Hycroft Mine (Crofoot-Lewis Mine; Standard Slag Mine; Crofoot Gold project; Crofoot project; Crofoot Mine; Lewis Mine; Hycroft-Lewis Mine; Brimstone; Gap; Graveyard [Boneyard]; North pit; South Central pit; South Extension pit; Peninsula), Sulphur, Sulphur District, Humboldt Co., Nevada, USA
|Latitude & Longitude (WGS84):||40° 52' 8'' North , 118° 41' 16'' West|
|Latitude & Longitude (decimal):||40.86889,-118.68778|
A Ag-Hg-Cu-Al-Au-S-Se-Zn-Sb mine located in secs. 25 & 36, T35N, R29E, MDM, 4.2 km (2.6 miles) E of Sulphur, on Bureau of Land Management administered land.
Initially, the Crofoot and Lewis mines reflected different ownership of portions the same continuously mineralized zone. Hycroft Resources acquired the Lewis mine in January 1987 and has since consolidated the two mines into the Crofoot/Lewis mine. This gold mine has been described in many earlier MRDS records: W700587, M242949, W700582, M242768, and M242743. The current record combines all information from the earlier records and supersedes those records. These earlier records should be deleted. In addition, there are several records that describe the sulfur, clay, and mercury deposits associated with the same system (D002144, M232693, and others) which should remain in the database.Initially, the Crofoot and Lewis mines reflected different ownership of portions the same continuously mineralized zone. Hycroft Resources acquired the Lewis mine in January 1987 and has since consolidated the two mines into the Crofoot/Lewis mine. This gold mine has been described in many earlier MRDS records: W700587, M242949, W700582, M242768, and M242743. The current record combines all information from the earlier records and supersedes those records. These earlier records should be deleted. In addition, there are several records that describe the sulfur, clay, and mercury deposits associated with the same system (D002144, M232693, and others) which should remain in the database.
The mine is situated off the NW flank of the Kamma Mountains. The current Crofoot/Lewis Mine is developed on a continous mineralized zone that was originally covered by Standard Slag's Lewis mine, on the north and by the Crofoot property to the south. The claims cover 12,230 acres (4,950 hectares).The mine is situated off the NW flank of the Kamma Mountains. The current Crofoot/Lewis Mine is developed on a continous mineralized zone that was originally covered by Standard Slag's Lewis mine, on the north and by the Crofoot property to the south. The claims cover 12,230 acres (4,950 hectares).
Mineralization is an epithermal vein deposit (Mineral occurrence model information: Model code 150; USGS model code 25c; Deposit model name: Epithermal vein, Comstock; Mark3 model number 16), hosted in rocks of the Sulphur group (aphanitic volcanic rocks, agglomerate) and the Camel Conglomerate. The ore body is disseminated, blanket in form with a length of 4,827.9 meters, strikes NE (faults) and dips steeply (faults). Controls for ore emplacement included structure and host-rock permeability. The NNE-trending central fault zone and related splays are mineralized continuously over a distance of 2.5 miles. The system is enriched in Ag southward and with depth, e.g. Ag:Au 4:1 to 10:1. Base metals increase with depth and As, Sb, and Hg decrease with depth. The system is high in Se.
There are two major types of alteration recognized at Sulphur: solfataric-near surface alteration by hot water, gases, acids; and silica-pyrite-(silica-alunite) characterized by dense siliceous flooding accompanied by fine-grained sulfides, formed at moderate depth. Alteration varies from intense silica-adularia-pyrite to weak propylitic with a widespread cap of late "acid-leach alteration," which appears to have descended along pre-existing structures. Local rocks include Tuffaceous sedimentary rocks.
Workings include surface openings comprised of an open pit. The property is developed as an open pit mine; with heap leach operation; single bench; and cyanidation facilities.
Reserves of the Crofoot/Lewis mine were reported in 1988 as 25 million tons of ore grading 0.025 opt Au. In 1991, reported reserves were 33 million tonnes of ore at 0.29 oz/t gold with a waste: ore stripping ratio of 1.6. There was an additional 7 million tons of reserves as refractory unoxidized material. In 1999, 23.8 million tons of ore grading 0.0204 opt Au - proven and probable reserves and 2.3 million tons, 0.0177 opt Au indicated reserves. Vista Gold in about 2000 indicated resources of approximately 25 million tons of ore at .021 opt Au, or about a half million ounces. In 2004, Vista Gold reported measured and indicated reserves of 41,865,000 tons grading 0.0196 opt gold for 819,000 contained ounces; and an inferred resource of 14,108,000 tons grading 0.0152 opt gold for 213,000 contained ounces. 2006 proven and probable reserves were reported as 33,320,000 tons of ore grading 0.02 ounces of gold per ton. Reported production for the Crofoot/Lewis Mine: 1988: 75,800 oz Au 1989-98: 868,544 oz Au, and 2,717,170 oz Ag 1999: 40,075 oz Au, 183,190 oz Ag 2000: 13,493 oz Au, 38,418 oz Ag 2001: 3,232 oz Au, 2,000 Ag 2002: 1,771 oz Au, 217 oz Ag totaling slightly over a million ounces of gold and 2.94 million ounces of silver between 1988 and 2002.
Sporadic mining activity has occurred in this area since about 1875, with early activity focused on sulfur, mercury and clay, and later silver and gold. This district was explored for precious metals by various companies in the 1970s through the 1980s. In 1981 Homestake did widely spaced drilling on 600 ft centers, and intersected deep silver values indicating 10-15 million s.t. of low grade mineralization at shallow depth. Homestake continued their exploration in 1982. This deposit was developed by Standard Slag as the Lewis Mine in 1984, employing about 150 workers. In 1985, Granges put down 12 RC holes, 6 of which were fill-in holes in the North zone adjoining the Lewis Mine, which confirmed Homestake's earlier results. Metallurgical results were encouraging enough to justify accelerating the program. In 1985, a 25-hole, 4000-ft RC. drilling program was in progress, 10 holes to be drilled in the central zone and 10-15 in the relatively unknown southern half of the property, which covers 3600 acres total. In October of 1991, the mine was producing at a rate of 45,000 tpd, 6 days a week. The mine was still listed as an active open pit heap leach gold-silver mine as of 2002, employing 7 persons. Mining in the Brimstone deposit was suspended in December 1998, pending better gold prices. Since that time the site has been on care and maintenance and all operating permits and mining claims have been maintained. Gold recovered from recirculating leach pad solutions has helped to defray holding costs. The owners, Vista Gold, completed a new feasibility study in about 2000 indicating resources of approximately 25 million tons of ore at .021 opt Au, or about a half million ounces. In August 2004, Vista Gold Corp. announced an updated resource estimate for its Hycroft Mine prepared by Mine Development Associates of Reno. The resource estimate, based on a database containing 575 drill holes, credits Hycroft with measured and indicated resources of 754,600 oz Au, and inferred resources of 133,600 oz Au. In 2005, the mine was owned by Vista Gold who recently entered into an option agreement to sell the mine to Canyon Resources.
Mineral ListMineral list contains entries from the region specified including sub-localities
36 valid minerals.
Rock Types Recorded
Rock list contains entries from the region specified including sub-localities
Select Rock List TypeAlphabetical List Tree Diagram
Entries shown in red are rocks recorded for this region.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
|Pliocene - Miocene|
2.588 - 23.03 Ma
|Younger tuffaceous sedimentary rocks|
Age: Neogene (2.588 - 23.03 Ma)
Description: Tuffaceous and other young Tertiary sedimentary rocks. Most of these rocks are sedimentary with a strong volcanic component - a few are tuffaceous with a strong sedimentary component. This unit includes rocks originally mapped as the High Rock sequence in Washoe County; the Horse Camp Formation in northern Nye County; the Esmeralda Formation in Mineral and Esmeralda Counties; older lake beds in Lincoln County; the Belted Range Tuff; the Indian Trail Formation (now abandoned); Timber Mountain, Paintbrush, and Crater Flat Tuffs; Wahmonie and Salyer Formations in southern Nye County; the Siebert Tuff in Esmeralda County; the Muddy Creek Formation in Clark County; and the Thousand Creek and Virgin Valley “beds” in Humboldt County; and other unnamed units. It corresponds to units Ts3 and Tts from the 1978 State map. It is present in all counties.
Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. 
Localities in this Region
- Humboldt Co.
- Sulphur District
- Humboldt Co.
Homestake Mining Co. (1983), unpublished report.
A.E.G. Field Trip Guidebook (1984), Sulphur Mining District.
Mine Search Annual (1984-1985), Volume 11: 111, Metals Economics Group, Boulder Co.
Pay Dirt (1984) (April).
Granges Exploration Lts. (1985) Annual Report, Vancouver, British Columbia.
Mining Journal (1985) (6/21/85).
Nevada State Inspector of Mines (1985), Directory of Nevada Mine Operations Active in Calendar Year 1984.
Tingley, J.V. (1985), Field Examination and Sample Analysis (6 Sep 1985).
Bonham, H. F. (1986), Nevada Bureau of Mines and Geology Map 91.
Tooker, E.W. (1986), USGS Bulletin 1646: 142-143.
Bonham, H. F. (1988) in Nevada Bureau of Mines and Geology Mineral Industries - 1987.
Duncan, D. and B. Wigglesworth (1988) Crofoot's Project Joins Nevada's Formidable List of Gold Producers. Mining and Engineering Journal, June, 1988: 40-41.
Beling, D. C., J. Harrington, and B. Wigglesworth (1991), Treating Low-Grade Solutions at the Crofoot Mine. Chapter 22 in GoldOLD '90, ed. by D. M. Hausen. Society of Mining Engineers, AIME: 231-234.
Doebrich, Jeff (1991), USGS Written communication 1991.
Nevada Department of taxation, Mining Division (1991). Hycroft Resources and Development Net Proceeds Tax records. Carson City, Nevada, February, 1991.
Miller, M.S. (1993), Emigrant Trail Study Area, U.S. Bureau of Mines, Open File Report MLA 7-93: 89.
Nevada Bureau of Mines and Geology (1994), Mineral Industries - 1993.
Nevada Division of Minerals (1994).
Ebert, Shane W, Groves, David I, Jones, J Kenneth (1995), Geology, alteration, and ore controls of the Crofoot/Lewis Mine, Sulphur, Nevada; a well-preserved hot-spring gold-silver deposit, in Geology and Ore Deposits of the American Cordillera; symposium proceedings, Geological Society of Nevada.
Granges Inc. (1995), 1995 Annual Report, 52 pp.
Granges, Inc. (1995), Form 10-K: 7-8.
Nevada Mines Directory (1995): 11, 24. (1995/1996 ??).
The Mining Record (1995), Production at Crofoot/Lewis Mine (February 22, 1995): 7.
Randol Mining Direcotry (1996-1997), U.S. Mines & Mining Companies: 238.
The Mining Record (1996): 107(42) (October 16, 1996): Pl.
The Mining Record (1996): 107(47) (November 20, 1996): Pl.
The Northern Miner (1996).
Wallace, Andy, Friberg, Robert S. (1996), Geology and mineral deposits of the Sulphur mining district, Humboldt County and Pershing County, Nevada; in Gold and silver deposits of western Nevada, Geological Society of Nevada, Special Publication vol.18, pp.67-76.
Economic Geology (1997): 92: 578-600.
Skillings Mining Review (1997): 86(39):14 (September 27, 1997).
The Mining Record (1997): 108(13) (March 26, 1997): Pl.
Long, K.R., DeYoung, J.H., Jr., and Ludington, S.D. (1998), Database of significant deposits of gold, silver, copper, lead, and zinc in the United States; Part A, Database description and analysis; part B, Digital database: U.S. Geological Survey Open-File Report 98-206, 33 p., one 3.5 inch diskette.
Davis, D.A. and J. V. Tingley (1999), Gold and silver resources in Nevada, Nevada Bureau of Mines and Geology Map 120.
Crafford, A.E.J., editor (2003), GSN Road Log 2, Interstate 80 Eastbound, Fernley (Exit 48) to Golconda (Exit 194).
Mine Development Associates (2004) 2004 Report (on website).
Vista Gold Corp. (2004), press release, 9/22/2004.
USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10310408.
U.S. Bureau of Mines, Minerals Availability System (MAS) file ID #0320130636, 0320130498, 0320130476 & 0320130635.