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Gwin Mine (Paloma Mine; Gwin Extension No. 1 Mine), Paloma, Paloma District, Mother Lode Belt, Calaveras Co., California, USA
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Latitude: 38°16'33"N
Longitude: 120°45'29"W
 
 

Longitudinal section of the Gwin gold mine..

A former lode Au-Ag-Pb-Cu-Zn mine located in secs. 21, 22, 27 & 28, T5N, R11E, MDM, 1.9 km (6,100 feet) N of Paloma and 4 miles SW of Mokelumne Hill, in Rich Gulch. Mine started about 1850 by Dr (MD) William McKendree Gwin, of Tennessee, later Senator from California. Operated during the periods 1850 to 1882, 1894 to 1908, 1927, 1941 and 1946. MRDS database stated accuracy for this location is 100 meters.

The Gwin Mine is in the Paloma District and most of the gold produced in the district came from this mine, and it was one of the most productive Mother Lode gold mines in Calaveras County with an estimated output of between 6 and 7 million dollars at period gold values. The mine earned millions of dollars for Gwin.

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 1960's, 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).

The Gwin mine is located in the same belt of Mariposa slate that contains the highly productive Mother Lode mines to the north in Amador County. This belt is approximately 2,000 feet wide in the Gwin mine area.

The following information is extracted from the U.S. Geological Survey Mother Lode folio: "South of the Mokelumne River, the Gwin mine is situated in a belt of Mariposa slates which is directly continuous with the slates of the Kennedy and Argonaut mines. The Gwin vein strikes with the slates and is generally parallel with their cleavage. It is accompanied by stringers and varies in width, but on the whole is entitled to be called a simple vein rather than a stringer lead.

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.

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 N12-35W and dips 60-75NE, 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 1,200 foot main shaft level, and was mined by the Gwin Mining Company. It's 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 1,500 feet (Clark, 1970).

Milling ore averaged about 0.25 ounce of gold per ton and yielded about $7 -$8 (period values) 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).

Mineralization is precious and base metals deposit (Mineral occurrence model information (Model code: 273; USGS model code: 36a; Deposit model name: Low-sulfide Au-quartz vein; Mark3 model number: 27), hosted in Late Jurassic slate of the Mariposa Formation and conglomerate. The ore body strikes N12W and dips 70NE at a thickness of 9.14 meters. It is tabular, pinch and swell in general form. The ore shoot is pyramidal in shape and vertical as far as exploited. Sulfides in slate have a high Ag (??) fraction and lower Ag (??) fraction. A conglomerate lies on the footwall of the vein and carries $0.50 (period values) per ton in Au. The E vein was most productive. A new ore body was encountered at the 2,400 level, 1,700 feet S of the crosscut from then main shaft. The vein minerals are quartz, pyrite, arsenopyrite, gold, chalcopyrite, galena, sphalerite, albite, sericite and calcite. Galena and sphalerite, when present in small amounts, are considered to indicate good ore. The arsenopyrite is found in both large and small crystals. the former are particularly prized, as they inclose beautiful arborescent masses of crystallized Gold. The new S ore shoot contained from 0.79 to 1.3 ounces of Au per ton. Local alteration included ankeritic and sericitic alteration of the wall rock, with disseminated auriferous pyrite mineralization. Local rocks include Mesozoic volcanic rocks, unit 2 (Western Sierra Foothills and Western Klamath Mountains).

Regional geologic structures include the Bear Mountain fault zone and the Melones Fault zone. Local structures include the Melones Fault zone.

The Gwin Mine was developed by a 2,533-foot vertical main shaft, an 1,800-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 1,200, 1,400, 1,600, 1,750, 1,900, 2,100, 2,300, and 2,400-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 1,000, old 1,200, old 1,400, 1,400, 1,600, and 1,750-foot levels. The shafts were connected on the old 1,200-foot level of the south shaft (approx. 1,000-foot level of the main shaft), and on the 1,400, 1,600, and 1,750-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.

Workings include underground openings with an overall depth of 868.68 meters. The mine is developed by the 2,533-foot vertical main shaft, the 1800-foot inclined south shaft, and several old shallow shafts. There are more than 25,000 feet of underground workings at the Gwin mine. The 100-stamp mill used in the last operations was driven by water power and was equipped with Frue vanners. No equipment remains on the property.

Production was some 300,000 ounces of gold worth $139,000,000 in todays dollars. Estimates for total output vary from $6 to $7 million at period values. $1,399,146 (period values) total bullion output 1871-1882, 1897-1901. 2,701,959 tons of ore yielded $1,121,425 (period values).

Free gold is coarse and was caught in a No. 16 screen on moprtars. 98% of Au was recovered in this manner - quite unique and high average.

Mineral List

Albite
Arsenopyrite
Calcite
Chalcopyrite
Galena
Gold
Muscovite
var: Sericite

Pyrite
Quartz
Sphalerite


10 entries listed. 9 valid minerals.

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References

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

Crawford, James John (1894), Twelfth report of the State Mineralogist: California Mining Bureau. (Report 12): 12: 93.

Ransome, Frederick Leslie (1900), Description of the Mother Lode district, California: USGS Geol. Atlas, Mother Lode (No. 63), 11 pp.: 8.

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

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

Logan, Clarence August (1934), Mother Lode Gold Belt of California: California Division Mines Bulletin 108, 221 pp.: 141-142, Pl. 6.

Logan, Clarence August and Franke, H. (1936), Mines and Mineral Resources of Calaveras Co.: California Division of Mines, California Journal of Mines & Geology, California Division of Mines (Report 32): 32(3): 307, Pl.

Clark, William B. & Lydon, Philip A. (1962), Mines and Mineral Resources of Calaveras County, California, California Division of Mines and Geology, County Report No. 2: 56-59, 156, Pl. B.

Murdoch, Joseph & Robert W. Webb (1966), Minerals of California, Centennial Volume (1866-1966): California Division Mines & Geology Bulletin 189: 86.

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

Pemberton, H. Earl (1983), Minerals of California; Van Nostrand Reinholt Press: 12 (map 2-3).

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.

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 pp.

USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10028712, 10284323 & 10310625.

U.S. Bureau of Mines, Minerals Availability System (MAS) file ID #0060090300.

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