Washington Mine

Site Name

Primary: Washington Mine
Secondary: Red Cloud
Secondary: Jenny Lind
Secondary: Washington No. 2
Secondary: Josephine

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Commodity

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

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Location

State: California
County: Mariposa
District: Hornitos

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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: 1850
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: Carbonate; ankerite, mariposite

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Rocks

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

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

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Analytical Data

Not available

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Materials

Ore: Gold
Ore: Pyrite
Ore: Chalcopyrite
Gangue: Quartz

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Comments

Comment (Geology): According to Bowen and Gray (1957), the principal vein at the Washington Mine strikes N40W, dips 70-75SW, and averages 6 to 8 feet wide. Castello (1921), however, reported the vein strikes north-south, dips 65E, and averages about 12 feet wide. The northwesterly orientation of the mine's shafts as reported by Bowen and Gray (1957) suggests that the strike of the vein is northwesterly. Vein matter is chiefly ankerite-quartz-mariposite rock in which large sheets and lenses of milky to glassy quartz several feet thick are locally developed. The presence of ankerite and mariposite suggests that country rocks were hydrothermally altered by carbon dioxide-bearing solutions. The presence of mariposite also suggests that country rock in places was ultramafic rock/serpentinite. The character of the vein closely resembles typical Mother Lode veins present seven miles farther east. Ore minerals are primarily pyrite and chalcopyrite with native gold. In addition to the quartz-vein ore, there are local masses of wall rock impregnated with auriferous pyrite (gray ore). This ore commonly runs about 0.09 ounces of gold per ton.

Comment (Commodity): Ore Materials: Native gold, pyrite, chalcopyrite

Comment (Commodity): Gangue Materials: Quartz

Comment (Geology): REGIONAL GEOLOGY The Washington 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 (Development): The Washington deposit was discovered in 1850 and prior to 1900 was probably the most productive mine in the Hornitos District. As early as 1851 it was equipped with a 6-stamp mill; in 1859, a 20-stamp mill was erected on the nearby Merced River to serve the mine. In the late 1860's and early 1870's, production was reportedly large. By 1881, the main shaft was reported to be about 1,400 feet deep. Records suggest that, although there were several attempts to promote and develop the mine between the 1880's and 1930's, there was relatively little production. The Mariposa-Washington Mining Company of Hornitos built a new mill in 1939 and milled about 4,000 tons of dump material between 1939 and 1940. Shortly afterward, the Lind Mining Company extracted a large tonnage of moderate grade in 1941 and 1942. Also during this period, the company renovated various older workings, conducted exploration, and developed new workings. The workings cleaned out or newly driven totalled nearly 8,000 feet; the main (Jenny Lind) shaft was deepened slightly to just below the 1500-foot level. Between 1943 and 1945, the mill and flotation plant were leased by the Hecla Mining Company to process zinc ore from the nearby Blue Moon Mine. Between 1945 and 1957, the mine remained idle. It is not known if additional work has been done at the mine since that time.

Comment (Economic Factors): Bowen and Gray (1957) reported an estimated production of $2,247,000 from this mine prior to 1900, $1,099,000 of which was recorded. The total estimated production to 1954 was $2,377,000. Between 1939 and 1942, a total of 17,356 tons of ore were milled, which yielded 3,131 ounces of gold, 2,436 ounces of silver, 1,060 pounds of copper, and 111 pounds of lead.

Comment (Commodity): Commodity Info: Ore reportedly ran about $25/ton in the 1870'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 Washington 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. Wall rocks are mainly micaceous and amphibole-bearing schists and dark quartz-biotite hornfelses (Bowen and Gray, 1957), which are mapped as part of the Mariposa Formation of Jurassic age. According to Castello (1921) the wall rock is slate. Storms (1896) reported that the vein is accompanied by a light-colored felsitic dike, which adjoins the vein on the footwall side in the southeast workings and on the hanging-wall side in the northwest workings. Small, irregular intrusions of hornblendic granitic rock penetrate to the surface west of the main shaft area. Much of the metamorphic rock in the vicinity of the main shaft was derived originally from tuffaceous sedimentary rocks.

Comment (Identification): The Washington 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 ten producers in Mariposa County and was the second leading producer in the Hornitos District. Despite its prominence, however, available information on the mine is sparse. This deposit is shown as the Red Cloud Mine on the USGS 7.5-minute Hornitos quadrangle.

Comment (Location): Location selected for latitude and longitude is the Red Cloud Mines adit symbol on the USGS 7.5-minute Hornitos quadrangle. Main workings were shafts rather than adits.

Comment (Workings): The Washington Mine was developed by standard shafts, drifts, and stopes. Two main shafts, the Jenny Lind (1,500 feet deep) and the Washington (at least 1,000 feet deep), are inclined at an angle of about 68o. It is not known if the reported depths are on the incline or vertical. Workings that connect the two shafts aggregate nearly two miles. Levels were developed down to the 1500-level. There are at least two other shafts on the property.

Comment (Deposit): The deposit at the Washington Mine consists of a typical gold- and sulfide-bearing hydrothermal quartz-vein system within metamorphic rock. Wall rocks are mainly micaceous and amphibole-bearing schists and dark quartz-biotite hornfelses (Bowen and Gray, 1957), which are mapped as part of the Mariposa Formation of Jurassic age. According to Castello (1921) the wall rock is slate. The vein reportedly is also accompanied by a light-colored felsitic dike, which adjoins the vein on either the footwall or on the hanging-wall side. Small, irregular intrusions of hornblendic granitic rock penetrate to the surface nearby. Much of the metamorphic rock in the vicinity of the main shaft was derived originally from tuffaceous sedimentary rocks. According to Bowen and Gray (1957), the principal vein at the Washington Mine strikes N40W, dips 70-75SW, and averages 6 to 8 feet wide. Vein matter is chiefly ankerite-quartz-mariposite rock in which large sheets and lenses of milky to glassy quartz several feet thick are locally developed. The presence of ankerite and mariposite suggests that country rocks were hydrothermally altered by carbon dioxide-bearing solutions. The presence of mariposite also suggests that country rock in places was ultramafic rock/serpentinite. The character of the vein closely resembles typical Mother Lode veins present seven miles farther east. Bonanza ore was found in the upper parts of the vein, whereas the lower parts yielded more moderate-grade ore ($6-7/ton) in the mid-twentieth century. Ore minerals are primarily pyrite and chalcopyrite with native gold. In addition to the quartz-vein ore, there are local masses of wall rock impregnated with auriferous pyrite (gray ore). This ore commonly runs about 0.09 ounces of gold per ton.

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References

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): 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.

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): 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): Storms, W.H., 1896, Mariposa County: California State Mining Bureau, 13th Annual Report of the State Mineralogist, p. 216-225.

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.

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