Favorable areas for prospecting adjacent to the Roberts Mountains thrust in southern Lander County, Nevada

GEOLOGICAL SURVEY CIRCULAR 563

Prepared in cooperation with the Nevada Bureau of Mines

C I R C U L A R 563

Prepared in cooperation with

Geological Survey

William T. Pecora, Director Abstract- - - - - - - - - - - - - - - - - - - - - - - - - - - - - Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - Ravenswood window- - - - - - - - - - - - - - - - - - - - - - - Callaghan window - - - - - - - - - - - - - - - - - - - - - - - - Toiyabe Range south of Austin - - - - - - - - - - - - - - - - - Petes Canyon window - - - - - - - - - - - - - - - - - - - - - - Suggestions for prospecting - - - - - - - - - - - - - - - - - - References cited - - - - - - - - - - - - - - - - - - - - - - - -

2. Geologic map of Ravenswood window - - - - - - - -
3. Geologic map of Toiyabe Range south of Austin - -
4. Geologic map of Petes Canyon window - - - - - - -

closely adjacent parts of Eureka and Nye Counties - - - - - - - - - - - - - - - - - - - - - - CONTENTS

Page The recent discoveries of large open-pit gold deposits at Carlin and Cortez in nonh-central Nevada (Hardie, 1966; Roberts, 1966; Roberts and others, 1967; Erickson and others, 1966; Wells and others, 1967) have created interest in prospecting for similar de p o s i t s in nearby areas. During the last 2 years, mapping by the U.S. Geological Survey of more than 2,500 square miles of a largely unstudied part of north-central Nevada directly south and southwest ofCortez has outlined areas with a stratigraphic and structural setting similar to that of the areas con t a i n i n g the Carlin and Cortez deposits. These favorable geologic factors indicate that further prospecting is warranted in these areas.

Most of the metallic deposits in sou the r n Lander County (table 1) and in north-central Nevada occur in pre-Tertiary sedimentary rocks. Many of these deposits are in carbonate strata exposed in windows in the Roberts Mountains thrust, a widespread fault along which siliceous and v o 1c ani c (western) assemblage rocks have been thrust eastward, perhaps as much as 100 miles, over carbonate (eastern) or transitional assemblage rocks (Roberts and others, 1958; Gilluly and Gates, 1965; Roberts, 1960, 1966). The new gold discoveries at Carlin and Cortez are both in such windows and lie below the thrust (Hardie, 1966; Erickson

Adiacent to the Roberts Mountains Thrust

in Southern Lande.r County, Nevada

By John H. Stewart and Edwin H. McKee

INTRODUCTION and others, 1966). In addition, both de p o s i t s occur mainly in altered or silicified platy limestone of the Silurian Roberts Mountains Formation. The ore bodies of the Bootstrap and Number Eight mines (Roberts, 1966; Erickson and others, 1966) near the Carlin mine are in breccias along the Roberts Mountains thrust itself as are the gold deposits at Gold Acr~~s (Gilluly and Gates, 1965, p. 134), about 8 miles northwest of the Cortez deposits.

The Roberts Mountains thrust and the Roberts Mountains Formation in the lower plate of this thrust seem, therefore, to be particularly favorable places for metallic ore deposits in north-central Nevad"' .. We describe here four areas within southern Lanrier County where the Roberts Mountains Formation crnps out below the thrust. Granitic plutons that might have been a source for mineralizing solutions crop out in two of these areas and are within a few miles of the two remaining areas.

Maps of three of these areas are given in this report; a map of the fourth has already been published (Stewart and Palmer, 1967). Detailed geolorfc maps of most of southern Lander County at a scale of I :62,500 are also available on open file at the U.S. Geological Survey at Washington, D. C., Denver, Colo., Salt Lake City, Utah, and San Francisco, Los Angeles, and Menlo Park, Calif., and material from which copy cr'll be made at private expense is available at the Neva<'\a Bureau of Mines, Reno, Nev. An index totheseopen-file maps is shown in figure 1.

The work on which this report is based is pan of the county mapping program of the U.S. Geological Survey in cooperation with the Nevada Bureau of Mines. We wish to thank R. H. Washburn of Juniata CoU~ge, Huntingdon, Pa.; D. B. MacLachlan, BureauofTooographic and Geological Survey, Harrisburg, P a.; and H. F. Bonham, Nevada Bureau of Mines, Reno, N~v., for use of their unpublished information.

The Ravenswood window (fig. 2) is in the Shoshone Range about 15 miles northwest of Austin. In this window Cambrian, Ordovician, and Silurian q u art zit e, limestone, and shale are exposed in the lowerplate of the Roberts Mountains thrust, and, nearby, Ordovician and Silurian chert, shale, sandstone, and quartzite are exposed in the upper plate. The Roberts Mountains Formation crops out in two small areas in the northern part of the window and in a fairly large area directly below the Roberts Mountains thrust in the southern part of the window. The strata of tl',~ lower plate have been intruded by porphyritic quartz monzonite that crops out over an area of about 2 square miles in the central part of the window. Tertiar_,, welded tuffs cap much of the higher part of the area, and Tertiary and Quaternary sedimentary strata and older alluvium occur both on the west and northeast partf' of the range.

The old Ravenswood mining district, ''rhich has produced only a small amount of ore, is in the northern part of the window. The deposits (Hill, 1916, p. 106- 113) consist of small lenslike quartz ,.~ins that are predominantly in shale, quartzite, and limestone of Early Cambrian age. Many prospect pits in the district show abundant secondary copper minerals. The deposits, according to Hill (1916, p. 109), contain chalcopyrite, galena, and tetrahedrite, all of which are said to carry silver and a little gold. Some prospects are in the southern part of the window, a few of them along the thrust where the Ordovician Valmy Formation is thrust over the Silurian Roberts Mountains Formation.

CALLAGHAN WINDOW The Callaghan window is in the Toiyabe Range about 15 miles north of Austin. Its geology has been described in detail previously (Stewart and Palmer, 1967) and is only briefly considered here. Lower plate strata are exposed in an area of about 40 square miles and consist of over 10,000 feet of quartzite, limestone, and shale of Cambrian, Ordovician, and Silurian age. The Roberts Mountains Formation crops out in the southern and northwestern parts of the window. Strata above the thrust consist of chert and quartzite of Ordovician and Devonian age. The strata in the window are unmetamorphosed and no granitic rocks crop out in the window. The quartz monzonite of the Austin pluton lies 3 miles south of the southern limit of the window, and granitic rocks also crop out in a small area about 3 miles north of the northern limit of the window and in a small area about 7 miles east of the eastern limit of the window. Except for a quicksilver prospect (Bailey and Phoenix, 1944, p. 113), no metallic mineral deposits have been reported.

TOIYABE RANGE SOUTH OF AUSTIN Lower plate strata ranging in age from Cambrian to Devonian are extensively exposed in the Toiyabe Range south of Austin (fig. 3). These strata have been intruded by several plutons, the largest of which contains the rich silver veins of the Reese River district at Austin.

Three thrust plates are recognized in the Toiyabe Range south of Austin. The lower and middle plates are separated by the Kingston Canyon thrust, a fault at present recognized only in this part of the Toiyabe Range; the middle and upper plates are separated by the Roberts Mountains thrust, which is of regional extent. The strata in the lower two plates lithologically resemble each other more than they resemble the strata of the upper plate, but they are not entirely the same. The most conspicuous difference is in strata of Middle Cambrian to Early Ordovician age, which are mostly black phyllite and argillaceous limestone in the lower plate and mostly light-gray limestone in the middle plate. The juxtaposition of these unlike facies suggests that considerable lateral transport has occurred along the Kingston Canyon thrust. Strata above the Roberts Mountains thrust in the Toiyabe Range consist of western assemblage black chert, shale, and quartzite assigned to the Valmy Formation of Ordovician age.

Five mining districts are 1ocate d in the Toiyabe Range from Austin on the north to the vicinity of the Lander-Nye County line on the south. The largest of these is the Reese River silver district at Austin, which has a recorded production of over $18 million. In the Birch Creek district along the east side of the range about 9 miles southeast of Austin, depos~ts of gold, silver, and lead occur in granitic rocks along the southern margin of the Austin pluton and in flanking metamorphosed lower Paleozoic strata. Beryllium, tungsten, arsenic, and molybdenum are also tnown from this district (table 1). The Big Creek distr~.ct is in the western half of the range from 7 to 12 miles southwest of Austin. Here antimony deposits occur in the dark shale and chert above the Roberts Mountains thrust, in the carbonate strata below the thrust, anclin a silicified fault breccia along the thrust. In the Kingston district in the eastern part of the range abo·1t 20 miles south of Austin, gold and silver have been produced from veins in lower Paleozoic rocks. The Washington district, which is mostly in Nye County, has produced silver and lead ores from veins cutting lower Paleozoic rocks. Some of these deposits occur close to the Aiken Creek and Vindicator plutons (fig. 3). In adiition, uranium has been mined from deposits in both intrusive rocks and metamorphosed Lower Cambria'i' sedimentary rocks along the southern edge of the Austin pluton on the west side of the range 3 miles south of Austin (Davis, 1954; Sharp and Hetland, 1954; Thurlow, 1956; Nye, 1958).

The Petes Canyon window in the Roberts Mountains thrust is in the Toquima Range about 20 miles southeast of Austin (fig. 4). This window covers about 20 square miles and exposes Ordovician a'1d Silurian carbonate strata; the Roberts Mountains Formation is the most widely exposed formation in the lower plate and occurs directly below the thrust. Upper plate strata consist of shale, quartzite, and chert of the Vinini Formation. No granitic intrusive rocks cccur in the window, but granitic rocks crop out ab')ut 5 miles southwest of the southern part of the window, and a small outcrop of granitic rock is about 4 miles northwest of the northwestern part of the window. No metallic mineral deposits have been reported fr')m the window, although tungsten has been mined in tactite adjacent to the previously mentioned small exposure of granitic rock northwest of the window. SUGGESTIONS FOR PROSPECTING Since the discovery of t11.e rich silver veins at Austin in 1862, southern Lander County has attracted the attention of prospectors. The numerous mines and prospects in the region attest to the thorough search for surface outcrops of readily recognizable mineralbearing veins and lodes. The possibility of finding a large mineral deposit by using the same methods as previous prospectors seems remote, and the search for metals in the region should be focused on new prospecting techniques and deep exploration by drilling.

Deposits of fine- grained gold s i m il a r to tho s e recently found at Carlin and Cortez, however, may have gone undetected in southern Lander County, in spite of extensive earlier prospecting, because exposures of such deposits show little, if any, metallic minerals detectable to the naked eye and the gold is not easily recognizable using ordinary prospecting methods. Exploration for obscure deposits of this type requires extensive sampling and abundant chemical data. The development of rapid and inexpensive methods for determining trace amounts of gold, silver, and mercury (Lakin and Nakagawa, 1965; Huffman andothers, 1967; Ward and others, 1963; Vaughn, 1967) has provided the tools for conducting this type of prospecting economically.

As outlined here, several areas in southern Lander County have a structural and stratigraphic setting similar to the areas where gold has been f o u n d at Carlin and Cortez; that is, carbonate rocks, including the Roberts Mountains Formation, are exposed below the Roberts Mountains thrust. The areas considered favorable for prospecting in southern Lander County include the Ravenswood window in the Shoshone Range northwest of Austin, the C a 11 a g h an window in the Toiyabe Range north of Austin, the Petes Canyon window in the Toquima Range southeast of Austin, and extensive exposures of lower plate rocks in the Toiyabe Range south of Austin.

Prospecting in the region could also be undertaken by drilling. The most likely possibility of finding hidden deposits seems to be by drilling through upper plate siliceous and volcanic rocks that contain ore deposits into lower plate carbonate rocks that are more favorable hosts for ore deposits; small deposits in upper plate rocks could indicate larger deposits at depth. Within the region, deposits occur in upper plate rocks in the Skookum district, 7 miles northwest of Austin, and along the north side of the Austin pluton, 4 miles north of Austin within the Reese River district; in the latter area, however, some supposed upper plate rocks may instead by assignable to the Cambrian Gold Hill Formation in the lower plate. These areas seem favorable for deposits at depth, but no information is available about the depth to the lower plate carbonate rocks. A further possibility for drilling is along the west flank of the granitic pluton in the Ravenswood wirdownorthwest of Austin (fig. 2), where carbonate rocks could occur adjacent to granitic rocks under a rresumably thin cover of Tertiary and Quaternary sedimentary rocks.

McCarthy, J. H., Jr., and Leong, K. W., 1966, Gold geochemical anomaly in the Cortez district, Nevada: U.S. Geol. Survey Circ. 534, 9 p. Ferguson, H. G., Muller, S. W., and Rob~rts, R. J., 1951, Geology of the Mount Moses qucdrangle, Nevada: U.S. Geol. Survey Geol. Quad. Map GQ-12, scale 1:125,000. Gilluly, James, and Gates, Olcott, 1965, Tectonic and igneous geology of the northern Shostone Range, Nevada, with sections on Gravity in Crescent Valley, ~ Donald Plouff, and Economic geo1og:y, ~ K. B. Ketner: U.S. Geol. Survey Prof. Paper 4~5, 153 p. Gilluly, James, andMasursky, Harold, 196~" Geology of the Cortez quadrangle, Nevada, with B' section on Gravity and aeromagnetic surveys~ r. R. Mabey: U.S. Geol. Survey Bull. 1175, 117 p. Hall, Robert, 1962, Sampling of Lynch Creek beryllium-tungsten prospect, Lander Count~·. Nevada: U.S. Bur. Mines Rept. Inv. 6118, 10 p. Hansen, H. J., 3d, 1960, Geo1ogyofthe Big Creek area, Toiyabe Range, Lander County, Nevada~ New York, Columbia Univ., M.A. thesis. Hardie, B. S., 1966, Carlin gold mine, Lrm district, Nevada: Nevada Bur. Mines Rept. 13, p•. A, p. 73- 83. Hill, J. M., 1915, Some mining districts in northeastem California and northwestern Nevadr~ U.S. Geol. Survey Bull. 594, 200 p. _____,1916, Notes on some mining districts in eastern Nevada: U.S. Geol. Survey Bull. 648, 214 p. Huffman, Claude, Jr., Mensik, J. 0., and Riley, L. B., 1967, Determination of gold in geologic materials by solvent extraction and atomic-absorption spectrometry: U.S. Geol. Survey Circ. 544, 6 p. Kay, Marshall, and Crawford, J. P., 1964, Paleozoic facies from the miogeosynclinal to the eugeosynclinal belts in thrust slices, central Nevada: Geol. Soc. America Bull., v. 75, no. 5, p. 425-454. Kral, V. E., 1951, Mineral resources of Nye County, Nevada: Nevada Bur. Mines Bull., v. 45, no. 3, Geology and Mining ser. 50, 223 p. Lakin, H. W., and Nakagawa, H. M., 1965, A spectrophotometric method for the determination of traces of gold in geologic materials, in Geological Survey research 1965: U.S. Geol. Survey Prof. Paper 525-C, p. C168-Cl71. Lawrence, E. F., 1963, Antimony deposits of Nevada:

Nevada Bur. Mines Bull. 61, 248 p. Lincoln, F. C., 1923, Mining districts and mineral resources of Nevada: Reno, Nevada Newsletter Publishing Co., 295 p. Lowell, J. D., 1958, Lower and Middle Ordovician stratigraphy in eastern and central Nevada: New York, Columbia Univ., Ph. D. thesis. McKee, E. H., 1968a, Geologic map of Ackerman Canyon quadrangle, Nevada: U.S. Geol. Survey openfile map. -~-_1968b, Geologic map of southwestern part of Lander County, Nevada: U.S. Geol. Survey openfile map. Means, W. D., 1962, Structure and stratigraphy in the central Toiyabe Range, Nevada: California Univ. Pubs. Geol. Sci., v. 42, no. 2, p. 71-110. Muller, S. W., Ferguson, H. G., and Roberts, R. J., 1951, Geology of the Mount Tobin quadrangle: U.S. Geol. Survey Geol. Quad. Map GQ-7, scale 1:125,000. Nye, T. S., 1958, Geology of the Apex uranium mine, Lander County, Nevada: Berkeley, California Univ., M.S. the~is, map scale 1:9,600. Roberts, R. J., 1960, Alinement of mining districts in north-central Nevada, in Short papers in the geological sciences: U.S. GeoLSurveyProf. Paper400-B, p. Bl7-B19. . --:--~1966, Metallogenic provinces and mineral belts in Nevada: Nevada Bur. Mines Rept. 13, pt. A, p. 47-72. Roberts, R. J., Hotz, P. E., Gilluly, James, and Ferguson, H. G., 1958, Paleozoic rocks of north-central Nevada: Am. Assoc. Petroleum Geologists Bull., v. 42, no. 12, p. 2813-2857. Roberts, R. J., Montgomery, K. M., and Lehner, R. E., 1967, Geology and mineral resources of Eureka County, Nevada: Nevada Bur. Mines Bull. 64, 152 p. Ross, C. P., 1953, The geology andoredeposits of the Reese River district, Lander County, Nevada: U.S. Geol. Survey Bull. 997, 132 p. Sharp, B. J., and Hetland, D. L., 1954, Preliminary report on uranium occurrence in the Austin area, Lander County, Nevada: U.S. Atomic EnergyComm. RME-2010, 16 p. Stewart, J. H., and McKee, E. H.,1968, Geologic map of the Mount Callaghan quadrangle, Lander County, Nevada: U.S. Geol. SurveyGeol. Quad. MapGQ-730 (in press). 1968a, Geologic map of southeastern part of Lander County, Nevada: U.S. Geol. Survey openfile map. --=---:1968b, Geologic map of west-central part of Lander County, Nevada: U.S. Geol. Survey openfile map. Stewart, J. H., and Palmer, A. R., 1967, Callaghan window-a newly discovered part of the Roberts thrust, Toiyabe Range, Lander County, Nevada, in Geological Survey research 1967: U.S. Geol. Survey Prof. Paper 575-D, p. D56-D63. Thurlow, E. E., 1956, Uranium deposits at the contact of metamorphosed sedimentary rocks and granitic intrusive rocks in western United States, in Page, L. R., and others, Contributions to the geology of uranium and thorium by the United States Geological Survey and Atomic Energy Commission for the United Nations International Conference on Peaceful Uses of Atomic Energy, Geneva, Switzerland, 1955: U.S. Geol. Survey Prof. Paper 300, p. 85-89. Trengove, R. R., 1959, Reconnaissance of Nevada manganese deposits: U.S. Bur. Mines Rept. Inv. 5446, 40 p. Vandenburg, W. 0., 1939, Reconnaissance of mining districts in Lander County, Nevada: U.S. Bur. Mines lnf. Circ. 7043, 83 p. Vaughn, W. W., 1967, A simple mercury vapor detector for geochemical prospecting: U.S. Geol. Survey Circ. 540, 8 p. Ward, F. N., Lakin, H. W., Canney, F. C., and others, 1963, Analytical methods used in geochemical exploration by the U.S. Geological Survey: U.S. Geol. Survey Bull. 1152, 100 p. Washburn, R. H., 1966a, Paleozoic stratigraphy in Toiyabe Range, southern Lander County, Nevada [ abs. J: Geol. Soc. America, Rocky Mountain Sec., 19th Ann. Mtg., Las Vegas, Nev., 1966, Program, p. 57. 1966b, Structure and Paleozoic stratigraphy of ----:--.

the Toiyabe Range, sou the r n Lander County, Nevada: New York, Columbia Univ., Ph.D. thesis. Wells, J. D., Erickson, Ralph, andStoiser, L. R., 1967, Geology and mineralogy of the Cortez gold deposit, Nevada [absJ: Mining Eng., v. 19, no. 12, p. 40.

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Figure 1.-Pre-Tertiary rocks, mining districts, and sources of information in southern Lander County.

SOURCES OF INFORMATION USED IN FIGURES 1-4 AND IN U.S. GEOLOGICAL SURVEY OP.EN-FILE MAPS OF SOUTHERN LANDER COUNTY. NEVADA Ferguson, and

Figure 2. -Geo!ogic map of the Ravenswood window, Shoshone Range.

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Figure 3.-Geologic map of the Toiyabe Range south of Austin.

Figure 4.-Geologic map of Petes Canyon window in northern Toquima Range.

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