Vitl Geology and Quicksilver” Deposits of the New Almaden District Santa Clara County ,6”? ., v. fiéo California W GEOLOGICAL SURVEY PROFESSIONAL' PAPER 360 K \— Prepared in cooperation with the State of California, Resources Agency, Department of Conservation, Division of Mines and Geology Geology and Quicksilver Deposits of the New Almaden District Santa Clara County California By EDGAR H. BAILEY and DONALD L. EVERHART GEOLOGICAL SURVEY PROFESSIONAL PAPER 360 Prepared in cooperation with the State of California, Resources Agency, Department of Conservation, Division of Mines and Geology UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964 Car for Larih Sci Lib. UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas. B. Nolan, Director The US. Geological Survey Library has cataloged this publication as follows: Bailey, Edgar Herbert, 1914— Geology and quicksilver deposits of the New Almaden dis- trict, Santa Clara County, California, by Edgar H. Bailey and Donald L. Everhart. Washington, U.S. Govt. Print. Off.,1963. vii, 206 p. illus., maps. diagrs., tables, and portfolio (fold. col. maps, fold. col. diagrs.) 29 'cm. (U.S. Geological Survey. Pro- fessional paper 360) Prepared in cooperation with the State of California, Dept. of Nat- ural Resources, Division of Mines. Bibliography: p. 195—201. , (Continued on next card) Bailey, Edgar Herbert, 1914- Geology and quicksilver deposits of the New Almaden district, Santa Clara County, California. 1963. (Card 2) 1. Geology—California—Santa Clara Co. 2. Quicksilver—Califor- nia—Santa Clara Co. 3. Ore-deposits—California—Santa Clara Co. 4. Mines and mineral resources—California—Santa Clara Co. I. Elverhart, Donald Lough, 1917— joint author. II. California. Divi- sion of Mines. III. Title: New Almaden district, Santa Clara County, California. (Series) For sale by the Superintendent of Documents, US. Government Printing Oflice Washington, D.C., 20402 CONTENTS Abstract ___________________________________________ Introduction _______________________________________ Scope of the report ______________________________ Location and accessibility ________________________ Topography ____________________________________ Climate and vegetation _________________________ Previous work __________________________________ Present investigation ____________________________ Mapping methods ______________________________ General geology ____________________________________ Franciscan group _______________________________ Bibliography of the Franciscan group _________ Clastic sedimentary rocks ____________________ Graywacke _____________________________ Siltstone and shale ______________________ Alta ___________________________________ Conglomerate __________________________ Organic and chemical sedimentary rocks _______ Limestone _____________________________ Chert _________________________________ Volcanic rocks (greenstones) _________________ Lavas _________________________________ Pyroclastic rocks _______________________ Nontachylitic tufl's and breccias ______ Tachylitic tufl's and breccias _________ Chemical composition of the greenstone- __ - Metamorphic rocks _________________________ Hornblende rocks _______________________ Glaucophane rocks ______________________ Chlorite-la. wsonite rock __________________ Quartz- muscovite schists _________________ Quartz-actinolite schist __________________ Actinolite-chlorite rocks _________________ Metachert _ _‘ ___________________________ Origin of metamorphic rocks _____________ Age of the Franciscan group __________________ Thickness of the Franciscan group ____________ Origin of the Franciscan group _______________ Serpentine _____________________________________ Gabbros and related rocks _______________________ Silica-carbonate rock ____________________________ Other Upper Cretaceous rocks _____________________ Eocene rocks ___________________________________ Temblor and Monterey formations ________________ Santa Clara formation ___________________________ Quaternary alluvium ____________________________ Landslides _____________________________________ Page 1 Geologic structure __________________________________ 3 Methods used in determining the structures ________ 3 Preliminary outline of the geologic structure _______ 3 Major structural blocks __________________________ 3 Santa Teresa block __________________________ 6 Los Capitancillos block ______________________ 6 E1 Sombroso block ___________________________ 7 Folds __________________________________________ 8 Folds in rocks of the Franciscan group ________ 10 Folds 1n post- -Franciscan rocks ________________ 10 Faults _________________________________________ 11 Shear zones ________________________________ 12 Strike-slip faults ____________________________ 13 Dip-slip faults ______________________________ 17 Faults due to intrusion of serpentine __________ 18 Tension fractures ___________________________ 20 Ore deposits ________________________________________ 21 Quicksilver ore bodies_________-____' _____________ 21 Mineralogy ________________________________ 25 Ore minerals ___________________________ 30 Accompanying sulfides, _________________ 31 Gangue minerals ________________________ 33 ‘ Character of the ores ________________________ 33 ‘ Size of‘ the ore bodies ________________________ 34 ‘ Grade of the ore bodies ____________ i __________ 35 Localization of the ore bodies _____________ .___ 37 Lithologic control _______________________ 39 Structural control ____________________ 1- _ _ 40 Examples of ore control _________________ 41 Genesis ________________________________________ 42 Age of_ the quicksilver ores ___________________ 42 Character of the mineralizing agents __________ 42 Depth of deposition of the quicksilver ores_ 42 j Pressure and temperature of ore deposition- 43 Relation to intrusive rocks____,_ ___________ 45 Relation to mineral springs ______________ 46 Summary of origin of the quicksilver ore 46' bodies_,_.____-________-g ______________ 47 Other metallic deposits ______________________ 57 COpper ............................... '— 58 Chromite _______________________________ Manganese _____________________________ 64 . . 4 Nonmetalhc mineral resources- _ _ _ _ _ ._ _________ 6,9 Building stone __________________________ 71 Limestone _____________________________ 75 Serpentine _____________________________ 76 , Gravel, sand, and loam- _ _ _ _ _ _ . _ _ _ _ 1 _____ 76 Road. meta1_______1 ____________________ 111 V 868 99 102 106 107 108 108 110 112 122 122 122 122 123 124 125 125 125 125 126 126 126 126 126 127 127 127 CONTENTS IV Page Mines—Continued Mines --------------------------------------------- 127 New Almaden Mines—Continued Page New Almaden mines ____________________________ 128 Senator mine _______________________________ 158 New Almaden mine _________________________ 128 Guadalupe mine ________________________________ 162 Cora Blanca area ----------------------- 130 Santa Teresa mine ______________________________ 169 Harry area —————————————————————————————— 133 Bernal mine ____________________________________ 169 V3135“) area ——————————————————————————— 135 Placer cinnabar deposit __________________________ 169 Central St'0pe area —————————————————————— 136 Future production __________________________________ 170 Victoria area ——————————————————————————— 139 Submarginal ore ________________________________ 171 North Randol area ___________________ V--- 141 High-grade ore _________________________________ 172 SOUth Randol area ---------------------- 144 Extensions of known ore bodies _______________ 172 San Francisco area ---------------------- 146 Satellite ore bodies __________________________ 172 Santa Mariana area --------------------- 148 Possible new ore bodies ______________________ 172 San Pedro-Almaden area _________________ 149 Other suggestions ___________________________ 176 America mine ______________________________ 150 Summary _______________________________________ 176 Providencia mine --------------------------- 151 History of the New Almaden mines ___________________ 176 Enriquita mine ----------------------------- 152 Annotated bibliography of the New Almaden mine__ 195 San Antonio mine ___________________________ 155 Literature cited _____________________________________ 199 San Mateo mine ____________________________ 156 Index ______________________________________________ 203 ILLUSTRATIONS [Plates are in separate volume] PLATE 1. Geologic map and sections of the New Almaden district. 2. Diagrammatic sketches showing the process of formation of the primary and placer quicksilver ores of the district. 3. Geologic map of the New Almaden mine area. 4. Composite map of the underground workings of the New Almaden and America mines. 5—10. Geologic level maps of the New Almaden and America mines. 11. Cross sections through the New Almaden mine area. 12. Geologic section through the Harry area and block diagrams of the Cora Blanca and San Francisco areas of the New Almaden mine. 13. Geologic map and section of the America mine. 14. Geologic map and sections of the Guadalupe-Senator mine area of the New Almaden district. 15. Composite map of the underground workings of the Guadalupe and Senator mines. 16—18. Geologic composite level maps of the Guadalupe and Senator mines. Page FIGURE 1. Index map showing the location of the New Almaden district ________________________________________ 4 2. Shaded contour map showing the main topographic features of the district and places referred to in the report _______________________________________________________________________________________ 5 3. Massive graywacke of the Franciscan group ________________________________________________________ 13 4. Deformed graywacke exposed in roadcut between Bald Mountain and the crest of the Sierra Azul ________ 14 5. Photomicrograph of feldspathic graywacke of the Franciscan group ___________________________________ 15 6. Photomicrograph of lithic graywacke of the Franciscan group ________________________________________ 15 7. Photomicrograph of siltstone of the Franciscan group _______________________________________________ 18 8. Alta lying above intrusive serpentine ______________________________________________________________ 19 9. Hand specimen of alta ___________________________________________________________________________ 19 10. Photomicrograph of alta _________________________________________________________________________ 19 11. Outcrop of Calera-type limestone occurring in the Franciscan group ___________________________________ 22 12. Thin beds of limestone of the Franciscan group interlayered with tuffaceous shale _______________________ 22 13. Calera type of limestone of the Franciscan group at locality from which Upper Cretaceous Foraminifera have been obtained ___________________________________________________________________________ 22 14. Oolitic limestone of the Franciscan group __________________________________________________________ 23 15. Photomicrograph of oolitic limestone of the Franciscan group ________________________________________ 23 16. Fossils from the limestone of the Franciscan group __________________________________________________ 25 17. Sharp folds in bedded chert of the Franciscan group _________________________________________________ 26 18. Vertical cut through ”cauliflower” of botryoidal chert, showing internal structure _______________________ 27 19. Photomicrograph of chert of the Franciscan group containing Radiolaria _______________________________ 28 20—23. Photomicrographs of greenstones of the Franciscan group ____________________________________________ 32 24. Polished sections of drill cores showing two kinds of tuffaceous greenstone in the Franciscan group ________ 34 25. Photomicrograph of calcareous tuff from the Franciscan group ________________________________________ 35 26. Polished surface on breccia composed chiefly of fragments of altered tachylite __________________________ 36 FIGURE 83. 84. CONTENTS . Photomicrograph of section of altered tachylitic tuff of the Franciscan group ___________________________ . Map showing distribution of metamorphic rocks of the Franciscan group and serpentine _________________ . Photomicrograph of hornblende-albite gneiss _______________________________________________________ . Photomicrograph of garnet-hornblende gneiss _______________________________________________________ . Polished surface on glaucophane~quartz schist ______________________________________________________ . Polished surface on chlorite-lawsonite rock _________________________________________________________ . Polished surface on silicified tuffaceous greenstone showing typical “orbicular jasper” structure ___________ . Photomicrograph of metachert containing needles of crocidolite _______________________________________ . Blocky serpentine exposed in fresh roadcut _________________________________________________________ . Typical boulder strewn surface developed on areas underlain by blocky serpentine ______________________ . View of margin of serpentine mass in the low foothills of the New Almaden district _____________________ . Polished surface on fresh serpentine derived from dunite _____________________________________________ . Photomicrographs of dunite partly replaced by serpentine minerals ____________________________________ . Photomicrographs of serpentine derived from peridotite ______________________________________________ . Silica-carbonate rock derived from sheared serpentine _______________________________________________ . Polished surface on silica-carbonate rock derived from slightly sheared serpentine _______________________ . Photomicrograph of altered serpentine showing early stage in development of silica-carbonate rock ________ . Photomicrograph of magnesite replacement of thick veins of chrysotile ________________________________ . Photomicrograph of silica-carbonate rock derived from serpentine that was derived from dunite __________ . Photomicrographs of silica-carbonate rock derived from sheared serpentine ______________________________ . Diagram showing gains and losses in alteration of serpentine to silica-carbonate rock _____________________ , Interbedded graywacke and shale of Late Cretaceous age in the Sierra Azul _____________________________ . Casts of Inoceramus sp ___________________________________________________________________________ . One of the smaller quarries in Upper Cretaceous sandstone in the Santa Teresa Hills _____________________ . Dome formed by spheroidal weathering ________ - _____________________________________________________ . Igloolike rock resulting from spheroidal and cavernous weathering _____________________________________ , Photomicrograph of arkosic sandstone showing glauconite that has replaced biotite _______________________ . View northeastward from Bald Mountain showing outcrops of Temblor formation _______________________ . Topographic map and section of landslide in the New Almaden mine area _________________________________ . Map showing distribution of Calera—type limestone and altered tachylitic greenstone _____________________ , Map showing structural blocks in the New Almaden district and the major faults in them ________________ Map showing the major structural features in the older rocks __________________________________________ , Plastically deformed and sheared rocks of the Franciscan group ________________________________________ Interbedded graywacke and siltstone showing typical irregularities due to rock flowage and minor faulting--- . Plastically deformed and faulted layer of greenstone __________________________________________________ . Map showing the major structural features in the post—Franciscan rocks ________________________________ . Fairly regular contact between serpentine and alta ___________________________________________________ . Irregular contact between serpentine and alta _______________________________________________________ . Photomicrograph of section across banded dolomite vein containing cinnabar, pyrite, and hematite _________ _ Stibnite in dolomite vein cutting earlier dolomite vein _________________________________________________ . Silica-carbonate rock cut by typical hilo ____________________________________________________________ . Postore dolomite vein _____________________________________________________________________________ . Photomicrographs of “froth vein” formed by deposition from immiscible fluids __________________________ . Polished specimen of rock ore from the New Almaden mine ___________________________________________ _ Large polished slab of ore from the New Almaden mine _______________________________________________ . Polished surface on ore specimen from Randol workings of the New Almaden mine _______________________ ‘ . Polished surface on rich ore formed by replacement of silica-carbonate rock, which, in turn, had replaced sheared serpentine _____________________________________________________________________________ , Extremely rich replacement ore from the New Inclined shaft area of the Guadalupe mine _________________ Key to figure 74 _________________________________________________________________________________ _ Photomicrograph of rich ore formed by replacement of silica-carbonate rock by cinnabar __________________ Photomicrograph of high-grade ore formed by replacement of silica-carbonate rock by cinnabar _____________ . Map of La Ventura stope of the New Almaden mine ________________________________________________ . Map of the New Ardilla stope of theNew Almaden mine showing the localization of ore along a swarm of hilos- . Map of a part of the 600 level of the Harry workings of the New Almaden mine showing an area of unmin- eralized silica-carbonate rock containing swarms of hilos ___________________________________________ . Map show distribution of stopes along the upper surface of the main serpentine sill in the New Almaden mine- . Map showing the localization of the Machine stope ore body of the New Almaden mine beneath a small dome on the upper contact of an intrusive sill ____________________________________________________ Map showing the localization of the New World stope ore body of the New Almaden mine beneath a plunging inverted trough on the surface of an intrusive sill _________________________________________________ Cross section through the Giant Powder-Ponce stope area of the New Almaden mine showing the localization of ore in an anticlinal warp on the upper surface of a sill of serpentine _______________________________ V Page 37 38 39 40 41 42 43 43 48 49 49 50 51 53 59 60 61 61 61 62 63 66 66 67 67 68 68 72 77 80 81 106 108 109 110 110 111 112 113 114 115 116 116 VI FIGURE 85. 86. 87. 88. 89. 90. CONTENTS Cross section through the Warren stopes of the New Almaden mine showing ore bodies formed in a thin apophysis diverging upward from a large intrusive sill _____________________________________________ Map of La Ventura stope area of the New Almaden mine showing localization of ore bodies along the lower contact of an intrusive sill _____________________________________________________________________ Map of the Curasco stope area of the New Almaden mine showing localization of ore along the lower side of ‘ a sill where it forms a plunging inverted trough ___________________________________________________ Cross section through the Far West, Santa Rosa, and E1 Collegio stope area showing ore bodies localized near the lower contact of an intrusive sill _____________________________________________________________ Map of the Moreno and Water stope area of the Guadalupe mine where ore bodies were formed along hilos that are not near intrusive contacts _____________________________________________________________ Map showing the upper stopes and workings of the Cora Blanca area of the New Almaden mine where the ore occurred in veins and in fault gouge in tuff _______________________________________________________ . Graph showing production of the New Almaden mines, 1847—1945 inclusive ____________________________ . Index map of the New Almaden mine area showing parts of the mine described as separate units in the text--- - . Perspective drawing showing the relation of the Randol and Victoria stopes to the upper surface of the upper serpentine sill in the New Almaden mine _________________________________________________________ , Tramming on the 1500 level of the New Almaden mine ---------------------------------------------- . Stope on the 1500 level of the New Almaden mine __________________________________________________ . Geologic map of the Enriquita mine --------------------------------------------------------------- . Geologic map of the more recent workings of the San Antonio mine ----------------------------------- . Map of the San Mateo mine _____________________________________________________________________ . Ruins of the mercury reduction plant at the Senator mine in 1947 ------------------------------------ . Generalized section through New Almaden mine along the Day tunnel --------------------------------- . New Almaden reduction works in 1851 ____________________________________________________________ . New Almaden Hacienda in 1852 __________________________________________________________________ . View of Mine Hill in 1854 ----------------------------------------------------------------------- . Mexican miners in 1854 at one of the underground shrines in the New Almaden mine ------------------- . Labor, or stope, in the upper levels of the New Almaden mines in 1854 ________________________________ . Reduction furnace in use at the Hacienda in 1854-__---__________-_---___--________- ---------------- 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. The Hacienda, or furnace area, in 1854- _. __________________________________________________________ Trammin g large pieces of rich ore through the Main tunnel in the 1860’s ______________________________ View of China dump and Spanish camp about 1870 _____________ , ____________________________________ View of China dump area in 1874-_____-_-----______-________-----_-__-_-__-_- -------------------- Opencut on east side of Mine Hill in the 1880’s ----------------------------------------------------- Shaft houses for the famous Randol shaft_- - _ - - _ _. __________________________________________________ Miners in two-decked cage in Randal shaft----_ _ - _ - - - - _ - - - - ________________________________________ The Hacienda in 1875 ___________________________________________________________________________ Santa Isabel shaft buildings in late 1880’s ---------------------------------------------------------- Building housing the boilers, steam engines, and pumps for the Buena Vista shaft ------------------------ Hoisting equipment used at the Buena Vista shaft about 1885 ---------------------------------------- View of the Randol shaft hoisting works about 1885 ------------------------------------------------- Washington shaft buildings in the middle 1880’s ---------------------------------------------------- Shaft house of the Victoria shaft _________________________ ‘_' ________________________________________ Harry shaft buildings ___________________________________________________________________________ Square-set timbering used atNew Almaden--_-_--_-_---___---------__-_-______________-_- _________ Opencuts developed on Mine Hill between 1940 and 1945 -------------------------------------------- Rotary furnace plant built on Mine Hill in 1942 ---------------------------------------------------- Mining equipment employed on Mine Hill in 1945 -------------------------------------------------- Page 117 117 118 119 120 121 129 131 140 143 144 153 155 157 160 174 178 179 180 181 181 181 TABLE 1. .N OJUINPW {000‘} 10. 11. 12. 13. 14. 15. 16. 17. CONTENTS ' TABLES Analyses of graywacke from the Franciscan group, with average of 30 graywacke and 40 granodiorite analyses parison ___________________________________________________________________________________________ . Analyses of alta and siltstone from the Franciscan group, New Almaden district, Santa Clara County __________ . Pebbles of a conglomerate of the Franciscan group exposed on Cemetery Hill, New Almaden district, Santa Clara- _ . Analyses of Calera-type limestone from near Permanente Creek, Santa Cruz quadrangle ______________________ . Bulk analyses of Calera-type limestone from quarries of the Permanente Cement Co. near Permanente Creek, Santa Cruz quadrangle _____________________________________________________________________________ . Analyses of cherts and shales of the Franciscan group ____________________________________________________ . Analyses of greenstones of the Franciscan group, with composite analyses of diabase and spilite for comparison__ . Analyses of serpentine rocks from the New Almaden district and from elsewhere in the California Coast Ranges, together with a composite of 24 serpentines from other regions for comparison ____________________________ Partial analyses of serpentine rocks from the New Almaden district and adjacent area ________________________ Analyses of rocks from the New Almaden mine showing change from serpentine to Cinnabar-bearing silica-carbonate rock _____________________________________________________________________________________________ Analyses of rocks from the New Almaden mine showing change from serpentine to silica-carbonate rock ________ Average chemical composition of serpentine and silica-carbonate rock from the New Almaden mine, together with calculations to show chemical changes involved in the formation of silica-carbonate rock from serpentine- _ Pebbles from conglomerate in the Upper Cretaceous rocks of the Sierra Azul and the Franciscan group, Santa Clara County _____________________________________________________________________________________ Composition and maximum indices of refraction of carbonates from the New Almaden district _________________ Approximate yield of quicksilver per linear foot of workings in some California quicksilver mines ______________ Annual production of the Guadalupe mine, Santa Clara County ___________________________________________ VII Page 1 7 18 20 20 24 54 54 62 63 63 65 100 107 163 GEOLOGY AND QUICKSILVER DEPOSITS OF THE NEW ALMADEN DISTRICT SANTA CLARA COUNTY, CALIFORNIA By EDGAR H. BAILEY and DONALD L. EVERI—IART ABSTRACT The New Almaden district, situated a few miles south of San Jose in Santa Clara County, Calif, has yielded nearly 40 percent of the quicksilver produced in the United States. The area mapped as the district for this report includes about 80 square miles, extending south from the flat Santa Clara Val- ley across the moderately low foothills containing the mines to the more rugged crest of the California Coast Ranges. The rocks underlying about three-fourths of the district, in- cluding all of the mineralized area, are assigned on the basis of lithology to the Franciscan group of‘Late Jurassic and Cre- taceous age. The only diagnostic fossils found indicate that the age of a part of the group is lower Upper Cretaceous (Cenomanian). The group consists of lithic and feldspathic graywacke, siltstone, dark altered volcanic rocks, chert, lime- stone, and a subordinate amount of metamorphic rocks, in- cluding glaucophane schists—an assemblage regarded as a typi- cal eugeosynclinal suite. The thickness of the group cannot be determined accurately because of structural complexities, but the part present in the New Almaden district is believed to be at least 10,500 feet thick and may be much thicker. The sedimentary rocks are believed to have been derived from the rapid erosion of a rising landmass, and deposited in a sub— siding trough which filled to wave base only in a few places near the end of the depositional period. The accompanying igneous activity included local outpourings of lava onto the sea floor and eruption of fragmental material of similar com- position which accumulated as pyroclastic beds. The chert is abundant only in the part of the Franciscan group that con- tains altered volcanic rocks, and it is believed to owe its origin to the reaction of hot lava with sea water. The post-Franciscan sedimentary rocks, which do not occupy large areas, range in age from Upper Cretaceous to Recent. Two sedimentary units, differing in both lithology and degree ' of deformation, have been assigned on the basis of a few fos- sils to the Upper Cretaceous. The next younger sedimentary rocks are dated as middle Eocene by fossils occurring in lime— stone lenses near the base of a sequence of sandstones and shales. Rocks of early, middle and late Miocene age contain- ing abundant fossils are divided into two formations, the rocks of which grade from sandstone through clay shale to diatoma- ceous shale. The sandy sedimentary rocks below the lowest diatomaceous bed have been assigned to the Temblor forma- tion, and this bed and the overlying rocks have been assigned to the Monterey shale. Some included salic volcanic material indicates igneous activity during the middle Miocene. The younger sedimentary rocks in the district are all gravel de- posits, which fill the larger valleys and occur as perched rem- nants on some of the lower foothills. They have been divided, largely on the basis of their dissection and topographic posi- tion, into the Pliocene and Pleistocene Santa Clara formation and Quaternary alluvium. Tabular masses of serpentine that have been intruded into the rocks of the Franciscan group are of particular interest because altered parts of them contained the quicksilver ore bodies. Some of these masses are conformable with the rocks of the group and are therefore sills; others are intruded along faults that dip more steeply than the bedding. The larger masses consist of blocks of unsheared serpentine embedded in a matrix of sheared serpentine; the smaller masses and the borders of the larger ones consist entirely of sheared serpen- tine. The internal structures and details of the contacts, to- gether with theoretical considerations, suggest that the ser- pentine masses were intruded as serpentine, not as peridotitic magma. The age of the serpentine cannot be closely placed by the evidence available in the district, but by comparison with other occurrences in the Coast Ranges it is believed to be Late Cretaceous. Silica-carbonate rock—the host rock for the quicksilver ore bodies—has been formed locally by hydrothermal alteration of the serpentine. 0n the thicker serpentine masses it commonly occurs as a thin peripheral shell, but some of the thinner masses are entirely converted to silica—carbonate rock. As the silica-carbonate rock was formed by replacement, it shows abundant relict textures inherited from the serpentine. The dominant minerals of the silica-carbonate rock in the New Almaden district are quartz and magnesite. The process of hydrothermal alteration that formed the rock consisted chiefly in bulk substitution of carbon dioxide for water, but a little magnesium was removed. The alteration took place in late Tertiary, probably early Pliocene time. Because of the differ- ence in age between the serpentine and the silica-carbonate rock, the hydrothermal solutions causing the change cannot be genetically related to the serpentine or its primary magma, but they may represent the early stages of the quicksilver mineralization. The structures within the district trend northwestward or westward, and are dominated by the structures in the rocks of the Franciscan group. As these rocks nearly everywhere show evidence of flowage, folding, or shearing, only the more continuous coarse structures can be traced through them. The Franciscan rocks dip in general to the north, but an anticlinal flexure helped to localize many of the large ore bodies of the New Almaden mine. The post-Franciscan rocks that antedate the alluvial gravels occupy synclinal troughs that are nearly everywhere separated from the rocks of the Franciscan group by faults, but they are less crumpled than the Franciscan. The gravels are locally tilted and cut by normal faults of rela- tively small displacement. 2 GEOLOGY AND QUICKSILVER DEPOSITS, The faults in the district are nearly parallel in strike to the rocks of the Franciscan group, but dip more steeply. The larger displacements are believed to be dominantly strike slip, the block southwest of a fault having moved northwestward, but the direction and amount of displacement can be directly determined for only a few of the faults. Along the largest faults there are wide shear zones formed before the deposi- tion of the late Upper Cretaceous rocks; so the first movement occurred in early Upper Cretaceous time. Other faults were active after the deposition of the upper Miocene rocks, and a minor amount of faulting took place after the deposition of the Pliocene and Pleistocene gravels. Many of the younger faults followed older shear zones and indicate recurrent movement along these zones. The most pronounced structural break is the Ben Trovato shear zone, which trends west-northwestward, parallel to the nearby San Andreas fault, through the central part of the dis- trict. It extends beyond the limits of the district, and in places it has a width of more than 4,000 feet. Its apparent horizon- tal offset is about 10 miles. A second shear zone, which is partly covered by younger rocks and partly obliterated by in- trusive bodies of serpentine, diverges from the Ben Trovato shear zone east of Los Gatos and extends eastward across the district. It has been followed by the post-Miocene Shannon fault. Many of the other faults diverge from the shear zones at small angles, and they are belie ed to have less offset. Steep fractures, trending nort ‘, to northeast, traverse only the silica-carbonate rock, but theSe are of special interest be- cause they bear a close relation to many of the ore bodies. These fractures generally extend into the silica-carbonate rock from the margins of the intrusive bodies for only a few score feet, and are widest close to the contact with the rocks of the Franciscan group. The fact that they have a uniform trend, re- gardless of the attitude of the silica-carbonate rock or of the serpentine sill from which it formed, indicates that they origi- nated in response to a regional force, and therefore they may be regarded as tension fractures. Quicksilver ore in place was recognized for the first time within the present confines of the United States on Mine Hill in 1845, but the bright red cinnabar that cropped out there had earlier attracted the attention of Indians and Mexican settlers. 'The subsequent development of the New Almaden mine, largely in Mine Hill, had resulted by the end of 1948 in a production of 1,046,198 flasks of quicksilver, which sold for $50,000,000. The ores of the Guadalupe mine, whose outcrops were found a little later, have yielded about 112,600 flasks of quicksilver, placing this mine sixth in rank among California producers. The other mines in the district were also first de- veloped many years ago, but although they have been inter- mittently active, they have made only a comparatively small production. Since 1890 the production from the district has been small as compared with its earlier output. The mineralogy of the quicksilver ore bodies is simple. The only ore mineral of much economic importance is cinnabar, although locally native mercury impregnates and enriches the ores. Accompanying sulfides, present in only small amounts, include pyrite, stibnite, chalcopyrite, sphalerite, galena, and bornite. The gangue minerals introduced by the mineralizing solutions are dominantly quartz and dolomite with some hy- drocarbons, but a few other minerals occur here and there. Most of the quicksilver ore is of primary origin, although one alluvial deposit containing nuggets of cinnabar ore has been mined. The typical primary ore bodies were composite; NEW ALMADEN DISTRICT, CALIFORNIA the cinnabar in them was deposited in part by replacement of silica-carbonate rock along steep northeastward-trending frac- tures, and in part by filling of the open spaces provided by the fractures. The replacement extended only a few inches out- ward from these fractures, but Within this limit it was so complete that commonly more than 50 percent of the replaced rock was cinnabar. In many of the ore bodies the steep frac— tures occurred in swarms, so closely spaced that much of the intervening silica—carbonate rock was converted to rich ore. In most places the fractures were filled with quartz and dolomite containing very little cinnabar, but in some places the vein filling was sufficiently mineralized to form ore even where the walls were not mineralized. The ore bodies that have been mined were large and excep- tionally rich. The largest was about 200 feet wide and 15 feet thick, and extended down the dip for about 1,500 feet. The ore furnaced in the first 15 years of mining at the New Alma- den mine contained more than 20 percent quicksilver, but to obtain this amazingly high grade the ores were cobbed and hand sorted. In the course of time the grade steadily declined to less than 0.5 percent, owing to the utilization of lower grade ores and less careful mining and sorting. The remarkable richness of much of the ore, however, is well indicated by the fact that the average grade of all the ore furnaced in the hundred years during Which the New Almaden mine was pro- ductive is only a little less than 4 percent quicksilver, or about a flask of quicksilver per ton. The ore bodies in the district are not distributed at random; nearly all are restricted to certain rocks and certain struc- tural environments. A consideration of these lithologic and structural factors together with the geology of the area indi- cates that some places in which there is a reasonable hope of finding ore remain unexplored. Nearly all the ore bodies were formed in silica-carbonate rock, although this rock occupies only a very small part of the district. Furthermore, only the silica-carbonate lying close to the contact with the rocks of the Franciscan group is particularly favorable for ore deposition, and most of the ore bodies were richest within a few feet of this contact. The distribution of the ore bodies along the contacts apparently was influenced by two other structural factors, whose importance varied with the steepness of the contacts. Where the contacts were steep, swarms of cross fractures took a dominant part in localizing ore bodies, but where the contacts were inclined at less than 45°, the shape of the contact itself was of equal or greater importance; along such contacts the ore bodies tended to form at the crests of domes or plunging anticlines. The quicksilver ore is believed to have been deposited during the Pliocene epoch by hydrothermal solutions rising from a deep-seated source. These solutions followed fractures which were best developed in the silica-carbonate rock near contacts with rocks of the Franciscan group. Deposition of the cinna- bar took place through a vertical interval extending from near the surface to a depth of about 2,600 feet, and in a tempera- ture range believed to have been from 50° to 150°C. The rich- est ore bodies were localized along gently dipping contacts, Where the solutions spread out and stagnated under a capping of relatively impervious sheared rocks of the Franciscan group; but along steep contacts replacement by solutions flowing through fractures took place even where structural traps were absent. Although most of the ore bodies are in the silica- carbonate rock formed along the top sides of serpentine sills, INTRODUCTION 3 some equally rich oneswere formed in similar rock along the lower side of the sills. The descriptions of the various mines of the district are intended chiefly to be of aid to those interested in their fur- ther development, but the descriptions also contain details of geology and ore controls that will be of service to those desir- ing a thorough understanding of the deposits. Following each description is a section devoted to a comprehensive analysis of the possibilities for further development. In this section we have considered the known sources of submarginal ore and have also tried to indicate where ore of high grade might per haps be obtained. As mining in the district was carried on during the last 40 years without adequate exploration and development, the amount of submarginal ore that is readily accessible is small. On the other hand, the geologic structures in someof the mines, when considered along with the factors responsible for the localization of the rich ore bodies, indicate that intelligent and aggressive development, supported by ade— quate funds, can be expected to reveal new ore bodies. Such ore bodies, if as rich as those previously mined, would be minable even during periods when the value of quicksilver is low. A history of the New Almaden mine extends through a pe- riod of more than 100 years. The cinnabar was first used by Indians as a pigment. While California was under Mexican rule, the mine was developed according to ancient Spanish mining methods. Later, after the admission of California to the United States, title to the property was obtained by an American mining company through a series of legal battles fought through State and Federal Courts and finally settled by international arbitration. The mining history is exception- ally interesting because it begins with primitive methods and extends through a period when new techniques for mining the ores and new methods of recovering mercury from them were first introduced at New Almaden; in essence, it provides a his- tory of mining and metallurgy for the mercury mining indus- try of the United States. INTRODUCTION SCOPE OF THE REPORT The first recognition of quicksilver ore in the United States was made in 1845 in the New Almaden district, and since then the district has yielded nearly 40 per— cent of all the quicksilver produced in this country. Most of its production came from the famous New Almaden mine, which is one of the great quicksilver mines of the world, but the district contains other formerly productive mines, including the Guadalupe, which ranks sixth among the quicksilver mines of California. In spite of its prominence, the district had been little studied until a comprehensive geologic investigation was made during and after World War II by the Geological Survey. The prime pur- pose of this study was to determine whether this district should be regarded as exhausted, or whether it may still contain hitherto unknown ore bodies. Because the New Almaden quicksilver deposits are similar in origin and environment to many others in California, parts of this report may be applied equally well to several other quicksilver districts in the State. The mines lie in an area of unusual sedimentary and volcanic rocks making up the Late Jurassic and Cretaceous Franciscan group, which is exposed in, or believed to underlie, at least 30,000 square miles in California. As the parts of the district containing these rocks are of great economic interest, they have been studied more intensively and mapped in greater detail; other parts containing only younger formations have been mapped and studied less thoroughly. LOCATION AND AG CESSIBILITY The area mapped for this study as the New Al- maden district includes about 80 square miles in the west—central part of Santa Clara County, Calif, about 9 miles south of San Jose and 50 miles southeast of San Francisco. (See fig. 1.) On the General Land Office grid it includes parts of T. 8 and 9 S., R. 1 W., and R. 1 and 2 E., Mount Diablo meridian, and it lies in the northern third of the Los Gatos 15—minute quadrangle. The mines of the district were once served by a branch line of the Southern Pacific railroad, but this line was abandoned and the track removed many years ago; the mines are now reached by good paved and ballasted roads extending from San Jose and Los Gatos. Branch roads lead to many ranches and sum- mer homes remote from the mineralized belt, so that few parts of the district are more than a few miles from a passable road. TOPO GRAPHY The topography in most of the district reflects the dominant northwesterly trend of the bedrock struc- tures, so that the main ridges and valleys trend north- westward. (See fig. 2 and pl. 1.) In the northern part of the district, however, the bases of the moun— tain ridges have been overlapped by the alluvium fill— ing the southern part of the Santa Clara Valley, which slopes gently northward to San Francisco Bay. The alluvium nearly everywhere separates the northern- most of the three principal ridges in the district from the other two, and alluvial tongues extend up some of the valleys in the next ridge to the south. The other two main ridges are less distinctly separated, because the longitudinal valleys between them lie above the general slope of the Santa Clara Valley and they are therefore sharply incised and devoid of alluvial filling. The northernmost ridge—the Santa Teresa Hills—— emerges from the alluvium at an altitude of about 200 feet in its western end and attains a maximum height of 1,150 feet at Coyote Peak near the eastern edge of the district. The next ridge to the south, across the valley of Alamitos Creek,,is of special in- GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 123° 122° 121° ‘I ‘ ”W I I I f c s I ' OLU A MENDOCINO ‘ SUTTER \ YI‘JBA \ ) ‘ I / . \l | ’- 1 — 39 I yr I ,1 I ‘T/‘L I YOLO ( T‘ \ DISTRICT GUERNEVILLE \—\,/"\""| S’Cf‘AMENTO DISTRICT \K NAPA / z I ‘ I g g “ O SONOMA ‘ \\ E ! ’ 5 \ \ -—‘ 2 —/ e \ \ é— a l 5-" \ \ g SOLANO ) ei/ ~— \"\ I z 2 ’ ' \ m3 (mm 24w: m_ 23.50 22 252:3 Emcmfimzc SEE? zmz xwazw _8_mo_oww ‘m j E0: mmmm brim mfg .‘ _, . um m_m, __ boumma .mQLNH m on 6 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA terest because it contains all the highly productive quicksilver mines. It is known, in its central part at least, as Los Capitancillos Ridge. At its northwest and it rises abruptly from the Santa Clara Valley to an altitude of about 800 feet, and to the southeast it rises gradually to 1,750 feet on Mine Hill. Farther to the southeast it is more dissected, but, nonetheless, its higher peaks reach approximately the same altitude. This ridge is sharply cut in three places by the trans- verse Guadalupe, Alamitos, and Llagas Creeks, which flow into Santa Clara Valley, and longitudinal tribu- . taries of these creeks separate it on the south from the third parallel ridge. This third ridge, the Sierra Azul, is a part of the backbone of the California Coast Ranges and is con- siderably higher than either of the others. It extends for several miles with altitudes only a few hundred feet above or below 3,400 feet, but near the western boundary of the district it also is breached by Los Gatos Creek, which flows at grade with the Santa Clara Valley. The slopes of the hills vary considerably in steep— ness. In general, the Santa Teresa Hills are fairly subdued, the Los Capitancillos Ridge moderately rug— ged, and the Sierra Azul decidedly rugged. In spite of the general ruggedness of the area, the crests of all the main ridges are characterized by general slopes and local flats. Landslides, ranging in length from a few tens of feet to a mile, are common topographic features on the Los Capitancillos Ridge and the lower slopes of the Sierra Azul. On these same ridges in areas where no distinct slides can be recognized, there are extensive slopes of excessively rocky soil which has moved downslope by creep for long dis- tances. The canyons in these areas are V—shaped, but their troughs are so charged with loose rock that they ofler very limited exposures of bedrock. CLIMATE AND VEGETATION The climate of the district is generally mild but varies somewhat with the altitude. In the Santa Clara Valley the temperature drops a little below freezing a few times each winter, and summer temperatures rarely exceed 100°F; the usual daily variation in tem— perature, however, is rather great. Precipitation gen- erally occurs only during the winter and spring, and the wet and dry seasons are reported (Clark, W. 0., 1924, p. 40—42, 49) to be more sharply contrasted in the Santa Clara Valley than in any other part of the United States. The precipitation in the valley, which averages 20 inches per annum (Grunsky, 1908, p. 496— 543), falls amost entirely as rain; snowfall is so rare that whenever it comes there is a virtual holiday in 4‘ San Jose. In the higher parts of the district the tem- perature range is somewhat greater, owing largely to colder winter nights; and the average rainfall is about 40 inches per year. Some snow falls in the mountains each winter, but generally it melts quickly. The vegetation reflects the climatic differences due to altitude, although it is also influenced in a smaller degree by other features, such as northerly or south- erly exposure, kind of soil, and drainage. The broad valleys, which apparently were once carpeted with grass or wild oats and studded with oaks are now largely covered with prune and apricot orchards. Some parts of the lower hills still retain the wild oats and oak trees, but other parts are planted with vine- yards. Higher ground supports a thicker growth of trees with an undergrowth of poison oak in many places. Everywhere, however, there are scattered patches of grassland, and extensive areas, particularly at altitudes above 1,700 feet, are blanketed with “Chaparral,” a dense head-high growth of shrubs inter- mixed with small trees; the more abundant species are— Eastwood manzanita (Archlostaphylos glanduloss) California scrub oak (Quercus dumosa) Wartleaf ceanothus (Oetmothus papfllosus) Chaparral broom (Baccham's consangui) Chamise greasewood (Adenostoma fasciculatum) The wetter parts of the stream valleys, at altitudes above 2,000 feet, support scattered growths of various conifers. The vegetation in some areas is so closely controlled by the underlying rock that the distribution of the rocks may be roughly traced by the character of the vegetation. Such special lithologic control of the vegetation is discussed with the appropriate rock de— scription. PREVIOUS WORK Despite the prominence of the New Almaden dis— trict as the foremost quicksilver producer in the United States for more than a century, very little has been published about the geology of either the mine or the district. The only lengthy discussion of the geology is the one by G. F. Becker (1888, p. 310— 331, 467—468) in his monograph dealing with most of the domestic quicksilver deposits known in 1888. How? ever, his broad statements concerning the geology and ores and his generalized surface map were based on fieldwork of rather brief duration. The most valuable contribution in Becker’s report so far as this district is concerned consists of the excellent planimetric maps of the New Almaden mine workings. Forstner (1903, p. 168~187 ) added a few geOlogic observations in 1903, but he was much handicapped by the inaccesibility of I INTRODUCTION 7 many of the workings. Bradley (1918, p. 154-168) likewise was unable to get into many of the mine workings in 1918, and as a result relied largely on previous publications for descriptions of the local geology. In 1929 C. N. Schuette (1931, p. 411-417), on the basis of his familiarity with the parts of the New Almaden mine accessible since 1916 and his study of available company maps and records, included a discussion of its geology and ores in a summary of the salient features of quicksilver mines of the world. More recently, Ransome and Kellogg (1939, p. 450— 457) briefly described the geology and ores of the district in a summary account of the quicksilver mines of California, but again because of the inaccessibility of the mines, their principal contribution to geologic knowledge of the area consisted of the inclusion of a reconnaissance geologic map of the district made by John V. S. Tolman. In addition to these published articles we have had the benefit of private reports to the mine owners pre— pared by Luther Wagoner,1 Samuel B. Christy (1889), John A. Church (1892), Charles C. Derby (1908), J. H. Farrell,2 H. W. Gould,3 and C. N. Schuette (1935). The second of these, based on Dr. Christy’s 16 years of intermittent study in the New Almaden mine, was of particular value because it contained some geologic maps of workings that have been in- accessible for nearly half a century. PRESENT INVESTIGATION The US. Geological Survey investigation leading to this report extended over a period of 61/2 years (1941—47), and many geologists contributed to the resultant product. The authors, however, assume full responsibility for the final maps and text. In 1939, as a result of the urgent need to develop quicksilver resources for wartime uses, the Survey began a co— ordinated investigation of quicksilver districts in the United States under the direction of Edwin B. Eckel. Because of the shortage of geologists and the more urgent requirements elsewhere, no specific study of the New Almaden district was then made, but half a dozen of the Survey geologists then studying other quicksilver deposits began reconnaissance mapping of the district during times when they could be spared from other assignments. This preliminary mapping served to delimit the potentially mineral-ized area, and it also indicated that an exceptional amount of de— tailed study would be required before the complex 1Wagoner, Luther. 1881, Unpublished report on the Guadalupe mine. 3Farrell, J. IL, 1923, Unpublished private report on the Senator mine, October. 3Gould, H. W., 1925, Unpublished private report on the Guadalupe mine, November. geologic features could be adequately understood. In mid-1941 Lowell S. Hilpert and Paul Averitt were assigned to study the district, and they were joined in June 1942, by G. Donald Eberlein. Their work, be- cause of its urgency and time limitations, was devoted to deciphering the important structural control for the ore deposits by rapid, but locally detailed, surface and underground mapping. In September 1942, at the request of the US. Bu- reau of Mines, a drilling and sampling project was conducted jointly by the Bureau and the Survey at the Guadalupe mine, and this was followed, late in 1943 and during the first half of 1944, by diamond drilling at the New Almaden mine. The geologists assigned to the project were kept so occupied by the work it involved that they could make little headway toward mapping either the district or the New Al— maden mine. In the spring of 1944 nine Survey geolo- gists, including the writers of this report, were as— signed to prepare, under the direction of Aaron C. Waters, detailed geologic maps of the extensive ac- cessible workings of the New Almaden mine and to complete a detailed map of the surface above the mine workings. Shortly after the reassignment of Waters to other work late in 1944, the project was suspended for 3 months. Early in 1945 the writers returned to the district to complete the underground mapping, to prepare detailed maps of the areas over- lying the New Almaden mine and the Guadalupe— Senator mines, to make a geologic map of the district, and to prepare this report. They carried on fieldwork and the necessary office work continuously until Octo— ber 1947, except that Everhart was assigned for 1 year to another job; throughout the last year they were very capably assisted in the geologic mapping by Donald H. Kupfer. Many Survey geologists have thus contributed to the final product. Where particular credit or respon— sibility for a geologic idea, part of a map, or a carto- graphic technique is due, their contributions are ac- knowledged; but as so many ideas “just grow” from informal discussions, the writers no doubt have failed in some cases to give due credit. They are grateful, however, to all their colleagues for their individual and collective contributions. Preliminary mapping of the district was done by Paul Averitt, Arthur E. Bradbury, James B. Cathcart, Robert R. Compton, G. Donald Eberlein, and W. Bradley Myers; aid in the detailed mapping of the accessible underground workings of the New Almaden mine was given by Randall E. Brown, Juanita Crawford, G. Donald Eberlein, Lowell S. Hilpert, David A. Phoenix, George W. Walker, Aaron C. Waters, and Robert G. Yates. 8 GEOLOGY AND QUICKSILVER DEPOSITS, The California State Division of Mines aided in financing a part of the field investigation leading to this report, and the writers are indebted to Dr. Olaf P. Jenkins, who was then Chief of the Division, for his unfailing interest in the geology and economic potentialities of the district. Many of the old photographs included with this report were supplied by Mr. Laurence E. Bulmore, of Berkeley, Calif, who has made an extensive collection of photographs and other historical material pertain- ing to the early history of the mine. He also kindly supplied information bearing on the local history dur- ing the period from September 1878 to December 1899, when his father, Robert R. Bulmore, was cashier and later general agent for the Quicksilver Mining Co. During the field investigations the Survey parties received wholehearted cooperation from the local min- ing companies and mine operators. The owners and operators of the New Almaden and Guadalupe mines made all their records and maps available to the Sur— vey. Mr. C. N. Schuette, manager of the New Al- maden Corp., and Mr. George F. Kirk, operator of the Guadalupe mine, both gave much information about the history of their respective mines and volun— teered all pertinent facts regarding their more recent operations. Mr. H. F. Austin, who operated an ill- ‘teresting cinnabar placer deposit in Almaden Canyon, provided information about that deposit. Many other miners and local residents, too numerous to mention individually, helped by their continued interest and assistance to facilitate the investigation. MAPPING METHODS The methods used in mapping geologic features as exposed on the surface or in a mine depend on a great many conditions, such as amount of exposure. complexity and attitude of structures, character and persistence of rock units; they also depend on the time available and the ultimate objective of the study. or proper evaluation of the resultant maps the critical reader must know something about these factors and about the mapping techniques that were adopted to take advantage of the favorable features and mini— mize those less favorable. Terrains underlain by rocks of the Franciscan group and the closely associated intrusive serpentine bodies present inherent difficulties to the geologist; quicksilver deposits likewise are char- acteristically erratic and irregular, and therefore hard to delinate. The following paragraphs briefly sum— marize the techniques used in the work on the New Almaden district and in the mines. NEW VALMADEN DISTRICT, CALIFORNIA Enough time was available for mapping in as much detail as seemed to be justified by the probable useful- ness of the results. Because of the difference in prac- tical application of the various parts of the geologic map of the district (pl. 1), it represents two different methods of mapping. In general, the area containing the mineralized belt, lying to the north of a line be- tween Los Gatos and a point 1 mile west of the south- east corner of the district, was mapped in detail, whereas the rest, except for a few important local areas, was covered only by reconnaissance mapping methods. Within the area studied in detail, only the Fran- ciscan rocks and serpentine were believed to be p0ten~ tial ore bearers; all the younger rocks, therefore, were examined and mapped less thoroughly. In the Fran- ciscan terrain exposures are poor, being estimated to amount to no more than» 0.1 percent of the area, and generally it was not possible to set up a stratigraphic succession or rely on “key beds.” Every contact was followed as closely as possible, however, by the use of available outcrops and by identifying rock fragments in the residual soil. The detailed part of the map is believed to be in accord with all outcrops and reliable float, although the way in which the outcrops are grouped into rock masses is subject to interpretation. In the area studied by reconnaissance methods no at- tempt was made to follow every contact. In extreme instances, particularly in the south-central part of the district, traverses were so widely spaced as to omit on-the-spot examination of areas more than half a square mile in extent; however, as is indicated by the recorded data on the district map, traverses were gen- erally no more than two thousand feet apart. Conse— quently, some small rock bodies, only a few hundreds of feet or less in area, have doubtless been omitted from the reconnaissance part of the map; but their omission probably does not seriously detract from a reasonable understanding of the regional geology. The accuracy of position of a contact as shown on a geologic map depends not only on the certainty with which it can be located in the field but also on the precision with which it can be placed on the field map. The topographic base used for the New Almaden dis— trict mapping was a 2.6 enlargement of the Geological Survey’s 15-minute topographic map of the Los Gatos quadrangle, surveyed in 1915—16 and published at a scale of 1:62,500. Aerial photographs were used in the field in conjunction with these enlargements, and much of the geology was first plotted on the photo— graphs, but owing to inaccuracies of the enlarged topographic base, some adjustment of contacts from INTRODUCTION 9 their real position to their apparent position in rela- tion to the contours was necessary” To get the best fit to the topography, and yet retain the proper inter- relationship of the various geologic units, most of these adjustments were made in the field. On the larger scale maps of the areas over the New Almaden mine (pl. 3) and the Guadalupe and Sena- tor mines (pl. 14), the geologic contacts are much more accurately placed than on even the more detailed part of the district map. On each of these larger scale maps the contacts were controlled by stadia shots in those parts where new topography was sketched; elsewhere they were as closely placed as possible by using Brunton compass bearings on known points on the topographic base, and by careful pacing. As the larger scale maps are accurate, no adjustment of con- tacts to obtain agreement with the topography was necessary. The geologic maps of the underground workings of the New Almaden mine are based on data from many sources, and, as on the surface maps, the accuracy of the geology shown on them varies from place to place. As might be expected in a mine more than 100 years old and containing more than 30 miles of workings, many parts are inaccessible. When the mine was mapped in 1944—45, practically all the workings be— low the 800 level were flooded, and the most ancient. workings, which lie near the surface, were largely caved or filled. However, between 1865 and 1904 the Quicksilver Mining Co. had prepared a very accurate and complete planimetric map of the open workings at a scale of 40 feet to the inch. This map, which was an outstanding example of mine mapping for its pe- riod, is preserved on a large roller in a maphouse on Mine Hill. It has been used as a base for all inacces- sible workings, and data from a few even older maps were incorporated to add workings which apparently were inaccessible by 1865; but even so, a few workings that are known to exist have been omitted for lack of any kind of map. The mining after 1904 was rather irregular and consisted mostly of cleaning out and enlarging old stopes; as a result the company map, although very good for the access workings, was not, reliable in the stopes. To verify the company’s map- ping and obtain control points in the stopes, the Sur— vey field parties ran several miles of closed traverses by means of tape, Gurley compass, and planetable, and also mapped the walls of all accessible workings. Altitudes throughout the mine presented greater problems than did the planimetric control because only a few points of known altitudes are indicated on the company maps. Because the shapes of gently inclined 6861-671 0—63——2 geologic contacts, particularly in the stopes, are of great importance in establishing the structural con- trols for the ore bodies, it was necessary to obtain rea— sonably accurate vertical control throughout the mine. For those parts of the mine that were open, levels were run by means of closed foresight and backsight tra- verses, using 6-foot folding wooden carpenter’s rules as handy level rods. In the inaccessible areas altitudes were generally known for all levels at the shafts and at important junction points, but elsewhere they were estimated on the basis of the exceptionally steep gra- dient of 11/2. feet per hundred at which levels were run during the period of mining. As is discussed in greater detail on pages 110—121, the ore-controlling contacts of the open parts of the New Almaden mine are generally irregular in both strike and dip, and in many places they dip at low angles. To show their true shape in the stopes, they have been contoured at known elevations at regular intervals of from 5 to 10 feet by means of hand level- ing from points of known altitudes. Detailed maps of the New Almaden mine prepared by using these methods are of great value to a few persons, but they were not believed to be of sufficient value to the gen- eral reader to justify publishing them with this report. However, 20 such large maps of the accessible work- ings of the mine at a scale of 40 feet to 1 inch have been placed in open file and can be consulted at the US. Geological Survey offices in Menlo Park, Calif, and Washington, D.C., or at the ofiice of the Califor— nia State Division of Mines in the Ferry Building in San Francisco, Calif. The major features of the geol- ogy shown on these maps have been incorporated in the smaller scale composite level maps accompanying this report, and especially suitable parts of them are used herein as text figures. The geologic features shown on the maps of the inaccessible workings of the New Almaden mine were compiled from monthly surveyor’s records, and although not accurate in detail, they probably serve to give the broad features of the ore control and geology. This could not have been accomplished without the efficient help rendered by Virginia S. Neuschel, of the Survey, who spent 2 months of the summer of 1945 in transcribing the notes from chron— ologic order to an order based on the space relations of the workings and in plotting the data on base maps. In the outlying mines, no attempt was made to use the very time-consuming contact-contouring tech- nique, but, instead, the geology was plotted waist high on levels or shown by sections and sketches in ir- regularly floored stopes. 10 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA GENERAL GEOLOGY The rocks of the New Almaden district range in age from Late Jurassic to Recent, but they represent a depositional history that was interrupted by several major breaks. The oldest assemblage consists of the herterogeneous Franciscan group of Late Jurassic and Cretaceous age, which occupies nearly three-fourths of the district. It contains graywacke, siltstone, mafic volcanic rocks, chert, limestone, and minor amounts of metamorphic rocks, including glaucophane schists. This assemblage is regarded as typical of eugeosyn- clinal accumulations in orogenic belts, and, as might be expected, the rocks nearly everywhere exhibit small-scale folds and shears and show abundant evi— dence of rock flowage. Intrusive into the rocks of the Franciscan group are many tabular bodies of serpentine, a few of which are of special interest be- cause they have been hydrothermally altered during the late Tertiary to silica-carbonate rock—the host rock for the quicksilver ore bodies- Upper Cretaceous deposits include two groups of rocks differing in both lithology and degree of de— formation. The more deformed group, and prob- ably the older, consisting of several thousand feet of conglomerates, graywacke, and shale, is exposed only in the higher parts of the Sierra Azul in the south—central part of the district. The other unit of Upper Cretaceous age, consisting of gently folded sandstone and shale, is found in the northern part of the area, principally in the Santa Teresa Hills. This unit may be the equivalent of part of the Chico for- mation, but because of the uncertainty of correlation, it is herein designated as the Upper Cretaceous rocks of the Santa Teresa Hills. Overlying this unit is a relatively thin sequence of limestone, sandstone, and shale of middle Eocene age, which is so poorly ex— posed that it has not been assigned a formation name. The next record of sedimentation is furnished by rocks of lower, middle and upper Miocene age. These are at least 3,800 feet thick, and grade upward from conglomerate and sandstone to diatomaceous shale. The older part of this sequence has been referred to the Temblor formation, whereas all beds above the lowest diatomaceous shale are included in the Mon- terey shale. Some included felsic volcanic material reveals igneous activity in the area during the middle Miocene. The post—Miocene deposits consist largely of al- luvium, which fills the Santa Clara Valley, floors major canyons, and in places lies along the base of the foothills as perched gravels. Largely on the basis of their dissection and topographic position these gravels have been assigned to two formations. The older deposits have been correlated with the Santa Clara formation of Pliocene and Pleistocene age, whereas the younger ones are grouped as Quaternary alluvium. The structural features of the district consist of folds and faults trending westward or northwestward. The oldest rocks in general dip northeastward, but they have yielded to deformational forces by exten— sive crumpling, folding, flowing, and faulting. The major faults are believed to have largely strike-slip displacement, and they are generally marked by shear zones rather than single planes of slippage. The most notable of these shear zones—the Ben Trovato, which trends obliquely through the central part of the district—attains a width of more than a half a mile. The rocks of intermediate age, though also deformed, have remained more cohesive and form simple continuous folds cut by narrow faults. The young alluvial formations are little tilted, and show traces of faults only by topographic scarps less than 100 feet in height. The following sections describe the 21 distinctive cartographic rock units mapped in the district. Some of these contain only a single kind of rock, whereas others, such as the greenstone of the Franciscan group, contain several distinct but related lithologic types. Correlations between isolated exposures of rocks of the different cartographic units were made largely on the basis of lithologic similarity, as fossils were too rare in nearly all of them to be of much aid. FRANCISCAN GROUP The oldest and most extensive assemblage of rocks. in the district has been assigned primarily on the basis of lithology to the Franciscan group, which in- cludes rocks of both Jurassic and Cretaceous age. All the diverse rock types commonly found in this group throughout the California Coast Ranges occur in the New Almaden district, and their relative abundance and general lithology in this district are believed to be typical of the central part of the . Coast Ranges. The group consists mainly of medium— to fine-grained graywacke (p. 13) and dark shale; it contains a somewhat smaller amount of generally altered mafic volcanic rocks usually classed as “green— stones,” and small amounts of conglomerate, lime- stone, and chert. Another rock, serpentine, which accompanies the group in many places, is considered by some as a part of the group; but it is somewhat younger, because it is intrusive into the other rocks. Because of the distinctive character of the group, both lithologically and structurally, and because of its importance to California geologists and quicksilver FRANCISCAN GROUP miners, it has been more thoroughly studied than any of the younger formations in the district, and is therefore described in more detail. All the rocks of the Franciscan group in the New Almaden district show at least incipient metamor- phism in some places, but most of the rocks of the group are so little changed that in the field they appear l ‘ unmetamorphosed. The typical rocks do not show slaty cleavage, foliation or schistosity, prominent de- velopment of stress minerals, or crystalloblastic tex- tures. In a few areas of relatively small extent, how- ever, some of the rocks of the group are crystallo- blastic, schistose, or even gneissic, having obviously been subjected to special metamorphic processes that have not affected the rest of the group. These dis- tinctly metamorphosed rocks consist largely of schist, amphibolite, and crocidolite—bearing metachert, prob- ably formed from the normal graywacke, greenstone, and chert. Because of their unusual and seemingly erratic distribution, these metamorphosed rocks of the Franciscan group are usually discussed together, rather than treated separately as special metamorphic phases of the various rocks from which they are de- rived, and they will be discussed together in this report. These metamorphosed rocks have attracted a disproportionate amount of attentionbecause of the interest aroused by a few uncommon minerals found in them, and this, together with the fact that the group was referred to in early reports as the Meta- morphic series, has led to a widespread but erroneous belief that the Franciscan group consists largely of crystalline schists. The name Franciscan (series) was first used by Andrew C. Lawson (1895a, p. 342—356; 1895b, p. 399— 47 6), and it has been adopted by all geologists work— ing in the California Coast Ranges. Other names, however, had previously been applied in several im— portant publications to rocks now included in the Franciscan group. As early as 1856, Blake (p. 153) used the name San Francisco or California sand- stone in a report containing a description and map of the sandstone around San Francisco Bay. Nine years later Whitney (1865, p. 19—108) described the entire group, together with the serpentine, using the terms San Franciscan sandstone, metamorphic rocks, and metamorphosed Cretaceous. Becker (1888) applied various terms, including Neocomian, Metamoprhic series, and Knoxville, to the rocks older than the Chico (Upper Cretaceous) formation of the Coast Ranges. Since these early studies a great many reports deal- ing directly or indirectly with the Franciscan group have appeared. 1855 1856 1858 1865 1885 1886 1888 1892 1893 1895 1895 1904 1909 1910 1914 1933 1936 1943 1943 1946 1948 1918 11 BIBLIOGRAPHY OF THE FRANCISCAN GROUP General Blake, W. P., Observations on the characters and prob- able geological age of the sandstone formation of San Francisco: Am. Assoc. Adv. Sci. Proc., v. 9, p. 220—222. Blake, W. P., Report of explorations in California for railroad routes: 33d Cong, 2d sess., EX. Doc. 78, Geol. Rept., p. 153. Blake, W. P., Report of a geological reconnaissance in California: New York, H. Bailliére, p. 145—159, map facing p. 145. Whitney, J. D., Geological survey of California: Geol- ogy, v. 1, p. 19—108. Becker, George F., Notes on the stratigraphy of Califor- nia: U.S. Geol. Survey Bull. 19, p. 7—25. Becker, George F., Cretaceous metamorphic rocks of California: Am. Jour. Sci., 3d ser., v. 31, p. 348—357. Becker, George F., Geology of the quicksilver deposits of the Pacific slope: U.S. Geol. Survey Mon. 13, p. 1—486. Fairbanks, H. W., The pre—Cretaceous age of the meta- morphic rocks of the California Coast Ranges: Am. Geologist, v. 9, p. 153—166. Fairbanks, H. W., Notes on a further study of the pre- Cretaceous rocks of the California Coast Ranges: Am. Geologist, v. 11, p. 69—84. Lawson, A. C., A contribution to the geology of the Coast Ranges: Am. Geologist, v. 15, p. 342—356. Lawson, A. 0., Sketch of the geology of the San Fran- cisco Peninsula: U.S. Geol. Survey 15th Ann. Rept, p. 399—476. Fairbanks, H. W., Description of the San Luis quadran- gle [California] : U.S. Geol. Survey Geol. Atlas, Folio 101, p. 2-3, 9. Branner, J. C., Newsom, J. F., and Arnold, Ralph, De- scription of the Santa Cruz quadrangle, California: US. Geol. Survey Geol. Atlas, Folio 163, p. 2, 8—11. Smith, J. P., The geologic record of California: Jour. Geology, v. 18, p. 216—227. Lawson, A. 0., Description of the San Francisco district: U.S. Geol. Survey Geol. Atlas, Folio 193, p. 4—7, 17, 19, 22. Reed, R. D., Geology of California: Am. Assoc. Petro- leum Geologists Spec. Pub., p. 27—59, 72—97. Reed, R. D. and Hollister, J. S., Structural evolution of Southern California: Am. Assoc. Petroleum Geolo- gists Spec. Pub., p. 55—57. Taliaferro, N. L., Geologic history and structure of the Central Coast Ranges of California: California Div. Mines Bull. 118, pt. 2, p. 119—162. Taliaferro, N. L., Franciscan-Knoxville problem: Am. Assoc. Petroleum Geologists Bu11., v. 27, no. 2, p. 109— 219. Allen, J. E., Geology of the San Juan Bautista quadran— gle, California: California Div. Mines Bull. 133, p. 22—26. Huey, A. S., Geology of the Tesla quadrangle, Califor- nia: California Div. Mines Bull. 140, p. 14—23. Sandstone Davis, E. F., The Franciscan sandstone: California Univ., Dept. Geology Sci. Bull., v. 11, p. 6—16. 12 1902 1933 1893 1894 1918 1943 1943 1856 1869 1896 1913 1930 1932 1937 1942 1942 1943 1948 GEOLOGY AND QUICKSILVER DEPOSITS, NEW Limestone Lawrence, A. C., A geological section of the Middle Coast Ranges of California: Science, new ser., v. 15, p. 415—416. Eckel, E. C., Limestone deposits of the San Francisco region: California Div. Mines, Rept. State Miner- alogist, v. 29, no. 3, 4, p. 348—361. Greenstone and chert Ransome, F. L., The eruptive rocks of Point Bonita: California Univ., Dept. Geology Sci. Bull., v. 1, p. 71— 114. Ransome, F. L., The geology of Angel Island, with a note on the radiolarian chert from Buri—buri Ridge, San Mateo County, California, by G. J. Hinde: Cali- fornia Univ., Dept. Geology Sci. Bull., v. 1, p. 193— 240. Davis, E. F., The radiolarian cherts of the Franciscan group: California Univ., Dept. Geology Sci. Bull., v. 11, p. 235—432. Taliaferro, N. L., and Hudson, F. 8., Genesis of the manganese deposits of the Coast Ranges of Califor- nia in Manganese in California: California Div. Mines Bull. 125, p. 221—234. Trask, P. D., Wilson, I. F., and Simons, F. 8., Manga- nese deposits of California, a summary report, in Manganese in California: California Div. Mines Bull. 125, p. 58. Paleontology and age Blake, W. P., Observations on the character and prob- able geological age of the sandstone formation of San Francisco [abs]: Am. Assoc. Adv. Sci. Proc. 9, p. 220—222. Gabb, W. M., Cretaceous and Tertiary fossils: Califor— nia Geol. Survey, Paleontology 2, p. 193, 246. Fairbanks, H. W., Stratigraphy at Slate’s Springs, with some further notes on the relation of the Golden Gate series to the Knoxville: Am. Geologist, v. 18, p. 350—356. Davis, C. H., New species from the Santa Lucia Moun— tains, California, with a discussion of the Jurassic age of the slates at Slate’s Springs: Jour. Geology, v. 21, p. 453—458. Stewart, Ralph B., Gabb’s California Cretaceous and Tertiary type lamellibranchs: Acad. Nat. Sci. Phila- delphia Spec. Pub. 3, p. 1—106. Nomland, J. 0., and Schenck, H. G., Cretaceous beds at Slate’s Hot Springs, California: California Univ., Dept. Geology Sci. Bull., v. 21, no. 4, p. 37—49. Reiche, Perry, Geology of the Lucia quadrangle, Cali— fornia: California Univ., Dept. Geology Sci. Bull., v. 24, no. 7, p. 134—137. Camp, C. L., Icthyosaur rostra from central California: Jour. Paleontology, v. 16, no. 3, p. 362—371. Thalmann, H. E., Globotruncana in the Franciscan lime- stone, Santa Clara County, California [abs]: Geol. Soc. America Bull., v. 53, no. 12, p. 1838. Thalmann, H. E., Upper Cretaceous age of the “Francis- can” limestone near Laytonville, Mendocino County, California [abs]: Geol. Soc. America Bull., v. 54, no. 12, p. 1827. Cushman, J. A., and Todd, Ruth, A foraminiferal fauna from the New Almaden district, California: Cush- ALMADEN DISTRICT, CALIFORNIA , man Found. Foram. Research Contr., v. 24, pt. 4, p. 90-98. Glaessner, M. F., Foraminifera of Franciscan (Califor- nia) : Am. Assoc. Petroleum Geologists Bull., v. 33, no. 9, p. 16154617. Schlocker, Julius, Bonilla, M. G., and Imlay, R. W., Ammonite indicates Cretaceous age for part of Fran— ciscan group in San Francisco Bay area, California: Am. Assoc. Petroleum Geologists Bull., v. 38, no. 11, p. 2372—2381. Kiipper, Klaus, Upper Cretaceous foraminifera from the “Franciscan Series", New Almaden district, Cali- fornia: Cushman Found. Foram. Research Contr., v. 6, pt. 3, p. 112—118. Metamorphic rocks Ransome, F. L., The geology of Angel Island, with a note on the radiolarian chert from Angel Island and from Buri-buri Ridge, San Mateo County, California, by G. J. Hinde: California Univ., Dept. Geology Sci. Bull., v. 1, p, 193-240. Palache, Charles, On a rock from the vicinity of Berke- ley containing a new soda amphibole: California Univ., Dept. Geology Sci. Bull., v. 1, p. 181—191. Smith, W. S. T., The geology of Santa Catalina Island: California Acad. Sci. Proc., ser. 3, Geol., v. 1, p. 1—71. Blasdale, W. 0., Contribution to the mineralogy of Cali- fornia: California Univ., Dept. Geology Sci. Bull., v. 2, p. 327—348. Nutter, E. H., and Barber, W. B., On some glaucophane and associated schists in the Coast Ranges of Cali- fornia: Jour. Geology, v. 10, p. 738—744. Holway, R. S., Eclogites in California: Jour. Geology, v. 12, p. 344—358. Smith, J. P., The paragenesis of the minerals in the glaucophane-bearing rocks of California : Am. Philos. Soc. Proc., v. 45, p. 183—242. Murgoci, G. M., I. Contribution to the classification of the amphiboles: II. On some glaucophane schists and syenites: California Univ., Dept. Geology Sci. Bull., v. 4, p. 359—396. Woodford, A. 0., The Catalina metamorphic facies of the Franciscan series: California Univ., Dept. Geology Sci. Bull., v. 15, p. 49—68. Pabst, Adolf, The garnets in the glaucophane schists of California: Am. Mineralogist, v. 16, p. 327—333. Hutton, C. 0., Stilpnomelane and pumpellyite, constitu- ents of the Franciscan series [abs]: Geol. Soc. America Bull., v. 59, no. 12, pt. 2, p. 1373—1374. 1949 1954 1955 1894 1894 1897 1901 1902 1904 1906 1906 1924 1936 1948 CLASTIC SEDIMENTARY ROCKS The most abundant and widespread of the Fran- ciscan rock types are the elastic sedimentary rocks, which in the New Almaden area amount to more than two—thirds of the assemblage. Although the sedi- mentary rocks are widely distributed throughout the district, no single exposure serves to Show the variety of rocks included in this single cartographic unit. However, one easily accessible, and fairly representa- tive, series of exposure is afforded by the roadcuts along the first 2 miles of State Highway 17 south of Los Gatos. FRANCISCAN GROUP 13 The elastic sedimentary rocks are characterized by a high content of feldspar which is mostly sodic plagioclase, and except in the conglomerate, by a marked angularity of grains; in many there is an uninterrupted gradation in grain size from the coarsest clasts to the finest matrix material. Rocks with the grain size of sandstone predominate, siltstone is com- mon, shale less so, and conglomerate is rare in the district. The rocks are poorly sorted; the sandstones are true graywackes and 'the siltstones and shales contain a large proportion of minute mineral grains rather than clay minerals. In spite of the variety of elastic rocks in the sedimentary pile, which is more than ten thousand feet thick, they are so distributed that it is impossible to divide the sequence into mappa— ble units consisting wholly, or even largely, of either coarse or fine sediments. Within sequences that are chiefly graywacke are local sections several hundred feet thick consisting largely of siltstone and shale, but attempts to map these failed because the shale is not persistent along the strike. Conglomerate lenses inter- bedded with the graywacke are also too thin and too limited in extent to provide mappable units. The physical characteristics of the elastic sedimen- tary rocks on microscopic, hand specimen, and outcrop scale indicate that they were deposited rapidly, in part at least, by turbidity currents. Such poorly sorted sedimentary rocks, together with the intercalated chert, minor limestone, and soda-rich volcanic rocks, make up an assemblage found in many parts of the world. They are regarded as typical of the accumula— tions that build up in a structural trough along the margin of a continent when rapid deformation accom- panies deposition, and are generally referred to as eugeosynclinal deposits. anywach Graywacke, which in most reports is referred to as Franciscan sandstone, makes up more than half of the Franciscan group. Nearly all varieties are dis- tinguished easily from the other rocks in the district even though the graywacke exhibits much greater variation in color, in mineral content, and in general appearance than do most sedimentary units. 'All varieties are dark-colored poorly sorted dirty rocks containing abundant grains of feldspar and some rock fragments. (See fig. 3.) The variations they show are partly the result of original sedimentation and partly the result of late processes leading to dif- fering degrees of induration, metamorphism, and deformation. Several thick sequences of graywacke separated by volcanic rocks are found in the district, and it was at first thought that these might be mapped as sepa- FIGURE 3.——Massive graywacke of the Franciscan group; size. To bring out the texture the specimen was polished and etched with hydrofluoric acid, which makes this dark-gray rock actual appear much lighter than it really is. Veinlets are calcite. table 1. Irregular white patches and An analysis of this rock is given in column 2, rate units or formations. An elaborate scheme of field classification based on proportions of feldspar to quartz, grains to matrix, light-colored grains to dark- rock fragments, and other criteria was attempted; but attempts to develop cartographic units by this means failed because the differences between sequences were no greater than the variations within a single sequence. In general, however, the older rocks oc- curring in the southern part of the district were found to be more feldspathic, whereas the younger rocks contain more lithic fragments. The exposures of graywacke are generally less than a hundred feet across, but locally, as on Mount Umun- hum, exposures are fairly continuous over half a square mile. In some large areas, notably in the Santa Teresa Hills, outcrops of these rocks are almost nonexistent. The average individual outcrop does not exceed 10 feet in length and consists of exceptionally massive rock cut by several sets of joints. In many such isolated knobs of graywacke bedding can be distinguished only with difficulty, although in places the alinement of mica flakes or shale fragments in- dicates the attitude of an otherwise massive rock. The true character of the sequence is better seen in arti- ficial exposures, as in roadcuts or mine workings. In these exposures the graywacke generally shows more bedding and can be seen to be somewhat folded and cut by faults, as shown on figure 4. Where inter- layered with shale it may also show individual beds 14; GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA FIGURE 4.—Deformed graywacke exposed in roadcut between Bald Mountain and the crest of the Sierra Azul. The irregular folds and loW- angle thrust faults, such as are shown here, are abundant throughout much of the district, but in many places su‘tficient shale accompanies the graywacke to permit rock flowage to predominate oversimple folding and faulting. that have been plastically deformed so that their present thicknesses are quite different from their original thicknesses. Original variation in the thickness of beds of graywacke, or the accompanying beds of shale or tuff, however, are the general rule, and no systematic relation between the thickness of the graywacke and other interbedded rock was observed. Most ex— posures show a completely erratic distribution of beds of difl'erent thicknesses, although locally there are sequences of thick or thin beds. The maximum thick- ness of beds seen in artificial exposures rarely exceeds 5 feet, though surface outcrops several times as wide without apparent bedding are not uncommon. Sharply limited thin layers of graywacke and shale also can be seen in some places. Lenticular beds were observed, especially where thin layers of graywacke are interbedded with shale or tuff, but more com- monly slippage along breaks that are nearly parallel to bedding has formed lenticles that do not neces- sarily reflect original lenticular bedding. Graded bedding, with the graywacke grading upward to shale by an imperceptible decrease in grain size, accom- panied by a darkening of color, is present in some of the sedimentary rocks but cannot be seen in most places. Current bedding on a small scale was ob- served in thin-bedded sedimentary rocks in some ex- posures in the mines, but apparently it is rare as it was not observed in, the poorer surface exposures. Flutings were noted on the lower surfaces of the graywacke layers, but they are rarely found. None of the sedimentary rocks contain ripple marks. Vein— . lets of either quartz or calcite, or both, are abundant, and the weathered surfaces of rocks partly replaced by calcite are covered with small irregular pits. The lithic graywackes that are most abundant in the central part of the district contain a high pro— portion of mafic rock fragments and seem to grade into tufts and breccias mapped as greenstone. They also contain in places erratically distributed green- stone pebbles, or even boulders of greenstone several feet in diameter. FRANCISCAN GROUP 15 Because of the scarcity of exposures, the geologist mapping in the Coast Ranges must often rely on the character of the residual soil, and therefore a few comments on the soils developed on graywacke may be of value. Where the graywacke is highly feld— spathic, the soils are light buff or tan and differ in color from the reddish soils yielded by most green- stones. Although we found a few areas of altered tufl'aceous greenstones that also yielded light—colored soils, these were so small that we believe mapping areas mantled with light—colored soils as graywacke will cause only a very occasional error. Where the graywacke contains abundant fragments of green- stone, it gives rise to reddish soils identical in color to some soils derived from tuffaceous or massive green— stone. In areas so mantled the geologist should rely on the small rock fragments that can commonly be found in the soil; in their absence no criteria is really reliable, but we found that soils developed on gray- wacke were generally more gritty than those on green- stone, probably owing to the persistence of quartz. Megascoplc features The most consistent features of the graywacke are its dirty appearance and its high content of feldspar, which normally slightly exceeds quartz in amount. It contains highly angular grains, which are generally monomineralic, and somewhat more rounded rock fragments. The monomineralic grains are mostly of feldspar and quartz and between 0.25 and 1.0 mm in diameter, but a few detrital grains of minerals of the epidote group and zircon are commonly present. Al— though all the graywackes contain rock fragments, the proportion is quite variable, ranging from a few percent to three-fourths of the rock. Most of the fragments are of basic lavas, but fragments of shale are generally present and locally abundant. Carbon- ized wood fragments, such as are abundant elsewhere in the Franciscan group, are uncommon in the New Almaden district. The indeterminate matrix is dark colored, and ranges in amount from thin films be- tween closely packed grains to perhaps as much as 20 percent. Where the matrix is highly siliceous, as in much of the more feldspathic graywacke, the frac- tures tend to go through the grains; where it is more chloritic or clayey, however, the breaks go around the grains. None of the rocks, however, contain more than a little clay, and none contain an appreciable quantity of red ferruginous oxides. The color gen- erally is some shade of gray or light green, but where the graywacke is weathered, as inmost outcrops, it ranges in color from dark gray to buff or light tan and locally is reddish. FIGURE 5.—Photomicrograph of feldspathic graywacke of the Francis- can group. Crossed nicols. Angularity of grains and lack of sort— ing are typical. Grains are chiefly plagioclase (p1) and quartz (Q). Microscopic features Enough thin sections were studied to indicate that the graywacke varies widely in relative proportions of component minerals and in amount of matrix, but many more sections would have to be studied before definite limits could be assigned to the variations. (See figs. 5, 6.) All the sections contain angular grains of sodic plagioclase, quartz, and rock frag- ments, separated by a fine-grained matrix composed of the same materials and recrystallized fine-grained FIGURE 6,—Photomicrograph of lithic graywacke of the Franciscan group. Crossed nicols. Contains monomineralic grains of plagio- clase (p1) and quartz (Q), and grains of greenstone (G), altered tachylite (T), argillite (A), and chert (C). 16 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA aggregates of chlorite, sericite, and perhaps other minerals. A few detrital grains of augite, epidote, clinozoisite, or zoisite are generally present, and some varieties contain several percent of biotite or musco- Vite or both. Other detrital minerals occurring in minor quantity are chlorite, sphene, zircon, garnet, ilmenite, leucoxene(?), and magnetite. Tourmaline and hornblende, reported from other localities by Taliaferro (1943b, p. 135), were not found. The quartz grains, which make up from 10 to 35 percent of the graywackes, are commonly angular, but some are subangular. A few well—rounded grains were observed, as was a single grain showing a definite euhedral shape. The quartz is clear, generally con— tains liquid and gas-filled cavities, and rarely includes needles of zircon. Many of the grains show undula— tory extinction, and some are composites of several crystal units separated by sutured boundaries. Feldspar grains generally account for from one-third to two-thirds of the monomineralic grains, and feld- spars are also common constituents of the rock frag- ments. The monomineralic grains are generally angular or subangular, and a surprisingly small pro— portion show straight edges that coincide with the cleavage direction. In some sections many of the feldspar grains appear to be euhedral crystals modi- fied only to the extent of having slightly rounded corners. The feldspar is generally cloudy enough to be easily distinguished from the clear quartz but fresh enough to show sharp lamellae in the grains that are twinned. Much of the feldspar, however, is un— twinned, and this, together with the incipient altera— tion, makes the determination of the relative abun- dance of different kinds of feldspar diflicult. To ascertain the ratio of potash feldspar to plagioclase 10 thin sections were etched with hydrofluoric acid and stained with sodium cobaltinitrite solution, which differentially stains the potash feldspars a bright yel- low. Two of these thin sections contained a few grains of potash feldspar, but in the others none was found; this admittedly inadequate sample suggests that orthoclase is a minor constituent of only some of the graywacke. The relative amounts of albite, Oligo— clase, and andesine have only been approximated by comparing indices of the feldspars with the index of balsam along the edges of thin sections, which is a rather unsatisfactory method because many of the feldspars are so clouded with alteration products that a reliable comparison cannot be made. This method leads to the tentative conclusion that albite is by far the most common plagioclase, oligoclase is generally present, and andesine is rare. Features that might be suggestive of a granitic source for the feldspars, such as graphic intergrowths, myrmekite, or microcline, were not found. Rock fragments may make up from less than 10 percent to more than 75 percent of the clasts in the graywacke. Most of the fragments are mafic lavas or greenstones, similar in texture and mineral content to the massive greenstones of the Franciscan group, and a single thin section will generally include several varieties of greenstone fragments. Some of them consist of completely altered mafic glass, others con- tain albite and scattered relicts of pyroxene in an altered fine-grained or glassy groundmass, and still others are composed largely of plagioclase with only a little altered groundmass. Less common are frag— ments of shale, phyllite, and chert showing varying degrees of recrystallization. Shale fragments are gen- erally tabular and invariably bent around adjacent grains of quartz and feldspar, whereas the fragments of mafic volcanic rock have rounded and irregular shapes due to distortion of the original grains by flowage to form a better fit with adjacent grains. The term “matrix” as applied to graywacke needs to be defined before it can be discussed because in many of these rocks there is no clear break in grain size between the coarsest and finest material. The matrix, however, is generally considered, as it will be here, to include the material between grains that is itself so fine grained as to be only partly determinable under high magnification (about 0.002 mm in diame— ter) and the somewhat coarser material that is recrys- tallized from this fine—grained paste. The quantity of matrix in the Franciscan graywacke varies from an amount large enough to provide a- groundmass in which the other grains are clearly isolated to a thin, scarcely discernible film between closely packed grains. Much, and perhaps all, of the matrix is recrystallized. It contains quartz, sericite, chlorite and probably also albite and actinolite. In some varieties it replaces the margins of feldspar grains or rock fragments giving these a fuzzy outline. In addition to the normal ma- trix in some varieties there are small areas in which the matrix is replaced by calcite. Chemical features The few chemical analyses that have been made of graywackes of the Franciscan group are shown in table 1, and for comparison we have included an aver- age of 30 graywacke analyses from other localities and an average of 40 granodiorite analyses. The new analysis given in column 2 is of a specimen collected from the center of a 10-foot boulder that had been blasted apart, and it appears to be entirely fresh. A photograph of this rock is shown in figure 3. FRANCISCAN GROUP TABLE 1.——Analyses of graywacke from the Franciscan group, with average of 30 graywacke and 40 granodionte analyses for comparison 1 2 3 4 5 6 7 56. 84 67. 26 68. 50 68. 84 71. 72 68. 1 65. 01 n.d. 1. 76 .60 .25 .35 .7 .57 11.37 12.37 12. 82 14. 54 13.23 15. 4 15. 94 1.46 .59 1.29 .62 .30 1.0 1.74 4.95 4.03 3.37 2.47 3.58 3.4 2.65 .22 .08 .02 nil nil .2 .07 3.10 2.34 2.21 1.94 1.81 1.8 1.91 7.62 3.33 1.82 2.23 1.80 2.3 4.42 3.26 2.96 6.03 3.88 2.72 2.6 3.70 1. 86 1.17 1. 26 2. 38 1. 29 2. 2 2. 75 .45 .31 .28 . 5 .15 3.24 2.50 2.11 1.60 2.53 2-1 1'04 .10 .15 .16 .35 .09 .2 .20 5 10 .56 n.d. 14 n.d. n.d 04 11 d n.d. n.d 05 n n.d. n.d 15 99. 57 99. 41 100. 47 100. 13 Nora—Description of sample and locality as follows: 1. “Neocomian Sandstone" from headwaters of Bagley Creek, Mount Diabio, Calif. W. H. Melville, analyst. From Melville, 1891, p. 412. 2. New analysis. Graywacke (NA—450) from a point 3,450 ft N. 66%° E. of the apex of Mine Hill, New Almaden district, Santa Clara County, Calif. Mrs. A. C. Vlisidis, U.S. Geological Survey, analyst. 3. Sandstone of the Franciscan group from Sulphur Bank, Calif. W. H. Melville, analyst. From Becker, 1888, p. 82. 4. Fresh sandstone of the Franciscan group from quarry of the Oakland Paving Co., Piedmont, Calif. James W. Howson, analyst. From Davis, 1918, p. 22. 5. Fresh sandstone of the Franciscan group from junction of Buckeye Gulch and Hospital Canyon, Carbona quadrangle, Stanislaus County, Calif. Analyzed passilerdslgéan Laboratory, Glasgow; analyst not known. From Taliaferro, , D- - 6. Average of 30 graywackes. From ’I‘yrrell, 1933, p. 26. Fean given as 3.4 percent assumed to be in error, as that figure makes the total 102.4 percent. 7. Average of 40 granodiorites calculated to 100 percent by R. A. Daly. From Daly and others, 1942, p. 2. Except for the analysis given in column 1, which was made from a graywacke containing an abnormal amount of calcite that probably occurred as veins, the analyses exhibit the range in composition no greater than might be expected in these unsorted sediments. As compared with the “average graywacke,” the Fran- ciscan rocks contain less aluminum and more magne— sium and sodium, which suggests that they contain more fragments of soda-rich mafic volcanic rocks than does the “average graywacke.” The similarity of the graywacke to granodiorite in chemical composition has been pointed out by Tali- aferro (1943b, p. 137, 138), and may be checked by comparing columns 2—4 with column 6. The corre— spondence is fairly good except for three notable de— partures: the ratio of FeO to Fe203 is considerably higher in the graywacke than in the granodiorite, the CaO content is much lower, and the ratio of N320 to K20 is higher. SILTSTONE AND SHALE Siltstone and shale make up less than 10 percent of the Franciscan group in the New Almaden district. Two varieties are. common. One, a tan to light—gray phyllitic siltstone, is confined to the southwestern part of the area, where it is interbedded with the feld- spathic graywacke in the lowest part of the Franciscan group that is exposed in the district. The other, con— sisting of dark-gray to black siltstone and shale, oc- 17 curs throughout the section, but it is most abundant in a belt trending northwestward through the central part of the district, where it is associated with green- stones that belong to a younger part of the group. A less common third variety, containing considerable iron oxide and commonly having a red or green color, is everywhere closely associated with the cherts and is described with them on page 28. Megascoplc features The older light-colored siltstone crops out locally along the first ridge southeast of Los Gatos, in deeply worn trails or on ridge tops nearly devoid of soil or brush. These rocks are not abundant, and they gen— erally occur in thin beds intercalated with highly feldspathic graywacke; but locally, as in the upper drainage area of Limekiln Canyon, they apparently attain a thickness of several hundred feet without be- ing interbedded with coarser sedimentary rocks. The siltstone is generally more contorted than the surround- ing massive arkose. Bedding can be distinguished with certainty only in the coarser layers, for the silt— stone is invariably phyllitic, breaking into flakes along subparallel parting planes that only approximately coincide with its original bedding. The parting sur- faces are shiny, and under the hand lens they show a myriad of very small unoriented light-colored mica flakes. In thin sections these rocks are seen to con- sist mainly of well-sorted angular grains of quartz and feldspar; but they also contain several percent of muscovite, which appears to be partly detrital and partly authigenic. Clay minerals are also present in apparently small but undetermined amounts. The younger and darker fine-grained sedimentary rocks are believed to be largely siltstone because of their fissile to irregular fracture, but they include some shale and mudstone With conchoidal fracture. The color of the siltstone ranges from gray to black, but most of it where fresh has a somewhat greenish cast and Where weathered is lighter colored. Expo— sures of unaltered siltstone and shale without inter— bedded coarser rocks are rarely found except in arti— ficial cuts or in sharply incised ravines, but along borders of serpentine masses the siltstones have l0- cally been so hardened that they form good outcrops. In many exposures the siltstone occurs only as thin seams interbedded with graywacke, but in some places, as in Rincon Canyon 3,000 feet upstream from its junction with Guadalupe Canyon, sections as much as 500 feet thick are exposed that consist largely of silt- stone. In other places thin layers ‘of siltstone are interbedded with greenstone tufi; good examples of these intercalations can be seen in many of the upper workings of the New Almaden mine, but they are 18 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA FIGURE 7.—Photomicrograph of siltstone of the Franciscan group. Note the abundance of angular fragments, largely plagioclase and quartz, and the relatively small amount of clay or fine-grained matrix material. Bedding runs horizontally across photomicro- graph. rarely exposed at the surface. Very locally in the siltstone, small elliptical concretions of limestone 6 to 8 inches in diameter are developed. A few of these concretions found in a thin—bedded dark rock exposed at the base of the Calero Dam proved to be of un— usual interest because they contained fragments of marine megafossils. Microscopic features The small amount of microscopic work done on these fine-grained rocks shows that they have approximately the same mineral content as the graywacke, except that they contain a little more mica, clay, and car- bonaceous matter (fig. 7). Wood fragments large enough to distinguish with a hand lens are fairly com— mon in the Franciscan siltstones elsewhere, in the Coast Ranges, but not in those of the New Almaden district. Chemical features An analysis of black siltstone of the Franciscan group from Fern Peak (formerly called Fern Hill) in the New Almaden district is given in table 2, col‘ umn 1. As compared with the analysis of graywacke shown in the next column, the siltstone contains less silica and a little more aluminum, iron, and magne- sium. It also contains a little less calcium, even when the calcium necessary to form calcite from the carbon dioxide is excluded. These differences indicate that the siltstone contains more fragments of mafic rock and a little more clay than the analyzed graywacke. A comparison of the siltstone of the Franciscan group with average shales may be made by referring to table 2, column 3, which gives a composite of 78 analyses of shales. The analyzed rock from the New Almaden district contains more silica, magnesium, and sodium and less aluminum, ferric iron, calcium, potassium, and combined water than the average shale. It also has a potash-soda ratio of less than 1. These differ- ences are those that would be expected from the low clay centent of the siltstones of the Franciscan group and the soda-rich character of all the clastic sedimen— tary rocks of the group. TABLE 2.~Analysis of siltstone of the Franciscan group, with analyses of graywacke and a composite of 78 shales for com- partson. 1 2 3 Si02_______; ___________________ 62. 54 67. 26 58. 38 A1203 __________________________ 14. 81 12. 37 15. 47 Fe203 __________________________ 2. 02 . 59 4. 03 FeO ___________________________ 5. 47 4. 03 2. 46 MnO ___________________________ . 05 08 Trace MgO ___________________________ 3. 38 2 34 2.45 CaO ___________________________ 1. 40 3 33 3. 12 NaZO __________________________ 2. 90 2 96 1. 31 K20 ___________________________ 2. 13 1 17 3. 25 H20‘ __________________________ . 82 . 31 H20+ __________________________ 2; 91 2. 50} 5' 02 TiOz ___________________________ 87 l. 76 . 65 P205 ___________________________ 15 . 15 17 002 ____________________________ 08 . 56 2 64 S ______________________________ . 05 ________ . 26 Organic ________________________ . 96 ________ . 81 Total ____________________ 100. 54 99. 41 100. 02 Less O—S _________________ . 03 Total ____________________ 100. 51 99. 41 100. 02 NOTE—Description of sample and locality as follows: 1. Black siltstone (NA—315) from Fern Peak, New Almaden district, Santa Clara County, Calif. Mrs. A. C. Vlisidis, U.S. Geological Survey, analyst. 2. Graywacke, see table 1, this report. 3. Composite 0f 78 shales from Clarke, 1924, p. 631. SO; recast as S; BaO omitted ALTA A distinctive variety of rock composed largely of sheared shale envelopes the serpentine masses. Many of the quicksilver deposits in the Coast Ranges are closely associated with this rock, which has become widely known to California quicksilver miners as alta. It received‘this Spanish name, meaning “hanging wall,” frOm the Mexican miners in the early days, because it commonly overlay the ore bodies that were found along the upper margins of altered serpentine sills. The alta, however, has been found to be just as common along the lower sides of the serpentine sills, where in places it forms the footwall for ore bodies. Similar rock also occurs along fault zones that traverse rocks of the Franciscan group. The alta, like a fault breccia or mylonite, owes its character as much to shearing as to original lithology, but siltstone or shale is everywhere its most abundant FRANCISCAN GROUP , 19 FIGURE 8.——Alta lying above intrusive serpentine contact on the 500 level in the Harry workings of the New Almaden mine. Serpentine converted to silica-carbonate rock (so) and cut by a few dolomite veinlets. Alta contains larger fragments than in most places and consequently shows sheared texture somewhat better. constituent. In most places the alta also contains some of the other rocks of the Franciscan group, to— gether with small pods and lenses of serpentine or silica-carbonate rock. (See fig. 8.) Two processes seem to have operated together to form the structures that are characteristic of the alta. One is shearing, and as most of the serpentine bodies in the mine area are sill-like the shears in the alta, which are parallel to the intrusive contact, are also closely parallel to the bedding. The other process is compression—perhaps due to the intrusion—applied at right angles to the bedding. These two processes op- erating together have caused stretching of the indi- vidual rock layers and a flowage of the shale. Where thin layers of graywacke or tuff are interbedded in the alta they commonly have been drawn out into iso- lated lenticular pods forming a boudinage structure. Where thicker beds of massive greenstone or gray— wacke are included they, too, are broken and drawn apart, but the larger disconnected pieces commonly retain more angular shapes. As the alta grades from highly sheared rock near the intrusive contact to the less sheared normal Franciscan rocks, it is possible in places to observe all transitions from alta that resem- bles fault gouge to bedded rocks of the Franciscan group. (See figs. 9, 10.) In mine workings the appearance of the alta is striking because its texture is emphasized by the varied colors of the rocks in it. The shale, which predominates, is all jet black; pods of tufi are altered to light cream—colored clays; and pods of serpentine FIGURE 9.—Hand specimen of alta showing fragments of graywacke and siltstone. This specimen is not typical of most alta; it was selected because the shearing has not obliterated all the original bedding, as it has in more typical specimens. or silica-carbonate rock are generally green. The gray- wacke, although not very light colored, is enough lighter than the black shales to make a contrast. In most surface exposures, however, the intense black color of the shale has been lightened by weathering to such a degree that the augenlike texture of the alta is not conspicuous. Chemical matures Largely because of the prevalent jetblack color of the shale in the alta, the writers suspected that it might diifer from the normal shale of the Franciscan FIGURE 10.—Photomicrograph of alta. indurated and have yielded by breaking, whereas the finer material has flowed. \The same structural relations are commonly seen in exposures, where dimensions are measurable in feet rather than in fractions of a millimeter as in this thin section. The coarser grained layers are 20 TABLE 3.—Analyses of alta and siltstone from the Franciscan group, New Almaden district, Santa Clara County, Calif. IA. 0. Vlisidis, U.S. Geological Survey, analyst] GEOLOGY AND QUICKSILVER DEPOSITS, l 2 SiOZ ____________________________________ 54. 67 62. 54 A1203 ___________________________________ 14. 91 14. 81 F8203 ___________________________________ . 15 2. 02 FeO ____________________________________ 5. 37 5. 47 MgO ___________________________________ 3. 77 3. 38 CaO ____________________________________ 3. 26 1. 40 Na20 ___________________________________ 1. 93 2. 90 K20 ____________________________________ 2. 95 2. 13 H20_ ___________________________________ . 50 . 82 H201L ___________________________________ 3. 42 2. 91 TiOz ___________________________________ . 76 . 87 P205 ____________________________________ . 19 . 15 MnO ___________________________________ . 10 . 05 002 ____________________________________ 6. 22 . 08 S ______________________________________ . 23 . 05 Organic _________________________________ 1. 22 . 96 Total _____________________________ 99. 65 100. 54 Less O—S _________________________ . 12 . 03 Total _________________________ ' s___ 99. 53 100. 51 Norm—Description of sample and locality as follows: I. Aléa 19114-412) from the Relief drift, New Almaden mine, Santa Clara County, a 1 2. Black siltstone (NA—315) from Fern Peak, New Almaden district, Santa Clara County, Calif. group in having a higher content of microcrystalline pyrite or dark chlorite; chemical analyses werethere— ' fore made to see whether any constituents had been introduced from the bordering serpentine. Table 3 gives an analysis of alta taken from close to the in— trusive contact, where the serpentine had been con— verted to silica-carbonate rock; for comparison the table also gives an analysis of siltstone from the Franciscan group. These analyses indicate that per- haps the specimen of alta for analysis was poorly se- lected, for it apparently was carbonatized by the hy— drothermal solutions that formed the silica-carbonate rock. Otherwise, the only significant features are the lower silica and ferric iron content of the alta, which may also be attributed to hydrothermal alteration. No difference that would adequately account for the darker color of the alta is apparent, and it seems likely that this darker color is due to the wider dis- persal of the organic matter by intimate shearing. CONGLOMERATE True conglomerate constitutes only a very small part of the Franciscan group in the New Almaden district, although graywacke beds containing scattered rock pebbles or pieces of shale are not uncommon. The conglomerate that was noted formed relatively thin lenses which could nowhere be traced for more than a hundred feet. Most of these lenses lie in the central part of the district, and conglomerates are NEW ALMADEN DISTRICT, CALIFORNIA apparently very rare or lacking in the lower part of the Franciscan group exposed in the southern part of the district. ' Megascoplc features A typical conglomerate lens no more than 10 feet thick is exposed on Cemetery Hill about 1,400 feet southeast of the New Almaden furnace (coordinates 50 N., 3,500 W., pl. 3). This conglomerate consists of well-rounded pebbles and boulders as much as 9 inches in diameter, but averaging about 2 inches, set in a fairly abundant graywacke matrix that appears similar in all respects to the feldspathic graywacke of the Franciscan group. The rock is cut by fractures which pass through the pebbles without deviation, and some of the fractures are lined with quartz. Where the rock is weathered the pebbles, which are somewhat more resistant than the matrix, protrude to form a knobby surface. A count of a hundred of the pebbles gave the results shown in table 4. Pebbles of variously metamorphosed sedi- mentary and igneous rocks are about equally abun— dant. Only about 10 percent of them are pebbles that could possibly have been derived by erosion of slightly older strata in the Franciscan group, and it is very likely that all of them were derived from pre— Franciscan formations. Pebbles of such distinctive rocks as glaucophane schist or amphibolite, which are found elsewhere (Taliaferro, 1943b, p. 141—143) in TABLE 4.——Pebbles of a conglomerate of the Franciscan group exposed on Cemetery Hill, New Almaden district, Santa Clara County, Calif. . Number SedImentary rocks: ofpebbles Black fine-grained feldspathic quartzite ______________ 9 Gray medium-grained silicified graywacke ____________ 5 Light-gray fine-grained arkose ______________________ 13 Tan fine-grained clayey arkose _____________________ 4 Gray silicified(?) siltstone __________________________ 1 White quartz conglomerate, metamorphosed _________ 1 Gray to black chert _______________________________ 21 Total __________________________________________ 54 Igneous rocks: Aplite ___________________________________________ 1 Quartz porphyry __________________________________ 3 Lavas with quartz phenocrysts _____________________ 4 Tuffs and breccias with quartz _____________________ 5 Mafic brcccias and tuffs ___________________________ 1 Light—colored altered lavas _________________________ 9 Greenstoncs, mostly metamorphosed and silicified _____ 19 Diabase _________________________________________ 3 Total __________________________________________ 45 Vein quartz __________________________________________ 1 Total included in count __________________________ 100 FRANCISCAN GROUP 21 conglomerates of the Franciscan group, were sought but not found; serpentine pebbles also are lacking. Another variety of conglomerate was found as float in several places in the district. It consists of smooth round pebbles of black chert, 1/8 to 1/2 inch in diameter, closely packed in a siliceous matrix. The rock as a whole is gray, very hard, and resistant, and fragments of it are found in stream canyons far below its outcrops. The rock is unusual in being largely composed of a single variety of rock pebbles, which apparently were derived from a distant land mass. ORGANIC AND CHEMICAL SEDIMENTARY ROCKS LIMESTONE Limestone constitutes only about 0.1 percent of the Franciscan group in the New Almaden district, but it is nevertheless important for several reasons. To geologists the limestone is helpful because it forms a discontinuous key-horizon that aids in deciphering «the structure of the area, and because it has yielded the few fossils that give some local evidence regard— ing the age of the Franciscan group. The limestone also possesses economic interest because it has sup— plied several limekilns in the district, and in the future it may be used in making cement, for a similar lime- stone is now used at the Permanente Cement Co. plant a few miles to the northwest of the Almaden area. The limestone exposed in the New Almaden district is doubtless equivalent in part to the Calera lime- stone of the Franciscan group (Lawson, 1914, p. 5, 22), which was observed within a few miles of the northwest corner of the district in mapping the Santa Cruz quadrangle (Branner and others, 1909). A similar limestone is prominent also southeast of the district, in the adjoining San Juan Bautista quadrangle (Allen, 1946, p. 25). About 100 separate bodies of limestone are shown on the geologic map of the district (pl. 1). They occupy a belt that extends southeastward from the hills south of Los Gatos to Longwall Canyon, beyond which it swings northward and northwestward to reach the vicinity of the Calero Reservoir, where the limestone occurs in scattered blocks. (See fig. 56.) Farther east, in the adjacent Morgan Hill quadrangle, the same limestone crops out in more continuous ex- posures 2 miles south of the mouth of San Bruno Canyon. Nearly all the limestone bodies are comparatively small, and the size of many of the smaller mapped outcrops had to be exaggerated on the map to make their position apparent. The most extensive out— crops, none of which are quite 2,000 feet long, occur in two general areas—one close to the southeast corner of the district and the other near its western edge and south of Los Gatos. The thickest body, which crops out on Mine Hill about 800 feet southwest of the modern furnace, appears to be about 10 feet thick, but it may be isoclinally folded. Most of the lime- stone masses that have been quarried are only about 50 feet thick, although in some of the quarries they appear thicker because of repetition by faulting. Many of the outcrops, however, appear to represent beds less than 15 feet thick, and in a good many the limestone forms isolated, roughly equidimensional blocks less than 8 feet in diameter. Because of the variation in thickness of the limestone from place to place, it is believed to have been deposited as lenses, of which only a few were as much as 50 feet thick; these lenses, in turn, may subsequently have been broken and pulled apart by orogenic movements to form the smaller blocks. Megascople features The limestone is one of the most easily recognized rocks in the area. It forms some characteristic bold outcrops that, because of their striking white color, can scarcely be overlooked, and it gives rise to boul- dery float that serves to indicate its presence on grassy or wooded slopes. Probably, therefore, very few outcrops of limestone are omitted from the geologic map (pl. 1). The limestone shows some variation from place to place, and it includes two principal varieties, which seem to have been deposited at slightly different times. The more widespread of the two varieties, which is the older, corresponds to the typical Calera lime— stone; it generally shows fairly well—developed bed- ding, and in most exposures it contains gray or white chert in elongate discontinuous thin lenses flattened parallel to the bedding (see fig. 11). The poorly de- veloped parting layers, which are from a few inches to several feet apart, commonly contain a thin film of shale and may be either fairly regular or stylolitic on a small scale. In most outcrops these thin layers provide the only record of any interruption in the deposition of the limestone, but locally the limestone occurs as thin lenses intercalated with tuffaceous and calcareous shales. (See figs. 12, 13.) The color of the freshly broken surfaces is generally black or dark gray, but in some varieties is white or pink. The darker varieties have a strong fetid odor, due to petroliferous material and hydrogen sulfide. Most of the lighter colored varieties contain minute Forami- nifera, which appear to the unaided eye as small transparent dark specks but can be seen under a hand 22 GEOLOGY AND QUICKSILVER DEPOSITS. NEW ALMADEN DISTRICT, CALIFORNIA FIGURE 13.—Calera type of limestone of the Francmcan group inter- bedded with tuifaceous and calcareous shale, as exposed in roadcut 1 mile west of horseshoe bend in Guadalupe Canyon. Beds on the left side of the photograph yielded the Upper Cretaceous Foraminif— era described by Cushman and Todd and by Kiipper (p. 25). FIGURE 11.—Bold outcrop of the Calera-type limestone occurring in the Franciscan group. The more resistant lenses are dark chert, which accompanies the limestone in most places. FIGURE 12.—Thin bed 5 of limestone (ls) of the Franciscan group interlayered with tuffaceous shale (T). Roadcut along Los Gatos-Santa Cruz highway, onehalf mile south of Los Gatos. FRANCISCAN GROUP ‘ 23 lens to have organic shapes. Much of this limestone shows no recrystallization even in thin section, except that the replaced Foraminifera may consist of sutured calcite grains somewhat coarser than the rest of the rock; however, locally it is recrystallized. Coarsely crystalline calcite fills fractures, and in recemented breccias it amounts to nearly half of the rock. Pyrite, generally altered to limonite, is fairly common, and the more impure varieties contain altered tachylite and a small amount of quartz and plagioclase. The second variety of limestone, which is strati- graphically a little higher in the section, crops out on the south slope of Los Capitancillos Ridge as isolated bodies, of which the most prominent is about 2,000 feet southwest of the apex of Mine Hill. This limestone differs from that found lower in the section in that it is everywhere fairly coarsely crystalline, contains numerous lenses and pellets of glauconite, and lacks chert and Foraminifera. An unusual oolitic limestone, unlike any known to have been reported heretofore from the Franciscan group,4 forms large conspicuous outcrops in Longwall Canyon about 1 mile west of its junction with Llagas Canyon. These outcrops are massive, and where the limestone is purest they show no bedding. Solution - has commonly roughened their upper surfaces, and where they are jointed, the cracks have been enlarged by solution. This oolitic limeStone is dull battleship gray, and the purest consists almost entirely of oolites about 2 mm in diameter, embedded in ‘a'matrix of smaller oolites averaging about 0.1 mm in diameter (figs. 14, 15). The substances forming the nuclei of the larger concentrically layered oolites are shell fragments, bryozoan( ?) fragments, carbonatized ve— sicular mafic glass, and aggregates of smaller oolites. N0 Foraminifera were found in the rock. The more common impure facies, which grade into tuffs and shales, contain shale fragments, considerable glauco- nite, and sparse grains of quartz and feldspar; they also contain some interesting but generally fragmen- tary fossils of gastropods and echinoi‘ds. The oolitic limestone could not be traced continuously to typical Calera—type limestone beds and cannot be definitely correlated with either of the two limestone units, but because it crops out along their projected strike, the writers believe that the oolitic limestone is merely an unusual variety of the Calera~type limestone, de- posited at the same time and under nearly the same conditions. 4This oolitic limestone may possibly be the same rock described in California State Mining Bur., 12th Rept. of the State Mineralogist, p. 394, 1894. FIGURE 14.——Oolitic limestone of the Franciscan group from Longwall Canyon in the southeastern part of the New Almaden district. An analysis of this limestone is given on page 24. Insoluble residues A study of the insoluble residues of the limestone from several parts of the district has been made by Pantin5 to determine whether the isolated outcrops in the New Almaden district could be correlated by this means with the thick section of Calera limestone exposed in the quarry of the Permanente Cement Co. several miles to the north. He found the insoluble residues to consist of allogenic gray silt, very fine grained sand, clay, and chert, and authigenic glau- conite, pseudocubic crystals of quartz, barite, chert, lignite, limonite pseudomorphs after pyrite, and limo- nite replacements of microfossils. In different parts of the section these insoluble minerals were present in dif- ferent proportions, and their aggregate quantity ranged from 2 to 15 percent. On the basis of their distribution Pantin was able to correlate the sampled outcrops of FIGURE 15.—Photomicrograpn of oolitic limestone of the Franciscan group. Note the groups of small oolites that form the cores for some of the larger oolites. 5 Pantin, José Henrique, 1946, Insoluble residues of the Calera lime- stone in Santa Clara County, California: Stanford Univ., unpublished master’s of science thesis, November. 24 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA the New Almaden district with various parts of the better developed section at the Permanente quarry in the Santa Cruz quadrangle. He concluded (p. 74) that “. . . the zones in the limestone can be cor— related over considerable distances by means of in- soluble residues * * *” and “the correlation means that the limestone in different areas must have been laid down simultaneously, under similar environmental conditions, if not as a part of a continuous bed.” Chemical features The limestone of the Calera type, exclusive of the interlayered lenses of chert, consists largely of cal— cium carbonate with only a very small quantity of magnesium, iron, aluminum, and phosphate. No anal— yses of this type of limestone from the New Almaden district are available, but three analyses of the similar limestone quarried near Permanente Creek, a few miles northwest of the district, are shown in table 5. The bulk analyses shown in table 6 are of the same limestone; they were kindly provided by the Perma- TABLE 5.—Analy8es of Calera—type limestone from near Permanente Creek, Santa Cruz quadrangle, California l 2 3 330(2) ___________________________ 2. 08 1. 56 1. 52 Feio::‘_'::::::::::::::::::::::::} -56 :33 } -47 anOa _________________________ ll.d. . 05 1Ld. CaO ___________________________ 54. 44 54. 43 54. 84 MgO ___________________________ . 20 . 21 . 14 002 ____________________________ 42. 92 42. 99 43. 33 §?6;::::::::::::::::'::::::::“} Trace ”6% } Trace H20 ___________________________ . 05 . 15 [1.11- Total ____________________ 100. 25 99. 99 100. 45 C8003 _________________________ 97. 36 97. 42 98. 17 Nora—Description of sample and locality as follows: 1. Lilrgfitonefilrom Permanente Canyon. W. L. Lawson, analyst. From Lawson, , D- - 2. “Black Mountain” limestone from Permanente. S. A. Tibbetts, analyst. From Huguemn and Costello, 1921, p. 185. Recast to show oxides rather than carbonates. . 3. Same as 1. nente Cement Co. These latter analyses include the siliceous chert lenses, but extrapolation to a silica-free rock shows perhaps even better than any single analy- sis the low magnesium content of the calcite, for such an extrapolation yields a theoretical rock containing 55.5 percent CaO and but 0.1 percent MgO. An analysis by A. C. Vlisidis, US. Geological Sur- vey, of the unusual oolitic limestone of the Franciscan group from the north bank of Longwall Canyon in the southeastern part of the New Almaden district fol— lows. Slog 0.91 A1203 .61 F8203 - .30 MgO .77 0310 55.11 Ti02 .10 P205 _ .31 MnO .01 002 _ ___ 42.24 Total 100.36 CaCO3 96.0 This analysis shows the similarity in chemical com- position between the Calera-type limestone and the oolitic limestone. Minor differences are due to the fact that the oolitic rock contains fragments of dark volcanic rock, whereas the impurities in the other limestone are chiefly quartz and clay. As the oolitic limestone contains Virtually no chert lenses, it offers good commercial possibilities, even though the ex— posed bodies are somewhat smaller than some of the bodies of Calera—type limestone in the district. Fossils A microfauna and a few megafossils were obtained from the limestone of the Franciscan group in the New Almaden district. The microfossils, which were all Foraminifera, occur in abundance in most of the Calera-type limestone, but generally they cannot be freed from this rock for study. To Senor J osé Pantin, TABLE 6.—Bulk analyses of Calera-type limestone from quarries of the Permanente Cement 700. near Permanente Creek, Santa Cruz quadrangle, California 1 2 3 4 5 6 7 8 9 10 SiOz ___________________________ 29. 23 18. 4 17. 5 16. 7 15. 9 15. 2 13. 4 11. 3 7 24 4. 18 A1203 __________________________ 1. 26 1. 5 1. 4 1. 5 1. 5 1. 5 1. 4 1. 3 60 . 66 F8203 __________________________ .54 .7 .7 .7 .7 .7 .6 .6 .42 .32 030 ___________________________ 38. 04 43. 3 43. 8 44. 4 44. 9 45. 4 46. 4 47. 8 50. 96 52. 74 MgO __________________________ .24 .3 .4 .3 .4 .4 .4 .3 .04 .05 Loss on ignition _________________ 3o 66 34.3 34. 7 35. 1 35. 7 36. 1 37. 1 38. 3 40. 48 41.90 Total ____________________ 99. 97 98. 5 98. 5 98. 7 99. 1 99. 3 99.3 99. 6 99. 74 99.85 C3003 _________________________ 68. 4 77. 3 78. 2 79. 3 80. 2 81. 0 82. 9 85. 3 91. 39 94. 67 1, 9, and 10) From Logan, 1947, p. 315. 2-8) Obtained from Permanente Cement Co. by the writers. FRANCISCAN GROUP 25 a former graduate student at Stanford University who worked on the Calera-type limestone at the writers’ suggestion, goes the credit for noticing the locality shown in figure 13 in which some Forami— nifera had been freed from the limestone by weather- ing; subsequent investigation resulted in finding other Formainifera in tuffaceous limy shales interbedded with the limestone. This locality yielded the Forami- nifera which have been described by Cushman and Todd (1948, p. 90—98) and Kiipper (1955, p. 112—118). It lies in a road cut in the SW14 of sec. 24, T. 8 S., R. 1 W., and it can easily be reached by traveling south 0.23 mile along a poor dirt road that branches from Shannon Road at a point 0.7 mile west of Guadalupe Creek. Cushman and Todd believed the fauna to be diagnostic of a Lower Cretaceous age, and Glaessner (1949, p. 1615—1617) upon comparing their illustration with forms found in the Mediter- ranean region suggested that the age be restricted to the upper part of the Lower Cretaceous (Albian- Aptian). Kiipper, who obtained better material than that available to Cushman and Todd, reported, “The evidence thus favors correlating * ¢ * with strata classified as Cenomanian in Europe and Africa.” Rather poorly preserved megafossils were found in the oolitic limestone along the middle part of Longwall Canyon in the southeastern part of the dis- trict. Although small fragments of fossils, and also wood fragments, are fairly common in this rock, few of the fossils are complete, and as the rock is brec- ciated and recrystallized, only weathered surfaces 1 inch I 1 inch B A yielded useful fossils. A diligent search resulted in the finding of only two specimens of the gastropod Nem‘nea, and several spines and a fairly complete test of an echinoid, Uidam's (fig. 16). According to R. W. Imlay (written communication, Oct. 29, 1948), the Nerz'nea must be either Jurassic or Cretaceous, but the Uidam's has not been identified closely enough to indicate its age. In addition to the above fossi]s from the limestone, fragments of Inocemmus were found in limy concretions in shale of the Franciscan group near the north end of the Calero Dam, but they were too small to be determined specifically. cum Chert constitutes less than 0.5 percent of the rocks of the Franciscan group in the district, but because of its resistance to weathering, it seems much more abundant than it really is. Its areal distribution in the district, and in the geologic column, is not uni— form, for it is much more abundant along the belt of greenstone trending westward through the central part of the district than it is elsewhere. This belt includes all the major quicksilver mines, but, although quicksilver mines have been developed in the chert of the Franciscan group elsewhere in California (Bailey, 1946, p. 219—221), only minute amounts of cinnabar have been found in the chert in the New Almaden dis- trict. Manganese ores also have been mined from cherts of the Franciscan group elsewhere in the State, but in this district only a few scattered occurrences of manganiferous chert have been prospected. C FIGURE iii—Fossils from the limestone of the Franciscan group in the New Almaden district. A, Oidaris 31)., side view. Natural size. B, Cidan‘s sp., spine. Natural size. 686-671 0——63——3 C, Nerinea sp., ground to show internal structure. X2. 26 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA Megasooplc features The cherts of the Franciscan group show considera- ble diversity of character, probably because they do not all have the same origin. Davis (1918, p. 235— 432) has well described the field occurrences and lithology of the various kinds of chert in other parts of the Coast Ranges, and because his descriptions could apply equally well to the cherts of the New Almaden district, only the more significant features will be repeated in this report. The typical chert of the Franciscan group is well bedded, generally rhythmically bedded, and red or green. A more un- usual kind, locally associated with the well—bedded chert, exhibits botryoidal surfaces and distinctive internal structures which are described below. Less abundant but more typical of the older part of the Franciscan group is a massive nearly white chert, which also occurs sporadically in the younger part of the sequence. Other light—gray to white chert occurs as thin lenses or nodules in the limestone of the Fran- ciscan group, and is not separated from the limestone on the maps accompanying this report. Still other chertlike rocks clearly formed by silicification of mafic tufi's are not considered to be true cherts and are included with the greenstone on the geologic maps. The most striking feature of the good outcrops of bedded chert is their ribbed appearance, due to rhyth- mic bedding of layers of chert and shale of the same color, either red or green. The layers of chert are generally between 1 and 2 inches in thickness, whereas the shale partings are in most places not more than a quarter of an inch thick and are commonly much thinner. The individual chert layers are blunt—ended lenticules that extend at most only a few tens of feet; some of these layers vary in thickness, the bulges of one layer commonly being compensated by thinning in an adjacent layer. Sharp chevron folds a few feet across and minor faults at low angles to the bedding planes are common. At sharp bends in the folded cherts as at the axes of chevron folds, the chert is likely to be appreciably thicker but unfractured, sug- gesting that the deformation may have taken place while the chert was still plastic (see fig. 17). Fractures, not related to the folds and found in almost all the well-bedded chert, form two systems of joints, both of which are nearly perpendicular to the bedding and commonly intersect on the bedding surfaces at angles of about 120°. In many places these fractures are small faults that make little steps on the bedding surface; in other places no displace— ment is noticeable. Some cracks are open near the upper bedding surface like mud cracks; elsewhere they FIGURE 17.—~Sharp folds in bedded chert of the Franciscan group. The absence of fractures through the axial parts of the folds is typical, and has been cited as evidence of folding during submarine slumping before lithification. are tight. Locally, the cracks are filled with silica, which resists weathering and forms ribs; or they are filled with calcite, which weathers away and forms grooves. Surfaces of the chert broken across the bed— ding are generally smooth and conchoidal; bedding surfaces are slightly pitted or pimply. Examination under the hand lens shows the minor surface irregu- larities to be due, in part at least, to the unequal weathering of small grains of clear silica that has replaced, or filled in, the Radiolaria that abound in some of the cherts. The chert exhibiting botryoidal surfaces is uncom- mon in the district; it is exposed only in a group of outcrops half a mile north of the summit of Fern Peak and is found as float on a small knob about a quarter of a mile south of Tulare Hill. Its color at both places is brownish red with greenish patches. The outcrops near Fern Peak are small, not exceed— ing 20 feet in diameter and generally much smaller, and are roughly equidimensional. These are made up of smaller bodies that are shaped somewhat like a cauliflower. Each cauliflower has a nearly flat lower surface that shows some flattened botryoidal bumps and a roughly hemispherical upper surface on which are superposed smaller hemispheres or more irregular flattened bumps and ridges not more than a couple of inches across. The bumps are separated by sharp but wide V—shaped grooves, and in most places they appear to have been packed on top of each other so that the surface closely resembles that of certain kinds of spatter cones. Equally distinctive are the internal structures of the botryoidal chert (fig. 18). These feathery arcuate structures appear to have resulted from shrinkage of an originally very hydrous gel FRANCISCAN GROUP 27 forming openings, which later were largely filled with white quartz; small quartz crystals line cavities that were not completely filled. In some specimens the botryoidal surfaces are visible within the mass, whereas in others, which have an equally well-devel- oped botryoidal shell, the internal structures do not indicate successive botryoidal layers. Conversely, some chert breccias not exhibiting the surface struc- tures show internal structures that appear to have resulted from dehydration shrinking (fig. 18). These evidences of dehydration, however, are found only in small, generally isolated, masses of chert, and they are not typical of either the rhythmically bedded or massive chert lenses that account for 99 percent of the chert in the Franciscan group. The light-colored chert that occurs mainly in the lower part of the Franciscan group is exposed in small outcrops generally less than 30 feet long and only a few feet wide. These outcrops are too small and too erratic to be shown on the map of the district, but chert, interbedded with arkose, was observed in several places along the ridge extending northwestward from El Sombroso. This chert is massive, without shale partings, and only in a few places does it contain separation planes that indicate its bedding. Light- colored quartz is the dominant mineral, but some lenses contain a little oxidized pyrite, which has mis- led prospectors into believing that the siliceous lenses were gold—quartz veins. No Radiolaria were noted in any of these light-colored older cherts. Microscopic features In thin section the bedded chert is seen to be largely a mixture of quartz and chalcedony, which generally is clouded with red iron oxide dust. Some varieties are composed of a very fine grained aggre- gate of quartz, others contain chalcedony, and still others have a matrix of silica that appears almost isotropic but has an index of refraction slightly greater than that of balsam. This material, which apparently has puzzled others (Lawson, 1895b, p. 423) and deserves further study, is probably a crypto- crystalline aggregate of chalcedony and quartz, with overlapping crystals which tend to compensate each other between crossed nicols. In thin section, clear areas having the outlines of Radiolaria stand out in contrast to the clouded matrix, and where the Radio- laria are unusually well preserved, the spines and mesh structures, as well as the general outlines of the microfossils, can be distinguished (fig. 19). Most of these clear areas consist of quartz a little coarser grained than that composing the matrix, but some consist of chalcedony, whose fibers may radiate from one or. more centers of growth. In a few thin sec- FIGURE 18.——Vertical cut through part of a “cauliflower” of botryoidal chert showing internal structure. specimen is oriented as it occurs in nature. The Dark is red-brown chert; light is milky quartz. FIGURE 19.——Photomicrograph of chert of the Franciscan group con- taining numerous remains of Radiolaria. tions cut normal to the bedding, thin seams of chlorite deformed after the fashion of stylolites are visible. Most thin sections of the chert contain at least a few veins of clear sutured quartz, and some of these are veined and partly replaced by later calcite. GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA Associated shales The shaly rocks associated with the well-bedded chert are distinctive owing to their content of iron and the resulting color, which is red or less com- monly green, in contrast to the tan, gray, or black of typical siltstone and shale of the Franciscan group. .They are faintly laminated. Although some are ac- tually shale, others are so coarse grained as to feel gritty and should be classed as siltstone. They con- tain chlorite and very fine goethite or hematite dust imbedded in an unidentified fine—grained matrix which makes up most of the rock. Chemical features N0 analyses of either the cherts or the shales that accompany them in the New Almaden district have been made. Complete and partial analyses of the typical chert and shale of the Franciscan group made by Davis (1918, p. 268—269), together with analyses of a graywacke and black shale of the Franciscan group and a composite of 78 shales for comparison, are shown in table 7. Comparison of the three cou- plets of chert and associated shale indicates that a mere addition of silica to the shale might yield the corresponding chert, but a very large quantity of silica would be required. The shales associated with the cherts differ chiefly from the average shale, column 10, in having a higher content of silica, and ferric iron, and a lower content of aluminum, cal- cium, and potassium. They compare more closely TABLE 7.—Analyses of cherts and shales of the Franciscan group 1 2 3 4 5 6 7 8 9 10 Chert Shale Chert Shale Chert Shale Graywacke Average of Black Composite 2, 4, 6 siltstone of 78 shales SiOz _____________________ 93. 54 69. 98 95. 08 63. 47 96. 37 68. 22 64. 61 62. 2 62. 54 58. 51 A1203 ____________________ 2. 26 11.69 2. 17 10. 14 2. 17 11.31 12. 92 10. 9 14. 81 15. 55 e203 ____________________ . 48 6. 23 2. 82 14. 60 2. 82 8. 75 1. 66 9. 7 2. 02 4 03 e0 _____________________ . 79 1. 08 n.d. n.d.. n.d. n.d. 5. 63 1. 1 5. 47 2 50 MnO ____________________ . 23 . 49 n.d. . 08 n.d. . 13 . 25 . 2 .05 __________ MgO ____________________ . 66 l. 29 n.d. 2. 41 n.d. 2. 39 3. 53 1. 9 3. 38 2. 44 CaO _____________________ . 09 . 38 n.d. . 74 n.d. . 99 1. 27 . 7 1. 4O 2. 99 NazO ____________________ . 37 . 73 n.d. n.d. n.d. n.d. 3. 71 . 7 2. 90 1. 28 20 _____________________ . 51 3. 72 n.d. n.d. n.d. n.d. . 98 3. 6 2. 13 3. 28 20‘ ____________________ . 21 1. 03 n.d. 1. 44 n.d. 1. 39 1. 65 1. 3 . 82 1. 31 H20+ ____________________ . 72 2. 92 n.d. 3. 91 n.d. 4. 42 3. 68 3. 7 2. 91 3. 69 P205 _____________________ n.d. n.d. n.d. n.d. n.d. n.d. . 11 __________ 15 __________ Total ______________ 99. 86 99. 64 100. 07 96. 79 101. 36 97. 60 100. 00 100. 0 98. 58 95. 58 Nora—Description of sample and locality as follows: 6. Partial analysis of red shale accompanying 5. E. F. Davis, analyst. From ,_. . Brownish-red chert from Bagley Canyon, Mount Diablo. W. H. Melville, analyst. From Melville, 1891, p. 411. . Brownish-red shale associated with 1. W. H. Melville, analyst. From Melville, 1891, p. 411. . Silicegus red chert from Red Rock Island. E. F. Davis, analyst. From Davis, 191 , p. 268. i . Partial analysis of red shale accompanying 3. E. F. Davis, analyst. Davis, 1918, p. 269. . Red chert from Point Richmond. E. F. Davis, analyst. p. 268. From (”#th From Davis, 1918, Davis, 1918, p. 269. 7. Altered graywacke of the Franciscan group from headwaters of Bagley Creek, Mount Diablo. W. H. Melville, analyst. From Melville, 1891, p. 412. Recast by the writer to exclude 5.10 percent 002, calculated as 021003, and remainder calculated to total 100 percent. 8. Average of 2, 4, and 6; recalculated to total 100 percent. 9. Black siltstone of the Franciscan group, New Almaden district; minor oxides omitted, see table 2. 10. Composite of 78 shales from Clarke, 1916, p. 546. Noncomparable constituents (002, $03, P205, 1330, T102) omitted. FRANCISCAN GROUP 29 with the analyzed graywacke, column 7, and black siltstone, column 9. However, they contain more total iron, which is also more oxidized, ‘and they contain less aluminum and magnesium. Unlike many of the other rocks of the Franciscan group, the shales occur- ring with chert have a potash-soda ratio of more than 1. Fossils Although the Radiolaria of the chert in some places are remarkably well preserved, they are not suf— ficiently diagnostic to date the Franciscan group more closely than Jurassic or Cretaceous. Hinde (1894, p. 236), who studied the Radiolaria from Angel Island in San Francisco Bay and Buri—Buri Ridge in San Mateo County, reported as follows: The majority of them evidently belong to simple spheroidal and ellipsoidal forms, included in Haeckel’s suborders Sphere- roidia and Prunoidia * * * The most distinctive feature * * * [is] the number and variety of forms of the genus Dictyo— mitm present. Some of the better preserved Radiolaria found in the New Almaden district are shown in figure 19. Origin The interesting problem of the origin of the chert in the Franciscan group has been dealt with by several of the geologists most familiar with the rocks of the California Coast Ranges. It has not been particularly studied during this investigation, but a summary of the various suggested origins is given here, along with some pertinent data gathered in the course of our work, for the benefit of those who may not wish to read the extensive literature on this subject. Although many of the early workers described ob— servations bearing on the origin of the chert, or sug- gested possible sources for the silica, the first to make a thorough field and laboratory study of the problem was Davis (1918, p. 353—408). He concluded, after dismissing many other possibilities, that the silica came from submarine siliceous springs of magmatic origin, that most of the silica was chemically precipi- tated and the Radiolaria were only incidental fossils, and that the rhythmic bedding and wedging out of beds were best explained by colloidal segregation of silica from the intermixed shaly material. Taliaferro (1933, p. 51, 54) believed that the source of the silica must lie in siliceous water that accompanied the out- pourings of volcanic material. At first he suggested that the greater part of the silica was supplied by magmatic water, but some silicic acid may have been liberated by the interaction of hot basic lavas with sea water. In a later report Taliaferro (1943b, p. 147— 148) stated, “a considerable amount [of the silica] might have resulted from the interaction of hot lava and sea water.” Trask and others (1943, p. 58) and Trask and Pierce (1950, p. 219—221) in their reports on the manganese deposits of California gave evidence suggesting that the chert was deposited in local basins, but reached no conclusion regarding the source of the silica. Bramlette (1946, p. 55), who studied intensively the cherts of the diatomaceous Monterey formation in California, suggested that possibly the solution of the thinner shelled siliceous organisms may have provided the silica for the chert during diagenetic changes or low-grade metamorphism of the Franciscan rocks. Another suggestion formerly entertained, that the cherts result entirely from the accumulation of radio— larian tests under abyssal conditions, does not seem to fit observations made in the Franciscan rocks, and it has been generally abandoned. ‘Most of our observations having a bearing on the origin of the chert consist of relationships between the chert and greenstone. These can be seen on the geologic map of the district, which shows that chert is most abundant in areas containing greenstone, and that more than three-fourths of the chert lenses are either enclosed in greenstone or border a mass of greenstone. Probably equally significant is the fact that nearly all the chert in contact with greenstone lies on the upper side of the greenstone masses. Some chert lenses, however, are apparently enveloped by clastic sedimentary rocks, and although some of the lenses are stratigraphically only a few hundred feet above the greenstone masses, others would seem to be truly far removed from any igneous rocks. The map also indicates that the ratio of chert to greenstone is about 1:250, which is pertinent if one considers the possibility of the silica being derived by reaction of sea water with hot lava. Whether or not this ratio is representative of the Franciscan group cannot now be determined because of the lack of detailed maps of much of the extensive area occupied by Franciscan _ rocks. The association of the chert and greenstone strongly suggests that the cherts in the Franciscan group are in some way genetically related to the greenstone. The writers believe it likely that the silica and iron of the chert and accompanying shale was derived chiefly, if not entirely, by reaction of the hot mafic lava with sea water. At shallow depth sea water, even though heated nearly to its boiling point, is incapable of dis- solving much more silica than can be retained on cool— ing to normal sea temperature; but, under pressures that exist at a depth of 10,000 feet or more, many times as much silica could be dissolved. Hot silica- ladened waters so formed would rise and be cooled to 30 GEOLOGY AND QUICKSILVER DEPOSITS, normal sea temperature giving up their excess silica, which would probably settle as silica gel. Thus, a re- action between sea water and erupting lava in deep water offers a possible mechanism for both introducing silica into sea water and getting it out again in the form of silica gel. The outpourings of mafic lava that accompanied deposition of the Franciscan sediments were chiefly submarine, and reaction between 'magma and sea water would be expected. Violent submarine eruptions giving rise to the glassy fragmental green- stones probably provided optimum conditions for re- action between the magma and sea water, and even the submarine flows with pillow structure would pro- vide large areas of contact between magma and water. That a reaction between hot basalt and sea water yield- ing silicic acid can take place was demonstrated by Van Hise and Leith (1911, p. 515—516, 525), butthe writers are not aware of controlled experiments which would indicate the quantity of silica that might be released by this reaction at elevated pressures. How- ever, if the lavas before reaction contained about 50 percent silica and a little less than 1 percent of it was lost by reaction with the water, this would supply sufficient silica to form all the chert in the district. Although the exact quantity Of silica taken up by the sea water is unknown, it is reasonable to assume that the water when cooled to sea temperatures would be supersaturated and would yield silica gel. This would flocculate and settle by gravity to form a mass with a density greater than the adjacent sea water, and such a mass might be expected to flow as a density current if it formed on even a very low angle slope. This flowage into basins may explain why some chert masses apparently occur a considerable distance from volcanic rocks. The process of collection Of the silica, expulsion of the water and iron-rich compounds, and the develop- ment Of rhythmic bedding has been commented on by many geologists who have studied rhythmically bedded cherts in various parts of the world. NO theory has met with wide acceptance. Davis treats the problem of the rhythmic bedding in the cherts of the Francis- can group exhaustively and gives the results of several experiments in which he obtained rhythmic separation of silica gel from clay (Davis, 1918, p. 386—402). As a result of his experiments and careful field Observa- tions Davis concluded that the bedding in these cherts owed its origin to colloidal segregation, but it is not clear whether he believed the entire sequence of beds in a lense of chert was formed by segregation of a single mass of gel or from several superposed masses. Although the peculiar lenticular character Of the beds in the rhythmically bedded sequences were duplicated in part by Davis’ experiments, he was unable to form NEW ALMADEN DISTRICT, CALIFORNIA more than a few such layers, whereas in nature se— quences involving hundreds of beds are not unusual. Taliaferro believes each layer was deposited separately and solidified before the deposition of subsequent lay- ers, and as evidence for this he cites the occurrence of chips of chert in the partings between layers (Talia- ferro, 1943b, p. 149). The unusual botryoidal cherts of the New Almaden area are believed to be most easily explained as having been built up by rapid con- solidation of silica gel oozing from a submarine ori— fice, and they also suggest rapid solidification of the silica gel. VOLCANIC ROCKS (GREENSTONES) The varied mafic volcanic rocks interbedded with the sedimentary rocks of the Franciscan group in the New Almaden district are believed to be fairly repre- sentative of the volcanic rocks found in the group throughout the California Coast Ranges, although they do not include quite \all the varieties that have been reported. Because they are so chloritized' and otherwise altered that precise field classification is impossible, they have been grouped under the general term of “greenstone,” in accordance with the usual practice of geologists of the Coast Ranges. They crop out over an area of about 20 square miles, or one—third of the part of the New Almaden district that is under— lain by rocks of the Franciscan group. They form bodies that» are lenticular but well defined, and these bodies are mapped as cartographic and stratigraphic units. (See pl. 1.) The greenstones are apparently all derived from ex- trusive rocks, and many, if not all, are of submarine origin. They differ widely in grain size and texture: the coarsest are of Ophitic or diabasic texture, others are variolitic or pilotaxitic, and still others are pyro- clastic breccias or tuffs. In spite of textural differ- ences all contain nearly the same mineral assemblage, a fact suggesting similarity in the chemical composi— tion of their parent magmas. The typical primary minerals are sodic plagioclase and subcalcic or titanian augite, but a few of the lavas contain a little Olivine. In addition, material formed by alteration of mafic glass (or tachylite) is found in the diabasic rocks and is very abundant in the finer grained volcanic rocks; in part of the district altered mafic glass is the major constituent of accumulations of tuff and breccia more than 500 feet thick. The alteration of the volcanic rocks to greenstones has been widespread, and varied in both kind and de- gree. In much of the greenstone deuteric alteration has formed minerals of the epidote group from pyrox- enes, antigorite from the olivine, and a saussuritic ag- gregate from the plagioclase. In some of it the origi- FRANCISCAN GROUP 31 nal glass is replaced by albite, indicating a type of spilitic alteration. Very low grade regional metamor- phism may be responsible for extensive chloritization, and may have also caused the widespread irregular veining of the rocks with quartz and calcite. The greenstones of small areas have been still further meta- morphosed to form amphibolites, which are discussed with the other more highly metamorphosed rocks of the Franciscan group on pages 39—40. In the descriptions given in the following para- graphs the greenstones are grouped according to whether they were originally lava flows or pyroclastic rocks. LAVAS The greenstones derived from mafic lava flows are more abundant and more Widely distributed than those derived from pyroclastic rocks. The lavas differ widely in texture, and their groundmasses range from holo- crystalline to glassy. The coarser varieties doubtless grade into the aphanitic varieties in many places, but owing to the poor exposures such transitions were ob- served only in individual pillows, some of which have diabasic centers and glassy rims. An attempt to sub- divide the lavas into diabasic and aphanitic textural types was made during the field mapping in hope of establishing a stratigraphic sequence, but this attempt failed because of the intermixing of the varieties and the scarcity of exposures. It was evident, however, that the aphanitic rocks are the most abundant, and that the diabasic rocks occur most commonly within the larger masses of greenstone. The character of the exposures of the massive green— stone varies with differences in the original character of the lava and in its mode of alteration. The best exposures are those formed by lavas exhibiting pillow structure, for these are among the most resistant rocks in the district; in several places they form rugged cliffs. Bold outcrops are also found Where the lavas are silicified, but generally these are small and errati- cally distributed. The more widespread diabasic and amygdaloidal lavas are well exposed in some canyon bottoms, but elsewhere they underlie large areas that contain only widely spaced subdued outcrops, sur— rounded by a characteristic red-brown soil containing fragments of the altered greenstone. Megascopic features The unweathered greenstones are dark green to black, but in outcrop they are generally weathered and reddish brown. The coarser varieties are readily recognized, for they have diabasic textures in which the tabular crystals of feldspar, oriented at random, are large enough to be distinguished with the naked eye. The feldspar is usually the only mineral that can be identified megascopically, but because of its gener— ally altered character, it is greenish white and opaque rather than glassy; and although it is plagioclase, its twinning is not always visible. In the finer grained lavas the textures are not apparent and individual minerals cannot be determined even with a hand lens. These rocks, however, commonly show such features as pillow structure, flow banding, and vesicles, which aid in their recognition. Vesicles, where present, are generally filled with either calcite or deep-green non— tronite. Irregular veins of both quartz and calcite are abundant in some of the greenstone. Microscopic features Although thin sections of the lavas reveal only minor variation in kind or proportion of the primary miner- als, they show wide differences in texture and grain size and in the character of their secondary minerals. (See figs. 20—23.) The principal primary minerals are sodic plagioclase and either subcalcic augite or titan- augite; accessory minerals are magnetite, ilmenite, leu— coxene, sphene, rutile, and apatite. Mafic glass was originally present in most varieties, but has everywhere been altered to chlorite. Olivine was once present in a few of the greenstones, but it has been completely replaced by either antigorite or iddingsite. Other common secondary minerals identified are albite, chlo- rites, epidote and clinozoisite, quartz, calcite, and non— tronite. A more thorough study of some of the finer grained aggregates of secondary minerals would no doubt reveal other species. The textures of most of the greenstones as seen in thin section are well preserved in spite of the advanced stage of alteration. Most of the coarse-grained lavas are holocrystalline; the coarsest contain plagioclase tab- lets 3 mm long and pyroxene prisms a little longer. In the aphanitic rock, however, glass was an impor- tant constituent, amounting in some to as much as 40 percent. Among the holocrystalline rocks an inter— granular texture is more common than a truly diabasic texture; in the aphanitic rocks the texture is widely varied and no single kind of texture variety predomi- nates. Porphyritic textures are uncommon, although a little of the greenstone has an aphanitic groundmass containing scattered phenocrysts of albite as much as 4 mm long. The plagioclase, which before secondary alteration amounted to from 30 to 60 percent of the lavas, is generally euhedral in form, although in those rocks that have undergone severe deuteric alteration re- newed growth of plagioclase has formed crystals that are somewhat sutured and interlocked. Much of the plagioclase is partly saussuritized or replaced by chlo- rite, epidote, or calcite, but nearly all the greenstone 32 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA FIGURE 20.~Greenstone without vesicles and only a little altered FIGURE 21l.—Greenstone without vesicles but with considerable al- glass. Ophitic texture formed by radial aggregates of somewhat tered glass. Texture is largely intergranular, but some augite en- altered andesine (and) enclosed in larger crystals of subcalcic augite velopes crystals of albite. Mafic glass altered to nontronite (N). (A). Dark areas are leucoxene formed at the expense of ilmenite. Plagioclase contains minute crystals of epidotc. Groundmass of altered glass consists of chlorite, nontronite, and small crystals of epidote. FIGURE 23.—Greenstone with abundant vesicles filled with calcite (C). Section shows microlitic texture of minute laths in glassy groundmass, but also contains a few phenocrysts of albite, replaced along borders and cleavages by calcite, and still fewer equant crys- FIGURE 22.——Greenstone with vesicles but little altered glass. Both andesine and augite are in thin needles forming a pilotaxitic tex- ture. Vesicles are filled with chlorite locally replaced by calcite. tals of augite. PHOTOMICROGRAPHS OF GREENSTONES OF THE FRANCISCAN GROUP SHOWING VARIATIONS IN TEXTURE, CONTENT OF ALTERED CLASS, AND VESICULARITY ~- FRANCISCAN contains unreplaced feldspar that gives sharp extinc- tions so that its composition can be determined. Albite twinning is prevalent; zoning was observed in some sections but is not common. Although the feldspars of these rocks are invariably too soda rich to be nor— mal for such mafic rocks, the individual crystals in most specimens show no ragged borders, irregular patches, or other features suggesting that the sodic plagioclase has replaced a more calcic feldspar. In the coarser grained lavas and phenocrysts the plagio- clase is invariably albite (A1124). In a little of the aphanitic lava plagioclase as calcic as An40 was noted, but inmost the plagioclase is near albite. Pyroxenes are important constituents of the green— stones of the Franciscan group, making up from 20 to 35 percent of their volume. They deserve more thorough study than has been made for this report, but enough work has been done to indicate that they belong to several varieties. The optic-axial angle (2V) of a few crystals of augite was determined by Dr. C. M. Swinney, of Stanford University, by means of universal stage. The angles noted in 2 sections ranged from 36° to 43°, indicating subcalcic augite (Benson, 1944, p. 111—118), and estimates of the optic— axial angle of other perxenes in' thin sections we examined indicate that this is the most common va— riety. Another variety of pyroxene, found only as stubby euhedral crystals in the finer grained lavas, shows the purplish tints of titan-augite. Still a third variety having a plumose or radial habit did not yield sufficiently good interference figures to permit a reli- able estimate of the optic-axial angle, which appears to be large, and this pyroxene remains undetermined. Basaltic glass is believed to have been abundant originally in much of the lava, although it now has everywhere been completely converted to a mixture that includes chlorite, probably nontronite, and other fine-grained minerals not determined. The material believed to have once been glass has a mottled appear— ance, and encloses a myriad of skeleton crystals and microlites 0f magnetite and plagioclase. The most abundant secondary minerals are chlorites, which commonly make up from 15 to 25 percent of the altered lavas. They replace the original basaltic glass and augite and form a minor part of the saus— suritic aggregates replacing plagioclase. Clinochlore is the predominating variety, and although several other varieties have been distinguished, they have not been sufficiently studied to warrant assigning specific names to them. Epidote and clinozoisite are wide- spread in fine—grained aggregates replacing plagio— clase, and in some rocks they occur as secondary eu— hedral crystals large enough to be readily identified. GROUP , 33 Both quartz and calcite locally replace much of the groundmass of the lavas, or fill vesicles in them, but these minerals are more common in veinlets. Where the age relation between the two minerals is clear, the calcite is invariably the later. PYROCLASTIC ROCKS Pyroclastic rocks, including breccias, tufl’s, and al— tered tachylitic tuffs, make up at least a fifth of the greenstone of the New Almaden district. They are commonly much altered and are recognized chiefly by their fragmental or bedded character coupled with their mafic composition. They are exposed principally in two bands. One band, which is narrow but per— sistent, extends eastward from a point about 1 mile north of the Calero Reservoir through the eastern part of the Santa Teresa Hills, and continues beyond the eastern border of the district. The other, which is broader but less continuous, extends from a point on the west edge of the district just south of Los Gatos and crosses the central part, passing through the Guadalupe and New Almaden mine properties, and continuing nearly to the southeast corner. The pyroclastic. rocks are divided into two groups for description. In one group the main original con— stituent was obviously tachylite, whereas in the other group it was not. _ NONTACHYLITIC TUFFS AND BRECCIAS The nontachylitic tuffs and breccias make up most of one thick body of greenstone that extends from the vicinity of the Almaden Reservoir, in Almaden Can— yon, northwestward through the central part of Mine Hill and along the north slope of Los Capitancillos Ridge to the vicinity of the Senator mine. As these rocks are readily weathered, they are exposed on the surface only in sharply incised canyons and roadcuts. In good exposures, however, particularly those in the workings of the New Almaden mine, the tuffs are seen to be generally interbedded with black shale, but in some places they are interbedded with graywacke, chert, and lavas of the Franciscan group. In the New Almaden mine a good exposure of alternating thin beds of tuff and shale can be seen in the upper part of the raise that extends from the 700 level to the Far West stope, and a well—exposed thick section of tufi' containing only a little shale is cut by the Day tunnel just south of the crosscut to the Santa Rita shaft (pl. 4). Megascoplc features The appearance of the nontachylite breccias and tufts is so varied as to defy simple description. They exhibit a wide range of colors and textures, and they have suffered varying degrees and kinds of alteration, 34 GEOLOGY AND QUICKSILVER DEPOSITS, which has affected their other physical properties, such as hardness and manner of fracturing. Their colors, which seem to reflect those of the rocks with which they are associated, range from dark to light green or brownish green in the chloritized tufl’s associated with massive greenstone to lighter buffs or reddish or pur— plish browns in the tuffs associated with sediments. Those tufts that have been hydrothermally altered and contain abundant clays, such as many exposed in the workings of the New Almaden mine, are light buff, light gray, or locally nearly white in color. W'here the textures are least obliterated the tuifs are marked by very fine banding, which is due in some places to different concentrations of light and dark material, and elsewhere to differing degrees of oxidation of their iron oxides or to differing sizes of their minute angu~ lar clastic grains. (See fig. 24.) In the coarser tuff breccias the textures are more obvious, with poorly to moderately well-sorted angular fragments, generally less than an inch in diameter, embedded in a matrix of fine-grained elastic material. Microscopic features As seen in thin section the original pyroclastic na- ture of most specimens is masked by alteration, but in some sections layering and vague elastic texture are discernible. The original minerals identified in a few of the fresher rocks are broken tablets of plagioclase, subhedral clinopyroxene, magnetite, and a little quartz; where the texture is coarse, fragments can be distin- guished (fig. 25). Material that has replaced mafic glass is present in some, but no curved shards, such as are commonly found in tufts, were recognized. The commonest alteration products include chlorite, quartz, calcite, celadonite, and clays. The earliest of these is the chlorite, which is derived mainly from the ferro- magnesian minerals and glass but also to some extent replaces the feldspars. Quartz, which has been the next mineral to form in some of the altered tuifs, oc- curs as granular aggregates replacing the rock and as veinlets. Later calcite, as replacements and veins, is common. Near the ore bodies of Mine Hill hydrother— mal solutions extensively altered the tufi's to clay min— erals before the introduction of most of the quartz and calcite. TACHYLITIC TUFFS AND BRECCIAS Tachylitic rocks, both tufts and breccias, composed almost entirely of altered basaltic glass, have not been reported from other areas underlain by rocks of the Franciscan group so far as we know, but in the New Almaden district they underlie extensive areas and make up a considerable part. of the greenstone 0f the district. They occur in two principal areas: one is a NEW ALMADEN DISTRICT, CALIFORNIA wide band that extends eastward from Los Gatos Creek to the Guadalupe mine, the other is a narrower band containing scattered bodies and extending along the north side of Longwall Canyon. The rocks are particularly well exposed both along Los Gatos Creek, south of Los Gates, and near the middle fork of Long- wall Canyon, in the southeastern part of the district. The fresher tachylitic rocks are readily recognized from their unusual textures, dark— to light—green color, and the serpentinelike appearance of their fragments. The fine—grained varieties, especially where weathered, closely resemble graywacke; but they can be distin- guished from it by their general lack of quartz, their slippery feel, and their relict textures. FIGURE 24.—Polished sections of drill cores showing two of the more common kinds of tutfaceous greenstone in the Franciscan group. Core on left shows cream-colored tuff bleached by hydrothermal so— lutions interlayered with black shale. Right core shows layers of red, purple, and green tuff containing a few larger fragments iden- tifiable as altered mafic glass. FRANCISCAN GROUP 35 FIGURE 25.—Photomicrograph of calcareous tuft from the Franciscan group. The fragments are various maflc rocks, most of which con- tain some altered tachylite; the matrix is calcite (C). Megascoplc features The appearance of outcrops of the tachylitic rocks depend upon the grain size of the rock and the degree of weathering. The finer grained rocks are generally little weathered, moderately well exposed, massive, and dark green; the more fissile varieties tend to part much like a schist, giving rise to indistinctly bedded outcrops. In hand specimen these tuifaceous rocks appear massive, breaking with a hackly fracture and possessing a distinctive but fine—grained texture that is easily recognized though hard to describe. This texture is characterized by irregular roughly elliptical masses of greenish altered glass so closely packed in a matrix of chlorite that they appear to have deformed one another. Many of the elliptical masses contain smaller flattened black or deep-green amygdules, and almost all show concentric bands of various shades of green. In the typical tuffs the fragments are about 1 mm in diameter, but these rocks grade to breccias having fragments, dominantly of vesicular basalt, as much as 6 inches in diameter. (See fig. 26.) In out- crop the fragments of vesicular lavas are generally weathered reddish brown, although where fresh they are green to black. The breccias show only crude bed- ding and sorting, and their having contained a glassy matrix could hardly be recognized except in the fresh— est specimens. Microscopic features Thin sections of the tachylitic tuffaceous rocks are striking in appearance because of the relict textures imparted by the original glass, as is shown in figure 27. The tachylitic glass has all been replaced by micro- crystalline aggregates of doubly refracting minerals, some of which are extremely fine grained and have not been identified with certainty. Some of these altera- tion products are chlorite, and some appear to be mi- crocrystalline feldspar; none appears to be palagonite. The few mineral grains found in the tuffs and appar- ently of primary origin are crystals of augite and very sodic albite. The deep-colored filling of the small vesicles that can be seen with a hand lens is probably nontronite, and other smaller vesicles are filled with albite (A112). A matrix of very fine grained, nearly isotropic chlorite is generally present. The coarser breccias contain, in addition to shards, tachylitic la- pilli and glassy and diabasic volcanic fragments that are similar in composition and microscopic texture to other greenstones of the Franciscan group. Some of the tachylitic rocks are extensively replaced by calcite, and others contain veins and replacement masses of albite associated with a little quartz. CHEMICAL COMPOSITION OF THE GREENSTONE Two new chemical analyses of greenstone of the Franciscan group, together with three older ones, are presented in table 8, which also includes some average analyses of diabase and spilite for comparison. With so few available analyses of these rocks, which are altered in such various ways, it is unsafe to draw any general conclusions regarding their composition; it does seem advisable, however, to point out a few of the problems awaiting further study. Mafic rocks associated with the eugeosynclinal ac- cumulations of graywacke, black siltstone, and chert, like the sedimentary assemblage of the Franciscan group, in other parts of the world are commonly rich in sodium, contain abundant albite, and are classed as spilites. Pillow lavas, such as are common in the Franciscan group, likewise, are in many places spilitic. Largely because of these facts the greenstones of the Franciscan group are regarded by many as spilites (Reed, 1983, p. 83). In the New Almaden district much of the greenstone contains albite, and the most calcic plagioclase identified in the greenstones was andesine. In other areas, however, the greenstones are reported to contain only labradorite (Weaver, 1949, p. 113—114), or original andesine-bytownite lo- cally altered to albite-oligoclase (Taliaferro, 1943b, p. 145). Thus, from a consideration of the feldspars alone, one might conclude that the greenstones of the Franciscan group were normal diabases locally en— 36 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA FIGURE 26.——Polished surface on breccia composed chiefly of fragments of altered tachylite. individual fragments is typical of the altered mafic glass in the district; dark horseshoe~shaped band is due to Weathering. riched with sodium, and that all of them in the New Almaden area were so enriched. The analysis given in column 1 is of the least altered diabasic greenstone found in the district. In thin section the rock can be seen to contain completely fresh augite, plagioclase that is somewhat clouded by alteration products but still fresh enough to give good measurements of ex- tinction angles, and a groundmass that was originally glass but is now completely chloritized. The plagio— clase, which appears to be primary, is andesine (An35). If olivine was ever present, it was in small amount and is now replaced by chlorite. A few thin veins of quartz and chlorite cut the rock, and a few small patches of calcite are. also present. The analysis con— tains such a moderate amount of sodium and such a large amount of calcium that one would expect the plagioclase to be more calcic if the normal process of crystallization had been followed. The analysis given in column 2 is of the altered tachylitic tufl' shown in figure 27. It consists chiefly of chlorite derived from the original glass, but it also Color zoning within contains a few crystals of plagioclase and augite. The plagioclase is albite (A112), and although it is com- pletely unaltered, some grains appear to have recrys- tallized. The effect of metamorphism that has taken place since the glass reacted with sea water cannot be determined, and the present composition could, of course, be very different from that of the chilled glass originally deposited. If there has been no postdepo— sitional loss of sodium, the present low value for sodium and the low sodium—calcium ratio are indeed surprising. These two analyses from the New Almaden district are inadequate to show anything other than the prob— lem involved in trying to learn from chemical analyses the original chemical character of the now—altered vol— canic rocks. These two rocks are not rich in sodium in spite of the soda-rich character of their plagioclase, and they are not chemically comparable to either nor- mal diabase 0r spilite. The three other analyses of greenstones of the Franciscan group given in columns 3—5, table 8,‘sh0w a higher sodium content, but in FRANCISCAN GROUP FIGURE 27.——Photomicrograph of section of altered tachylitic tuft of the Franciscan group. The shards, which were originally mafic glass, are completely altered to a fine-grained aggregate of chlorite. A few small crystals of albite and augite are visible in the shards. Vesicles are filled with nontronite and secondary albite. The matrix is fine-grained chlorite similar to that which has replaced the origi- nal tachylite. analyses 3 and 4 at least the sodium-calcium ratio is considerably less than in normal spilites. Many more analyses will be needed before one can safely state that the greenstones of the Franciscan group are soda- 37 rich spilites, or can draw conclusions regarding a process of spilitization that they may have undergone. In the New Almaden area we know that the plagio— clase, which appears in sections to be primary, con— tains an abnormal amount of soda for such mafic rocks; whether or not this reflects a magma rich in sodium, or even sodium—rich rocks, remains an open question. METAMORPHIC ROCKS Metamorphic rocks having a truly gneissic or schist- ose texture, and consisting largely of minerals formed by metamorphic processes, constitute an exceedingly small, but very interesting, part of the Franciscan group. These metamorphic rocks are so strikingly different and so much more highly metamorphosed than the typical sedimentary and igneous rocks of the Franciscan group, which show only incipient meta— morphism, that they must have been formed by some additional metamorphic processes. Two varieties, hornblende rocks and glaucophane rocks, occur in suf— ficiently large areas to be shown on the map of the district, plate 1. Other varieties described below are of such local occurrence that they are not represented on the map, but elsewhere in the California Coast Ranges they are Widespread, though not abundant. The distribution and character of the metamorphic rocks in the New Almaden district is believed to be representative of those occurring in the Franciscan TABLE 8.———Analyses of greenstones of the Franciscan group, with composite analyses of diabase and spilite for comparison Altered Hornblende Pseudo- Average Average Average Diabase tachylitic Fourchite Pseudo- diabase diabase spilite spilite tuft diabase 1 2 4 5 6 7 8 $102 ____________________________ 48 19 44. 90 46. 98 49. 08 51. 28 50. 48 51. 22 46. 01 A1203 ___________________________ 13 85 9. 94 17. 07 14. 68 15. 05 15. 34 13. 66 15. 21 Fe203 ___________________________ 2 82 3. 10 85 1. 95 2. 42 3 84 2. 84 1. 35 FeO ____________________________ 7 82 8. 82 7 02 9. 63 8. 01 7 78 9. 20 8. 69 MgO ___________________________ 7 28 14. 25 8. 29 6. 69 6. 07 5 79 4. 55 4. 18 08.0 ____________________________ 11 28 9. 38 12. 15 10. 09 7. 08 8 94 6. 89 8. 64 NaZO ___________________________ 2 67 1. 17 2. 54 4. 60 4. 43 3. O7 4. 93 4. 97 H28 ____________________________ 32 27 53 . 20 . 12 . 97 75 34 2 “ ___________________________ . 15 86 28 2. 97 H20+ ___________________________ 3. 26 4. 95 } 4 86 i 1 18 . 39 } 1' 89 88 2 48 T102 ____________________________ 1. 94 1. 84 __________ 1. 72 1 33 1. 45 3. 32 2 21 P205 ____________________________ 20 17 09 . 23 13 25 . 29 61 MnO ___________________________ . 17 15 __________ . 15 .25 .20 . 25 .33 002 ____________________________ . 24 O2 ________________________________________ . 94 4. 98 Total _____________________ 100. 19 99. 82 101. 38 100. 48 *99. 53 100. 00 100. 72 100. 00 " 0.1 NiO omitted. 3. Fourchite from Angel Island, Marin County, Calif. From Ransome, 1894, p. 231. 4. Hornblende pseudodiabase, from near Mount St. Helena, Calif. W. H. Melville, Nora—Description of sample and locality as follows: analyst. From Becker, 1888, p. 98. 1. Diabasic greenstone (N 15424), from crest of ridge 3.45 miles S. 700 E. of apex of 5. Pseudodiabase, from near Knoxville, Calif. W. H. Melville, analyst. From Mine Hill, New Almaden district, Santa Clara County, Calif. A. c. Vlisidis. Beekerv 188M)- 99- analyst. Plagioclase is Abefi. 6. Average of 99 drabases, calculatedhy Daly and others, 1942, p. 2. 2. Tachylitic tufiaceous greenstone (NA—325), from point of 1,100 ft alt, 3.44 miles 7- Average Splute according to Sundlusv 1932 D- 9- S. 55%" E. of apex of Mine Hill, New Almaden district, Santa Clara County, 8- Average Splhte accordlng to A- K- Wells, 1923' p. 69' Calif. A. C. Vlisidis, analyst. Phenocrysts are Abel! and vesicles locally filled with Abel. CALIFORNIA GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, 38 Join—Eu E6354 BSA 05 5 2.353% «in 9:8» anomwondurm 23 «0 wave 03933308 no Gaga—5.5mm. ”53.on aafi'fim samba—h .mvoamfl .. o {axe :...\. o J \ _ _ _ _ _ o m 932 32: we Smog—=8 meEEE «Ea 95.3 93..» :nom_u:§.m 2: .«o 338 955 :mum_o:u._,m of mo 932 35:00 5323:“; 2: «c 32: $50 uEEoESoE uctawnézfiaoosflw uEEoESoE wctmwaéuzwfifiom mmmmmmmmmmmmmmmmmmmmm VWNQN OV§ ‘ , \ ‘‘‘‘‘ \\ V bm FRANCISCAN GROUP 39 group throughout much of its wide extent, although in a few areas, such as one a few miles west of Healds— burg, in Sonoma County, the metamorphic rocks are more abundant and better developed. HORNBLENDE ROCKS Hornblende schists, and similar but less schistose rocks called amphibolites, are among the most com- mon metamorphic rocks in the district, but because they are not so striking as the beautiful blue glauco- phane rocks they are more easily overlooked. Most of them crop out on Mine Hill or in the vicinity of the Guadalupe mine, but small bodies also occur along the margins of serpentine masses extending southeast of Mine Hill and in the central part of the Santa Teresa Hills. (See pl. 1 and fig. 28.) Their field re- lations are commonly obscure because these rocks weather readily to form a characteristic red soil, through which the more resistant kinds protrude as low knobs. Where roadcuts or adits offer good ex- posures, one may observe that the distribution of the hornblende rocks is spotty, and in areas of knobby outcrops not all of the intervening rock is metamor- phic. Consequently, most of the larger areas mapped as hornblende rock actually contain a “matrix mate- rial” of greenstone tufi' as abundant as the hornblende rocks, although in some areas, such as the one about half a mile west of the Guadalupe mine, the amount of amphibolite is considerably greater than the amount of admixed greenstone. Megasooplc features Although typically the hornblende rock is entirely dark green or almost black, some that contains albite is flecked with light areas elongated parallel to the schistosity. The diameters of the constituent mineral grains are variable, but in most specimens the average grain size is a little more than 1 mm. Surfaces broken parallel to the poorly developed schistosity are rough and irregular, but owing to the amphibole cleavage planes, they exhibit a sparkle by Which the hornblende rocks may be distinguished from the coarser varieties of greenstone. Less typical, but more easily recog- nized, are foliated hornblende gneisses With alternat— ing bands rich in hornblende, albite, or epidote, and a few varieties contain porphyroblasts of pink garnet, which form an obvious flaser texture. Irregular veins of albite, quartz, epidote, or chlorite cut these rocks in many places. Microscopic. features The principal minerals found in thin sections of the hornblende rocks are hornblende, epidote, chlorite, al— bite, and garnet; minerals found in smaller quantity include common actinolite, blue-green actinolite, zois— ite, clinozoisite, zircon, apatite, sphene, leucoxene, pyrite, and magnetite. The hornblende forms stubby subhedral crystals that are generally oriented only to the extent of having their 0 axes nearly in the plane of schistosity. It is common hornblende and shows little variation among the specimens examined; most of it is moderately deep colored and pleochroic with X pale yellowish green, Y olive green, and Z faintly bluish green; birefringence ranges from 0.020 to 0.028; the extinction angle between the slow ray and the (2 axis is about 20°; 2V is large, and the optic sign is negative. The other amphiboles occur in minor amounts as needles in albite or as thin overgrowths on the hornblende. (See fig. 29.) The plagioclase in all specimens examined is albite; it occurs mainly as intricately sutured interlocking crystals in irregular veinlets or as interstitial patches. The minerals of the epidote group generally form subhedral crystals which tend to be concentrated in layers. Garnet forms fractured porphyroblasts several millimeters in diam- eter, and in many specimens it is partly converted to Hornblende (H), albite (A), epidote (E), and blue-green sodic amphibole (sa). Note that sodic amphibole occurs in crystal continuity with horn- blende along path of albite veinlets. FIGURE 29.—Photomicrograph of hornblende-albite gneiss. 40 GEOLOGY AND QUICKSILVER DEPOSITS, chlorite, as shown in figure 30. Chlorite also occurs in irregularly sheared veins and as abundant sharply bounded pseudomorphs, probably after epidote. Sphene is very abundant, amounting to more than 4 percent of some specimens. Quartz occurs only in veins. The relative proportions of the component minerals vary considerably, but hornblende is the most abun— dant. It makes up more than 95 percent of some specimens, and none of the other minerals makes up more than a third of any specimen examined. GLAUCOPHANE ROCKS Metamorphic rocks containing glaucophane are somewhat more abundant in the district than those containing nonsodic amphiboles. They are also more widespread, and in a general way the area in which they crop out surrounds the area containing the horn- blende rock. Most of the outcrops of the glaucophane rocks lie in the eastern half of the district in two zones that run parallel to the strike of the surround— ing rocks, but scattered outcrops occur outside these zones, as shown in figure 28. The northern zone trends westward through the east-central part of the Santa Teresa Hills where the glaucophane rocks crop out in an area of graywacke as prominent isolated knobby boulders generally not exceeding 15 feet in diameter. The southern zone trends northwestward from near the junction of Berrocal and Almaden Can— yons for a distance of more than 8 miles, and although glaucophane rocks within this zone are exposed only in patches, the lack of outcrops of other rocks neces- Garnet Note chlorite replacing FIGURE 30,—Photomicrograph of garnet-hornblende gneiss. (G), hornblende (H), and chlorite (C). borders of garnet porphyroblasts indicating some retrograde meta- morphism. NEW ALMADEN DISTRICT, CALIFORNIA sitated mapping parts of it as continuous bodies of glaucophane rock. In this zone, a little less than 2 miles from the eastern edge of the district, an espe— cially well-developed glaucophane-lawsonite schist forms a readily accessible isolated knob a few hun— dred feet in diameter adjacent to the road in Llagas Canyon. Megascoplc features The glaucophane-bearing rocks are characterized by a gray-blue color imparted to them by the soda am— phibole glaucophane, and by wetting the rock this color can be considerably emphasized. These rocks are mostly foliated, with alternate bands rich in glau- cophane or lawsonite, but in some specimens there is no orderly segregation of the minerals. In small part they are true schists, but much of the rock that is foliated shows little tendency to split parallel to the foliation. Some of the nonfoliated rock contains relict diabasic textures indicating derivation from mafic ig- neous rocks, but most of the glaucophane rocks are so completely recrystallized that they show no relict textures of any kind. (See fig. 31.) Locally, por- phyroblasts of albite are developed in the schists. Microscopic.reatures The minerals found in the thin sections examined include glaucophane, crossite, lawsonite, albite, mus— covite, chromian muscovite or fuchsite, chlorite, stilp— nomelane (Hutton, 1948, p. 1373—1374), garnet, quartz. ' calcite, rutile, sphene, leucoxene, and magnetite. The form and arrangement of the constituent mineral grains varies with the structure of the rock. Glauco- phane in the schists is commonly in long prismatic crystals without terminal faces, whereas in some of the more massive rocks it tends to form stubby sub- hedral crystals with frayed borders. In the schists most of the prismatic crystals lie with their 0 axes in the plane of the schisto‘sity and also show a linear orientation, but in the more massive rocks they have no obvious preferred orientation. The glaucophane prisms are rarely more than 2 mm long, but in some specimens their apparent length is increased by cross frac- tures filled with later sutured quartz. In the coarser grained nonfoliated rocks mottling and zoning shown by differences in the color of single glaucophane crys- tals is common, and overgrowths of small needles in optical continuity are not unusual. Crossite was found in a single section as an overgrowth on glaucophane. Lawsonite occurs in equant anhedral crystals as much as 2 mm in diameter, but more commonly forms ag- gregates of minute crystals or subhedral tablets less than 1 mm in diameter. Several sections contain inter— growths of lawsonite and quartz simulating graphic FRANCISCAN GROUP 41 FIGURE 31,—Glaucophane—quartz schist cut and polished at right angles to plane of schistosity. The light- colored augenlike areas are quartz, as are most of the transverse veinlets. The variations in composition and grain size suggest relict are similar veinlets of glassy albite. bedding. structure. Polysynthetic twinning occurs in the law— sonite, but it. is so uncommon as to be of little value in identifying the mineral. The stilpnomelane is all of the ferric variety, with intense pleochroism in yel- low to yellow brown. Fuchsite was tentatively iden— tified only by its brilliant green color and blue to green pleochroism, and, if present, it is rare. Garnet also is generally rare, but many small subhedral crys— tals of garnet form a conspicuous part of some quartz- itic schists derived from chert. Calcite occurs chiefly in irregular veins, but it is also found in ragged in- terstitial patches which are thought to have formed during the recrystallization of the rock. The relative proportions of the minerals in the glau- cophane rocks vary considerably. Glaucophane makes up almost all of some of the massive rock; it consti- tutes 25 to 60 percent of the common glaucophane— lawsonite schists but only a few percent of the com- paratively rare quartz~lawsonite schists. Lawsonite rarely amounts to more than 40 percent, but some is present in all thin sections of the glaucophane rocks. The micas rarely exceed 5 percent of the rock, and in some sections sphene is equally abundant. 686‘6710—63—4 In some specimens there The relationship between the original bedding and the foliation of the glaucophane rocks can be deter- mined with certainty in only a few outcrops, and in these the two are parallel. It seems likely that the planar orientation of the mineral grains and the folia- tion of the rock is controlled by the bedding, as has been suggested by T aliaferro (1943b, p. 168), for the foliated rocks are derived from sediments or tuifs, whereas the nonfoliated rocks seems to be derived from diabasic greenstone or massive grayWacke. The glau— cophane needles in some of the schists have a linear arrangement for which no explanation has been found. CHLORITE-LAW SONITE ROCK An unusual chlorite-lawsonite rock occurs near the mouth of Almaden Canyon, 10,100 feet N. 57° E. of the top of Mine Hill, as a single 15-foot rounded boulder surrounded by graywacke. This boulder is gray-green, and its surface is studded with lawsonite tablets as much as .15 mm in diameter. As seen along a transverse split in the boulder, the schistosity of the chlorite is well developed and nearly flat in the cen— tral part, but curves to follow the outline of the boulder near its periphery. Although the lawsonite 42 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA is scattered all through the boulder, it is coarsest and most abundant in a layer that follows the periphery. Where the tablets are abundant, the rock superficially (See fig. 32.) resembles a coarse. diabase. FIGURE 32.—Polished surface on chlorite-lawsonite rock showing de- ceptive pseudodiabasic texture resulting from random orientation of euhedral tablets of lawsonite. Thin sections of various parts of the boulder show it to be composed almost entirely of variable propor— tions of lawsonite and chlorite, with the latter gener— ally conspicuous. Most of the chlorite is of a nearly isotropic variety, but another variety showing anom- alous gray-green interference colors locally forms coarse decussate groups. The lawsonite occurs as sharply bounded unoriented tabular crystals with large (001) and narrow (110) faces, but it also ap— pears in thin sections as broken fragments lying along fractures that are healed with chlorite. Leucoxene, occurring as fine dust, is the only other mineral rec- ognized. omnrz-muscovrrr scnrs'rs Quartz—muscovite schists, both with and without sodic plagioclase, were found on Mine Hill, where they are closely associated with hornblende-bearing metamorphic rocks. Except for their schistosity and shiny muscovite-covered parting planes, they resemble the sheared feldspathic graywacke of the Franciscan group, from which they doubtless were derived. In thin section the quartz, which is the dominant mineral, is invariably seen to be sutured, to have undulatory extinction, and to contain many liquid inclusions. Albite occurs in one section as clastic grains with over- growths of the same mineral in optical continuity. The muscovite forms tabular crystals largely concentrated along the parting planes. Each thin section of quartz- mica schist that was examined also contains several percent of euhedral crystals of garnet, although in some sections they are largely replaced by chlorite. A little chlorite is also found with the muscovite along the parting planes, but it does not exceed 5 per— cent in any of the sections examined. QUARTZ-ACTINOLITE SCHIST Quartz—actinolite schist containing nearly equal amounts of fine-grained sutured quartz and minute needles of actinolite, together with a little chlorite and leucoxene, is interbedded with graywacke of the Franciscan group in the upper part of Longwall Can— yon. It is an unusual rock in the district, and prob— ably was derived from an impure tuff. ACTINOLITE-CHLORITE ROCKS Small nodular masses of semioriented closely packed actinolite prisms with interstitial chlorite are found along the margins of serpentine bodies in a few places in the Santa Teresa Hills; similar rocks occur in areas of hornblende schists along the road a little less than a mile south of the junction of Guadalupe and Los Capitancillos Canyons. Such nodular masses are com— mon elsewhere in the Franciscan metamorphic rocks of the California Coast Ranges, but they are exceedingly rare in the New Almaden district. Taliaferro (1943b, p. 181) suggests that they have been formed from serpentine by an endomorphic reaction. METACHERT Most of the chert in the district shows no obvious metamorphic change, but in places we found scattered boulders of varicolored “metachert” displaying un— usual minerals or textures. Some of the changes that have produced these metacherts probably took place at low temperature, during the transformation of a silica gel to a hard siliceous rock; other changes, resulting in the formation of minerals that indicate higher tem- peratures or pressures, are true metamorphic changes. Because of the inherent difficulty of separating the rocks showing diagenetic changes from those which are truly metamorphosed, all these unusual varieties of chert are herein considered together. “Orbicular jaspers” 6 and yellow cherts with a mi- crospherulitic texture are fairly common as isolated boulders. Most of these consist of massive chert, and they are believed to result directly from the radial growth of chalcedony (length-slow variety termed “lutecite”) during the crystallization of a silica gel. An unusual orbicular jasper rock, shown in figure 33, 6 “Orbicular jasper” is the term widely used for such rocks by ama- teur mineral collectors and lapidary enthusiasts. FRANCISCAN GROUP 43 FIGURE 33,—Polished surface on silicified tuffaceous greenstone showing typical “orbicular jasper” structure resulting from radial growth of chalcedony fibers in the replacing silica. indicates a two-stage process of formation in which a brecciated greenstone was partly replaced by ferrugi— nous silica and later it recrystallized to form an or- bicular jasper. Similar rocks occurring a few miles east of the district in the Morgan Hill quadrangle are valued by lapidary enthusiasts and have found their way into thousands of collections in California. With further recrystallization of the chert, the chalcedony is replaced by a mosaic of sutured quartz and the iron oxides generally are expelled to the periphery of the quartz grains. Where the rock is still more meta— morphosed, the iron forms scales of specular hematite as much as half a millimeter in diameter. Although the hematite is generally most abundant in areas of clear quartz or in vugs that also contain a little car— bonate, one thin section showed strings of minute hematite scales which appeared to have replaced cro- cidolite. Crocidolite-bearing chert, characterized by its tough- ness and patches of inky blue color, occurs in boulders along zones that also contain other varieties of meta— morphic rocks. Thin sections of these rocks show that the silica is present in the form of sutured quartz ap- preciably coarser grained than the silica of the un- metamorphosed chert. The crocidolite generally is concentrated in veinlets, but in some places it sur— rounds individual quartz grains and in others it has a random distribution (fig. 34). In some metachert, aegirine replaces the central part of clots of crocido- lite and forms the margins of quartz-aegirine veins. Such crocidolite-bearing cherts are fairly common in the California Coast Ranges, and have been studied in greater detail by Louderback and Sharwood (1908, p. 659) and by Ransome (1894, p. 211—219). ORIGIN OF METAMORPHIC ROCKS To emphasize the difficulty of explaining the origin of the metamorphic rocks in the Franciscan group, it is only necessary to point to the many and varied agencies to which the metamorphism has been ascribed. These include dynamometamorphism, regional meta- morphism, and additive contact metamorphism, and it has even been suggested that the metamorphic rocks are merely fragments of an older underlying forma- tion dragged up along faults. The most recent con— FIGURE 34.—Photomicrograph of metachert containing needles of cro— cidolite (dark) in a groundmass of quartz. Plane light. 44 GEOLOGY AND QUICKSILVER DEPOSITS, tribution to the problem is a noteworthy discussion by Taliaferro (1943b, p. 159—182), who concluded that the rocks owed their origin to selective and non— selective additive metamorphism along the contacts of basic and ultrabasic intrusives, being formed only in those places where the quantity of volatiles and solu— tions escaping from the magma during its emplace- ment was large. Primarily because of the areal dis- tribution of the metamorphic rocks in the New Al- maden district, we do not believe that the origin of all the various metamorphic rocks can be ascribed to contact metamorphism caused by the ultramafic in~ trusive rocks. The problem of the origin of these rocks is two- fold: first, what were the original rocks, and, sec— ond, under what conditions were those rocks reconsti— tuted? Geologists who have studied these rocks in recent years seem to agree that the more widespread and abundant types were formed by metamorphism of the various rocks in the Franciscan group, although it has been suggested that the chlorite—actinolite and chlorite-actinolite-talc rocks may have been derived from serpentine. In the New Almaden district the field relations and textures of many of the rocks indi— cate their derivation from rocks of the Franciscan group; and, it seems likely, though it cannot quite be proved, that they were all derived from these rocks. The hornblende schists and amphibolites can be traced directly to tufl'aceous greenstones; they occur in patches along a belt of tufl beds, which for reasons unknown are especially susceptible to metamorphism. Some of the glaucophane schists contain relict textures identical with those of the lavas of the Franciscan group, and others show the incipient development of glaucophane in normal-appearing graywacke. Still others, to judge from their mineral content and general appearance, seem to have been derived from cherts. The crocidolite-bearing rocks, also, are believed to be derived from normal cherts of the Franciscan group, for it was possible to collect speci- mens showing the alteration through small stages. The quartz—muscovite schists grade into feldspathic graywacke, from which they were probably derived. The actinolite-chlorite rocks, found only as polished nodules along the borders of a few serpentine bodies, may be fragments dragged up by the serpentine, or they may be altered mafic rocks; but it seems un- likely that they were derived from serpentine, which nowhere shows a marginal metamorphosed zone con- taining any actinolite or chlorite. Evidence as to the metamorphic process might be gained from the areal distribution of the rocks and from their texture, mineralogy, and chemical compo- NEW ALMADEN DISTRICT, CALIFORNIA sition. Of these, only the distribution and mineral- ogy are well known for the rocks of the New Almaden district, but a general idea of the bulk chemical com- position can be obtained from the mineralogic com- position. In regard to texture the preponderance of oriented platy and linear minerals in the metamorphic rocks is significant. All the larger masses of hornblende-bearing meta- morphic rocks are restricted to the central part of a favorable tuff bed on Mine Hill and to another tufi' bed, or possibly a faulted segment of the same bed, near the Guadalupe mine. Most of the masses border ser— pentine sills; but in a few places along the same fa— vorable bed small masses of hornblende rocks are found at a considerable distance from any known serpentine mass. Conversely, serpentine borders the bed in a few places without the development of any hornblende-bearing metamorphic rocks. Elsewhere in the district there are many other serpentine bodies that border tufi'aceous greenstones, yet they have pro- duced no apparent metamorphism. A particularly good example of the latter type can be seen along the Day tunnel of the New Almaden mine south of the New Ardilla stope (pl. 4) where a bed of unmeta- morphosed greenstone is included between two ser— pentine sills, each of which is several hundred feet thick. The textures of the hornblende—bearing metamorphic rocks shed little light on the problem of their origin. They are generally somewhat schistose, but they show little linear orientation. Foliation is developed in some, but most are apparently nonfoliate. Where field relations are clear, the foliation appears to be parallel to the bedding, and probably it was caused by differences in the chemical composition of indi— vidual beds. The hornblende-bearing rocks probably resulted from the recrystallization of the tufl’ beds under rela- tively low pressures, owing chiefly to their depth of burial. Without chemical analyses it is not possible to know whether the metamorphic rocks difl'er chemi- cally from the tufl's from which they were derived, and although they all have mineral assemblages that could conceivably result from simple recrystallization, volatile fluid escaping from nearby serpentine sills may have helped to cause the recrystallization. The general lack of metamorphic efl'ects along these sills, however, casts considerable doubt upon the ability of the serpentine sills to effect metamorphism; yet the general association of hornblende rocks with serpen— tine suggests that in certain places, probably where the composition or the water content of a tuff bed was particularly favorable, the serpentine may have FRANCISCAN GROUP 45 been effective in “triggering” the metamorphic re— action. The origin of the glaucophane-bearing metamorphic rocks is even more obscure, partly because these ap— pear to be derived from many of the rocks of the Franciscan group rather than from a single rock type or favorable bed and partly because of their erratic distribution. The largest masses in the district are associated with both greenstones and sedimentary rocks of the Franciscan group; some border serpen- tine intrusions but many others do not. More than 50 bodies, many of them less than 20 feet long, are isolated outcrops surrounded by unaltered graywacke or siltstone. The majority of these small isolated masses are probably in shear zones, but such an en- vironment cannot be demonstrated for all of them. Nevertheless, the most striking feature of the glau- cophane-bearing rocks is their close areal relationship to shear zones, faults, or structural “knots.” In texture the glaucoph-ane—bearing rocks are in general more distinctly foliated than the hornblende- bearing rocks, and although some show no obvious mineral orientation, many others show both planar and linear parallelism. The foliation is parallel to the original bedding, where the relation can be de- termined, but it has been seen in only a few outcrops. The textures of the rocks are thus of little help in as— certaining the metamorphic process by which they were formed. No chemical analyses of the glaucophane rocks of the New Almaden district are available, but several analyses of similar rocks from elsewhere in the Coast Ranges are given in an excellent paper by Smith (1906, p. 183—242). He concluded, on the basis of these analyses, that no new materials, except possibly water, had been added in the formation of the glau- cophane rocks. Taliaferro (1943b, p. 178—179), on the other hand, cites analyses to prove the introduc- tion of material during the metamorphism of a chert to a quartz-glaucophane rock. He apparently does not take into account, however, the shaly parting lay- ers, which are excluded from the analyses of chert but were doubtless incorporated into the glaucophane schist. It does seem likely, however, that in the for- mation of some glaucophane—bearing rocks of the Cali— fornia Coast Ranges small amounts of soda and other materials have been introduced, and that other con- stituents have been lost. On the whole, however, the rocks are not appreciably more sodic than the rocks of the Franciscan group from which most of them were derived; they could easily have been formed by a reconstitution of the elements in the original rock accompanied by a little metamorphic diffusion. Despite the schistose appearance and relatively com— plete recrystallization of the glaucophane—bearing rocks, the presence of stilpnomelane, epidote, albite, and chlorite indicates that they are probably the equiv— alent of the more usual assemblages of the greenschist facies and represent relatively low—grade metamor- phism (Turner, F. J ., 1948, p. 99—100). They appear to have formed in the New Almaden district along tectonic zones. This distribution may have resulted from the development of high-pressure areas during deformation, the generation of heat by shearing, the availability of solutions, or by several of these com- bined. In places the glaucophane rocks are adjacent to serpentine masses, but as these [serpentine bodies are of the type thought to have been squeezed into their present position as serpentine (p. 54—57), they were probably incapable of any pronounced thermal or hydrothermal metamorphism. In many more places in the district the glaucophane—bearing rocks do not lie adjacent to serpentine, and it would appear un- likely that they have any direct relationship to these rather impotent intrusives. The origin of the less common crocidolitic meta— cherts is not known. In areal distribution these rocks are comparable to the glaucophane-bearing rocks, and it seems likely that they have a common origin. The presence of aegirine, however, suggests a moderately high temperature of formation. AGE OF THE FRANCISCAN GROUP Fossils diagnostic of the age of the Franciscan group in the New Almaden area have been found only in the limestone, which on the basis of Foraminifera has been dated as early Late Cretaceous (Cenomanian), see page 24-25. The next younger rocks in the district are also Late Cretaceous in age, but we know neither the part of the Late Cretaceous they represent nor their stratigraphic relation to the Franciscan, because they are in fault contact with this group. The time span represented by the Franciscan grOup elsewhere in the State is imperfectly known for many reasons. Few fossils have been found. Correlations over a wide area are based on similarity of lithology, and it is possible that rocks assigned to this group in different areas were deposited at different times. The base of the formation is nowhere exposed; the upper contact is controversial. Taliaferro (1943b, p. 190— 202, 208—212) cites several localities where beds as— signed to the Franciscan group grade upward into fossiliferous Knoxville (Upper Jurassic) shales, with which cherts and greenstones are interbedded. On the basis of these relations, and previous fossil dis- coveries which he carefully summarizes, Taliaferro 46 GEOLOGY AND QUICKSILVER DEPOSITS, assigned the group to the Late Jurassic (Titho— nian). Subsequent to the publication of this sum- mary, an ammonite, Douvillez'cems sp., of unquestioned Early Cretaceous (Albian) age, was discovered in San Francisco, which is generally regarded as the type locality of the Franciscan group. This discovery and its significance is described in an article by Schlocker, Bonilla, and Imlay (1954, p. 2372—2381). The information now available indicates that some of the rocks of the Franciscan group were deposited in Late Cretaceous (Cenomanian) time, and some were deposited in Early Cretaceous (Albian) time. Other beds, correlated with this group chiefly on the basis of their content of greenstone and chert, are said to grade upward into, or be overlain by, Knox— ville shales, and therefore they are at least as old as Late Jurassic. It thus appears that we are dealing with either a thick formational unit ranging in age from Late Jurassic to Late Cretaceous, or that we are dealing with two similar sequences, one of Tithonian or older age and the other Albian and Cenomanian age, and have failed to find criteria by which they can be distinguished. Until this problem can be solved, it seems best to designate the Franciscan rocks as Late Jurassic and Cretaceous. THICKNESS OF THE FRANCISCAN GROUP The thickness of the Franciscan group cannot be determined accurately in the district because of its structural complexity, and because neither its base nor top is present. The part of the group that is present is believed from a study of cross sections to be at least 10,500 feet thick, and it may be consider— ably thicker. The minimum thickness of the part of the Fran— ciscan group present in the area can be approxi— mated by utilizing the partial sections exposed in the various fault blocks. The older part of the group, consisting of feldspathic graywacke, buff siltstone, and overlying greenstone and limestone, forms a block in which no major faults were found in the area between Blossom Hill and the Limekiln fault. (See section A—A’, pl. 1.) The sedimentary rocks are about 6,000 feet thick, and the overlying greenstone is about 1,500 feet thick. On Los Capitancillos Ridge northeast of the Enriquita mine another unfaulted section more than 4,000 feet thick contains 3 sequences of green- stone separated by sedimentary rocks. A positive cor— relation between these 2 partial sections cannot be made, but a minimum thickness for the rocks exposed in the 2 blocks can be obtained by assuming that the greenstone at the top of the older block is the equiva‘ lent of the lowest greenstone exposed in the Los Capi— tancillos block. Using this assumption, we find that NEW ALMADEN DISTRICT, CALIFORNIA there are about 3,000 feet of additional section pres- ent on Los Capitancillos Ridge, making a minimum thickness of 10,500 feet for the composite of the 2 blocks. Some of the deep workings 'in the New Alv maden mine, however, passed through thin greenstone layers lying deeper in the section than the greenstone exposed on the surface, and if this is correlated with the greenstone of the older block, the composite sec— tion would be 2,000 feet thicker. The relation of the rocks of the other blocks south of the Shannon fault to these two sections are not well known; but the blocks contain limestone, which can probably be used as a key bed, and this suggests that they contain equivalent sections. No limestone occurs in the Franciscan group north of the Shannon fault zone, and no correlation across this fault is possible in the New Almaden area. ORIGIN OF THE FRANCISCAN GROUP The assemblage of rocks in the Franciscan group is typical of eugeosynclinal accumulations found in oro- genic belts throughout the world. The geosyncline in California was at least 550 miles long and more than 150 miles wide. If the assemblage of more meta- morphosed but otherwise lithologically similar rocks found on the Palos Verdes Hills (Woodring and others, 1946, p. 12—13), on Santa Catalina Island, on the islands 011' Lower California, and in the Western Cape region of Lower California (Beal, 1948, p. 36— 37) are included in the group, the length of the geo— syncline was more than 1,000 miles. The feldspathic character and angularity of the grains in the gray- wacke indicate that this rock was derived from a rugged, probably actively rising, landmass not far distant from the site of deposition. Taliaferro (1943b, p. 187—188) has concluded from the composition of the pebbles in the conglomerates, and from a general coarsening of the sediments westward, that the prin- cipal landmass lay to the west of the depositional - trough. This conclusion may be justified, but the lack of fragments showing myrmekitic or graphic inter— growths in the sedimentary rocks of the Franciscan group, and the scarcity of orthoclase suggest that the Santa Lucia granodiorite was not an important source, either because it was not exposed by erosion or because it had not yet been intruded. That the group accumulated rapidly is indicated by the character of the sedimentary rocks, the scarcity of fossils, and the great thickness of the volcanic rocks. Despite this rapid accumulation, however, some pro- gressive changes are shown by the differences between the older and younger parts of the group. The older part contains little volcanic material, chert, or con- glomerate, and in general is better sorted and more SERPENTINE 47 feldspathic; the younger part contains a larger pro— portion of lithic graywacke, considerable greenstone and chert, and some limestone and conglomerate. These differences are believed to reflect an increase in orogenic and igneous activity during the deposition of the group. Probably the older sediments were de- posited in deeper water and farther from their source. The oolitic character of some of the limestone in the younger part, together with the abundance of shale flakes and the presence of conglomerate, indicates that some of the geosyncline was shallow, at least at times, during the accumulation of the younger part of the Franciscan group. SERPENTINE Serpentine occupies less than 10 percent of the New Almaden district; however, because some of it is hy— drothermally altered to silica-carbonate rock—which is the host for all the rich quicksilver ore bodies—it merits special attention. Of particular importance is the consideration of the character of the ultramafic material when it was intruded, for this influences the shape and nature of the walls of the intrusive masses along which the ore bodies were locallized. The un— usual opportunity to study the serpentine bodies and their contacts afforded by the perfect exposures in the mine workings has led us to conclude that all the masses were intruded as serpentine, rather than formed in place by hydration of an ultramafic igneous rock. Distribution The serpentine of the district is exposed along linear zones that trend eastward or southeastward, nearly paralleling the structures of the rocks of the Fran— ciscan group. Disregarding a small exposure in the extreme northeast corner of the district, the northern- most zone lies along the northern slope of the Santa Teresa Hills and is most prominent in the large mass of serpentine forming Tulare Hill, on the east edge of the district. The next zone to the south extends eastward from the vicinity of the Guadalupe and Senator mines to the east boundary of the district. It cannot be traced continuously, for about midway in its course it is covered by the alluvium in the broad valley of Alamitos Creek; the eastern end, however, is especially thick and well exposed in the Santa Teresa Hills. The next zone to the south branches from the last in the vicinity of the Senator mine and extends southeast to the Enriquita mine, east through the New Almaden mine area, and southeast from Mine Hill across Fern Peak. Beyond Fern Peak it swings eastward along the north side of Longwall Canyon, becoming more broken and irregular close to Llagas Canyon. Part of this zone seemingly crosses Llagas Canyon and extends southeast at least as far as Uvas Canyon, east of the mapped area; another part swings northward around an arc and then continues north— west to the prominent serpentine hill at the mouth of Almaden Canyon. The next main zone of serpentine bodies to the south extends from Los Gatos Creek, on the west edge of the district, to the upper part of Llagas Canyon. It is less continuous than the other zones, for near El Sombroso there are gaps of nearly a mile in which no serpentine was found, although a more thorough search in this heavily wooded area might reveal additional small bodies. The southern- most zone of serpentine extends northwestward from the upper part of one of the tributaries of Almaden Canyon and crosses the main divide of the Sierra Azul just south of Mount Umunhum. The serpentine masses vary widely in size and shape. The largest mass in the district extends along the Santa Teresa Hills, where it has an exposed length of 41/2 miles and an average width of about half a mile. Several others exceed 1 mile in length, whereas the smaller bodies range downward in size to isolated pods only a few feet long. The smallest masses can best be observed in the mine workings, where one may see sill-like apophyses and pods, in many places less than 1 foot thick, bordering the larger sills. The out— crop patterns of the serpentine masses vary according to their geologic structure. Some of the masses are sill-like bodies conformable with the enclosing rocks of the Franciscan group; others occupy fault zones and are unconformable. The conformable bodies are tabu— lar and generally tilted with the enclosing rocks, giv- ing rise to irregular outcrop patterns, whereas the bodies lying along faults are generally vertical or very steep and give rise to more linear patterns. Ex- amples of the conformable bodies are seen on Mine Hill and in the area to the east and southeast, whereas good examples of the fault-controlled bodies are found along the more southerly zone that extends eastward from Los Gatos Creek and lies north and east of El Sombroso. Megascopic features Two varieties of serpentine having different struc- ture and texture are common in the district. They grade into each other, but since most exposures can readily be classed as of one or the other variety, each merits a separate description. One of them, here termed “sheared serpentine,” is intensely sheared, fo- liate, and shiny; it ranges in color from white through light green to a moderately deep green in fresh ex- posures. It forms very few extensive bodies, but is a marginal phase of many of the larger masses. The other, termed “blocky serpentine,” contains massive 48 GEOLOGY AND QUICKSILVER DEPOSITS, rounded blocks of unsheared serpentine in a completely sheared matrix (fig. 35). The proportion of matrix to blocks, as seen in artificial cuts where exposures are perfect, varies within wide limits; the sheared matrix may form only thin separations between massive blocks that nearly touch one another, or it may be relatively abundant, containing only here and there a small rounded pod of unsheared serpentine. All gradations in the relative amounts of sheared matrix and un- sheared blocks may be found. The blocks, where fresh, are dark green, nearly black, in color and have a pseudoporphyritic texture wherein ragged crystals of bastite a quarter of an inch long, derived from py— roxene, are scattered through a matrix of deep—green granular serpentine derived from olivine. Many blocks also contain magnetite, either disseminated as individual crystals a little less than 1 mm across, or concentrated in veinlets. The field appearance of the serpentine masses de— pends on how much they are sheared and on whether they crop out in the low foothills or in the higher mountains. The large masses are generally blocky and can be distinguished as serpentine from a distance. In NEW ALMADEN DISTRICT, CALIFORNIA the lower foothills these give rise to distinctive green- ish or drab-colored slopes studded with groups and trains of closely spaced boulders as much as 20 feet in diameter (.fig. 36). From a distance many of these slopes display a crude banding, caused by alternation of bands of large and small boulders, or by a succes~ sion of more sheared and less sheared zones. This banding generally is nearly parallel to the contacts of the serpentine mass, but in parts of the large and ex- ceptionally well-exposed mass in the Santa Teresa Hills it can be seen to diverge from the contacts at angles as great as 30°. The margins of the masses in the foothills are sharply marked in many places by slight topographic bulges in the peripheral serpentine and a shallow, but perceptible, flattening of the lepes just below (fig. 37). This topographic expression tends to be obscured, however, by small landslides, which are common along these contacts and give rise to small seeps or springs. In some areas in which the ser- pentine is well exposed, linear grass-covered patches devoid of boulders are conspicuous; these patches are generally underlain by septa. of sedimentary or vol- FIGURE 35,—Blocky serpentine exposed in fresh roadcut. rounded corners and edges. 'T‘hey invariably show und Between the blocks the serpentine is intensely sheared The blocks are irregular in shape, but have sllckensided surfaces and istorted relict textures inherited from the original peridotite or dunite. and does not exhibit any relict textures. SERPENTINE 49 FIGURE 36.—Typica1 boulder strewn surface developed on area under- lain by blocky serpentine in the lower parts of the New Almaden district. This kind of surface results from the erosion of the sheared serpentine matrix leaving the unsheared blocks as residual boulders. canic rocks of the Franciscan group. In the lower parts of the district the blocky masses of serpentine support, in addition to sparse grass, a growth of scat— tered bushes and a few struggling oak or bay trees, Whereas similar bodies in areas of greater altitude generally support a dense, locally impenetrable, growth FIGURE 37.—View of margin of a serpentine mass in the low foothills of the New Almaden district. The short but pronounced steepen- ing of slope at the contact is typical. Note in lower right the water trough that utilizes the small flow from a contact spring; such springs and seeps are fairly common along the lower margins of serpentine masses. of manzanita bushes. By the unusual gray-green color of the manzanita leaves it is possible in many places to recognize serpentine masses from a distance even Where none of the boulders project above the bushes, and in a few places it is possible to delineate the masses fairly accurately by outlining the man- zanita thickets. The boulders that weather from the blocky serpen— tine range in diameter from about 6 inches to more than 20 feet, but the majority are between 1 and 4 feet in length. The largest boulders are mostly sub— rectangular, the smaller ones nearly spherical. In fresh exposures the boulders are coated with the sheared matrix material and are shiny and smooth, but in most places they are weathered and have rough dark-colored lichen-covered surfaces. The roughness of the surface isdue to differential weathering. The bastitic pseudomorphs after pyroxenes are more re— sistant than the serpentine derived from olivine and protrude from the surface. Most surfaces are parti- tioned by a rectangular network of narrow veins of antigorite which are perpendicular to the surface of the boulder and penetrate the rock for only about 1 inch; such veins are easily weathered, producing a surface that appears intricately cracked. Larger veins of chrysotile asbestos cut completely through the rock; these veins are more resistant and stand out on weath— ering, as do also some thick veins of porcelaneous ser— pentine. About 1 inch below the surface of the boul- ders, where the antigorite veinlets pinch out, there is commonly a zone of fractures or veins, and on intense weathering these tend to open, causing some spalling of the crust. Some boulders show parallel banding due to the concentration of bastite in layers 1/2 to 3 inches thick, and where the rock is weathered, these layers stand in relief. Sheared serpentine forms a large part of many of the smaller serpentine bodies, particularly the more linear ones, and some of the smaller bodies consist entirely of the sheared variety. Such masses, except where they are silicified, afford poor outcrops or are not exposed at all except in artificial cuts. Generally, however, they are covered with a distinctive soil, which is black and very sticky when wet, but when dry, is dark-gray, hard, and traversed by wide polyg— onal joint cracks. Fortunately, this soil contains mi- nute shreds of- sheared serpentine, for without these shreds it could easily be confused with the soil rest— ing on the black alta, which is derived from the sedi— mentary rocks of the Franciscan group along the margins of the serpentine. Landslides are common in the larger masses of sheared serpentine, and along the upper scarps of these landslides are found the best 50 GEOLOGY AND QUICKSILVER DEPOSITS, natural exposures of the serpentine. In the higher country the sheared serpentine, like the blocky vari— ety, supports a growth of manzanita, and in some places at lower altitudes tarweed is particularly abun- dant on it. During a few days in the early summer it is particularly easy to outline the masses of sheared serpentine on a soil covered slope, for the grass in the soil derived from serpentine remains green a little longer than it does in the soils derived from other rocks. By making use of these criteria, generally one may locate and crudely outline masses of sheared ser- pentine rather quickly, but determining their exact outlines is difficult and cannot everywhere be done with certainty. Where the sheared serpentine is silicified, whether in larger masses or in septa between boulders of un- sheared serpentine, it forms on weathering jagged, spired, and crudely tabular angular outcrops, which show at a glance the prevalent direction of shearing. In a few outcrops of silicified sheared serpentine snow-white pufi' balls of magnesite, generally a little less than 1 inch in diameter, are conspicuous; these balls of magnesite, however, being resistant to weath— ering, commonly fall out, leaving a pock-marked sur— face. Massive serpentine derived from dunite containing NEW ALMADEN DISTRICT, CALIFORNIA no pyroxenes is rare in the area, but it was found in several small scattered exposures. It can be distin- guished only where it is unsheared, but there it is readily recognized in both fresh and weathered expo- sures by its granular texture, lack of bastitic pseudo— morphs, and typical irregular veining by various ser- pentine minerals (fig. 38). Where it is slightly weathered, it assumes a light-gray, nearly white color and is not unlike a fine—grained sandstone; examina— tion with a hand lens, however, invariably reveals the presence of minute black crystals of picotite, easily recognized by their submetallic subadamantine luster. In most places, however, the serpentine derived from dunite is more intensely weathered, and part of the rock has been hardened through a Width of about a quarter of an inch along many irregular veinlets and fractures; the hardened zones are deep green and re— sistant, and the intervening serpentine is whitened and leached out, so that the surface is full of highly irreg- ular deep cavities from ‘14 to 1 inch in diameter. Microscopic features As the serpentine minerals are variable in optical properties and variously grouped under different names, it,is appropriate to discuss the nomenclature used in this report before describing the appearance of the serpentine in thin section. A survey of the 1 man FIGURI 38.—Polished surface on fresh serpentine derived from dunite. ‘El‘his porcelaneo‘us material is apparently What was originally referred to by that replaces older serpentine minerals. Lodochnikov as “serpophite.” (See p. 50.) The latest veins are a waxy porcelaneous serpentine SERPENTINE ‘ 51 extensive literature on serpentine and serpentine min- erals reveals that dozens of varietal names have been used, but many of these have been discarded. Some now used as mineral names were originally intended merely as varietal rock names. Other names conno— tated textures, some of which were inherited from the replaced minerals, Whereas others resulted from the crystallization of the serpentine mineral itself. Recent X-ray and thermal studies made of serpentine are still not reconciled, and there remains considerable uncertainty, and some disagreement, as to just how many serpentine minerals there are, and just what are the limitations to their variable physical and optical properties. Because it is not possible to differentiate precisely the various serpentine minerals, they will be divided in this report, according to the simplest optical tests, as follows: Fibers ________________ Length-slow- _ _ Chrysotile Length-fast_ _ _ _ Fibrous antigorite Plates ________________ Length-slow- _ _ Antigorite Length-fast- _ _ _ Not found Amorphous, or nearly so- ______________ Serpophite This grouping follows for the most part accepted usage, except that fibrous antigorite, the most com— mon of the serpentine minerals in the district, has often been misidentified as normal antigorite because of its low birefringence and the difliculty of ascertain— ing whether it is fibrous or platy. In addition, we include under the term “chrysotile” material with lower birefringence than is customary. “Serpophite” is used for designating the structureless nearly iso- tropic mineral that generally occurs as a pseudomorph after olivine, following usage that is generally ac- cepted by English-speaking geologists even though it differs somewhat. from the original intent of Lodoch— nikov (1936), who proposed the term. The senior au- thor is indebted to Mr. V. P. Sokolofl' for translating lengthy sections of this ponderous volume for him. Lodochnikov’s best definition of serpophite seems to be the one given on pages 34 and 35, but it leaves much to be desired. The term is loosely used for “macroscopi- cally dense, structureless, varieties of serpentine, having waxy or enamellike luster, and light to dark color.” According to Lodochnikov (p. 34), serpophite occurs chiefly as veinlets, and apparently he intended the term to be used in megascopic, rather than microscopic, de- scriptions. “Bastite” is used as a varietal term for any serpentine mineral that forms pseudomorphs after either an orthorhombic or monoclinic pyroxene with coincidence of c axes. The study of thin sections of serpentine derived from dunite, which consists almost entirely of olivine, provides the logical starting point for the study of the process of serpentinization because of the small num- ber of minerals involved. These sections will therefore be described first, even though in the New Almaden district serpentine derived from dunite is much less common than serpentine derived from rocks contain- ing pyroxenes as well as olivine. The least—serpentinized dunite found in the area con- tains less than 30 percent of residual olivine, as shown FIGURE 39.—Photomicrographs of dunite partly replaced by serpentine minerals, showing development of typical meshwork. Olivine (ol), serpentine minerals (S), and magnetite (M). Upper, Plane light. Lower, Section oriented so that olivine residuals are near the ex- tinction position. Note that all of these show the same illumina- tion, indicating that they are all parts of a single crystal and un- rotated. Mineral bordering olivine residuals is fibrous antigorite; narrow veinlets are chrysotlle. Crossed nicols. 52 GEOLOGY AND QUICKSILVER DEPOSITS, in figure 39; however, because many small unreplaced granules of olivine are evenly distributed throughout the rock, it is possible to deduce with a fair degree of certainty the character of the original dunite. Most of the olivine grains were anhedral, although some had a few crystal faces; the largest grains were more than 3 mm long, but the average length was about 1 mm. In shape they ranged from equant grains to slightly embayed prisms with a length four times their width. The resultant texture was xenomorphic gran- ular, like that normally found in fresh dunites. The only original mineral other than olivine is chromite or picotite, which occurs as subhedral grains about 0.1 mm in diameter. The first step in the process of serpentinization is fracturing, which is closely followed by replacement of the olivine by fibrous antigorite and the filling of cracks with chrysotile. The fractures are variously controlled; a few are parallel to cleavage within single olivine grains, others radiate from chromite grains, and still others extend continuously through several differently oriented olivine grains. The fractures all belong to a single system, for they terminate and bend at their junctions. The widest fractures are also the longest and straightest, and in section these form a polygonal pattern, each polygon of which may be fur— ther divided into small polygons by smaller and less continuous fractures. The resultant fracture pattern is the basis for the familiar mesh structure developed from olivine. The olivine bordering these fractures is replaced by a wave of fibrous antigorite growing from the cracks toward the olivine, and simultaneously the fractures are filled with a chrysotile of low bire- fringence, which generally encloses magnetite dust. When the chrysotile has filled the narrow fractures it replaces the fibrous antigorite ,previously formed along the walls. The development of these two serpentine minerals probably results from a single reaction, so thatrthe two are nearly contemporaneous. Nowhere was either mineral found without its companion, and the two occur in a fairly constant ratio of about three parts of fibrous antigorite to one of chrysotile. The reaction apparently stops when the fibers of antigorite attain a length of about 0.03 mm, unless, as happens in many places, additional fractures open parallel to the origi- nal one are filled with more chrysotile. If the process of serpentinization is stopped at this point, the partly serpentinized dunite will contain many residual sub— angular fragments of olivine, like “eyes,” in the mesh of serpentine minerals. The olivine fragments are fresh, and their contacts with the fibrous antigorite are sharp. The several remnants from each olivine NEW ALMADEN DISTRICT, CALIFORNIA crystal extinguish exactly together, which indicates that they have not rotated during serpentinization and suggests that the process has been strictly one of re— placement involving no significant expansion. With increasing serpentinization the remaining olivine is replaced by serpophite, which has a slightly higher index of refraction and much lower birefringence than the fibrous antigorite; and invariably the original con- tact between antigorite and olivine is preserved as the contact between antigorite and serpophite. At the same time the magnetite dust is collected into larger isolated crystals or strings of crystals along the wider veinlets. The borders of chromite grains become fuzzy, and in reflected light the grains are seen to be coated with magnetite. At the same stage in the alteration process magnetite also replaces the chromite along fractures. The serpentine of the district is mostly derived from a harzburgite, a rock containing olivine and orthopyroxene, but some of it is derived from lherzo- lite, which contains these minerals and also clino- pyroxene. The distribution of the harzburgite and lherzolite is unknown, for the two pyroxenes cannot readily be distinguished in the field; both rocks, how- ever, are known to occur in a single body of serpen— tine. Although the pyroxenes in most of the serpen- tine have been replaced by bastitic minerals, the amount of pyroxene originally present in an unsheared serpentine is easily estimated from the proportion of bastite pseudomorphs. In most of the serpentine this proportion is between 10 and 25 percent, but the dunite previously mentioned contains no pyroxene, and certain bands and segregations in the ultramafic rocks contain more than 85 percent of pyroxene replaced by bastite. No systematic variation in degree of serpentiniza— tion, either from the margins of a mass inward or from the surface downward, was found. The process of serpentinization of the typical peridotites, includ- ing both harzburgites and lherzolites, can be fairly well traced by studying various thin sections of rock collected throughout the district, if they are properly arranged in increasing order of serpentinization, as was done in this study; it should be emphasized, how- ever, that the result is a synthesis rather than a re— port of the changes effected in a single body of peridotite. Of the least serpentinized peridotite studied in thin section, a little less than half consisted of primary minerals, including olivine, enstatite, and a clinopyrox- ene that is probably augite. (See fig. 40.) The olivine originally occurred as rounded anhedral grains, where- as the pyroxenes generally show some crystal faces SERPENTINE , 53 and irregular or embayed outlines where they adjoin the rounded grains of olivine. Some of the enstatite poikilitically encloses small olivine crystals, but this is uncommon. The olivine grains are of about the same size as those in the dunite, and the pyroxene crystals are commonly from 1 mm to several milli- meters long. The alteration of the olivine in the fresh- est rocks has advanced only to the stage at which a few of the cores are replaced by serpophite. The py- roxenes are very slightly replaced along cleavage traces by bastitic chrysotile of low birefringence, forming jagged edges where these intersect crystal boundaries. The only accessory mineral found was a pale-yellow picotite, which is largely anhedral; this mineral tends to enclose olivine and to be enclosed by pyroxene, and even in the least altered sections it is slightly replaced and rimmed by magnetite. Sections of more altered peridotite show that the orthopyroxene is replaced by a pale—green magnetite-free chrysotile of low bire- fringence at the same stage in which the cores of the olivine grains are being replaced by serpophite. The clinopyroxene generally remains little altered until most of the orthopyroxene is completely serpentinized, and where it forms thin tabular intergrowths with the orthopyroxene, laminae of orthopyroxene may alter- nate with laminae of bastitic chrysotile. With still further serpentinization the clinopyroxene is converted to an intricate intergrowth of needles quite unlike the orderly bastite, but in most sections of completely ser— pentinized peridotite no pseudomorphs of clinopyrox- ene were recognized. Where the serpentinization of the clinopyroxene is complete, the picotite is largely replaced by magnetite, and the iron oxide freed from the olivine has collected into strings of fairly well formed magnetite crystals following the larger frac- tures. Some lenticular and otherwise irregular veins of normal chrysotile also are commonly found in these completely serpentinized rocks. The sheared serpentine generally shows in thin sec- tion only scattered fragments of bastitic pseudomorphs and a few grains of residual picotite or magnetite by which one may infer its ultimate origin from an ultra- mafic igneous rock. Because the shearing has de- stroyed all the original textures, one can draw no con— clusions regarding the serpentinization process from examinations of thin sections. The field relations, however, as well as the chemical composition, are such that there can be no doubt that the sheared serpentine has the same origin as the more massive varieties. To this point the serpentinization process is largely one of hydration, and, since the process appears to be a pseudomorphic replacement, some silica and mag- nesia, and perhaps also chromium, must have been re- FIGURE 40.———Photomicrograp\hs of serpentine derived from peridotite. Upper, Shows complete replacement of olivine and partial replace- ment of large pyroxene on left. Plane light. Lower, Partly re- placed pyroxene consists of parallel growth of orthopyroxene (in extinction position) and clinopyroxene (narrow llght band's) Crossed nicols. moved from the rock. Additional changes, which have affected only isolated areas, consist of further veining with chrysotile and recrystallization of the serpentine minerals to platy antigorite accompanied by shearing. The widespread alteration of serpentine to form silica- carbonate rock is a radically different change, believed to have been caused by hydrothermal solutions having their source outside the serpentine. This alteration, which took place at a much later time than the origi- nal serpentinization, is treated at length on pages 58-64, after the description of the silica-carbonate rock. 54 Chemical composition GEOLOGY AND QUICKSILVER DEPOSITS, Chemical analyses of 3 serpentine rocks of the New Almaden district are shown in table 9, along with 3 other analyses of serpentine rocks from elsewhere in the Coast Ranges and a composite of 24 serpentines from other regions for comparison. The 3 analyzed rocks from the district are from the underground workings of the New Almaden mine and are unweath— ered. Two of them are from the central parts of large massive blocks and contain no megascopic veinlets of chrysotile; the third, which also contains no veinlets, is a minutely sheared variety of serpentine from the border of a large sill. The analyses show no signifi— cant difference between the blocks and the sheared matrix material, and thus confirm our belief that the matrix is merely a sheared part of the rock rather than a foreign material that has engulfed the blocks. Also worthy of note is the high water content, which indicates, as do also the thin sections, how complete has been the serpentinization of the original rock. Comparison of all the analyses suggests that the ser- pentines of the New Almaden district are representa- tive of those in the California Coast Ranges, and that they are closely similar to the average of serpentines from other parts of the world represented in column 7. Some partial chemical analyses of other serpentine rocks from the New Almaden district and nearby areas are shown in table 10. These would seem to indicate that the fresh serpentine from the mine con- tains a slightly greater amount of magnesium and less silica than the other apparently fresh samples of ser— pentine taken from the surface and shallow cuts. Origin The problem of the origin of such an unusually hydrous rock as serpentine has been one of the more NEW ALMADEN DISTRICT, CALIFORNIA TABLE 9.—Analyses of serpentine rocks from the New Almaden district and from elsewhere in the California Coast Ranges, together with a composite of 24 serpentines from other regions for comparison ‘Determinations' made by J. J. Fahey, U.S. Geolozical Survey. tDetermined spectroscopically by K. J. Murata, U.S. Geological Survey. Norm—Description of sample and locality as follows: 1. Unsheared fresh serpentine from the New Almaden mine, Santa Clara County, Calif. (Coordinates 1535 N.—5130 W., alt 1,183 ft). F. A. Gonyer, analyst. . Unsheared fresh serpentine from the New Almaden mine, Santa Clara County, Calif. (Coordinates 2427 N .—4703 W., alt 974 ft). F. A. Gonyer. analyst. 3. Sheared but not veined fresh serpentine from the New Almaden mine, Santa Clara Cotinty, Calif. (Coordinates 1610 N.—5100 W., alt 1,183 ft). F. A. Gonyer, ana yst. . Serpentine from the Mayacmas Range, near Missouri mine, Sonoma County, Calif. H. E. Kramm, analyst. From Kramm, 1910, p. 329. . . Serpentine describedas “bastite with fine seams of chrysotile” from Mount Dlablo, Calif. W. H. Melville, analyst. From Turner, H. W., 1891, p. 406. . Serpentine from Mount Diablo, Calif. W. H. Melville, analyst. From Turner, 1891, p. 406. . Average of 24 serpentines of the “magma type.” Average given is average of other average analyses from Massachusetts, Finland, Newfoundland, Southern Rhodesia, and Cuba, given by Hess, 1938, p. 330. N \IOUIDB elusive problems of igneous and metamorphic geology for many years, and its solution is, of course, beyond the scope of this report (see Benson, 1918; Hess, H. H., 1933). Some suggestions as to what seems to be the most probable origin of the serpentine bodies in the district, however, are justified, even if they serve only to indicate that many features of these bodies as e2?- posed in the California Coast}. Ranges are not yet en- TABLE 10.—Partial analyses of serpentine rocks from the New Almaden district and adjacent area [All analyses by Permanente Cement 00.; provided through courtesy of Messrs. E. A. Hassan, Jr., and W. R. Woodman. Analysts unknown] 1 2 3 4 5 6 7 8 9 10 11 8102 ___________________ 44. 9 44. 0 42. 7 42. 4 41. 9 41. 4 41. 4 41. 13 40. 5 40. 5 42. 1 A1203 __________________ 1.1 1.9 1.7 .9 1.4 2.2 1.0 8 27 { 2.6 } 10 9 { 1.6 Fe203* _________________ 7. 6 7. 2 7. 5 7. 9 8. 3 8. 4 8. 0 ' 8. 5 ' 7. 9 MgO __________________ 34. 5 27. 7 33. 5 32. 0 31. 1 32. 2 32. 1 36. 12 30. 7 31. 9 32. 2 CaO ___________________ n.d. 1. 6 n.d. n.d. n.d. n.d. n.d. . 60 2. 2 t. 9 1. 4 H20+ __________________ 11.4 16.3 14.2 15.4 16. 6 14. 7 15.6 11.82 14.2 } 16 0 14.8 H20- __________________ .5 4. 7 2. 5 3. 8 5. 1 2. 4 4. 2 1. 88 3. 0 ‘ ........ Total ____________ 100. 0 103. 4 102. 1 102. 4 104. 4 101. 3 102. 3 99. 82 101. 7 100. 0 100. 0 ‘ Total Fe reported as Fe203, which results in totals being too high. T Reported as “rest.” Norm—Description of sample and locality as follows: 1. Fresh massive serpentine from large mass at mouth of Almaden Canyon. Con- tains some unaltered enstatite, augite, olivine, and picotite. 2. Highly sheared but fresh-appearing serpentine from railroad cut on north side of ’I‘ulare Hill, in northeast corner of the New Almaden district. 3. Shgarfied tserpentine from 1 mile northwest of Coyote Peak, New Almaden 15 1‘10 . 4. Fresh massive bastitic serpentine block embedded in sheared matrix; from hill north of Edendale, 2 miles north of central part of New Almaden district. 5, 6. Serpentine from Hillsdale area, north of the New Almaden district. 7. Highly sheared serpentine from margin of mass at mouth of Almaden Canyon 8. Massive bastitic block and sheared serpentine matrix from Hillsdale area, north of the New Almaden district. . 9. Sheared serpentine from one-half mile east of Guadalupe mine. . 10. Bulk analysis of serpentine quarried from north side of large intruswe body at mouth of Almaden Canyon. 11. Average of 1—10; H20- omitted. SERPENTINE 55 tirely explained and are well worth further study. The chemical reactions involved in the formation of serpentine are relatively simple. They can most easily be shown by equations based on reactions involving the pure magnesium member of the olivine group, for- sterite; and although the natural olivine occurring in dunite or peridotite does contain some iron, its pres- Forsterite Water ence will not alter the conclusions presented below. The pressure-temperature fields of stability of the minerals in the MgO—SiO2—H20 system have been in- vestigated by Bowen and Tuttle (1949, p. 439—460), and their results provide important data regarding the reactions of forsterite and water. The two perti— nent equations are as follows: Brucz'te Serpentine —— 450° 2(2MgO-Si02) +3H20 : 3MgO.2Si02-2H20+MgO-H20 +450° Silica Water —- 500 ° 3 (2MgO-Si02) + $0,. + 4H20 : 2 (3MgO-28i02-2H20) + 500 ° Forsterite Serpentine Bowen and Tuttle’s experiments were performed at pressures as much as 40,000 pounds per square inch, equivalent to the lithostatic pressure at a depth of about 6 miles. In this range, each of the reactions takes place near the indicated temperature, with pres- sure having very l.ittle effect, suggesting the tempera- ture of the reaction would be little changed even at greater pressure or depth. Two important conclusions can be drawn from these equations, namely: 1. Serpentine cannot exist at temperatures appreci— ably above 500°C, or in the presence of excess magnesia above 450°C. 2. Forsterite by hydration alone forms serpentine and brucite, but with the addition of silica (or loss of magnesia) forms serpentine. If volume rela- tions remain equal, as in the direct replacement of forsterite by serpentine, both magnesia and silica must be removed. Three different theoretical conditions for the state of the serpentine bodies when intruded would seem to be possible, and each of these has been proposed for serpentines in other areas (Benson, 1918). First, the material may have been a liquid or partly crystallized ultramafic magma, which, after solidification to perido— tite, became hydrated in place by solutions either having their origin in deeper parts of the magma chamber, or in the adjacent wall rocks, or in a com- pletely unrelated younger magma. Second, the mate- rial may have been intruded at a low temperature as an extremely hydrous magma which crystallized either directly or indirectly through an olivine stage to ser— pentine, without the addition of water (Hess, 1938, p. 321—344 and Sosman, 1938, p. 353—359). And third, the serpentine bodies may have been injected plasti- cally as serpentine forming the so-called “cold intru- sions” (Clark, B. L., 1935, p. 1060, 1074; Bailey, 1942, p. 150—151; Bailey, 1946, p. 211; and Eckel and Myers, 1946, p. 94). The first of these possibilities, involving the emplace— ment of a peridotitic magma, its solidification in place, and its subsequent hydration, has been accepted by many for the serpentine bodies in the California Coast Ranges (Kramm, 1910, p. 315—349) and seems to be required to explain the few masses which show differ— entiation banding. It also has been believed by some to explain satisfactorily the prevalent blocky variety of serpentine if one invokes expansion during the hy- dration process to account for the internal shearing in the masses (Taliaferro, 1943b; p. 154—155). The source for the water, if considered at. all, is generally regarded to be the same magma, for in most parts of the Coast Ranges there are no other intrusive rocks that can supply the large quantity of water required. Many objections to this theory of origin have been raised. One of them is that the melting point of pe- ridotite is so high that a peridotite magma would be expected to produce widespread metamorphic effects in the surrounding rocks, whereas these .effects are generally lacking. The temperature of such a magma would be about 1,400°C (Daly, 1933, p. 67), although it has been pointed out that the presence of abundant- water or other mineralizers would lower this perhaps a few hundred degrees. Such mineralizers, however, if present, would presumably migrate at least to a small extent into the wallrocks, and therefore would promote the development of metamorphic aureoles around the serpentine bodies. Any proposed source for the water can likewise be countered by objections in the light of experimental work by Bowen and Tuttle (1949, p. 439—460). If the water were included in a peridotite magma, on cooling to a temperature of 900°C the rock would be 56 completely crystallized as enstatite and olivine, and water remaining could only be in the vapor phase. It would, therefore, seem impossible for enough water to remain in the pore space of the rock, even if it con— sisted of loosely packed crystals, to produce more than incipient serpentinization, whereas the New Almaden rocks are thoroughly serpentinized. If the water were slowly added from deeper seated parts of the magma chamber, one might expect the serpentinization to be more complete; but one would also expect it to be most complete in structural traps or in places where the rock was excessively sheared. No such distribu- tion of more and less serpentinized rock is apparent, however, in this district; all the serpentine bodies, re- gardless of size or degree of sharing, are serpentinized to about the same degree. Younger intrusive rocks of appropriate age are lack- ing and cannot be depended upon as the source for the water. Assimilation of water from the surround- ing sediments, which are notable for their lack of porosity and permeability, seems also unlikely, espe— cially in sufficient quantity to serpentinize masses hun- dreds of feet thick. Moreover, unless the water were charged with silica, its reaction with olivine would form brucite as well as serpentine, not serpentine alone (Bowen and Tuttle, 1949, p. 452). Many phenomena seem to forbid our invoking ex- pansion by hydration to account for the internal shear— ing in the masses. As has been pointed out, the blocks in the serpentine bodies are massive and unsheared, and show pseudomorphic textures after peridotite with ap- parently unrotated remnants of olivine crystals, which would seem scarcely possible if expansion were effec- tive. To obtain serpentine from peridotite this pseu- domorphism, of course, requires a loss of material, principally magnesia and some silica, and the process is therefore one of replacement rather than simply hydration. Inasmuch as the matrix of sheared serpen- tine has exactly the same chemical composition as the blocks it encloses, it becomes unreasonable to suppose that the matrix owes its origin to a simple hydration process involving expansion, rather than to the same replacement process that formed the serpentine of the blocks. Other objections to the proposed expansion in place are found in the lack of local outward bulges along the contacts, and in the complete absence of any veinlets of serpentine minerals in the bordering wall- rock, such as might be expected to result from the squeezing out of any residual liquid by the expansion process. Intrusion of low-temperature serpentine magma has been shown by Bowen and Tuttle (1949, p. 453) to be impossible. It was also discarded by us because it GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA failed to explain the internal structures of the serpen— tine bodies. The theory of origin that best fits our observations assumes that the serpentine masses were plastically injected as serpentine. This allows two possibilities. The material may have begun its upward migration as a crystal mush resulting from the cooling of a ' magma, and have been serpentinized during intrusion by loss of temperature concurrent with hydration by absorption of water from the surrounding rocks. Or, as appears more likely, the serpentine masses may rep— resent plastic injections from a deeper seated mass, which had the composition of peridotite but had al— ready been serpentinized. The writers visualize the serpentine as having formed from solid peridotite at an unknown depth, largely by a process of replacement involving the es- cape of large amounts of magnesia and some silica into the walls of the surrounding chamber. Subse— quently the serpentine mass was broken up and squeezed plastically into its present positions. An exceedingly small amount of the serpentinization prob— ably did take place, however, after the brecciation of the rock; this is indicated by the peripheral develop- ment of serpentine minerals in small fractures around the margins of the blocks, generally at right angles to their margins but locally parallel to them. This concept is believed to explain adequately the hydration of all the serpentine, regardless of the size of the body in which it occurs or of position within the body. It explains the textures of the unsheared blocks, which could only have formed in a solidified rock, and it further permits the differentiation band— ing seen in some of the blocks. This concept also ex- plains the general lack of either thermal or hydro- thermal alteration along the walls of the intrusive bodies, for the temperature must have been less than 500°C, above which serpentine cannot exist. It will account for the blocky structure of the inner parts of the larger bodies, the more sheared condition of their margins, and the thorough shearing of the smaller bodies. That serpentine is. capable of plastic intrusion is shown by the known examples of “cold intrusions” of serpentine into some of the younger rocks of the Coast Ranges, by its ability to form extensive slides on sur- faces with a low angle of slope, and by the way the floors and walls of some mine workings in serpentine even at relatively shallow depths tend to move inward. The degree of plasticity required for the intrusion of the serpentine masses, particularly the small tongues and apophyses bordering some of the larger masses, is perhaps the chief objection to this concept, and is GABBROS AND RELATED ROCKS 57 something that should be determined experimentally. From the chemical nature of the rock, however, it seems not at all unlikely that the conversion of a minute fraction of the serpentine to magnesium-silicate gel may perhaps aid the intrusion by acting as a lubri- cant. Age The age of the serpentine bodies in the California Coast Ranges has been variously placed by geologists who have worked in different parts of the region. In addition, serpentine was formerly treated as an inte- gral part of the Franciscan group, much as the green- stones are treated, chiefly because it is particularly common in areas of rocks assigned to the Franciscan group. Taliaferro (1943b, p. 153) has noted serpentine in the late Upper Jurassic Knoxville formation, but Anderson (1945, p. 956—957) maintained that the ser— pentine masses in the Knoxville formation were all plastically injected. Yates and Hilpert (1946, p. 239) have shown that Knoxville sedimentary rocks contain detrital serpentine and are also invaded by serpen- tinized peridotite, and they conclude that there were serpentine intrusions of at least two ages. Near Wil— bur Springs, in southern Colusa County, in the Lower Cretaceous Paskenta formation there are extensive sedimentary beds hundreds of feet thick consisting al— most wholly of serpentine detritus. These are most easily explained by the extrusion of serpentine onto the sea floor during Early Cretaceous time. Still younger rocks have been invaded by serpentine else— where in the Coast Ranges (Clark, B. L., 1935, p. 1060, 1074; Bailey, 1942, p. 150-151; and Eckel and Myers, 1946, p. 94). Injections of serpentine into the younger rocks of the Coast Ranges are generally believed to be plastic injections, but the intrusions into the older rocks are commonly believed to represent intrusions of molten magma. Because the writers believe that it is likely most of the older serpentine bodies were injected plas— tically as serpentine, rather than as peridotitic magma, the time of intrusion is believed to be dependent upon orogenic forces rather than the time when molten magma was available. On this hypothesis, once the serpentine is formed at depth it is available for in— trusion at any time thereafter, although, of course, through geologic periods the thickening of the over— lying cover by sedimentation makes its injection from the substructure to high levels increasingly less likely. However, previously injected serpentine may be later remobilized, and in the New Almaden area there is some evidence that this has taken place. 686-671 O~63——~5 GABBROS AND RELATED BOOKS The intrusive body lying along the main divide south of Mount Umunhum and shown on the map of the district as serpentine contains perhaps as much gabbroic rock as serpentine. The gabbroic rocks are partly serpentinized themselves and are so intricately intermixed with the normal serpentines that mapping them as a separate unit, if it could have been done at all, would have required more time than it seemed to be worth. Because these gabbroic rocks are of rather small extent, they have not been studied in any great detail. Areas underlain by the gabbroic rocks have a dis— tinctive appearance. They generally have a rather subdued topography, are covered with a moderately dense growth of brush in which manzanita predomi— nates, and are overlain by a deep reddish soil. Large rounded boulders of relatively fresh rock protrude from the soil here and there, and, particularly on gen- tle slopes, smaller boulders and pebbles tend to form a surficial pavement. Natural exposures of rock in place are scarce, but some good exposures are af- forded by road cuts. Here the gabbroic rocks may be seen to vary widely in both texture and mineral, content within a few feet or even inches. Some patches are fine grained, others are coarse grained, and still others have a pegmatitic texture and contain large poikilitic crystals of pyroxene or hornblende. The rocks range in color from white to deep green or nearly black, depending on the amount of feldspar, intermediate varieties being speckled or mottled. A single exposure may show nearly all these variations, and in some places the rocks are distinctly banded, different minerals being concentrated in either layers or concentric zones. In most exposures they are also cut by autoinjection dikes, which tend to be somewhat more feldspathic than the average gabbro. The principal minerals that can be identified with a hand lens are feldspar, pyroxene, hornblende, ser— pentine minerals, and mafic accessories, such as mag- netite, chromite, and picotite. A limited amount of microscopic study showed that the original feldspars were calcic labradorite and bytownite, and that the other common minerals were olivine, augite, and horn- blende. Virtually all the original minerals have been altered in varying degrees. The plagioclase is not highly altered in most sections; in some, however, it is sausuritized, and in others fine—grained secondary albite forms a “groundmass” between the larger grains. The serpentine minerals antigorite and serpophite have replaced the olivine to such an extent that only a few residual cores of olivine were noted. A few 58 sections contain considerable fine-grained chlorite. The genetic relation between the gabbroic rocks and the serpentine remains an, unsolved problem, but the two rocks do not appear to have formed by any sort of segregation in place. GEOLOGY AND QUICKSILVER DEPOSITS, SILICA-CARBONATE ROCK “Silica-carbonate rock” is the term applied in this report to a rock that is derived from serpentine by hydrothermal alteration and is composed principally of silica (quartz, or chalcedony, or opal) and a car— bonate (generally ferroan magnesite). This rock is of special importance in the New Almaden district, as in many other quicksilver districts in the Coast Ranges, because it is the host rock of all the more productive bodies of quicksilver ore. The different varieties that occur in California are variously referred to by min- ers as “vein rock,” “ledge matter,” “quicksilver rock,” “ore rock,” “opalite,” “opaline,” and “silica-carbonate rock.” Because many of these terms have been used to apply to other kinds of rocks, it is fortunate that the most acceptable term, “silica-carbonate rock,” is also the most widely used. The term generally has been applied only to rocks derived from serpentine, but in recent years, “silica-carbonate rock” has also been applied elsewhere (Faust and Callaghan, 1948, p. 11—74) to rocks of different origin and mineral composition. Distribution The distribution of the silica—carbonate rock in the ' New Almaden district is much more restricted than that of the serpentine bodies from which it is derived. (See pl. 1.) Most of the outcrops of silica—carbonate rock are scattered along the Los Capitancillos Ridge, in a zone that includes Mine Hill and extends north- westward 1 mile beyond the Guadalupe mine. A sec— ond zone of outcrop, less continuous and partly cov- ered, diverges from the first one east of the Senator mine and extends eastward across the valley of Ala— mitos Creek into the Santa Teresa Hills. The first of these zones contains all the highly productive mines of the district; the second, although it contains some Cinnabar, is little prospected. A third zone, contain— ing only small pods of silica-carbonate rock, extends along the north side of the Santa Teresa Hills, where it has yielded a little quicksilver at the Santa Teresa and Bernal mines. Not only is the silica-carbonate rock restricted to the serpentine bodies lying in these zones, but it is even further restricted to certain parts of them. Although > some small serpentine bodies that are thin and sheared are completely replaced, the thicker, more massive ones NEW ALMADEN DISTRICT, CALIFORNIA are generally replaced only along their sheared mar- gins and have, in effect, an armorlike shell of the hard silica-carbonate rock. There is no relation between the size or thickness of the serpentine bodies and the thickness of the shell of silica—carbonate rock devel— oped around them, nor is there any relation between the thickness or extent of the shell and the occurrence of quicksilver ore. The largest exposed body of silica- carbonate rock—the one that lies north and west of the Guadalupe mine—is perhaps 1 mile long and a few hundred feet wide, but it is sparsely mineralized only here and there. On the other hand, many small bodies of silica—carbonate rock, as well as some of in— termediate size, contain extensive bodies of minable quicksilver ore. It must therefore be concluded that small bodies are as likely to contain ore as larger ones. Furthermore, the distribution of the silica- carbonate rock from the surface downward does not show any marked change within the depths explored by the mines. In the New Almaden mine a remark- ably large amount of silica-carbonate rock is found in the upper levels, where the serpentine sills are fairly flat, but, according to the company records, large masses of “vein rock” were also cut in workings lying as much as 500 feet below sea level, or 1,750 feet directly below the present erosion surface. Megascopic features The silica-carbonate rock varies widely in appear- ance, but most of it is readily recognized because of its pseudomorphic textures inherited from serpentine and because of its areal relation to serpentine masses. The variations in the silica-carbonate rock result partly from original differences in the mineralogy and texture of the parent serpentine and partly from vari— ation in the kind and grain size of the replacing silica and carbonate minerals. In the silica-carbonate rock of the New Almaden district the silica is nearly all quartz; opal and chalcedony are so uncommon that in a general description they can be disregarded. In ad- dition, the source rock is mainly peridotite, and silica- carbonate rocks derived from dunite are too uncom- mon to merit more than a brief mention. The princi- pal variations in the rock in this district, therefore, depend chiefly on the quantity of shearing in the origi— nal serpentine, on the coarseness of the component minerals, and on the relative abundance of quartz and carbonate. Most of the silica-carbonate rock is derived from the sheared serpentine, and in many places it retains the sheared structure and also contains residual un- altered minute crystals of chromite or picotite. Where fresh, silica-carbonate rock of this origin has a lenticu— SILICA- CARBONATE ROCK 59 FIGURE 41.—-Silica-carbonate rock derived from sheared serpentine. The white veinlet is dolomite. This specimen is typical of the rock that is the host for nearly all the quicksilver ore bodies in the district. lar or streaked-out appearance, although it shows very little tendency to break parallel to the shear planes inherited from the serpentine. (See fig. 41.) L0— cally, it may show textures interpreted as resulting from complete replacement of chrysotile veinlets or bastite pseudomorphs, but more commonly these tex- tures have been removed by shearing before the altera— tion of the serpentine. The different lenticules and streaks are generally gray and green of various shades, and the sheared texture is accentuated by veining with light-colored dolomite or quartz. Hence the overall color of most of the silica-carbonate rock is green or greenish gray, but an unusually siliceous kind found only west of the Guadalupe mine is almost black. The hardness and mode of fracturing 0f the rock depend upon the proportion of silica to carbonate, and in a less degree upon their grain size and distribution. Carbonate-rich rock resembles fine-grained marble, be- ing fairly soft and having an irregular fracture, whereas rock rich in silica is more like chert, being hard and having a relatively smooth conchoidal frac— ture. The more common intermediate varieties are in most places surprisingly hard and tough, but they break with a rough fracture. Weathering generally makes a radical change in the appearance of the silica—carbonate rock, because the ferroan magnesite is removed by weathering, leav- ing only hydrated ferric oxides and silica. All de- grees of exposure are seen in the district. In a few places, where the weathered rocks contained little silica, they do not crop out at all, but instead give rise ‘to an ocherous soil containing only a few siliceous 60 GEOLOGY AND QUICKSILVER DEPOSITS, fragments. Such material, as seen in shallow adits or opencuts, is a porous, locally brittle ocher, not easily recognized. Siliceous skeletons of bastite pseudo— morphs or vaguely lenticular textures are discernible in places, and nearly everywhere the ocher will yield chromite or picotite if panned. Where the rock is somewhat more siliceous, it crops out as white to brown rounded boulders having pitted surfaces, due to the carbonate having been removed and a frame— work of silica left behind. As might be expected, the blocky siliceous variety of silica-carbonate rock forms prominent knobs or ledges. Silica-carbonate rock, derived from blocky serpen— tine is not abundant, but excellent exposures of it can been seen in the Day tunnel of the New Almaden mine, about 2,000 feet from its portal. Here the walls are unevenly encrusted with various white secondary salts in a manner that brings out the structure of the rock. Where the walls are not too heavily coated, one can see every small detail of the sheared matrix and un- sheared blocks nearly as well as in the best exposures of unaltered blocky serpentine, even though the ser— pentine is completely converted to a hard variety of silica-carbonate rock. The part derived from the sheared matrix is similar in all respects to the silica- carbonate rock previously described, but that derived from the unsheared blocks retains a porphyritic ap— pearance resulting from the selective replacement of the bastitic pseudomorphs occurring in the serpentine. In some of the blocks, however, the serpentine de- rived from olivine is largely replaced by fine—grained gray magnesite, and the accompanying bastite pseudo— FIGURE 42.—Pollshed surface on silica-carbonate rock derived from slightly sheared serpentine. The light areas on the left are pseudo- morphs after crystals of pyroxene. NEW ALMADEN DISTRICT, CALIFORNIA morphs are replaced by more coarsely crystalline white carbonate enclosing a bright—green platy mineral, probably a chlorite. In such rock the pseudoporphy- ritic texture is very pronounced because of the color contrast between the grayish “groundmass” and the pale apple-green “phenocrysts” (fig. 42). A few blocks of fine-grained silica-carbonate rock that has replaced serpentine derived from dunite were found in the same general part of the Day tunnel, and in these it was possible to distinguish a mesh texture inherited from the serpentine. Such examples of silica-carbonate rocks derived from massive serpentine are unusual and of small extent. They are of special interest, however, because of what they reveal about the process through which the silica—carbonate rock was formed. It replaced the serpentine with so little volume change that no effects due to expansion or contraction are apparent. Microscopic features Thin sections that show all stages of alteration from serpentine to silica-carbonate rock were examined. Those sections showing the least replacement contain in addition to the serpentine minerals only a little carbonate, whereas sections that show more advanced alteration contain quartz as well as carbonate and lit- tle or no serpentine. Although the alteration does not everywhere take place in the same way, the fol— lowing statements will describe the general process as inferred from a study of the entire suite of sections. To show most clearly how some of the serpentine min— erals are replaced before others, the rock derived from the unsheared serpentine is described first, even though it is not the most common variety. The alteration of unsheared serpentine begins with the crystallization of magnesite in the mesh of anti- gorite and chrysotile that has replaced olivine. This carbonate first forms a network of irregular veinlets, which have ragged edges because the crystals grow outward from cracks as replacement of the serpentine advances. (See fig. 43.) The bastitic pseudomorphs are not generally attacked at this stage, but in places the carbonate fills narrow sharply bordered cracks that traverse them. The larger veins of chrysotile, on the other hand, are especially susceptible to replacement by a carbonate that retains a fibrous aspect even though it is actually in rather equant crystals. (See fig. 44.) Further alteration of the serpentine results in rosettes of magnesite in the serpophite cores of the mesh and in the bastite pseudomorphs, and at the same time the meshwork of carbonate becomes better defined and coarser. With continued alteration more carbonate may be added, but a more striking change SILICA-CARBONATE ROCK 61 FIGURE 43.———Photomicrograph of altered serpentine showing early stage of development of silica-carbonate rock in which magnesite (dark) has replaced the chrysotile veinlets and a little of the fibrous antigorite bordering them. FIGURE 44.—Photomicrograph of magnesite replacement of thick veins of chrysotile; note typical fibrous aspect of the carbonate thus formed. is the replacement of the remaining serpentine by quartz. The earliest quartz fills in between the car- bonates as an aggregate of minute grains with rounded outlines, but in a more advanced stage of alteration the quartz recrystallizes into larger crystals showing many straight sharp crystal faces. (See fig. 45.) Where alteration has been very intense, as in some of the ore bodies, quartz locally replaces the carbonate, and both late quartz and dolomite fill small fractures in the silica—carbonate rock. The magnetite of the serpentine disappears at an early stage, the iron doubt— less being incorporated into the ferroan magnesite; but chromite or picotite generally remains unaltered. The alteration of sheared serpentine to silica-car— bonate rock probably is a very similar process, al- though the selectivity of the replacement is not so marked (fig. 46). As in the unsheared serpentine, replacement by carbonate takes place first along frac— tures, but here the more open fractures are generally those that have resulted from shearing, rather than from the original replacement of the olivine by ser— FIGURE 45.—Photomicrograph of silica»carbonate rock derived from serpentine that was derived from dunite. Coarse veins are magnesite (M) ; remainder is largely quartz (Q) with finely divided dark iron oxides. Note the meshwork typical of replacement of olivine by ser- pentine minerals is now outlined by the iron oxides in the quartz. pentine minerals. Again the carbonate replacement is followed, and also partly overlapped, by replacement by quartz, the earliest quartz being very fine grained and the latest coarse. Chemical composition Chemical analyses of some silica—carbonate rocks, and of parts of the unaltered serpentine bodies from which they were derived, are given in tables 11 and 12. These show the silica—carbonate rock to be composed chiefly of about 35 percent silica and 60 percent mag- nesium carbonate, with several percent of picotite or chromite. The analyzed sample of silica-carbonate rock is typical of that in which most of the ores have been formed and represents the most common variety in the district. Some of the rock, of a kind best seen west of the Guadalupe mine and everywhere barren, would show on analysis a much larger percentage of silica and less magnesite. Where carbonate is espe- cially abundant an appreciable part of it may be do- lomite rather than magnesite, as such occurrences of 62 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA FIGURE 46.—Photomicrographs of silica-carbonate rock derived from sheared serpentine. Magnesite (M), Quartz (Q), and magnetite (mgt). Section also contains minute needles of mlllerite too small to show in the photomicrographs. Upper, Plane light. Lower, Crossed nicols. dolomite have been reported in other quicksilver dis— tricts. This, however, is probably uncommon, for these rocks are derived, by simple carbonatization, from serpentines containing little or no calcium. Considerable dolomite is present in the “dolomitic transitional rock” of tables 11 and 12. This rock, which is coarsely crystalline and relatively incoher- ent, occurs in the New Almaden mine only between the unaltered serpentine and the normal variety of silica-carbonate rock, and because of this relationship it was regarded as representing an intermediate stage in the alteration and was mapped as a “transitional rock.” The chemical analyses, however, show that the rock is not intermediate in chemical composition, as was first supposed, for it has an abnormally high con— tent of lime and low content of silica. It was probably formed in a late stage of rock alteration, during which TABLE .11.——Analyses of rocks from the New Almaden mine showing change from serpentine to cinnabar—bearz‘ng silica- carbonale rock 1 2 3 4 5 ‘ “Dolomitic Barren Silica-car- MaSSIve Sheared transi- silica-car- bonate rock serpentlne serpentine tionalroek" bonate rock with Cinnabar Major elements determined chemically [All analyses by F. A. Gonyer except those marked (*), which are by J. J. Fahey, U.S. Geological Survey] Si02.. 35. 98 37. 36 3. 36 35. 32 34. 78 A1203. 3. 19 *1. 42 1.04 . 84 . 72 F6203- 6. 36 *3. 69 1. 11 1. 74 2. 06 FeO.. 1. 72 *3. 65 3. 56 3. 70 3. 08 MnO . . 09 .09 .03 .08 . 09 MgO. 37. 60 38. 54 23. 14 25. 87 26. 14 Ca 0 . _ None None 22. 46 . 98 1.56 NazO. None None None None None K20... None None None None None H20+ 14. 16 13. 50 1. 73 .37 . 002.. . .84 . 42. 74 30. 34 30. 22 P205. _ None None None None None ______________ .04 .07 n.d. n.d. .61 Cran ....... 29 . 25 . 18 . 15 16 $03 _________________ None n.d. .45 .64 n d 100. 27 I 99. 45 ‘ 99. 80 l 100. 03 I 100. 06 Minor elements determined spectrographically [K. J. Murata, US. Geological Survey, analyst. Elements sought but not found: An, Ag, Pt, Pd, Be, Zn, Cd, Ge, In, Tl, Pb, Sn, La, Y] Location of samples referred to mining company coordinates: 1535 N.—5l30 W.; alt 1,183 ft. . 1610 N.-5100 W.; alt 1,183 ft. . 1650 N.-5080 W.; alt 1,183 ft. . 1660 N.-5075 W.; alt 1,183 ft. . 1685 N.-5035 W.; alt 1,183 ft. OIDBWMH the open fractures in the silica—carbonate rock were filled with dolomite. As it is present only locally and nowhere contains any ore, it has been included with the serpentine on the composite geologic maps of the New Almaden mine (pls. 5—10). Origin The process whereby serpentine is converted to silica-carbonate rock has already been partly implied in the description of the microscopic features of these rocks. It is chiefly a replacement process, wherein the structures of the serpentine are retained during the conversion of serpentine to a mixture of quartz and magnesite. The process is not, however, entirely a simple molecular replacement; for at an early stage the crystals of both magnesite and quartz become too large to preserve the finest serpentine structures that can be seen in thin sections, and, moreover, a limited TABLE 12.—Analyses of rocks from the Ne SILICA-CARBONATE ROCK 63 w Almaden mine showing change from serpentine to silica—carbonate rock 1 2 3 Massive “Dolomitic Silica-carbonate serpentine transitional roc rock’ ’ Major elements determined chemically [All determinations by F. A. Gonyer, except those marked (*), which are by J. J. Fahey, U.S. Geological Survey] SiOz _______________ *36. 43 27. 64 31. 22 A1203 ______________ *3. 04 3. 44 . 94 Fe203 ______________ *3. 68 3. 74 2. 22 FeO _______________ *3. 72 5. 22 2. 58 MnO ______________ . 09 . 07 . 08 MgO ______________ 36. 99 31. 86 28. 78 CaO _______________ None 2. 42 . 04 NaZO ______________ None None None K20 _______________ None None None H20+ ______________ 15. 00 10. 03 . 59 CO2 _______________ . 54 15. 04 33. 16 P205 _______________ None None None ________________ -_ . 14 Trace . 02 Cr203 ______________ *. 38 . 36 . 24 803 ________________ n.d n.d. n.d. Total ________ 100. 01 99. 82 99. 87 Minor elements determined spectrographically [K. J. Murata, U.S. Geological Survey, analyst. Elements sought but not tound Au, Ag, Pt, Pd, Be, Zn, Cd, Ge, In, T1, Pb, Sn, La, Y] gu ________________ 0. 002 i _________________ . 2 Co _________________ . 006 V205 _______________ 007 Ti02 _______________ 04 SrO ____________________________ BaO _______________ <. 001 B203 _______________ . 05 0. 004 ____________ . 2 0. 08 . 009 . 004 . 008 ____________ 09 . 008 008 . 003 001 . 002 01 . 02 Location of samples referred to mining company coordinates: 1. 2427 N.—4703 W.; alt 974 ft. 2. 2410 N.-4710 W.; alt 975 ft. 3. 2385 N.—4725 W.; alt 980 ft. TABLE 13.—Average chemical composition of serpentine and silica— carbonate rock from the New Almaden mine, together with calcula— tions to show chemical changes involved in the formation of silica—carbonate rock from serpentine 1 2 3 4 5 Gain or Silica- Percent Percent loss serpen- Serpentine carbonate X sp gr X sp gr tine to rock serpentine silica- silica-car- carbonate bonate rock rock (g per cc) 36.59 33. 77 90. 01 95. 23 +5. 22 2. 55 .83 6 27 2. 34 —3. 93 4. 54 2. 01 11. 16 5. 67 —5. 49 3. 03 3.12 7 45 8.80 +1.35 . 09 . 08 . 22 . 23 +. 01 37.71 26. 93 92 77 75 94 —16. 83 None . 86 00 2. 43 +2. 43 14. 22 . 53 34. 98 1. 49 -33. 49 .75 31. 24 1 84 88. 10 +86. 26 .31 . 18 76 .51 —. 25 99. 79 99. 55 __________________________________ Avg sp gr ........... 2. 46 2. 82 _________________________________ . Column IXAvg sp gr of 2.46. . Column 2XAvg sp gr of 2.82. . Column 4 minus 3. cue-www- . Average of3 serpentines: 1 and 2 of table 11 and 1 of table 12. . Average of 3 silica-carbonate rocks: 4 and 5 of table 11 and 3 of table 12. SERPENTINE SILICA-CARBONATE ROCK Sp gr=2.46 Sp gr=2.82 /l / / / / / / / COZ \ \ \ \ \ H20 \ \ \ \ \ SiO2 \ \ \ \ \ \ \ MgO _1_ (E): \\ ‘Feo ‘ ‘ g _ __ _Fe203 ‘ A|203 FIGURE 47.—Diagram showing gains and losses by weight of principal oxides in hydrothermal alteration of a unit volume of serpentine to silica-carbonate rock, assuming volume for volume replacement. amount of migration of the constituents is indicated by the arrangement of the replacing minerals. Never- theless, the preservation of the larger textures and structures indicates there has been no appreciable change in volume, and a consideration of chemical analyses along with specific gravities shows that in spite of limited migrations the change is principally one of simple dehydration and carbonatization. The chemical changes are shown in table 13 and also in figure 47. Columns 1 and 2 of the table present the averages of three analyses of serpentine and of silica- carbonate rocks derived from nearby parts of the same serpentine bodies. As the specific gravity of serpen- tine and silica-carbonate rock is different, columns 3 64 GEOLOGY AND QUICKSILVER DEPOSITS, and 4, based on percentage times the average specific gravity of the rocks, show the relative amounts of the various oxides present in a unit volume of the average rocks. Column 5 shows the gains and losses of the oxides involved in the change from serpentine to silica-carbonate rock, and the same is presented graph- ically in figure 47. It is readily apparent that the principal changes are loss of water and a gain of car- bon dioxide, but there is also some loss of magnesia. Other minor changes indicated are a loss of aluminum and ferric iron and a gain of silica and ferrous iron, H4Mg38i209 +3002 "‘A 3MgCO3 (276 g; 110 cc) If the process were strictly one of replacement, however, the quantities indicated on the right side of the equation would be too large, and it is necessary to assume that H4Mggsi209 +3002 _% 2.34Mg003 (276 g; 110 cc) Using the values given and the amount of magnesium carbonate in the average silica—carbonate rock (column 2, table 13), one obtains a theoretical amount of 34 percent silica if only magnesium is lost and there is no volume change. This agrees so closely with the analytical value of 33.77 percent that there can be little doubt that the process of replacement of serpentine by magnesite and quartz also involved a loss of magnesium. , That the solutions responsible for the conversion of serpentine to silica—carbonate rock are not genetically related to the serpentine itself is indicated by two lines of evidence (1) the distribution of the silica—carbonate rock relative to the serpentine bodies in the district and (2) the age relations of the two. The geologic map of the district (pl. 1) shows that the silica—car— bonate rock is virtually confined to two of the serpen- tine zones, in both of which it is abundant; the map shows also that some of the largest areas of serpentine are not accompanied by silica-carbonate rock. If the rock were formed by solutions that had their source in the serpentine or a related magma body, one might expect it to have a more widespread and regular dis— tribution. Furthermore, the serpentine was largely intruded in the Late Cretaceous, whereas the silica- carbonate rock was not formed until after the middle Miocene. Age The determination of the age of the silica~carbonate rock is based in part on the abundance of pebbles and boulders of serpentine, without accompanying silica- carbonate rock, in a conglomerate of middle Miocene (252 g; 84 cc) NEW ALMADEN DISTRICT, CALIFORNIA although these changes are so small that they might be due to sampling errors or an insufficient number of analyses. Some calcium also has been added, but this was largely introduced in late veinlets of dolo- mite. The calcium, therefore, represents an addition, rather than an essential part of the reactions involved in the conversion of serpentine to silica-carbonate rock. This conversion of serpentine to magnesite and quartz can be represented by the following equation (after Turner, F. J., 1948, p. 135): + 2Si02 +2H20 (120 g; 44.5 cc) silica or magnesium, or both, have gone off in solution. As the foregoing chemical analyses suggest the loss of magnesium, the equation may be revised as follows: + 2Si02 (198.6 g; 65.5 cc) (120 g; 44.5 cc) Goes Off in 501W” age exposed in a road cut about 5,000 feet southwest of the summit of Mine Hill. Supporting evidence is afforded by a small contorted mass of unbroken silica- carbonate rock enclosed by Miocene shale, exposed in a roadcut about 5,000 feet west of the bend in Guada— lupe Canyon below the Guadalupe mine. This mass of silica—carbonate rock is believed to have formed after the injection of serpentine into the shales, for it is hard to imagine how so brittle a rock could have been so extremely contorted without being shattered. The contortion probably took place in the serpentine be- fore it was altered. The upper limit to the age of the silica-carbonate rock can be placed only relatively to the quicksilver .ores, which are believed to be Pliocene in age. Be- tween the formation of the silica-carbonate rock and the formation of the ores contained in it the rock was extensively fractured, but whether or not this resulted from forces applied during a long-extended period of hydrothermal alteration, responsible for the formation of the host rock and in its later stages the ores, could not be determined. OTHER UPPER CRETACEOUS ROCKS Rocks apparently younger than the Franciscan group, but also of Late Cretaceous age, crop out in two widely separated parts of the district. Because they differ in lithology and degree of deformation, they were mapped as separate cartographic units and are believed to have been deposited at different times: one of these is exposed only in the Sierra Azul in the south—central part of the district; the other is exposed OTHER UPPER CRETACEOUS ROCKS 65 chiefly in the Santa Teresa Hills in the northern part. As the two units are nowhere in contact and have yielded only a few fossils, none of which are closely limited in range, their relative age is not known. The writers, however, regard the rocks of Sierra Azul as being the older, because they are more indurated and more intensely deformed than the rocks in Santa Te— resa Hills. Uppper Cretaceous rocks or the Sierra Azul The Upper Cretaceous rocks found in the Sierra Azul consist of several thousand feet of interbed— ded conglomerate, feldspathic graywacke, and shale. Within the mapped area graywacke constitutes more than half of the unit, and conglomerate and shale each constitute a little less than a fourth, but shale is much more abundant farther south. These rocks un— derlie only a small area along the northern slope of the Sierra Azul in the south-central part of the dis- trict (pl. 1), but, because they extend south into Santa Cruz County, their total area of outcrop is much larger than is indicated by the map accompanying this report. As these rocks are poorly exposed, and con- tain few fossils within the district, and as they are being studied by others in the area of better expo- sures to the south, we have not assigned a formational name to the unit. The fresh graywacke is medium grained and light colored, but where it is weathered, it is speckled with White feldspars in a red matrix. The grains are sub— angular and subrounded and are only moderately well sorted. Locally, they are admixed with small peb- bles or fist-sized clay balls. The principal minerals observed in the single thin section that was made are quartz, orthoclase, and albite; these are accompanied by a little muscovite, biotite, chlorite, and calcite. Staining tests made on a half—dozen specimens indicate orthoclase amounts to from 5 to 20 percent. Frag— ments of chert and mafic lava amount to about 10 percent, and some of the feldspars show myrmekitic intergrowths. The feldspathic graywacke differs from that of the Franciscan group in being slightly cal- careous and in containing much more orthoclase, more matrix, and more clayey material in the matrix. It is poorly exposed, but is somewhat better exposed, on the average, than the graywacke of the Franciscan group; on hillsides it forms characteristic brushy slopes that are generally mantled with talus, and on hilltops it yields a reddish or pinkish soil. The conglomerate forms beds commonly more than 10 feet thick, and in places these occur in groups sepa- rated by foot-thick beds of graywacke. Locally, the conglomerate beds crops out in bold relief, but more commonly they are subdued and give rise to a red— dish bouldery soil. Some of the boulders are as much as 1 foot in diameter, but the average is between 2 and 3 inches. The result of a pebble count made on the conglomerate exposed on a ridge road just south of the serpentine body near Mount Umunhum is shown in table 14, and a similar count from a con- glomerate of the Franciscan group is given for com- parison. The 2 conglomerates are notably different in the proportion of igneous and sedimentary rock pebbles; igneous rocks make up 90 percent of the pebbles in the conglomerate of the Sierra Azul, but less than 50 percent of those in the conglomerate of the Franciscan group. Only 16 of the 100 pebbles identified in the Sierra Azul conglomerate could pos— sibly be derived from the Franciscan group, and it is likely that most of these are not. The matrix of the conglomerate of the Sierra Azul is similar to the gray- wacke with which it is interbedded, but in many places it is more silicified. The conglomerate is much frac— tured; in the more silicified parts the fractures cleave the pebbles and in the unsilicified parts they break around them. Calcite fills some of the fractures and locally replaced part of the matrix. TABLE 14.—Pebbles from conglomerate in the Upper Cretaceous rocks of the Sierra Azul and the Franciscan group, Santa Clara County, Calif. Sierra Franciscan Azul group Number of Number of Rock pebbles pebbles Sedimentary rocks: Sandstone and quartzite _______________ 7 32 Chert _______________________________ 1 21 Conglomerate _____________________________ 1 Slate ________________________________ 1 _______ Total sedimentary rocks _____________ 9 54 Igneous rocks: Granite ______________________________ 4 _______ Aplite _______________________________ 2 1 Quartz porphyry ___________________________ 3 Volcanic rocks with quartz phenocrysts" 32 9 _ Volcanic rocks with feldspar phenocrysts and no quartz phenocrysts ___________ 35 20 Diabase _____________________________ 7 3 Greenstone without phenocrysts ________ 11 9 Total igneous rocks _________________ 91 45 Vein quartz ___________________________________ 1 Total pebble count __________________ 100 100 The shale in the Upper Cretaceous rocks of the Sierra Azul is exposed in the New Almaden area in few places other than artificial cuts. In some road- cuts one may see beds of shale less than 1 foot thick rhythmically interbedded with graywacke, as is shown 66 GEOLOGY AND QUICKSILVER DEPOSITS, FIGURE 48.~—Interbedded graywacke and shale of Late Cretaceous age in the Sierra Azul as exposed in roadcut south of Lorna Prieta and just outside the New Aimaden district. in figure 48; in other cuts the shale forms massive sec— tions more than 100 feet thick with only a few inter— bedded layers of graywacke a few inches thick. The shale is olive drab, and in places is interlaminated with lighter colored silt. It commonly breaks with a NEW ALMADEN DISTRICT, CALIFORNIA conchoidal fracture; but where it is shattered, it has a splintery or shoe-peg fracture. White or gray ellip— soidal limy concretions are scattered through the shale, but are not common in the district. Most of those observed were 1 or 2 inches thick and several inches long, but some measure more than 1 foot in length. Within the district no useful fossils were found in the limy concretions, but on Mount Chuai, 1 mile south of the district, similar shale beds contain septarian concretions in which we found fragments of Inoceramus. No fossils were found in any of the other rocks of this unit within the mapped areas, but in the upper part of the southern branch of Almaden Canyon, at an altitude of 2,300 feet, the graywacke yielded speci- mens of a large Inocemmus (fig. 49) and many pelecy— pods similar to Buchéa. Farther south; along the Loma Prieta road where it crosses the Santa Clara— Santa Cruz County line, fossils are fairly abundant. A few collected in this area by C. M. Gilbert and by us were examined by Dr. S. W. Muller, of Stanford University. He found no forms well—enough preserved to be determined specifically, but he recognized Tri- gom'a (cf. T. emmsz'), Glycz’meris, Spondylus, Den— talz’um, and various small thin oysters~an assemblage that he regarded as indicating Late Cretaceous age. FIGURE Alia—Casts of Inocemmus sp. from rocks of Late Cretaceous age in the upper part of the south branch of Almaden Canyon. OTHER UPPER CRETACEOUS ROCKS 67 Uppper Cretaceous rocks or the Santa. Teresa Hills A sequence of sedimentary rocks of Late Cretaceous age, consisting of massive gray medium-grained arko- sic sandstone and tan or greenish-gray shales, is ex— posed in the western part of the Santa Teresa Hills and in thin fault silvers north of the Senator mine. The sandstone is of special interest because it was used in the construction of the buildings of Stanford Uni- versity and also of several well—known public buildings in San Jose and San Francisco. The sandstone of this unit is fairly well exposed, even though it. is deeply weathered as can be seen in quarries more than 30 feet deep. The weathered rock is generally found in clusters of large rounded boul— ders scattered along lines of strike, but continuous exposures measuring several hundred feet along the strike are exposed in the vicinity of the quarries. Where outcrops are sparse, the areas underlain by sandstone can easily be distinguished from those un— derlain by shale because they support a heavy growth of brush. The bouldery outcrops show little bedding, as the individual boulders commonly are derived from a single bed, but in places the attitude of the bedding is indicated by alinement of shale fragments or by thin shale partings. The best exposures of the sandstone are found in the vicinity of the westernmost rock quarry in the Santa Teresa Hills shown in figure 50. This quarry, having been cut into a dip slope of mas- sive sandstone, is ideally situated to take advantage of both the jointing and the bedding in the rock. The individual beds as exposed in the quarry are as much as 6 feet thick and are separated either by thin shale partings or by layers containing abundant large flakes of shale. Some bedding planes are marked by thin layers containing small pebbles, most of which are of dark chert, but the rock shows little tendency to part along these pebbly layers. The shaly partings, where exposed by stripping, exhibit many large worm trails, and they commonly also show poorly formed ripple marks. The characteristic, and locally spectacular, weather- ing of the sandstone is best developed on the slope above the quarry. Along this dip slope the least weathered rock is cut by a rectilinear pattern of joints, perpendicular to the bedding and spaced at intervals of 8 feet or more. Upslope from the quarry the joints have been widened by weathering, and the slope is more and more deeply dissected toward the crest of the hill. At the crest the edges of the dip—slope beds are exposed and the joint pattern gives way to piles of grotesque spheroidally weathered boulders. Some of these boulders have a diameter of about 40 feet, though most of them are somewhat smaller, and in FIGURE 50.——One of the smaller quarries in the Santa Teresa Hills from which Upper Cretaceous sandstone was taken for use as a build- ing stone. The thick beds and barren dip slope combined to make this an ideal site for a small quarry. several places they are precariously perched on the dip slope of an underlying bed. Casehardening, due to a concentration of limonite near the surface of the rock, has formed on the rounded boulders crusts about 1 inch thick, which are generally cracked in a polyg- onal pattern“ so that they resemble bread crusts, as is shown in figure 51. Additional weathering along these surficial cracks widens them until only small knobs representing the centers of the polygons remain and when these knobs have weathered away the process is apparently repeated. Another type of weathering re- sults in the formation of flat-bottomed caves. This process can be followed from the development of small flat basins with overhanging rims, resulting from the enlargement, by standing water, of natural depressions FIGURE 51.—Dome formed by spheroidal weathering in massive sand- stone of the Upper Cretaceous rocks in the Santa Teresa Hills. The casehardened surface and “bread crust" fractures are best developed on these domes. GEOLOGY AND QUICKSILVER DEPOSITS, FIGURE 52.—Igloolike rock in the Upper Cretaceous sandstone of the Santa Teresa Hills resulting from spheroidal and cavernous weather- ing. The lower surfaces of the caverns are flat and act as a collect- ing basin for rainwater, which, on standing, dissolves the cementing material from the rock beneath. When dry, the loosened sand is apparently removed by wind or animals. on the gentle dip slopes, to flat-bottomed caves and pits that are found in the bouldery area. An extreme type of weathering, resulting from the hollowing of a spheroidal boulder, is the igloolike rock shown in figure 52. I Lithologically the Upper Cretaceous sandstone of the Santa Teresa Hills is fairly uniform throughout its extent. Where fresh it is light gray in color, but the more common weathered sandstone is bufl' colored. Some outcrops have a uniform reddish tinge, and some exhibit concentric or wavy bands of red and brown FIGURE 53.—Photomicrograph of arkoslc sandstone of Late Cretaceous age from north of the Senator mine, showing glauconite that has replaced biotite. Glauconite (G), quartz (Q), orthoclase (0), plagio- clase (pl), microcline (M), and calcite (C). NEW ALMADEN DISTRICT, CALIFORNIA iron oxides. The sandstone is homogeneous, moder- ately well sorted, and medium to coarse grained. Most of the grains are subangular, a few are angular, and others are subrounded. Their average diameter is a little less than 0.5 mm, and nearly all the diameters fall Within a range of 0.2 to 1.0 mm. Rounded grains of chert are found in some specimens; these are gener- ally a little larger than the other grains, some of them being as much as 3 mm in diameter. Cementing mate— rial is present only in small amount and appears to consist of clay and limonite. Judging from a study of only a few thin sections of the rocks, they contain from 50 to 75 percent of quartz; up to 30 percent of orthoclase, myrmekite, and oligoclase; a few percent of microcline; and a little biotite, muscovite, glau- conite, sphene, magnetite, and cloudy limonite. Rock fragments also are present but are not abundant. One variety of the sandstone, occurring north of the mouth of Almaden Canyon and also in one of the fault slivers north of the Senator mine, weathers to a rock with a striking chocolate-brown porous peripheral zone that is sharply separated from a core of unweathered gray fine—grained sandstone. In thin section this sandstone is seen to have a calcite cement and to include more than 5 percent of brown biotite which is largely al— tered through a green biotite stage to glauconite (Gal— liher, 1935, p. 1351—1365). (See fig. 53.) The shales of Late Cretaceous age in the Santa Teresa Hills are not exposed, but they are believed to underlie several areas of rolling grasslands that have a sticky deep—brown soil containing sparse frag- ments of dark shale and scattered limy concretions. In one of the fault slivers north of the Senator mine, shale is exposed in a sharply incised canyon. In this small area at least, the shale is dark greenish gray and thin bedded, and breaks with a hackly or curved frac- ture. It resembles some of the shale and siltstone of the Franciscan group; however, it is more distinctly bedded and more clayey, and lacks the sheen charac- teristic of cleavage surface of the shales of the Fran- ciscan group. At this locality, and also in the lowest shale beds in the Santa Teresa Hills, limy concretions averaging about 1 foot in diameter are characteristic. Two varieties of these concretions are common. One variety is generally dark brown and septarian, and has crack fillings of yellowish calcite; some of these concretions contain a few fossil fragments. The other variety is chalky on the surface and White or bufl“ on the inside. These concretions contain minute spherical transparent bodies, which probably represent organic remains that are too poorly preserved to be identified. Fossils are exceedingly rare in the Upper Cretaceous rocks of the Santa Teresa Hills. A fragment of a fos- EOCENE sil identified by L. W. Stephenson, of the US. Geo- logical Survey, as Baculites chicoemsz’s Trask was found in a concretion just above a small saddle 0.38 mile east of the summit of the 430-foot hill southwest of the Santa Teresa mine. In addition, two casts found in the sandstone of the Stanford University buildings have been identified by S. W. Muller as Turritella chicoensis Gabb. Both of these fossils are found in the Chico formation (Coniacian to Cam— panian) at its type locality, but they hardly seem to justify correlation over so great a distance. The stratigraphic thickness of this unit in the New Almaden area, as determined from cross sections, is at least 1,200 feet; but the total original thickness was greater, for its base is not exposed and its upper limit is an unconformable contact with rocks of middle Eocene age. The thickness even of the rocks that are exposed is uncertain, because the unit is cut by faults of small but unknown displacement. Moreover, an exact measurement of the exposed thickness in any one place would have little significance, for the sequence is characterized by lenticular sandstone beds that thin out within short distances along their strike. Possible correlation and age Although isolated specimens of the rocks of Upper Cretaceous age from the Sierra Azul and the Santa Teresa Hills might be easily confused, the differences between the rock of the two areas are considerable. The Sierra Azul sequence contains thick beds of con- glomerate, which are lacking in the Santa Teresa Hills. The fresh feldspathic sandstones of the two formations look much alike in hand specimens, but they are readily distinguished in the field because they weather differently and have different types of expo- sure. The graywacke found in the Sierra Azul weath- ers to form hard angular pieces of reddish or pinkish color, whereas the arkosic sandstone in the Santa Teresa Hills weathers to a tan—colored and generally friable rock; the graywacke of the Sierra Azul is poorly exposed, whereas the sandstone of the Santa Teresa Hills generally forms prominent outcrops of spheroidally weathered and casehardened boulders. The shales of the two formations differ chiefly in their mode of occurrence: in the Sierra Azul they are gen— erally interbedded with many thin layers of gray- wacke, whereas the Santa Teresa Hills sequence con- tains thick strata of homogeneous shale. The two formations generally differ, also, in the character of their folding. The rocks in the Sierra Azul are so tightly folded that they show dips as high as 70°, whereas the beds of the Santa Teresa Hills rarely dip so steeply as 45°. This, however, expresses only a part of the difference in degree of deformation, ROCKS 69 for in finer detail the Sierra Azul rocks are much more crumpled and are cut by a multitude of small frac- tures and faults that are not found in the rocks of the Santa Teresa Hills. Because of these differences, the two formations are believed to be distinct, even though both contain fos— sils indicating Late Cretaceous age. The differences in their lithology and deformation suggest that the Sierra Azul sequence is the older. Perhaps this se- quence corresponds to the Pacheco group of Taliaferro (1943a, p. 130—134), and the Santa Teresa Hills se- quence corresponds to a part of his Asuncion group, which was deposited after his Santa Lucia orogeny. EOCENE ROCKS A sedimentary sequence, of early middle Eocene age, consisting of fissile shale, fossiliferous limestone, and coarse-grained sandstone underlies the central part of the Santa Teresa Hills and a group of low hills east of the point where Guadalupe Creek emerges from the mountains onto Santa Clara Valley. Because this se— quence occupies less than a square mile and is very poorly exposed, it is not assigned a formational name in this report. In the Santa Teresa Hills it lies un— conformably on the previously described sediments of Late Cretaceous age with a small angular discordance, and it is overlain only by alluvium. Its thickness cannot be accurately determined, but it appears to be at least 900 feet thick. The limestone is much like the Eocene Sierra Blanca limestone of Nelson (1925) of the Santa Ynez Mountains, in Southern California, and the entire sequence is probably equivalent in age to the upper part of the Meganos formation or the Capay formation of Weaver and others (1944:). The nearest known exposures of Eocene rocks represent the equivalent of the Domengine or Capay formation northeast of Morgan Hill, described by Gilbert (1943, p. 640—646). _ The lowest part of the Eocene sequence in the Santa Teresa Hills consists chiefly of shale, and unexposed layers of shale are probably intercalated between the younger beds of sandstone. Shale underlies most of the eastern part of these hills, but even here it is nowhere exposed; it forms subdued grass-covered hills riddled with ground-squirrel burrows, and the largest observable shale fragments are found in the material excavated from these holes. The shale is light tan, fissile, and exceptionally powdery. Examined under the microscope, it appears to consist of clay minerals, calcite, some limonite, and only a few grains of quartz and feldspar. Sandstone makes up most of the unit in the western part of the Santa Teresa Hills and also in the low 70 hills near the mouth of Guadalupe Canyon. Most of the sandstone lies stratigraphically above the shale, but the lowest beds seem to grade into shale along the strike towards the east. The outcrops of the sandstone are marked by concentrations of weathered low-lying rounded light—bufl’ or light-gray boulders, the largest more than 5 feet. in greatest diameter, and in most places the sandstone shows only a trace of bedding. Accumulation of iron oxides on and near the weath- ered surfaces results in casehardening, and beneath the surface it gives rise to brownish-red and pinkish blotches and bands. Iron oxide also imparts a pink- ish-brown color to a distinctive sandy soil that forms on the sandstone. In parts of the area this soil sup- ports thick growths of brush which contrast with the grassy areas underlain by shale. The typical unweathered sandstone is light gray, coarse—grained, and poorly sorted. It contains angular clasts of glassy quartz, feldspar, and rock fragments cemented by interstitial fine-grained calcite. In the weathered rocks, the close—packed clasts are in contact, and the interstitial calcite has been leached away pro— ducing a porous texture. The clasts in some places are as much as 1.5 mm in maximum diameter, but they average only a little more than 0.5 mm. Quartz is relatively more abundant in the sandstone of Eocene age than in that of the Upper Cretaceous rocks of the Santa Teresa Hills, though sodic plagioclase, myrmek- ite, microcline, and orthoclase together may make up as much as 20 percent of the rock. In some places the feldspar has been partly altered to clay minerals, but elsewhere it is fresh. Detrital sericite, hornblende, and epidote occur in very small amount. Scattered throughout the rock are shale fragments that are com- monly rimmed with stains of iron oxides. A special phase of the sandstone, commonly occur- ring near lenses of limestone and referred to in the field as the “white sandstone,” resembles the “glass sand” of the Meganos formation north of Mount Diablo. It contains about 20 percent of finely crys— talline calcite cement, which is more than is found in the typical sandstone of the formation, and on weathered surfaces the grains stand out in stronger relief. Thin sections show large clastic grains of quartz and feldspar, surrounded by a calcite matrix which contains some quartz and feldspar fragments less than 0.1 mm in average diameter. Rocks repre- senting all gradations between this “white sandstone” and sandy limestone may be observed, and in many places the sandstone contains echinoid spines, broken shells, and a few large F oraminifera. Several lenses of limestone are interbedded with the GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA shale near the base of the unit in the Santa Teresa Hills. The thickest of these, exposed in a quarry near the Bernal mine, measures about 25 feet in thickness, but most are only 3 to 5 feet thick. Some of the lenses are several hundred feet long, but many are so small that each is indicated only by a few boulders. As these lenses contain several different kinds of lime- stone, their exposures difl'er in character. In general, however, the limestone is more resistant to erosion than the surrounding rocks, and its outcrops are marked by rounded boulders, by trains of low boul— ders, or more rarely by inclined rounded tablets flat- tened parallel to the bedding. The different kinds of limestone tend to grade into one another, but they may be divided for description into three varieties. One variety of limestone, best exposed in the quarry near the Bernal mine, is flinty, generally massive, and poorly bedded. It weathers in such a way that the surface of its bouldery outcrops, and even most of the quarry faces, are encrusted with a dazzling-white fine— grained chalk; but fresh exposures of the rock are commonly mottled or locally banded in shades of tan or light gray. In places it is blotchy, containing ir- regular masses of medium-gray coarsely crystalline limestone, which, when freshly broken, has a faint odor of crude petroleum. Veinlets of clear coarsely crystalline calcite cut all the rock, and it contains nu- merous vugs, some of them as much as 1 inch in diam- eter, which are lined with crystals of calcite. In gen— eral, this variety of limestone is not very fossiliferous, but it does contain some small gastropods and pelecy- pods, the most abundant of which is Pitar of P. Cali— fomianus (Conrad). A second variety of limestone occurs interbedded with the coarse-grained sandstone in the Santa Teresa Hills and appears to be stratigraphically higher than the limestone exposed in the quarry. It is generally characterized by a fragmental appearance, but its tex— tural and mineralogic makeup differs from place to place. The rock appears to have been formed by mix- ing of coarse arenaceous material with highly fossilif— erous calcareous material, for it is made up of lenses and irregularly interfingered masses of sandstone, fine conglomerate, and elastic limestone. The elastic lime- stone contains irregularly broken fragments of gas- tropods, pelecypods, F oraminifera, and reef-forming organisms, such as coralline algae and bryozoans, to- gether with rounded chert pebbles and fragments of other rocks. T. Wayland Vaughan, of the US. Geo— logical Survey, examined a small collection of fossils from three different limestone masses in the Santa Teresa Hills and identified Asterocyclim aster TEMBLOR AND MONTEREY FORMATIONS 71 Woodring, Pseudophragmma, (Propovrocyclz'na) psila (Woodring), Gypsinw? sp., and Opewulmoz’des sp. He writes (written communication, November 7, 1946): “These three lots represent a single horizon, that from which Woodring originally described the two species listed above.” The horizon referred to is the Sierra Blanca limestone (Nelson, 1925) of the Santa Ynez Range in southern California believed by ‘Woodring (1930, p. 145—170; 1931, p. 371—387) to be “probably well down in the middle Eocene or in the upper part of the lower Eocene.” The fossil content of this limestone, which is remarkably like that of the limestone in the New Almaden area, was also studied by Keenan (1932, p. 53—84), who described it as con- taining echinoid fragments, small brachiopods like Terebratalia, oyster-shell fragments, Discocyclina, Globigem‘na, N odosam'a, nummulitoid Foraminifera, bryozoans, and calcareous algae. Keenan placed its age as “younger than Martinez (lower Eocene or Paleoéene) and older than Tejon at its type locality, probably either upper Meganos or lower Domengine.” A third variety of limestone that is characterized by abundant tests of the large orbitoid Discocyclina is exposed in the low hills east of the point where Guadalupe Creek first reaches the Santa Clara Valley. It forms rounded, locally knobby outcrops on which the Discocyclz'mw stand out in relief, in an area where no other rocks are exposed. The weathered surfaces are light tan to brownish gray in color, but the fresh rock is dark gray or grayish brown. This limestone is well stratified into beds about 3 feet thick, but in- dividual beds are massive or somewhat brecciated. In some outcrops the limestone is largely made up of closely packed Foraminifera tests, but in others the fossils are more sparsely scattered through a matrix of finely crystalline calcite, together with small scat- tered angular grains of plagioclase, microcline, ortho— clase, and pyrite. Glauconite also commonly occurs interstitially to the fossils, in fine aggregates up to several millimeters in diameter. No fragments of fer— romagnesian minerals were found in this rock. This limestone was studied by Schenck (1929, p. 224—227), and is the type locality of Discoc‘yclim califomica Schenck. Other forms that Schenck described as oc— curring in this limestone are—“numerous smaller Foraminifera, bryozoans, calcareous algae (cf. Ar- chaeolithothanmion), nummulitic Foraminifera, Gyp— sina, perhaps a stellate Discocyclim, gastropods, pele- cypods, crustaceans, and an occasional brachiopod.” The fossil content suggests that this limestone is a little younger than the strata in the Santa Teresa Hills; because of its isolated position and poor expo- sure, this cannot be verified by stratigraphic relations. TEMBLOR AND MONTEREY FORMATIONS Sedimentary rocks of early, middle and late Miocene age, including fossiliferous elastic limestone, conglom- erate, coarse-grained sandstone, and siliceous and dia— tomaceous shales, interbedded with a few lenses of volcanic rocks, occur in the central and northwestern parts of the New Almaden district. They are assigned on the basis of lithology and a few fossils to the wide— spread Temblor and Monterey formations, which in other areas include early to late Miocene deposits. As the two formations intergrade, the placing of the con- tact is somewhat arbitrary; in mapping, the Monterey shale was regarded, in accordance with the suggestion of Bramlette (1946, p. 3—5), as including the lowest beds of the typical white porcelaneous shales, and the rocks lying below are regarded as a part of the Tem- blor formation. The maximum aggregate thickness of the Miocene sedimentary rocks in the district, as determined from cross sections, is about 3,800 feet (pl. 1) ; but, because of the probable time overlap of different facies, it is possible that the sedimentary rocks now exposed were not quite so thick in any one place. On the other hand, the upper part of the section is everywhere un— conformably overlain by other formations or removed by erosion so that the full thickness of the Monterey shale cannot be determined. The rocks of Miocene age in the New Almaden dis- trict crop out in two main areas occupying about 5 square miles. One area is a broad band that extends eastward from Los Gatos through the low hills along the margin of Santa Clara Valley for nearly 7 miles. The other area, which contains several segments iso- lated by faulting, is a narrow band trending southeast— ward; its northwest end is 1 mile west of the Guada— lupe mine, and its southeast end 1 mile west of Almaden Canyon. In some places, one of them east of the Senator mine, these rocks can be seen to lie unconformably on the older rocks of the district, but, for the most part, they form blocks bounded by faults. They are generally thrown into fairly open folds ex— cept close to faults, where they are steeply tilted or crumpled. Temblor formation The Temblor formation consists largely of calcite— cemented conglomerate and medium- to coarse—grained arkosic sandstone, the latter containing, in some places, numerous fragments of pelecypods, oysters, and bar- nacles; the upper part of the formation, however, is more heterogeneous than the lower, and contains, in addition to the rocks mentioned, some chocolate—col- ored organic shale, greenish-gray shale, and glauconitic 72 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA FIGURE 54.——View northeastward from Bald Mountain. of the Temblor formation. mine on, right. sandstone. The arkosic sediments of the lower part are best exposed along a road that extends between Guadalupe and Almaden Canyons across the long ridge lying between Bald Mountain and Mine Hill. The roadcuts through a stratigraphic sequence at least 2,150 feet thick of predominantly massive and coarse-grained, White or light-buif, well-sorted sand— stone, mostly in beds from 12 to 30 inches in thickness. Sequences of several resistant beds as much as 20 feet thick in aggregate thickness are exposed almost con- tinuously along their strike in white, moss—covered out— crops, which give the hill the banded appearance shown in figure 54. The sandstone are all well ce— mented with calcite, and many beds contain numerous fragments of mollusca shells, which in some places are so concentrated as to form beds of organic limestone. Most of the fragments are of thick—shelled oysters, but two fairly complete pelecypods from the sandstone have been identified by Miss Myra Keen, of Stanford University, as Mitch's ewprmsus Arnold(?) and M. Zoe‘lz' Grant. Intercalated with the sandstone are beds of conglom— erate having a matrix similar to the sandstone in grain size and mineral composition. The pebbles and larger blocks in the conglomerate consist principally of a Banded aspect of hill in center is due to outcrops of more resistant sandstone beds Crest beyond is a part of Los Capitancillos Ridge; America mine is on left and opeucuts of New Almaden typical assemblage of the older rocks found on the slopes of the Sierra Azul. In order of apparent abun- dance are pebbles of graywacke, chert, shale, amphib— olite, serpentine, and greenstone. The pebbles are di- visible on the basis of their origin into two classes. One class consists largely of typical rocks of the Franciscan group, together with some serpentine boul- ders which are peculiar in that. their outer surfaces are weathered and stained a deep red. All sizes be- tween pebbles less than half an inch in diameter and boulders as much as 2 feet in diameter are common; the shapes range from rounded to angular. The other ' class of pebbles consists of hard rocks, dominantly black chert, white vein quartz, and quartz porphyry; these are well rounded and very smooth, some having a polished surface. Some pebbles are more than 4 inches in diameter, but most are less than 1 inch. They are probably reworked pebbles from older conglomer- ate beds, such as the conglomerate of the Sierra Azul. Similar smooth fragments of hard rock are found in much of the sandstone in the lower Temblor, though in these rocks they are generally in smaller grains. Stratigraphically, about 1,500 feet above the oldest exposed coarse-grained sandstone and conglomerate beds are layers of medium—grained buff sandstone of TEMBLOR AND MONTEREY FORMATIONS 73 different aspect. The lowest of these are widely spaced and thin, but within a hundred feet strati- graphically they become so numerous as to dominate the sequence. This medium-grained sandstone is a friable limonite—rich sand that forms faintly laminated beds about 5 feet thick. It crops out in only a few places, and where it is exposed in the ridge south of Mine Hill, it is deeply weathered. Quartz, orthoclase, and subordinate plagioclase in grains averaging about 0.5 mm in diameter are the predominant minerals, but the rock also contains a little muscovite and glauconite. It has a weak cement apparently consisting of limo- nite—stained clay; and oxidized grains of magnetite and hematite seen under the microscope offer a pos— sible source for the ocherous iron oxides. The sand- stone is not highly fossiliferous, but inch-long echinoid spines are conspicuous in some layers, and in others a few well—preserved molds were found. These were identified by Miss Myra Keen, of Stanford University, as formed by Pecten discus Conrad, Pecten andesom’a Arnold, and Nation sp. ' Above the medium-grained buff sandstone south of Mine Hill lie interbedded organic shale, siltstone, and glauconitic arkosic sandstone. The total thickness of these beds may be as much as 400 feet, but they crop out in only a few places and are well exposed only along the road leading eastward from the junction of Guadalupe and Rincon Creeks. In the roadcuts the organic shales and siltstones form well-defined persistent beds up to 4 feet in thickness. Where fresh, they are light to dark gray and flecked with minute White spots, but they are mostly weathered to a gray- ish brown or chocolate color. They are fairly mas— sive and break along irregular or conchoidal frac— tures as readily as along bedding planes. The larger mineral grains are of quartz, orthoclase, plagioclase, microline, and a little calcite, but a large part of the rock is clay and organic matter. Foraminifera are abundant, and fish scales are not uncommon. The interbedded arkosic sandstone, occurring in beds of comparable thickness, is a fine- to medium-grained greenish-gray rock characterized by a relatively high percent of glauconite and calcite cement. A few of the thinner beds contain enough calcite to be termed “arenaceous limestone.” In the Blossom Hill area, and farther east along the north edge of Los Capitancillos Ridge, the upper part of the Temblor formation consists mainly of light-colored well-sorted medium-grained sandstone; but for about 4 miles eastward from a point a little west of Guadalupe Canyon, the formation includes a comparatively thin but mappable bed of dacitic tuff— breccia. (See pl. 1.) This bed deserves particular 686-671 O—63-———6 attention because of the importance attributed it by Becker (1888, p. 314, 468), who believed that it was an intrusive dike, probably of late Pliocene age, and that it was the source of the solutions that deposited the quicksilver ore. The dacitic tuif—breccia has a maximum thickness of about 40 feet north of the Senator mine, but it thins along its strike both eastward and westward to a thickness of only a few feet. It is more resistant to erosion than the Miocene sedimentary rocks, and along much of its length it forms an irregular scarp on the steep side of a hogback. Its outcrops are rugged and irregularly rounded, and they give rise to bouldery float that is strewn over the slope below. In general, they are mostly white and streaked or mottled with red and yellow iron stains; their sur- faces are both pitted and embossed. In detail the tuff—breccia shows considerable variation from place to place, but it is everywhere fragmental. The frag- ments are mostly angular and less than 1 inch in diameter, but some are lenticular and of greater size; these are generally less than 2 inches long, but some are as much as 1 foot long. The lenticular fragments are oriented parallel to the bedding and are rudely banded parallel to their length; they are believed to be collapsed pumice. Most of the larger angular frag- ments are composed of volcanic rocks, but some of them, particularly in the lower part of the tuff—brec- cia, are sandstone and shale. The matrix material is so altered in most places that its original nature can— not be determined, but, largely because of the abun— dance of volcanic fragments in the rock, it is believed to be pyroclastic. If it is, the original tuflaceous ma— terial must have contained many small, whole crystals of plagioclase and some quartz, for locally, especially where the rock is silicified, the rock resembles a fine— grained dacitic lava. Alteration of the rock has everywhere been intense. The ferromagnesian min- erals have been completely leached and locally replaced by jarosite; the feldspars in some areas were com- pletely leached out, whereas in others they remained fresh and glassy; and the quartz generally escaped alteration. The alteration process was accompanied by the filling of veins with quartz, opal, and jarosite. The tuff—breccia is believed to be a pyroclastic de- posit rather than either a dike or a flow for the fol— lowing reasons: 1. It contains angular fragments throughout. 2. It is underlain by somewhat tuffaceous sandstone, and locally grades upward through tufi'aceous sandstone to normal sandstone of the Temblor formation. 74 GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 3. There is no evidence of baking either above or below. 4. It is everywhere apparently conformable with the other sediments. The most prominent mass of Miocene volcanic rocks forms Lone Hill an elliptical knoll rising 120 feet above the floor of the Santa Clara Valley about 4 miles northeast of Los Gatos. The hill has been re— garded as a volcanic neck, but it is merely an erosional remnant of eruptive rock. It is underlain by three varieties of volcanic rock which cross the hill in ir— regular layers that strike slightly west of north and dip, in general, steeply eastward. The southwestern base of Lone Hill and a rise just south of it are under— lain by altered well-bedded whitish rocks that are re— garded as pyroclastic tuifs, though they differ little from the more tuflaceous varieties of shale in the Monterey. Lying unconformably on the pyroclastic rocks is a layer of perlitic dacite, which in some places is as much as 200 feet thick. Where fresh, the perlitic rock is light gray and exhibits a glassy matrix flecked with plagioclase phenocrysts, a few quartz crystals, and deformed cavities filled with secondary minerals. The plagioclase has cores of andesine and rims of albite-oligoclase. Stratigraphically above the perlite and underlying the greater part of the hill is a body of massive and flinty gray dacite. It is exposed in rather small blocky weathered outcrops that break along irregular, somewhat conchoidal fractures. Some of it is vesicular, and some exhibits irregular flow banding. The dacite contains varying proportions of small phenocrysts of quartz and glassy andesine, which are generally unoriented, and slightly larger dull- white fine—grained fragments that are probably tufl’. F erromagnesian minerals were originally present in small amount, but are completely replaced; the shapes of pseudomorphs indicate that the rock contained both a pyroxene and an amphibole. All the various kinds of volcanic rocks on Lone Hill are in most places ex— tensively altered. They are in large part kaolinized and pyritized, and much of the more porous rock con— tains high-temperature forms of silica. Both cristo- balite and tridymite occur so abundantly in the groundmass and in cavities as to have attracted the attention of mineralogists and mineral collectors. The other smaller bodies of dacite shown on the map (pl. 1) are composed of similar rocks all greatly altered. Their occurrence along faults and as isolated bodies suggests that they are intrusive, but most, of them are so brecciated and silicified that their original character is obliterated. The dacite that projects through the alluvium at the east end of the hill of Eocene rocks north of the Senator mine is of special interest, because it locally contains Cinnabar in suffi— cient quantity to be readily visible without the aid of a hand lens. Monterey shale The sedimentary rocks of the Monterey in the dis— trict conformably overlie the sedimentary and volcanic rocks of the Temblor formation. They consist largely of extremely fine grained dazzling-white siliceous rocks which are mostly diatomaceous, but near the base of the formation these are interbedded with fine— to medium-grained light—gray sandstone and massive white sandstone. The exposed thickness of the for— mation is 1,300 feet, but its top is nowhere exposed. The exact time range of the sedimentary rocks doubt— less could be ascertained from their foraminiferal con- tent, but such a study has not been made; the age of the formation is regarded as late Miocene only because most of the diatomaceous shale in the Coast Ranges was deposited at this time. Between the Temblor and Monterey formations in part of the area is a transitional zone about 100 feet thick in which porcelanite and diatomaceous shale are intercalated with bentonitic shale, coarse—grained bufl’ sandstone, and reddish conglomerate with pebbles up to 4 inches in diameter. The rocks of this zone gen- erally do not crop out, but rather disintegrate to form a pale ocher-colored sandy soil containing diatomite fragments and rounded pebbles. They are well ex- posed at only one place in a cut on a road branching south from Shannon Road, about 2 miles east of Los Gatos. The bentonite exposed here is a homogeneous greenish-gray plastic clay occurring in beds up to 18 inches thick. Under the microscope no fragmental material or shards were found in the bentonite, but from its refractive index it is believed to be largely montmorillonite. The bufl’ sandy layers average about 1 foot in thickness and are poorly consolidated; they contain conspicuous, though not abundant, small well— rounded pebbles of black chert. The conglomerate beds, also poorly consolidated, consist predominantly of cobbles of dark-brown medium-grained sandstone and blocky fragments of indurated shale. In other parts of the area the contact between the Temblor and Monterey formations coincides with the top of the tufi-breccia, and in still other places there is no gradation between arenaceous and siliceous sedimen- tary rocks. The typical rocks of the Monterey are most exten- sive in the Blossom Hill area east of Los Gatos, where they underlie steep~sided rounded hills that are ex— tensively planted in vineyards and fruit orchards. Al- though the siliceous shales are well exposed in many artificial cuts, they form few natural outcrops, because SANTA CLARA FORMATION 75 they readily disintegrate to yield a light- to dark-gray porous soil containing only scattered fragments of rock. In good exposures the shales can commonly be seen to be rhythmically bedded in thin layers, averag- ing about 2 inches and locally exceeding 4 inches in thickness; but in places they form massive exposures several feet thick so dissected by closely spaced ir— regular fractures that it is difficult to distinguish any bedding. Where the shales are well bedded, however, the fractures are generally normal to the bedding planes and thus cause the rock to break out into rec? tangular blocks. Neither cherts, formed by extensive silicification of the siliceous shales, nor opaline con— cretions, both of which are common in other areas, are found in the Monterey in the New Almaden district. The siliceous shales of the Monterey in the New Almaden district are light colored, porous, and punky to hard, depending on the amount of siliceous ce— ment. In greater part they are finely laminated and correspond to the “porcelaneous shales” of Bramlette (1946, p. 15); the term “porcelanite” has been ap— plied, however, to rocks that are thin bedded, but not conspicuously laminated (Taliaferro, 1934, p. 196). True diatomites, composed chiefly of the tests of di— atoms, are uncommon in the New Almaden district. The most striking feature of the siliceous rocks is the dazzling-white to pale-cream color they assume on weathering. Their bedding planes, which are sharply defined and smooth, are flecked in places with fish scales, and in many places they show rings and bands of limonitic stain. The texture of the shales is much too fine to show individual grains under a hand lens, but in many specimens Foraminifera, or pores result— ing from the removal of Foraminifera by weathering, are visible. Even in thin sections under the highest power objective, the groundmass of the rock looks like a fine cloudy mud; it consists chiefly of opal in the siliceous rocks and of clay in the more argillaceous shales. Scattered through this groundmass are angu— lar fragments of quartz, small amounts of glauconite and zircon, and a few fragments of fine-grained sili— ceous rock. The fossils observed in the shales include numerous Foraminifera and diatoms, fish scales and bones, the thin-shelled Pecten peckham‘z’, and a few large fragments of silicified vertebrate bones. SANTA CLARA FORMATION The Santa Clara Valley contains alluvium of at least two ages. The older alluvium is exposed mainly in the northwest corner of the New Almaden district, in and near Los Gatos, but small patches of it are scattered along the northern foothills of the Los Capitancillos Ridge as far east as the mouth of Al— maden Canyon. From the area around Los Gatos it may be traced northwestward into the Santa Cruz quadrangle, where it has been mapped as the Santa Clara formation (Branner and others, 1909, p. 6), and although the other scattered patches shown on the map of the New Almaden district may differ a bit in age, all probably were deposited within the deposi— tional interval represented by the Santa Clara forma- tion as mapped elsewhere. The formation has been dated as Pliocene and Pleistocene in age on the basis of a few fresh-water fossils (Branner and others, 1909, p. 6), some plant remains (Hannibal, 1911, p. 329—342), and a correlation with marine beds of the Merced formation lying south of San Francisco (Law- son, 1914, p. 14). Although no diagnostic fossils have been found in the formation in the New Almaden dis— trict, this age determination is accepted because it seems consistent with the local physiographic devel— opment of the landforms on the formation. The Santa Clara formation in the New Almaden district consists largely of fine to very coarse alluvial material, deposited by streams that differed little from those now eroding the mountains that border the Santa Clara Valley. It is poorly sorted in most places and locally shows irregular bedding due to gullying. The finest material is a coarse silt, and the largest observed boulders are a little more than 2 feet long. In shape the pebbles and boulders are commonly more flat than spherical, and because they lie with their flat surfaces parallel to the bedding, they provide in many places the only means of estimating the attitude of the for— mation. The pebbles and boulders are of rocks found in the adjacent range, and although they are domi— nantly of the harder rocks, they include some pieces of soft siltstone. The formation contains boulders of silica-carbonate rock in several places, and on Blossom Hill, in an exposure almost 400 feet above the present valley floor, it also contains an appreciable amount of , detrital Cinnabar. Beds of light-brown limestone con— taining abundant fresh-water gastropods were found in the formation at a single locality, south of Los Gatos and half a mile north of St.—J0sephs Hill. The formation as a whole is poorly exposed, except in roadcuts or streambanks, for it rapidly weathers to a reddish bouldery soil. The small patches of Santa Clara sediments lying on spurs of the Los Capitancillos Ridge are recognized only by the abun— dance of rounded boulders of the hardest rocks, such as diabase, gabbro, silica—carbonate rock, and chert, strewn over the surface. The attitude of the beds was measured accurately in only a few places, where the steepest recorded dip was 20°, and although in some 76 places outside the New Almaden district the forma- tion is highly tilted, most of it within the district is believed to be tilted no more than a few degrees from its original attitude of deposition which, of course, was nowhere quite horizontal. GEOLOGY AND QUICKSILVER DEPOSITS, QUATERNARY ALLUVIUM Quaternary alluvium covers the floor of Santa Clara Valley, and narrow tongues of it extend from the edge of the valley for several miles up the larger tributary canyons. The alluvium consists of silt, sand, and gravel, and contains pebbles and boulders of rocks evidently eroded from the nearby hills and moun— tains. As it forms the principal storage reservoir for the all-important ground water required for irriga- tion in the Santa Clara Valley, it has been extensively studied by W. 0. Clark (1924). The reader desiring more details as to the thickness of the alluvium, the relative proportions of the various sizes of materials it contains, or its porosity or permeability is referred to Clark’s report. Terrace gravels only slightly older than those on the floor of Santa Clara Valley are perched on terraces along the lower courses of the larger canyons as high as 100 feet above the present canyon floor. These were mapped in places along the Guadalupe and Los Gatos Canyons, where they are fairly extensive, and similar terrace gravels were also noted in Alamitos, Cherry, and Llagas Canyons. LANDSLIDES More than 50 landslides are shown on the map of the New Almaden district, and many more were noted in the field but considered either too small or too shallow to be worth mapping. Most of the larger slides lie in a belt that strikes diagonally northwest- ward across the district, from the southeast corner to Blossom Hill. That so many landslides were mapped in this belt is partly because the belt includes the area that was mapped in greatest detail. But land- slides are in fact especially numerous here, because the rocks in this belt were originally less massive, and are now more altered and sheared than those in other parts of the district. The recognition of landslides is important in plan- ning the location of adits, roads, or dams, because on slides there is always danger of renewed movement. Most of the slides in the district are easily recognized by some physiographic expression, such as cirquelike heads, hummocky surfaces, and distorted drainage; a few of the older ones, however, have long been stag- nant and can best be distinguished by their abnormal NEW ALMADEN DISTRICT, CALIFORNIA overlapping onto different kinds of rocks. If a slide has descended a canyon, as many have in this district, it is generally easy to recognize; where a canyon crosses a slide it is more sharply incised than else- where, and where a stream has passed around the toe of a slide, its channel is obviously bent out of its nor— mal course. _ The largest slides occupy nearly half a square mile, and from these there are all gradations down to small earth slumps. Their shapes are irregular, for many of them are composites of several parts which have slid at different times. The heads of the slides are commonly rounded and somewhat wider than their lower parts, but many of the more irregular ones have several heads and join downward to form a sin— gle mass. Most of the larger slides are of the types classed as rockslides, consisting chiefly of bedrock and of debris— slides (Sharpe, 1938, p. 74, 76) composed largely of talus and soil. The distinction between these two types, however, must necessarily be rather arbitrary in areas underlain by highly broken or crushed rocks of the Franciscan group, because there is so little dif- ference between the broken rock in place and the débris derived from it. A good example of an earthflow, which moved more slowly than landslides normally do, was mapped topographically, on a scale of 200 feet to 1 inch, as a part of detailed mapping of the surface above the New Almaden mine. This flow is especially interest- ing because it originated in 1927 and is therefore little eroded; moreover, as the area had been accurately mapped before 1927, on the same large scale, by the mining company surveyors, the form of the surface before and after the movement can be closely com- pared. The change is illuStrated by the profiles shown in figure 55 which indicate the supposed position of the sole of the slide. This sole is doubtless somewhat more irregular than shown, but the angle of its slope (14°) cannot be far from correct. This flow, like many slides in the area, shows many of the features generally associated with glaciers. Its head is cirque- like, and a basin near the head contains a small pond. The surface of the lower part is cut by many open cracks like the crevasses of a glacier, and along the lower margins in places there are low, linear mounds resembling lateral moraines. The surfaces of slip- page that are exposed show striae indicating the di- rection of movement. Some of the small slides shown on the map of the New Almaden mine area (pl. 3) originate in the large mine dumps, but as the waste rock moves, it gener— ally drags along the upper 5 to 10 feet of the under- GEOLOGIC STRUCTURE 77 \ ) \_,\\/J y \ I \ , ~. \ 00 § f0 s8 / \ § \ a: /: I bill \ / : //// /’ 6° I / /’ // Q]? / / j/ / ,7, . ‘ Q -. " .5. /:/ /// // A, - \Pond -... " ~-" I... .. Ill" “((4, . // . ., \ " \ A Q \ \ . .. .- " l , \ l m E: . I. 'n. u. .. . h -. I I A I I l”) /\ A 4/ EXPLANATION A’ 1500' _ d m -------------- Top of scarp Outline of recent 1400’— /Former profile 130°1_ T r.. V I 1200’— 1100’— 1000'— 900’ landslide —- Contour before re- cent landslide Mass which moved in recent landslide Topography by EH. Bailey and D‘ L. Everhart 250 0 l I A l . I 250 500 F E ET I 4 J CONTOUR INTERVAL 10 FEET DATUM IS MEAN SEA LEVELv FIGURE 55.—4Topographic map and section of landslide in the New Almaden mine area, 1946. lying hillside. These slides, which were only a few years old when mapped, are much like rock streams or rock glaciers, having crenulated surfaces and very steep arcuate fronts. They move very slowly, appar— ently when saturated with water, by solifluction; in this climate freezing can play no part in their move- ment. GEOLOGIC STRUCTURE The geologic structure of the California Coast Ranges, amid which the New Almaden district lies, has been intensively studied so far as the post-Fran- ciscan (post-Cenomanian) rocks and structures are concerned, by petroleum geologists and others, with the result that the geologic history of the region from 78 this time on is well known. Petroleum geologists, however, have a habit of classing the older rocks as granitic and metamorphic basement or Franciscan com- plex, and consider them only as much as seems re— quired to determine possible oil-bearing areas and structures. What detailed knowledge there is of the older rocks of the Coast Ranges is largely a result of work by geologists connected with various universities or with State or Federal surveys engaged in the search for economic deposits commonly associated with the older rocks, notably the ores of quicksilver, chromium, and manganese. In the New Almaden area the post-Franciscan rocks do not cover large areas, and important structures were developed before any younger rocks were de— posited; many of the structural features, therefore, must be deduced from the attitudes and distribution of the rocks of the heterogeneous Franciscan group. When this has been done, it becomes clear that these older structures have affected profoundly the subse— quent deformational pattern of the younger rocks in the district, and this also seems to be true in other parts of the Coast Ranges (Clark, B. L., 1932, p. 385— 401). Consequently, the study of these older struc- tures is of vital importance in fully understanding even the later structure and geologic history of the Coast Ranges. A brief summary of the previous geologic studies of terrains underlain by rocks of the Franciscan group is desirable to orient the. reader as to the present knowledge of these older structures. Such areas in the central Coast Ranges were studied largely from a lithologic standpoint near the turn of the century, and in the following decade the major geologic struc- tures of limited areas were briefly described in three geologic folios of the US. Geological Survey (Fair— banks, 1904; Branner and others, 1909; Lawson, 1914). During the last 20 years much larger parts of the Coast Ranges have been ably studied by Taliaferro (1943a, b), of the University of California, and many students working under him, but although several ex— cellent summary reports on this work have been pub- lished, much of the more detailed structure remains undescribed. A diflerent approach to the structure of the Coast Ranges has been a synthesis of the available published data by such eminent students of the geology of the Coast Ranges as Willis (1927, p. 34—37 ; 1946, p. 1885—1886) and Reed (1933, p. 11—14,27—59,86—88). Some European geologists (Kober, 1925, p. 139—144) have compared the Franciscan sedimentation and de— formation to that of major synclinal areas elsewhere in the world. As a result of all these varied ap- GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA proaches, as Taliaferro (1943b, p. 151) aptly points out, “Almost every assertion regarding the fundamen— tal control of Coast Range structure has been met with a contradiction.” Some geologists have emphasized the dominance of folding over faulting whereas others have believed that the Coast Ranges were comparable to the Alps in containing major overthrusts and “mo— bile belts”. Some (Clark, B. L., 1930, p. 747—828; 1935, p. 10264034) have considered the Coast Ranges to be fault blocks which periodically move up or down, whereas others, though also believing them to be broken into blocks, considered the dominant movement along the bordering faults to be relative northwestward shifts of the blocks on the southwest side of each fault. Obviously, then, much remains to be done before the older structures of large parts of the California Coast Ranges are understood. The detailed mapping of the New Almaden district, according to the methods de- scribed, (p. 8—9) reveals structures that lead to a rea— sonable analysis for this limited area; but whether they are representative of typical structures of the Coast Ranges will not be known until the results of geologic study of areas of Franciscan rocks in a far larger part of the Coast Ranges have been published. METHODS USED IN DETERMINING THE STRUCTURES The difficulties encountered in finding, mapping, and explaining structures of the rocks of the Franciscan group are due partly to the generally poor exposures and partly to lithology and structure. Lithologically, the sequence of interbedded greenstones and feld- spathic sedimentary rocks varies greatly from place to place, yet few individual beds or groups of beds possess distinctive features by which they can be cor- related across even short intervals. Coupled with this is the general lack of fossils in the rocks. As a result, it is not generally possible to set up a “standard sec~ tion” before completion of the areal mapping, nor can one rely on fossils for aid in correlation. These diffi— culties due to lithology and lack of fossils would not be so insurmountable were it not for the complications added by the way the Franciscan group has responded to deformation. The rocks have yielded by shattering, or by crumpling accompanied by extensive rock flow- age, to such a degree that nearly every part of the group is broken and deformed. Consequently, an iso— lated exposure may show an attitude that departs widely from the attitude prevalent in the surrounding area, and one cannot rely on the significance of any local attitude observed in the field. Furthermore, the rocks have not developed cleavage or systematic linear elements by which one may determine the direction of GEOLOGIC STRUCTURE 79 axial planes or plunges of folds. As an additional structural complication, most of the large faults that cut the rocks form wide shear zones that. are not obvi- ous in the field, and most of them strike nearly paral— lel to the bedding. Many of these faults are believed to have a large component of strike—slip displacement. Some of the difficulties can be minimized by the tedious and time—consuming process of following out in detail all contacts between major types of rocks, and that was done in mapping the mineralized north- east half of the district; but even after the distribu- tion of the greenstones and sediments was known, many important structures were not obvious. In at- tempting to further unravel the structure of the New Almaden district an additional refinement was made tentatively by distinguishing varieties of greenstone, such as tuff, breccia, pillow lava, and tachylitic rock, and also by trying to draw fine distinctions in the elastic sediments. We found that the greenstones could be fairly well divided into three groups—tachy— litic rocks, mafic tuffs, and more massive lavas—but we were unable to make subdivisions of the elastic rocks that were persistent enough to aid in correla- tion. By means of this additional refinement two partial stratigraphic sections were recognized in the area. Only one of these was valid over a sufficiently large extent to be of much use. It consisted of feld— spathic graywacke, tachylitic rocks, and thin discon— tinuous lenses of foraminiferal limestone, which, as they generally lay close to the tachylitic rocks, were believed to be at a single horizon rather than at sev- eral horizons. The distribution of these units which L aided in understanding the structure is shown in fig— ure 56. The other sequence, which consisted of tuff, graywacke, and massive lavas, was recognized only on Mine Hill, where it was useful in determining local structures of economic importance. The mapping of the distribution of these partial sections led to the delineation of some folds, several faults, and the major shear zone of the district, and when this had been done the structural pattern of the area was outlined. Other faults were then added on the basis of exposures of abnormally sheared rock, topographic expression, apparent linear features in the pattern of rock distri- bution or in the topography, and the alinement of ser— pentine bodies that appeared by their shapes to have been controlled by faults. The resultant map of the district we believe to be as reliable as the exposures allow, and although it may omit faults that strike virtually parallel to the bedding of the rocks of the Franciscan group, it probably depicts all the dominant structures of the district. along faults. PRELIMINARY OUTLINE OF THE GEOLOGIC STRUCTURE . The geologic structure of the older rocks of the New Almaden district is so complex in detail that most exposures exhibit at least minor folds or faults. Despite this complexity, the larger lithologic units have retained considerable continuity, and the major structures are reasonably simple ones, which can be worked out with a fair degree of assurance. (See fig. 58.) All major structures have trends falling between west and north-northwest. The rocks of the Francis- can group in general strike about west-northwest in the southern two-thirds of the district and nearly east— west in the northern third; in much of the district they dip to the north. A major anticlinal fold trends west-northwest along Los Capitancillos Ridge and ex- tends southwards to a point near the junction of Long— wall Canyon and Llagas Creek; however, it is offset by several faults and so poorly represented by the atti- tudes of the rocks exposed at the surface that it might have remained undiscovered had we not had the bene- fit of the many exposures provided by the quicksilver mines. _ The larger faults that traverse the Franciscan strata are nearly parallel to the strike of the strata but somewhat steeper in dip. The most remarkable of these is the Ben Trovato shear zone, which strikes west-northwest through the center of the district, at- tains a maximum width of 4,000 feet, and has an apparent offset of about 10 miles. From this zone, faults with thousands of feet of offset diverge north- ward, and others with large, but less positively known, offset diverge westward. Other faults that parallel the Ben Trovato zone, both to the north and south of it, have a strike-slip component of at least several hundred feet. In addition, there are many discon- tinuous and generally unmappable faults resulting from folding, crushing, or the intrusion of serpentine. All these structures are believed to have originated shortly after the deposition of the Franciscan rocks, but many have been modified by later movement. Probably all the larger faults originated in response to the same forces that produce right lateral displace— ment along the San Andreas fault, which is only a few miles southwest of the district. (See fig. 57.) The serpentine associated with the Franciscan group is partly in sills and partly in steep tabular bodies The time of the intrusion of the ser- pentine to its present position is probably not the same for all of the masses. The sill—like bodies ante— date most of the faulting, whereas those lying along faults were dragged or squeezed into their present GEOLOGY AND QUICKSILVER DEPOSITS, NEW ALMADEN DISTRICT, CALIFORNIA 80 vaNN .om .Eflao no .353:me wEBonm mafildm 553% _ _ _ [1]. m SEN Roam 9.2» 5,365.5 I! ll I 2: «o £08 uwfio 5:5 33:00 ’l)! I mafia :mommozfim m5 .3 @2533» 9.23. 1331 0:335: EEEEEEom we mumvom €333 SSSwaiahc «L31: ‘32an :92 9.8.. Bass» 5:: 8.3.80 2:355 33:3 3 big so: .. :1 33.5w 9.8m caoflofith 2: we w=3m=wo§ 953—9953 we macaw ZOC.32 .25 E 3:53 H5533? Maison? nufilfim aubfih gauge 22?: @3on 338:8 32:: €er 385 :23 EE>==a Ex: MES 52% 23¢ 3:85 335 .550 9:5 m::a:mwm 33mg— »uano LEN—5 5:: 33o £qu E2>==a EaEwfisa (I.\H(.llv '5 ml ............ Z O . F< Z <4 n. X m _ _ _ _ a N & v or 6‘ J 3N< (Elma 5.65. .0: «Em £5 E 232:5 82 position during movement of the faults. Most of the serpentine occupying a fault zone appears to have been emplaced before the deposition of the post-Fran- ciscan rocks, but, locally, small masses have been squeezed into rocks that are clearly post-Franciscan. Structures involving rocks younger than the Fran— ciscan group include both faults and folds, and their positions and trends are governed largely by the struc- tures previously developed in the rocks of the Francis- can group. Most of the folds are fairly open and strike nearly east. The most important of the faults following older structures are the Shannon fault, which separates the northern third of the district from the remainder, and a braid of more discontinuous faults, displacing Miocene rocks, along the southwest- ern border of the Ben Trovato shear zone. Compara— tively young normal faults strike northwest across the displaced gravels of the Santa Clara formation in the northwest corner of the district. GEOLOGY AND QUICKSILVER DEPOSITS, MAJOR STRUCTURAL BLOCKS For description, the New Almaden district may be divided into three major blocks separated by major faults as shown in figure 57, and each of these ex— hibits a different structural pattern (fig. 58). The northernmost, called the Santa Teresa block, has an indefinite northern limit covered by the alluvium of the Santa Clara Valley, but is bounded on the south by the Shannon fault, which trends a little south of east across the entire district. The middle, or Los Capitancillos block, lies south of the Shannon fault, and is separated from the southern, or E1 Sombroso block, by the Ben Trovato shear zone, which diverges from the Shannon fault in the Blossom Hill area and extends southeastward to the southeast corner of the district. SANTA TERESA BLOCK The northern, or Santa Teresa, block contains struc- tures that strike generally eastward, but in its western part it is cut by faults that swing northwestward from the Shannon fault. The rocks exposed in this block range in age from the Upper Jurassic and Cretaceous rocks of the Franciscan group to the Recent alluvium filling the Santa Clara Valley. The alluvium, which itself is not deformed, covers the older structures in a large part of the block. The Pliocene and Pleisto- cene gravels of the Santa Clara formation exposed in the northwest corner of the district have been slightly deformed and are cut by normal faults that produced only minor topographic scarps and irregularities in the drainage pattern. The formations of Miocene, Eocene, and latest( ?) Cretaceous age have been thrown into open eastward-trending folds, which are cut by NEW ALMADEN DISTRICT, CALIFORNIA eastward-trending faults. The basement rocks of the Franciscan group, which are largely graywacke in this block, are poorly exposed, but the few attitudes that could be obtained indicate that they are plicated into folds with a similar strike. The rocks of the Franciscan group also are traversed by two eastward— trending shear zones intruded by thick masses of ser— pentine. LOS CAPITANCILLOS BLOCK The middle, or Los Capitancillos, block consists chiefly of folded interbedded sedimentary rocks and greenstones of the Franciscan group intruded by ser- pentine. The major anticline of the district follows the Los Capitancillos Ridge, but it is cut obliquely by several widely spaced faults which diverge northwest— ward from the Ben Trovato shear zone. (See fig. 57.) All these faults on which one can determine the dis- placement have a large strike-slip component, the west side moving northward. The westernmost fault, named the Enriquita fault, extends from the Ben Trovato shear zone to a zone of serpentine bodies believed to have been intruded along an old fault zone that has been followed fairly closely by the younger Shannon fault. The other faults are roughly parallel to the Enriquita; but as their courses are marked only in part by serpentine intrusions, they are harder to fol— low. The Almaden fault, for example, diverges south— ward from the precursor of the Shannon fault 7,000 feet east of the Enriquita fault. Its course near its northern end is indicated by offsets of the sedimentary rocks and greenstones of the Franciscan group, but on reaching the New Almaden mine area it becomes more nearly parallel to the strike of the older rocks and cannot be traced on the surface. It was, however, apparently reached by some of the underground work- ings of the New Almaden mine. Along its southward projection are intrusive bodies in line with it that may mark its continuation or be in shear zones parallel to it. Similarly in the area south and west of the apex of Mine Hill, the north-northwesterly alinement of bodies of serpentine and silica-carbonate rock suggest they were squeezed into, or dragged along, fractures parallel to the Enriquita and Almaden faults, but this cannot be demonstrated by offsets of the surround rocks. The easternmost parallel fault in this block, termed the “Calero fault,” likewise probably diverges from the Shannon fault, though the point of junction is covered with alluvium. From the Calero fault branch two other faults, which are more nearly paral- lel to the Ben Trovato shear zone than the other oblique faults are. The Franciscan strata in the fault— bounded areas west of the Calero fault are variously deformed, but before the development of the oblique 3 8 £3.5me £31954 33A 95 a: $33 .868 93 E @35de flanges“: .3de 95 wagonw ndfillwn gunman . _ _ . . _ mg: v m o N mum; .823 we 3.18 23h M. 85ch was a. E. a; 8:5 § “W 3 mr\\\\\ \\ 2:3 \o 3:33 953 zaommogum m5 ‘8 $.qu 3:5 2:3 NEH: \: was: 3:33;; B i 2* \\ wnzuzzm Jr \\ .l *- |' 35 Swain—.5968 «a 2.353% 0v r? \\ z , i Jl V I SEE NEE»? 8a.: asiexm mm on 2 3:0 E< 2:5va :93 pow—=5» uza 82> -l?‘ .=:n 2.35320 52: 32c mxuom Mm, mu V ¢¢¢¢¢¢¢ 5 § \l.\ 822:? 235320 H .o I km E ZO_._.