Keystone-Union Mine (Calaveras Copper), Copperopolis Mines (Copperopolis group), Copperopolis, Foothill Copper Belt, Calaveras County, California, USAi

Regional Level Types
Keystone-Union Mine (Calaveras Copper)	Mine
Copperopolis Mines (Copperopolis group)	Group of Mines
Copperopolis	Census-designated Place
Foothill Copper Belt	Belt
Calaveras County	County
California	State
USA	Country

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%.

Other Databases

Link to USGS MRDS:	10028813

Other Regions, Features and Areas containing this locality

References