Gwin Mine

Site Name

Primary: Gwin Mine
Secondary: Paloma Mine

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: California
County: Calaveras
District: Paloma (Mokelumne Hill)

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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: Calaveras County Planning Dept.

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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: M

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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, pinch and swell

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Structure

Type: L
Description: Melones Fault zone.

Type: R
Description: Bear Mountains fault zone, Melones fault zone

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Alterations

Alteration Type: L
Alteration Text: Ankeritic and sericitic alteration of wall rock with disseminated auriferous pyrite mineralization

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Rocks

Name: Slate
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: Arsenopyrite
Gangue: Quartz
Gangue: Slate
Gangue: Sphalerite
Gangue: Galena
Gangue: Sericite
Gangue: Albite

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Comments

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 Eastern Belt is dominantly argillite, phyllite and phyllonite, chert, and metavolcanic rocks of Paleozoic-Mesozoic age. The phyllite and phyllonite are dark to silvery gray. The chert is mostly thin bedded with phyllite partings. The Paleozoic-Mesozoic metasedimentary and metavolcanic rocks of the Eastern Belt have been assigned to the Calaveras Complex by most investigators (Earhart, 1988). Older Paleozoic metamorphic rocks have been assigned to the Shoo Fly Complex. The metamorphic complexes have are intruded in places by Mesozoic plutonic rocks. Lode deposits of the East Belt consist of many individual gold-bearing quartz veins enclosed in metasedimentary 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 gold-bearing veins in the Paloma district and Gwin Mine lie within the same belt of Mariposa Formation (Upper Jurassic) slate that contain the larger and more famous Mother Lode gold mines of the Jackson District in Amador County (Argonaut, Kennedy, Plymouth mines etc.). This slate belt is about 2,000 feet wide in the Gwin Mine area.

Comment (Workings): The Gwin Mine was developed by a 2533-foot vertical main shaft, an 1800-foot inclined south shaft that follows the foot wall of the main vein, and several old shallow shafts. There are more than 25,000 feet of underground workings. Levels were turned on the main shaft at 1200, 1400, 1600, 1750, 1900, 2100, 2300, and 2400-foot levels. A 450-foot winze was driven 450 feet south of the main shaft from the 2400-foot level to the 2600 and 2800- foot levels. The south shaft developed the shallower portion of the mine with levels at 100, 300, 440, 600, 800, Old 1000, Old 1200, Old 1400, 1400, 1600, and 1750-foot levels. The shafts were connected on the Old 1200-foot level of the south shaft (approx. 1000-foot level of the main shaft), and on the 1400, 1600, and 1750-foot levels. A longitudinal cross section of the Gwin Mine is shown in Clark and Lydon (1962, page 58, Figure 8). There are also numerous open cuts and prospect holes on the mine property (Clark and Lydon, 1962). At its peak, the water-powered mill consisted of two Blake crushers, 100 stamps, twenty 6-foot Frue vanners, and sixteen 4-foot Frue vanners. A chlorination works was on site.

Comment (Identification): The Gwin Mine is located in the Paloma District, about 4 miles southwest of Mokelumne Hill and one mile northeast of the old mining town of Paloma in northwestern Calaveras County, California. The mine was named after U.S. Senator William Gwin, who purchased the property in 1867. Most of the gold produced in the district came from this mine, which was also one of the most productive Mother Lode mines in the county. The mine was operated on a large scale during the 1860s and 1870s, and from 1894 to 1908. The value of its total output is unknown, but information compiled from old reports put the total somewhere between $6-$7 million (Clark and Lydon, 1962).

Comment (Geology): There are two gold-quartz veins in the Gwin Mine; the East, or main, vein from which most of the gold was obtained, and a west vein. Both veins vary in width and are accompanied by stringers. The East vein strikes N 12?-35? W and dips 60?-75?NE, generally parallel to the cleavage of the enclosing slate country rock. Its width ranges from 10 to 30 feet. It has a soft black slate gouge on the footwall, with a black slate hanging wall. The ore shoots of the East vein are of great length, varying in width from a few inches to 20 feet. The vein is usually of a banded structure and exhibits a branching tendency with many spurs running into the hanging wall (Tucker, 1914). The two principal ore shoots on the main vein were the Old South and the North ore shoots. The Old South shoot extended from the surface to the 1200-foot main shaft level and was mined by the Gwin Mining Company. Its stopes averaged 250 feet in length with a maximum width of 22 feet. The North shoot was the principal source of production for the Gwin Mine Development Company. The North ore shoot in this mine had a horizontal stoping length of up to 800 feet and a pitch length of 1500 feet (Clark, 1970). Milling ore averaged about 0.25 ounce of gold per ton and yielded about $7 -$8 of free gold per ton. Occasionally, much richer pockets of auriferous quartz were found. Ore consists of white and banded quartz carrying free gold and auriferous pyrite and arsenopyrite with occasional admixtures of sphalerite and galena. The arsenopyrite occurs in both large and small crystals. The smaller crystals are highly prized as they surround beautiful masses of crystallized gold (Ramsome, 1900). Albite, sericite, and calcite are also present. All the free gold is coarse. Recovery from the plates ran as high as 98%. Sulfides comprise about 1.5 percent per ton and concentrates yielded about $100 per ton. Gold ranges in fineness from 850-875. The enclosing wall rock slate is impregnated with sulfides, but these sulfides are less rich than those in the vein and carry a larger proportion of silver (Ransome, 1900).

Comment (Location): Location selected for latitude and longitude is the Gwin Mine shaft symbol on the USGS 7.5 minute Jackson quadrangle

Comment (Deposit): The Gwin Mine produced approximately $7 million from two principal ore bodies within the main Gwin gold-quartz vein. The vein is a typical Mother Lode mesothermal gold-quartz vein and lies within the same narrow Mother Lode belt of Mariposa Formation black slate that yielded over $180 million from similar veins in the Plymouth-Jackson district to the north. The East (or main) producing vein strikes N 12?-35? W and dips 60?-75?NE and ranges from 10 to 30 feet wide. The two principal ore shoots on the main vein were the Old South and the North ore shoots. The Old South shoot extended from the surface to the 1200-foot main shaft level with stopes averaging 250 feet in length and a maximum width of 22 feet. The North shoot had a horizontal stoping length of up to 800 feet and a pitch length of 1500 feet. Ore consists of white and banded quartz carrying free gold and auriferous pyrite and arsenopyrite. Average yield was about 0.25 fl ounce of gold per ton. Sulfides comprise about 1.5 percent per ton and concentrates yielded about $100 per ton. Gold ranges in fineness from 850-875.

Comment (Geology): REGIONAL GEOLOGY The Paloma district is within the Sierra foothills, where bedrock consists of north trending tectonostratigraphic belts of metamorphosed sedimentary, volcanic, and intrusive rocks that range in age from late 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 Tuolumne County, lode gold deposits occur in three distinct lithologic belts - the West Gold Belt, the Mother Lode Belt, and the East Gold Belt. The Mother Lode Belt is responsible for most of the gold produced and includes the Gwin Mine. However, there has also been substantial gold production from the West Belt and East Belt. The West Belt consists of widely scattered gold deposits located west of the Mother Lode vein system. 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 can be divided into an eastern component composed of an ophiolitic melange and a western component composed of Jurassic rocks of the Copper Hill volcanics, Salt Springs slate, and Gopher Ridge volcanics. The Bear Mountains fault zone separates the melange from the Copper Hill volcanics. The Mother Lode Belt traverses western Calaveras County and consists of the upper Jurassic Logtown Ridge and upper Jurassic Mariposa formations. The Logtown Ridge Formation consists of about 6,500 feet of volcanic and volcanic-sedimentary rocks of island arc affinity. These rocks are mostly basaltic and include flows, breccias, and a variety of layered pyroclastic rocks. The overlying Mariposa Formation contains a distal turbidite, hemipelagic sequence of black slate, schist, amphibolite and chlorite schist, fine-grained tuffaceous rocks, and subvolcanic intrusive rocks. The thickness of the Mariposa Formation is difficult to ascertain due to structural complexities, but is estimated to be about 2,600 feet thick at the Cosumnes River (Earhart, 1988). Mother Lode 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, 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): Commodity Info: Average ores contained 1/4 ounce of gold per ton. Sulfides comprised 1.5% of ore with sulfide concentrates yielding about $100 per ton. Gold was 850-875 fine

Comment (Commodity): Ore Materials: White and banded quartz carrying free gold and auriferous pyrite and arsenopyrite. Hydrothermally altered wallrock with disseminated auriferous pyrite

Comment (Commodity): Gangue Materials: Quartz, slate, spalerite, galena, sericite, albite

Comment (Economic Factors): Clark (1970) reported that the Gwin Mine produced about $7 million. Milling ore averaged about 0.25 ounce of gold per ton and yielded about $7 -$8 of free gold per ton. Concentrates, which comprised about 1.5 percent per ton, yielded about $100 per ton.

Comment (Development): One of the first gold-quartz mines to be opened in California, the Gwin Mine (formerly the Paloma Mine) was originally prospected in 1850 when $100,000 was recovered from surface pockets. Soon afterward a 200-foot shaft was sunk, and some high-grade ore was encountered. After lying idle for many years, U.S. Senator William Gwin purchased the mine in 1867 and the property was incorporated under the name of the Gwin Mining Company. He later acquired the adjoining Alexander and Smith claims and consolidated them with the Gwin Mine (Clark and Lydon, 1962). At its peak, the property comprised the North Paloma, Gwin, Gwin Mine Mill site, Alexander, Smith, Queen Consolidated, Boston Hill Placer, and Leonid Consolidated Placer claims. In the early 1870s, this was the most productive mine in Calaveras County, and one of the most productive in the State, the yield being as much as $1000 per day. By 1877, the mine had been developed to an inclined depth of 1200 feet. In that year, the so-called "main chimney" or North ore shoot was encountered, which yielded large quantities of ore between the 1200 and 1500-foot levels. (Clark and Lydon, 1962). The mine was shut down in 1882, the estimated value of production to that date ranged from $2 - $3 million, largely from ore from the Old South ore shoot. In 1894, the newly organized Gwin Mine Development Company purchased the property and reopened the mine (Clark and Lydon, 1962). A new vertical main shaft was sunk north of the old workings, and a new 40-stamp mill (later upgraded to 100 stamps) was erected. An exploratory drift was driven south on the 2400-foot level which encountered a new ore body about 1700 feet south of the crosscut from the main shaft. This was known as the New South ore shoot. It had a length of about 95 feet and was reported to have contained from 0.79 to 1.3 ounces of gold per ton. The mine was operated on a major scale until 1908 after which it was idled. During this last period of operation, 984,442 tons of ore was mined and milled, which yielded approximately $4,044,000 from the (Clark and Lydon, 1962). Most of this production came from North ore shoot. Apparently some work was done on the property in 1927 as the Mineral Resources of the United States for that year reported a yield of 100 ounces of gold (Min. Res. of U.S., 1927, part 1, p. 273). In 1936, it was leased by the Belmont-Osborn Gold Mining Company. They rehabilitated the main working entries and mined some ore, which was treated at the Belmont-Osborn mill a half-mile to the northwest. In 1941, the surface was prospected by a Robert Gallagher of Valley Springs, and in 1946 by John Folsom of Valley Springs. Later several individuals prospected the upper workings in search of high-grade pockets. As late as the early 1960s, intermittent surface prospecting work was being done on the property. One 20 feet wide prospect hole, 400 feet west of the main shaft exposed a 10-foot zone of parallel quartz stringers containing abundant iron oxide and small amounts of free gold (Clark and Lydon, 1962).

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References

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 104-105.

Reference (Deposit): Clark. W. B., Lydon, P.A., 1962, Mines and mineral resources of Calaveras County, California: California Division of Mines and Geology County Report No. 2, p. 56-59.

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): Hanks, H.G, 1886, Gwin Mine: California State Mining Bureau 6th Annual report of the State Mineralogist, p. 30-34.

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

Reference (Deposit): Ransome, F. L., 1900, Mother Lode district folio, California: U. S. Geological Surevey geological Atlas of the U.S., folio 63, 11 p.

Reference (Deposit): Tucker, W.B., 1914 Calaveras County, Gwin Mine: California State Mining Bureau, 14th Annual Report of the State Mineralogist, p. 84-85.

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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California Gold

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