 mm) to a coarse-grained quartzite (grain size, 0.04 to 7 mm). The grain size gradually decreases away from the contact until, at a distance of 1,000 to 2,500 feet from the complex, the recrystallized character of the rock is no longer detectable with the hand lens. Some of this recrystallized novaculite, although resistant to weathering, is very friable and can be crumbled with the fingers to a sand. Outcrops east of the town of Magnet are of this nature. In thin section the recrystallized rock is composed of quartz anhedra of varying sizes; the larger crystals have sutured contacts. ‘ The shale in the Arkansas novaculite is altered to a CONTACT ZONE TABLE 42.——Analyses, in percent, of apatite-pyrite vein Ir116-e \ L—llfi—i ‘ Ir116—1 \ L—116—2 L—116 \ Irll6—b ‘ L—llG—c Spectrographlc analyses [Analystz Sol Berman, 11-116, L—ll6—b, L—116-c, L-116—e, IrllG—f; H. J. Rose, L- 116—1 and L—116—2] 0. 0008 0. 002 0. 0003 0. 004 0 . 005 . 03 . 08 0 . 005 >5 >5 5 n.d. n.d. . 2 . 9 9 n.d. 002 03 . 2 5 n.d. 0 0 . 005 0 n.d. n.d. . 01 . 01 . 007 0 0 . 07 . 3 . 03 0 . 9 2 . 1 24 0 1 1 . 5 n.d. n.d. .006 02 . 008 n.d. n.d. 1 . 6 . 4 2 . 008 01 . 06 . 2 0 0 xx xx >10 0 04 . 1 . 08 . 08 . 002 02 . 06 09 0 0 002 . 004 0008 0 . 0004 02 . 002 0 0 002 . 02 . 03 .08 . 004 >5 >5 5 n.d >5 01 . 003 0 0 . 05 002 0 . 003 0 0 . 7 . 2 . 009 . 01 . 04 . 002 0 0 0 . 0003 . 001 . 002 . 001 0 . 001 . 01 . 02 . 009 0 0 Radiometric analyses [Analyst, B. A. McCall] 8U ....... l 0012‘ 0.020 \ 0.013 ‘ 0.014‘ 0.003 ‘ 0.098‘ n.d. Chemical analyses [Analyst, J. Dubinsky] \ n d U ........ ‘ n.d. n.d. n.d. n.d. n.d. \ .059 n.d. = not determined Looked for (spectrograjihically) but not found: Hf. Th. Ta. W. U. Re. Ru. Rh. Pd. Os. Ir. Pt, Au. Zn, Cd. Hg. 111, T1. Ge, Sn, As, Sb, Bl, Te. EXPLANATION OF SAMPLES Sample X-ray Analyst _______ __/__.._.._ _‘_’_._._———- / L—116 _____ W e a t h e r e d apatite-pyrite vein, NWM sec. 20. L—l16—b. _ Fresh apatite-pyrite vein, NWVl sec Irllc-c ________ do _________________________ L-llG—e... W e a t h e r e d apatite—pyrite vein, NWV; sec. 20. L—llS—f ________ o _________________________ L—116—1..- Monazite and plumbogurn- Monazite plus 13‘. A. Hilde- mite group(?) vein, NWV; plumbogummite brand. sec. 20. group mineral(?) L—116—2___ Pyrite from L—116-c ___________ / fine-grained hornfels composed of orthoclase, perthite, brown biotite, and quartz. METAMORPEOSED STANLEY SEALE The Stanley shale is composed of shale and sandstone. Some of the shale is quite siliceous and the sandstones are quartzite, feldspathic quartzite, arkosite, subgray— wacke, and graywacke. The shales at the outer limit of the contact zone are characterized by black spots as much as 2 mm in di- ameter. In thin section the spots appear to be segre- _______ —————i 57 TABLE 43.—-Spectrographic analyses, in percent, of minerals from oetns L—75di L-75d: L—338—1 1r-165-1 M 0-8—1 W—5—l M O— MC~ 162-1 178-1 0. 00X 0. 00x 0. 01 >1 0.005 0. 01 0. 02 . 00x . 0x n.d. n.d. n.d. n.d. . 01 n 0 . 08 0 . 004 . 0008 n (1 >10 0 . 004 .02 6 05 0 0 n d n.d. n d n 0 0 0 0 . 004 0 0 0 . 0X 0 . 02 0 0 0 0 . 0X 0 . 01 0 0 0 0 n.d. n.d. n.d. n.d. 0 n.d. . 0X .01 . 8 . 004 . 03 . 03 012 0 0 . 008 . 003 003 . 002 035 0 0 0 . 000): 0007 . 009 . 0009 . 0009 . 00008 X . 00X . 0006 . 1 001 . 003 . 02 n.d. .0): >10 >10 09 2 . 3 0 0 . 007 0 0 0 0 . 002 0 . 00x . 000x . 001 0009 . 001 002 0007 0 0 0 0002 . 0001 . 0001 0 0 0 001 0 . 002 003 . 0006 n.d.=not determined. Looked for but not found: Be, B, Zr, Hi, Th 1?, Ta, Mo, W, U, Re, Pd, Ir, Pt, Au. Zn, Cd, Hg, In, 'l‘l, Ga, Ge, Sn, Pb, As, st, Bl. Analysts: H. J. Rose—MC—s-l, W-5—1, MO—162—1, L—105-1; J. D. F1etcher-L—75d1, L—75d2, MC—178—1, L—838—1. L—75db Ilmenite float, SEX sec. 17. L—75dgt.1nmenite float, SEM sec. 17. Analyses from diflerent parts of the same crys a . L—165—1. Pyrite, NEV sec. 18. L—338—1. Barlte, NE 4 sec. 19. MO—s—l. Ferroan dolomite, NEH sec. 18. W—5—1. Microcline from feldspar-rich vein, NWM sec. 20. MC-162—1. Microcllne from feldspar-rich vein, SWX sec. 17. MC—178—1. Microcline trom feldspar-rich vein, NWM sec. 20. gations of sericite partially altered to brown or green biotite, opaque minerals, minor recrystallized quartz, and chlorite. The groundmass is composed of recrys- tallized quartz, sericite partially altered to brown or green biotite, opaque minerals and chlorite. As the contact with the igneous complex is approached. the amount of biotite increases, and the grain size of both biotite and quartz increases slightly. The argillaceous portion of the sandstones is similarly affected, but in the cleaner sandstones the only effect is a minor recrys— tallization of the quartz. On the north edge of the complex the narrow zone near the contact in which biotite is visible was also de- limited. In the outcrop some of the rock with visible biotite is gneissic and some is massive. The mineralogy is dependent upon original material, distance from the contact, and the nature of the igneous rocks at the contact. Good exposures of garnet-pseudoleucite syenite and garnet-nepheline syenite in contact with Stanley shale, are found in the stream that cuts the outer rim of ig- neous rock in the north—central part of sec. 18. In this area the metamorphosed sediments are fine grained gneisses cut by a multitude of alkalic syenite aplite dikes from V4 to 12 inches wide. About 15 feet from the concealed contact With garnet-pseudoleucite sye— nite, the fine grained gneiss is composed of orthoclase, microperthite, green hornblende, brown biotite, mag- netite, and pyrite. About 20 feet away from the contact the gneiss is 85 percent oligoclase, 8 percent reddish- i 58 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. TABLE 44.—Analyses, in percent, of metamorphosed sediments of the contact zone I L—247a I L—178 I L-243 I L—38 I L—173 I L—234 I L—235 L—247b L—149 I Range Spectrographic analyses 0. 0001 O. 0003 0. 0003 0. 0003 0. 0004 0. 0002 0. 0002 0. 0001 0. 0001—0. 0004 X X . X X X X X . X . X—X .2 >5 .2 .2 .07 .2 X .1 .1—>5 .01 .3 .04 .1 .01 .02 .008 .01 .01—.3 .07 .5 .1 .2 .09 .2 .04 .1 .04—.5 . 003 . 001 . 002 . 003 . 003 . 007 . 005 . 002 . 001—. 02 . 0009 0 . 0009 . 0009 . 002 . 002 . 0009 . 0004 0—. 002 . 004 . 007 . 004 . 006 . 004 . 007 . 005 . 004 . 004—. 007 . 01 . 02 . 009 . 009 . 009 . 01 0 0 0—. 02 O . 0002 . 0001 . 0004 0 . 0004 . 0001 . 0005 0—. 0005 . 4 1 . 4 . 5 . 5 . 5 . 3 . 2 . 2—1 . 006 . 05 . 04 . 02 . 02 v . 02 . 03 . 02 . 006—. 05 . 005 . 07 . 007 . 01 . 01 . 008 . 002 . 002 . 002—. O7 0 . 03 0 0 . 009 0 0 0 0—. 03 . 008 0 . 007 . 01 . 01 . 02 . 004 0 0—. 02 0 0 O . 0008 0 0 0 0 0—. 0008 .06 .2 .03 .03 .1 .03 .06 .01 .01—.2 3 >5 2 3 3 3 3 . 7 . 7—>5 . 002 . 002 0 . 001 0 . 0008 . 001 0 0—. 002 . 003 0 0 . 002 . 003 . 005 . 0009 0 0—. 005 . 01 . 02 . 0009 . 002 . 004 . 001 . 002 . 0005 . 0005—. 02 0 . 0001 0 0 0 0 0 0 0—. 0001 0 0 . . 002 . 003 . 002 . 002 . 002 . 003 . 002 . 002 . 002—. 003 . 0 0 . 004 . 001 0 . 004 0—. 03 Radiometric analyses [Analyst, B. A. McCall] eU _____________ I 0. 003 I 0. 003 I 0. 003 I 0. 004 0. 003 0. 002 I 0. 002 I 0. 001 I O. 001 I Chemical analyses [Analyst, Roosevelt Moore] 0.0011 I 0.0012 I 0.0014 I 0.0010 I 0.0010 x=found, quantity not determined. IP38. Hornblendeoligoclase gneiss, N W% sec. 18. Looked for (spectrographically) but not found: Ce. Hf, Th, P. Ta. W, U, Re. Ru , L~173. Biotite gneiss, N W% sec. 18. Rb, Pd, Os, Ir. Pt. Au. Zn, Cd. Hg, In. Tl. Ge, Sn, As, Sb, Bi, Te. L—234. Acmite hornfels, NWK sec. 28. L—235. Quartz-biotite hornfels, N WV4 sec. 28. L—247—a. Spotted biotite-quartz argillite, SWV4 sec. 9. L—247b. Feldspathic quartzite, SWX sec. 9. L—178. Spotted biotite~quartz argillite, N W% sec. 18. L-149. Lithic turf in Stanley shale, wyz sec. 28. L—2 Hornblende hornfels, N E‘/4 sec. 17. brown biotite, 4 percent pale dirty green hornblende, No consistent visible biotite zone was developed 1 percent aegirine, 3 percent magnetite, and a trace of along the southern half of the complex, probably be- pyrite. Albite, aegirine, brown biotite, greenish—brown cause the Stanley shale is more siliceous or sandy along hornblende, magnetite, and pyrite are found about 30 this contact. On Stone Quarry Creek in the north— feet from the contact. About 40 feet from the contact western part of sec. 28, a medium—grained hornfels the gneiss is composed of orthoclase, andesine, horn- composed of orthoclase, microperthite, albite, quartz, blende. sphene, apatite, and pyrite. About 125 feet and acmite is developed at the contact and grades into from the contact the composition is 58 percent ortho- a fine-grained hornfels composed of quartz, biotite, ser- clase plus minor perthite and plagioclase, 26 percent icite, and pyrite, about 3 feet from the contact. On a brown biotite, 7 percent sericite, 7 percent green to color- small tributary of Cove Creek in the northwestern part less anhedral garnet(?). 1 percent magnetite, and 0.1 of sec. 30, the fine grained subgraywacke 1 foot from percent pyrite. At the outer limit of the visible bio- the contact with the pegmatite shows a partial recrys- tite zone (about 300 ft) the very fine grained gneiss is tallization of the quartz and only incipient alteration composed of quartz, brown biotite, sericite, and pyrite. of sericite to biotite. GEOCHEMSTRY Spectrographic analyses of several examples of the foregoing rocks, table 44, suggest that, of the trace ele- ments, Sr, Ba, Zr, La, Nb, and V have been introduced into the sediments. Spectrographic analyses of most of the Paleozoic formations outside of the contact metamorphic zone, table 45, show no significant additions of the trace ele- ments that characterize the Magnet Cove igneous com- plex (Ti, Zr, Nb, Ba, Sr. V, and rare earths). GEOCHEMISTRY BOOKS The igneous rocks of the Magnet Cove complex are high volatile, high lime, alkalic and subsilicic; they contain intrusive carbonatite masses as well as the iron, titanium, zirconium, and phosphate minerals that char— acterize similar alkalic rocks throughout the world. High niobium substitution in titanium minerals and rare-earth substitution in apatite and perovskite are also typical. The weighted average of the major oxides for all the analyzed rocks from Magnet Cove (table 46) show that the average chemical composition can be expressed ————i 59 in rock terms as a melanocratic phonolite. It is well to remember, however, that the weighted average com- position is based essentially on analyses of two very dissimilar rock groups—the mafic jacupirangite-ijolite group and the felsic phonolite-nepheline syenite group. The most significant difference between these rocks and the average igneous rock (table 46) are the high content of soda, lime, titania phosphate, carbon dioxide, chlorine, fluorine, sulfur and sulfur trioxide, and the low content of silica. The high soda is reflected in occurrence of nepheline and sodic pyroxene and am- phibole; high lime and titania produces abundant titaniferous pyroxene, melanite garnets, sphene, and perovskite; the high carbon dioxide and phosphate combine with lime to make carbonatite and apatite; chlorine and sulfur trioxide are in sodalite and cancrinite alterations of nepheline; high fluorine is in fluorite and biotite. The trace elements which show significant concentra- tions in the Magnet Cove rocks as compared to average igneous rocks (table 46) are strontium (in the high lime minerals), barium (in potassic minerals), lanthanum (in apatite and perovskite), zirconium (in eudialyte and TABLE 45.——Analyses of sedimentary rocks in the Magnet Cove area, in percent Spectrographic analyses (Analyst, Sol Berman] BF—209 Ir332 11-316 L—3l5 P—278 11-333 L—309 L-331 L—224 W—780 L-295 Range 0 0. 0002 0. 0002 0. 0002 0. 0003 0. 0002 0. 0002 0. 0001 0-. 0003 .03 .4 .005 X .3 X X .6 .X .X .07 .0- <.01 x <.01 x (.01 .01 .4 .4 .1 .2 03 <.01~x 0 . 001 0 . 01 0 . 001 . 004 . 004 . 003 . 002 0 0-. 01 .001 .001 0 .1 .06 .1 .1 .08 .09 .01 .002 0-.1 0 0 0 . 005 . 01 . 06 . 02 . 004 . 009 . 02 . 005 0—. 00 0 0 0 . 004 . 002 . 006 . 006 . 002 . 001 . 0009 0 0-. 006 0 0 0 . 004 . 003 . 006 . 008 . 004 . 005 . 006 0 0-. 008 0 0 0 0 0 . 01 0 , 0 0—. 01 0 0 0 . 0004 . 0005 . 0004 0005 . 0003 . 0004 . 001 0 0-. 001 .01 .008 0 .3 .2 .8 .3 .4 .5 .1 0—1 0 0 0 . 008 . 004 . 01 . 02 . 02 . 04 . 08 02 0—. 08 0 0 0 . 003 . 02 . 008 . 004 0 . 002 . 002 001 0—. 02 .002 0 0 .006 .004 .01 .01 .004 .005 .003 .003 0—.01 0 0 0 0 - . 002 0 0 - » 0—. 002 0 . 02 0 . 03 . 004 . 02 . 05 . 03 . 02 . 02 008 0-. 05 .09 .07 .06 4 .3 >5 2 .06->5 0 0 0 .001 .001 .001 .0007 .009 0 0-.009 0 0 0 *. 003 . 003 . 004 . 000 . 001 . 001 . 0005 0 0-. 006 . 0003 . 0004 . 001 . 007 . 003 . 0009 . 003 . 005 . 0009 . 0003 0002 0003-. 005 0 0 0 . 001 . 001 . 003 . 003 . 002 . 002 . 002 0 0-. 003 0 0 0 0 0 0 0 0 0 . 000 0 0-. 006 Radiometric analyses [Analyst, B. A. McCall] eU ____________________________ n.d. n.d. n.d. ‘ n.d. n.d. n.d. n.d. l n.d. n.d. <0. 001 0.001 <0.001 _____,_.__..._—— Chemical analyses [Analystsz Grafton Daniels—~B-1, BF.209, 11-332, L—316, L—315, P—278, L—333, L—309, L—331; Roosevelt Moore—b224, W—780, L—295.} U ____________________________ <0. 001 <0. 001 <0. 001 ‘ <0. 001 ‘ <0. 001 ‘ 0. 003 l <0. 001 ‘ 0. 001 <0. 001 0.0012 0. 0011 0. 0010 n.d.-not determined. Looked for (spectrographically) but not found: Ce. Hf. Th. P, Nb, Ta, W. U. Re, Ru, Rh. Pd. Os. Ir, Pt. Ag, Au. Zn, Cd. Hg, In. T1. Ge, Sn. As, Sb. Bi, Te. 13-]. Bigfork chert. BF—209. Bigfork chert. L—332. Arkansas novaculite (black). 11-316. Arkansas novaculite (white). L-—315. Shale from the middle division of the Arkansas novaculite. P5278. Polk Creek shale. ' _ . . 7 669634—62 5 L~333 . L—309. Missouri Mountain shale. Stanley shale. Sandstone from Stanley shale. . Quartzite from Stanley shale. . Blaylock sandstone. . Hot Springs sandstone. * 60 TABLE 46.——Computed values for Magnet Cove rocks and the average igneous rock, in percent Chemical Spectrographic Chemical a l 1 23 1 2a 44. 97 0.0009 0.0006 U _____ 0.00030 0.0004 17. 61 .25 .03 Th._-_ .00091 .00115 3. 74 .025 3. 93 0005 .28 00281 2. 46 00183 10.06 ......... . 19 . 022 6. 30 .015 4. 43 . 0024 1. 46 .020 1. 76 .00025 2. 13 .0015 .60 .0023 . 11 .0080 . 14 .0070 .23 .0015 . 54 1. Bulk composition weighted by area. 2. Average igneous rock (Clarke, 1924). 2a. Average igneous rock (Rankama and Sahama, 1950). of rocks of the complex excluding sedimentary inclusions, ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. garnet), vanadium (in all mafic minerals), and niobium (in perovskite and sphene). Some of these elements (particularly niobium) are also concentrated in late rutile and brookite veins. Chromium, nickel, and copper show significantly less concentration in the Magnet Cove rocks than average igneous rocks. The chemical characteristics of the igneous complex as shown in table 47 are: 1. Average silica content is low (45 percent) and ranges in the analyzed rocks from 34 percent in jacupirangite and olivine melagabbro to 54 percent in late formed tinguaitic dike rocks. The most significant break in silica content occurs between the jacupirangite—ijolite group (average, 38 per- cent) and the phonolite-nepheline syenite group (average, 48 percent). 2. Alumina ranges from 9 to 22 percent. gite (9.21 percent), moderate in lites and nepheline syenites. It is 10W in jacupiran- ijolite, and high in phono- TABLE 47.~Analyses of igneous Intermediate ring Outer ring Phonolite-trachyte-breccia Syenites Altered phonolite J acupi- Undi- Garnet Garnet Garnet Sphene rangite Banded vided Feldspathoidal Weighted pseudo- nepheline psuedo- nepheline Weighted .trachyte trachyte leucotrachyte average leucite syenite leucite syenite average syenite syenite 40. 93 49. 62 50. 48 52. 60 54. 67 48. 5 47. 31 48. 88 49. 43 51. 27 48. 08 35. 42 15. 13 19. 13 20. 98 22. 18 20. 83 19. 7 20. 10 19. 77 20. 17 20. 42 20.16 9. 21 1. 32 1. 04 2. 29 . 29 . 20 1. 31 3. 57 4. 20 4. 00 2. 12 3. 45 8. 94 6. 86 5. 59 5. 16 3. 83 3. 69 5. 33 2. 62 1. 72 1. 71 2. 60 2. 21 7. 17 .29 .23 .29 .27 .32 .28 .30 .31 .32 .20 .28 .29 1. 74 1.68 1. 65 .33 .20 1.3 .89 .72 .64 1.22 .86 7.77 10. 71 6. 52 5. 11 3. 67 2. 59 6. 0 6. 67 5. 38 5. 08 4. 69 5. 55 20. 83 .17 .14 .18 .29 .20 .21 .36 .32 .27 .18 .29 .00 7. 19 6. 93 5. 96 5. 61 5. 27 6. 2 7. 98 7. 95 8. 30 8.05 8. 09 1. 47 2. 82 4. 81 3. 39 7. 41 8. 43 4. 9 6. 19 6. 80 6.19 6.10 6. 32 . 62 .84 .63 .99 1. 61 1.60 1.20 1. 73 1. 39 1.42 .94 1. 11 1.05 .04 .07 .11 .10 .09 __________ .10 .30 .10 .06 ............ .11 2. 23 1. 97 2. 21 . 71 .35 1. 8 . 88 . 76 . 63 1. 21 .85 4.05 .48 .38 .37 .07 .03 .3 .21 .14 .13 .20 .17 2.23 7.85 .05 .31 .00 .60 1.77 1.07 1.09 .96 .11 .81 .11 .22 .10 .01 .11 .01 .15 .01 .00 .13 .00 .05 .12 .42 .25 .00 <01 .01 .34 .04 .07 .08 .46 .15 .02 .24 .22 .55 .36 .23 .34 .19 .28 .20 .29 .23 .17 .88 1. 57 .36 1.02 1. 48 1. 00 .06 .04 . 03 .18 .07 .38 Spectrographic analyses 0. 0005 0. 001 0 0. 002 0. 0002 0.0008 0. 0004 0. 0007 0. 0007 0. 0004 0. 0006 0. 0008 .071 .5 .08 .5 . 6 .24 .044 .054 .014 .065 .04 .5 . 0005 . 0003 0 0 __________ . 0001 . 0003 . 0003 . 003 . 0005 . 0003 . 002 . 003 . 007 007 0 __________ . 004 . 0038 . 0024 . 0036 . 0028 . 008 . 032 .02 . 02 . 02 .009 .02 .018 .024 .027 .024 .023 .03 . 0002 . 0004 . 0002 0 0 . 0001 . 0002 ............ . 0002 . 0003 . 0002 . 001 . 056 . 03 . 04 . 02 . 03 . 039 . 021 . 03 . 022 . 040 . 027 . 04 . 052 . 029 . 02 . 021 . 03 . 027 . 045 . 052 . 046 . 018 . 040 . 07 . 015 .02 . 01 . 01 .03 . 013 . 0074 . 0099 .0096 .0087 . 0087 .002 . 000 . 001 001 . 0004 0 . 0007 . 0005 . 002 . 0034 . 013 . 0042 . 0001 0 .0007 0 0 .0003 0 0 .0006 . 0001 . 001 . 0021 . 002 . 001 0 0 . 001 . 0009 . 0008 . 0008 . 0012 . 0009 . 005 . 0009 .001 0 0 . 0002 .0006 0 .0012 .0005 .001 . 0074 . 003 0008 0 . 001 . 0023 . 0008 . 0009 . 0010 . 0021 . 0012 . 02 . 0036 . 001 . 002 . 001 004 . 0021 . 0027 . 0033 . 0027 . 0022 . 0027 . 002 _________________________________________________ 002 002 0 0 _______-__-. _-_-__-_-_-_ 0 <0. 001 <0. 001 0.002 <. 0001 . 0001 . 0002 <. 001 . 001 . 001 . 015 . 015 . 010 Radiometric analyses eU ___________________ 0. 005 0. 003 0. 004 0. 003 0. 006 0. 005 0. 004 0. 002 0. 005 0. 003 0. 004 0. 003 <0. 001 U . 00040 . 00033 . 00029 . 00025 . 00043 <. 001 . 00034 . 00020 . 00027 . 00027 . 00021 . 00049 . 00110 . 00043 . 00053 . 00069 . 00101 .......... . 00073 . 00055 . 00045 00100 . 00070 00339 it GEOGHEMISTRY 61 3. 13.5.0. is higher than FeO except in those rocks which contain consistently high and more abundant than potash in all appreciable pyrite. Fersman (1937) pointed out the analyzed rocks except two. Potash is low in the ijolite prevalence of high valency cations in the Khibine and group and increases sharply in the phonolite-nepheline Lovazero alkalic plutons. 4. Magnesia is highest in the jacupirangite and Olivine mela- 7. Titania is relatively high in all the rocks, a major constituent gabbro (7 to 8 percent), moderate in t percent, average), and 10 The average magnesia complex (2.46 percent), is lower than a syenites. (3.49 percent). 5. Lime is consistently high in all the rocks an percent for the entire complex. in tinguaite dike rocks to over 20 percent in jacupirangite. ficant break in lime content (13.49 percent) and phono- 5-5 Percent average) The high lime, alkalic character of the Magnet Like silica, the most signi occurs between the ijolite group litenepheline syenite group (about It ranges fro he ijolite d averages 10 m 2.5 percent syenite group. , group (4'2 in jacupirangite (4.05 percent), and averages w in the phonolites and nepheline for the entire complex. content for the entire verage igneous rock average of the entire complex. 1.76 percent 8. Phosphate is high in the olivine melagabbro dike (2.72 per- cent), but only moderately high in the other rocks. 9. Carbon dioxide and fluorine are high in all the rocks; chlorine is high in nepheline syenites, tinguaites, and phonolite but low in psuedoleucite syenite and the ijolite group. Sulfur and sulfur trioxide are erratic but are high for the 6. Total alkali content is high in both the mafic and felsic rock Cove complex is well illustrated by the sum of 0&0, groups. jacupirangite to almost 1 Twenty of the an rocks from Magnet Cove alyzed rocks contain more than 9 percent alkali. It ranges from about 2 percent in Na20, and K20 in these rocks compared to this sum 7 percent in tinguaite. Soda is in other 100k types (table 48). Inner core Late dikes Ijolite group Tlnguaite Pegmatite Analcirne Carbona- Biotite- Garnet Nepheline Eudialyte olivine tite garnet Fine-grained ijolite Garnet ijolite biotite Weighted Tlnguaite Sodalite Trachyte syenite nepheline melagabbro ijolite melteigite average trachyte porphyry pegmatite syenite pegmatite 35. 46 36. 89 87 29 41. 19 43. 36 40. 08 38. 65 52. 85 53. 61 53. 89 52. 00 52. 18 34. 51 1. 90 18. 60 13.28 14 41 17. 99 19. 12 11.74 17.26 19. 44 20. 32 21. 63 20.80 19. 28 10. 24 . 33‘ 7. 90 7. 58 4 23 5. 63 2. 85 7.19 5. 94 4. 82 2.84 1. 74 2. 36 2. 89 5. 58 .42 3. 27 3. 71 6 10 1. 76 1. 44 4. 76 3. 06 .57 1. 98 1. 82 1.15 1.97 8.48 .32 .25 .33 32 .30 .25 .32 .28 .37 .36 .15 .22 .19 .22 .26 4.16 4.22 4 15 3.06 5. 21 6. 72 4. 20 .13 .15 . 43 .28 .91 8. 26 1. 05 12. 66 14. 80 13 69 14. 61 12. 62 15. 69 13. 49 2. 53 2. 49 3. 08 2. 86 4. 17 15. 62 53. 37 .10 .18 14 .06 .01 .14 .09 .35 .19 .08 1.52 .10 .14 .00 6. 34 5.29 5 61 6.85 7. 88 3. 89 6. 47 8. 87 8.20 6. 62 7.87 7. 43 1.62 . 00 3. 07 4. 62 4 22 2. 38 2. 82 3. 47 3 23 7 96 7 66 7.80 8 18 7 21 . 70 . 16 3.46 1.11 69 3.13 2.56 .87 2 53 78 1 12 1.09 l 00 1 64 1.06 .12 . 33 . 14 04 . 40 . 58 .06 ............ 07 06 . 10 04 08 . 51 . 04 2. 47 3. 32 3 46 1.67 .56 3. 44 2 20 33 .31 .60 24 1 02 4. 88 . 10 1. 08 . 99 77 . 41 05 . 87 69 04 . 10 . 08 05 19 2. 72 2. 00 . 17 1.54 3 29 . 46 66 . 18 89 13 14 .26 1 10 47 5. 36 39. 41 . 03 .80 19 . 07 06 . 13 16 03 . 02 .07 17 17 . 03 .02 . 00 .41 33 . 01 00 .04 .09 .60 . 52 .09 32 06 . 02 . 00 . 16 .40 32 .04 03 . 24 17 42 . 19 .32 13 05 . 18 . 15 . 97 . 95 2 43 . 41 16 . 65 . 87 . 04 . 03 . 35 05 12 . 26 . 09 Spectrographic analyses 0. 0008 0. 004 0. 0002 0. 002 0. 004 0. 0004 0. 002 0. 0005 0 0. 0002 0. 0004 0. 0002 - 0 0. 0001 .5 .5 .9 .08 .6 .044 .33 .066 .08 .2 .091 .2 .2 .5 . 0303 . 0009 . 002 0 . 001 . 0009 . 0007 0 0 0 0 0 . 007 0 - .004 .005 003 . 003 0 .0022 . 0029 0 0 .001 0 .002 .003 .004 . 01 . 009 . 005 0 .037 .0042 021 . 01 . 007 .011 . 009 .005 . 03 . 0002 . 0003 . 0003 . 0008 0 . 0003 0 . 0002 0 . 0003 . 0002 . 0005 . 01 . 03 . 04 . 03 . 02 . 062 . 022 027 . 03 . 03 0082 . 04 . 007 . 001 .04 .05 .1 . 1 . 02 . 096 076 014 .008 . 02 016 03 . 04 . 02 005 . 01 02 . 002 0 . 014 007 . 022 . 02 . 009 0053 01 . 002 0 0 . 0007 . 003 0 .0004 . 018 0009 0007 001 0 0004 001 . 05 0 0 . 0008 0 0 0001 002 0 0007 0 . 001 .003 . 002 . 002 .0005 . 001 .0045 0019 0 0 0 0 0006 .004 0 0 . 001 . 0007 0 0 . 0065 . 0003 0 0 0 0 0 . 02 0 .004 .005 . 01 .003 .0009 .024 . 005 .0006 . 001 .001 .005 .002 . 008 .001 . 003 . 003 . 003 . 003 . 003 . 003 . 003 . 0045 . 003 . 002 . 0030 . 003 . 002 0 0 0 0 0 0 0 0 . 0072 ............ . 002 <. 0005 0 0 0 Colorimetric analyses ”HM; <0. 001 <0. 001 ___________ <0. 001 <0. 001 0. 002 ............ <0. 001 . . 000 . 0001 . 0003 . 0001 ____________ <. 0001 ______________________ (.001 ._-___-_.__- .004 <. 002 . 018 ____________ . 025 Radiometric analyses 0. 002 0. 002 ............ 0. 006 0. 010 0.004 0. 004 0. 006 <0. 001 ____________ . 00021 . 00019 . 00032 . 00043 . 00066 . 00069 . 00010 . 002(7) . 00004 ____________ ‘ . 00028 . 00055 . 00050 . 00054 . 00077 . 00136 . 00044 ............ ' . 00038 ............. 62 TABLE 48.~Lime-alkah' sums of igneous rocks compared to Magnet Cove rocks 1 Cao NaxO K20 Sum Rhyolite 1. 22 3. 34 4. 58 9. 1 'I‘rachyte ......................................... 3. 09 4.43 5. 74 13. 3 Andesite _________________________________________ 5. 80 3. 58 2. 04 11. 4 Phonolito ._ 1. 50 8. 84 5. 23 15. 6 Basalt ______________________________ 8. 95 3. 11 1. 52 13. 6 Olivine basalt-.. ___________________ 10. 2 3. l .9 14.2 Granite ____________ _ 1. 99 3. 48 4. 11 9. 6 Granodiorite _____________________________________ 4. 42 3. 70 2. 75 10. 9 Diorlte __ 6. 74 3. 39 2. 12 12. 2 Gabbto __________________________________________ 10.99 2. 55 . 89 14. 4 Average for igneous rocks __________________ 5. 08 3.84 3 13 12.0 Magnet Cove complex: I elite group _________________________________ 13. 49 6.47 3. 23 23. 2 eldspathoidal syenites ______________________ 5. 55 8.09 6. 32 20.0 Phonolites ___________________________________ 5. 94 6. 34 4. 91 17. 2 Average for entire complex _________________ 10. 06 6. 30 4. 43 20. 8 l Based in part on material from Daly (1933, p. 9—28). It is noteworthy that the alkali-lime sum from Mag— net Cove rocks increases with the postulated decrease in age of the rocks. Thus, first formed rocks, phonolite— trachyte associated with volcanic activity, have the low- est alkali-lime sum. The ijolite group which is postulated to be the youngest of the major rock groups has the highest. This apparent relationship may re- flect progressive desilication and increasing alkali—lime content from early to very late crystallized rocks. This possibility is discussed in the chapter on origin. In some cases the lime and alkali are separated as high lime-low alkali in jacupirangite and low lime-high alkali in tinguaite dikes. The high alkali content of the Magnet Cove rocks is also emphasized by the high ratio of total alkali to alu- mina. Ussing (1911) used the term “agpaite” for nepheline syenites that have excess alkali in propor- tion to alumina in accordance with the equation: na+k ~ a1 where na, k, and al are the relative amounts of Na, K, and Al atoms in the rock. The average agpaitic ratio for Magnet Cove rocks is 0.91. Tinguaite is the only rock of the 29 analyzed rocks that is a true agpaite (agpaitic ratio: 1.3). Chemical analyses of the rocks (table 47) are grouped according to rock type which corresponds roughly to their position and sequence of intrusion in the complex. The intermediate ring of phonolite—trachyte, the old— est igneous rocks in the complex, show a wide range in chemical composition particularly in Si02, A1203, CaO, and 002. This wide range, however, is caused chiefly by the high content of 002 and CaO as calcite and FeO and S as pyrite in the breccias. Thus, the other ox- ides, particularly SiOz and A1203, are depressed. Note also the high content of total volatiles in the breccia which will be discussed further in relation to the origin of the complex. The clean, unbrecciated trachytes 1.2 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. and phonolites show a fairly uniform composition characterized by the lowest alkali-lime sum of any of the Magnet Cove rocks. The weighted average composition of this rock group most closely approaches the average composition of the feldspathoidal syenites of the outer ring. How— ever, the higher total alkalies and significantly lower strontium in the syenites set them apart. The jacupirangite-ijolite group compared with the phonolite-nepheline syenite group is characterized by lower silica and total alkali and higher lime, magnesia, titanium, and phosphate. The late tinguaite and pegmatite dikes are charac- terized by very high total alkali, and lowest lime and magnesia content of any igneous rocks in the complex. The distribution of trace elements in the major rock groups is more erratic than the distribution of the major oxides because, for the most part, they do not form independent minerals. Distribution is controlled not only by the normal order of crystallization but also by the availability of a favorable mineral structure to accommodate the trace element. For example, niobium normally tends to concentrate in the residual magmas, however, it will enter a favorable host mineral such as perovskite that may crystallize in early differentiates. In general, however, Be, Sr, Sc, and V are higher in the ijolite group and Ba, La, and Nb are higher in the phonolite—nepheline syenite group. tholz’te-nephelln: wemte Ijolite Be _________________________ 0. 0007 0. 002 Sr _________________________ . 14 . 33 Se _________________________ . 0002 . 0007 V __________________________ . 033 . 076 Ba _________________________ . 22 . 08 La _________________________ . O2 . 004 Nb ________________________ . 011 . 007 The distribution of trace elements in the common rock-forming minerals at Magnet Cove, (table 49) is discussed in the section on “Geochemistry of minerals.” VARIATION DIAGRAMB Several variation diagrams were constructed to deter- mine the trends of the major elements and the chemical character of the Magnet Cove rocks. The ternary diagrams emphasize the high alkali-lime content and indicate that the lime content would provide the most significant abscissa for construction of a variation diagram. . The ternary diagram for the major elements, figure 5, shows the close grouping of the Magnet Cove rocks and emphasizes the high alkali-lime character (20 to 36 percent). The separate rock groups appear to have their own distinctive trends. The most interesting GEOCHEMISTRY ‘9 6 Feldspathoida «° «9 I A syenite group « )i a? . 63 Si+Al FIGURE 5.——Temary diagram showing igneous rocks of the complex plotted against atomic proportions of the major elements, and informative feature of all the variation diagrams is the distinctive, independent trends within each rock group. The jacupirangite-ijolite group shows greatest variation in the Mg—total Fe content with very little change in the alkali-lime total. The phonolite-tra- chyte group shows maximum variation toward the Si—Al apex. The nepheline syenites and late dike rocks show very close grouping. The Si—Ca—Na+K diagram, figure 6, shows that the chief variation is toward the lime apex. Silica and alkali show only moderate changes. The greatest variation within rock groups is exhibited by the phono— lite-trachyte group; however most of this variation is caused by the high calcite content of the altered phono: lite and breccia. The gap from jacupirangite to ijolite is particularly noteworthy and supports our concept that jacupirangite is a crystaldifferentiate that was effectively separated from the rest of the magma so that gradational rock compositions Could not form to fill this gap. The K—Na—C-a diagram, figure 7, emphasizes the uniformly high total alkali content with the major trend toward the calcium apex. Again the abnormally great effect of jacupirangite in the trend is noteworthy. Both the ijolite and phonolite-trachyte groups show marked differentiation; the ijolite group moves toward the soda apex With increasing nepheline content, and the phonolite—trachyte group moves toward the potas- sium apex with increasing sodic orthoclase content. The Na+K—total Fe—Mg diagram, figure 8, shows the general trend toward the alkali apex. Again jacupi- rangite is by itself as a crystal differentiate. The rock groups show very marked difierentiation; the ijolite group trends toward total Fe apex with increasing diopside-garnet content and the phonolite-trachyte group moves toward the alkali apex. . 64 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. 90 it .‘x Late dike rocks SI 50 w Phonolite-trachyte Feldspathoidal syenites | Na+K FIGURE 6,—Ternary diagram showing igneous rocks of the complex plotted against atomic proportions of Ca~Na+ K—Si. The lime variation diagram, figure 9, does not show smooth curves; however, the diagram is significant because it suggests separate periods of intrusion and differentiation within the rock groups, particularly the ijolite group. The lime content ranges from 2 to almost 21 percent; total alkali is high and N ago is consistently high while K20 shows a much steeper curve. The greatest scattering of points occurs in the Si02, A1203, and total Fe curves. This is caused by the wide range in abundance of garnet, biotite, and nepheline in ijolite. The jacupirangite position causes the steep part of the curves. The feldspathoidal syenite group that forms the outer ring dikes shows a much closer grouping than any other rock type. Although altered phonolite shows lower SiOz and A1203 and higher total Fe, it approaches the average composition of Magnet Cove rocks. If the volatile constituents are removed from the phonolite, this comparison would be closer. Note that N a20 and K20 offset each other so that total alkali would fall on the curve. A separate but parallel curve can be drawn for the high total iron in the phonolite-trachyte group; these rocks also have a high sulfur content (table 47). The most significant interpretations of the variation diagrams are summarized below: 1. All the rocks are very closely related and probably formed by differentiation of a single magma. 2. Rocks were intruded at separate periods and show varying amounts of differentiation within the rock groups. , 3. Total alkali and lime contents are high; soda is consistently high in all rock types, whereas potash exhibits a. much steeper curve. ' ' GEOCHEMISTRY 65 FIGURE 7,—Ternary diagram showing igneous rocks of the complex plotted against atomic proportions of Na—K—Ca. MINERALS The distribution of minor elements in the important rock—forming minerals and some chemical analyses of minerals are discussed in this section. The average minor-element content of each mineral is shown in table 49; variation in minor-element content within mineral species is discussed under mineral sub headings. The following observations are made from table 49. 1. Be is concentrated in thomsonite (0.01 percent), a zeolitic alteration of nepheline, and is therefore, most abundant in the nepheline—rich ijolitic rocks (up to 0.004 percent in garnet ijolite). Be also occurs in idocrase, melilite, and tobermorite in concentration greater than 0.001 percent. It is noteworthy that all these minerals as well as thomsonite, are late cyrstallizing lime silicates; alkali feldspars, nepheline, micas, and alkali pyroxenes that commonly accumulate Be are deficient in the ele— ment. Borodin (1957) suggests that Be separated out during the early stages of rock crystallization in the Khibina alkalic rock massif in Russia because of its ab- sence in secondary rock minerals and its constant presence in the primary rock—forming minerals. The opposite situation‘appears to be true at Magnet Cove. Be is chiefly in secondary lime silicate minerals that are alteration products of previously existing minerals and therefore has probably concentrated in the late residual solutions. If the Be-rich thomsonite alteration of neph- eline is deuteric, it suggests that the ijolites are a late differentiate as nepheline from ijolite contains more Be than nepheline from syenite. If thomsonite alteration is hydrothermal or pneumatolytic, Be has been introduced ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. 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So. 5 or o mooo. do.“ 3. oo. n. dd ,moo. moo. o dd mo. d. dd Hood d. ............. dimmed o o So. o o woo. woo. So. 2 5* o oooo‘ o 3. mo. 5. dd 3 o. So. dd 5.. o. S odm {iodomdowdédtfiwd o o moo. o o moo. moo. moo. S no. o No. o oo. mo. mom wooed o Ho. woo. dd 8. m. E Boo." E ................. a: -wdwndwdodédwmdoa o o o o o woo? Boo wood 2 No o dooo o Bo mood Ho dd o o. mooo dd mood «to S voooo o ............. wdmmooa "is; o o 8o. o o moo. o o H moo. Boo. Boo. o 8o. o 8. o o o o So. d. my 3. do.‘ n .yl------8_domdod8 o o moo. o o 8.1 o o w‘ ooo. moo. dd o coo. dd 8. o o o o moow moo. mo. mu. Boo. o ............ SEEwZ o o «8. o o woood o o 3o. moo. o 38. o do. do. no; o o ,o o dd mo. oo. No. o m ......... , ..mE€Ed< o o oooy o o mooo. o o dd, ooo. o «ooo‘ o 8.. o no. o o o o 8o. NH. mm. dd mooo. n .......... 2:3de o o o o o ooo. o o m. 58. o wooo. o o o d. o o o o dd my mu? no. o d i: ---.8tcda5aA o o moo. o o 3o. o o R. woo. o oooo. o moo. o no. o o o o dd 2. Ho. S. o o ........... $56882 o o moo. o o 38. o o mm. moo_. o wooo. o moo. No. «o. o o o o dd o4... may Bo. o a liomfloodto 535 o mood. «8o o o Hood o o mad nooo Sod. voooo o ooo. Boo moo o o o o dd no“ mmo no. o m ..... wmfloonfio Edam 93¢..— , 313.305 dm Am «C dN w< do “2 oo oh d2 3% do a? > .N F a; . «A W ow m am pm 3% am «a non Hfiodfia -852 at? .250 “wood: 3 flawdwfi .3 33:8 EwES» r333. dodgwov‘ldou dddme GEOCHEMISTR Y Feldspathoidal syenite group V 67 Na+K FIGURE 8.—Temary diagram showing igneous rocks of the complex plotted against atomic proportions of Fe—Mg—Na+K. by reaction of a residual magma of high water-high volatile content (carbonatite?) with earlier formedijolite. 2. Strontium is one of the characteristic elements at Magnet Cove; it occurs both in felsic and mafic minerals chiefly as a substitution for Ca“. The highest concen- tration occurs in labradorite, apatite, thomsonite, eudi- alyte, melilite, calcite, and barian orthoclase. 3. Barium, like strontium, occurs in both felsic and mafic minerals but it has a much more restricted choice of host minerals because it follows potassium. It is most concentrated in feldspar and biotite. 4. Boron was detected only in natrolite (0.005 per— cent) and thomsonite. 5. Scandium is most abundant in kimzeyite (zirco- nium garnet—0.09 percent 80205) and augite (0.01 per— cent). Both minerals are from late—forming rocks—— carbonatite and analcime olivine melagabbro dikes, respectively. 659634—62—6 6. Yttrium and lanthanum occur chiefly in late crystal- lized titanium minerals: anatase, kimzeyite, perovskite from the carbonatite. Lesser amounts occur in tita- nium garnet, sphene, and apatite. The concentrations of these two elements, however, do not show a direct relationship; the greater enrichment of yttrium is in anatase and the greater enrichment of lanthanum is in perovskite. 7. Ytterbium was detected only in apatite, sphene, and diopside-hedenbergite. 8. Titanium was found in all of the analyzed minerals at Magnet Cove. In addition to its occurrence in such minerals as rutile, brookite, perovskite, anatase, and sphene, it is a major constituent in titanium garnet, and kimzeyite, and appears as significant minor—element substitution in magnetite and pyroxene. Titanium is enriched in the residual solution to such an extent that the oxide minerals were formed; rutile and brookite occur 68' OXIDES. IN PERCENT 54 52 50 48- 4e- 44- 42 4o- 38 36»l 34 22 - 20, - 18- 16 14' 12 10' ALKALIC IGNEOUS COMLEX AT MAGNET COVE, ARK. Feldspathoidal Ieucosyenite 74/ Trachyte porphyry‘ Tinguaite Eudialyte-nepheline syenite pegmatite” Feldspathoid syenites Alkalic trachyte Banded trachyte Ndpheline H’ syenne pegmatiie // é/ Average syenite Average composition {.\ Altered phonolite \9 . Total Fe as FeO {/‘4‘ 1:} x ' . I V, ao\.\ L/ a , Na20 ..a'.‘ o ‘3 '. E. .—"'“-‘--.--'. . 222018161412108 6 4 2 0 CaO.IN PERCENT FIGURE 9.—Lime variation diagram. in late veins; beautiful euhedral crystals of perovskite and anatase occur in the carbonatite. 9. Vanadium follows titanium and is most abun- dant in anatase, rutile, brookite, magnetite, and garnet; however, the relationship is not direct. Vana- dium is also consistently present in apatite (0.08 per- cent average) probably in the phosphate position. 10. Chromium was detected in all of the analyzed minerals but is most abundant in magnetite and pyroxene. 11. Small amounts of molybdenum are captured in the early crystallized felsic minerals, particularly sodic orthoclase, but it also concentrates in the very late residual fluids and crystallizes as molybdenite in veins. 12. Manganese is most abundant in monticellite, particularly the late—forming euhedral crystals in car- bonatite. It also occurs in magnetite, sodic pyroxene, pectolite, and eudialyte. Thus manganese appears to concentrate in the residual fluids perhaps, in part, in the higher valency states. 13. Cobalt and nickel are concentrated chiefly in pyrite; however, the late formed vein pyrite is deficient in cobalt and nickel relative to early pyrite in the major rock types. Appreciable cobalt (0.01 percent) was also detected in magnetite and magnesiofen‘ite. 14. Copper was detected in all minerals at all stages of crystallization. It is most abundant (0.03 percent average) in pyrite. The overall distribution, however, suggests that most of the copper is captured during early stages of crystallization. 15. Silver occurs in amounts up to 0.004 percent in augite from a late dike rock and in aegirine. Trace amounts (0.0001 percent) were also detected in individ- ual samples of biotite, zeolite, feldspar, pyrite, and calcite. 16. Zinc was detected only in magnetite and is highest in the early crystallized magnetites in the major rock group; however, the lower limit of spectrographic sen- sitivity for zinc is 0.02 percent. 17. Gallium occurs in two unlike mineral groups: in felsic minerals (nepheline—feldspar) associated with alu- minum; in magnetites, associated with Fe”. 18. Lead is concentrated in sodic orthoclase illustrat- ing the well known relationship of lead and potassium in the crystal lattice of feldspar; however, lead also occurs in anatase, pyrite, pectolite, and eudialyte. Borodin ( 1957) suggested “a connection betWeen lead and calcium and especially strontium,” for the Khibina alkalic rocks because “the greatest quantity of lead is not connected with the chief rock-forming minerals, but is connected with such minerals as eudialyte, lampro- phyllite,’ rinkolite and sphene.” GEOCHEMISTRY 19. Tin is chiefly in the zirconium-rich minerals, eudialyte and kimzeyite. Tin was detected also in some, but not all, rutile and brookite crystals and in sodic amphibole. From the foregoing observations it appears that the following conclusions are justified: 1. Most of the minor elements are concentrated in either the mafic or the accessory minerals. Pb, Ga, Sr, and Ba may also be concentrated in felsic minerals; M0 is concentrated in the felsic minerals only. 2. Be, Y, La, Ti, Zr, Nb, Mn, and Sn are concentrated in the residual magma and are most abundant in the late-formed minerals in carbonatite and pegmatite. 3. Concentration of some minor elements in the re- sidual magma became sufficiently high to produce independent minerals such as the titanium minerals perovskite, rutile, brookite, the zirconium minerals—— eudialyte and kimzeyite; molybdenum and lead as molybdenite and galena in veins. 4. Co, Ni, Cu are most concentrated in the early- formed minerals; Sr, Ba, Ga, and Cu occur in all stages of crystallization. 5. The distribution of minor elements in the same minerals from different rock types emphasizes the 69 FEL'DSPAR Rapid chemical analyses were made on two sodic orthoclases from sphene nepheline syenite and nephe- line syenite pegmatite, table 50. The orthoclase from the pegmatite has a low summation chiefly because barium was not determined. The spectrographic analysis of this feldspar (table 51) shows a barium content of 2.4 percent. The Na20 content is equiva- lent to 29 percent normative albite. Feldspar from the nepheline syenite is silica deficient and has an abnor- mally high soda content caused by sodalite contamina- tion and kaolinite alteration. Spectrographic analyses of feldSpar, table 51, are divided into three groups—sodic orthoclase, barium orthoclase, and microcline. All groups are poor in minor element content except for barium and strontium, both of which show a wide but systematic range in abundance; low in microcline, moderate in sodic orthoclase, and highest in barium orthoclase. Ga occurs in all feldspars; Pb was detected in two sodic orthoclases, and abnormally high concentration of Mo (0.02 percent) was detected in one sodic orthoclase. Small quantities of Fe and Ti occur in all feldspars. TABLE 50.——Chemical analyses of sodic orthoclase [Rapid chemical analyses by P. L. D. Elmore and K. E. White] . . . s h N 11 1111 close compos1t10nal relatlon of all the rocks at Magnet Parent rock neghgiiiie 55$}... 0 . . syenite pegmatlte Cove and strongly md1cates a common parent magma for the Wide variety of rock types. For example, the Sample No. (MC—l—lO) (MO—12041) same suite of minor elements appears in perovskite . f . . .t t d 1 't 't d 5102 ________________________________ 57. 9 60. 0 rom Jacuplrangl e, garne pseu o euc1 e syenl e, an A1203 _______________________________ 23_ 1 21. 6 carbonatite. However, the absolute amounts of some £300 ------------------------------- 3- 8 ll 2% . . . . . 2 -------------------------------- - - minor elements in perovsklte Show a progresswe ln- Ignition loss _________________________ 2. 2 . 48 crease (Nb and rare earths) or decrease (V ' and Cu) Total _________________________ 99_ 2 97_ 1 in samples from rocks of early to verylate crystalhzatlon. TABLE 51.—-Spectrographic analyses for minor elements in feldspar Rock type Sample No. Be Mg Ca Sr Ba Tl Zr V Cr Mo Mn Fe Cu Ag Ga Pb Sodlc onhoclase Sphene nepheline syenite ..... MC-l-l ......... 0. 0001 0. 02 .d. 0. 3 0. 4 0. 03 0 0. 01 0. 0005 0 0. 01 0. 3 0. 001 0 0.002 0. 000 D0 0 .06 .28 .14 .3 .02 .003 .001 0 0 .003 .24 .002 0 .005 .002 Tlnguaite .............. 0 . 004 .d. . 4 . 2 . 01 0 . 007 . 0006 . 02 . 002 . 3 . 0009 . 0001 . 004 0 Average ________________ 0 . 03 ...... . 28 . 3 . 02 ....... . 006 . 0004 ...... . 005 . 28 . 001 ________ . 004 . 003 Bar-inn orthoclase Pegmatite ____________________ MC-l20—d ...... 0 0. 01 0. 13 0. 61 2. 4 0. 01 0. 001 0 0 0 0. 005 0. 26 0. 0001 0 0. 003 0 Garnet nepheline syenite ...... MC—112—4 ....... 0 . 02 n.d. . 5 9. 5 . 03 . 04 .01 . 0007 0 . 001 . 9 . 0008 0 . 003 0 Microcline Sphene pyroxenite ____________ MC-l72-1 _______ 0 ....... n.d. 0. 07 0. 09 0. 03 0 0. 001 0 0 0. 003 n.d. 0. 002 0 0. 002 0 Pegmatibe ............ 0 0. 02 n.d. . 01 . 07 . 006 0 . 01 . 001 0 . 007 . 4 . 001 0 . 004 0 D0 ...... __ ._ 0 . 006 n.d. . 0009 . 06 . 006 0 . 009 . 0009 0 0 . 09 . 0009 0 . 004 0' Vein.-. 0 . 01 ILd. . 004 . 6 . 03 0 . 003 . 0009 0 . 003 . 2 . 002 . 0001 . 003 0 0- 0 .005 n.d. 0 .02 . 004 0 .003 .0009 0 .001 .09 .001 .0001 .002 0 0- 0 . 02 0. 01 . 0008 . 05 . 03 0 . 002 . 0001 0 . 02 . 3 . 0007 0 . 0006 0 Average- -_ 0 . 01 ...... . 01 .15 .02 0 .005 .0006 0 .000 .21 .001 ........ .003 o n.d.—not determined. 70 NEPHELINE Chemical analyses of nepheline, table 52, show a ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. TABLE 52.—Analyses of nepheline [Standard chemical analyses: MC—1-9 by P. L. D. Elmore and K. E. White; MO— 216—3 and L—123-3 by L. N. Tenant] very similar soda-potash ratio in the nepheline from two very dissimilar rock types—nepheline syenite Chemical analyses Atomic ”“05 and ijollte. Calculatlon of nephelme and kaliophihte Spfieflfi 33191;? G t nap e e 10 1 arms end members shows a kaliophillte content of 20 to 23 Parenmck _________ syenite Home Home percent in the analyzed nepheline. The calculatlons for nepheline from the sphene nepheline syenite show Sample N0 ---------- MC‘1‘9 M041” ”123-3 Moms—3 5123-3 an appreCIable excess 0f Slllca’ WhICh sug-geSts CODtaHll- Sio, _________________ 543(2) g; 92 33:9 1.975}2 003 1.8;5} 006 natlon of nepheline by orthoclase, poss1bly present in 11.3. 1. $2 i g '83 . 33(1) 11. . . . . . SOhd solution. 14.2 15.09 14. 37 .718 .690 Spectrographic analyses of nepheline, table 53, show i113. 8'89 3'82 0'215 ‘954 0'215 '935 moderate concentrations of Sr and Ba. Ga 1s hlgh as fijdj :3; :3; '002 0 expected, particularly in nepheline from sphene nephe— 1'30 0‘22 _27 line syenite (0.011 percent). T1 and V were detected in n'd' 1'14 1'80 all samples; Pb was detected in one sample; and boron “ml “““““ 99'5 100'“ 99'” in two samples—probably indicating zeolitic alteration. n.d.=n0t determined. TABLE 53.—Spectrograph7‘c analyses for minor elements in nepheline Rock type Sample No. Be Sr Ba B Sc Ti Zr V Cr Mo Mn Fe Cu Ga Pb Garnet biotite ijolite. __ MC—216—3 ..... 0.0005 0.4 0.06 0.001 0 0.01 0 0. 03 0.0004 0.0004 0.007 n.d. 0.0004 0.003 0 Ijolite ______________ _ 14-123-3 ....... .0003 .2 .01 0 .001 .006 0 .03 .0004 0 .004 n.d. 0 .004 0 Do ___________________ - 11—17—4 ________ 0005 .2 .04 .003 0 .007 0 .02 .0004 0 .01 n.d. .0002 .006 0 Sphene nepheline syenite _________ MC—1—9 _______ .12 .2 0 0 .06 .002 .001 0 0 .001 .40 .0001 .011 .002 Do. __ MC—1—2 .0001 .2 .3 0 0 .03 0 .01 .0007 0 .006 5 .001 .004 0 Average ____________________________________ 0003 . 22 . l2 ________________ . 02 ________ . 02 . 0004 ________ . 006 ______ . 0003 . 006 ______ n.d.=not determined. ZEOLITE N atrolite and thomsonite occur as zeolitic alteration products of nepheline, particularly in the ijolite; anal- cime occurs in the groundmass of the analcime olivine melagabbro and as an alteration product of nepheline. A chemical analysis of zeolite from ijolite (table 54) was calculated after the method of Hey (1932) and fits thomsonite closely; specific gravity and refractive indices also suggest thomsonite. However, X-ray anal- ysis of the zeolite by F. A. Hildebrand showed natrolite rather than thomsonite thus suggesting that this may be a mixture or solid solution of natrolite-thomsonite. The strontium and barium contents also suggest that the analyzed zeolite (table 55, L—l23—2) belongs to the natrolite group. Significant differences of concentrations of certain minor elements occur between the various types of zeolite. Sr, Ba, and Be are strongly concentrated in the high lime thomsonite; B and Ga are in natrolite. These differences are more marked than differences in minor element content of a single type of zeolite from unrelated rock types. However, the natrolite from jacupirangite has an unusually high concentration of Mo and Cu. PYROXENE The pyroxenes are all monoclinic and can be divided into four types: 1. Diopside—— chiefly in ijolite group rocks. 2. Diopside-hedenbergite— chiefly in syenite and phonolite; and jacupirangite. 3. Aegirine-diopside—chiefly in syenite and phonolite as rims around a diopside-hedenbergite core. Some aegirine crystals occur in the syenite phonolite group and pegmatite. 4. Augite—found only in the late analcime olivine melagabbro dike rock. The most characteristic features of the pyroxenes are: (1) the high lime content—the greatest proportion of the pyroxenes fall in the diopside-hedenbergite group, TABLE 54.—Analysis of zeolite from ijolite [Sample L—123—2. Standard chemical analyses by L. N. Tar-rant] Chemical _ . (20130501811125) Atomic ratlo 8102 ______________________________ 38. 70 20. 80 A1203 _____________________________ 30. 21 19. 12 39. 99 F8203 _____________________________ . 18 . 07 C30 ______________________________ 6. 88 3. 95 N320 _____________________________ 10. 57 11. 00 20 ______________________________ . 21 . 13 15. 22 MnO _____________________________ . 02 . 01 MgO _____________________________ . 16 . 13 Ti02 _____________________________ . 01 ____________ 111168 """"""""""""""""" 2' 9% "0'50 """ 1130+ ____________________________ 10.47 18. 72}22 52 Total _______________________ 99. 55 ____________ GEOCHEMZISTRY 71 TABLE 55.—Spectrographic analyses of zeolites Rock type Sample No. Be Mg Sr Ba B Ti Zr V 01' Mo Mn Fe Cu Ag Ga Annlcime Garnet-pseudoleucite sye— MC—111—1 _____ 0.00008 0.01 0.02 0.003 n.d 0.008 0. 02 0.007 0.0007 0 0.001 0.03 0.0006 0 0.003 nite. Melagabbro ________________ L—lla—3 ________ 0 . 03 1 . 03 0 . 05 . 004 0 0 0 . 002 . 06 . 001 0 . 0007 Natrolite J acupirangite _______________ MC—173—9 _____ 0. 0003 0. 05 0. 01 0. 002 0. 005 0. 02 0 0.009 0 0. 007 0. 003 0. 3 0. 1 0 0.002 L—81—5 ________ .0005 .08 .01 .008 .004 .02 0 .004 0 .004 .2 .007 0 .003 MC—216—1 _____ 0005 3 01 001 01 . 009 0 . 005 0 . 002 . 01 . 3 . 0002 0 . 002 o ____________ MC—216—2 ..... 0006 3 06 001 . 02 . 008 0 . 008 0 . 003 . 005 . 3 . 0006 0 , 002 Ijolite ______________________ L—123—1 _______ 0005 2 004 . 01 . 006 . 004 0 . 007 0 . 001 . 01 . 07 . 0003 0 . 007 Garnet nepheline syenite. _- MC—112—5 _____ . 0001 04 1 . 009 n.d. . 01 .01 .01 . 002 0 . 005 . 2 .0008 0 . 008 L—167—6 _______ 002 02 . 05 . 02 n.d. . 02 0 . 006 002 0 . 005 . 02 . 0009 0 . 002 0009 06 . 007 . 007 n.d. . 005 0 . 004 0006 0 . 001 . 08 . 0007 . 0001 . 002 Ijolite ________ _ 0005 01 . 01 . 004 . 005 0 . 004 0 . 002 . 008 . 1 . 0003 0 . 003 Average , _____________________________ . 0007 13 03 . 008 . 005 . 01 ........ . 006 .......... . 002 . 006 . 2 . 01 ________ . 003 Thomsomte Jacupirangite __-. L—81-4 ________ 0 005 0.04 0 7 0.1 0.02 0 0.0008 0.00009 0.001 0.003 0.2 0.005 0 0.0006 Ijolite ________ ____ L—17—2 ________ 02 .04 1 5 .2 002 .009 0 .001 0 .0005 .004 .07 .0007 0 0007 Do _________ ____ L—17—3 ________ .01 .03 8 .06 .004 0 .001 .00009 0006 .003 05 .0005 0 0007 Average ______________________________ . 01 .04 8 . 1 ........ . 01 0 .001 .0001 0007 . 003 1 .002 0 0007 n.d.=not determined. and (2) the high soda content of late-formed pyroxene. Normal augite is rare. The analyzed pyroxenes fromijolite and jacupirangite, table 56, fit very Well into the diopside and salite divi- sions as defined by Hess (1941, 1949). The indices of refraction, however, appear to be too high. The titanium content of the salite is also high but certainly not unexpected in salite from jacupirangite (magnetite pyroxenite). Unfortunately the pyroxene mixtures in the nephe- line 'syenites could not be sufficiently separated for chemical analysis. Most of the grains have a colorless to pink tinted (titaniferous) core of diopside-heden- bergite (probably salite or ferrosalite) rimmed with green aegirine-diopside and aegirine. The high ferric iron and soda in the chemical analyses of the composites support this conclusion. The high fluorine and com- bined water suggest that biotite or amphibole is included in the green sodic rims of the grains. The totals of the chemical analyses are particularly low; unfortunately the amount of sample available for analysis was too small to permit further analytical work. Sample MC—lll—l probably also contains substantial fluorine and combined water. The analyzed aegirine shows the characteristic high ferric iron and soda content of this mineral. Magnesia is very low—less than titania. Spectrographic analyses of the pyroxenes for minor elements, table 57, show some interesting relationships both for the pyroxene group as a Whole and for indi- vidual types of pyroxene. Ti, Sr, Mn, and V, show significant concentration in all pyroxenes but the degree of concentration has a Wide range among the different types of pyroxene. Titanium and manganese are diagnostic because they show significant changes from one group to another whereas Sr and V are consistently high in all pyroxene groups. However, the more sodic pyroxenes contain somewhat higher Sr and V particularly if analyses from the lime silicate and pegmatite bodies are omitted. Following is a summary of Ti and Mn concentrations relative to pyroxene groups. Ti Mn Diopside-hedenbergite _________________ 2. 2 0. 07 Aegirine-diopside _____________________ . 7 . 7 Aegirine _____________________________ . 5 . 87 Diopside _____________________________ . 11 . 19 Although highest Ti correlates with lowest Mn, the relationship is not exactly inversewhighest Mn does not correlate with lowest Ti. Higher Ti in aegirine than the calcic diopside reflects higher FeO content of aegirine. The marked increase in manganese content as well as ferric iron in aegirine suggests that a part of the Mn is trivalent. Fersman (1937) has noted the prevalence of highest degrees of oxidation of the elements such as Fe”, Mn”, and Mn+4 in the rocks of the Khibina alkalic massif in Russia. The diopside group has captured the least amount of minor elements and is characterized by high Sr and V; most of the other elements except Mn have lower con- centrations than in the other pyroxenes. Note that diopside from the lime silicate body is particularly defi- cient in Sr, V, and Mn. This diopside is probably a secondary mineral formed from alteration of melilite. The Nb and Ti contents of diopside from melteigite sug- gest possible perovskite and sphene contamination. Diopside-hedenbergite is characterized by high Ti and low Mn relative to the other pyroxenes. Yb was detected only in this group. The high Cr, Ni, and Sc ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. 72 dag—am. .Z .A .3 $383 .550 :3 ”maxim .M .2 ha .NIEALH 398.3 .8335“ 32330 933% 6255306 Sandi .985 oufisnuognégaoe uo 32:88 3550535 .oumam _ 8 .SA 8 .SA on .SA 8 .9: 8 .2: w .3. ..... “Z III. .||.l - c HA. N .5 A. .3 an A. E. .w Hawk-Tom A. A 5 .mm 0 .mm mm .9 5. .NA. - -...m2 c .2 A An c .8 an .3 we .2. ..... a0 mA .3 .......... wn . .--....... .---.-.--. .----...-. --.--...-. .-.-...-.. .........0 £3 «a do ”A. .3 E .mm mm .8 cw .3 ca .8 E. .8 - - A309 .UAH m5 . .6.“— UHA .UAH .UJH AVG ............. 9H 3 . an. A; 6.: dd 6d 6d ......... +o.mA 8. 8. no. mo. no. ac. 8. H ........ loam so. go. woo woo. 8. mm. 3A AN. 3. 3. 3. ......... OOH «ma Amt. . S A So . man. was . A8 A awe . mg . mmo . on .3 AN .0 Am A. 3 . ch . AA. . an . ......... Caz now. 13 . Ana . o5. mam . .3 A. «A .NA S .5 AA .mm hm .Nm 3 .vm «N .am .......... OaD «no . Bo . woo . m8. m8 . Ah. cm A aw A «a . an . Ac . Aw . ......... on: SA . amA . «A. 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Ho. moo. m. .............. 2H o o wooo. mooo. o mooo. m. mooo. o H. o wo. o o o o moo. m. ............... 9Q o o o wooo . o mooo. m . mooo. o no. o mooo. o o o o moo . m . .............. 2H o o o wooo. o mooo. m. Hoo. o mo. o wooo. o o o Hoo. moo. w. ............... oQ o o mooo . mooo . o mooo . m. mooo. o mo . o no . o o o o moo. n . ................ 3:02 o o o Ho. woo. hoo. m. wooo. o m. o m. o o Hoo. moo. mo. w. .......... ofidonfihm o o o moo .o ooo .o moo .o oo .o moo. mo .o Ho .o moo .o o .o o o o Hoo .o 8 .o o .o ........... 35332 35.55 am u< a0 H5 H2 o0 n2 .5 AZ > pm 5. 2V 3 V um um um em .02 Sufism 35 “com gouging a: 398$»? 83.2: 3x. mwmfiogo nooaoxooécwamlknm @348 74 of diopside—hedenbergite from the sphene nepheline syenite is particularly noteworthy; a similar concen— tration of the three elements was found in the relatively rare augite from analcime olivine melagabbro. Note that the minor element content of this group is relatively uniform regardless of the parent rock type. The most distinguishing feature of the aegirine-diop— side group is the increase in Mn content as the composi— tion of the pyroxene shifts from diopside to aegirine. The wide range in the other minor elements reflects the variation in proportions of aegirine and diopside. Aegirine is characterized chiefly by high Mn. The occurrence of small amounts of Nb and Ag are also diag— nostic. An unusual and unexplained concentration of Pb was detected in one sample (MC—115—6). Again it is interesting to note that aegirine formed as a secondary mineral in the lime silicate body contains appreciably more Y and Zr and generally lower concentrations of Sr, Ti, V, and Mn than primary aegirine; the large aegirine crystals in the late eudialyte nepheline syenite pegma— tite approach concentrations of these elements similar to secondary aegirine. Augite which occurs only in one dike rock, analcime olivine melagabbro, is characterized by high Sc, Cr, and Ni. GARNET Garnet is one of the common rock forming minerals in the Magnet Cove complex and ranges in composition from the light—colored andradite to dark—colored mela- nite (titanium garnet). Chemical analysis of a dark-brown garnet from ijolite, table 58, shows the high TiO2 content characteristic of melanite or schorlomite garnet. The analysis was cal- culated to fit the formula of andradite, R”3R”§ (81003, because melanite is a titaniferous andradite; however, there is only approximate agreement as can be seen from the atomic ratios. Some of the divalent cations should be brought into the R” ’ group to balance quad- rivalent titanium. Impurities (diopside, sphene, and perovskite) were less than 3 percent. TABLE 58.——Analysis of dark-brown garnet from biotite-garnet tjolite [Sample MC—216—8. Standard chemical analysis by L. N. Tarrant] Chemical analysis Atomic ratios (percent) 8102 _____________ 27. 89 2 352 . 0 983 { 648 } 3' 000 T102 _____________ 15. 51 ' 335 A1203 ____________ 2. 12 211 1. 711 Fe203 ____________ 18. 32 1. 165 FMEO ____________ 2. 57 . 040 e _____________ . 91 . 206 03.0 _____________ 31. 79 2. s70 3- 268 MgO ____________ 1. 22 . 152 100. 33 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. The spectrographic analyses of light- and dark-colored garnets, table 59, show surprisingly small diflerences in minor element concentrations except for significantly higher Zr, Nb, Y, and Sc in the titanium garnets. This is to be expected as these elements commonly are associated with high titanium in the Magnet Cove rocks. La occurs in four of the dark-colored garnets. Mn, V, and Sr were also detected in appreciable quantities but not as unusual enrichments. The zirconium garnet, kimzeyite, has a relatively high Nb, Sc, and Y content compared to the other garnets. The published computed analysis (table 26) also shows appreciable Sn (0.09 percent Sn02). BIOTITE Chemical analyses and atomic ratios of three biotites, table 60, show that the biotites are all of the phlog— opite type; the major difference between individual samples is the FeO-MgO ratio. The high-iron biotite from jacupirangite with about equal amounts of F e0 and MgO has a composition of about the midpoint in the annite-pholgopite series; it also contains the highest amount of T102 and the lowest amount of fluorine. The other biotites contain less FeO and are closer to pure phlogopites. The lime content, particularly of the biotite from jacupirangite, is due chiefly to minor amounts of apa- tite, sphene, pyroxene, and zeolite in the analyzed samples. Spectrographic analyses, table 61, show that the bio- tites are relatively free of minor elements; Ba is most abundant (0.45 percent average) as expected in a high potash mineral but it has a wide abundance range. Mn, Sr, and V are the only other elements present in average amounts greater than 0.01 percent. High Cr and Ni were found in biotite from melteigite. APATITE The analyzed apatites (table 62) are relatively free of minor elements; strontium is most abundant (0.83 per- cent average for 11 apatites), and V, La, and Y also show significant enrichment. Yb is more consistently present and at higher amounts in apatite than any other of the analyzed minerals. B was detected in six of the nine samples. The standard phosphate analysis of phosphate-rich saprolite from the carbonatite (table 24) show quantities of fluorine ranging from 0.26 to 1.9 percent indicating that the apatite in the saprolite is probably fluorapatite. MAGNETITE Spectrographic analyses of magnetite, table 63, show that some of the analyzed samples contain more than 10 percent MgO and should be called magnesioferrite. The high concentration of iron in jacupirangite magma 75 GEOCHEMISTRY .woflfiuwueo Honfldd m8 .o 88 .o o o me .o H8 .o m .o Ho .o SA a .m S .o H .c S .9 «8 .9 Ho .o m .o o . . ..... Sufisfiao Sagan— HS. m8. m8. H8. a. H8. 8. a. 2. ....................... no. as. 8. m. H; o - - .................. mushy: H8. H8. «8. o a. H8. 3. m. m. N .v a mo. woo. «8. m8. H: o ........... $8ng ........................ £05m 25A «8 . H8 . 88 . o H. . o E . N . ma . SH 9 o 88 . we . H. . m . o ........ $8702 ........ Szaauum 88. so. 88. 38. S. Hos. 8. a. 3. SH o mo. o H5. m. H.. o ......... $2-02 .----:--:H-wumwwwwmmfimeWEan 88. 8o. *8. H8. m. 88. as. m. m. SH 8. H. a mo. m. .H o ........ £leon - ................. on «8. m8. «8. o o. 28. H. m. a. SH 8. 8. o H. .H H. o ........ HTHHTOE .......... on 88. «8. H8. «8. m. 88. mo. N. ma. oHIH S. 8. o no. u. .H o ......... oIHHH|oH>H .H. ..... 339% meommfiwawmmfian o «8. 8. So. a. c He. m. a. 2A o No. 88. m8. mo. .HZH o .......................... H8. «8. Ba. 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Snow mac—rim was—8.3M: 90 :0 Hz co :2 5 :2 > E E. 5 V am am Hm mg am .oz 299% 83 «com “85%. gm mEm§£a SS2: .Sx. mowflsgs umfiasxweéowamldm aqmfifi 659634—62——7 76 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. TABLE 60.——Analyses of biotite [Standard chemical analyses by L. N. Tarrant] Chemical analyses Atomic ratios Parent rock Blotite from Phlogopitetrom Phlogopitefrom jacuplrangite meltelglte iiolite MC—l73—3 MC—llB—b MC—2l6—4 Sample No. MC—l73—3 MC-113—b MC-216—4 SiOz _______________________________ 35. 06 36. 38 38. 67 2. 71 2. 69 A1an ______________________________ 15. 70 16. 26 14. 11 4. 00 4. 00 4. 00 1. 29 1. 31 1. 43{_14 1.42 _11 F8203 ______________________________ 2. 46 1. 70 2. 08 . 14 . 10 e0 _______________________________ 14. 47 6. 95 6. 08 . 94 2. 97 . 43 2. 97 37 2. 95 MgO ______________________________ 12. 96 19. 13 21. 81 1. 49 2. 11 MnO ______________________________ . 36 . 12 . 45 . 02 . 01 T102 _______________________________ 4. 06 3. 79 1. 14 . 24 . 21 08.0 _______________________________ 1. 28 . 22 . 33 11 02 NagO ______________________________ . 40 . 36 . 46 . 06 1. 0 . 05 1. 00 l. 00 K20 _______________________________ 9. 05 9. 88 9. 76 . 89 . 93 H20+ _____________________________ 2. 68 3. 08 4. 08 1. 38 1. 52 _________________________________ . 26 . 95 . 85 1. 4 1. 2. 18 1130— _______________________________________ 52 . 00 . 06 . 22 98. 74 99. 34 99. 82 Total ________________________ 1 1 . 40 . 36 Less 0 _______________________ 98. 63 98. 94 99. 46 TABLE 61 .——Spectrographic analyses for minor elements in biotite Rock type Sample No. Ba B So Y Zr V Cr Mn Co Ga JaCupirauglte ................. MQ—l73—3 _________ 0. 09 0 0. 0002 0. 002 0 0. 02 0 0. 2 0. 01 0. 0 0. 002 Melteiglte.--. -_ . MC—ll3-b- . . 4 0 . 0008 0 . 002 . 01 . 06 . 1 004 . 0 . 002 Ijolite ________ - MC—216-4" .006 . l _ 002 0 . 002 0 . 009 . 3 . 005 0 0 . 003 Lime silicate __________________ L—l ___________ . 005 1. 2 n.d. . 004 0 . 008 . 02 0006 . l . 001 0 . 003 Average - _- . 45 ________ 0. 001 0. 001 0.002 0. 015 ________ 0. 2 0. 005 0. 006 0. 003 n.d.:not determined. TABLE 62.-—Spectrographic analyses of apatite Rock type Sample No. Mg B Al Sc Y La Yb Ti Zr V Cr Mn Pb MC,-l73-7.-_ 0. 01 0. 005 0. 05 0 0. 02 0. 09 0. 02 0 0. 04 0 0. 01 0 0 MC-l73-8..- . 005 002 . 03 0 . 02 . 01 0 . 01 0 . 04 0 . 01 0 0 L—81—1 ...... . 02 0007 n.d. 0 . 03 . 01 . 0009 . 03 0 . 06 0 . 006 0 . 002 L—81—2. _ _-_- . 03 0007 n.d. 0 . 03 . 02 . 0008 . 08 0 . 08 0 . 006 0 0 MC—ll3—a.-. . 04 . 002 n.d. . 0005 . 03 . 2 . 0007 . 05 0 . 04 . 0001 . 008 0 0 MQ—216—6..- . 01 . 002 .04 0 . 02 . 04 0 . 006 0 . 1 . 0009 . 01 0 0 _ L—123—5 ..... . 01 . 05 0 . 004 . 05 0 . 003 0 . 1 0 . 01 0 0 Do ............ L—l7—l---___ .002 n.d. 0 .01 .01 .0002 0 . 1 0 .02 0 0 Melagabbro _______ L—ll a-l _____ . 1 n.d 0 . 03 . 009 . 0008 . 008 0 . 02 0 . 04 0 0 Carbonatite ....... L—166—2_ . ___ . 03 n.d. n.d . 0006 . 02 . l n.d. . 005 0 . 2 . 0007 . 002 . 0 Lime silicate ...... L-168-12_ _ _. . 1 n.d. n.d 0 . 01 . 01 n.d. . 004 . 004 . 1 . 0009 . 002 0 0 Average ................... 0. 02 __ 0. 02 0. 03 0. 0006 0. 02 _______ 0. 08 0. 0002 0. 01 ______ n.d.=not determined. TABLE 63,—Spectrographic analyses of magnetite Rock type Sample No. Sr Ba Sc Y La Ti V Nb Cr Mn 00 Ga Jacupirangite. MO—l73—10" 0 007 0. 002 0 0. 006 0 . 4 0 0. 2 0 0. 0005 l. l 0. 01 0 0. 004 D9.--" L—81—9 ______ 0 02 . 002 . 0003 . 006 0 . 0 0 . 2 0 . 0003 1. 1 . 02 0 . 006 Melte1g1te..__ . MC—ll3—l.-. 002 .005 0 0 0 . 8 0 . 3 0 . 2 1. 0 . 008 . 2 . 004 Garnept pse MC—lZl—l... 0009 .005 0 0 0 .4 0 .5 0 . 004 1.0 .008 .2 .003 syem e. Sphene nepheline syenite. L—24—1. _ . _.- 01 .006 0 0 .02 .2 . 5 0 .009 . 5 . 008 .005 Carbonatlte ______________ L—166—5- 0 . 001 0 0 . 0 . 07 . 03 . 0007 2. 7 . 01 . 009 Lime silicate ............. L—168—2_ . M 0 .004 0 0 6 .2 0 .0009 . 8 .02 .004 Average __________________________ 0. 006 0. 003 0. 0008 _____________ 2. 9 0. 008 . 3 _____ 0. 03 1. 2 0. 01 0. 005 GEOCHEMISTRY forced early crystallization of magnetite, and magnesia assumed the role of a minor element until sufficient iron had been deposited to lower its relative concentra— tion with respect to magnesia and permit crystallization of magnesia-rich minerals such as pyroxene. No significant differences in minor element content could be detected between the normal magnetite and magnesioferrite. Ti, Mn, V, Zn, Co, Ni, and Ga, in order of abundance, are concentrated in both types. Although Ga usually accompanies aluminum in igneous rocks and is in all of the felsic minerals at Magnet Cove, its greatest concentration occurs as sub- stitution for ferric iron in magnetite (0.009 percent) and zirconium garnet (kimzeyite—0.008 percent) from the carbonatite. It is noteworthy that the magnetite from melteigite contains very little magnesia, yet the rock contains almost as much MgO as jacupirangite. In this case, MgO accompanies K20 in the formation of early biotite phenocrysts in melteigite. PEROVSKITE Spectrographic analyses of eight perovskites (CaTiOa) show significant enrichment of Fe, Nb, Sr, La, Y, V, and Cu, in this mineral (table 64). A very marked dif— ference in concentration of Nb and La occurs between perovskite separated from the major rock types (0.38 Nb; 0.14 La) and perovskite from carbonatite (8.0 Nb; 1.0 La). This relationship demonstrates the marked tendency for Nb and rare earths to concentrate in re- sidual magmas. However, the uniformity of the total suite of minor elements concentrated in perovskite from different rock types sharply points up the direct 77 genetic relationship of carbonatite to the major rock types. Sr and rare earths substitute for Ca; and Fe”, Nb, and V substitute for Ti. SPHENE Nine spectrographic analyses of sphene, table 65, show that Fe, Nb, Zr, Sr, and V occur in sphene in average concentrations greater than 0.1 percent. Y and La show moderate enrichment. Sr, Y, and La probably substitute for Ca; Fe, Nb, Zr, and V substitute for Ti. All the minor elements detected in sphene show fairly uniform concentration regardless of the host rock type—except that Nb is more abundant in sphene from nepheline syenite-phonolite group than the j acupirangite-ijolite group. Mo and Pb were detected only in the sphene from sphene nepheline syenite. PYRITE Cobalt, nickel, and copper are concentrated in pyrite (table 66). Pyrite from the sphene pyroxenite and ijolite contains the highest concentrations of these met- als whereas the vein pyrite is notably lower. This sug- gests that the metals enter early formed pyrite in the major rock types and are not concentrated in the resid- ual vein-producing solutions. The analyses of pyrite from the phonolite indicate considerable contamination from other minerals in this fine-grained rock. CALCITE Spectrographic analyses of four calcite samples from different host rocks, table 67, show that the calcite is unusually pure; magnesia averages only 0.05 percent TABLE 64.——Spectrographic analyses of perovskite Rock type Sample N 0. Mg Sr Ba Sc Y La V Nb Cr Mn Fe Co Ni Cu Pb Jacupirnnaifn M 04734 n.d. 0. 2 0 0. 0004 0.057 0. 1 0. 045 0. 27 0 0. 03 n.d. 0 0 0.012 0 Do- MC—173—5 n.d. .2 0 .0004 .050 . 1 .039 .30 .03 n.d. 0 0 .033 0 L—81—10 ___________ .04 .4 .001 .003 .04 .4 .04 .2 .0005 . 02 1.4 . 002 0 .007 .002 MC—113-d ........ .002 .1 .002 .001 .04 .2 .04 .3 .001 .04 >100 .003 0 .01 0 be MC—216—10 ________ n.d. .2 .002 .0004 .02 .1 .04 .9 0 .04 n.d. 0 0 .01 0 Garnet pseudoleuclte syenlte ...... MC—121—c ......... . 02 . 3 . 002 . 1 . 05 . 1 . 3 . 0008 . 04 >10. 0 . 003 0 . 009 0 Average- . 2 . 001 . 0008 . 05 . 15 .05 . 4 . 0003 . 03 ________ . 001 0 .013 ...... Carbonatite _______________________ L—129—c ___________ . 0x . x . 0x . 00x . 0x x. . 025 8. 2 . 000x 0x 2. 6 0 . 00x . 00x 0 Do. L—129—d_ 0x . X .OX .OOX .0x x. .025 9. 2 . 000x 0x 4. 2 0 . 00x .00x 0 n.d.:not determined. TABLE 65.—Spectrographic analyses of sphene Rock type Sample No. Sr Ba Sc Y La Yb Zr V Nb Cr Mo Mn Fe Co Ni Cu Pb Jacupirangite ____________ L—81—8 ______ . 8 0.02 0 0. 02 0. 04 n.d. 0. 1 0.07 0. 1 0.0007 0 0.008 0.8 0. 002 0.002 0.004 0 Do MC—l73—6... .05 .0004 .029 .04 n.d. .19 .07 .15 0 0 .009 n.d. 0 0 .0075 0 MC—176 _____ .x .00): 0 .0): .OX 0.00X .Ox .29 .22 .000): 0 .0x 2.5 0 0 .00x 0 MC-177 _____ .X .001! 0 .OX .X .00): .0X .12 .20 .000x 0 .0X 2.0 0 0 .00): 0 MC-l72—4... .05 .002 .0004 .037 .02 n.d. .14 .15 . 12 0 0 .01 n.d. 0 .006 .010 0 MC—ll3—3... .3 .009 0 0 .06 n.d. .08 .1 .2 .02 0 .03 1.7 0 0 .003 0 L—l7—8 ______ .07 .0001 .009 .01 n.d. .6 .4 .2 0 0 .01 n.d. 0 .01 .001 0 yte MC—l71—1-.- .05 .002 0004 .04 .04 n.d. .4 .2 .3 .0008 0 .07 n.d. 0 .006 .02 0 Sphene nepheline syenite. MC—l—c ..... .3 . 1 .0002 .04 . 04 . 0008 . 05 .04 . 7 . 0002 . 0006 . 001 . 4 . 0008 . 01 . 005 .005 Average __________________________ 0. 2 0.02 0. 0002 0. 025 0.04 ________ 0. 22 0. 16 0. 24 0. 005 ________ 0. 02 1. 5 ........ 0. 004 0. 007 ...... n.d. ==not determined. 78 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. TABLE 66.—Spectrographic analyses of pyrite Rock type Sample No. Mg Ca Sr Ba Ti Zr V Cr Mn 00 Ni 011 Ag Ga Pb MC—172-5 _________ n.d. n.d. 0. 002 0. 004 0. 2 0 0 0 0. 02 0. 22 0.12 0. 090 0 0 0.007 . . n.d. 0 .002 .004 0 0 0 .005 .03 .01 .02 0 0 .002 n.d. 0 .004 .004 0 0 0 .02 . 1 .004 .02 0 0 0 0.2 . 0003 . 07 . 06 . 01 0 0 . 02 . 02 . 002 . 006 0 0 . 01 n.d. 0 0 . 01 0 0 . 0007 . 0006 . 007 . 002 . 001 0 . 001 0 0 _______________________ n.d. . 002 0 . 008 0 . 002 . 0004 . 004 . 05 . 04 . 0003 001 0 Average ___________________________ 0. 001 . 01 0.05 _____________ 0.0002 0. 01 0. 07 0. 02 0. 03 _______________ 0. 003 n.d.=not determined. TABLE 67,—Spectrographic analyses of calcite Rock type Sample No. Be Mg Sr Ba B A1 Sc Y La Ti V Cr Mn Fe Cu Ag Ijolite _________________________ 0 0.05 0. 84 0. 10 0. 001 0. 08 0 0 0.002 0.002 0.003 0 0. O4 0. 1 0. 0003 0 D0 _________ ___- . 0003 .03 . 84 . 1 . 001 . 09 0 0 . 002 . 007 . 003 0 . 03 . 1 . 0004 0 Lime silicate 0 . 02 . 2 . 02 n.d. n.d. 0 0 0 . 002 . 009 . 003 . 01 . 001 . 0008 0 Carbonatite---- 0 . 1 . 5 . 02 n.d. n.d. 0 0 0 . 005 . 03 . 003 . 01 . 001 . 0006 0 Vein ___________ MC—8—1 l 0 1 . 08 . 004 n.d. n.d. . 004 . 02 . 01 . 8 . 008 . 009 . 1 >10 . 0009 . 0002 Average ____________________________________________ 0. 05 0. 06 0. 6 _ _ _ __________________________ 0.004 0. 01 0. 002 0. 02 0. 05 0. 0005 ....... 1 Iron-rich dolomite. n.d.=not determined. Sr is the most abundant minor element (0.2 to 0.84 per— cent) followed by Ba, Fe, Mn, and V. Calcite from ijolite contains 0.001 percent boron and is higher in Sr, Ba, Mn, and Fe, and lower in V, Cr, and Cu than calcite from the carbonatite and lime-silicate body. The iron-rich dolomite, from a rutile-bearing vein, differs markedly from the analyzed calcites—particu- larly in its low Sr and Ba and high rare earths, Ti, and bin content. GEOCHEMISTRY 0F NIOBIUM Previous workers have firmly established the fact that the highest concentrations of niobium in rocks occur in alkalic rocks, particularly nepheline syenite, and that titanium minerals are the most favorable hosts for niobium. Therefore the occurrence of significant quantities of niobium (0.01 percent) in the igneous rocks of the Magnet Cove complex and (0.1 percent) in the rutile and brookite deposits associated with the com- plex is not surprising. In these rocks niobium is con- centrated in rutile, brookite, perovskite, and sphene, and to a lesser extent in titanium garnet, aegirine, and hornblende. RUTILE AND BROOKITE The description and economic considerations of the rutile and brookite deposits have been discussed pre- viously by Fryklund and Holbrook (1950) and Fryk— lund, Harner, and Kaiser (1954), and will not be repeated here. Spectrographic analyses of rutile, brookite, and paramorphs of rutile after brookite, tables 68, 69, and 70, show that the niobium content in these minerals ranges from less than 1 percent to more than 6 percent. The average niobium content of rutile and brookite is about 2 percent; paramorphs of rutile after brookite have an average niobium content of 1.3 percent. It is interesting to note that the highest niobium concentra- tion in rutile occurred in cyclic twins; 6 of 7 such crystals analyzed had more than 5 percent niobium, 1 contained 1.3 percent. However, most of these crystals were from the carbonatite area on Perovskite Hill in NW% sec. 19. Niobium concentration is probably greater in this part of the complex. For example, perovskite from the same area contains more than 9 percent niobium. Light-colored (red) rutile, rutile paramorphs, and brookite contain less niobium than dark—colored crys- tals—consistently less than 1 percent. Light—colored (red) patches can be seen in many of the crystals, and these patches are particularly abundant in paramorphs of rutile after brookite. The contact of light- and dark-colored patches is gradational for the most part, but in a few crystals the contact appears sharp. In one thin section of a feldspar—carbonate- rutile vein, there appears to be at least two ages of rutile. The older rutile may be a paramorph after brookite. The crystal boundaries of the paramorph are encrusted with later formed rutile needles which have grown with their long axis normal to paramorph crystal faces. In transmitted light the paramorph(?) is pleochroic (yellow-orange to green) and contains purple patches. We suspect that the deep-purple, almost opaque areas are the richest in Nb and they appear to replace the highly pleochroic yellow-orange to green variety. The rutile encrustations are also purple and show rhythmic zoning in shades of purple. The abundance of light-colored patches in paramorphs suggests that in the transformation from orthorhombic brookite to tetragonal rutile there may be some shift- ing of the Nb ions—perhaps tending to force them out or concentrate them in clusters. However, there is some color variation in brookite and in nonparamorph GEOCHEMISTRY 79 rutile but this is a more rhythmic variation. Why is Nb erratically distributed? Does it tend to cluster around particular centers in the crystal? According to Goldschmidt’s capture principle, Nb, if quinquiva- lent, should concentrate in the earlier phases of rutile or brookite. So far, we have no evidence of this. Correlation coefficients were determined for Nb, V, and Fe in rutile and brookite with the following results: Nb—V =+0.60 Nb—Fe = + .72 Fe—V =+ .27 For this group of 28 samples anything above +0.4 is a significant correlation. There is a moderately strong positive correlation of V and Fe with Nb but very poor correlation of Fe—V. This might be explained by the valence states of the elements concerned which are sub- stituting for Ti+4 (0.68 A). Nb is probably quinquiva— lent (0.69 A)——thus needs a trivalent atom to balance which could be Fe+3 (0.64 A). If V is present as V+4 (0.63 A), correlation with Fe+3 is not expectable. The V—Fe ratios in the rutiles (0.1 to 0.3) are probably much higher than they are in the solutions from which rutile formed. The V—Fe ratio in the rock from Magnet Cove would average 0.004. This suggests that vanad- dium, like niobium, moves into the rutile structure preferentially more than Fe”, and that the amount of Fe” present is directly related to the amount of N b” that substitutes for Ti“. Other trace elements consistently present are Mn, Ca, Mg, Cr, Cu, Sr, W—all in the 0.00X range. Mo and Sn were found in some samples in 0.00x concentration. W, Mo, Sn, and possibly Cr substitute in the Ti+4 position. ' Semiquantitative spectrographic data for Se, Y, and Zr are erratic and appear to vary with the analyst; how— ever the values given are near the limit of sensitivity for the semiquantitative method. Ni was detected only by the quantitative method. CARBONATITE The highest niobium concentration (9.2 percent) noted in any mineral in the complex occurs in perov— skite from' the carbonatite. These crystals, usually octahedrons modified by a cube, have been described in detail by Williams (1891). More recently Fryklund, Harmer, and Kaiser (1954) mapped the Kimzey calcite quarry geologically and analyzed perovskite crystals and channel samples of the carbonatite. They report a range of 5.1 to 8.8 percent niobium in individual per- OVSkite crystals and a range from 0 to 0.07 percent nio- bium in channel samples of carbonatite. Niobium was detected in only 6 of 21 channel samples. Anatase, formed by alteration of this perovskite, contained 6.8 percent niobium. The high content of niobium in per- ovskite from carbonatite indicates the strong tendency for niobium to concentrate in very late residual mag- mas; perovskite in the igneous rocks contains less than 1 percent niobium. IGNEOUS ROCKS AND MINERALS The niobium content of the analyzed rocks ranges from 0 to 0.03 percent. Feldspathoidal leucosyenite and one sample of sphene pyroxenite are high, with 0.03 percent. Garnet ijolite, jacupirangite, and anal- cime melagabbro are all low in niobium. Perovskite (0.2 to 0.9 percent Nb), sphene (0.1 to 0.7 percent Nb), garnet (0 to 0.1 percent Nb), aegirine (0 to 0.03 percent Nb) and hornblende (0.007 and 0.01 percent Nb) are the minerals in which niobium is con- centrated. Niobium (0.1 percent) was detected in only one of the light-colored garnets whereas it was detected in all but one of the dark-colored titanium-rich garnets. Although niobium usually substitutes in titanium min- erals, it may be seen from table 47 that no direct rela- tionship exists between the niobium and titanium content of the rock. Dark basic rocks such as jacupiran- gite and olivine melagabbro which contain abundant titanium are low in niobium. Titanium in ijolite is principally in perovskite which apparently affords a favorable structure for niobium substitution and the tendency for niobium to stay in the residual magma is overcome. Generally the more felsic rocks such as sphene-nepheline syenite which contain small quanti— ties of titanium have the greatest amount of niobium. These results are in accord with the general observa- tion that niobium, because of its high valence, is con- centrated in residual magmas. Titaniferous pyroxene crystallizes very early in the igneous rocks at Magnet Cove, but although it may con- tain from 1 to 10 percent titanium, it does not contain niobium in detectable amounts. Late-formed sodic pyroxenes, however, which contain less than 1 percent titanium have as much as 0.02 percent niobium. On the other hand, early crystallized sphene and perovskite contain as much as 0.7 percent niobium. Thus it is apparent that at least 2 factors determine the distri- bution of niobium in the rocks. 1. Niobium tends to concentrate in residual magma. 2. Availability of favorable mineral structure to ac- commodate niobium. If perovskite or sphene crystal- lizes even in early differentiates such as jacupirangite, a part of the niobium will preferentially enter the struc- ture rather than stay in residual magma. It is impor- tant, however, that niobium substitution is not as great in the minerals from early crystallized rocks as in the same minerals from late formed rocks. Sphene from sphene pyroxenite contains 0.2 percent niobium; from 80 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. TABLE 68—Spectragraphic Sample No. V Nb Fe Be Mg Ca Sr Ba Sc Y Yb Zr Cr 3. 6 0. 84 0 0. 00x 0. 00x 0 00x 11 d 0 0 0 0 0. 00x .76 .45 0 .00x .00x 00X nd 0 0 0 0 .0011 2.1 2. 2 0 . 00X . 00x 00x 00x 0 0 0 0 .00x 2. 8 1. 4 0 .001: . 00x 00x 00x 0 0 0 o . 00x 1. 4 .77 0 .002: . 00x 0 0 0 O 0 0 . 000x 1. 7 1. 3 0 .00)! .001: . 00x 00x 0 0 0 0 .00x 1. 6 .92 0 . 00x . 00x . 00x 00x 0 0 0 0 . 00x 2. 3 1. 5 . 000x . 00x . 00x . 00x 00x 0 0 0 0 . 00x 2. 2 . 96 0 00): . 00x .00x . 00x 0 0 0 0 . 00x 1. 4 . 78 0 00x 00x .000x 00x 0 0 0 0 . 00x 2. 2 2. 2 0 00X 00x .00x 00x 0 0 0 0 .00): 3.0 1. 9 0 0x 00x .00x 00x 0 0 0 0 .00x 3. 8 1. 5 0 00x 00x n.d. n d 0 0 0 0 .00x 2.0 . 72 0 00x 00X 11 d n d 0 0 0 0 .00x 2. 2 .94 0 00x 00x 11 d n d 0 0 0 0 .001: 2. 2 1.1 0 0x .0x n.d. n d 0 0 0 0 .0x 2. 2 . 66 0 001: . 00x n.d. n d 0 0 0 0 . 00x 1.0 .92 11.11 11 d n.d. n.d. n d n.d. n.d. n.d. n.d. n.d. .22 .72 0 00x . 00x n.d. n d 0 0 0 0 0 1. 4 .92 0 00X 00X . 00X 00x 0 0 0 0 . 00x 5. 3 1. 6 0 00: . 00X . 000x 00X 0 0 0 0 .00x 1. 3 . 86 . 000x 00X . 00X . 00x 00x 0 0 0 0 .00X 5. 6 1. 5 0 0x . 00X . 000x 00X 0 0 0 0 . 00x . 78 . 88 0 00x 00X . 000x 00x 0 0 0 0 . 00x .11 .66 .000): 00x 00X 0 n d 0 0 0 0 .0x .90 56 0 00x . 00X 00x 11 d 0 0 0 0 . 00x 1. 6 1 1 .0002: 00x .00X 00x 11 d 0 0 0 0 .00): . 42 96 . 000x 0x .00): . 000x 00x 0 0 0 . 0x . 0x 26 1 1 .000): .00x . 00x 000x 00x 0 0 0 0 .00X 11 82 000x 00x . 00x 00X 11 d 0 0 0 0 .00}! 2. 0 .78 .0001: .00X .00x .00x n.d. 0 0 0 0 .00x . 90 1. 3 0 00x 00x . 000x 1) d 0 0 0 0 . 00x 6.6 1. 8 0 00x 00x . 000x n d .000x 0 0 0 .00}: 2.8 1. 6 0 00): 0x .00x 00x 0 0 0 0 . 0x 2. 6 1.0 0 00X 00x n.d. n d 0 0 0 0 .00): 1. 5 . 20 0 .00x .00X .00): n.d. 0 00x 0 00x ox 1. 5 . 28 0 00X .00x .00x n.d. 0 . 00x 0 00x 0): 1. 5 0.08 0 0 00x 0. 00x 0.00): n.d. 0 0.00): 0 0 00x 0 OK 1.4 .18 0 00x .00x .00x n.d. 0 .00X 0 00x 00x 1. 3 .34 0 0x .1: .Ox n.d. 0 .0x . 00x 0: 0x .8 . 54 0 .0x .00X .00): n.d. 0 00x 0 .00x .Ox 1. 5 .24 0 .0x . 00x .00x n.d. 0 00x 0 . 0x . 00x 6.0 n.d. 0 n.d n.d. 0 0 .0001 0 n.d. 0 .0008 5.1 n.d. . 0003 n.d n.d 0 0 0001 0 n.d. 0 .0008 L—136b ___________________________ 5. 6 n.d. 0 [Ld 11 d 0 0 0001 0 n.d. 0 .0008 Range _____________________ 0. 038—2. 8 0. 11—6. 6 0. 08—2. 2 0—. 000x 0. 0011—. OK 0. 00X—. 0x 0—. 0X 0—. 00): 0—. 000x 0‘ 00X 0—. 0x 0—. 0x 0—. 0x Average ___________________ . 6 2. 2 1. 8 __________________________________________________________________________________________________ Number of samples ______________ 45 \ 45 l 42 l 44 ‘ 41 ' 41 l 37 ‘ 21 44 44 41 44 44 1 Tin robably introduced during separation with methylene iodide. 1 An yst to: elements V, Nb, and Fe,R. S. Harmer. Looked for but not found: B, La. Th, Ta, U, Go, Rh, Pd, Ir, Pt, Ag, Au, Zn, 0d, Hg, Ga, In. Ge, As, Sb, B1. n.d.=not determined. GEOCHEMISTRY 81 analyses, in percent, of while Mo W Mn Ni Cu Sn Pb Analyst Description 0 0. 00x 0. 00x 0 0.0x 0 0 I. D Fletcher ......... Float.‘ SM: sec. 17. 0 .00x .000x 0 .0x 0 0 do Float. Light colored. SM sec. 17. . 00x 0 .000x 0 . 00x 0 0 From feldspar vein. N WM sec. 17. 0 0 .000x 0 .00x 0 0 - Float. NEH sec. 18. 0 0 . 000x 0 . 00x 0 0 Float. Center 01 sec. 17. 0 0 .000x 0 .00x .00x 0 Float.0 Efi sec. 18. 0 0 .000x 0 .00x .00x 0 .000x .0x .00x 0 .0x .00x 0 Float.0 NWM sec. 17. 0 0 . 000x 0 .00x .00x 0 Float. SWM sec. 17. 0 .00x .000x 0 .00x 0 0 Float. SEX sec. 17. 0 .00x . 00x 0 .00x .00x 0 Do. 0 0 .00x 0 .00x 0 Float. NEV; sec. 20. 0 .00x . 000x 0 .0x 00x 0 Float. Geniculated twins. NEK sec. 18. 0 .00x . 000x 0 .0x 00x 0 Float. Geniculated twins. SEM sec. 13. 0 .00x .00x 0 .0x 001: 0 Float. Geniculated twins. N EM sec. 18. .00x .00x .0x 0 .0x 0 .00x ..... do ................ Float. NWM sec. 17. 0 .00x .000x 0 .00): 0 0 From feldspar-carbonate vein NEH sec. 18. n.d. n.d. n.d. n.d. n.d. n.d. n.d. Float. Light colored. 8% sec. 17. 0 0 . 000x 0 . 0x 0 0 Do. 0 0 .00x 0 . 0x .00x 1 0 Float. NWV4 sec. 17. o 0 .0x 0 .0x .0x 1 0 Float. Cyclic twin NEV; sec. 29. 0 0 .000x 0 .0x 0 0 _ Float. Cyclic twin 8% sec. 20. ' 0 0 .0x 0 .0x .0x 0 _ Float. Cyclic twin N WM sec. 19. 0 0 . 00x 0 0x 0 .00x From feldspar vein. Light colored. SW14 se 0. 0 0 .00x 0 0x 0 0 F§>$yteldslizgr-mrbonate vein. Light- colored 1geniculated twins. 4 sec 0 0 .0001: 0 .0x 0 0 Float. Light-colored geniculated twins. NWM sec. 19. 0 0 .000x 0 .0x 0 0 - Float. NWV4 sec. 19. 0 0 . 000x 0 .0x 0 .00x . From feldspar vein. SE54 sec. 20. 0 0 .00x 0 .0x 0 .00x _ Fromfeldspar-carbonate vein. SE54 sec. 20. 0 .x .000x 0 .0x 0 0 Float. SE14 sec. 20. 0 .000x 0 .0x 0 0 D0. 0 0 .0x 0 .0x 0 0 _ Fromfeldspar-carbonate veins. Geniculated twins. NEH sec. 24. 0 0 .0x 0 .0x 0 0 Float. Cyclic twin. NWV4 sec. 19. 0 0 .00x 0 .0x .00x I .00x _ Fromieldspar vein. SEM sec. 17 0 .00x .000x 0 .0x .00x 0 Float. NWM sec. 17 0 0 .00x 0 .00x .00x 1 .00x From feldspar-carbonate vein. W14 sec. 17. 0 0 .00x 0 .00x .00x 1 0 - From‘feldspar-carbonate vein. SWM sec. 17. 0 0 0.01: 0 0. 0011' 0. 00x 1 0 Do. 0 0 . 00x 0 .00x 0 0 From feldspar vein. Center of sec. 17. 0 0 .00x 0 . 00x .00x 1 00: Fromtiledspar vein. SE34 sec. 17. Apparently contaminated with ape. 1 e. . 00x 0 . 00x 0 . 00x 0 0 From feldspar-carbonate vein SW14 sec. 8. 0 0 .00x .00x 0 0 _ From feldspar vein. SWM sec. 17 0 0 . 03 .080 .016 .02 0 _ FloatD. Cyclic twin. NWM sec. 19. 0 0 . 02 . 052 . 016 . 02 0 Do. 0 0 . 02 . 065 .016 . 01 0 Do. 0—. 00x 0—. x 0.000x-.0x 0—.03 0 0011-. 0x 04.01: 0—.00x 44 44 44 44 44 44 44 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. 82 .358586 8:.."6: .5 .nm .3. 5m .00 .3 .45 .60 .:N .5: $4 .3 .._H .6: .:fi .oz 60 .D 62 .3. .m :3. .9». .mA .m ”6:68 go: one. :8 638°: .5:qu .w .mlom .n Z .> 35830 .8“ 3.32:. . oh . m .H on . ..oufio>< H .HIH . o .TB. v .Nuomo .o .-..ow:w.m .onH ............. o6 ..... o moo . oooo . o woo . o o o Ho. o 6.: moo . Hooo . H . «a . H. ......... oomHLH .3 .03 NB: 6228.32: $2.: - ...... $8: .: .: o ooo. so. o So. o o o s. o 6.: 8o. 88. 1 «o. 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AanmoLH .NH .03. wA m 43lo o o wo. woo. woo. woo. o o o o 6.: woo. wo. woo. vm . o.m woo . -1 ........ pi .Hfimio 3an «0 ”tom o o wo . woo . woo . woo. o o o o 6.: wooo . woo . wo . o .H o .m mH . ....... Amwaonlfl SH .03 X 32 432% o o wo.o wood wood wood o o o o 6.: woooo woo woo H.H wfi HH .o ........ Q wonLH :oooaiomoa am am so :2 3 HO 5 Sr V am am Hm a0 MS or: AZ > .oZ 295m 2.6333 .8 agent: o... .ommooooo oooagoooooamluoo H.548 AGE RELATIONS nepheline syenite—0.7 percent niobium. Perovskite from garnet melteigite contains 0.3 percent niobium; from late-formed carbonatite—4 to 9 percent niobium. Some interesting speculations can be made on the relative ages of Magnet Cove rocks as a function of niobium concentration in minerals. On the basis of of niobium concentration in perovskite (table 64), the relative age of perovskite bearing rocks would be from oldest to youngest: J acupirangiteesyenite aij olite—>carbonatite. On the basis of niobium concentration in sphene, table 65, the relative age of sphene-bearing but perovskite- free rocks would be from oldest to youngest: {Sphene pyroxeniteasphene-nepheline syenite. Trachyte It seems possible that the variations in trace element concentrations in minerals common to more than one rock type may provide a useful guide in the interpre- tation of relative ages of closely associated rocks. Con- centration in minerals would be more informative than concentration in rocks because trace element concentra- tion is controlled not only by concentration in the magma but by the availability of favorable host minerals. RADIOACTIVITY Several small radioactivity anomalies of low intensity occur in the Magnet Cove area. The strongest anom- aly occurs in an irregular vein, 2 to 8 inches wide, of earthy, fine-grained, pale greenish—yellow monazite, that contains 0.098 percent equivalent uranium and 0.059 percent uranium; thorium was not detected by spec- trographic analysis. Other anomalies were detected in 1954 by an Atomic Energy Commission airborne radio- metric reconnaissance. Field check of these anomalies by E. P. Beroni and Roger Malan showed that the high- est radioactivity is in feldspar-rich rutile-bearing veins; lesser amounts were found in phosphate residuum over carbonate bodies such as the “tufa hills” in sees. 19 and 20 and at Perovskite Hill. Thorium probably is the chief radioactive component of the veins and phosphate residuum; uranium is es- sentially absent. One sample contained 0.14 percent eU; none of the others contained more than 0.03 percent eU and none of the samples contained more than 0.001 percent uranium. The thorium-bearing mineral has not been identified as yet. Uranium analyses of vein material are given in tables 40, 41, and 42. Low-level uranium and thorium determinations were made on most of the chemically analyzed rocks at Magnet Cove (table 47). For the complex as a whole, thorium (9 ppm average) is about 3 times as abundant as uranium (3 ppm average). The uranium content ranges from 2 to 7 ppm; it is highest in the late tinguaitic dike rocks and lowest in the feldspathoidal syenites OF THE ROCKS 83 (2 ppm). Thorium ranges from 2.8 to 33.9 ppm; it is highest in jacupirangite and lowest in the ijolite group (5 ppm average). Although the analytical differences are not great, uranium appears to be highest in the late- formed rock whereas thorium,with the exception of veins and weathered carbonatite, is highest in early formed rocks. Radioactivity in the nepheline syenite body at Potash Sulfur Springs, about 6 miles west of Magnet Cove, is due chiefly to uranium and its daughter products; thorium has not been detected. Small amounts of uranian pyrochlore (about 3 percent equivalent ura- nium) were found in one sample, as tiny, honey-yellow octahedrons; it is isotropic, index > 2.00, specific gravity > 3.95. A black mineral in the same gravity fraction gives an X-ray pattern of rutile. The rutile probably also contains Nb. AGE RELATIONS OF THE ROCKS The age relations of the rocks at Magnet Cove can not be clearly established from the geologic mapping. Rock outcrops are scarce and deeply weathered. Only a few well-exposed contacts between the various rocks can be seen. However, the available field evidence together with our interpretation of a great amount of chemical work on the rock and minerals suggests the following sequence from oldest to youngest: (1) phono- lite and trachyte; (2) jacupirangite; (3) alkalic syenites; (4) ijolite; (5) carbonatite, minor dike rocks and veins. Field evidence 5. Minor dike rocks, and carbonatite, and veins. The pegmatite, tinguaite, melagabbro and re- lated dike rocks out all the major rock types. Carbonatite intrudes ijolite and contains large inclusions of ijolite. 4. Ijolite. No good exposure of the contact of the large mass of ijolite in the center of the igneous complex with adjacent rocks was found. How- ever, in the northeast part of sec. 29 on the valley floor of Stone Quarry Creek, the garnet- pseudoleucite syenite is cut by a fine—grained ijolite dike (L—89), about 4 inches wide which strikes N. 50° W. and dips 85° SW. About» 450 feet downstream from L—89 the garnet- pseudoleucite syenite is cut by a fine-grained ijolite porphyry dike (MC—145). This vertical dike (up to 16 inches wide) strikes N. 85° W. In the northeast part of sec. 18, a vertical, fine-grained ijolite dike (MC—85) about 4 feet wide strikes N. 55° W. in cutting the garnet- nepheline syenite. In the western part of sec. 17, another vertical, fine-grained ijolite dike (MC—91) about 10 inches wide strikes N. 80° E. 84 and cuts the garnet-nepheline syenite. Thus at least part of the ijolite is younger than the garnet—pseudoleucite syenite and garnet-nephe- line syenite. Field evidence to suggest that ijolite is older than garnet-pseudoleucite syenite can be found in the face of the Diamond Jo quarry, where rocks that have an ijolitic composition but different physical appearance than the main mass of ijolite occur as large inclusions in garnet-pseudoleucite syenite. Inasmuch as these inclusions do not look like the typical ijolite and were observed only in the quarry more than one-half mile from the main ijolite mass, they are of little help in determining the age relation of the principal bodies of ijolite and garnet-pseudoleucite syenite. In the northwestern part of sec. 19 a piece of float, MC—179, was found showing a contact between altered phonolite and fine-grained ijolite. A chilled border in the ijolite at the contact indicates that the fine-grained ijolite is younger than the altered phonolite. In the north—central part of sec. 29, a fine-grained ijolite dike, L—103, cuts the undivided trachyte. The dike is about 3 feet wide and strikes N. 25° W. and dips 75° NE. 3., Alkalic syenites. The alkalic syenites (sphene- nepheline syenite, garnet—pseudoleucite syenite and garnet-nepheline syenite) form the outer ring dike of the Magnet Cove complex and probably were all emplaced about the same time. The writers’ prejudice is that sphene—nepheline syenite is slightly older than the other syenites. In the western part of sec. 18, about 200 feet upstream from sample locality MC-114, the garnet-pseudo- leucite syenite has a fine-grained chilled border in contact with the altered phonolite, and is thus inter- preted to be younger than the altered phonolite. Swarms of sphene-nepheline syenite dikes ranging from 1 inch to 3 feet in width cut jacupirangite along Cove Creek in sec. 17 and thus are younger than jacu- pirangite. 2. J acupirangite. J acupirangite is older than sphene- nepheline syenite and ijolite as pointed out in no. 3 above“ Its age relation to the phonolite- trachyte rocks of the intermediate ring is unknown because these rocks do not have mutual contacts. 1. Phonolite and trachyte is older than garnet- pseudoleucite syenite (no. 3 above) and ijolite (no. 4 above). Geochemical evidence The above age sequence of the Magnet Cove rocks is about the same sequence that would be shown if the rocks were placed in order of decreasing silica and increasing dominance of the undersaturated minerals. ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. This general plan of gradual desilication of the rock suite from oldest to youngest has been noted by Backlund (see discussion p. 87). The distribution of niobium in the rocks also shows this same age sequence if we assume that niobium tends to concentrate in residual magmas (see discussion p. 83). If the rocks are arranged in order of increasing niobium content in perovskite separated from these rocks, the sequence is: J acupirangite —>syenite ~9ij olite—>carbonatite On the basis of niobium content of sphene from sphene-bearing but perovskite-free rocks, the sequence is: Phonolite-trachyteasyenite Although neither the field evidence or geochemical evidence is conclusive, they mutually support each other. ORIGIN The origin of the igneous rocks at Magnet Cove is a part of the general problem of the origin of strongly alkaline undersaturated rocks. The origin of similar rocks in other parts of the world has been ascribed to various processes which include: (1) modification (desil- ication) of granitic magma by assimilation of limestone (Daly, 1910, 1918; Shand, 1945); (2) metasomatism of wall rocks, particularly migmatites, by carbonate magma (von Eckermann, 1948); (3) modification of granitic magmas by a “streaming of gases” rich in alkali (Smyth, 1913, 1927); (4) derivation from deep—seated fusion of a limy shale—evaporite sequence (Jensen, 1908); (5) dif- ferentiation of peridotite magma (Holmes, 1932, Holmes and Harwood, 1932, Strauss and Truter, 1951); (6) fractional crystallization of primary olivine basalt magma (Kennedy, 1933; Barth, 1936). The writers believe that the Magnet Cove complex of alkalic igneous rocks was derived by differentiation and fractional crystallization of a residual melanocratic phonolite magma rich in alkali, lime, and volatiles.- The high concentration of volatiles is believed to be of great importance in the development of the many varie- ties of unusual rock types—jacupirangite, ijolite, car- bonatite, etc. This residual magma is believed to have been derived by fractional crystallization from aregional undersaturated olivine basalt magma. Processes involving assimilation, metasomatism, or fusion may occur but they are not a necessary condition for generation of the Magnet Cove magma. Facts that apply to alkalic undersaturated rocks in general are summarized by Turner and Verhoogen (1951): 1. “Magmas of nepheline syenite composition, though not uncommon, typically develop in volumes very much ORIGIN smaller than is the case with basaltic and granitic magmas.” 2. “Nepheline syenite magmas belong to the low tem— perature end of the evolutionary series, for in their high con- tent of alkalies, low combined CaO, MgO and FeO, and high FeO/MgO ratio, nepheline syenites resemble both granites and the low-temperature residues of fractional crystallization recorded in laboratory investigations of silicate melts.” 3. “The high content of volatile elements (P, F, Cl) and of Zr, Ti, Nb, Ta, and rare earth metals in many nepheline syenites suggests analogy with pegmatites.” From these observations comes the obvious conclu- sion: Nepheline syenite magmas are residual magmas—0r as stated by Turner and Verhoogen * * * “end fractions of the processes of fusion or differentiation by which magmas are generated.” The question follows: residual magmas of what parent (‘1’) Holmes (1932) has suggested that alkali rocks are derived from parental peridotite magma by crystalliza- tion difierentiation—abstraction of olivine, enstatite, and diopside. The geochemical similarities of kimber- lite and ijolite are pointed out by Holmes as indicating a common peridotite parent. Strauss and Truter (1951) tentatively adopted Holmes’ hypothesis for the origin of the alkali complex at Spitzkop in the Eastern Trans- vaal. They further point out from their review of the literature that this hypothesis finds ample support in the association of alkali rock with ultrabasic types at the Premier mine, Transvaal; Magnet Heights, Transvaal; Eastern Uganda; Southern Rhodesia; and N yasaland. Moor (1957) believes that alkalic basalt dikes from the northern margin of the Siberian platform are closely related to ouachitites and alnoites, and that there is a possible genetic connection between the alkalic and ultrabasic complexes and the flood basalts. Alkalic and ultrabasic rocks were formed from those portions of magma which because of depth of the chamber, richness in volatile constituents, assimilation, etc., have undergone a great deal of differentiation; whereas the common flood basalts came from shallower parts of the magmatic cham— ber from a magma relatively poor in volatile con- stituents. The case for an olivine basalt parental magma receives support from several workers Who have demon- strated in the field that undersaturated feldspathoidal rocks are characteristic of olivine basalt associations. Barth (1936) has shown that differentiation of an undersaturated olivine basalt magma will produce an undersaturated residual liquid; further he has shown that an approximately saturated parent magma can produce either a phonolitic or rhyolitic residue de- 85 pending upon the extent of fractionation of early- formed olivine and pyroxene. Thus only a very slight change in the original composition of olivine basalt magma can produce Widely divergent end fractions. Kennedy (1933) believes that under- saturation in parental basalt persists into the end fraction. The initially low silica content is used up in the formation of olivine and pyroxene, so that While alkalies and alumina are increasing, silica barely manages to hold its own and even may decrease slightly in the end fraction from its original concentration in the olivine basalt magma. Turner and Verhoogen (1951, p. 340) have concluded that the normal course of evolution of undersaturated olivine-basalt magma commonly leads to development of a small proportion of alkaline magma which would crystallize under plutonic conditions as nepheline syenite. This can occur in an oceanic environment where, so far as we know, neither granite, limestone, nor any rock other than basalt is available for reaction with the evolving magma. The most likely mechanism is one of fractional crystallization accompanied, and perhaps modified, in the final stages by high concentration of volatiles. How does the alkalic complex at Magnet Cove fit into this picture? Can any association with olivine basalt rock be demonstrated? The answer is an emphatic “yes.” The Magnet Cove complex is not an isolated geologic curiosity but only a small part of a regional belt of strongly alkaline undersaturated rocks. This belt of igneous rocks of similar chemical composition and of similar age extends in an are from West Texas to Central Mississippi and, as previously noted by Miser (1934), fall within or near the b0und- aries of the old Ouachita geosyncline (pl. 2). This Paleozoic structure provided zones of structural weak- ness for the intrusive and extrusive rocks. Undersaturated rocks, many of them strongly alkalic, have been found during deep drilling in the search for oil and gas in the northern Gulf Coastal Plain (Moody, 1949). These rock types include peridotite, olivine basalt, nepheline syenite, phonolite, tinguaite, monchi- quite, fourchite, and much alkalic and basaltic pyro- clastics. Moody reports that virtually every well in western Mississippi and in eastern Louisiana which has been drilled through the Upper Cretaceous section has pierced beds which are in part, at least, composed of fragmental igneous rocks produced as lapilli or ash in the throats of volcanoes which dotted the late Mesozoic landscape. Lonsdale (1927) described the igneous rocks of the Balcones fault region, Texas, which consist of peridotite, limburgite, olivine basalt, nepheline basalt, nepheline- melilite basalt and phonolite. The age of the alkaline rocks is Cretaceous(?) and early Tertiary Closer to Magnet Cove pulaskite and fourchite occur to the northeast, and'peridotite to the southwest. 86 Magnet Cove rocks Olivine basalt A 18— 16 — 20.; l4 — \ D 12 — 0&0 ._ Z M 0 E 10 ~— 0. .2. m” __ DJ 0 2 O 8 .- 6 _ 4 _ 2 _ Kg "4 o I I I 46.5 470 47.5 48.0 . SiOz. IN PERCENT FIGURE 10.—Variation diagram showing composition of material (A) subtracted from an olivine basalt to produce magma of the weighted-average composition of the Magnet Cove complex. We conclude that the regional magma generated in the Ouachita geosyncline was an undersaturated olivine basalt, and further, that fractionation of this magma produced peridotite and other ultrabasic rock on the one hand, and a highly alkaline undersaturated phono- litic end fraction on the other. Lonsdale (1927) con- cluded. that the parent magma in the Balcones fault ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. region was olivine basalt. Moody (1949) concluded that two different magmas appeared in the northern Gulf Coastal plain. The earlier was pro—Cretaceous basalt and the later was alkaline and basaltic and both reached to the surface in certain areas and gave rise to volcanic activity. With this regional picture in mind we can now discuss the Magnet Cove igneous complex as a residual magma derived from a parental regional undersaturated oli- vine basalt. The weighted average composition of the Magnet Cove igneous complex, table 46, is best described as that of a melanocratic phonoh'te. The graph (fig. 10) shows that a residual magma of this composition can be obtained by subtraction of 70 parts of an olivine melagabbro from 100 parts of parental olivine basalt. The parental olivine basalt used in these calculations is the average of four chemical analyses of olivine basalt given by Lonsdale (1927) for the Balcones fault region—— a part of our regional belt of undersaturated rocks. Unfortunately these analyses did not include 002, F, Cl, and minor element concentrations. As may be seen from the graph (fig. 10) the composition of the sub- tracted olivine melagabbro (or material A) is very similar to the parent olivine basalt. MgO and CaO are slightly higher; A1203, N a20. and K20, are slightly lower. Thus the subtracted magma seems to have a very reasonable and predictable composition; no unique or unusual crystal separation need be made. N epheline in the norm indicates that the parental olivine basalt is undersaturated and that differentiation of this under- saturated parental magma would produce a highly alkalic, silica—deficient end fraction even without crystal fractionation. Note that the end fraction would also be rich in titanium and phosphate. As previously pointed out, no data were available for volatile and minor element content of the parent olivine Chemical analyses End fraction Parent A (material Magnet Cave olivine basalt subtracted) composition 47. 88 48. 88 46. 69 14. 66 13. 77 18. 28 2. 51 2. 16 3. 88 9. 24 10. 63 4. 37 6. 96 7. 58 2. 55 12. 60 13. 22 10. 64 3. 03 2. 09 6. 64 . 99 . 00 4. 60 1. 54 1. 49 1. 83 . 59 . 58 . 62 ORIGIN basalt. However, a perovskite concentrate from the peridotite near Murfreesboro, Ark., contained 2.5 per- cent columbium. Perovskite and sphene also occur in the basalts and nepheline basalts in the Balcones fault region of Texas (Lonsdale, 1927). Rankama and Sahama (1950) note that “remarkably much columbium is present in ultrabasic rocks.’ ’ Carbon dioxide, water, and sulfur are common emanations from volcanic vents. Thus, it would appear the parental olivine basalt does contain all the essential typical elements of Magnet Cove rocks—Ti, P, Nb, CO2, and S in particular. We have now derived our Magnet Cove magma. The next question: how were the widely divergent rock types formed from this magma and in what sequence of intrusion? Williams (1891) concluded the complex is the result of a series of intrusions: (1) ijolite, (2) lamprophyres, and (3) syenites. Washington (1900) concluded the complex is the result of a laccolith differentiated in place. Landes (1931) concluded the complex is the result of two injections: (1) ijolite as a stock differentiated in place with the carbonatite as an included block, and (2) syenites differentiated in place from a later intrusion of an acidic differentiate of the main magma. Wash— ington and Landes followed Williams’ terminology in calling the phonolite—trachyte ring metamorphosed sediments. The present writers conclude that the Magnet Cove alkalic igneous complex is a series of ring dikes, intruded at different times with varying degrees of differentiation during individual intrusion phases. This conclusion is based upon the arcuate dike shape of the exposed rocks, the sequence of intrusion, the chemical relationships of the rocks, and striking similarity to ring complexes in other parts of the world (for details, see section on “Structure”). The sequence of intrusion is interpreted as: (1) trach- yte—phonolite and breccia of the intermediate ring, (2) jacupirangite, (3) feldspathoidal syenites of the outer ring, (4) ijolite forming the central core, (5) carbonatite, pegmatite, tinguaite and similar late dike rocks. Back- lund (1933), in his excellent review of the common de— nominators of alkalic rocks throughout the world, noted that the common principle governing the distribution of rocks seems to point to a gradual desilication of the rock suite from the wall inward and from the top downward: the core thus represents the most perfect grade of desilication. Further he concludes that the general plan of rock sequence is that of increasing dominance of the undersaturated mineral association within the whole rock series, the most pronounced undersaturation being shown by the latest ones. This undersaturation proceeds step by step towards complete desilication of the rock suite, the carbonatic (Alno, Fen, 87 Spitzkop), apatitic (Umptek, Palaboro, Spitzkop, Alno) and iron-ore (Spitzkop, Alno, Fen) rocks being in each case the last more or less independent member of a complete alkali suite. The Magnet Cove rock suite closely fits this pattern of increasing undersaturation from the walls inward: syenites, phonolite, and trachyte in the outer and inter- mediate rings; ijolite core and almost complete desilica- tion in very late carbonatite, apatite, and magnetite concentrations in the central part of the core. Backlund (1933) points out that although there exists a certain sequence of more or less independent magmatic intrusions, no real time distinction can be made between the different rock groups. This problem is further com- plicated at Magnet Cove because of deep weathering and poor rock exposures. However, the phonolite- trachyte breccia of the intermediate ring gives sufficient evidence to indicate its early role in the sequence of intrusion at Magnet Cove. Further, we believe that the ring fracture occupied by these fine-grained rocks reached the surface and gave rise to explosive volcanic activity at Magnet Cove. This early period of ex- plosive volcanic activity is suggested by the abundance of volatiles particularly 002, the presence of miarolitic cavities and amygdaloidal textures, and the absence of recognizable syenites or ijolite inclusions in the breccia. The chemical composition of the phonolites and breccia most closely approach the aVerage composition of the igneous rocks (fig. 9)—a condition to be expected if this material was the first liquid to be suddenly released from the magma reservoir and extruded. That part of the magma trapped in the conduit at the end of explosive periods would react with 002, H20, and S to produce the altered phonolite, Most of the breccia pieces have the same composition as the groundmass which suggests repeated movement in the early ring fracture zone and repeated injections of heavily gas charged magma into this fracture zone. AbOut 29 percent of the igneous rocks were emplaced during this period, assuming that the area exposed is proportional to volume. Line B on fig. 11 shows the composition of the remaining magma after removal of the phonolite-trachyte to form the intermediate ring. Note that the remaining magma has not changed ap- preciably; SiOz and A1203 have depressed slightly and CaO and Fe203 have been increased. Further, note that throughout the diagram, the composition changes in the magma are much less severe than the composition changes in rocks formed from the magma. The second recognizable period of intrusive activity was the emplacement of jacupirangite which comprises about 10 percent of the exposed igneous rocks. It is cut by swarms of narrow nepheline-syenite dikes and is, therefore, interpreted as older than the outer ring of feldspathoidal syenites. However, its appearance at 88 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, this stage was probably accidental. The field evidence suggests that crystallization in the now slower cooling magma follows the agpaitic series of Ussing (1911)— early crystallization of feldspars and nepheline which because of their lower specific gravity than the residual liquids, are forced to rise. The mafic residue and volatile constituents accumulate downward. Backlund (1933) suggested that “agpaitic differentiation leads to decreasing viscosity of the cooling magma” whereas in the commoner granodioritic differentiation, increas- ing viscosity of the residual liquid is effected by the loss of volatiles that together with the more acid com- ponents have accumulated in the top portion of the the magma chamber. In some exposures of garnet-pseudoleucite syenites, euhedral pseudoleucite phenocrysts are concentrated in a fine-grained groundmass; these crystals persist even in chilled border phases of the syenite. This suggests that pseudoleucite (nepheline and sodic orthoclase) formed early and were rising and collecting in the magma (agpaitic differentiation). Ijolite and mel— teigite segregations and inclusions in the same rock, however, were probably crystallizing and sinking and were carried along in the rising current of magma, when it was intruded. It seems probable then, that the lower part of the reservoir may have been tapped by fractures so that a part of the downward accumu- lating gas—charged mafic residue, with its pyroxene and magnetite crystals, streamed up the fractures and was emplaced as satellite bodies of jacupirangite. Line 0 of figure 11 shows the composition of the residual magma after removal of jacupirangite. As expected, Si02, A120, and alkalies have increased slightly, and CaO, FeO, and MgO have decreased. Emplacement of the feldspathoidal syenites of the outer ring, chiefly sphene-nepheline syenite and garnet- pseudoleucite syenite, formed the third period of intru- sive activity. The “agpaitic” series is particularly well-demonstrated during this period. The lighter magma at the top of the magma reservoir with its early crystallized feldspar and nepheline is tapped by fractures and this magma rises to form the outer, almost complete ring dike of feldspathoidal syenites. About 30 percent of the rocks were emplaced during this period. The residual magma is now largely mafic but with ever increasing amounts of volatile constit- uents, P, Ti, Fe”, Zr, and rare earths. Sl02 and A1203 have been depressed; CaO increases sharply; and the alkalies show minor change. The stage is now set for intrusion of most of the remaining magma as ijolite. The magma has been so desilicated that al— though the alkali content is high, not enough silica is present to form feldspar—only nepheline can crystallize. The pyroxene is now almost pure diopside; excess Fe, > :u N ‘ ' syenite Parent olivine basalt Phonoliturachyte Ijolite Weighted averag: composition-Magnet Cove locks a 0‘ \v - I r . 1 I/. \ \ \ \ \ \ Q— Q P \\\ \ /// ,. FeO I \\ ”— l \ \ / > Fe O z”’ XI? / k<<7k \ \>\ ONahO‘m OXIDES. IN PERCENT S E 5“ 5 8 . / ._. O oméomaa‘monho‘m ... l3: ‘0 I \ l 1225' 100 70 60 29 VOLUME 0F RESIDUAL MAGMA, IN PERCENT A FIGURE 11.——Varlation diagram showing composi- tion of rocks (dashed line) and residual magmas (solid line) for main periods of intrusion. ORIGIN Ti, and Ca must be used in the very low silica garnet; excess K20 forms the silica—poor biotite. Effects of the high volatile content appear in the primary alter- ation of nepheline to cancrinite and sodalite. Much of the rock unit is pegmatitic and monomineralic segre— gations are common. Large masses of garnet, nephe- line, magnetite, biotite (phlogopite) attest to the importance of the gas phase in prolonging fluidity of the magma and permitting development of giant crystals. Niobium content of perovskite crystals in- creases; beryllium concentrates in the zeolites. Thus the chemical analyses of the ijolites show a wide compositional range and a consequent wide scattering of points on any variation diagram. The composition of the remaining end fraction (9 parts of the original 100, line E, fig. 11) appears unique. SiOz has increased again but CaO and 002 show very marked concentration and A1203 a very marked de- crease. In this highly volatile end fraction, the elements are relatively free to move about and form even stronger monomineralic segregations than in the ijolite; this is the period of late magmatic, pneumatolytic and hydrothermal activity. Most of the alkali combines with remaining silica and alumina to form pegmatites rich in K—feldspar, nepheline, and giant aegirine crys- tals. Zirconium concentration is now great enough to form eudialyte. The CaO combines with P205 to make abundant apatite, with Ti to make perovskite, and with silica to make monticellite. Finally the CaO still in ex- cess combines with 002 to form crystalline carbonatite bodies. Niobium concentrates in the perovskite up to 9 percent; rare earths and fluorine are concentrated in the apatite, zirconium forms kimzeyite and iron forms euhedral magnetite crystals. Probably much of the titanium and niobium is transported in the gas phase (fluorides?) or as hydrothermal solutions to form niobium-bearing rutile and brookite veins. Perhaps even small amounts of silica are transported in the gas phases to form the very rare occurrences of quartz- bearing veins. We have now completed our picture of the petro— genesis at Magnet Cove. The derivation of rock types has no doubt been oversimplified. We stress the effect of gaseous phases without benefit of much supporting laboratory experimental data but certainly complex mineral assemblages may be expected to form in the presence of magmatic gases—particularly in an alkali- rich, undersaturated magma. ECONOMIC GEOLOGY Potentially commercial deposits of titanium, nio- bium, molybdenum, phosphate, agricultural lime, and iron ore occur in the igneous complex at Magnet Cove. The titanium deposits have been known for many 89 years and through 1944, more than 5,000 tons of rutile (Ti02) concentrate had been produced from the rutile pits in sec. 18. No production has come from the brookite (Ti02) deposits in recrystallized novaculite along the northeast border of the complex. Detailed descriptions, and economic considerations of the tita- nium deposits have been given by Fryklund and H01— brook (1950). Because of the association of niobium with titanium minerals, the US. Geological Survey in 1952 studied, sampled, and in part mapped the rutile, brookite, and perovskite (CaTiO3) areas at Magnet Cove (Fryklund and others, 1954). Table 71 reproduced from the report of Fryklund and others summarizes the work of the US. Bureau of Mines, Fryklund and Holbrook (1950), and the US. Geological Survey. Nieberlein and others (1954) reported that the ore reserves in the three columbium-bearing titania deposits investigated by the Bureau of Mines (Magnet Cove Titanium Corp. property, Christy brookite deposit, Hardy-Walsh property) are estimated to be at least 8 million tons containing 4 to 8 per- cent TiOz and 0.05 to 0.15 percent Cb. The columbium con- tent of these reserves is estimated to be 12 million pounds. TABLE 71.—Summary of analytical data from the principal titanium deposits at Magnet Cove, Ark. [Data from Fryklund, Hamer, and Kaiser, 1954, p. 49, table 10] Estimated average recoverable grade of known material Estimated average grade of titanium Mineral of concentrates Locality possible (percent) (percent) value T102 Nb V105 TiOa Nb V105 Magnet Cove Titanium Rutile _______ 3 0.04 009 92 1 2 05 Corp. property. Ohristy property __________ Brookite _ . . _ 5 9 . 2 5 Kllpatrick (Hardy-Walsh) _____ do ______ 5 9 . 06 21 92 2 . 5 (?) property. Mo-Tl Corp. property ..... Rutile. _ A __ _ ______________________________________ Do _____________________ Perovskite. . ______ . 03 .......................... Klmzey calcite quarry area. _____ do. . __ _. 20 . 01 ____________ 6 ________ Milling tests on rutile and brookite ore from Magnet Cove have produced high-grade concentrates of about 92 percent TiO2 but have given low overall titania recovery, due largely to the occurrence of Ti02 in several unrecovered minerals such as ilmenite, leucoxene and silicates. Fryklund, Harner, and Kaiser (1954) point out, however, that “it is probable that processes. for making metallic titanium from the rutile and brookite concentrates will allow separation and recovery of niobium and vanadium.” There has been no production of molybdenite ore from Magnet Cove. Holbrook (1948) has described a small molybdenite vein deposit (Mo-Ti prospect) in fractured j acupirang- ite in the NW1/4 sec. 17. He states that the veins range in thickness from less than )4 inch to 5 feet, trend northwest, dip sharply northeast, and have 90 ALKALIC IGNEOUS COMPLEX AT MAGNET COVE, ARK. a total strike length of about 400 feet. The most significant concentration of molybdenite is in an area about 225 feet long and from 10 to 35 feet Wide. N o tonnage estimates were made because of the unreli- ability of samples obtained by diamond drilling. Other small molybdenite-bearing veins occur in the floor of the carbonatite quarry (NW1/4 sec. 19), in a stock pond in the western part of sec. 20, and with rutile veins about 500 feet south of the Mo—Ti prospect in sec. 17. Weathering of the carbonatite produces a phosphate- rich residual material with a rare-earth content near 1 percent. Standard phosphate analyses (table 24) show that the grade is adequate for commercial phos- phate. These residual phosphate areas are shown on the geologic map, but more drilling and sampling must be done to determine the size of the deposit. The parent carbonatite also offers possibilities for produc— tion of agricultural lime. N iobium-rich (4 to 8 percent) perovskite crystals in the carbonatite might be recovered as a byproduct. Magnetite has been mined sporadically from the Kimzey magnetite pit in the N W1 /4 sec. 20. It occurs as irregular rounded masses up to 1 foot across, some alined like beads on a string, in an idocrase—diopside matrix. No data are available on past production or estimated reserves. Geologic mapping and the aerial magnetometer survey, however, do not suggest an ex- tensive magnetite ore body. Low—grade uranium-bearing material and zeolite (thomsonite) with as much as 0.02 percent beryllium have been found but not in potentially commericial amounts. REFERENCES CITED Backlund, H. G., 1933, On the mode of intrusion of deep-seated alkaline bodies: Univ. 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W., 1940, Lower Cretaceous and Jurassic formations of southern Arkansas and their oil and gas possibilities: Arkansas Geol. Survey Inf. Circ. 12, 64 p. Jensen, H. I., 1908, The distribution, origin, and relationships of alkaline rocks: New South Wales Proc. Linn. Soc., v. 33, p. 585—586. Johannsen, Albert, 1938, A descriptive petrography of the igneous rocks: v. 4: Chicago, Chicago Univ. Press, 523 p. Keller, Fred, Jr., and Henderson, J. R., 1949, Total intensity aeromagnetic map of Magnet Cove area, Hot Springs County, Arkansas: US. Geol. Survey Geophys. Inv. Prelim. Map. Kennedy, W. Q., 1933, Trends of diflerentiation in basaltic magmas: Am. Jour. Sci., v. 25, p. 239-256. Kinney, D. M., 1949, The Magnet Cove Rutile Company mine, Hot Springs County, Ark.: U.S. Geol. Survey open-file report. Kouvo, J. A. O., 1952, A variety of monticellite from Crestmore, Ca1if.: Finlande Comm. Géol. Bull., no. 157, p. 7—11. Landes, K. K., 1931, A paragenetic classification of the Magnet Cove minerals: Am. Mineralogist, v. 16, p. 313—326. Lonsdale, J. T., 1927, Igneous rocks of the Balcones fault region of Texas: Texas Univ. Bull. 2744, 178 p. Macrery, J., 1806, A description of the Hot Springs and volcanic appearance in the country adjoining the river Ouachita in Louisiana: New York Medical Depository, v. 3, p. 47—50 (communicated in a letter to Dr. Miller, according to Williams (1891)). REFERENCES CI’I‘ED 91 McConnell, Duncan, 1942, Griphite, a hydrophosphate garnetoid: Am. Mineralogist, v. 27, no. 6, p. 452—461. McConnell, Duncan, and Gruner, J. W., 1940, The problem of the carbonate-apatites. III. Carbonate-apatite from Magnet Cove, Arkansas: Am. Mineralogist, v. 25, no. 3, p. 157—167. Melville, W. H., 1892, Mineralogical notes, Natrolite from Magnet Cove, Arkansas: US. Geol. Survey Bull. 90, p. 38. Milton, Charles, and Blade, L. V., 1958, Preliminary note on kimzeyite, a new zirconium garnet: Science, v. 127, no.3310, p.1343. Miser, H. D., 1912, New areas of diamond-bearing peridotite in Arkansas: US. Geol. Survey Bull. 540, p. 534—546. 1934, Relation of Ouachita belt of Paleozoic rocks to oil and gas fields of midcontinent region: Am. Assoc. Petroleum Geologists Bull., v. 18, no. 8, p. 1059—1077. Miser, H. D., and Glass, J. J., 1941, Fluorescent sodalite and hackmanite from Magnet Cove, Arkansas: Am. Mineral- ogist, v. 26, p. 437—445. Miser, H. D., and Stevens, R. E., 1938, Taeniolite from Magnet Cove, Arkansas: Am. Mineralogist, v. 23, p. 104—110. Moody, C. L., 1949, Mesozoic igneous rocks of northern Gulf Coastal Plain: Am. Assoc. Petroleum Geologists Bull. , v. 33, no. 8, p. 1410—1428. Moor, G. , 1957, Alkalic basalt dikes from the northern margin of the Siberian platform (Norilsk region): Acad. of Sciences of the U.S.S.R., Proc. Geol. Sci. Sections (Consultants Bur., Inc.; Eng. trans), v. 116, no. 1—6, p. 803—805. Murata, K. J., Rose, H. J., Jr., Carron, M. K., and Glass, J. J., 1957, Systematic variation of rare-earth elements in cerium- earth minerals: Geochim. et Cosmochim. Acta, v. 11, no. 3, p. 141—161. Murdoch, Joseph, 1951, Perovskite: p. 573—580. Nieberlein, V. A., Fine, M. M., Calhoun, W. A., and Parsons, E. W., 1954, Progress report on development of columbium in Arkansas for 1953: US. Bur. Mines Rept. Inv. 5064, 23 p. Nockolds, S. R., 1954, Average chemical compositions of some igneous rocks: Geol. Soc. America Bull., V. 65, p. 1007—1032. Parks, Bryan, and Branner, G. C., 1932, A barite deposit in Hot Spring County, Arkansas: Arkansas Geol. Survey Inf. Circ. 1, Pecora, W. T., 1956, Carbonatites: A review: Geol. Soc. America Bull., v. 67, no. 11, p. 1537—1556. Penfield, S. L., 1894, Anatase von Magnet Cove, Arkansas: Zeitschr. fiir Kristallographie, V. 23, p. 261. Penfield, S. L., and Forbes, E. 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A Page Access to report area .......................... 2 Accessory minerals in, garnet-pseudoleucite syenite ........................... 11 jacupirangite ................ l7 sphene-garnet-nepheline syenite .......... 16 sphene-nepheline syenite ................. 8 tinguaite .................. 40 Acknowledgments ............................ 4-5 Age of the rocks ............................ 5, 83-84 Agpaite ...................................... 62, 88 Altered phonolite, chemistry .............. 24—25, 63 distribution and description ........... 23—24, 84 origin ..................................... 25 Analcime, spectrographic analyses ............ 71 Anaicime—olivine melagabbro, analyses, norm, and mode _________________________ 43 chemistry ................................ 4344 distribution and description .............. 42—43 Apatite, analyses ............................. 74, 76 veins ..................................... 54 Apatite-pyrite vein, analyses ................. 57 Aplite ..................... 47 Arkansas novacuiite ........................ 5, 18, 56 B Backlund, H. G., quoted ..................... 87 Balcones fault region ...................... 85, 86, 87 Barium, mineral occurrences ............ 67 Beryllium, mineral occurrences.. 65 Bigiork chert ........................ 5 Biotite, analyses .............................. 74, 76 Biotite, garnet ijolite, analysis of garnet from. 74 analyses, norms, and modes .............. 29 description ............................... 28 spectrographle analyses of mineral separ- ates of ............................ 31 Blaylock sandstone ............. 5 Boron, mineral occurrences- 67 Brookite, analyses .......... 82 deposits .............. 54 niobium in ............................... 78—79 C Calcite, analyses .............................. 77—78 Carbonatite, analyses ........... 35 chemistry .................. .. 37-38 distribution and description .. 34-37 niobium in ................. 79 origin ............................. 38—39 Chemical analyses, altered phonolite_. 24 analcime-oiivine melagabbro ...... 43 biotite .................................... 76 biotite-garnet iiolite and garnet iioiite ..... 29 carbonatite ............................... 35 saprolite of ........................... 36 dikes outside complex .................... 53 eudialyte-nepheline syenite pegmatite. --. 48 feldspathoidal leucosyenite ............... 16 fine-grained ijolite ........................ 33 garnet-biotite melteigite .................. 22 garnet-nepheline syenite .................. 13 garnet-pseudoleucite syenite .............. 12 jacupirangite ............................. 18 nepheline... . ................ 70 nepheline syenite pegmatite .............. 45 pyroxenes ................ 72 sedimentary rocks ........................ 59 INDEX Page Chemical analyses—Continued sodalite trachyte .......................... 42 sodic orthoclase ........ . 69 sphene-nepheline syenite. . 9 sphene pyroxenite ..... . 19 tinguaite .......... . 41 trachyte porphyry... . 46 undivided trachyte-phonolite . 27 veins ...................... . 55, 56 zeolite ........... . . 70 Christy deposit ................ . . - 54, 89 Chromium, mineral occurrences. . 68 Cobalt, mineral occurrences. . . - 68 Colorimetric analyses, altered phonoiite ...... 24 analcimeoollvine melagabbro _________ 43 biotite-garnet ijolite and garnet ijolite ..... 29 carbonatite ............................... 35 fine-grained ijolite ........................ 33 garnet—biotite melteigite .................. 22 garnet fourchite .......................... 49 garnet-nepheline syenite .................. 13 garnet-pseudoleucite syenite .............. 12 igneous rocks ............................. 60-61 jacupirangite .................... 18 nepheline syenite pegmatite .............. 45 sphene-nepheline syenite ................. 9 sphene pyroxenite ..... 19 tinguaite ................................. 4i undivided trachyte-phonolite ............. 27 veins ...................... . 56 Contact zone ................................. 56—59 Copper, mineral occurrences .................. 68 Core drilling ............... 34 Covite ........................................ 7 D Diamond Jo quarry ........................ 10, 11, 54 Bikes, outside the complex, analyses. _. . 53 outside the complex, general features ..... 49 location .............................. 50 rock descriptions ..... .. 51—52 swarms cutting jacupirangite ............. 17 Within the complex .................... 39—49, 83 Economic geology ...... Eleolite mica syenite ......................... 17 Eleolite syenite ............................... 7 Eudialyte-nepheline syenite pegmatite, anal- yses, norm, and mode ............. 48 chemistry ................................ 4849 distribution and description .............. 47—48 F Feldspar, analyses of ......................... 69 Feldspathoidai leucosyenite, description and analyses .......................... 15—16 Felsic rocks, minor-element content of minerals in ................................ 66 Fine-grained ijolite, analyses, norms, and mode ............................. 33 chemistry ................................ 33 distribution and description .............. 30-31 Fryklund, V. 0., Jr.. and others, quoted ..... 89 G Gallium, mineral occurrences ................. 68 Garnet, analyses. . . .......................... 74, 75 Garnet-biotite melteigite, analyses, norm, and mode. ________________ 22 chemistry ................................ 22~23 distribution and description .............. 21 Page Garnet fourchite, description and analyses--. 49 Garnet ijolite, analyses, norms, and modes.__- 29 contact with banded trachyte ....... _- 26 description _______________________________ 28 spectrographic analyses of mineral separ- ates of ____________________________ 32 Garnet-nepheline syenite, age ................. 84 analyses, norm, modes .................... 13 distinguishing features .................... 11 Garnet-pseudo leucitesyenite, analyses, norms, modes ____________________________ 12 chemistry ................................ 13—15 distribution and description ________ 10—13, 84, 88 Geochemistry, minerals ...................... 65—78 niobium, general features _________________ 78 rocks _____________________________________ 59—62 Geology, general features _____________________ 5 H Hardy-Walsh deposit ......................... 54 Hot Springs sandstone ........................ 5 I Igneous complex, age of rocks ................. 83—84 chemistry and analyses._ _ 60—61 dikes outside ............................. 49—54 dikes within ______________________________ 39—49 distribution of trace elements in. 62 general features ___________________________ 6—7 inner core ________________________________ 27—39 intermediate ring._ 23—27, 62 general features _______________________ 23 niobium in rocks of _______________________ 79,83 origin ___________ . 84—89 outer ring ................................. 7—23 general features ....................... 7 Ijolite, age .................................... 83—84 average and type, chemical analyses and modes ____________________________ 30 chemistry ______________ - 28—30, 63,64, 65 distribution and description ______________ 28 See also Fine-grained ijolite. Index map ................................... 3 Indices of refraction, aegirine .......... 15 aegirine-diopside __________________________ 14 diopside ____________ ._ 21, 31, 32, 40 diopside-hedenbergite ______________ 10, 14, 15, 20 nepheline ......................... ._ 31, 32 salite _____ 20 Introduction ............. 1—2 Intrusive activity, periods of ................ 87—88 Isogonic chart ................................ 4 J J acupirangite, analyses, norm, and modes. . . . 18 chemistry .......................... 19—21, 63, 64 distribution and description .............. 17, 84 emplacement of ........................... 87-88 Joints in garnet-pseudoleucite syenite ........ 11 K Khibina alkauc massii.. ............... 65, 68, 71 Kimzey magnetite pit ........................ 4, 90 Kimzeyite, analysis for minor elements ....... 75 composition .............................. 38 93 94 L Page Lanthanum, mineral occurrences _____________ 67 Lead, mineral occurrences ______________ 68 Lead-alpha age determinations- - - ________ 5 Lime-alkali sums of igneous rocks- ________ 62 Lime-silicate rock, description ________________ 39 spectrographic analyses of mineral sep- arates from _______________________ 40 Lime variation diagram ______________________ 68 Location of report area ___________ 2 Logs of core holes _____________________________ 34—35 M Maflc rocks, minor-element content of min- erals in ___________________________ 66 Magnetite, analyses... _._ 74, 76, 77 deposits ............. . ...... 90 Magnetometer map, distribution of jacupi- rangite ........................... 17 Manganese, mineral occurrences. . 68 Mapping methods ........... . 2, 4 Mazarn basin ............. .. 2 Mazarn synclinorium ......................... 5 Melanocratic phonolite ....................... 59 Melteigite, average and type, chemical anal- yses and modes ................... 30 Methods of work ........................ 2, 4 Microcliue, spectrographic analyses ........... 69 Mineral deposits in veins.... Miser, H. D., quoted ........ Missouri Mountain shale ..................... Mode, analcime-olivine melagabbro .......... 43 biotite-garnet ijolite and garnet ijolite. 29 eudialyte-nepheline syenite pegmatite. .. 48 feldspathoidai leucosyenite ........ 16 flue-grained ijolite ........................ 33 garnet-biotite melteigite .................. 22 garnet-nepheline syenite ..... 13 garnet-pseudoleucite syenite .............. 12 jacupirangite ............................. 18 sphene-nepheline syenite . . - . . 9 sphene pyroxenite ............... . 19 undivided trachyte-phonolite..-. _ 27 Molybdenite veins .................. . 89—90 Molybdenum, mineral occurrences ........... 68 Molybdenum-titanium prospect .............. 54 Monchiquite ................................. 42 Moody, C. L., quoted ________________________ 85 N Natroiite, spectrographic analyses ............ 71 Nepheline, analyses .......................... 70 Nepheline syenite pegmatite, analyses and norm ............................ 45 chemistry ..................... 44—46 distribution and description. ......... 44 Nickel, mineral occurrences ................... 68 N ieberlein, V. A., and others, quoted ......... 89 Niobium, in titanium minerals ............ 78-79, 83 Norm, altered phonolite ...................... 24 analcime-olivine melagabbro .............. 43 biotite-gamet ijolite and garnet ijolite ..... 29 eudialyte-nepheline syenite pegmatite- - ._ 48 feldspathoidal leucosyenite ............... 16 fine—grained ijolite ........................ 33 gamet-biotite melteigite- - ..... 22 garnet-nepheline syenite.- ..... 13 gamet—pseudoleucite syenite. . .. 12 jacupirangite ............................. 18 nepheline syenite pegmatite .............. 45 sodalite trachyte ........... . 42 sphene-nepheline syenite .......... .. 9 sphene pyroxenite ................. 19 tinguaite ........... 41 trachyte porphyry-- - 46 undivided trachyte-phoneme ............. 27 INDEX 0 Page Olivine basalt, as parental magma ............ 85 Origin of igneous rocks ..................... 84—89 Orthoclase, barian and sodic, spectrographic analyses .......................... 69 Ouachita geosyncline ....................... 5, 85, 86 P Pakiser, L. 0., J12, quoted .................... 6 Pecora, W. T., quoted ........................ 38 Perovskite, analyses .......................... 77 niobium in ............................... 79 Phonolite. See Altered phonolite. Phosphate analyses, saprolite of carbonatite.- 36 Phosphate deposit ............................ 90 Polk Creek shale- .. . 5 Potash Sulfur Springs- 83 Previous work ................................ 2 Pyrite, analyses ............................ 77, 78 Pyroxenes, chemistry and analyses ..... 70—74 in garnet-pseudoleucite syenite ..... 11 in sphene-nepheline syenite ............... 8 R Radioactivity ................................ 83 Radiometric analyses, altered phonolite ...... 24 analcime-olivine melagabbro .............. 43 biotite—garnet ijolite and garnet ijolite ..... 29 carbonatite ________ 35 saproiite of ...... 36 dikes outside complex ................... 53 eudialyte-nepheline syenite pegmatite.. .. 48 fine-grained ijolite ........................ 33 garnet fourchite ....... .. . 49 garnet-nepheline syenite--- _._ . 13 garnet-pseudoleucite syenite .............. 12 igneous rocks ............................. 60—61 jacupirangite .......... 18 nepheline syenite pegmatite ______________ 45 sedimentary rocks ........................ 59 sodalite trachyte ............. 42 sphene—nepheline syenite ...... 9 sphene pyroxenite ..... . 19 tinguaite ................................. 41 trachyte porphyry ........................ 46 undivided trachyte-phonolite.. 27 veins ............................ -. 55, 56 References cited ..................... .. 90-91 Ring dikes, formation of ______________________ 6 Rutile, analyses. ........................ 80-81, 82 niobium in .............................. 78—79 S Saprolite, of analcime-olivine melagabbro, minerals in ........ - 43 o! carbonatite, analyses of- . - . 36 oi’ jacupirangite, minerals in ..... .. .. 18 of nepheline syenite pegmatite, minerals in ................................ 44 of sphene-nepheline syenite, minerals in ................................ 8 Scandium, mineral occurrences ............... 67 Silver, mineral occurrences ................... 68 Sodalite trachyte, description, analyses, norms ............................ 42 Spectrographic analyses, altered phonolite. .. 24 analcime-olivine melagabbro. . . 43 mineral separates of ..... . 44 apatite ................................... 76 biotite, minor elements in ................ 76 biotite-garnet ijolite and garnet ijolite. 29 mineral separates of .................. 31, 32 brookite .................................. 82 calcite.-.- ...... 78 carbonatite .............. 35 mineral separates of. . 37 saprolite of ........................... 36 Page Spectragraphic analyses, altered phonolite—Con. dikes outside complex .................... 53 eudialy‘te-nepheline syenite pegmatite- - . . 48 feldspar, minor elements in ............... 69 feldspathoidal leucosyenite ............... 16 fine—grained ijolite .................. 33 garnet, minor elements in.. ......... 75 garnet-biotite melteigite- . ..... 22 mineral separates of .................. 22 garnet fourchite .......................... 49 garnet-nepheline syenite... ..... 13 mineral separates of- . ..... 15 garnet-pseudoleucite syenite. ..... 12 mineral separates of .................. 14 igneous rocks ............................. 60—61 jacupirangite ......... 18 mineral separates of .................. 20 lime-silicate rock, mineral separates of- . . . 40 magnetite ................................ 76 metamorphosed sediments.. . 58 nepheline ................... . 70 nepheline syenite pegmatite .............. 45 perovskite ................................ 77 pyrite .................... . 78 pyroxenes, minor elements in. .. 73 rutile ....................... 80—81, 82 sedimentary rocks ........................ 59 sodalite trachyte .......................... 42 sphene ..................... . 77 sphene-nepheline syenite ................. 9 mineral separates of ................... lo sphene pyroxenite ........ . 19 mineral separates of- . . . . 21 tinguaite ................... - 41 trachyte porphyry ........................ 46 undivided trachyte-phonolite ............. 27 veins ......................... _ 55, 56 vein minerals .................. 57 zeolites ........................... 71 Spectrographic sensitivities of elements ...... 7 Sphene, analyses ........................... 77 Sphene-cancrinite syenite, description and distribution ...................... 16 Sphene-garnet-nepheline syenite ............. 16—17 Sphene—nepheh‘ne syenite, analyses, norm, and modes .................. chemistry ........................ distribution and description .......... 7—8, 84, 88 Sphene pyroxenite, analyses, norms, and modes. ......................... l9 description ............................... 18 Stanley shale ............................... 5, 57—59 Strontium, mineral occurrences ......... Structure ............... Syenites, miscellaneous. See also various syenite rock types. T Ternary diagram, igneous rocks and Fe—Mg~ Na+K ........................... 67 igneous rocks and major elements ......... 63 igneous rocks and Na—K—Ca ...... 65 igneous rocks and Si—Ca—Na+K ......... 64 Thomsonite, spectrographic analyses ......... 71 Thorium ............................... 83 Tin, mineral occurrences. . . . ...... 69 Tinguaite, analyses and norm. ...... 41 chemistry ............................. 41—42, 62 distribution and description .............. 39-41 Titanium, deposits .......... 89 mineral occurrences ....................... 67 Tobermorite, chemical analyses ............... 48 Trace elements, altered phonolite. 25 analcime—olivine melagabbro 43 carbonatite ................... .. 37, 38 distribution in igneous complex ........... 62 feldspathoidal leucosyenite ............... 16 fine—grained ijolite ......... . . 33 gamet—biotite melteigite- - . . . 22 Page Trace elements, altered phonolite—Continued gamet—pseudoleucite syenite.. guide in age interpretation“ 14—15 ijolite group ...................... jacupirangite and sphene pyroxenite-_ nepheline syenite pegmatite ...... 45 significant concentrations.-- 59—60 sphene-nepheline syenite. 9—10 tinguaite ............... 42 veins _______________________ 55 Trachytes, miscellaneous _____________ _... 47,84 See also various trachyte rock types. Trachyte-phonolite. Sec Undivided traohyte phonolite. Trachyte porphyry, chemistry ............... 47 description, analyses, and norm __________ 46,84 INDEX Page Trap Mountain anticlinorium ................ 5 Trap Mountains _____________________________ 2 Turner, F. J ., and Verhoogen, Jean, quoted" 84—85 U Undivided trachyte-phonolite, analyses, norms, and mode __________ -_ 27 chemistry and origin _______ __ 26-27 distribution and description ........... 25—26, 84 Urtite, average and type, chemical analyses and modes ________________________ 30 V Vanadium, mineral occurrences _______________ 68 Variation diagrams ..................... 62—65, 86, 88 O 95 Page Veins ......................................... 54-56 Volcanic activity ........................... 6, 25, 87 W Womble shale ________________________________ 5 Y Ytterbium, mineral occurrences ______________ 67 Yttrium, mineral occurrences _________________ 67 Z Zeolite, analyses .............................. 70—71 Zigzag Mountains ___________________________ 2, 5 Zinc, mineral occurrences _____________________ 68 PROFESSIONAL PAPER 425 IED STATES DEPARTMENT OF THE INTERIOR 7 ' . I I GEOLOGICAL SURVEY . I ’ PLATE 1 92°53'45” _ _ 7 , ,, . . "WW... ,, 7 I' I 7 52' ’jl’ 50' 9204940” 7 7 7 I I" ' I ’ II II I II III I "IIII ‘I "I E III II II I I I II I ' IT I TIII II I I I " ~ I I I" , , , 7”" ,/ ,r \V 7 / V ’ " ' " V/ ‘ " ' "’ " 4/” " " " ' ' \ 7 7 77 77 77 7737777 7V 7 77 7 7 7 1:777:17" 7 7 V 77 - 7 7 7 7 7 7 7 77 77:7 7 V777777 7 777 77777 77777 7 7 7 7 7 77 7777 777777 77 W 7,\ .7 7 “11°28’40” 7 7 7 » s . _ 7 . / . V, . . 3 EXPLANATION IGNEOUS COMPLEX 9 DlKES OUTSIDE COMPLEX B rp } O /(W/l E Residual and secondary phosphate 0 Weathered igneous W Carbonatlte Tinguaite a Tinguaite Aplite a I p Aplite Pegmatite ep Eudialyte-nepheline syenite pegmatite Trachyte ”D "\IZ‘\T: T? Nepheline syenite pegmatite Trachyte porphyry 7.: 07‘ ‘K—I‘T WARS " I ‘ ‘ ' 7 ' ‘ ’ ' ' ‘ ' 7- " ' ‘7 ~ " ‘ " 77 ' 7. 7 ' 7 4 7. -. ; Trachytes, miscellaneous Syenite tp ’an~ Trachyte porphyry Andesite . \V \\_ \\ ~V , 7, IV I, V: VV V» ~ , VV :I V _V .j 7 V7/ ‘AE V\V‘ d \l \ II I l I I I I I V ‘ 7 II 7 , I‘ If I’ I II 3‘ ‘ g ‘I 7 '4 ' ' .~ . ,/ Sodalite trachyte Diorite 77\,_ __ ‘ . 1/ ,. 7V V .I Vr V . V “I V . V , , . , I. . I VV 7V ’ 7 VV,.' a V” 77 7 77 V‘ l VV , i / /I I ' ' ‘ ' I- 7 ‘ ~ ‘7 ' " , 7 ' ' I' “I I‘ I‘ 7 .- 7’ 7: I .7 " I V , .1 ‘1 Garnet fourchite MOHZOHite aom ..,\ 28' Analcime-olivine metagabbro Lamprophy re Lime—silicate rock q ' Quartz BiotiteI—IIgarnetI ij olite qf I II " Quartz-feldspar Garnet 1J011te a qb I) fgi L LO” Quartz—brookite-rutile U . . .. . < Flne-gramed IJOIIte iii f D: Garnet-biotite melteigite fc 5m Feldspar—carbonate . /I _ II ' . : .\ ‘_ _ V/ . V .~ _ _ _. . _ 7. . . . _ Syenites, miscellaneous ap 7- , x . .7 ‘ " x ' ' V 7 ‘ , .7 ’ scs Apatite Sphene—cancrinite syenite sgn Contact metamorphic zone T4 3 8- Sphene—garnet—nepheline syenite AS FLOAT OR IN VEINS ' an; ., Brookite Sphene—nepheline syenite 9 Rutile gns . V . Rutile paramorphs after brookite . Garnet-nepheline syenite \ \I IV . I:_ *7 \ l V 'II- V V I II ‘, V II . , , ‘I:\ V II- / gps ‘ I 1 x ‘ . * ' V " \ 7 / " ‘ . * 7 7 _ \ \._ J x l __ ' 2 7 : Garnet—pseudoleucite syenite V/ L-136 ls; '\I 7 ., ‘ 7 7 ‘ \ ‘ I ‘i- . I . ’ . ‘2 I. .' , I ' ‘ , ’ 7 ‘ I 7 I' ; ///“’” A 1/ / . ‘ 7 I .HI I III‘, I‘ ‘7‘ . . \ I b , ‘ I‘ I '- 'I I I II I 7 I j I \I I I I III\ I ' I‘I- 7: I I II "- II II .; / ; V. , 7. S ., , _ . , . . . . . . ; V , , - ’. _ _7 .V V , V V .7: , _ ,,; V ~. . _ ‘ ; Contact V I I I‘ I ’ I‘ I' - I l I E J , I‘ II 7 7 7 ” '_ , . . ~ 7- .~ , 'I _ _ 'I 7 ‘ i . Dashed where approximately locate \) / ’ I _ 7 ’I I ’ I Sphene pyroxemte dashed where indefinite r/‘l 7./ l \ l" E)I‘Mc-138 I / / I /./-777--"’ 4 9 \\ / VV/ / V “' , . .‘ . , ‘ . I _ ., _ V . ) V /,/‘/I I ’ ' ' -‘ _ ' I V 7 . ; . ' ' I I. I V l ‘ '. ‘7 I - I ‘ . ‘~ \ 3 / .3 / . ~I _ , 7 £ 7 7 _ . Gradationalcontact , , , ,7 . 7 3V _ 7/7 ; -. 7 ., ,. Jacupiranglte fi—“T—T _ Thrust or low angle reverse fau‘ 7' ’ - Dashed where approximately located. M aggstciove Feldspathoidal leucosyenite teeth on side of upper plate \7. t ‘. ‘ i _,__._——-——- //’T’ I“ I' Axis of anticline Banded trachyte Fold axes approximately locaten .. //%777" Trach e— honolite, undivided Axis Of Wadi“? yt p Fold axes approx1mately 10031364 - ///Q”——I’— ; 27’ Altered phonolite and breccia J Axis of overturned anticline, approxi located METAMORPHIC ROCKS L) Arrow shows direction of dip of lia Q _ ms 8 ’////9’ _| Axis of overturned syncline, approxi Metamorphosed sedimentary rocks E located Arrow shows direction of dip of liq SEDIMENTARY ROCKS <2: 79/ E \ Strike and dip of beds Ms E $ U) /Q/70 Stanley shale F) 8 Strike and dip of overturned bed . Q E .V \\ \.\’\\ L—157 2 Ii- ){90 I A 7. z Strike of vertical beds . I I DZ 0 Hot Springs sandstone Z < m V . . < E D: 70/ ‘ If" .i . Z n_ ‘1 Strike and dip of foliation . ‘MD‘a ‘ < a U , 2 U) Arkansas novaculite Q a) X > u) Strike of vertical foliation Lu .— Q E 70/ _ . . Z Strike and dip of joints Mlssourl Mountain shale <_z Di 3 2" Sb! fl Strike of vertical joint 0') Blaylock sandstone 0 L'246 ses were performed by personnel of the Quality of Water Branch in Washington and Denver. Special thanks are extended to Messrs. R. L. Nace and J. D. Hem, who originally supervised this work and who have been continually helpful even though no longer directly associated with this project. COLLECTION AND EXAMINATION OF SAMPLES SAMPLING PROCEDURES Field personnel were given considerable freedom to select specific sampling sites within their areas of operation because their knowledge of local conditions would aid in selecting sources that would best meet the needs of this study. Most samples were obtained from wells and springs: however, a few samples from lakes and streams were collected for purposes of com- parison. Sites at which samples were collected are shown on plate 1. SELECTION OF SAMPLING SITES A sampling network was designed to give each State as wide a geographic and stratigraphic coverage as possible. When specific sites were being selected for certain stratigraphic units or for a particular region, emphasis was placed first on sources furnishing water for public supplies provided the desired geologic and hydrologic requirements were also satisfied. Some sampling sites, which otherwise would have been desirable, were excluded because well logs or other geo- logic data were insufiicient to identify the aquifer, because wells or springs were dry owing to a drought, or because wells were not equipped with operating pumps. METHOD OF SAMPLE COLLECTION A set of 2 samples of about 2 or 4 liters each and 2 samples of about 100 ml (milliliters) each was collected at each sampling site. To aid in keeping the dissolved uranium and radium in solution until the time of analysis, one large sample was acidified with 8 ml of glacial acetic acid immediately after collection to lower the pH, and at the same time 2 m1 of chloroform was added to contrOI algae and fungi growth. The other large sample and the two small ones, to be analyzed for ordinary chemical constituents, were not treated. The samples were shipped to the laboratory as soon as feasible, usually within 3 days after collection. Samples from wells or developed springs were obtained at the point of discharge or, if the sample was collected from a pipeline, as near the source as possible. Samples from pumped wells were collected after pumps had operated long enough to clear the water that had been standing in the casings; samples from domestic pressure systems, or other sources where storage tanks were used, were obtained if possible after there had been recent turnover of water in the tank. Samples from undeveloped springs and seeps were obtained at their orifices; care was used to avoid sediment and other contaminating material. CHEMICAL AND BADIOCEEMICAL ANALYSES After receipt in the laboratory, the unacidified samples were analyzed for the common chemical con- stituents and physical properties according to methods regularly used by the Geological Survey (Rainwater and Thatcher, 1960). A few samples also were ana- lyzed for some of the more uncommon chemical constit- “uents, including sulfide, arsenic, boron, zinc, copper, bromide, iodide, and barium, according to standard methods. The acidified samples were analyzed for uranium, radium, and gross beta-gamma activity according to methods described briefly below. URANIUM DETERMINATION Concentrations of uranium in the samples were determined by the fluorophotometric method (Thatcher and Barker, 1957). A suitable volume of the, sample was evaporated to dryness and fused with a fluoride- carbonate flux. After the melt had solidified and cooled, the intensity of the fluorescence excited by near- ultraviolet light was measured photometrically and was compared with that from standard melts containing known amounts of a pure uranium salt. The precision of this method is about $15 percent of the reported value or i0.1 ppb (parts per billion), whichever is greater. The accuracy depends some- what on other constituents of the sample—especially heavy metals—but it is probably within a few percent. RADIUM DETERMINATION Concentrations of radium in the samples were determined by coprecipitating the radium in a suitable volume of sample with barium sulfate and measuring the alpha activity of the precipitate (Barker and Thatcher, 1957). This activity was compared with . that of similar precipitate containing a known fraction of a National Bureau of Standards radium-226 standard. The method described is almost equally sensitive to the three alpha-emitting isotopes of radium when they are present in the precipitate. The amounts of radium— 226 and radium-224 in the precipitate when the activity is measured are representative of the concentrations in the remainder of the sample at that same time. The concentration of radium-223 in the sample may not be strictly represented by its activity in the precipitate. However, because of the low natural abundance of radium-223 (less than 1 percent as abundant as radium- 226) in terms of radioactivity and its usual close association with radium-226, errors in the reported quantities of radium probably are negligible. The beta-emitting isotope, radium—228, is not measured by this method; the results, therefore, apply only to the three alpha-emitting isotopes. The concentration of radium-226 does not measurably change between the time of collection and the time of analysis of the sample, provided there is no precipitation or adsorption within the sample container. Radium- COLLECTION AND EXAMINATION OF SAMPLES - 3 224, however, has a short half-life; therefore, the concentration depends both on the time between collection and analysis and on the amount of its long- lived parent, thorium—228, in the sample. Although the results given in the report apply only to the total radium in the sample at time of analysis, usually 2 to 8 weeks after the time of collection, they can be con- sidered as the maximum concentration of radium-226 at the time of collection and, in fact, often serve as a good approximation to the actual radium-226 concen- tration. Because of the importance of radium-226 to public health, this limit may be of great significance. , The precision of this method varies with the amount of radium present, but for most samples it is about 3:20 percent or $0.1 puc per 1 (micromicrocuries per liter), whichever is greater. The accuracy is limited by interferences from other alpha-emitting nuclides, the; most important of which are polonium—210 and the alpha-emitting isotopes of thorium. These isotopes give rise to errors respectively equivalent to abOut one- tenth and one-fourth their concentrations in micromij crocuries per liter. These interferences do not detract greatly from the value of the data because the chemistry, of these nuclides is such that their concentrations in most natural waters are expected to be rather low. The errors caused by these interferences are no more serious than those caused by interferences in some of the common chemical determinations, such as the usual method of analysis for bicarbonate. BETA-GAMMA ACTIVITY DETERMINATION The gross beta-gamma activities of the samples were determined by measuring the beta-gamma activity of the residue left upon evaporation. A volume of the sample containing about 100 milligrams of solids was evaporated to dryness. The residue was then made into a slurry with distilled water and quantitatively transferred to an aluminum planchet. After the water was removed by drying under an infrared lamp, the activity of the residue was measured with an end window Geiger-Muller counter having a window thick- ness of 1.4 milligrams per square centimeter and mounted inside an iron shield 2 inches thick. Such a counter is about 100 percent efficient for the beta particles that penetrate the window. However, of the beta particles that are emitted by the sample the fraction penetrating the window depends partly on absorption in the window, in the air between the sample and the window, and in the sample itself, and partly on scattering from the sample and the planchet. The absorption and scattering vary with the energy of the beta particles; therefore, the overall efficiency is energy dependent. 4 DATA ON URANIUM AND RADIUM IN GROUND WATER IN UNITED STATES, 1954 TO 1957 The efliciency of a Geiger-Muller counter for photons (mainly gamma rays) depends upon the energy of the radiation; the amounts of absorption and scattering, though small, are also energy dependent. However, the efficiency of the counter is so low for photons, generally only a few percent, that the overall error is increased only slightly. ’ Each radionuclide is thus counted with a difi’erent efficiency which depends on the energy of the radiations and on the ratio of beta particles to gamma rays emitted. Counting data can be exactly transformed into units of radioactivity only when the radionuclides present are known, and even then only with difficulty if the mixture is complex. Thus, the selection of one refer- ence nuclide, in terms of which all data could be reported, was deemed desirable. Thallium-204 was chosen for this study because of its availability in standardized form, its rather average beta energy, and its widespread use as a standard of comparison for unknown fission product mixtures. All beta~gamma results listed in this report, therefore, represent the amount of thallium-204 activity that would produce the same counting rate as the sample when measured by the techniques and instruments described. The results thus serve for intercomparison of samples, though they cannot be interpreted in absolute terms. The fact that the results depend on the instrumentation and sample mounts precludes exact comparison of results obtained by the Geological Survey laboratories with those obtained by other workers using different instruments. However, the order of magnitude of the results should be comparable. ‘ The precision of this determination depends largely on the background counting rate of the instruments and on the level of activity in the sample volume used. The standard deviation of a net counting rate (Friedlander and Kennedy, 1955, p. 252—265) is given by 0=v (R/tr) -|- (13/15:) a=standard deviation of the net counting rate R=gross counting rate, sample plus background B=background counting rate t,=time during which sample was counted where tb=time during which background was counted. A given net counting rate (N =R —-B) is considered to be significant only when it lies more than two standard deviations above zero; that is N—20>0 N>2V (R/tr) 'i" (B/tb)- For counting rates near the detection limit (RzB) and where both counting times are approximately the same (the condition under which these Samples were counted), the equation reduces to . N>2,f—2(B/£)'. For the instruments and counting times used in col- lecting these data, the minimum detectable activity is found to be from 3.5 to 5 ##0. The sample volume used depends on the solid content of the sample; thus, the minimum concentration that may be detected varies with the total amount of mineral matter in solution. The certainty is less than 95 percent that a significant amount of activity is present if the counting rate of a sample is not greater than that indicated above. Such a sample is reported to contain less than (<) the detect- able activity. Zero could not be reported because almost all samples contain at least a few micromicro— curies of activity owing to the amounts of potassium and radium daughters in most water. The precision of a measurement lying below the detection limit is of little concern; by definition, the probability that the true value exceeds the reported figure is less than 50 percent. The precision of a measurement slightly above the detection limit may be taken as about 50 percent. The accuracy of the method is controlled largely by the uniformity of the deposit on the planchet. How- ever, accuracy probably is less afl'ected by laboratory techniques than by the uncertainties arising from the counting statistics for most of the analyses. REGIONAL DISTRIBUTION CHARACTERISTICS To facilitate statistical interpretation of the data, the Conterminous United States was divided into the 10 geotectonic regions shown on plate 2. The bound- aries of these regions were based upon considerations of tectonics (National Research Council, 1944; Eardley, 1951), geology (Stose and Ljungstedt, 1933), physi- ography (Fenneman, 1946), and ground-water provinces (Meinzer, 1923, 1939). The grouping of areas having similar characteristics permitted delineation of regions in which the geology and hydrology are for the most part homogeneous and which are suitable units for deriving statistical data to describe the natural regional distribution of uranium and radium. The regional characteristics of the waters were determined from only 509 of the 561 analyses available. Data from those samples having more than 3,000 ppm (parts per million) dissolved solids were not used in the statistical computations because it is probable that such W ter may represent only local conditions or may have any characteristics that are not obtained from the ho t rock from which the samples were collected. Analys s of samples obtained from mining areas also were 0 itted because mining operations commonly alter t e local geochemical regimen. INTERPRETATION OF DATA The anium and radium concentrations in samples from t ose geotectonic regions where sufficient data were vailable to justify statistical treatment were plotted as histograms, on a logarithmic base, as shown in figures 1 to 6. The class intervals for the statistical treatment were chosen in a manner to be consistent with the precision of the analyses (one significant figure at low concentrations), to cover the full range of con- centrations with a reasonable number of intervals, and still be of reasonably uniform width. The reported concentrations and their probable ranges, together with the logarithmic class intervals are-— Concentrations Class interval Reported Probable range Logarithmic base Width <01 0— 0.069 <—1. 16 _______ 0. 1 0. 070— . 149 —-1. 160———0. 826 0. 334 0. i—— 0.3 . 150— .349 — .825—— . 457 .368 0. 4—— 0. 8 . 350— . 850 — . 456——— . 071 . 385 0. 9- 1. 8 . 851— 1. 850 — . 070- . 267 . 337 1. .l— 4. 4 1. 851— 4. 450 . 268— . 648 . 380 4. .— 10 4. 451— 10. 50 . 649— 1. 021 372 11 — 23 10. 51 — 23. 49 1. 022— 1. 361 . 339 24 —- 54 23. 50 — 54. 50 1. 362— 1. 736 . 374 55 —120 54. 51 —125. 0 1. 737— 2. 097 . 360 130 —290 125. 1 —294. 9 ' 2. 098- 2. 470 . 372 >290 >294. 9 >2. 470 _______ Although the class intervals are not of exactly equal width the maximum variation is less than i8 percent from the average of 0.362; this is considered to be sufficiently good for the present needs. Sm )othed log-normal frequency distributions were calculated from the data and the curves representing these smoothed distributions are superimpOSed on the histograms for the appropriate regions. ,These fre- quency distributions and the log-normal curves were calculated as follows: v The cumulative frequencies of the concentrations were plotted on logarithmic-probability paper, and the best straight line through the points was deter— mined by the method of least squares. The mean value and standard deviation of the best-fit lag-normal curve were determined from the straight lme and substituted into the Gaussian equation. The resulting equation was normalized to the scale of the histogram for the appropriate region, and the 634405—62—2 INTERPRETATION or DATA 5 curve representing the equation was plotted in superposition on the histogram. The distribution of those samples lying below the detection limit was estimated from the best-fit log- normal curve and is shown as the dotted portion of the histogram. This technique makes p0ssible an esti- mation of the distribution of those concentrations below the detection limit, provided that 50 percent or more of the samples are above the detection limit. Although there is little reason to expect the universes from which the samples were taken to be distributed exactly according to a log-normal law, it can be seen from the figures that the approximation is sufficiently good that a. log-normal distribution can be used as a model of each universe. To illustrate the differences between universe and sample distributions, a synthetic universe was sampled in a manner corresponding to the actual experiments. A universe having a log—normal distribution was constructed from a table of random numbers; 80 samples were then withdrawn at random from this universe and divided into 10 class—intervals. The histogram representing the 80 samples, together with the curve corresponding to the universe, are shown in figure 7. It will be observed that this fit is about the same as that observed in the plots representing the geotectonic regions; therefore, the proposedlog—normal distributions are satisfactory models. The antilog of the mean of the log-normal distribution curve corresponds to both the median and geometric mean of the concentrations in the model universe described by that curve. The antilog of the value lying two standard deviations above the mean of the log- normal distribution represents the concentration that would be exceeded by only about 2 percent of the mem- bers of the model universe. Therefore, a sample exceeding this value might well represent an anomaly, and this critical concentration is called the “anomaly threshold” for that universe. Values of the geometric mean and anomaly threshold of the model universe for each geotectonic region susceptible to statistical treat- .ment are given in table 1, together with the range and "median for the sets of samples from each geotectonic region. Uranium, as well as its daughter, radium, is in almost all rocks; hence, these elements probably are in all ground water as well as most surface water even though the amounts in many samples Were below the detection limit. Samples of water having concentrations of radio— elements greater than the anomaly threshold suggest local areas in which the rocks may be somewhat enriched in uranium. 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H.H Nn NH H .m H .V «mV n N can H EN me oH . n .w w , CNN 8H c Now HH HNH NH 2. ........... co . no . o . Nm NH N. H. «HV ws «NH. NHN mmN H . N .N c. m .m «H c NmN N.N ed NN on ........... 8. No. H . N .N HH H.V H. mV mic an m 0N H. NH H. w. Him o m wH oH Hi 0N ........... co. oo. o. w.m S N NH 5 NH H.V NHHV H.w NNN HNH NNN o. N.N N. HH 3. o NH: O m 0H 3 NH. ........... co. 8 N. «H N . m A oHHHHh Cocoon T .30 .mod . A 35 :Bom ........................ v.» 3c EN «3 ...... w .H N . NH HmH a 8N m .N 3 «N no ........... No. on. ....... mN D H. H. NV. NH m5 11.. ............................... NH ......... NNN -11-... -111..- 1.1... 1-1... ......................................... m L “3an H. 9N 8m. mV as 2643 NHN w SNNNH co. ........ Nd condo oHN.m o N: o5 H oomfiw 81H ONN H ........... 8. NH N. mm H. . H M 5 “588mm «d N. NHV N .5 $4 EN EN N .H Tm H . NH NH o 3N e .H 3 NH Hw ........... on. a No ._ N. n NH e I H.V Hi NHV : NNH. EH NNN n. «.w o. 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N Z SE 08.55. ..... oHv ..... Ho «mo? maHHE «H NH |1|| , 1954 TO 1957 DATA ON URANIUM AND RADIUM ‘IN GROUND WATER IN UNITED STATES 106 436.5 .5585 23 30 5:» A: “8&3 33.3535 65% .825m and ”a: «we 8 Rm 95 835230 65” a ma 8 g Ram Sums 32 .m mug. no .wE SEQ mfiflfizm M 3:68 am 3:8qu Emma: 25:. nwl «Tea «nu com N US .......... :85 Ho Emu -5 ..... 535; m . was .EaESw E2 4 25H 8 ......... on .............. ow ..... .M ................. ow ..... :«2: End Sn] 3: c: on? 2: :5 Z 535 mEM ...... “58 «83 n .5nt 238.3 .m8 $58 4.38 :23 EE 5%.: 25. 8255an Saw @5332 .582 Ho 38 $2 .2 .Eom me .385 39.5 ......... 8 ..... .M :5 EanmfiaE ....................... v 33 n .H O a £35 anaefims £38 £8 H v _ .macoumESw—v ER mascaang 3:82 .358%& :mnm “:8 Egan ”a New 8 32 .8 has 8 ......... on ..... ‘ 35m 35;: SE 3% SW oxaenwmono .533 >32 mm] mug mam a3 Em .......... :95 .Moafioood .7 585:0 BwZ m .fiofimhafi .momnfl ~e>mfim SaaBéofiuzm 38 m 28 .933 was moment 35% .Emoia 9,59 .253 33 q 255 an .385 825 tuna 3.835 M 5 “ES 35 “Hagan in»? was. aEl a gm 2 .2 . .,hmuoo...:§> .33on --wEanoEovoum a .wnoumoE: 3.8% and 832 3.8m .Eam £5 38 5; 3883: ”SEE—ow d8 SEQSEE .23? #835” 93 macaw $33 Mo ”was 32 K 28a mm .29 82R” $8: 3:52 Q $8: waxwaeooonoo ........................................... 8m 2 ........ “55m -:Em=w=< his 3:5 e H 55??» :55 qu -2232: I no + own macadamia «a Sana ”Bum Anemone Comb menswzoo a av .533 .8 vfldhafi 9&3 mfihigo £3:on”. “Sofia €qu 8.58 .3580 E08 a mmfifiom 295% 9:: 85cm .80 .3852? 6:8 Z 4Q 333 8D no #58:: .3 Ho San .23 22395 BR: “35$ 32% Soc S 252 no .395 .84 f! ”:5. mEEBLEwB momummufloauano :85 .3383 muvflflmufionululihdun— OHUOROHQMN 92¢ OHGOAOHU vvsmmpqoolnuESS b. 333:». ESSEE 3:» «3.8 msfiksg .8 3% QESEEQ fies SmSSQ'd Hamish 107 GEOLOGIC, HYDROLOGIC, AND CHEMICAL DATA 355338 338mg «o Sum . 6835030 a 632mg 523 ficflsAOm E _ » A.V A.V mnV ad So u we. 5a m. ad mm «A o Ann m6 2; A. 5., o; 3 M. £2 a .23 touuoaoo 3 ..... I n .w Eh w m3 A ...... m . o .N on wa Av «in m .m N? m A c A ....... mg m. A.V an N‘w wt: m 1-1M--- NA ........ od 5 3 c afi Am: N. w. ....... R m. A. aA SAV N.m 8w N so can A N. A. mA mg «a o nAm ea As «A. 3 A . . E w A . A .V mnV c.w new «a 3m o m . a . 3 N. .w 9 mg «A E m: mm A . A mm m A.V w. MA NA. A8 me Q; m. A. . A. A .w wA. 9 n9 ed 3 pd E o. _ on a md AdV mNV us 2% 3m 5m o6 wd Nd «A aA o 3N mA mA an Aw Ad“ as A SEES 8 03 V “a 6 Q08 83 835 823 8:85 508 08:5 SEE 20 A59 305 as 36 99 A55 :5 A 3393A Cd A35 3933 mg 85 mammwu 2.285 -monn GOZV «Eu 90V Q08 3» Eamon 83m 373 33$: 33 35 was 323» 35 Ed: Q99 .E 32¢ 855 555% .83. 63m $58 -25 82:2 ABE oESEO 3.83m .55 L85 -wmuom Eivom -uaSA SSQEO BAN .30 .52 55 48:2. 85m 5 .95 Lam .aaem flow 32% .5 .50 62 958 m .EQ dodflmufloolm Hmwd<2< A40 AEHNO |\| 1954 TO 1957 1' DATA ON URANIUM AND RADIUM IN GROUND WATER IN UNITED STATES 108 _ £2 .Ndmm 32 6; con. 8 ......... 8 .............. 8 ..... a3 -1: Nam 8 mm: 6236 38:85 §1 3 cam 25 .fi 2 .5 £85 ..... €20 ...... 558mg a £035» .55 .5239 436% Eran _ .833m .302» 622333 28 5:3“ 432m 33% ”a 2: $2 .mfim 531% a; mm. .583 :25 afizuosso .5 8 8 :5 5m 5&3 hsfifim mm: m z: 5 2 .252 £35 ..... £33 ....... @52qu m 4353 .voatum was new Aura... 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HH o. o .w H. ,w o mm c .m H .m w .w GN ........... co. oo. o. S a H .V H .V NV w .o 9: he mmH mm. N .H c . c .w m .H o «a w .H H. .m m .H. «H ........... 8. mo. 9. mm m H .V H .V HHV b .w own m EN o. H». o .H S «N S $H HH an N . m .N ........... co. oo. c. mm H. H .V H .V NH w H 5H mm HmH nN . m . N . o .N m .H o cNH o .N o .w N .o mH ........... co . i. . H» . mm c N . H.V mV H .w H: «9 “NH 5 . o . m . o .m o .N o moH w .w w .m w .h mH ........... we . co . o . Nw m H ‘V H .V wV N .a mmm m NwN o lo o .H a .H ”H «m cN Nm o .N cm 9 . N .H ........... oo . oo . H . mm + H. .H H . HHV N. .5 wa NvN wow ...... N .H H . n .H mw o *mN w .N H d o .o Nw ........... oo . H co . H . wH m H. H .V mV m .N. mHH mm mm ...... m. N . o .H. nN o 5N HN .N H. .w m .m « .w ........... 8; No. H. mm N ‘ H .o H .o HHV w .w mam w me ...... o .o N .o 3 HH o NON a .o H@ o .o N .m ........... 8 d _ no .o H .o HH H ESE—Ema? 6 on“ . an 6 H58 83 2313 323 8:55 505 03H an 323 HB H69 305 96 36 89 H5): :5 H mxHanm ADV A36 3323 H3 8:.» ma mmo: ongmmuv $23 @073 03H AHOV Ago 9 8d. Eamon €us 376 EHHHmuHH $9 35 Hon 3wa A95 Eu: GOHmv .HHH Bin 8:6 5:83 $36 -Himm 3:8 .93 33$ Z 63m eEHoHHHO 833m .55 )6on .maaonH 823m .maSH EHHHonO oEN .aoO .52 :9: .552 Saw no .ED .SH .Ecm .59 33% Lo -30 62 323% -mHnH , \i fivSflmufloolmeVQ E93: 55 won—E8 .8356 ES #9:: 3:83 .wnopm flak :oSEEB .28: 65a 5 com 8 :3 .mnmu 5.8m damage A 3:25 E2 .3 .Evm mm .29 325 6:3 1.835 m .a @5338 $2269 “385 ml a Ba Ba H $1352“? ....... 8-: Ho £8: one 3 n T .wfimoamc $33? A 55 >9 amigo D ”woman; BEwESm .noo Savanna L 3:82 can 23% A .368 65% wovuonhfifi C 2520 firs Esau 33» .«m3 .82. .anSB m «R: .NN .Eam mm ......... cc ..... 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N. ~ m: 9 ON x 3 w. o A oNV N .w o3 EN EN _ o. m . N . o .N w .m o wNm a A m .m aN so 3. co. co. Nm. H. _ ma m . a Ens—H85 m . n .N mNV mg. mg 98 Non o. m . a A o .w cm» 0 SN a 4‘ «A NN SH NH . 8 . 8. Nu . c . a a .c w w. m. 3V 9c 3N «NH n5 c. w o. 96 mN o oNH wé m w m.w mm 8. co. mo. #5 , cJ N m m . a .V 9 N .h in may me o . N .m o . o A S o 53 m. . N. N an an 8 . co . co . no . H . ~ 8 N w d w .o owV a .5 Km NS wvm H d n .o m .o 8N 2: c on a .m mNH ma 3 S o 8 .o 8 d «ed ~ .c H m .m a Emmflévmmg 6 omN . _ a 6 co .5 A35 £015 923 8:55 50.3 oofl as SEE A5 A59 A505 06 A36 99 A55 :3 H mxaafiom ADV EM: 3333 HE 2:3 mm mmwn ”Hammad .3an 90 ZV out 90v Q0 9 3a $54.83 53w 37d 358a 30v ENV 3n @3an 3,5 Baa Amome .3 EB: EEU 3:an #26 623% 3:8 .ofl 83: Z ABE 2:830 SEEm -59 L85 .mauom Egcow -waz 83060 oEN A80 .52 nouH ABS: 85m no .SD .SH -weom .m8 “538 L0 -30 62 059on -mMQ U®5Qmaflou|zm HmVA< Z4 Q HHSHHEHE 3.:va Hanan—E6 mflfifinoo 2.8% .w HES mgififiom a -95 325 ......... 8 ..... EH. -Hiamwfiaesfii :32 .33 8| 2.3 com o? 2 $23.33 ..... 83m ...... H.553; HH .|Lli PROFESSIONAL PAPER 426 PLATE 1 UNITED STATES DEPARTMENT OF‘ THE INTERIOR GEOLOGICAL SURVEY E X P LA N AT! 0 N 1. Atlantic and Gulf Coastal Plain J; O M E II. Appalachian orogenic belt 111. Appalachian Plateaus V F (I) IV. Canadian shield L f V. Eastern stable region VI. Western stable region 0 O VII. Ozark-Ouachita system VIII. Rocky Mountain orogenic belt 0 IX. Colorado Plateau X. Pacific orogenic belt MAP OF THE CONTERMINOUS UNITED STATES SHOWING SAMPLING SITES SCALE 1: 7 000 000 o 100 2 300 MILES 100 Y , 00 E 634405 0 - 62 (In pocket) 1:7 ° 115' 39' 3.7. 35' 3 3' 31' 2 9' 27' UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY 129" 127' 125 ‘1 \ ,\ \ H \ \\ ’i . ‘ l: »\. I! l ‘ \ \ll §\‘\ \' “A ° 'f \\\x \J \ N J' , “Ilia A) 29.,» F wx 1% __ EXPLANATION . Atlantic and Gulf Coastal Plain . Appalachian orogenic belt III. IV. . Eastern stable region VI. VII. VIII. . Colorado Plateau . Pacific oregenic belt Appalachian Plateaus Canadian shield Western stable region Ozark-Ouachita system Rocky Mountain orogenic belt 121° 119° 117' PROFESSIONAL PAPER 426 PLATE 2 A f f i .. L/ \ \_ 9 » LG‘LE‘ 4 f. :‘é:\ \ \ GEOTECTONIC REGIONS OF THE CONTERMINOUS UNITED STATES SCALE 112070000 000 634400 0 v 62 (In pocket)