Mount Gaines Mine

Site Name

Primary: Mount Gaines Mine
Secondary: Barfield
Secondary: Frenchman
Secondary: Bearfield

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Commodity

Primary: Gold
Secondary: Lead
Secondary: Copper
Secondary: Silver

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Location

State: California
County: Mariposa
District: Hornitos District

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Land Status

Not available

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Past Producer
Deposit Type: Hydrothermal vein
Operation Type: Underground
Discovery Year: 1868
Years of Production:
Organization:
Significant: Y
Deposit Size: S

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Physiography

Not available

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Mineral Deposit Model

Model Name: Low-sulfide Au-quartz vein

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Orebody

Form: Tabular

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Structure

Type: R
Description: Bear Mountains Fault Zone

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Alterations

Alteration Type: L
Alteration Text: Reported, but not specified

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Rocks

Name: Slate
Role: Host
Age Type: Host Rock
Age Young: Late Jurassic

Name: Greenstone
Role: Host
Age Type: Host Rock
Age Young: Jurassic

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Analytical Data

Not available

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Materials

Ore: Proustite
Ore: Arsenopyrite
Ore: Pyrite
Ore: Chalcopyrite
Ore: Sphalerite
Ore: Galena
Ore: Gold
Ore: Argentite
Ore: Barite
Gangue: Quartz

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Comments

Comment (Commodity): Commodity Info: From 1932 to 1947, the ore averaged more than 0.5 ounce/gold per ton.

Comment (Geology): REGIONAL GEOLOGY The Mount Gaines Mine is within the Sierra Nevada Mountains, where bedrock consists of northerly trending tectonostratigraphic belts of metamorphosed sedimentary and volcanic rocks and associated intrusive rocks that range in age from Paleozoic to Mesozoic. The structural belts, which extend about 235 miles along the western side of the Sierra, are flanked to the east by the Sierra Nevada Batholith and to the west by sedimentary rocks of the Cretaceous and Jurassic Great Valley sequence. The structural belts are internally bounded by the Melones and Bear Mountains fault zones and are characterized by extensive faulting, shearing, and folding (Earhart, 1988). From El Dorado County southward into Mariposa County, lode gold deposits occur in three distinct belts - the West Belt, the Mother Lode Belt, and the East Belt. The Mother Lode Belt is responsible for most of the gold produced. However, there has also been substantial gold production from the West Belt and East Belt. The West Belt in Mariposa County consists of widely scattered gold deposits located west of the Mother Lode vein system, which represents the Mother Lode Belt. Gold occurs in irregular quartz veins and stringers in schist, slate, granitic rocks, altered mafic rocks, and as gray ore in greenstone. The West Belt is cut by the northwest-trending Bear Mountains Fault Zone, which separates an assemblage of metavolcanic and metasedimentary rocks of Jurassic age on the southwest from a more disrupted and diverse assemblage of metavolcanic, metasedimentary, plutonic, ultramafic, and melange rocks on the northeast. The metavolcanic rocks consist generally of volcanic and volcanic-sedimentary rocks of island-arc affinity. These rocks are mostly mafic to intermediate in composition and include flows, breccias, and a variety of layered pyroclastic rocks. Some silicic rocks are present also. Various formation names assigned to the metavolcanic assemblages include Gopher Ridge, Copper Hill, Logtown Ridge, and Penon Blanco. The metasedimentary rocks are dominantly distal turbidites and hemipelagic sequences of black slate. Assigned formation names include Mariposa, Salt Spring Slate, and Merced Falls Slate. The northwest-trending Mother Lode Belt traverses western Mariposa County and is associated with the Melones Fault Zone. The rocks of this belt are typically metavolcanic, metasedimentary, and ultramafic, some of which have been hydrothermally altered to assemblages as described below. Mother Lode Belt mineralization is characterized by steeply dipping gold-bearing quartz veins and bodies of mineralized country rock adjacent to veins. Mother Lode veins are characteristically enclosed in Mariposa Formation slate with associated greenstone. The Mother Lode belt vein system ranges from a few hundred feet to a mile or more in width. Within the zone are numerous discontinuous or linked veins, which may be parallel, convergent, or en echelon. The veins commonly pinch and swell. Few can be traced more than a few thousand feet. Mother Lode type veins fill voids created within faults and fracture zones and consist of quartz, gold and associated sulfides, ankerite, calcite, chlorite, limonite, talc, chromium-bearing mica, and sericite. Stringer veins are commonly found in both adjacent footwall and hanging walls. Mother Lode ores are generally low- to moderate-grade (1/3 ounce of gold or less per ton), but ore bodies can be large. Ore shoots are generally short, 200-300 feet being the average stope length. However, they persist at depth, some having been mined to several thousand feet (Clark and Lydon, 1962). Ore shoots are commonly localized at bulges in veins, shear zones, vein intersections, or near abrupt changes in strike or dip.

Comment (Commodity): Ore Materials: Native gold, galena, sphalerite, chalcopyrite, pyrite, arsenopyrite, barite, proustite, argentite

Comment (Commodity): Gangue Materials: Quartz

Comment (Geology): The main vein strikes about N35E and dips at an average of 20SE; the relatively shallow dip of the vein is notable. The vein system has a known length of about 9,000 feet. It varies from a few inches to more than 15 feet wide with an average of about 5 feet. Vein matter is chiefly milky quartz, but some ore is present in quartz veinlets in fractured, slaty to chloritic greenstone along the footwall. Much of the early work on the vein system was done on shallow pockets and stringers of sulfide-bearing quartz. In the main vein, which is sinuous, the best ore tends to be concentrated on the east sides of arcuate irregularities, particularly along the parts of the vein that have the gentlest dip. Ore shoots vary from 40 to 450 feet long and contain about 3-4% sulfides. Ore minerals include native gold, galena, sphalerite, chalcopyrite, pyrite, arsenopyrite, barite, proustite, and argentite. Native gold is observable in fractures in the pyrite, which indicates the gold was deposited later than the pyrite.

Comment (Deposit): The deposit at the Mount Gaines Mine consists of a typical gold- and sulfide-bearing hydrothermal quartz-vein system within metamorphic rock. The vein system is not well-defined at the surface. Much of the early work on the vein system was done on shallow pockets and stringers of sulfide-bearing quartz; the main branch of the system was discovered at depth. In the main vein, which is sinuous, the best ore tends to be concentrated on the east sides of arcuate irregularities, particularly along the parts of the vein that have the gentlest dip. Ore shoots vary from 40 to 450 feet long. The main vein strikes about N35E and dips at an average of 20SE; the relatively shallow dip of the vein is notable. The vein system has a known length of about 9,000 feet. It varies from a few inches to more than 15 feet wide, with an average of about 5 feet. Vein matter is chiefly milky quartz, but some ore is present in quartz veinlets in fractured, slaty to chloritic greenstone along the footwall. Ore shoots contain about 3-4% sulfides. Ore minerals include native gold, galena, sphalerite, chalcopyrite, pyrite, arsenopyrite, barite, proustite, and argentite. Native gold is observable in fractures in the pyrite. Wall rocks are chiefly massive pyroxene-andesite greenstone of Jurassic age with minor slate and hornfelsic metasedimentary rocks. Shearing and hydrothermal alteration along the vein caused widespread development of schistose to slaty chloritic material, particularly on the footwall side of the vein. A dike is present along the hanging wall of the vein against which the best ore in the mine was generally found; as of 1940, no ore had been found above the dike. Most ore was mined from the hanging wall beneath the dike.

Comment (Identification): The Mount Gaines Mine is part of what Clark (1970) described as the most productive gold district in the West Belt of the Sierra Nevada. It is also one of the top five producers in Mariposa County.

Comment (Location): Location selected for latitude and longitude is the large building symbol on the USGS 7.5-minute Hornitos quadrangle. The main working is a shaft, not an adit as shown on quadrangle map.

Comment (Development): The Mount Gaines Mine has a long history of development and production. Nearby placers on Eldorado and Burns Creeks are said to have been worked from 1853 to 1873 and to have produced a large amount of gold. This activity may have led to the beginning of lode mining at the Mount Gaines deposit in 1868 when shallow workings of rich but narrow stringers were developed. Mining continued off and on at least to the early 1950's under various companies, several of which encountered financial difficulties. In 1956, the mine was undergoing exploration and development. A dike is present along the hanging wall of the vein against which the best ore in the mine was generally found; as of 1940, no ore had been found above the dike. Most ore was mined from the hanging wall beneath the dike. Some ore was also produced from overlapping lenses, irregular quartz masses, and cross veins through slate that terminated at the dike. Modern exploration was conducted beginning in the 1980's and continued at least to 1988 as described in various trade publications of the mining industry. Both amalgamation and cyanidation processes were used at this mine.

Comment (Economic Factors): Production was estimated by Bowen and Gray (1957) to be at least $3.59 million up to the 1950's.

Comment (Geology): Wall rocks have invariably been hydrothermally altered, having been partially to completely converted to ankerite, sericite, quartz, pyrite, arsenopyrite, chlorite, and albite with traces of rutile and leucoxene (Knopf, 1929). The mineralization is generally adjacent to the veins in ground that has been fractured and contains small stringers and lenses of quartz. Locally, greenstone bodies adjacent to the quartz veins contain enough disseminated auriferous pyrite in large enough bodies to constitute what has been called "gray ore.? Altered slate wall rock commonly contains pyrite, arsenopyrite, quartz, chlorite, and sericite with or without ankerite (Zimmerman, 1983). Large bodies of mineralized schist also form low-grade ore bodies throughout the Mother Lode. This ore consists of amphibolite schist that has been subjected to the same processes of alteration, replacement, and deposition that formed the greenstone gray ores. The altered schist consists mainly of ankerite, sericite, chlorite, quartz, and albite. Gold is associated with the pyrite and other sulfides that are present. Pyrite comprises about 8 percent of the rock. The average grade of mineralized schist is about 0.1 oz per ton. The Melones Fault Zone separates the Mother Lode Belt from the East Belt. The East Belt is dominantly argillite, phyllite and phyllonite, chert, and metavolcanic rocks of Paleozoic-Mesozoic age. Carbonate rocks (marble) are also present locally. The phyllite and phyllonite are dark to silvery gray. The chert is mostly thin-bedded with phyllite partings. The Upper Paleozoic-Lower Mesozoic metasedimentary and metavolcanic rocks of the East Belt have been assigned to the Calaveras Complex by most investigators (Earhart, 1988). The Lower Paleozoic metamorphic rocks farther east have been assigned to the Shoo Fly Complex. More recently, some geologists have reinterpreted certain assemblages along and immediately east of the Melones Fault Zone as separate Jurassic units (Schweickert and others, 1999). The metamorphic complexes are intruded in places by Mesozoic plutonic rocks. Lode deposits of the East Belt consist of many individual gold-bearing quartz veins enclosed in metamorphic rocks of possible Jurassic age, metamorphic rocks of the Calaveras Complex, metamorphic rocks of the Shoo Fly complex, or in granitic rocks. Most of the veins trend northward and dip steeply. An east-west set of intersecting faults may be a controlling factor in controlling deposition of ore. Ore deposits of the East Belt are smaller and narrower than those of the Mother Lode, but commonly are more chemically complex, and richer in grade. Gold is generally associated with appreciable amounts of pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, and arsenopyrite. LOCAL GEOLOGY The deposit at the Mount Gaines Mine is part of the West Belt of gold mineralization in the Sierra Nevada foothills. It consists of a typical gold- and sulfide-bearing hydrothermal quartz-vein system within metamorphic rock. The vein system is not well-defined at the surface; the main branch of the system was discovered at depth. Wall rocks are chiefly massive pyroxene-andesite greenstone of the Penon Blanco Volcanics unit of Jurassic age, with minor slate and hornfelsic metasedimentary rocks, which possibly belong to the Mariposa Formation of Jurassic age. Shearing and hydrothermal alteration along the vein caused widespread development of schistose to slaty chloritic material, particularly on the footwall side of the vein. A dike is present along the hanging wall of the vein.

Comment (Workings): The Mount Gaines Mine was developed by standard shafts, crosscuts, drifts, raises, and stopes. In the 1950's, the main shaft was 1,322 feet deep on the incline and reached a vertical depth of about 400 feet. The shaft varied in dip, but averaged about 30 degrees and had 13 levels. There were more than 11,000 feet of drifts and 1,200 feet of raises. Most stoping was done from the 900-level to the surface and was concentrated in the portion of the deposit northeast of the shaft. All drifts as of 1940 followed the hanging wall. Bowen and Gray (1957) show diagrams of the workings as of 1948.

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References

Reference (Deposit): Schweickert, R.A., Hanson, R.E., and Girty, G.H., 1999, Accretionary tectonics of the Western Sierra Nevada Metamorphic Belt in Wagner, D.L. and Graham, S.A., editors, Geologic field trips in northern California: California Division of Mines and Geology Special Publication 119, p. 33-79.

Reference (Deposit): Wagner, D.L., Bortugno, E.J., and McJunkin, R.D., 1990, Geologic map of the San Francisco-San Jose Quadrangle, California: California Department of Conservation, Division of Mines and Geology Regional Geologic Map Series, Map No. 5A, scale 1:250,000.

Reference (Deposit): Zimmerman, J.E., 1983, The geology and structural evolution of a portion of the Mother Lode Belt, Amador County, California: Unpublished M.S. thesis, University of Arizona, 138 p.

Reference (Deposit): Castello, W.O., 1921, Mariposa County: California State Mining Bureau, 17th Annual Report of the State Mineralogist, p. 86-143.

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 65-66.

Reference (Deposit): Clark. W. B., and Lydon, P.A., 1962, Mines and mineral resources of Calaveras County, California: California Division of Mines and Geology County Report No. 2, p. 72-73.

Reference (Deposit): Earhart, R.L., 1988, Geologic setting of gold occurrences in the Big Canyon area, El Dorado County, California: U.S. Geological Survey Professional Paper 1576, 13 p.

Reference (Deposit): Julihn, C.E., and Horton, F.W., 1940, Mineral industries survey of the United States - Mines of the southern Mother Lode Region, Part II - Tuolumne and Mariposa counties: U.S. Bureau of Mines Bulletin 424, 179 p.

Reference (Deposit): Knopf, A., 1929, The Mother Lode system of California: U. S. Geological Survey Professional Paper 157, 88 p.

Reference (Deposit): Koschmann, A.H., and Bergendahl, M.H., 1968, Principal gold-producing districts of the United States: U. S. Geological Survey Professional Paper 610, 283 p.

Reference (Deposit): Logan, C.A., 1935, Mother Lode gold belt of California: California Division of Mines Bulletin 108, 240 p.

Reference (Deposit): Metals Economics Group, 1983, The MineSearch annual: California, Oregon, and Washington, vol. VIII, 636 p.

Reference (Deposit): Bowen, O.E., Jr. and Gray, C.H., Jr., 1957, Mines and mineral resources of Mariposa County, California: California Journal of Mines and Geology, v. 53, nos. 1-2, p. 35-343.

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