STATE OF CALIFORNIA EARL WARREN. Governor DEPARTMENT OF NATURAL RESOURCES WARREN T. HANNUM. Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO 11 OLAF P. JENKINS. Chief SAN FRANCISCO SPECIAL REPORT 20 AUGUST 1952 GEOLOGY OF THE SUPERIOR TALC AREA DEATH VALLEY, CALIFORNIA By LAUREN A. WRIGHT Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/geologyofsuperio20wrig GEOLOGY OF THE SUPERIOR TALC AREA, DEATH VALLEY, CALIFORNIA By Lauren A. Wright * OUTLINE OF REPORT and an upper 200 feet of alternating quartzite, shale, and dolomite Page layers. A diabase sill, 200 to 600 feet thick, lies at or near the Abstract 8 base of the massive dolomite. The intrusion was probably earlier Introduction __ 'A than the Keck Spring dolomite, which is composed of about 1300 Previous investigation 4 feet of dolomite and contains no diabase bodies. The Kingston Peak Purpose of current investigation 5 formation, at least 1900 feet thick, consists of a lower one-fourth Acknowledgments 5 of quartzite and shale and an upper three-fourths of conglomeratic Physical features ;> quartzite locally containing abundant diabase debris. Geology t The alteration zone, of which the Superior deposit is a part, lies Archean rocks ___ beneath the sill and is at least 1S00 feet long. The commercial Algonkian rocks material has been removed from a body that is 800 feet long at the Crystal Spring formation •) surface and 75 feet in maximum width and has been mined down- Heck Spring dolomite. .__ 12 cl i i> for more than 400 feet. In its thickest part it is composed of Kingston Peak formation __ 12 £ W0 talcose layers separated by a tremolitic layer. Mining has been Diabase __ 14 confined to the talcose rock. Paleozoic (?) rocks _ 15 The alteratil)11 ., n , e t amorphic effect related to the intrusion of Quaternary rocks _. . !•> diabase, has involved the introduction of MgO, Si0 2 , and H 2 0, and Metamorphism 10 the remova] of Ca0 and qq 2 The diabase appears t he most likely Regional metamorphism ___ _ lu source of the additive material, but H 2 and MgO may well have ( ontact metamorphism _ _ 1.) ])oon derived from ground water in the underlying noncarbonate Mining operations P) Rtrnrn Methods 10 ' U ' Ua - Mine workings 21 — — — ..-_. ,*»■ Suggestions for prospeeting___. 21 INTRODUCTION \^Zav"!,v^^^^o^tr^^^:\ "H 21 „ The Superior mine area, northern San Bernardino County, California, contains talc 1 deposits representative illustrations of a closely related group scattered throughout the south- page ern Death Valley-Kingston Range region. Each deposit Figure 1 index max showing location of Superior mine.. 4 f th . associated with a diabase sill ; each has 2. Index map showing lines ol measured sect ions .__ (> ° ' ' :{. Columnar sections of Pahrump series 7 altered from carbonate beds near the base of a predom- 4. Photo across superior mine area to southern end of inantly dolomitic member of the Crystal Spring forma- Death Valley _. 8 tion. The member is the lowest carbonate unit in the 5 ' Pb *5^^ ngw ^ lomerateatb ^ olCiar8talSpri,, « n Pahrump scries, a pre-Cambrian (Algonkian) sequence G. Photo 1 Rho'wing feidspatbicmart/.iie niembeVof Cry* of marine sedimentary rocks and diabasic intrusives. In tal Spring formation 10 all about 40 localities, distributed in a 70-mile northwest- 7. Photo showing feldspathic quartzite member of Crys- trending belt athwart the San Bernardino-Inyo County tal Spring formation __ 10 line ( . (llltain ta]( . deposits of this tvpe. 8. Photo showing exposure ot purple shale member of n . . . ' n . * ntn Crystal Spring formation— 11 l' ie Superior mine, wlncli was first operated in ly40, 9. Photo showing view northeast along strike of Supe- is one of the district 's most recently developed proper- rior deposit ... 14 fc . b t fe fh end f 1950 it had yielded a total f 10. Photo showing View southeast of thickest part of ' „ , , , - s r™ • • ,i ■,-,,-, Superior deposit 15 73,587 tons of talc (table 1). This is comparable with the 11. Photo showing underground exposure of Number Two output of any of the other deposits during a single decade. 10 ai-""iT"i c~~ ■ rVT"" " it' The White Cap mine, which is nearby, has yielded a 12. Microsketches of commercial talc specimens 1< <. ^ y , , , j 13. Microsketches of commercial talc specimens .__ 17 total of 4,797 tons in intermittent operations during the 14. Claim map of Superior and White Cap mine areas is period 1947-50. In recent vears relatively continuous 15. Underground workings of Superior mine 20 mining operations have been maintained in four of the Plate 1. Geologic map of the area of the Superior and White region's other talc-bearing localities. These operations Can lni, "' s - In i" l,k1,1 ar e the Warm Spring group in Warm Spring Canyon, abstract southeastern Panamint Range; the Ibex-Monarch group near Ibex Springs in the Ibex Hills; the Western mine „h V'rVTsnnno ?" T'' !*« '"n"' Wh w '" " 7**™*°'* '""", ™ the Alexander Hills; and the Excelsior mine in the ation totaled SO ,00(1 tons, has been obtained from a deposit typical T ,. T1 A . , ,, ,-.7- , in most respects to numerous other commercial talc deposits in the eastern part of the Kingston Range. Of these, the Western southern Death Valley-Kingston Range region of California. Kach mine, which since 1918 has yielded over 250,000 tons of has formed by alteration of carbonate strata in the basal part of f a ] ( . ] las | )v f ar t ne largest total output. Other properties a massive carbonate member which is near the .enter of the Crystal h beeii'operated intermittently ; still others have been spring formation. 1 his formation and the successively overlying ' . Reck Spring dolomite and Kingston Peak formation comprise the prospected 01ll\ . Pahrump series of Algonkian age. In the mine area the three forma- 1 The term "talc", strictly applied, refers to a mineral species with the tions have an aggregate thickness of more than 5500 feet, strike formula HsMg3(SiOs)«. But in commercial usage the term com- northeast and dip moderately to steeply southeast. monly alludes to mineral mixtures in which the mineral talc is Ti ,, ,, , 1 ■. ',- • r 1 ,,«, f . ordinarily, although not necessarily, a prominent constituent. Here the ( rystal Spring formation consists of a lower 800 feet other minerals in commercial talcs are mostly magnesian silicates, of conglomerate, quartzite, and shale, a middle 000 feet of dolomite, l,ut carbonate minerals are also common. In the discussion to fol- low, lli«' term "talc", unless otherwise qualified, will be used in its ♦Senior Mining Geologist, California Division of Mines. Manuscript commercial sense, but actually the material marketed from the submitted for publication January 1952. Superior mine area consists predominantly of the specific mineral. (3) Special Report 20 O 5 IO a *■ —_ A "6 Scale 20 miles -* 115' Fig. 1. Index map of part of eastern California, showing location of Superior mine. Tn recent years the combined annual production of the talc deposits in the southern Death Valley-Kingston Range region has been in the range of 40,000 to 60,000 tons, or approximately two-thirds of California's total annual talc yield, and about one-eighth of the national total. The Superior deposit, therefore, is a significant unit in one of the nation's principal talc-producing areas. Relatively large quantities of talc are also obtained in California from deposits in the Inyo Range region of Inyo County, and from the Silver Lake-Yucca Grove area of San Bernardino County. Most of these deposits are likewise alterations of carbonate strata, but they have no apparent genetic relationship with the talc deposits of the southern Death Valley-Kingston Range region. The Inyo Range deposits are late Jurassic or younger in age; the Silver Lake-Yucca Grove tale probably formed in Archean time, whereas the southern Death Valley- Kingston Range talc alteration appears to have occurred in Algonkian time. Previous Investigation The Superior mine area is of historical interest to stu- dents of Death Valley geology, for it was near here in the early eighteen-seventies that G. K. Gilbert 2 was the first to note the rocks that were later to be known as the Pah rump series. In 1902 the locality was briefly men- tioned by M. II. Campbell :! who presumed the rocks to be of Cambrian or pre-Cambrian age. The series was named in 1940 by D. P. Ilewett, 4 who in the Kingston Range some 30 miles to the East, had studied the most easterly of its known exposures. The name "Pahrump" was derived from the Pahrump Valley lying north of the range. Ilewett indicated a pre-Cambrian age for the ^Gilbert, O. K., Report on the geology of portions of New Mexico, j Nevada, I'tah, California, and Arizona : U. S. Geog. and Geol. Sur- veys W. 100th Mer. Rept., vol. 3, pp. 34, 170, 1875. j I 'Campbell M R., Reconnaissance of tbe borax deposits of Death V al- I ley and Mojave Desert : U. S. Geol. Survey Bull. 200, p. 14, 1902. * Hewett D V., New formation names to be used in the Kingston Range, Ivanpah quadrangle, Calif. : Washington Acad. Sci. J'"" vol. 30, no. G, pp. 23U-240, 1040. Superior Talc Area, Death Valley Table 1. Talc produced at the Superior and White Cap mines, 1940-50. Year Short tons of talc produced Superior mine White Cap mine 1940 388 3,974 5,292 4,126 7,152 10,512 9,908 8,101 8,252 6,094 9,788 1941.. 1942 . 1943 1944 1945 1946. 1947 1948 1,519 1,797 1,409 73 1949 1950 Totals 73,587 4,798 series, and recognized three component formations, which, from bottom to top, are the Crystal Spring formation, the Heck Spring dolomite, and the Kingston Peak formation, ;In a description of the geology of the Ivanpah qnad- rangle, Ilcwett 5 has subsequently provided detailed sec- tions of several of the Kingston Range Pahrump occur- rences, and a map showing their distribution and struc- tural features. Most of the talc-bearing belt, however, (lies to the west in an area to which L. F. Noble has (devoted many years of study. The broad stratigraphic and structural features of this area were described by Noble 6 in 1941. Both Noble and Ilcwett noted the presence of talc bodies at contacts between diabase and dolomite of the Crystal Spring formation. A few of the deposits were ;briefiy described by Sampson 7 in 1937, and by Tucker and Sampson 4 in 1!)4.'5; but there appears to have been no previously published mention of the talc deposits in 'the Superior mine area. Purpose of Current Investigation Because of the relative lack of detailed data on the oc- currence of talc in the southern Death Valley-Kingston Range area, and the increased commercial importance of talc, particularly to the ceramic, paint and rubber indus- tries of ( lalifornia, the study of these deposits has become part of a state-wide talc commodity survey by the Cali- fornia Division of .Mines. This talc program lias proceeded intermittently since its inception in October 1947; a pre- liminary report was published in 1950. 9 The first phase of the study entailed the preparation of detailed plane-table maps of several of the active de- posits. In general, the properties chosen for such study were ones that were both (dearly exposed and extensively developed underground. An attempt was made also to se- lect deposits that showed somewhat dissimilar structural and mineralogic features, but that, in a broad manner, ■Hewett, I>. 1", The geology and mineral resources of the Ivanpah quadrangle: l\ S. Cool. Survey Prof. Paper, in press. 6 Noble, I.. F., Structural features of the Virgin Spring area, Death Valley, Calif. : Ceol. Sue. America Hull., vol. 52, pp. 941-999, 1941. • Sampson, R. J., Mineral resources of the Resting Springs region, Inyo County: California DIv. .Mines Kept. 33, pp. 269-270, 1937. 8 Tucker, W. B., and Sampson, R. .1 , Mineral resources of San Ber- nardino County : California Div. Mines Rept. 39, pp. 44.">-4 49, 1943. •Wright, L. A., California talcs: Am. Inst. Mill. Met. Kng. Tech. Trans., vol. 1ST, pp. 122-128, 1950. ^yere representative of the region's large talc concentra- tions. The Superior mine, which fulfills these qualifications, was mapped by plane table in September and October of 1948, during periods that totaled about three weeks. The area was visited briefly several times in 1949 and 1950. Thin section studies of the talc and associated rock were made at intervals during these years. Acknowledgments The California tale program, of which the study of the Superior mine area is a part, was instigated by Olaf P. Jenkins. Chief of the California Division of Mines, and has been helpfully supervised by him and by Ian Camp- bell of the California Institute of Technology. The study was also made possible by the willing cooperation of the personnel of the Southern California Minerals Company, which operates both the Superior deposit and the nearby White Cap and Pongo deposits. Many facilities of the Company were placed at the writer's convenience by Walter K. Skeoch, owner, Frank T. Roberts, district man- ager, Charles F. Joy, superintendent of mines, and Benny D. Gomez, mine superintendent. Plane table control was established with the assistance of Mort D. Turner of the California Division of Mines and Refugio Ruiz, an employee of the Southern Califor- nia Minerals Company. Helpful suggestions were con- tributed by Mr. Skeoch and Drs. Jenkins and Campbell and by < lordorj B. < lakeshott of the ( lalifornia Division of .Mines and Richard II. Jahns of the California Institute of Technology, cadi of whom read the manuscript criti- cally. PHYSICAL FEATURES The Superior mine area, sees. 25 and 26, T. 19 N., R. 5 Iv. S. B. M., is in a group of hills in the southeastern corner of Death Valley National .Monument. The hills, commonly known as the Saratoga Hills, are bordered on the west by the southern end of the valley, and lie south of the Ibex Hills, which extend with increasing rugged- nrss and relief northward for about 16 miles. The Superior deposit is exposed in a saddle at an ele- vation of about (i")l) feet. The While Cap mine, as shown on the accompanying map. is at an elevation of about 250 fort. It is about 1500 feet west nf the Superior mine and is low on the south side of a small canyon that drains from the saddle. other talc occurrences, not shown on the map, are discontinuously exposed along the western flank of the ridge. < If these the Pongo (fig. 2) and Saratoga are being worked. Although Death Valley is in one of the nation's most arid and least inhabited regions, the Superior mine area is accessible from three small desert communities: Baker, about :*li miles to tin' southeast. Tecopa, about 14 miles to the northeast, and Shoshone, about 20 miles to the north-northeast. A paved road. State Highway 127. con- nects Baker with Shoshone and passes about ck will be included under the term "shale" in the descriptions i" follow. /\**w Conglomeratic quartzite mem ber Green quartzite member Dolomite ss; EggJ Upper sedimentary units Massive cbert member SUPERIOR DEPOSIT-^" Carbonate member GREEN TBCMOLITIC BOO Diabase T/UC - TREHOUTE BOCK /Fino-ffrained quartz- ite member Purple shale member Feldspatluc quartz- ite member Quartzite, schist, and" c/neiss Fk;. .'?. Generalized columnar sections of the Pahrump series in the Superior mine area. The Beck Spring dolomite is a monotonous sequence of evenly layered beds with no major lithologic breaks. The lower one-fourth of the exposed part of the Kingston Peak formation consists of thinly bedded quartzite and shale; the upper three-fourths is a conglomeratic quart- zite. The Pahrump strata generally have distinct bedding planes, and abrupt, vertical variations in lithology. The quartzitic and shaly members in the lower part of the series commonly display cross-bedding, ripple marks, and mud cracks. These features, together with the over-all lithologic sequence of quartzite and shale to carbonate rocks to more quartzite, are typical of sections ordinarily attributed to the transgression and regression of a shallow sea. Except for irregular bodies of probable algal origin noted in Crystal Spring carbonate rocks and the Beck Spring dolomite, the Pahrump series appears to be un- fossiliferous. Special Report 20 % ft i ..» * c *, Superior Talc Area, Death Valley The major rock units trend generally north-northeast, but in the northern part of the mine area their strike curves to the north-northwest in a broad anticline. In a few places steeply overturned beds dip westward, but in general the section dips eastward. At the surface, dips range from about 35° to nearly vertical. Despite the presence of numerous shear planes, several eross-faults that offset contacts as much as 200 feet, and an intimate, open-fractured "crackling" of the rock units, the section appears relatively intact. Much of the faulting, however, is nearly parallel to bedding planes and causes part of the variations in thickness. The tale- bearing zone in particular has localized such movement. Most of the fracturing and faulting is post-talc alteration in age, although some of the irregularities in the diabase bodies appear to be attributable to intrusion along frac- ture- or fault-planes. The strata from which the Superior and White Cap deposits have altered were originally the basal beds of the massive dolomite unit in the Crystal Spring formation; but the diabase sill now lies on the hanging wall side of the deposits and separates them from the main dolomite mass (fig. 4). The sill and the alteration zones associated with it are continuously exposed for a lateral distance of about 2 miles southwest of the mine area; but only locally is the talc in concentrations of economic interest. The region-wide, close spatial relation between the talc alteration and the diabase, together with their persistenl occurrence at or near the base of the massive carbonate unit in the center of the Crystal Spring formation justified a rather close scrutiny of the sediments of the Pahrump series. Because the talc-bearing zone character- istically occurs low on the ridges and mountain fronts, an understanding of the stratigraphy may well lead to the discovery of deposits hidden beneath relatively shallow dune and alluvial material. The stratigraphic descriptions that follow are based in part on observations within the area of the accompany- ing map, and in part on data gathered in the measurement of a section about one-half mile south of the Superior mine (fig. 2). Archean Rocks The Crystal Spring beds in the immediate vicinity of the Superior deposit rest with a pronounced unconformity upon much older and much more highly metamorphosed pre-Cambrian rocks of Archean age. A similar contact is exposed about one-half mile to the southwest, but it is possible that in this locality movement along the contact has caused a faulting out of the formation's lowermost beds. The unconformity separating these two types of pre-Cambrian rocks, as noted by Noble, 11 is the most pro- found in the southern Death Valley region, and strongly resembles the unconformity in the Grand Canyon of Ari- zona, which separates Algonkian rocks of the Grand Canyon series from Archean rocks of the Vishnu scries. The most abundant of the Archean rocks nearest the Superior deposit is a bluish-gray quartzite which ranges from massive to thinly layered, and is locally schistose. This rock encloses numerous quartz lenses and veinlets that contain subordinate amounts of feldspar and do not exceed a few inches in width. Most of these quartzose bodies parallel the layering in the quartzite, but some 11 Noble, L. F., op. cit. show cross-cutting relationships. At the locality to the southwest. Crystal Spring beds are underlain by gran- itic gneiss. Algonkian Rocks Crystal Spring Formation Conglomerate Member. In the mine area the lowest unit of the Crystal Spring formation is a pebble-to- boulder conglomerate (fig. 5). For most of its exposed length the conglomerate ranges in thickness from one or two feet to a maximum of about 20 feet ; locally it is miss- ing altogether. The conglomerate is not present at the Crystal Spring- Archean contact to the southwest; but it is the basal Crystal Spring unit at numerous localities throughout the southern Death Valley-Kingston Range region. The matrix of the conglomerate at the Superior mine forms about one-half of the volume of the rock, ami con- sists of light greenish-gray to medium dark gray, poorly sorted, fine- to coarse-grained quartzite. It is feldspathic and commonly contains a high proportion of ferromag- nesian minerals. The pebbles, cobbles, and boulders, all well-rounded, consist of quartzite and subordinate massive quartz. The (piartzite is generally light gray, very fine-grained, dense, and vitreous. The massive quartz is white to pinkish-gray in color. The boulders have a maximum observed dia- meter of about 1 \ feet. £ $$&> fr^ i'k.. Conglomerate ;ii base of Crystal Spring formation. /•', Idspathic Quart:i!< .1/* mfo r. The feldspathic quart- zite overlying the basal conglomerate in the mine area is present in all of the known occurrences of the lower part of the formation. It shows very little lithologic varia- tion from one locality to another, but ranges in thickness from about 350 to about 1300 feet. The (piartzite and the conglomerate at its base are consistently more resistant than the underlying, older pre-Cambrian rocks, and their contact with the older rocks is ordinarily marked by a moderate topographic rise. In the mine area the member is 800 feet thick and is typical in lithology. It grades upward from a medium light gray, coarse-grained pebbly (piartzite occurring in poorly defined beds into lighter gray to pale-yellowish- green, fine- to medium-grained (piartzite with relatively distinct bedding planes. Distinct textural and color varia- tions, however, commonly occur within several-foot inter- vals. Cross-bedding (fig. 7) is well shown throughout the member's thickness. Ripple marks (fig. 6) are common 10 Special Report 20 Fig. G. Exposure of feldspathic quurtzite member of Crystal Spring formation showing ripple mark. in its upper parts. Prominent in its lower half are massive coarse-grained beds that weather to a dark yellowish brown. Its upper third contains subordinate beds of gray- ish-blue, fine-grained quartzite and shale as well as a minor amount of brown dolomite in elongate lenses. Rarely do these lenses exceed 6 inches in thickness. In thin section the quartzite is seen to contain from 10 to 25 percent feldspar consisting largely or wholly of microcline. A very fine-grained material composed mostly of clay and sericite is ordinarily present in proportions of 2 to 5 percent. Quartz composes the remainder of the rock. The mineral grains of the quartzite are character- istically angular and poorly sorted. Some have recrystal- lized; others retain their original clastic outlines. Purple Shale Member. Throughout the southern Death Valley-Kingston Range region the feldspathic quartzite grades upward into a purple to grayish-blue, shaly to quartzitic unit (fig. 8) to which the name "purple shale member" is applied. The member is less resistant than the quartzites that enclose it; its distinctive color causes it to be readily distinguished from points several miles distant. The gradation between the two members ordinarily occurs through several tens of feet of alternating layers Fig. 7. Exposure of feldspathic quartzite member of Crystal Spring formation showing cross-bedding. of shale and quartzite. The lower limit of the purple shale member, as shown on the accompanying map, was placed at the top of the uppermost prominent quartzite layer. Thus defined, the member has a thickness of about 150 feet in the mine area. The rock- comprising the member ranges from massive to fissile. Most of the layers are actual strata; secondary schistosity is only a local feature of the finer-grained rocks. Well-preserved ripple marks and mud cracks are common. In this, the southernmost part of the Ibex Hills, most of the member is colored various shades of grayish-blue; purple, its most distinctive color elsewhere, is less com- mon. Characteristic of the member, in the mine area as well as throughout the region, are irregularly distributed patches colored pale bluish-gray to pale bluish-green that resemble splashes of paint upon the darker background. Many of them contain concentric cores of still lighter shades. Most of these are spheroidal to ellipsoidal, but many have tabular to very irregular outlines commonly with long dimensions parallel with the bedding. The more equant patches are ordinarily between one-half inch and three inches in diameter. The elongate ones average sev- eral times larger. Under the microscope the patches are Superior Talc Area. Death Valley 11 Table 2. Section of part of Crystal Spring formation measured at Superior mine. Crystal Spring Formation (Upper Part Not Measured) Feet Carbonate member. 12. Dolomite: pinkish-gray, locally grading into white or moderate red colors. Fine- to medium-drained, mas- sive. Weathers to light brown 200-f- Diabase and related alteration zones. (Altered rocks appear to have been lower part of overlying dolo- mite unit.) Altered carbonate rock : white-to-pale green, mostly platy. Fine- to coarse-grained composed mostly of calcite and tremolite in various proportions. .Much of the calcite-rich rock is thinly bedded 50 Diabase sill: similar to lower body 180 Altered carbonate rock: pale-yellow-green, very fine-grained. Composed mostly of tremolite. Mas- sive to platy. Forms screen between diabase sills 20 Diabase sill : dark-greenish-gray to greenish-black, medium-grained. Top and bottom show fine- grained selvage several feet thick 200 Talc: white, fine grained, mostly schistose and very friable; but locally a thinly layered, blocky rock- 15 Tremolite-calcite rock : light-greenish-gray, mostly line-grained, crudely layered. Composed mostly of tremolite blades in a calcite matrix 25 Talc: white, finegrained, predominantly schistose, very friable 25 Fine-grained quartzitc member. 11. Quartzite: light-brown to moderate-yellowish-brown, mostly finegrained, but locally ranges to coarse- grained, poorly bedded. Commonly grades into sandy dolomite. Contains layers of massive shale in upper part 135 Purple shale member. 10. Shale: grayish blue to grayish purple, platy to fissile. Spotted with abundant light-bluish gray spheroidal to lenticular patches. Contains very subordinate quartzite layers and thin dolomite lenses lot) Feldspathic quartzite member. Feet 9. Shale: grayish-blue, mostly thinly bedded. Contains abundant grayish-orange, fine-grained quartzite in layers that average about two feet thick 35 8. Quartzite : similar to unit no. 2, but with bluish gray, shaly layers, and very subordinate, thin dolomite lenses 30 7. Quartzite : grayish-pink, grayish-orange - , and pale- yellowish brown. Mostly fine-grained. More evenly bedded than underlying rocks. Contains subordinate shaly layers 70 C. Quartzite: similar to unit no. 4. Pale-yellowish-green color prominent, and dark-brown-weathering layers less abundant upward I'M 5. Quartzite: feldspathic, white to medium-gray to pale- yellowish-gray, fine- to coarse-grained, contains sub- ordinate pebbly layers that weather brown 120 4. Quartzite: feldspathic, white greenish gray, medium- to coarse-grained. Massive, poorly layered and indis- tinctly cross-bedded. Gradational into underlying unit. Some layers weather dark brown 50 .'{. Quartzite: similar to unit no. 2, but becoming lighter gray upward : 100 2. Quartzite: pebbly and feldspathic, mostly medium- light gray, medium- to coarse-grained, poorly sorted. Beds are massive but crudely layered and cross- bedded. Individual layers several feet thick com- monly weather to dark-yellowish-brown 80 Conglomerate member. 1. Conglomerate: matrix of light greenish-gray to medium-dark gray, poorly sorted, tine to coarse- grained feldspathic quartzite. Pebbles, cobbles, an.! boulders form from one-third to one-half of volume of rock. Dense, vitreous quartzite most abundant ; vein quartz also common. All pebbles are well- rounded 20 (max.) Older Pre-Cambrian Rocks Quartzite: medium bluish-gray to dark bluish-gray, mas- Bive to thinly layered, locally schistose. Contains pods and veinlcts of quartz. shown to contain opaque metallic grains apparently formed by reduction to magnetite of Limonitic material abundant in the body of the rock. The lighter colored cores contain very little ferruginous material, even as opaque grains. Thin section studios of the shale show thai it consists mostly of finely divided, angular grains of quartz, sericite and semi-opaque, limonitic material. These three male- rials occur in a wide range of proportions. Feldspar, principally microcline, is present in quantities that prob- ably do not exceed 5 percent. Fine-grained Quartzite Member. A stratigraphic unit, composed mostly of fine-grained quartzite overlies the purple shale member throughout the known extent of the Crystal Spring formation. From place to place the color of this member ranges from brown to various shades of green and dark red; but in the Superior mine area it is mostly a yellowish brown. Here, as elsewhere, it forms a resistant ledge above less steep slopes underlain by the purple shale member. Although it ordinarily lies immed- iately beneath a diabase sill, at the Superior mine the quartzite is separated from diabase by about 100 feet of alteration rocks and dolomite, and grades imperceptibly upward into dolomite. In the Superior mine area the member ranges from thinlv layered to massive and eommonlv contains lenses of sandy dolomite. The upper part of the formation locally encloses tabular bodies of grayish-green shale. In its average observed development the quartzite appears to be between 100 and 150 feet thick. In the mine area, the member attains what may well be its maximum thickness. Immediately north and east of the Superior deposit the member is at least 250 feet thick, but within a distance of 1500 feet to the southwest it apparently thins to slightly less than 100 feet. A FlO. 8. Exposure of the purple shale member of the Crystal Spring formation. 12 Special Report 20 Massive Carbonate Member. In the middle of the Crystal Spring formation is a thick, persistent carbonate member, at or near the base of which the diabase sill was intruded, and the larger concentrations of talc formed. The carbonate member is characteristically cliff-forming above the less resistant diabase. In the mine area this member, including the altered rock next to the diabase, averages about 470 feet thick. Its unaltered strata, comprising a 20-foot to 30-foot thick- ness beneath the principal talc-bearing zone and a 300- foot to 350-foot thickness above the sill, are wholly dolo- mite. The parent rock of the alteration zones probably was also dolomitic. The unaltered rock is colored a pinkish-gray to grayish- orange-pink and ordinarily weathers to a light brown. Most of the rock is fine-grained, but locally it has recrys- tallized to a medium-grained marble. Massive in appear- ance, it is devoid of the cherty layers so common in the lower part of the member at other localities. Except for quartzite lenses, common in the uppermost part of the member, bedding features are rare. Massive Chert Member. A prominent, dark-colored layer of massive chert, overlying the carbonate member, can be traced laterally from the mine area to its disappear- ance beneath alluvium about two miles to the south. In these exposures the layer maintains a 100-foot to 150-foot thickness. Masses of similar chert exist above the car- bonate member at most of the observed occurrences of the Crystal Spring formation, but generally are less uni- form in thickness. Unlike the Saratoga Hills occur- rence, the chert ordinarily lies beneath a diabase body and forms irregular masses that lens into the uppermost strata of the carbonate member, or into shaly strata higher in the formation. The massive chert member in the mine area ranges in color from pale brown to blackish red, and is commonly a mottled, jasperoid rock. It contains a few dolomite layers in its lower part, and locally shows uniform lami- nations that appear to be bedding features ; but for most of its thickness the member is structurally homogeneous and shows only minor A'ariations in mineralogv and tex- ture. One of the most noticeable variants is distinguished by evenly scattered chloritic clots. These rarely exceed one-eighth inch in diameter and have the appearance of widely spaced polka dots. Upper Units. The uppermost 400 to 450 feet of the Crystal Spring formation exposed in the Superior mine area is composed of alternating layers of several sedi- mentary rock types. Individual lithologic units have thicknesses within the 10- to 60-foot range, and are composed variously of shale, quartzite, or dolomite. Such layers can be traced laterally for several thousand feet within the mine area, but cannot be correlated from locality to locality among the known exposures of the upper part of the Crystal Spring formation. These rock units, therefore, cannot be designated as persistent mem- bers, and will be referred to merely as the "upper units" of the formation. In the stratigraphic section measured one-half mile south of the Superior deposit, gray shale is the most abundant rock of the upper units. The shale is platy to thinly bedded and characteristically shows irregular patches and veinlets of orange to red iron oxide stain. Also common is gray to grayish purple, shaly to medium- grained quartzite. Massive, gray dolomite occurs in sev- eral 5- to 60-foot layers. Two of the lower layers are of massive chert similar in appearance to the chert of the underlying member. A 10-foot thickness of pebble con- glomerate occurs as one of the middle layers. Beck Spring Dolomite The 1300-foot section of gray dolomite that overlies the Crystal Spring formation in the vicinity of the Su- perior mine is the most westerly of the known occurrences of relatively undeformed Beck Spring dolomite. The thickness of the dolomite at this locality is essentially the same as the thicknesses of the formation in its exten- sive exposures in the Alexander Hills and Kingston Range, which lie to the east. In these more easterly occur- rences, the dolomite is a massive, cliff-forming unit; whereas in the mine area it occurs in even beds, and is not noticeably more resistant than the underlying and overlying units. Here the Beck Spring dolomite is a monotonous se- quence composed mostly of gray to light-brown, fine- grained dolomite, but containing subordinate olive-gray to light-brown quartzite and shale. The dolomite beds ordinarily are 2 to 10 feet thick and locally contain thin siliceous layers parallel with the bedding. The quartzite and shale layers range from a few inches to as much as 20 feet in thickness, and are most abundant in the upper 200 feet of the formation. In spite of these variations, the formation is not readily divisible into members and is here treated as a single unit. Kingston Peak Formation In the Superior mine section, as in the Kingston Range sections described by Ilewett, 12 the Kingston Peak forma- tion is broadly divisible into a lower part composed of fine-grained quartzite and shale, and an upper part com- posed mostly of conglomeratic quartzite and conglomer- ate. Although the complete thickness of the Kingston Peak formation is not visible in this westerly occurrence, its unexposed, uppermost part is probably small. This is suggested by a comparison of the exposed part with Ilewett 's description of the formation's occurrence near Beck Spring in the Kingston Range. Both localities show exposed thicknesses of about 2000 feet, and at both, the lower one-fifth to one-fourth is fine-grained. The re- mainder is conglomeratic. Green Quartzite Member. In the measured section about one-half mile south of the Superior mine, the lower part of the Kingston Peak formation consists of 450 feet of light olive-gray to greenish-gray, fine-grained quartzite and shale. A very evenly bedded unit, its individual strata are ordinarily a small fraction of an inch thick, but some are as much as one foot thick. The rock commonly con- tains irregular stains of reddish-brown iron oxide. At the top of the member is a 15-foot layer of black, thinly bedded limestone. Conglomeratic Quartzite Member. The conglomeratic quartzite and conglomerate that overlie the green quartz- ite member in the Superior mine area have an exposed thickness of approximately 1500 feet, which is only about 200 feet thinner than similar beds measured by Ilewett in the Kingston Range. 12 Hewett, D. F., op. cit. Superior Talc Area, Death Valley 13 Table 3. Section of Pahrump series measured eas Pahrump Series, Pre-Cambrian Kingston Peak formation. Feet Conglomeratic sandstone member (not studied in detail). Conglomeratic sandstone : greenish-gray to dark green- ish-gray. Matrix fine- to coarse-grained very poorly sorted. Bedding indistinct, locally platy to schistose. Pebbles, cobbles, and boulders generally form less than one-tenth of volume of rock ; these are poorly sorted. Dense, vitreous quartzite most abundant ; granite, granitic gneiss, vein quartz, and limestone also common. Quartzite very well-rounded ; but all are moderately well-rounded 1400 + Green sandstone member. Sandstone and shale: light olive-gray to greenish-gray, thinly and very evenly bedded. Sandstone predomi- nantly fine-grained. Most beds in member are a frac- tion of an inch thick, but some are as much as one foot thick. Reds commonly show reddish-brown, iron oxide stains 450 Beck Spring dolomite. 1850 + 17 16 15, 11 13 12 11 10 18. Dolomite: medium-gray, fine-grained. Occurs in mas- sive, but evenly layered beds several feet thick. Dolomite beds alternate with beds of light brown shale 95 Dolomite : medium gray to medium bluish-gray, fine- grained. Occurs in well-layered, but massive beds several feet thick. Contains a minor proportion of silica in thin layers 50 Dolomite: in alternating light gray and dark gray layers. Fine-grained. Layers average about 5 feet thick. Unit contains numerous layers of pale yellow- ish-brown shale 75 Dolomite : light gray, fine-grained, massive 30 Dolomite: medium gray, fine-grained, massive to thinly-bedded. Much of the unit is composed of platy layers that weather light brown. Unit locally con tains thin, siliceous layers 170 Shale: grayish-orange -pink, platy to fissile 15 Quartzite: light brown, medium-grained, massive 5 Dolomite: medium to dark gray, finegrained. An evenly layered rock with light gray shaly layers prominent in the midpart 50 Dolomite: light to dark gray, fine-grained. Occurs in massive, but evenly layered beds several feet thick. Contains subordinate layers of light brown slabby dolomite and shale 175 9. Shale: light brown, thinly layered. Lower part of unit contains abundant, light brown, fine-grained quartzite 20 8. Dolomite: dark gray to brownish gray, fine-grained. Occurs in massive layers generally several feet thick. Interbedded with light brown, thinly layered 25 7. Dolomite: medium gray to dark gray to yellowish gray, fine-grained. Mostly massive, but contains subordinate layers of light gray to light brown platy, impure dolomite and shale SO G. Dolomite : similar to unit no. 5, but with prominent, brownish gray layers 20 5. Dolomite: medium to dark gray, fine-grained. Occurs in massive, but distinct, beds several feet in thick- ness 110 4. Dolomite: similar to underlying dolomite, but more slabby 1 130 3. Dolomite: similar to unit no. 1, but becomes more thinly bedded upward. Contains local, thin, sili- ceous layers 12", 2. Shale: light olive gray, massive, locally platy 10 1. Dolomite: medium gray to dark gray, locally light brownish gray, fine-grained. Occurs in massive layers several feet thick. Locally shaly or platy. Contains subordinate layers of brownish gray, thinly bedded quartzite and sandy dolomite 140 1325 tward along line one-half mile south of Superior mine. Crystal Spring formation. Feet Upper units. 32. Shale : light gray to pale yellowish-green to light brown. Thinly layered. Shows abundant, reddish- brown iron oxide mottling 50 31. Dolomite: yellowish-gray, fine-grained, evenly layered. Weathers to light brown 15 30. Shale : medium gray to yellowish-green, thinly bed- ded. Shows reddish-brown iron oxide mottling 20 29. Quartzite: grayish-purple, fine- to medium-grained, well-layered, semi-vitreous. Contains subordinate layers of pale yellowish-brown shale and pale yel- lowish-green quartzite 40 28. Dolomite : pale orange pink, fine-grained, massive. Weathers to pale yellowish-brown 30 27. Shale: pale green, massive. Contains thin layer of pebble conglomerate 10 2G. Quartzite: very light gray to pale purple to pale green. Generally compact. Contains subordinate layers of green shale 35 25. Shale: grayish purple, thinly layered 5 24. Dolomite: moderate brown, fine-grained, massive. Contains thin siliceous beds in upper part 25 23. Shale: pale green to grayish-green, thinly layered to massive. Contains subordinate layers of fine- grained sandstone 20 22. Shale : similar to unit no. 22 20 21. Dolomite : light olive gray, fine-grained, massive 5 20. Shale: light olive gray to grayish green, thinly lay- ered. Contains abundant reddish-brown iron oxide mottling "50 19. Dolomite: light olive gray, finegrained distinctly bedded. Contains fine- to coarsegrained sandy streaks 15 IS. Quartzite: medium gray, fine-grained, vitreous 5 17. Chert : similar to unit no. 15 15 16 Shale: medium gray, thinly layered 25 15. Chert : similar to unit no. 15 10 14. Shale: dark red to light brown, generally thinly bed- ded. Grades into fine grained sandstone of same color 30 Massive chert member. 13. Chert : similar to underlying unit, but without dolo- mite beds. Mostly massive, but locally well-layered 115 12. Chert: pale brown to blackish-red, massive. Com- monly has jasperoid appearance. Contains numer- ous thin dolomite beds 25 11. Dolomite: similar to unit no. S. Lower part contains chert layers as much as one foot thick. Contains Uppermost diabase body, a pod about 4 feet thick 40 10. Chert : in thin layers colored various shades of yellowish gray and brownish-black 5 !l. Dolomite: pinkish-gray to grayish-orange-pink, line- grained, massive. Weathers to light-brown. Contains very minor proportion of silica in small seams 250 Diabase and related alteration zones. I Altered rocks prob- ably originally formed lower part of overlying dolomite unit.) Diabase sill: similar to lower bodies 50 Altered carbonate rock: palc-yellow-green, lined grained, massive to platy. Contains one-half foot to one-foot layers of very light brown, less altered carbonate rock. Composed mostly of tine grained tremolite 105 Diabase sill: similar to lower body 385 Dolomite: grayish-orange-pink, fine-grained, massive to thinly laminated. Partly altered to talc. This zone con- tains the Saratoga talc deposits several thousand feet south of line of section. Diabase sill: dark -greenish-gray to greenish-black, me- dium-grained. Top and bottom shows fine-grained sel- vages several feet thick 7:> Limestone: pinkish-gray, medium-grained crystalline. Partly altered 15 14 Special Retort 20 Fine-grained quartzite member. Feet 8. Quartzite: pale-yellowish-brown to dark-gray, fine- grained, thinly layered to massive. Contains abun- dant massive shaly layers, and thin, impure dolomite lenses. Forms prominent layer above less resistant underlying purple shale 135 Purple shale member. 7. Shale: dusky-blue to grayish-blue, thinly layered. Becomes lighter colored and more fissile upward. Locally contains pale-blue-green patches in spher- ical to irregular shapes. Patches range from fraction of inch to more than two feet in Ions dimension. Fine-grained sandstone abundant in lower parts. Member is less resistant than underlying quartzite 100 Feldspathic quartzite member. 6. Quartzite : light brown to medium gray, medium- to fine-grained. Contains subordinate dark-bluish-gray shaly layers and thin dolomite lenses 4."i 5. Quartzite: moderate brown, fiine-grained, massive— 15 4. Quartzite: brownish-gray, fine-grained, massive. Con- tains light-gray to dark bluish gray shaly layers and thin dolomite lenses 45 3. Quartzite: medium-dark-gray, medium- to fine- grained, thinly bedded. Locally shows cross-bedding and ripple marks. Weathers to brownish-gray. Con- tains subordinate, pale blue layers of fine-grained quartzite and shale 50 2. Quartzite: similar to underlying unit, but dark brown- weathering beds more abundant. Greenish tints less prominent 250 1. Quartzite : light-olive-gray to light-greenish-gray, poorly sorted, mostly coarse-grained. Beds massive, commonly show well-developed cross-bedding. Con- tains abundant feldspar. Subordinate, thin layers of pebble conglomerate, pebbles mostly of fine- grained quartzite and vein quartz. Slightly more resistant than underlying gneissie rocks. Dark- brown-weathering beds common 125 2255 Older Pre-Cambrian Rocks Granite gneiss. The member uniformly consists of quartzite matrix containing a pebble to boulder fraction that rarely ex- ceeds 10 percent of the volume of the rock. Bedding- ranges from massive to platy. The matrix is greenish-gray to dark greenish-gray, fine- to coarse-grained, and poorly sorted. The grains, angular in outline, consist of quartz with subordinate feldspar. The rock also contains an abundance of green, very fine-grained material intersti- tial to the quartz and feldspar grains. Some of the finer- grained matrix shows a rude schistosity. Most of the pebbles, cobbles, and boulders are of a dense, vitreous quartzite that resembles the quartzite in the conglomerate at the base of the Crystal Spring forma- tion. Much less abundant are fragments of diabase identi- cal in appearance with the material in the Crystal Spring sills. Other rock types include fine- to coarse-grained granitic rocks, granite gneiss, and vejn quartz, which were probably derived from the Archean complex. Also pres- ent is limestone, a brown, punky rock, resembling none of the carbonate rocks lower in the section. The largest boulder noted was about 1| feet in diameter. Individuals of all of the sizes and rock types are moder- ately to well rounded ; but the quartzite fragments con- sistently show the best degree of rounding. Diabase The diabase of the mine area, confined largely to a 200- to 600-foot thick sill near the base of the Crystal Spring carbonate member, is a greenish-black rock that, on the barren desert slopes, looms boldly against the lighter colors of the enclosing strata. The other diabase bodies, as noted above, exist as smaller sills and tabular masses within the member and as small dikes cutting the quartz itic and shaly units lower in the formation. The large dia base bodies, common at or near the upper margin of the carbonate member at other localities in the region, are not present in the mine area ; but the lateral dimensions of the large lower sill are apparently region-wide, and the char acter of the diabase shows little variation from locality to locality. The large sill is mostly medium- to coarse-grained, but is bordered, top and bottom, by fine-grained selvages sev eral feet thick. The smaller diabase bodies are ordinarily entirely fine-grained. The "chilled" upper margins to- gether with the occurrence of contact metamorphic zones, above the diabase bodies indicate an intrusive rather than extrusive origin. Fig. 9. View northeastward along strike of Superior deposit, 1 diabase on right, lower units of Crystal Spring formation on left. The large sill actually is composed of several intrusive bodies. Its multiple character is best shown by narrow elongate septa or screens of altered carbonate rock thai parallel its general attitude. These are as much as 100( feet long and occur at various levels within the sill. Th< diabase bordering the septa is fine-grained through thick nesses of several feet. Except for the differences in grain size, the diabase bodies show little lithologic variation. Differences in th( proportions of mafic to felsic minerals do exist from plact to place, but such variations are erratic and cannot be attributed to gravitational settling of the heavier compo; p nents. Petrographie Features. Thin section studies of spec! mens gathered from the central part of the sill show thai more than two-thirds of the volume of the rock is composed it of secondary minerals. Of these, uralite, chlorite, sericite and clinozoisite(?) are the most abundant. The relict! texture is medium-grained and typically diabasic. The original rock was composed essentially of two-thirdi ferromagnesian minerals and one-third plagioclase feld spar (sodic labradorite). The plagioclase laths, though' highly altered, do locally show well-defined polysynthetit twinning. The plagioclase has altered principally tc sericite and elinozoisite ( ?)> but is also commonly corroded by irregular shreds of chlorite. Augite grains, which now form less than 5 percent oil the rock, appear to be remnants of the original ferromag fri Superior Talc Area, Death Valley 15 nesian fraction. The augite is partly altered to uralite and may well bo, the mineral that all of the uralite lias replaced. Most of the uralite, however, is unassociated with remnants of earlier mafic minerals. It occurs principally in grains from 5 to 10 millimeters in diameter. Felty aggregates of the mineral are also common. The uralite has been partly to wholly chloritized, an alteration particu- larly well shown along cleavage cracks in the larger uralite grains. Opaque grains, with observed lengths of as much as 2 millimeters, and with irregular, angular outlines, gen- erally form from 2 to 10 percent of the rock. Many of these appear to consist wholly of magnetite, but intergrowths of magnetite and ilmenite are common. Irregular quartz grains of undetermined origin comprise as much as 3 per- cent of the rock and are interstitial to grains of other minerals. Biotite shreds, thinly scattered through the rock, may be remnants of primary grains. These also are partly chloritized. Apatite is an abundant accessory. Traces of sphene are present. A specimen typical of the selvage zone immediately above the Superior talc body, when viewed in thin section, is shown to be distinctly inure felsie than specimens col- lected in the main body of the sill. The selvage specimen s a very fine-grained aggregate composed, in general, of about 60 percent feldspar thoroughly altered to sericite find clinozoisite ( ?)> •'■"> percenl partly chloritized biotite. and 5 percent quartz. .Minor minerals include scattered opaque grains, augite( .' ), and apatite. Diabase specimens gathered at other Crystal Spring localities in the region differ from the Superior specimens in that they generally contain several percent hvper- sthene and little or no quartz. The all-pervasive character of the alteration indicates deuteric processes rather than alteration by hydrothermal solutions unrelated to the intrusion. The most noteworthy bulk chemical changes produced by the alteration seem to have an enrichment in water ami potash and a decrease in silica. Silica thus released may well have been trans- ferred to the enclosing sediments. Age. If, as seems virtually certain, the abundant dia- base detritus in the Kingston Peak formation was derived from intrusive bodies in the Crystal Spring formation, the intrusions were pre-Kingston Peak. Moreover, the absence of intrusive diabase in all post-Crystal Spring strata, pro-Cambrian as well as Paleozoic, strongly sug- gests that the intrusion had taken place before the basal Beck Spring dolomite beds were deposited. Paleozoic (?) Rocks A prominent hill immediately north of the Superior deposit is capped by an outlier of light brown, medium- to coarse-grained, highly brecciated and unstratified quartzite resting with a nearly horizontal contact on up- ended Crystal Spring strata. It is tentatively correlated with quartzite that locally occurs as a subordinate phase of the Noonday dolomite (lower Cambrian), the region's lowest Paleozoic formation. Quaternary Rocks Bedrock exposures on the ridge containing the Superior mine are partly hidden beneath dunes and talus slopes, but the area of the acccompanying map lies well above the alluvium that flanks the ridge on the east, south and west. Southwest of the mine area, near Saratoga Springs, the talc zone extends beneath the alluvium. Dunes and alluvium probably also cover talc-bearing rocks in the complexly faulted northern part of the ridge. The dunes are particularly abundant in hollows on the ridge's north- west Hank, where they lie at various levels and appear relatively thin. Metamorphism Regional Metamorphism As noted by Noble 13 and Hewett, 14 the effects of re- gional metamorphism in the Pabrump series are no more pronounced than in the overlying Paleozoic section. In the Superior mine area regional metamorphism has trans- formed most of the sandstone to quartzite, but has not destroyed the clastic outlines of all of the quartz and IVldspar grains. Much of the shale has been changed to argillite, but rarely does it show a secondary schistosity. Though carbonate sediments have locally recrystallized to medium-grained rocks, most of them remain fine- grained. I'i«.. 10. View southeastward of surface exposures of thickest part of Superior deposit. Layer of tremolitie rock separates stoped layers of talcose rock. Contact Metamorphism In contrast with the slight regional metamorphism of the Pahrump scries, pronounced and widespread silicated zones have developed by alteration of Crystal Spring carbonate strata at or near diabase contacts. The zones are as much as 11)0 feet thick and are composed of fine- grained, massive to thinly layered rock. Talc, tremolite, orthoclase, and albite are the most abundant alteration minerals and exist in a wide range of proportions. Phlo- gopite, serpentine, and quartz are less common constit- uents. The zones also contain a carbonate (calcite and dolomite) fraction, which is generally in the range of three to fifteen percent. In the Superior mine ana. silicated zones occur above and below the multiple sill, as well as in the septa within the sill. All are alterations of strata of the carbonate member; all lie either against or within the sill, a dis- position indicating contact metamorphism genetically related to the diabase. At the Superior mine, as well as throughout the south- ern Death Valley-Kingston Range area, the alteration zones of present commercial interest consist of a snowy- white rock composed mostly of talc or tremolite, or 13 Noble, L. F., op. cit., p. 950. » Hewett, D. F., op. cit. 16 Special Report 20 mixtures of both of these minerals. This rock, which occurs in a wide variety of textures, will be referred to under the general name " talc-tremolite rock" (figs. 12 and 14). Serpentine, ealeite and dolomite, also are char- acteristic constituents, but the marketable material ordinarily contains a carbonate fraction of less than 10 percent. Feldspar is rare, or absent. A second general type of alteration rock is pale green to pale bluish-gray, and is persistently highly tremolitie. It can conveniently be called "green tremolitie rock." Feldspar (albite and orthoclase) is characteristically abundant. Other minerals include carbonates, quartz, and sepentine Finely divided, semi-opaque particles are common, but generally in very subordinate proportions. This rock is not a commercial material. The talc-tremolite rock and green tremolitie rock ordi- narily occur in separate bodies as alterations of different stratigraphic layers, but locally bodies of the two rock types interfinger. In general, the occurrences of talc- tremolite rock, including all of the bodies of commercial interest, are confined to the lower strata of the carbonate member, whereas the green tremolitie rock is most abun- dant in alteration zones higher in the member. In the Superior mine area, this relationship is shown by the localization of most of the talc-tremolite rock to an altera- tion zone at the base of the large sill, and by the pre- ponderance of green tremolitie rock in the septa within the sill and in the overlying alteration zone. This higher zone also contains talc-tremolite rock, which is, in general, too incompletely silicated to be of commercial interest. The Superior Deposit. In the vicinity of the Superior and White Cap mines, the silicated zones along the foot- wall of the sill are ordinarily less than five feet thick or are missing altogether. An exception, however, is the Superior body, which is composed entirely of talc-tremo- lite rock, and in plan measures about 750 feet long and 75 feet in maximum width. It has been mined down-dip to depths of 300 to 400 feet. This body, as shown on plate 1, tapers from a thick, two-pronged eastern end to a point where it pinches out between diabase and partly altered dolomite. West of this point the zone again widens and extends on the surface for an additional 200 feet. This body averages less than 10 feet thick and has not been explored underground. The silicated zone is also exposed northeastward from the deposit, but is only a few feet thick and composed largely of non-commercial rock. The Superior deposit dips southeastward from a maxi- mum observed angle of about 60° at its northeasternmost surface exposures to a minimum of about 20° in the lower workings at its southwest end. In general, the dip lessens downward and southwestward, and averages about 45°. The northeastern edge of the deposit appears to plunge southwestward at an average angle of 35° ; the south- western edge is essentially normal to the strike of the deposit. As explored to date, therefore, the deposit shortens with depth. Worked for a strike-distance of nearly 800 feet on the 65-foot level, the mineable talc- bearing zone was found to be 600 feet long on the 300-foot level. Underground exploration, however, may well reveal commercial talc beyond the limits of the body currently being worked. The foot-wall lies against fine-grained, reddish-brown dolomite; the hanging wall against diabase or against partly altered dolomite separating the zone from diaba: Knife-edge contacts, polished by movement, are cha acteristic. The thickest part of the Superior deposit co: sists of three layers; two layers of soft, highly talcose rock separated by a layer of tough, massive, tremolit: rock. The tremolitie layer, which averages about 40 fe< wide and is as much as 400 feet long, in some places termi nates against diabase, in others, by a juncture of the two talcose layers. The layer shortens with depth and is miss^lm: ing in the lowest part of the mine. It is by far the mostj regular of the three, and its borders form walls, the evenness and firmness of which have facilitated the min- ing of the talcose rock. \ ■r Fig. 11. Underground exposure of Number Two vein of commercial talc. II The talcose layer farthest from the diabase has been named the "Number One vein"; the "Number Two vein' (fig. 11) is next to the diabase. Irregularities in the atti tude of the foot-wall of the Number One vein cause marked rolls; thicknesses range from a maximum of about 30 feet to a minimum of a few inches. In a few places horses of country rock lie athwart the Number One vein; but in general, it maintains a 5-foot to 15-foot thickness. Less continuous, and ordinarily thinner, is the Number Two vein, which in some places wedges out alto- gether between the diabase and massive tremolite rock. It is about 12 feet in maximum width. Its variations in thickness are much less abrupt than those of the Number One vein, but it also contains horses of country rock. The southwestern end of the deposit, for lateral dis- tances of from 200 to 300 feet beyond the edge of the; tremolitie layer, is composed mostly of highly talcose: rock. At this end the body is as much as 20 feet thick, but, like the Number One vein, it shows marked varia- tions in thickness. The two talcose layers and their southwestern extension as a single layer have yielded virtually all of the market- able material mined thus far. Also of commercial interest is a massive tremolite rock that, in thicknesses of from two to four feet, borders both sides of the tremolitie layer. This rock is composed predominantly of tremolite and is comparable with rock mined elsewhere in the 'i Southern Death Valley-Kingston Range region and mar- keted for use as ceramic raw material. Mainly because it is difficult to remove, little of this rock has been mined in the Superior deposit. Superior Talc Area, Dkath Valley 17 Both the Number One and Number Two veins are com- )osed predominantly of talc schist ; but a massive talcose ■ock also forms parts of both and is particularly abundant n the Number Two vein. Although the massive rock )reaks into irregular blocks, much of it is very evenly and hinly layered. The schist and massive rock of the Num- >er One vein have a characteristic dull, chalky luster, vhereas the schist of the Number Two vein has a porcel- aneous sheen and translucency. The schistose rocks are riable ; the massive rock is relatively tough. All varieties ire white and fine-grained. The tremolite-rich rock of the middle layer is fine- grained and compact. It ranges in color from white ;hrough various shades of pale green and gray. The most xemolitic material, which occurs along the borders of the aver, is very hard, tough, and mostly white; the material n the center of the layer is somewhat softer and is darker n color. Thin-section examination of ten representative speci- nens of these various types of talc-tremolite rock- shows ;hat they consist mostly of grains with maximum dimen- sions in the 0.01 millimeter to 0.2 millimeter range. Much :>f the tremolite. however, occurs in grains from 0.2 to 3.5 millimeter long. The schistose specimens (figs. 12 and 13) contain from 75 to 9~) percent of the mineral tale ami from a trace to about ->."> percent carbonate material. Tremolite was not abserved in the schist in proportions greater than one or two percent, and is absent in some of the thin sections. Serpentine, sparsely disseminated through some of mate- ial observed under the microscope, appears much less abundant than in the average talc deposit of the region. The serpentine grains ranee from irregular to plumose n outline. Although somewhat similar to talc in its ap- pearance, the serpentine's habit and low birefringence are distinctive. ]\Iost of the thin sections are clouded with extremely fine-grained, semi-opaque material that may compose as much as five percent of some of the rock The talc in the schist occurs as dimensionally aligned shreds, producing a wavy schistosity. That the shreds are commonly also in crystallographic alignment is shown by their nearly simultaneous extinction over areas of as much as a square centimeter. The relatively small propor- tion of tremolite in the schist occurs as needles that aver- age several times longer than the tale grains. These needles lie at various angles with the schistosity and show- little or no parallelism with the talc grains. The carbonate grains in the schist, which, like the tremolite grains, are somewhat larger than those of the tale, occur in minute veinlets, or are disseminated indi- vidually or in mosaic-like clusters. Some of the veinlets He across the schistosity ; others are parallel with it. Under the microscope, specimens of massive talcose rock are seen to have essentially the same mineralojric composition and fineness of grain as the talc schist. The thin banding is caused by differences in grain size from layer to layer and the relative abundance of carbonate grains and opaque and semi-opaque material along certain layers. Must of the talc grains show no marked dimen- sional alignment. Not uncommonly, however, they lie normal to the layering. Observed in thin section, the tremolite blades of the rock in the massive tremolitic layer ordinarily show a decussate arrangement. Carbonate material forms from a trace to more than one-fourth of the rock's volume and is mostly interstitial to the tremolite. Also present in amounts of several percent is the semi-opaque, very fine- grained material common in the talcose rocks. Talc is locally abundant The Whiti Cap Deposit. The White Cap deposit, as explored to date, is a relatively small, single body of com- mercial talc-tremolite rock, but it is part of an alteration zone that is at hast 1 ,000 feet long and as much as 50 feet wide Mosl of this zone is beneath the sill, but the proved commercial deposit lies in a finger of altered rock that extends into the lower part of the sill (pi. 1). Fn;. 12. Microsketches of commercial talc specimens. I. Tremolite and carbonate grains in :i matrix of dimensionally aligned tnlc era ins. li. Tnlc schist containing lenticular aggregates of carbonate grains. Fig. I'i. Microsketcbes of commercial talc specimens. I. Talc schist containing semi-parallel carbonate veinlets. /.'. Talc schist containing, in lower part, carbonate grains and much finely divided, semi opaque material. 18 Special Report 20 N \ ip \/ # ^ ^ / // ■3? «/ y 9 / loV / #^> -Inclined / rvV SHAFT / yJO 4/ / y / / / y / **>■ sN^ v^ ■>p / / 4 / / / / / ^ A / V MAP OF SUPERIOR AND WHITE CAP GROUPS SARATOGA MTS SAN BERNARDINO COUNTY, CALIFORNIA SCALE 100 200 400 600 800 1000 FEET Basis of bearing on ossumed course of 5 57"30' W on center line Wonder Figure 14. Superior Talc Area, Death Valley 19 The deposit is lenticular, showing a maximum width f about 20 feet, and outcrop length of about 400 feet. n the underground workings, however, it has been )rofitably mined for a maximum of about 150 feet along- trike. The footwall of the deposit is bordered mostly by liabase. The hanging wall is ordinarily separated from liabase by green tremolitic rock and by sub-commercial alc-tremolite rock. At the southwestern end of the deposit liabase forms both Avails. In its surface exposures, the deposit lenses northeast- ward into sub-commercial alteration rock, and narrows jetween the diabase walls southwestward to a very thin point. It dips to the southeast, at angles of 30° to 50° and las been explored down-dip for a slope distance of 200 feet. The deposit, which is highly brecciated and contains abundant fault planes, is composed of material ranging from friable schist to tough massive to evenly laminated rock. In general, the schist is the most talcose, and is most abundant along the foot-wall. The massive or evenly lam- inated rock is ordinarily rich in tremolite and is best developed along the hanging wall. Northeastward extensions of the White Cap deposit have been sought in bulldozer excavations in the shallow alluvial cover east of the mine shaft. Several shallow cuts and pits have explored exposures of talc-tremolite rock along the base of the sill and between the two deposits; but none of these have outlined deposits large enough to be of current interest to the operators. Paragenesis and Origin of the Silieated Zones;. Obser- vations in the Superior mine area, as well as at numerous Other talc deposits in the region, have shown that the silieated zones 1 ) have a close spat ial relation with diabase bodies, 2) have thicknesses roughly proportional to the thicknesses of the diabase bodies with which they are associated, and 3) commonly exhibit similar sequences of alteration rock layers outward from diabase contacts. 15 These features indicate a contact metamorphic origin for the zones. The most, clearly shown and persistent age relations between minerals of the contact zones are the following: 1. Corrosion and veining of carbonate grains by tremolite, alkali feldspar, serpentine, and talc, each apparently a direct replace- ment of the carbonate. 2. Corrosion and veining of tremolite by serpentine and talc, commonly showing pseudomorphism. 3. Rimming and veining of serpentine grains by talc. 4. Corrosion of tremolite by carbonate and transection of all other minerals by carbonate vcinlets. In the talc-tremolite rock, therefore, the general mineral- ogic sequence is original carbonate, tremolite, serpentine. talc, each of the silicates having formed at the expense of each mineral listed before it. Other carbonate material post-dates the silicates. At several localities where the silieated zones can be traced laterally into unaltered strata, the alteration ap- pears to have produced no detectable change in volume. Moreover, it appears that the differences in the major alteration rock types do not, in general, reflect composi- tional differences in the original strata, and that most, if not all, of the silication has involved the introduction of most of the silicate mineral-forming material. '■ All example is the very common, layered seqiienre of diabase, thinly laminated tremolite rock, talc schist, and an outer zone composed principally of carbonate, potash-rich micas, albite, orthoclase, talc, and tremolite. The commercial talcs of the southern Death Valley and Kingston Range region average about 54 percent SiOo, 28 percent MgO, 5.5 percent CaO, 4.5 percent C0 2 , 4 per- cent H 2 0, 1 percent A1 2 3 , and less than 1 percent each of K 2 0, Na 2 0, and iron oxide. 16 To form a rock of this composition from a relatively pure dolomite, the most common parent rock, would require the abundant intro- duction of MgO, Si0 2 , and H 2 0. Similarly an introduc- tion of AI0O3, K 2 0, and Na 2 is implied for the forma- tion of the alkali feldspar and mica common in the non-commercial alteration rocks. Corresponding amounts of CaO and C0 2 would have been removed. It can be reasonably assumed that a deuteric hydration of the diabase was at least partly contemporaneous with the silication of the bordering rocks, and that the se- quence tremolite to serpentine to talc formed under suc- cessively lower temperatures as the diabase cooled. The diabase may well have been the source of additive mate- rial. If, as seems likely, the diabase was intruded soon after the Crystal Spring beds were deposited, ground water may also have provided appreciable amounts of MgO. The possibility that some of the MgO was leached from dolomite at depth is probably precluded by the rarity of dolomite in the Archean rocks and the lower ( Jrystal Spring members. The localization of the large bodies of talc-tremolite rock to the basal beds of the carbonate member may well be attributable to a former abundance of ground water in the underlying strata. Such water may have facilitated the transfer of material from the cooling diabase. Less abundant water in the carbonate member may have been the cause of the less complete silication and less complex mineralogic history, which characterize contact zones of diabase bodies higher in the member. MINING OPERATIONS Methods The Superior deposit has been mined mainly by over- hand stoping from drifts appended to inclined shafts. To avoid removal of waste material, the workings have been Confined, insofar as possible, to talcose layers more than .'! feel thick. \o single shaft connects the surface with the lowest workings. Most of the talc removed in recent years has 1 n hoisted two or three limes before reaching the surface. Tunnels are driven by standard drilling and blasting methods. Mechanical loaders are employed. Stoping pro- ceeds by the enlargement of raises driven between levels and spaced at intervals of about 30 feet. < Mice begun the stopes arc mined mostly with spade and bit points mounted on jackhammers. The talc is fed by gravity to chutes and thence to mine cars. The walls of the stopes are generally quite solid and are supported by pillars and by widely spaced props. The operators estimate that these methods permit the removal of 60 to 75 percent of the commercial material. They anticipate a future recovery of at least one-half of the remaining talcose rock, as parts of the mine are abandoned. Although relatively little timber is used in the stopes, the maintenance of tunnels beneath stoped ground requires closely spaced sets and lagging. 10 These percentages are averages obtained from chemical analyses of about :u> samples of commercial talc. The analyses, by various analysts, were supplied by the Southern California Minerals Com- pany, Sierra Talc and Clay Company, and Western Talc Company. 20 Special Report 20 Superior Talc Area, Death Valley 21 Mine Workings Superior Mine. The Superior mine workings, as they aisted in mid-1951, contained 6 levels. These have been Iriven at depths of 65, 125, 180, 215, 260, and 315 feet /ertically lower than the collar of a shaft near the north- ast end of the workings. The levels consist mostly of Irifts trending about N. 45° E. Another shaft, now bandoned, is near the center of the deposit. Each of the ive upper levels ends in non-commercial rock at a dis- ance of about 750 feet southwest of the centerline of the urrently-used shaft, but only the 65-foot and 125-foot evels actually connect with the shaft. The 315-foot level vas being developed in September 1951, when the writer ast visited the deposit. The northeastern end of each level ies 50 to 100 feet farther southwest than the end of the evel above, thus reflecting the southwest ward plunge of his end of the deposit. Where the deposit contains the wo mineable layers of talcose rock the levels consist of wo parallel drifts joined by cross-cuts. During the period 1940-41 the deposit was stoped to he surface from an adit (65-foot level) with a portal tear the southwest end of the deposit. The original shaft vas sunk on the Number One vein at a point about 250 eet northeast of the adit portal. This shaft, sunk at an ncline of about 45°, intersected the adit and was extended o the 125-foot level. In 1941, caving between the surface and the 65-foot evel caused the abandonment of the upper part of the outh shaft, and led to the opening of the shaft now in ise near the northwest end of the deposit. This shaft, nclined about 60°, was constructed by the enlargement >f a raise between the 65-foot level and the surface. It vas later bottomed in dolomite at the 125-foot level, and vas joined to the talc-bearing zone by a 60-foot cross-cut o the northeast. The 180-foot level was first reached by a steep winze ollowing the Number One vein from the 125-foot level. jater the two levels were joined by a less steep winze cut rom the Number One vein on the 120-foot level to the dumber Two vein on the 180-foot level. In 1949 the 215-foot level was opened by means of a 0° winze sunk near the southwest end of the 180-foot ?vel. This winze was subsequently extended upward as raise to the 125-foot level. In 1950 the 260-foot level /as reached by deepening the same winze. From a point n the Number Two vein on the 260-foot level 280 feet ortheast of the bottom of this winze, another winze was unk to open the 315-foot level. The slope distance from the •ottom of this winze to the surface is 560 feet. By mid-1951 the operators had recovered most of the nown talcose rock that existed between the surface and he 260-foot level and that could be removed without aving parts of the mine. White Cap Mine. The White Cap mine workings are omposed of an inclined shaft, 200 feet long, with ppended levels at slope distances of 50, 100, 1 Id, 160, and 00 feet. The shaft trends S. 20° E. and is inclined aboul 5° in its upper half and about 45° in its lower half. The n'els are drifts following the talcose zone and trend from 1 50° E. to N. 70° E. The talc has been removed chiefly by verhand stopes driven from the 100- and 200-foot levels. 3ach of these two levels, about 360 feet and 240 feet long espectively, extends for nearly equal distances on both sides of the shaft. The 50-foot level extends about 60 feet to the northeast and 40 feet to the southwest; the 140- foot level, about 60 feet to the northeast ; and the 160- foot level, about 50 feet to the southwest. SUGGESTIONS FOR PROSPECTING Because the talc bodies of proved commercial interest lie near the base of the Crystal Spring carbonate member, the search for talc in the southern Death Valley-Kingston Range region should involve a recognition of this part of the section and an inspection of the contacts between it and the lower diabase sill. The base of the carbonate member in many places is covered with talus from the more resistant dolomite above or with shallow sand dunes. Talc bodies may lie thus hidden, particularly in areas where the alteration is known to be intensive. It should be remembered, however, that at many places the alteration rocks are subcommercial or exist in bodies too small to be profitably mined. TREATMENT OF TALC Prom Dunn siding the talc is hauled in covered cars to mills in Los Angeles and in Ogden, Utah. Treatment, a dry process, consists of 1) crushing, in a jaw crusher, to fragments less than one inch in diameter, and 2) grind- ing in a Raymond mill in closed circuit with a Raymond air separator. The fineness of grinding depends largely upon the intended use. Talc to be marketed to the paint and textile industries generally is ground to 99 5-99.7 per- cent minus 325 mesh ; talc to be used as a wall tile ingre- dient is ground to 96 to 99 percent minus 200 mesh. The Superior and "White Cap tales ordinarily are blended with other talcs to meet commercial specifica- tions. The ground talcs, either blended or unblended, are packed by a semi-automatic packer into 50-pound bags. USES AND PROPERTIES OF THE GROUND TALC In general the uses and properties of the ground Superior and White < Jap talcs are similar to those of talcs from numerous other deposits in the region. Considerably more than one-half of the total output of all these deposits has been used as a raw material in the manufacture of wall tile. The remainder has been marketed chiefly as an extender in paints, and in smaller amounts as a filler in textiles and paper, as a filler and dusting agent in rubber and in roofiii"; and as an insecticide carrier. The use of tales from the Death Valley-Kingston Range region as wall tile ingredients steins largely from their high fusion point, low shrinkage, resistance to heat shock, and their whiteness when fired. Their value as paint ex- tenders is derived from whiteness of the ground material, hi, 27-28 percent MgO, 1.5-2 percent A1 2 3 , 5-7 percent CaO, less than 0.5 percent iron oxide, and less than one percent alkalies. Because commercial talcs commonly contain sili- cates of similar compositions, chemical analyses are noi reliable guides to mineral content. Analyses are used chiefly in the control of carbonate and iron oxide fractions Ground talcs from the southern Death Valley-Kingstor Range region have acicular or platy particle shapes. Mos1 are mixtures of botli types. Particle shapes are thought tc be significant in such properties as the rate of settling and durability of paints, and shrinkage of ceramic bodies. printed in California state printing office 58351 3-52 2M