^f ^2 ^ 8 STATE OF CALIFORNIA DEPARTMENT OF NATURAL RESOURCES ECONOMIC GEOLOGY OF THE CASA DIABLO MOUNTAIN QUADRANGLE CALIFORNIA SPECIAL REPORT 48 UNlVtnSiTV ! iF CAUrOkiNlA JUL 17 1957 LI6KARY DIVISION OF MINES FERRY BUILDING, SAN FRANCISCO SPECIAL REPORTS ISSUED BY THE DIVISION OF MINES l-A. Sierra Blanca limestone in Santa Barbara County, Cali- fornia, by George W. Walker. 1950. 5 pp., 1 pi. Price 25 (J 1-B. The Calera limestone, San Mateo and Santa Clara Counties, 24. California, by George W. "Walker. 1950. 8 pp., 1 pi., 6 figs. Price 25^. 25. 2. Geology of part of the Delta-Mendota Canal near Tracy, California, by Parry Reiche. 1950. 12 pp., 5 figs. Price 25^. 3. Commercial "black granite" of Diego County, California, 26. by Richard A. Hoppin and L. A. Norman, Jr. 1950. 19 pp., 18 figs. Price 25 (K. 4. Geology of the San Dieguito pyrophyllite area, San Diego 27. County, California, by Richard H. Jabns and John F. Lance. 1950. 32 pp., 2 pis., 21 figs. Price 50^ 28. 5. Geology of the Jurupa Mountains, San Bernardino and Riverside Counties, California, by Edward M. MacKevett. 1951. 14 pp., 1 pL, 14 figs. Price 25(!. 29. 6. Geology of Bitterwater Creek area, Kern County, Califor- nia, by Henry H. Heikkila and George M. MacLeod. 1951. 21 pp., 2 pis., 15 figs. Price 35j{. 30. 7-A. Gem- and lithium-bearing pegmatites of the Pala district, San Diego County, California, by Richard H. Jahns and Lauren A. Wright. 1951. 72 pp., 13 pis., 35 figs. Price $2.50. 31. 7-B. Economic geology of the Rincon pegmatites, San Diego County, California, by John B. Hanley. 1951. 24 pp., 1 pi., 5 figs. Price 35«i. 32. 8. Talc deposits of steatite grade, Inyo County, California, by Ben M. Page. 1951. 35 pp., 11 pis., 25 figs. Price 85(f. 9. Type Moreno formation and overlying Eocene strata on the 33. west side of the San Joaquin Valley, Fresno and Merced Counties, California, by Max B. Payne. 1951. 29 pp., 5 pis., 11 figs. Price 60^. 34. 10-A. Nephrite jade and associated rocks of the Cape San Martin region, Monterey County, California, by Richard A. Crippen, Jr., 2d printing. 1951. 14 pp., 14 figs. Price 25#. 35. 10-B. Nephrite in Marin County, California, by Charles W. Ches- terman. 1951. 11 pp., 16 figs. Price 25(4. 10-C. Jadeite of San Benito County, California, by H. S. Toder ^6. and C. W. Chesterman. 1951. 8 pp., 6 figs. Price 25(}. 11. Guide to the geology of Pfei£Eer-Big Sur State Park, Mon- terey County, California, by Gordon B. Oakeshott. 1951. 37. 16 pp., 1 pi., 28 figs. Price 25 ^ 12. Hydraulic filling in metal mines, by William E. Lightfoot. 38. 19.j1. 28 pp., 15 figs. Price 50^. 13. Geology of the saline deposits of Bristol Dry Lake, San Bernardino County, California, by Hoyt S. Gale. 1951. 24 39. pp., 1 pi., 2 figs. Price 35^. 14. Geology of the massive sulfide deposits at Iron Mountain, Shasta County, California, by A. R. Kiukel, Jr., and .1. P. 40. Albers. 1951. 19 pp., 6 pis., 6 figs. Price 75(f. 15. Photogeologic interpretation using photogrammetric dip cal- culations, by D. H. Elliott. 1952. 21 pp., 9 figs. Price 50(}. 41. 16. Geology of the Shasta King mine, Shasta County, Califor- nia, by A. R. Kiukel, Jr., and Wayne E. Hall. 1951. 11 pp., 3 pis., 4 figs. Price 50(i. 17. Suggestions for exploration at New Alniaden quicksilver 42. mine, California, by Edgar H. Bailey. 1952. 4 pp., 1 pi. Price 250. 18. Geology of the Whittier-La Habra area, Los Angeles 43. County, California, by Charles J. Kundert. 1952. 22 pp., 3 pis., 19 figs. Price 50 #. 44. 19. Geology and ceramic properties of the lone formation, Buena Vista area, Amador County, California, by Joseph A. Pask and Mort D. Turner. 1952. 39 pp., 4 pis., 24 figs. 45. Price 75{!. 20. Geology of the Superior talc area, Death Valley, California, by Lauren A. AVright. 1952. 22 pp., 1 pi., 15 figs. Price 500. 46. 21. Geology of Burruel Ridge, northwestern Santa Ana Moun- tains, California, by James F. Richmond. 1952. 1 pi., 11 47. figs. Price 500. 22. Geology of Las Trampas Ridge, Berkeley Hills, California, by Cornelius K. Ham. 1952. 26 pp., 2 pis., 20 figs. Price 750. 23. Exploratory wells drilled outside of oil and gas fields in 48. California to December 31, 1950, by Gordon B. Oakeshott, Lewis T. Braun, Charles W. Jennings, and Ruth Wells. 1952. 77 pp., 1 pi., map. Price, map and report, $1.25 ;ii alone, $1. Geology of the Lebec quadrangle, California, by Johi CroweU. 1952. 23 pp., 2 pis., 10 figs. Price 750. Rocks and structure of the Quartz Spring area, norlli( Panamint Range, California, by James F. McAllister. I'l 38 pp., 3 pis., 13 figs. Price 750. Geology of the southern Ridge Basin, Los Angeles Co., California, by Peter Dehlinger. 1952. 11 pp., 1 pi., 7 i Price 500. Alkali-aggregate reaction in California concrete aggregjt by Richard Merriam. 1953. 10 pp., 12 figs. Price 350. Geology of the Mammoth mine, Shasta County, Califoa by A. R. Kinkel, Jr., and Wayne E. Hall. 1952. 15 pp., pis., 5 figs. Price 750. Geology and ore deposits of the Afterthought mine, Sli County, California, by John P. Albers. 1953. 18 pp., ( 9 figs. Price 750. Geology of the southern part of the Quail quadrangle, Gi foruia, by Charles W. Jennings. 1953. 18 pp., 2 pis., IGfii Price 750. Geology of the Johnston Grade area, San Bernarii County, California, by Robert Barton Guillou. 1953. 18 p 1 pi., 19 figs. Price 750. Geological investigations of strontium deposits in souie California, by Cordell Durrell. 1953. 48 pp., 9 pis., 126. Price $1.25. Geology of the Griffith Park area, Los Angeles Com California, by George J. Neuerberg. 1953. 29 pp., 1 pi, figs. Price 500. Geology of the Santa Rosa lead mine, Inyo County fornia, by Edward M. Mackevett. 1953. 9 pp., 2 pis., Price 500. Tungsten deposits of Madera, Fresno, and Tulare Couiti California, by Konrad B. Krauskopf. 1953. 83 pp., 4p 52 figs. Price $1.25. Geology of the Palen Mountains gypsum deposit. Rivers County, California, by Richard A. Hoppin. 1954. 25 pp. pi., 32 figs., frontis. Price 750. Rosamond uranium prospect, Kern County, Californii, George W. Walker. 1953. 8 pp., 5 figs. Price 250. Geology of the Silver Lake talc deposits, San Bernard County, California, by Lauren A. Wright. 1954. 30 pp. pis., 18 figs. Price $1.00. Barite deposits near Barstow, San Bernardino Com California, by Cordell Durrell. 1954. 8 pp., 4 pis., 1 : Price 500. Geology of the Calaveritas quadrangle, Calaveras Cora California, by Lorin D. Clark. 1954. 23 pp., 1 pi., 7fi Price $1.75. Geology of the Angels Camp and Sonora quadrausl Calaveras and Tuolumne Counties, California, by John Eric, Arvid A. Stromquist, and C. Melvin Swinney. ^'' 55 pp., 4 pis., 21 figs. Price $3.75. Geology of mineral deposits in the Ubehebe Peak qn gle, Inyo County, California, by James F. McAllister 64 pp., 3 pis., 26 figs. Price $2.00. Geology of a portion of the Elsinore fault zone. Call by John F. Mann, Jr. 1955. 22 pp., 2 pis., 5 figs. Pri^ Bibliography of marine geology and oceanography, ( nia coast, by Richard D. Terry. 1955. 131 pp., - Price 750. Exploratory wells drilled outside of oil and gas fie!''- California to December 31, 1953, by Charles W. Jo and Earl W. Hart. 1956. 104 pp., 2 figs., map. Price > Geology of the Huntington Lake area, Fresno Coiiii' California, by Warren B. Hamilton. 1956. Price 750. Economic geology of the Bishop tungsten district, fornia, by Paul C. Bateman, with a section on tln' Creek mine, by Paul C. Bateman and Lawson A. Wag 1956. 87 pp., 27 figs., 14 photos, 14 pis. Price $4.00. Economic geology of the Casa Diablo Mountain quadrais California, by C. Dean Rinehart and Donald C. Ross. S 17 pp., 4 figs., 5 pis. Price $1.00. I STATE OF CALIFORNIA GOODWIN J. KNIGHT, Governor DEPARTMENT OF NATURAL RESOURCES DeWITT NELSON, Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO 11 OLAF P. JENKINS. Chief FRANCISCO SPECIAL REPORT 48 AUGUST 1956 ECONOMIC GEOLOGY OF THE CASA DIABLO MOUNTAIN QUADRANGLE CALIFORNIA By C. DEAN RINEHART AND DONALD C. ROSS Geological Survey. U. S. Department of the Interior Price $1.00 Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/economicgeologyo48rine ECONOMIC GEOLOGY OF THE CASA DIABLO MOUNTAIN QUADRANGLE, CALIFORNIA * By C. Dean Rinehart ** and Donald C. Ross ** OUTLINE OF REPORT Pa,Te ,ct ^ uction ^ units ' '^ amorphic rocks of early Paleozoic age or older 6 ornfels and schist ^ arble » actite 1^ iilc-hornfels " onic rocks of Cretaceous age 6 ranitic and related fine-grained rocks 6 iorite and gabbro ^' ■anic rocks of Tertiary and Quaternary age 7 asalt * hyolite '^ Light-colored glass, obsidian, and pumice 7 Bishop tuff l iments of Quaternary age 7 al deposits T' tory of mining 7 igsten ■ 7 listribution and general relations 7 uggestions for tungsten prospecting 11 lack Rock mine 11 Geographic setting, history, and production 11 Workings ^ — 11 Rock types H Structure 12 Ore deposits 13 Future exploration 16 d and silver 16 1 16 ustrial minerals 17 erences 17 Illustrations Pag-e e 1. Economic map of the Casa Diablo Mountain quad- rangle In pocket 2. Aerial photograph and explanation of Black Rock tungsten mine Between 9-10 3. Geologic map of Black Rock tungsten mine--In pocket 4. Composite map of underground workings. Black Rock tungsten mine In pocket 5. Cross section of Black Rock tungsten mine show- ing fault interpretation and fold interpreta- tion In pocket e 1. Index map showing location of Casa Diablo Moun- tain quadrangle 4 2. Sketch showing quartz veins in Casa Diablo Moun- tain quadrangle 14 3. Geologic map and section of Sierra Vista mine 15 4. Geologic map, adit level, of Gold Wedge mine 16 ABSTRACT e Casa Diablo Mountain quadrangle is in the southern part of County, California, about 12 miles northwest of Bishop, jrnia, and 18 miles southeast of Mono Lake. In most parts of uadrangle, the relief is moderate but in the southwest corner e quadrangle, along the Sierra Nevada front, the local relief much as 4,000 feet. out one-half of the quadrangle is underlain by basalt and rhy- of Tertiary and Quaternary age, one-fourth by alluvial and il deposits of Quaternary age, and one-fourth by metasedimen- and plutonic rocks. The oldest rocks are folded and faulted sedimentary rocks of early Paleozoic age or older. The most ion metasedimentary rocks are micaceous hornfels and schist; common rocks are marble, calc-hornfels, and tactite. The sedimentary rocks were intruded in the Cretaceous period by epared by the U. S. Geolog-lcal Survey as a part of a coopera- tive project with the California Division of Mines, ■ologist, U. S. Geological Survey. a number of plutonic bodies of slightly different ages that range in composition from gabbro to granite. Tungsten, the principal metal produced, occurs in scheelite-bearing contact-metamorphic tactite deposits. The Black Rock mine has ac- counted for almost all of the tungsten production in the quadrangle. In 1953 it ranked second in the production of tungsten concen- trates in California, and sixth in the United States. At the Black Rock mine the ore bodies are localized in the crest of a fold, a structural setting unique among tungsten deposits. Other minor scheelite-bearing tactite deposits are in the quadrangle. In addition to tungsten deposits, small, low-grade quartz veins containing gold, silver, lead, and copper were worked in the Benton I'ange intermittently from the early 1870's until the late 1930's. Total production of these mines was probably less than $1,000,000. Minor amounts of pumice, sand, and gravel have also been produced. INTRODUCTION The Casa Diablo Mountain quadrangle was mapped during the summers of 1952 and 1953 by geologists of the U. S. Geological Survey in cooperation with the Cali- fornia State Division of Mines, as part of a study of potential tungsten-bearing areas. The south boundary of the quadrangle is about 10 miles north of the Bishop district, one of the leading tungsten-producing areas in the United States. Geography. The Casa Diablo Mountain quadrangle (fig. 1) is in the southern part of Mono County, Califor- nia, about 12 miles northwest of Bishop, California, and about 18 miles southeast of Mono Lake. The quadrangle is in the extreme western part of the Basin and Range Province and includes parts of the Sierra Nevada, the Benton Range, the Volcanic Tableland, and Long Valley. In most parts of the quadrangle, the relief is moderate and only in the southwest corner of the quadrangle along the Sierra Nevada front, and in the northwest corner does the relief exceed 2,000 feet. Relief in the Sierra Nevada is as much as 4,500 feet. The maximum relief of the area is 6,742 feet, ranging from 5,000 to 11,742 feet above sea level. Pifion pine and juniper form a sparse forest over much of the Benton Range. Jeffrey pine grows on some parts of the Volcanic Tableland, and along with lodgepole pine, is abundant in the Sierra Nevada. Sage brush and other desert vegetation are sparsely distributed over much of the rest of the quadrangle. Most of the precipitation falls as winter snow, but local summer thundershowers are common, particularly in the Sierra Nevada. Rock Creek and Hilton Creek are the only permanent streams in the quadrangle. Owens River, although formerly a permanent stream, has been dammed to form Crowley Lake. At intervals, usually during the fall and winter, the entire discharge from the lake is diverted through aqueducts to power plants sev- eral miles below the dam. In the Benton Range early spring runoff from the normally light winter snowfall and scattered springs provide the only surface water. In 1954, the major economic activities in the quadrangle — besides the operation at the Black Rock mine— consisted of cattle and sheep grazing in the Benton Range, and the operation of small fruit ranches east of Wheeler Crest. Recreational facilities on Crowley Lake, operated by the Los Angeles Department of Recreation, and a store, serv- ice station, restaurant, and other facilities at Toms Place, f 3 ) 2—3380" Special Report 48 II9°30' 38°00'- 37°30' AREA OF NDEX MAP 37°00'i SCALE IN MILES 37°30' II9°30' II9°00 II8°30' Figure 1. Index map showing the location of the Casa Diablo Mountain quadrangle Casa Diablo Mountains Quadrangle mt for considerable tourist activity in the summer ;hs. e area is easily accessible from U. S. Highway 395 h traverses the southwest corner of the quadrangle, ■om U. S. Highway 6, six miles east of the quad- le. An asphalt road connects the Black Rock mine U. S. Highway 6, and several gravel and dirt roads to other parts of the quadrangle. The nearest rail ection is about 10 miles southeast of the quadrangle iws, the northern terminus of a narrow-gauge line connects with a standard-gauge line farther south le Owens Valley. Western Truck Lines, Southern fie, and Railway Express have truck terminals in ap. Passenger service to the area is by Greyhound ilong U. S. Highway 395 and the only station within uadrangle is at Toms Place. 'rpose and Scope. This report deals with the ore sits in the Casa Diablo Mountain quadrangle, and the geologic factors that bear on their discovery and )itation. Tungsten deposits in the quadrangle served le chief impetus for the work, although all of the 'n mineral deposits in the quadrangle were examined, icular attention was given the Black Rock mine, the productive and in 1954 the only active tungsten in the quadrangle. The mine was studied in detail I attempt to comprehend the structure of the deposit to evaluate the factors that may have controlled the ization of the ore. As the work also included map- the entire quadrangle, it was possible to relate the )gy of the mine to the regional geology. 16 major part of the text consists of a description of Black Rock tungsten mine. Data pertaining to the ler, less productive tungsten deposits as well as the silver, lead, and copper deposits, none of which exploited in 1954, are recorded in tabular form. I descriptions are included of the various rock units emphasis on those units of economic importance, ■cause of the economic emphasis of this paper, all rial not directly related to economic geology has omitted for the convenience of those interested pri- ly in mining or prospecting. This omitted material, iding a more detailed discussion of rock units, struc- geomorphology, and a more comprehensive geologic of the quadrangle, with cross sections, is incorpo- 1 in a report now in preparation, to be published by ]. S. Geological Survey, le economic geologic map of the Casa Diablo Moun- quadrangle shows the location of known mineral sits and the distribution of the major rock units, lasizing those units and relations which are most nent to the occurrence of tungsten deposits. The ibution of the quartz veins that have been exploited rold and silver in the Benton Range is shown on a lified bedrock map on which the volcanic and allu- deposits are removed to give a clearer picture of listribution of the metamorphic and plutonic rocks. ^ of the major workings of the Sierra Vista and '< Wedge mines are included as examples of the gold s of the quadrangle. evious WorJc. No comprehensive report of the geol- )f the Casa Diablo Mountain quadrangle has previ- ■' been published, although the ore deposits and some icted aspects of the regional geology are described iveral papers. One of the earliest published references to the ore de- posits of the area is in the 8th Annual Report of the State Mineralogist, California Mining Bureau (Whit- ing, 1888), which includes brief descriptions of the Ban- ner, Tower, and Wildrose mines. Further description of the fissure-vein deposits and some mention of the geology of the surrounding area is found in the 18th, 23rd, and 30th Annual Reports of the State Mineralogist (Boalich, 1922; Tucker, 1927 ; Tucker, 1934). The most recent pub- lication describing the mineral deposits, including some of the tungsten deposits, is a report by Sampson and Tucker (1940) on the mineral deposits of Mono County. The earliest reference to tungsten in the area is appar- ently that of Hess and Larsen (1920) ; they discuss briefly the setting of the Benton Range tungsten de- posits. Lemmon (1941) has published a preliminary re- port in connection with the Strategic Mineral Investiga- tions of the U. S. Geological Survey describing the tungsten deposits and the geology of the Black Rock mine area, and other tungsten deposits in the Benton Range. The regional geology is discussed incidentally in some of the references to ore deposits mentioned above, but the only reports that stress regional geology are those of Gilbert (1938, 1941). He discussed in detail the geol- ogy of the Bishop tuff and the late Cenozoic history of the region. A report by A. L. Ransome (1940) on the geology and ore deposits of Blind Spring Hill (adjacent to the northeast corner of the quadrangle) contains brief descriptions of the rocks and structure in the northeast- ern corner of the quadrangle. A paper on the origin of Rock Creek gorge and Owens River Gorge is in prepara- tion by Putnam (Abstract, 1952). Acknowledgments. The Wah Chang Mining Corpo- ration alloAved the writers to use company maps and other data that facilitated the detailed geologic study of the Black Rock mine ; discussions with company person- nel were also helpful. J. H. Foelkel, Joseph Main, Sam Morris, and Guy Way have supplied much information on the other mines of the district. This information is particularly helpful as most of the mines have been idle for many years ; much of the data concerning them have not been published, and the mines are now inaccessible. H. K. Stager of the U. S. Geological Survey examined the accessible mines in the quadrangle, except the Black Rock mine, and compiled most of the data on mines that are incorporated in this report. ROCK UNITS About one-fourth of the total area included in the quadrangle is underlain by pre-Tertiary metamorphic and plutonic rocks, one-half by Tertiary and Quater- nary volcanic deposits including rhyolite, rhyolite tuff, volcanic glass, pumice, and basalt, and the remaining one-fourth by Quaternary sedimentary deposits includ- ing lake beds, glacial till, pediment gravels, talus, valley fill, and alluvial-fan material (pi. 1). The oldest rocks in the quadrangle are the folded and faulted early Pa- leozoic or late pre-Cambrian metasedimentary rocks that underlie part of the Benton Range. Small, isolated inclusions of metamorphic rocks of early Paleozoic or late pre-Cambrian age also occur in the plutonic rocks of the Benton Range and the Sierra Nevada. The meta- morphic rocks were intruded in the Cretaceous period Special Report 48 by a number of plutonie bodies of slightly different ages. In the Benton Range the plutonie and metamorphic rocks are cut by a swarm of north-trending, steep- dipping, porphyritic rhyolite and aplite dikes and sills of Cretaceous( ?) age. Numerous quartz veins, locally sulphide-bearing, also cut the rocks of the Benton Range. Metamorphic Rocks of Early Paleozoic Age or Older The largest body of metamorphic rock underlies about 15 square miles of the Benton Range. Smaller bodies occur west and south of the Benton Range and in the Sierra Nevada. Hornfels and schist are the dominant rocks, and cal- careous rocks are less abundant. The calcareous rocks have been subdivided on the economic geologic map (pi. 1) into: tactite, the host rock of the tungsten deposits; marble, which is commonly associated with tactite ; and calc-hornfels and mixed calcareous rocks, a unit that in- cludes some tactite bodies too small to show on the geo- logic map. Inclusions of metamorphic rocks are found in the plu- tonie rocks ; the inclusions are particularly common near the contact of plutonie rocks with metamorphic rocks and also in bodies of diorite and gabbro. In the Sierra Nevada, plutons are locally separated by thin layers (septa) of metamorphic rocks, the calcareous members of which locally contain scheelite. Such septa were not found in the Benton Range. The age of the metamorphic rocks is unknown. Rocks of early Paleozoic and late pre-Cambrian ages are found in the White Mountains east of the quadrangle, and rocks of Ordovician and Pennsylvanian ages are found in the Sierra Nevada west of the quadrangle. The metamorphic rocks are probably of early Paleozoic age or older. Hornfels and Schist All of the noncalcareous metamorphic rocks of the quadrangle have been grouped under this heading. The predominant rock is a buff to brown, fine-grained, spotted, sericite( mica) -quartz hornfels that locally is schistose. Dark-gray to black, andalusite-graphite-quartz liornfels also is abundant. Other varieties of schist and hornfels consisting of varied proportions of sericite, bio- tite, chlorite, quartz, andalusite, and feldspar are present in lesser amounts. The original bedding locally is well preserved, particularly in the more siliceous rocks; the original sediments were probably shale, siltstone, and fine-grained, impure sandstone. Marble The marble is gray to white, fine- to coarse-grained, and the original bedding is commonly preserved as composi- tional layering, which is best shown by different amounts of carbonaceous impurities in adjacent beds. The marble is generally pure except for the minor amount of car- bonaceous material, the local development of silicate min- erals, and some irregular, siliceous pods that probably are chert. Tactite Tactite is dark colored, generally medium to coarse grained, and is composed chiefly of one or more of the following minerals: dark-green pyroxene, dark-green amphibole, red-brown garnet, epidote, and quartz. The tactite has been formed during thermal metamor] chiefly by the addition of material to marble, altl locally tactite has been formed from both fine-gr calc-hornfels and siliceous hornfels. Economically the most important metamorphic rock in the quadr as it serves as the host rock for scheelite. Calc- Hornfels The calc-hornfels is typically a fine-grained, pale-green to gray rock that consists chiefly of pyre epidote-clinozoisite, plagioclase, amphibole, and gi This rock occurs as interlayers of different thicknes marble and as separate units. It was formed b; thermal metamorphism of impure calcareous rocks Plutonic Rocks of Cretaceous Age Plutonic rocks vinderlie large areas in the B' Range and the Sierra Nevada and also crop out a lated bodies in the volcanic and alluvial deposits. I colored granitic rocks predominate, but dark-colorec bro and diorite are common along the west side o Benton Range. Most contacts between plutonie bodies and bel plutonie rocks and metamorphic rocks are not exp The observed contacts between granitic rocks are s whereas contacts between granitic rocks and gab rocks are gradational. The contacts of the metamo rocks with granitic rocks commonly are sharp, but are marked by a gradational zone of dark-colored broic rock. Exposed contacts are too few, howevf permit generalizations about contact relations. The age of the plutonie rocks is based on a correl with the granitic rocks of the Bishop district which been tentatively determined to be Cretaceous in age Granitic and Related Fine-Grained Rocks The granitic rocks include eight mappable rock ranging in composition from granodiorite to granit( because the different units seem to bear no relation t distribution of ore deposits, they have been gro together on the economic geologic map. Each granitic unit has the same general compos and fabric throughout and can be distinguished other units by differences in the appearance of the The granitic rocks contain abundant quartz, v amounts of microcline and plagioclase, and generally than 10 percent of either hornblende or biotite. Mc the granitic rocks are medium to coarse grained equigranular, but some are porphyritic. Dikes, sills, and small stocks of porphyritic rh^ and aplite are abundant in the Benton Range. The trude both the metamorphic and plutonie rocks am probably satellitic bodies related in age and orig the granitic rocks. Most of the dikes and sills dip sti and strike generally north. Float from the resistant ■ and sills is so abundant along the ridge east of Ba Springs that locally the country rock is completely ered. Diorite and Gabbro Diorite and gabbro resemble granite but contain dark minerals and therefore have a much darker ap ance. Most specimens are composed primarily of pi elase and hornblende, with lesser amounts of clii roxene (augite or diopside) and epidote; the la Casa Diablo Mountains Quadrangle J in the Sierra Nevada contains, in addition, olivine jhypersthene. Calcareous inclusions including tactite jibundant in the diorite and gabbro. The largest in- jons of tactite found are northwest of the Gold ge mine, northwest of Wildrose Canyon, and at ered localities along the ridge east of Banner lags. I Volcanic Rocks of Tertiary and Quaternary Age iie volcanic rocks consist of three mappable units li in order of decreasing age: (1) basalt, (2) light- ed rhyolite glass, obsidian, and pumice, and (3) •hyolitic Bishop tuff of Gilbert (1938). le basalt and light-colored rhyolite glass, obsidian, pumice are probably Pliocene in age, as they prob- can be correlated with similar rocks north and east 16 quadrangle that are associated with lake beds lining fossils of late Miocene or early Pliocene age oert, 1941). The Bishop tuff is mid-Pleistocene in as attested by its stratigraphic position between two tocene glacial tills. Lsalt crops out in an area of approximately 5 square ! along the north edge of the quadrangle, in an area than 1 square mile in the Owens River Gorge, and ;attered spots in the Benton Range. Most of the It is gray to black, vesicular, and locally porphy- Phenocrysts of olivine, augite, and plagioclase par are set in a felty plagioclase-rich groundmass. lite .ight-colored Glass, Obsidian, and Pumice ass Mountain Ridge is underlain by a great variety lyolite which includes light-colored, locally perlitic , obsidian, and pumice. In places near the ridge, and obsidian crop out boldly, but farther south rd Watterson Troughs outcrops are scarce and the is mantled by a considerable thickness of rhyolite le. ishop Tuff le Bishop tuff (Gilbert, 1938) covers about 75 square i in the south half of the quadrangle and forms the [y sloping plateau of the Volcanic Tableland. The is various shades of gray, salmon, pink, brown, and lie. The thickest section is exposed just west of the ock outcrop in the Owens River Gorge and measures t 800 feet from the rim to the bottom of the gorge, tuff is rhyolitic and consists of well-formed pheno- s of quartz and feldspar set in a pumiceous and y groundmass. The rock is typically soft and is Y quarried. Sediments of Quaternary Age diments of Quaternary age consist mainly of valley illuvial-fan material, and glacial till; less extensive alus, lake beds, and gravel, which cap terraces and nents. Most of the sediments occur in Long Valley, longate alluviated valleys in the Benton Range, the on of Rock Creek, and at the base of the Sierra da. MINERAL DEPOSITS e contaet-metamorphie tung.sten deposit exploited le Black Rock mine, through September, 1953, has ed 272,591 tons of ore containing approximately 0.5 percent WO.3.* The other tungsten deposits in the quadrangle have yielded a total of only a few thousand tons of ore. A number of quartz veins in the Benton Range that were worked primarily for their gold and silver content have also yielded minor amounts of lead and copper. A deposit of iron probably too small to develop has been prospected in the Benton Eange and expansible rhyolite occurs northwest of Wilfred Canyon. Inaccessibility of the expansible rhyolite hinders pros- pecting and development. Other commercial mineral deposits include pumice, sand, and gravel. Data on all of the mineral deposits of the quadrangle are summa- rized in table 1. Locations of the mineral deposits are shown on plate 1. Inasmuch as most of the gold and silver mines have been idle for 12 years or more, many workings are caved or flooded; consequently only a limited amount of sub- surface geologic study and mapping was possible. Much of the material contained in table 1, including history, ownership, production, and development, has been taken from the reports of the State Mineralogist. These de- scriptions were checked and supplemented wherever possible by H. K. Stager during the course of field map- ping in the quadrangle. History of Mining The first mining in the quadrangle was in the Benton Range in the early 1870 's, although mining was going on in the nearby Blind Spring Hill area (northeast of the quadrangle) as early as 1862. The district was known as the Chidago or Indian mining district, and the gold and silver mines were active intermittently until the late 1930 's. In 1917 scheelite was discovered at the present site of the Black Rock mine, and in 1928 the first produc- tion of tungsten from the mine was reported. The Black Rock mine was active intermittently until 1950 and has been in continuous operation since. During 1953 it was the sixth largest producer of tungsten concentrates in the United States. A few small tungsten properties were operated in the Benton Range during and following World War II but were all idle in 1954. The only pro- duction of nonmetallic deposits in the quadrangle has been the recent limited use of pumice and alluvial de- posits for construction material. Tungsten Distribution and General Relations The tungsten deposits of the Casa Diablo Mountain quadrangle are contaet-metamorphie deposits. Scheelite, the only tungsten-bearing mineral, occurs locally in tac- tite and is sporadically distributed in grains ranging in size from a fraction of a millimeter to several millimeters across. The tactite is typically associated with calcareous rocks, although it occurs locally with diorite and gabbro, and is sparsely distributed throughout the metamorphic rocks of the Benton Range. It also occurs locally south- east of Glass Mountain Ridge, in Rock Creek gorge, and on Wheeler Crest in the Sierra Nevada. The largest local concentration of tactite found in the quadrangle is at the Black Rock mine and in the adjoining claims (Morris Claims) to the south. Random testing of tactite specimens from beyond the Black Rock mine area with an ultraviolet light indicates • Production data furnished by the Wah Chang Mining Corpora- tion. Special Report 48 ^ s £ o3 :3 O Developed by five open pits and more than a mile of level workings, raises, winzes, and stopes. Substantial produc- tion. Three short adits, open pit, shallow inclined shaft; work- ings mostly caved. No pro- duction. 26-foot shaft, short adit, and several shallow pits. Minor production. 60-foot adit, two small open cuts, several shallow pits, and bulldozer scrapings. No pro- duction. ^^ a ^iz; ii a a a .2 a 2 0) a ^ TJ 111 a 3 -d a u § 'a "3 a CO a M a "3 J) cj > CD 3 Several small inclined shafts to a depth of about 150 feet on vein. Development consisted of about 1.000 feet of work- ings, but inasmuch as mine active only from 1870 to 1884, most workings caved. 700 feet of workings on an adit level; 2 raises, several stopes, and a winze. Portal caved almost shut, but adit open beyond portal in 1952. Numerous adits, most are now caved Main adit (May Brooks adit) crosscuts all three veins ; considerable drifting and stoping from this level. "o 6 E3i g oj B g g §2 = s ° Sf "-^ CO.S <0 -S i-S'S^ ■ - -S ?, ^ 4J a « 0) OS M :3 QJ i " " s 5^-3 -« ■3 -^ a lu CO £ .§ ^ >> '1 -5 -2 "s ■s-3|ig -a =« S— ^ a ,• ^ i3 i ■^S: 3-3 a Mo " b _ •^ a 0. & §"^ <= C:2 3 01 % . . .„m-g'S-a.a '^ • M "^ " .2.ii - --^ fl ^ ° .a =■ > O CB-'I 1 g 3 3 ga ^^ ^.s § a .So"." •r- . o3 -a 43 CD a .15 _f-" ~ t« -*^ 5 o o:a -s g"? § ^ Mg-3 -3 CO CO "^ 08 a ~ ■-3 > a . S 2 3 -g S3 ^ T3-^ 'C 03 1-3=3 a2 §§a8a O" :s a o a M_g 'i -S " o'^ a "?0 M 212-3 •" Gi O M S „ .3 S >- io 03 g |Z § S3 c3 rS ^ ^ §»^§ gill |a>. -3 o a a o a 1 a -i 3 a a) M a 3 a M a 3 H a S 1 3 H a -2 1 3 CD bO a 3 > O 3 o OJ o — ■ o S3-| "tJ' 3 g "o — ' O 1 1 £-"gi o3 i.&.S 1 11.2^^1 . a b a3 5°=--- xoi Q £ 2 .2 X ja . « eg CD Cl> -^ 3) 5 » a-^ ^3 .> O CO CD^ 3 -g -ts ca T3 a 0" < a -" -i >o 1 — ? (N t- a o cs a ^c .« > 0) . 0) - ri ^ > T3 o =» -^ ?^ g &• 22l2a-a Q a o •a 'J 03 og. ■§0 J, >> J 13 a 0) -r ^ a ->^ m a S; ■« M c ^ D M a a .a m $ •-3 a 3 .2 i-. . S3 CO Is" CB^'2-g CO r o aO S a «" « a3 O 03 03 -SO o ^^ ft .11 |i P5_° 1-5 a a CO pi . COo <2 . 03 CCS 4J a _, .a 3 03 r a H ° .-« 03 CO a «6 ..M to M . (N oS +i CD CO ^8 ^ 00 t* ^4 .0 CO 2 "t • u «t2 OJ CO <" 2 .-0 § "(2 -^ pi -so r c CD CC .5 CD . B CD CO - M 03 CD la 3 03 'a .a " .- CD H g Pi'^ CO -So r-c" ^ * g a< a^ oj - H-9 rt a CO to pj S3 cip; go" +:> O -".as O CDt^ CO •So ■£ o O "5 a<^ r ** CO a" P^l r 03 ^ o c30 0) to « o J »! 1 H oa w a 1 m 1 "a 3 2 '53 a < f.1 ca a 'u »— ( i m 6 ,^ £| o CO s — OJ o J rt ■3 '^ « u M13 n (u 'o rf =3 ■? CJ CI ?m OJ o c3 'S c3 S ■;3 -a '3 "C a a C >, r. " O D. aaOiJ „; . 03 43 ^ -a « 3 9 bC S d ■StS ^ if tin a ?. c3 a — ' ,^ n o a hJ ^ o -a o ^ -a o ;3 _D ■- ■-< « • _^ 4J '"' 4) o a 0) a; "1 S5 T3 T3 fe 3 C Q. 03 oj o. 04 ; v 0) ™ ^ "•«*-. _, Ml 5^ a S J'gS ■^ -O J^ "2 § S s "3 C3 Q 03 o a> ^, U.PI =«-a "S ^.o > OJ cs --- iJ "m a > 5 o " a"" » 2 c; i- -a ■g g 03 s ■» g .s rt s s; ■S S .s i _2 ^ S ^ ^6 «= > « fl "f I 2 '^ 3 W g^ • 03 IS lO o3 ffl ii .- C3 pes. 2 00 feet th sto nly crc (Tl .a M J3 CO (1) s 5 and ut wi g vei In 19 •^ " c • 3 S m ° " OS "* 0) g 03 ca !« O ^".^5 E: 03 T3 -^ .ti c a ►; d ta •JJ «- O OJ '^ ■e -S 3 M =a 3 f C: a CO g > M c3 J3 03 ° ■ • - N O . - ff _^ IS V '" s ■". .5 u kj :3 ja o '^ So V, Ct- 03 M bc a> ,' S S-- S ■is -isH i o t- o > g oj , ^ =3 .- a 5? 'o 'a o c3-a'2 -2 C 03 °5 O . ^ . • O c3 O W g _^- g o F O "2-^-3 •n c3 >> « GO r: -^ 2 fl ■-1 fl (- ^ 5:; e3 . ■t^ O oj J o S S rt -*^ to _3 3 to fl '3 '5 J3 . C " u .- O -^ 3 0) .-to . -I-] -3 ;3 =4 tu :^ £ i ^^ O to o a I ^g S .S M .3 ^ «*-. o rt Q 3 tu ^ •S .a to IN "O C -I "3 3 a -^ M > 03- c3 ^ 3 3 ■3W sS " o a '2 ^ o -a >> to '3 a t« g OJ QJ °3 ,« - »0 m u .a i'^ >^ 3 ta. O '3 H 03 3 +^ . « . 3 be ♦^ O OJ 'C N -a o [^ 10 ^ "3-2 a^ M S 2 ° 3 03 u 0; S s "3 O J3 o _ „ >> 3 -r J3 3 -43 t- O c3 o " e ■-:B & a S — -w 2 oj o o !S O o3 ° ^ o o3 iS t. w ' QJ O >^ C CU ^ St fl ■« .2i ! o oOl J cfl ^-' !; J « QJ .2 03 3^ t- o »2 e -° .3 .5 cs ^ ^ £ ■ ~ 3 J ■ M oj ;S ^:e 3 S =« ^^ 3 c3.2 I 0) U. — « is ^ a 03 c . ^ .« -•^ "o & s-a > to t3 !s .2 >> fl S =a j :2 ^- " -S «-° > 03 >'o C .J 13 a ° t^ 3 g J) 03 g .a _g oj fe . >> « ^ rJi .Si a to a 3 ° opmen a in 1876 floodec S82. Re 5 s *3 =5 <= " -a .2 c fe a =« tu -*^ ■0 o3 fl^O — 00 eve; begu mine in 1 ■g c3 t) 0" M .2 "^ ^ -§2 03 n fe "S ° aco dj e^ to ° CO 01 a >iw Q Q go M to" 03 ^ m j3 . a fl Wo CO aj . fl Pi 'a r;3 fl w 2 to "4i CO to 2 • =« S QO" 1^ +3 u a<; a; CO a I 03 " ...2 t- aim o "to CO 10 w r^ CO -Jl ti ■Jfl ro li. CO a H-S (N :a &0 WO •a S3 S ." ho « 3 uj J3 a d ja 03 Eh tl CO T3 oO ^ t^ a a CO o . IS co'oii "So H _5 to co" M *j a a< tu CO .So .0 • (30 ^^' hO « O H « I— I O w 02 10 SpEciAii Report 48 g 3.0 C3 5 o s Z m 0) OT M bl bl n M a a a a all •C.2 ^, S « ,a n ^1 o 0) a •=i^ gr S -a -2 =^ 43 CI fl u rf II o 1^- .■S3" -3 oS ,« c3 a> .2 i C3>2 « M-" S d 3 A ■» ? and he along and c epidot sugges to 3 .■§-3H •S-i ■s*«= s s til OTSo ^■3 to a »o ca 03 1> g .2 ° •S 8 o to S-q 3 ^ =s ft O OT o a »5 ja >», a m-a O 3 — ' 03 O u 3 (U a, 2 C 0.2 a »i .9 S^ MJ3-3 B g "S "ft •§ a a o 3 o o .1i to 'o .•s >. o-S ■S-9ii .23^ J3 O 03 1 1"! 2s II 12 ° So M^fl a a ^ M .2 a^ -^ -o > ^ :a o I'm C3H „ MH. 03 E-i ^ . a g J3 03 Sh " 03 o ^^ a ^ O [« -^ oj o ao c S- 03^^ 0) o *^ OJ OJ g g M S ° 9 Q. 3 S ■§^ : ^ H s-S r o io":S o CO 3 o ■ 09 r dJ « a '^ o u 3 to OJ H w ^ CO s< . i3 w 6 CC^ ^% -^ 5? co" d CO S S(5 dH Casa Diablo Mountains Quadrangle 11 ?lite is absent, or present in only small amounts. In 31ack Rock mine area, the tactite is locally seheelite- ing and no apparent structural or mineralogic dif- ice was found between scheelite-bearing and barren te. Tactite, though the host rock for scheelite, thus it uniformly scheelite-bearing. estions fop Tungsten Prospecting •ospecting for scheelite-bearing tactite in the quad- le should first consist of examining: marble outcrops particular attention to contacts with plutonic rocks. 3le not in contact with plutonic rocks, however, Id not be overlooked in view of the absence of ex- i plutonic rock at the Black Rock mine. Calcareous 5 are also found as thin discontinuous wall-rock rem- s (septa) along the contacts between granitic bodies, environment is particularly favorable for the de- )ment of tactite. Septa have not been found in the on Range, but they are significant in the Sierra ida. Calcareous rocks may also be present as inclu- I within granitic bodies. Diorite and gabbro bodies Id also be examined as they commonly contain cal- ms inclusions. Small bodies of diorite and gabbro le ridge east of Banner Springs are associated with 3S of calcareous rocks and both are enclosed in a )lex of porphyritic rhyolite dikes and sills. The ridge thickly mantled by porphyritic rhyolite float that necessary to Avatch carefully for small calcareous exposures, as these may be the only hint of much ;r, concealed bodies. < Rock Mine leographic Setting, History, and Production le Black Rock mine is on the dissected east slope of Benton Range 22 miles northwest of Bishop and 8 3 south of Benton, at an altitude of about 7,000 feet, ird-surface all-weather road connects the mine with I. Highway 6, which is 8 miles east of the mine. ; 2 shows the mine setting. heelite was first discovered in the mine area in 1917 L. E. Beauregard, but development was not under- 1 until 1928. Beauregard worked the property inter- 'ntly from 1928 until 1936 when it was purchased le Tungsten Corporation of California. The construc- of a 150-ton mill on the property was completed in mher 1937, and the mill was operated at about half city until the company went bankrupt in 1941. The erty then reverted to Beauregard who shipped a 1 tonnage of ore during the period 1942 to 1948. 1 production to 1948, inclusive, was about 83,000 In 1949 the property was purchased by the Tung- Corporation and about 2,000 tons was mined during -ear. The Black Rock Mining Corporation leased the erty in 1950 and operated it through 1954, although lame of the organization was changed in 1954 to the Chang Mining Corporation. The company mined )91 tons of ore to July 31, 1954, bringing the total uction to 272,591 tons having an over-all average e of approximately 0.5 percent WO3; data are in- iient to give a more precise grade. In 1953 the mine led 63,042 tons of ore that averaged 0.49 percent ;.* This was the second largest production of tung- information on production since 19.50 was furnished by Mr. J. Strutzel, Jr., General Manager of the Wah Chang- Mining srporation, and is published with his permission. sten in California and the sixth largest in the United States for that year. The following tabulation supplied by R. B. Maurer of the U. S. Bureau of Mines shows the nationwide rank of the Black Rock mine in 1953. 1. Tungsten Mining Corp., Hamme mine, North Carolina. 2. United States Vanadium Co., Pine Creek mine, Calif. 3. Nevada Massachusetts Co., Tungsten group, Nevada. 4. United States Vanadium Co., Riley mine, Nevada. 5. Getchell Mine Inc., Getchell mine, Nevada. 6. Black Rock Mining Corp., Black Rock mine, Calif. The accompanying map of the mine workings does not show the considerable amount of development work since September 1953. During the period 1951 to 1954 exploration involving diamond- and churn-drilling and underground work was performed, through the facility of the U. S. Gov- ernment's Defense Minerals Exploration Administra- tion, which resulted in the discovery of a substantial tonnage of ore. Workings The mine has been developed by means of large open cuts and more than a mile of underground workings. Most of the large-scale open cutting has been done by the Wah Chang Mining Corporation at four principal localities, designated on plate 3 as the Glory hole. Senior pit, No. 9 pit, and Junior pit. The open cuts are irregular in shape, and range in size from the Glory hole, which is 300 by 400 feet in plan, to the Junior pit, 100 to 150 feet in plan. Most of the underground work has been done on level C (pi. 4) which consists of 3,300 feet of level workings, 800 feet of accessible raises and stopes, and probably several hundred feet of raises, stopes, and level workings now inaccessible. Two sub- levels (E and F) are accessible from level C and com- prise a total of 650 feet of workings. An additional 2,300 feet of level workings is divided among 5 rather widely separated levels, A, B, D, H, and a small adit level. Rock Types Rocks exposed in the mine area include a layered sequence of fine-grained calc-hornfels, marble, and tac- tite with a total thickness of at least 400 feet, which is overlain stratigraphically by several thousand feet of quartz-sericite hornfels. The basal contact of the calcareous strata is not exposed. All the metamorphic rocks are folded into an asymmetric north-plunging anticline with a gently undulating crest. Tactite is com- mon throughout the mine area and is not restricted to the ore zones. Scheelite, the only tungsten-bearing min- eral present, is sporadically distributed through the tactite in the crestal part of the fold, in the gently dip- ping west limb, and locally in the east limb. The deposit is of the contact-metamorphic type al- though the contact of the calcareous rocks with the in- trusive granitic rocks is not exposed. The nearest expos- ure of granitic rock is about 600 feet west of the mine area. Tactite is the most abundant rock in the mine area. It is generally dark red brown to dark green or a mix- ture of these two colors. Most of the tactite is fine to medium grained, but locally it is coarse grained and somewhat vuggy. Nearly all of it is massive, but some of it is irregularly layered. The tactite is composed of 12 Special Report 48 varied amounts of brown to dark red-brown garnet, dark-green pyroxene of the diopside-hedenbergite series, dark-green amphibole, epidote, wollastonite, quartz, cal- cite, and locally pyrite, pyrrhotite, and scheelite. The taetite formed during thermal metamorphism chiefly by the addition of iron, magnesium, aluminum, and silicon to relatively pure marble. This conclusion is supported by the common occurrence of irregular-shaped masses of marble enclosed in taetite. Locally, however, taetite formed by the replacement of both quartz-sericite hornfels and calc-hornfels. This is demonstrated by (1) remnants of calc-hornfels and quartz-sericite hornfels which are enclosed in massive taetite, and (2) by vein- lets of taetite that are developed along fissures in the. calc-hornfels and quartz-sericite hornfels. Taetite that was formed by the replacement of marble contains gar- net and pyroxene that are considerably darker than the same minerals in taetite formed by the replacement of calc-hornfels and quartz-sericite hornfels. Seheelite is associated almost exclusively with the dark-colored tae- tite. All gradations exist between light- and dark-colored varieties, however, and it was not possible to map color differences in the field. Marble, exposed at several places in the area, is light gray, medium to coarse grained, and generally massive. It is most abundant in the southeast part of the area where it crops out almost continuously along a strike length of nearly 1,300 feet. Elsewhere in the area the marble is less extensive, commonly occurring as irregu- larly shaped blocks or lenses isolated within massive taetite. Calc-hornfels is interlayered with taetite and locally occurs as lenses within it. The rock is commonly gray to pale green, thinly laminated to massive, and fine grained. The most common minerals are pyroxene, epi- dote, clinozoisite, amphibole, plagioclase, garnet, quartz, and ealcite. Thin beds of quartz-sericite hornfels are locally interbedded with the calc-hornfels. The parent rock from which the calc-hornfels formed was probably impure limestone or dolomite which was interbedded locally with silty and shaly rocks. The massive, tan to gray quartz-sericite hornfels that stratigraphically overlies the taetite, marble, and calc- hornfels is fine grained, commonly spotted, and is com- posed of sericite, biotite, quartz, plagioclase, and anda- lusite. The spots or knots range in size from 1 to 5 mm across and are composed of extremely fine-grained aggre- gates of sericite, biotite, and, locally, corroded relics of andalusite crystals. This rock has been derived, by thermal metamorphism, from shale or siltstone. Dikes of porphyritie rhyolite and fine-grained diorite cut the metamorphic rocks in several places. Although the relative ages of the rhyolite and diorite dikes were not established by observation in the mine area, else- where in the quadrangle fine-grained diorite dikes cut dikes of porphyritie rhyolite. Structure Bedding is preserved in much of the calc-hornfels and locally in the sericite hornfels, but the taetite and much of the marble is massive. Secondary layering present locally in the taetite consists of thin alternating discon- tinuous layers of garnet and epidote which have devel- oped parallel to faults. The rocks are folded into an asymmetric north-pj ing anticline with a gently undulating crest, a \\ dipping west limb, and a steeply dipping east limlj nearly symmetrical outcrop of the calcareous layei [ 3) suggests a simple anticline with the trace of the ; plane coincident with a line that longitudinally 1 the outcrop. The actual trace of the axial plane, ho-\ is 130 to 200 feet west of this line. Two possible int' i tations of the structure are shown on plate 5, A e In the cross section shown in plate 5, A, a n fault or fault zone dipping steeply east and downtl on the west is inferred to account for the anon I position of the trace of the axial plane of the ant! The minor, post-mineral faults in the area of the ini' fault strike dominantly north to slightly west of and dip steeply to the east. The inferred fault is c parallel to these minor faults on the assumption they may reflect earlier major faulting. The ext< replacement that has undoubtedly taken place i: mine area would certainly tend to obscure pre-mi faults. Possibly, pre-mineral faults provided acces replacement solutions. The interpretation of the structure in plate shows a minor, tight, overturned syncline west o crest of the major anticline. In comparing the t'w terpretations of the structure it is apparent that interpretations accomplish the same ends, and be the syncline shown in plate 5, B, is necessarily isoc the question of which interpretation is correct be( academic, especially as little or no ore occurs along ( the inferred fault or trace of the inferred synclinal plane. Minor folds in the gently dipping west limb o major structure are particularly well shown in the part of level C, where reversals in the dip of gentlj ping beds occur laterally within a few feet. Minor are also visible in outcrops along the roads in the s west part of the map area. Bedding attitudes in tl cinity of the Glory hole, and in the west part of le-\ indicate the presence of a secondary anticline plui north 15 to 30 degrees, although it is not sufficiently exposed to determine its exact configuration. Many post-mineral faults with small displacemeni the rocks in the mine area; the largest amount oj placement recognized is in No. 9 pit where a bed o: tite has been offset about 10 feet. Fault breccia is ex] at several places but generally can be traced for short distances. A marble breccia exposed in the nort most part of level C is of uncertain origin. The bt consists of large, angular blocks of marble in a matrix and may be collapse breccia caused by gn water solution. Similar marble breccia is associated marble and cave-fill material along the south side c Glory hole. The faults shown on plates 3 and 4 formed afte taetite, although some probably reflect recurrent i ment along older faults. Evidence of recent move along older faults is shown by some of the thin grained diorite dikes, which were probably inti along older faults and have been slickensided by recent movement since the emplacement of the dik< i Casa Diablo Mountains Quadrangle 13 > Deposits tite, the host rock of the ore bodies, is widely dis- ;d in the mine area in elongate layers that probably rt reflect original bedding (pi. 3). Three thick exposed along the steeply dipping east flank of iticline may be in part correlative with the two layers of the crestal zone and the gently dipping imb of the anticline. Scheelite, the only tungsten- g mineral known to occur at the mine, is sporad- loealized in the tactite. Typically it is disseminated mte crystals a fraction of a millimeter in maxi- iimension, although grains as large as 6 or 8 milli- I across may be found locally. Insofar as was lined, the distribution of scheelite in the tactite iom and bears no recognized relation to local fea- )f the tactite, such as grain size, planar structure, al mineralogic change. Scheelite in quantities of ble grade (0.5 percent "WO3 or greater) is for the )art restricted to the crest and gently dipping west if the fold (pis. 3, 4, 5), and in this feature the t is believed to be unique among contaet-metamor- ungsten deposits. Most of the ore averages about rcent "WO3, but local occurrences containing as as 2 percent WO3 were reported by the operators.* olybdenum content of the ore is negligible. On sur- xposures (pi. 3) the grade was estimated from nation in ultraviolet light ; underground the grade id largely on company assay data. bodies have been exploited at six major localities : ory hole. Senior pit, No. 9 pit, Junior pit, level C, vel B. Ore bodies at all localities excepting Junior ay be in the same relative stratigraphic position gh only two, the Glory hole and level C ore bodies, us far known to be continuous. Except at Junior I the ore bodies are in the crest or west limb of the ne. Beyond the suggestion that they are parallel to yering, little is known regarding the shane of h\- lal ore bodies. The Junior pit ore body is in the y dipping east limb of the fold, and its strati- ic position relative to the other ore bodies is un- a. largest surface exposures of ore have been ex- i at the Glory hole. Senior, and No. 9 pits. The dy cut by the Glory hole is almost completely ob- l by dump material. Data obtained from recent nd drilling and from old company maps indicate he lower contact of the mineralized bed is only a ;et below and parallel to the floor of the pit. The at least 30 feet thick, but it is not uniformly ore ig and contains considerable amounts of marble, lior pit the ore body is exposed at the south end of t over a strike length of 50 feet and in a nearly al face 70 feet high. In No. 9 pit the ore body is t thick and is exposed over a strike length of about set. Of six vertical-surface borings drilled near the pits in order to explore the down-dip extent of the aly one intersected ore. This hole, located 100 feet )f Senior pit, cut 16 feet of ore containing about rcent WOa 170 feet below the surface. Since map- was completed in September 1953, a crosscut has driven to intercept the ore cut by this hole, and Cliang Mining Corporation. Published with permission of ers. the company geologist reports that an ore body was encountered in the crosscut. Another surface outcrop of ore, smaller than those described above, is in the steeply dipping east limb of the anticline and has been exploited at Junior pit. Ore exposed in the pit is submarginal, but ore exposed im- mediately west of the pit appears to be of mineable grade. The maximum depth of this ore body is probably not great as two diamond-drill holes, which cut tactite at 50 and 100 feet below the surface about 100 feet south of the pit, reveal no mineable ore. The most extensive underground mining has been done from level C (pi. 4). The ore in the Avesternmost part of the level is probably along the same bed as that exposed in the Glory hole. The underlying bed of ealc-hornfels can be traced from a point 80 feet above and 50 feet west of the south end of level H, where it was inter- sected by a drill hole, to level D, a few feet below the floor of the Glory hole. Other ore bodies exposed in level C and on level F at the bottom of a winze sunk from the westernmost part of level C are probably from strati- graphically lower beds. Many of the old stopes and raises from level C are now inaccessible, but old maps indicate that they are in scheelite-bearing tactite. Most of the raises and stopes extend 20 to 60 feet above level C and one extends to the Glory hole. A nearly horizontal ore-bearing bed of tactite is ex- posed in level B. The thickness of the bed is not known, but the upper contact with a layer of calc-hornfels is exposed at several places in the back. The calc-hornfels layer is probably rather thin, as tactite crops out on the surface a few tens of feet above the level. The lateral extent of the ore body is unexplored except for several vertical churn-drill holes about 150 feet west of the level. Although traces of scheelite were found in the cuttings, none of the material is of mineable grade. Most of the accessible part of level D is in ealc-horn- fels, probably the same bed as that exposed in the west part of level C. Old maps of level D show more than 300 feet of workings northwest of those shown on plate 4, most of which are in scheelite-bearing tactite. The only ore in level H is in the small raise near the south end of the level in which ore is exposed across a 5-foot face. Since the completion of mapping, however, several hundred tons of ore have been mined from raises and stopes driven northwest and southeast near the south end of the level. Lithology is the chief ore control at the mine, as ore bodies are restricted to the dark-colored tactite, which presumably formed by the additive metamorphism of relatively pure limestone. A control of nearly equal importance, however, is the folded structure of the rocks, which restricted the locali- zation of the major ore bodies to the crest and gently dipping west limb of the fold. The only exception to this structural environment is the ore body at Junior pit, which lies just east of the crest in the steeply dipping east limb. Data available at present indicate that ore bodies that extend down the west limb of the anticline are restricted to the Glory hole and Senior pit ore bodies. Drilling west of both No. 9 pit and level B has failed to reveal ore, but drilling has not been extensive enough to thoroughly explore these areas. 14 Special Report 48 II8*45' 37 "45 37° 30' II9°45 Dip 30 33 veins EXPLANATION Calcareous metomorphic roc Hornfels and schist Granitic rocks V|T3VH| Diorite-gabbro Strike and dip of quartz vein' Strike of vertical quartz vein Strike of quartz veins; dip unk Contact I Limit of reasonable exfrapola! bedrock beneath Tertiary on Quoternary rocks 4 Miles Figure 2. Quartz veins in the Casa Diablo Mountain quadrangle \ Casa Diablo Mountains Quadrangle 15 Note; Scotfered outcrops of tiornfels and dike rock not shown COMPOSITE MAP ''''':<'^vpm^ 6700 LEVEL EXPLANATION Sierra Vista or West "Vein" Breccia ond gouge containing fragments of porphyritic rhyoiite, hornfels, marble, and quartz. E z^^^^se'so-'' iV ^ ^,j^J?^:I>/,EVEL ^^^ X- ><678 5 LEVEL ';'ri' _^-^<=^"^ ETi^iiifjr •,>^ Dump B Shaft ot surface ISI Foot of raise or winze Head of roise or winze logy ond topogrophy by: Rinehort and D.C.Ross, October 1952 Rinehart and H.ttStoger, October 1953 Caved or filled workings 200 Contour infervol 50 feet Datum is approximately mean sea level Figure 3. Geologic map and section of the Sierra "Vista mine 16 Special Report 48 The data are not sufficient to permit an explanation for the localization of ore bodies within the favorable structural environment described above. No relation between the post-mineral faults and the ore bodies was recognized, although evidence of important pre-mineral faulting could well be obscured by the effects of additive metamorphism, and the post-mineral faults may bear little relationship to the distribution of this earlier fault- ing. Local differences in the permeability or composition of the parent rock may also have been important factors in the localization of ore. Future Exploration Most of the major ore bodies are in the crest of the anticline with two (the Senior pit and Glory hole ore bodies) extending down the west limb about 100 and 500 feet respectively. Hence, exploring the crest at depth very likely presents the best possibility for the discovery of new ore bodies. The exploration of the crest probably should include further exploration of the tactite beds now being mined, as well as exploration for possible tactite beds stratigraphically below those now exposed in the workings. Exploration of the west limb probably should not be undertaken until after the crest has been explored. A series of vertical core-drill holes spaced along the crest from No. 9 pit approximately to coordi- nates 2800 N-1600 E and drilled to the altitude of level H should adequately test this ground. The unexplored crest below level C could be explored by drilling several steep to vertical holes from this level. As ore encountered by drilling could be readily mined by extending level H, it is suggested that the ground closest to level H be explored first. In general, the outlook for continued successful mining is good. Mineable ore is exposed at many places through- out the mine, and a considerable amount of favorable ground remains to be explored. Logging - EXPLANATION Andolusite hornfels Micoceous hornfels Colc-hornfels and marble Quartz vein, showing dip (locally sulphide-bearing) Fault, showing dip Breccio zone 70y' Strike and dip of beds Geology by- CD Rinehort D.C.Ross October, 195? Figure 4. Geologic map, adit level, of the Gold Wedge mine Gold and Silver Most of the known gold and silver deposits i' quadrangle are of the fissure-vein type and occur area about 6 miles wide and 12 miles long in th tonic and metamorphic terrane of the Benton Rai the eastern half of the quadrangle (fig. 2). A few have been exploited west of the Benton Range p in Wildrose Canyon, and west of Banner Sprinj most of the veins the gangue material is quartz, 1 some it is fault breccia and gouge, with no apparen quartz. Most veins are 2 to 3 feet thick, but they from thin stringers to massive quartz veins as mi 20 feet thick. Most veins strike north to northwes dip 40 to 70 degrees west. • The most common mineral assemblage is pyrite, 1' ite, and galena in quartz with some free gold as ■« gold and silver contained in the sulfides. Ang sphalerite, chalcopyrite, azurite, malachite, and cl colla were also identified by the writers, and arg< pyrargyrite, cerussite, and tetrahedrite were rei by other workers (Tucker, 1927; Sampson and Ti 1940). Data on the grade of the deposits are few, bi cited reports indicate that the best ore contained '. ounces of gold and 25 to 50 ounces of silver per toi richest ore shoot in the Lone Star mine averaged 1' cent in lead that contained 26 ounces of silver pe and a 300-ton shipment of ore from the Long C mine averaged 1.85 ounces of gold per ton. These ■< represent extreme maxima for the district and not age grade. Development of the steeply dipping veins consisi shaft sinking and driving of drifts from the she suitable levels, or crosscutting from the hillside to { and then drifting, stoping the vein where it eont sulfides. Inaccessible workings limited the mapping of ni' the mines, but the Sierra Vista mine and part c Gold Wedge mine were mapped. The Sierra Vista (fig. 3) is an example of a gold-silver deposit in a zone with only subordinate vein quartz. The map ( longest level of the Gold Wedge mine (fig. 4) sh( typical quartz-vein deposit. The total production from the gold and silver i in the quadrangle has probably been less than $1,001 Individual production records for most of the min( unavailable, but the Tower mine is said to have yi about $150,000 worth of ore and the Wildrose about $100,000 (Sampson and Tucker, 1940). The Crown, Casa Diablo, and Sierra Vista mines pro yielded as much or more, and the Banner, Bed Lone Star, Gold Wedge, and Long Chance have pro yielded somewhat less. The other properties in the - rangle have had minor production. Iron A small deposit of magnetite and hematite occi the northeast part of the quadrangle, northeast c Black Rock mine. The prospect was explored by a adit and moderate scraping with a bulldozer, but has been no production, and no sample data are able. The deposit consists of a magnetite- and hem rich layer 70 feet long and 50 feet wide along a striking N. 50° E. and dipping southwest 75 dC; I Casa Diablo Mountains Quadrangle 17 1 cuts schist and hornfels. Ore is also exposed in ict of a short adit which cuts 22 feet of ore 60 feet ' the surface outcrop. The metalliferous layer is sed in an alteration zone 400 feet long and 200 feet which consists of irregular blocks of country rock matrix of gouge and talc. Some of the magnetite hematite has apparently replaced fragments of ry rock, but, in places, epidote-garnet tactite is ayered with the iron-bearing minerals suggesting the host rock was in part calcareous and that the lit is contact metamorphic in origin. Industrial Minerals Qmetallic materials in the quadrangle have been ex- ;d but little. Pumice is quarried and made into ing blocks at the Delta Placer mine on Whiskey I just south of U. S. Highway 395. Additional pum- larries and prospects are present in the eastern part e quadrangle, but production has been minor and ere idle in 1954. The pumice east of Long Valley Lonly contains fragments of light-colored glass as as 4 inches across and numerous small pellets of ian; therefore, it is probably not commercially val- , while clean pumice is available nearby. Expan- rhyolite is present in unknown amounts in the id volcanic country northwest of Wilfred Canyon i northwest corner of the quadrangle, but explora- to determine the grade and extent of the deposits ot been attempted because of the inaccessibility of rea. Sand and gravel are used locally for construc- material and road metal; the largest pit is in the alluvial fan of Rock Creek east of Toms Place. Pumice, sand, and gravel can all be produced in greater quantity from the quadrangle, but the distance to population cen- ters, and the resulting high cost of shipping these mate- rials, precludes much development under present conditions. REFERENCES Boalich, E. S., 1922, Benton district and Casa Diablo district, Mono County, California : California Min. Bur. 18th Kept., p. 418-419. Gilbert, C. M., 1938, Welded tuff in eastern California : Geol. Soc. America Bull., v. 49, p. 1829-1862. Gilbert, C. M., 1941, Late Tertiary geology southeast of Mono Lake, California: Geol. Soc. America Bull., v. 52, p. 781-816. Hess, F. L., and Larsen, E. S., 1920, Contact metamorphic tung- sten deposits of the U. S. : U. S. Geol. Survey Bull. 725, p. 245- 309. Lemmon, D. M., 1941, Tungsten deposits of the Benton Range, Mono County, California: U. S. Geol. Survey Bull. 922-S, p. 581-593. Putnam, W. C, 1952, Origin of Rock Creek and Owens River Gorge, California [abs.] : Geol. Soc. America Bull., v. 63, p. 1291-1292. Ransome, A. L., 1940, General geology and ores of the Blind Spring Hill mining district. Mono County, California : California Jour. Mines and Geology, v. 36, p. 159-197. Sampson, R. J., and Tucker, W. B., 1940, Mineral resources of Mono County, California : California Jour. Mines and Geology, V. 36, p. 116-157. Tucker, W. B., 1927, Mono County, California: California Min. Bur. 23rd Rept., p. 374-406. Tucker, W. B., 1934, Last Chance mine, and Lone Star mine, Mono County, California : California Jour. Mines and Geology, vol. 30, p. 319. Whiting, H. A., 1888, Benton mining district, Mono County, Cali- fornia : California Min. Bur. Rept. 8, p. 376-382, r 5-56 2M printed in California state phinting office STATE OF CALIFORNIA DEPARTMENT OF NATURAL RESOURCES )VGofd C own ' r. J ^ ^ EXPLANATION SEDIMENTARY ROCKS West-Towe Tl 8 ocJ< Roc^'^ Mi I i , •*y ^=~viJ^ -- .- t'^j ' „ A Wpb ,Au A, Pb Cu ; ^ ,_ i „Au A5 Pb Cu I J UUc Soulh Lunil.iiM ^•yW ^ Vend / r^ , Placer L-^^ "■ ««n» ["uicctio., 10?J NBIh »rr.c"(on ECONOMIC AND GKOLOCIC MAP OK THK CASA DIABLO MTN. QUADRANGLE. CALIFORNIA L By C. D. Rinehart and D. C. Ross '7§ '• ■— '-— SCALE 1 6Z500 Includes numerous bodies of slightly different ofM ranging in composition from granite lo quartz diorile. Locally Ihe gramlie rocfca contain calcareous melugions including tactile. Gabbro and dioriie Darlt-ealoTed roda of tarioble teiturc that commonly contain calcareous inch- tioru, including laclile. METAMORPHIC ROCKS '" ene, tpidott, molybdeuile. Tactile Rtd-brown to dark-green rock compoud chwfly of garnet, pyroi amMbole, and quarts; loeatty contains sehtetite, powellite, and pyrite. a- n. marble iot separable O o •i Minor lenses of lactilt locally Marble neluded in the •' < Calc-li. rnfHl Included in this iiml are areas on the scale of Ihe map Jma-bU.eak-h ■rnjils andtactife < ^ ^.y.m,w Sehiat and homfels Chiefly mica-quarti, and andalumtt-quarts mela.icdimentary rocks amounts of marble, ealc-honifcU, and laclile. Contact Dashed where approximalely located. >- Strike and dip of gently dipping beds (as much as 46°) >-~ Strike and dip of steeply dipping beds imore than 46°J >^ Strike of vertical beds C/idoco ■/> Pb, Au Pit; sg, sand and gravel; pu, pumice XFe Prospect, locally showing principal metals (•: Porphyritic rhyolite }Tr.^ Quortz- sericite hornfels SYMBOLS Contact, dashed where approximate or inferred Inferred foult zone Q Mine workings ALTERNATE INTERPRETATIONS OF STRUCTURE ALONG SECTION A-A' BLACK ROCK MINE, MONO COUNTY, CALIFORNIA