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Keystone-Union Mine (Calaveras Copper), Copperopolis Mines (Copperopolis group), Copperopolis, Foothill Copper Belt, Calaveras Co., California, USA

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Latitude & Longitude (WGS84): 37° 58' 35'' North , 120° 38' 34'' West
Latitude & Longitude (decimal): 37.97639,-120.64278
GeoHash:G#: 9qf0n9qs7
Locality type:Mine
Köppen climate type:Csa : Hot-summer Mediterranean climate

A former Cu-Ag-Au-Pb mine located in the NE¼SW¼ sec. 34, T2N, R12E, MDM, in the town of Copperopolis, on private (patented) land. Operated during the period 1861 to 1946. MRDS database stated accuracy for this location is 100 meters.

Mineralization is a replacement deposit (Deposit model: Model code: 184; USGS model code: 28a; Deposit model name: Volcanogenic massive sulfide, Kuroko; Mark3 model number: 93184), hosted in slate, metavolcanic rock, and schist. The ore body is lenticular at a thickness of 10.06 meters and a length of 106.68 meters. The ore bodies lie parallel to foliation, with a steep N rake, pitch length 600 feet or 1,200 feet, similar to the North Keystone Mine area, with dark blue-gray argillaceous slate, green schist and greenstone, bedded tuff, hornstone and volcanic breccia. Most have well-developed schistosity or cleavage dipping steeply eastward. 200 feet W is a large granodiorite intrusion, lenticular, 2,000 feet longh and 240 feet wide. A serpentine body 1,730 feet long and 85 feet wide, lies E of the granodiorite body. Controls for ore emplacement included: 1.) The zone of chloritization, and, 2.) A fault. Local alteration includes chloritization. Associated rocks include Late Jurassic serpentinite. Local rocks include Mesozoic volcanic rocks, unit 2 (Western Sierra Foothills and Western Klamath Mountains).

Ore characteristics:

A shallow (thirty feet or less in depth) zone of oxidized copper ore occurred. There was apparently no significant supergene enrichment zone. The deeper, unoxidized ore consisted of veinlets of chalcopyrite and pyrite enclosed in the host rock ("chlorite schist"). Chalcopyrite is mentioned as the important ore mineral by Reid (1907) and Copper, Curb and Mining Outlook (1916), and rich masses of nearly pure chalcopyrite occurred. The chalcopyrite was amenable to concentration by flotation.

Local structures include a fault which dips E on the eastern boundary of the serpentine body. It appears continuous with the footwall fault of the North Keystone Mine. Talcose gouge with disseminated magnetite is present. Displacement decreases southward, with 100 feet of displacement. There is also a quartz diorite intrusion.

Workings include underground openings with an overall depth of 411.48 meters. There are 6 shafts, which are, N to S: 1.) The Keystone-Discovery; 2.) the Keystone; 3.) the Union No. 1; 4.) the Union No. 2; 5.) the Union No. 3, and, 6.) the South Union, developed along a 2,000 foot length of the mineralized zone.
Production data are found in: Heyl, G.R. (1948): 96-97.

The total production of the Copperopolis copper mines (primarily from the Keystone and Union mines) for the period 1861 to 1946 was approximately 73,000,000 pounds of copper (Heyl 1948).

Production statistics: Year: 1861 (period: 1861-____) (ore): Accurate: ^317 ounces (8,950 grams) Au; 28,945 ounces (817,300 grams) Ag; 62,600,131 pounds (2,795 metric tons) Cu.

Ore treatment:

Rich ore (10 to >20% copper) was smelted on-site or shipped to smelters. In 1865 and 1866, approximately 23,000 tons (per year) of rich ore (at least 20% copper) were shipped to Swansea, Wales for smelting (Copper, Curb and Mining Outlook 1916). Some ore was also partially smelted to a matte, which was then shipped to smelters for refining.

Low-grade ore (~2% copper) was concentrated by flotation. Concentrates were shipped to smelters (including Tacoma, Washington), or smelted on-site (Copper, Curb and Mining Outlook 1916; Robie 1921).

Analytical data results: Tenor: 1 to 18% Cu, most higher than 2%.

Mineral List

3 valid minerals.

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on

145 - 201.3 Ma

ID: 2932359
Mesozoic volcanic rocks, unit 2 (Western Sierra Foothills and Western Klamath Mountains)

Age: Jurassic (145 - 201.3 Ma)

Stratigraphic Name: Copper Hill Volcanics; Gopher Ridge Volcanics; Logtown Ridge Formation; Mariposa Formation; Monte de Oro Formation; Oregon City Formation; Peaslee Creek Volcanics; Penon Blanco Formation; Brower Creek Volcanic Member; Smartville Complex

Description: Undivided Mesozoic volcanic and metavolcanic rocks. Andesite and rhyolite flow rocks, greenstone, volcanic breccia and other pyroclastic rocks; in part strongly metamorphosed. Includes volcanic rocks of Franciscan Complex: basaltic pillow lava, diabase, greenstone, and minor pyroclastic rocks.

Comments: Western Sierra Nevada and western Klamath Mountains. Mostly basaltic to andesitic breccias, flows, and tuffs, metamorphosed but with primary volcanic features generally recognizable. Minor associated sandstone and conglomerate. Largely or entirely of marine origin. Includes some rocks interpreted as ophiolites (Smartville complex)

Lithology: Major:{mafic volcanic}, Minor:{felsic volcanic}, Incidental:{chert, sandstone, conglomerate}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Jurassic - Triassic
145 - 252.17 Ma

ID: 3189515
Mesozoic sedimentary and volcanic rocks

Age: Mesozoic (145 - 252.17 Ma)

Lithology: Mudstone-carbonate-sandstone-conglomerate

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

Data and map coding provided by, used under Creative Commons Attribution 4.0 License

This page contains all mineral locality references listed on This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.


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Reid, J.A. (1907) The ore-deposits of Copperopilis, Calaveras Co., California. Economic Geology: 2: 380-417.
Copper, Curb and Mining Outlook (1916), Calaveras´ flotation success. 23-25.
Robie, E.H. (1921) The Smelter of the Calaveras Copper Co. Engineering and Mining Journal: 111(24): 984-987.
Manual of War Minerals of Counties of Northern California, 2nd edition (1944) (September 1, 1944): 26.
Heyl, George Richard (1948b), Ore deposits of Copperopolis, Calaveras County, California: California Division Mines Bulletin 144: 96-97, 106-108.
Taliaferro, N. L., and Solari, A. J. (1949), Geology of the Copperopolis quadrangle, California: California Division of Mines Bulletin 145: Pl. 2.
California Division of Mines Economic Mineral Map of California No. 6 (194_).
California Division of Mines (1957), Economic Mineral Map of California No. 7.
Goodwin, Joseph Grant (1957) Lead and zinc in California. California Journal of Mines and Geology, Division of Mines (Report 53): 53(3&4): 427.
O'Brien, J.C. (1957), Copper, in: Wright, L.A., editor, Mineral commodities of California: California Division of Mines Bulletin 176: 170, 171, 174.
Clark, Wm. B. & P.A. Lydon (1962), Mines and mineral resources of Calaveras County, California: California Division of Mines & Geology County Report 2; [… Geological Society of America Proceedings, 1933: 312-313, 1934]: 24-27, 196, Pl. D.
Clark, Wm. B. (1970a) Gold districts of California: California Division Mines & Geology Bulletin 193.
Moore, Lyman (1970) Data from 1970 Evaluation.
Pemberton, H. Earl (1983), Minerals of California; Van Nostrand Reinholt Press: 88 (map 3-3).
USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10028813 & 10235156.
U.S. Bureau of Mines, Minerals Availability System (MAS) file ID #0060090349.
Map GP-561.
USGS Mineral Investigative Resources Map file MR-34.

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