Eagle-Shawmut Mine

The Eagle-Shawmut Mine is a gold mine located in Tuolumne county, California.

About the MRDS Data:

All mine locations were obtained from the USGS Mineral Resources Data System. The locations and other information in this database have not been verified for accuracy. It should be assumed that all mines are on private property.

Mine Info

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Site Name

Primary: Eagle-Shawmut Mine
Secondary: Belmont Shawmut

Commodity

Primary: Gold
Secondary: Silver
Tertiary: Copper

Location

State: California
County: Tuolumne
District: Jacksonville District

Land Status

Land ownership: Private
Note: the land ownership field only identifies whether the area the mine is in is generally on public lands like Forest Service or BLM land, or if it is in an area that is generally private property. It does not definitively identify property status, nor does it indicate claim status or whether an area is open to prospecting. Always respect private property.
Administrative Organization: Tuolumne County Planning Department

Holdings

Not available

Workings

Not available

Ownership

Not available

Production

Not available

Deposit

Record Type: Site
Operation Category: Past Producer
Deposit Type: Hydrothermal replacement vein, hydrothermal stockwork, hydrothrml vein
Operation Type: Surface-Underground
Discovery Year: 1850
Years of Production:
Organization:
Significant: Y
Deposit Size: M

Physiography

Not available

Mineral Deposit Model

Model Name: Low-sulfide Au-quartz vein

Orebody

Form: Irregular, tabular

Structure

Type: R
Description: Melones Fault Zone

Type: L
Description: Melones Fault Zone

Alterations

Alteration Type: L
Alteration Text: Silicic; carbonate (ankerite)

Rocks

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

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

Analytical Data

Not available

Materials

Ore: Pyrite
Ore: Gold
Gangue: Quartz
Gangue: Ankerite
Gangue: Muscovite

Comments

Comment (Deposit): The ore deposit at the Eagle-Shawmut Mine consists mainly of large, low-grade bodies of mineralized rock (pyrite-bearing ankerite-quartz rock and mariposite rock) with numerous pyrite-bearing quartz stringers. Massive quartz veins typical of the Mother Lode, along which the deposit formed, are present also, but these are relatively barren. Smaller quartz veins, probably associated with later episodes of mineralization, carried native gold in sufficient quantities to justify mining. Auriferous sulfides, chiefly pyrite, constitute about 3-7% of the ore. At depth, the ore was composed principally of ankerite-sulfide rock.

Comment (Development): Discovered in 1850, this deposit was mined on and off until 1892, at which time mining activity became continuous except for an idle period from 1926 to 1936. From 1897 to 1942, the Eagle-Shawmut was operated on a large scale. It was allowed to continue work during World War II because of its capability to process copper and zinc ores from the Penn Mine in adjacent Calaveras County; such ores were important for the war effort. The mine was shut down in 1947. Ore was reportedly still present in the mine at the time of closure. Amalgamation and cyanidation processes were used at this mine. The mill was nearly at stream level along the east edge of Woods Creek before construction of Don Pedro Dam and Reservoir. Tailings were released directly into the creek. In 1948, chlorination tailings were shipped to the Selby smelter in the San Francisco Bay Area for use as flux.

Comment (Economic Factors): Logan (1949) and Clark (1970) reported a production value of $7.4 million for this mine.

Comment (Workings): This mine was largely an underground operation, although open-pit mining was conducted at least as early as 1936. Underground, the mine was developed by standard shafts, drifts, crosscuts, and stoping as well as a few glory holes. As of 1940, the vertical depth of mining had reached about 2,400 feet and there were about seven miles of workings altogether.

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 usually 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 some 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 usually associated with appreciable amounts of pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, and arsenopyrite. LOCAL GEOLOGY The Eagle-Shawmut deposit is along the famous Mother Lode gold belt, which is defined here by the Melones Fault Zone. Julihn and Horton (1940) reported the zone to strike N20-45W and dip 70NE. It is bounded on the west by Jurassic metasedimentary rocks of turbidite character (dominantly slate) and on the east by rocks mapped as Jurassic greenschist. The zone itselt is occupied by a hydrothermally altered assemblage that consists of quartz veins and veinlets, mariposite rock, a large dike (described by Knopf, 1929, as a pyroxenite that was later altered to a sheared and silicified chlorite schist), and schist that is locally heavily ankeritized and silicified. The complexity of the zone suggests that it has had a history of episodic movement and mineralization. The zone contains both auriferous sulfides and native gold. At least three separate veins were recognized during mining. Tucker (1916) described two of these veins as the Shawmut (33 feet wide with a dip of 72NE) and the Eagle (12 feet wide with a dip of 63NE). Julihn and Horton (1940) also discussed the Shawmut Vein as well as an East Vein and West Vein. Most production was reportedly from the Shawmut system.

Comment (Identification): Prior to opening of the Jamestown Mine in the late twentieth century, the Eagle-Shawmut Mine was the single most productive gold mine in Tuolumne County. It represents a consolidation of several older mines along the east side of the Woods Creek branch of Don Pedro Reservoir.

Comment (Geology): The Eagle-Shawmut Mine is within the Sierra Nevada foothills, 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 Tuolumne County consists of sparse, 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 Tuolumne 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 (Location): Location selected for latitude and longitude is the shaft symbol just east of elevation 1267 on the USGS 7.5-minute Chinese Camp quadrangle.

Comment (Commodity): Commodity Info: the ore generally contained 1/7 ounce of less of gold per ton; for many years, the mill heads at the Eagle-Shawmut averaged about $2.75 per ton. Production here was mainly from auriferous sulfides rather than native gold. Net recovery of gold for the entire mining operation averaged about 83% of assay value.

Comment (Commodity): Ore Materials: Auriferous sulfides (pyrite), native gold

Comment (Commodity): Gangue Materials: Quartz, ankerite, mariposite rock

References

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 77.

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): 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., 1921, Tuolumne County: California State Mining Bureau, 17th Annual Report of the State Mineralogist, p. 479-481.

Reference (Deposit): Logan, C.A., 1928, Tuolumne County: California State Mining Bureau, 24th Annual Report of the State Mineralogist, p. 14-16.

Reference (Deposit): Logan, C.A., 1949, Mines and mineral resources of Tuolumne County, California: California Journal of Mines and Geology, v. 45, no. 1, p. 47-83.

Reference (Deposit): Moore, L., 1968, Gold resources of the Mother Lode Belt, El Dorado, Amador, Calaveras, Tuolumne, and Mariposa counties, California: U.S. Bureau of Mines Technical Progress Report 5, p. 1-22.

Reference (Deposit): Higgins, C.T., 1997, Mineral land classification of a portion of Tuolumne County, California, for precious metals, carbonate rock, and concrete-grade aggregate: California Division of Mines and Geology Open-File Report 97-09, 85 p.

Reference (Deposit): Knopf, A., 1929, The Mother Lode system of California: U. S. Geological Survey Professional Paper 157, 88 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): 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): Tucker, W.B., 1916, Tuolumne County: California State Mining Bureau, 14th Annual Report of the State Mineralogist, p. 132-172.

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): 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.

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