Alto Mine (Alta Mine), Alto Mining District, West Belt, Calaveras County, California, USAi

Regional Level Types
Alto Mine (Alta Mine)	Mine
Alto Mining District	Mining District
West Belt	- not defined -
Calaveras County	County
California	State
USA	Country

A former lode Au mine located in sec. 36, T1N, R12E, MDM, along Scorpion Gulch (7 miles SSE of Copperopolis). Operated during the periods 1886 to 1896, and 1896 to 1907. Owned and operated by Glamis Gold Inc. (100%), California (1990). MRDS database stated accuracy for this location is 100 meters.

The Alto Mine was discovered in 1886 by George Blazer. The mine was operated on a limited scale until 1896 when the Utica Mining Company acquired the property. This company developed the mine on a major scale and erected a 10-stamp mill. In 1902, the firm of Wright and Lane gained control of the mine. They increased the mill to 40 stamps and initiated development via extensive glory hole mining. In one year, the mine reportedly produced 75,000 tons of ore.

The mine was shut down in 1907 and the mill was destroyed by fire shortly afterwards. In 1910, the Calaveras Mining Company reportedly recovered some gold from the property. The mine remained idle until Glamis Gold reopened the Alto Mine in 1989 as a small open pit heap/leach mine. It operated only a few years and was reclaimed in 1993. At the time, estimated remaining ore reserves were 455,000 tons grading 0.047 ounces of gold per ton.

Mineralization is a vein 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 Salt Springs slate and greenstone. The ore body is tabular, pinch and swell in form, strikes N50W and dips NE at a width of 22.86 meters. There are two principal veins, several hundred feet a[part, but converge to the N. The West vein is a dike-like body of mineralized greenstone which is massive, fine-grained, dark, and impregnated with fine-grained pyrite. A relatively small amount of quartz and comb structures are common. Controls for ore emplacement included free gold within mesothermal gold-quartz stringers. Local alteration is hydrothermal pyritization of greenstone. Local rocks include Jurassic marine rocks, unit 1 (Western Sierra Nevada and Western Klamath Mountains).

Regional geologic structures include the Bear Mountains fault zone and the Melones fault zone.

The Alto Mine is located in the West Gold belt of the Sierra Nevada. The mine occurs within a zone of sheared grayish-black Salt Springs slate that strikes N 45?- 75? W and dips steeply to the northeast. The mine developed two principal "veins", ("west vein" and "east vein"). In the primary workings, the veins are several hundred feet apart, but converge to the north. The "veins" are composed of gold-quartz stringers within mineralized bodies of greenstone.

The west vein, which was the most extensively worked, consists of an extensive dike-like body of mineralized greenstones striking N 50? W and dipping steeply to the northeast. Gold occurs as free gold in quartz stringers and in fine-grained disseminated auriferous pyrite within greenstone. The ore body is 50 to 75 feet thick and was mined for a distance of at least 600 feet along the strike. The mineralized zone reportedly extends for 2 miles on strike (Miller, 1940). The ore contains a large number of thin parallel quartz stringers veinlets that are somewhat folded and faulted. Comb structures are common in some of the veinlets (Clark and Lydon, 1962).

No specific information is available regarding the east vein.

During the period of glory hole mining between 1902 and 1907, both vein material and wall rock were reportedly milled with mill heads having an average value of about $1.20 per ton. Ore from the quartz stringers, however, contained up to $10 per ton. Sulfides, primarily auriferous pyrite, comprised about 2 percent of the ore. Concentrates usually averaged $35 - $40 per ton. Some high-grade ore was encountered.

Workings include surface and underground openings with an overall depth of 137.16 meters, overall length of 152.4 meters and an overall width of 76.2 meters. There is a glory hole on the West vein with a small one on the East vein. Levels in the mine are at 100, 200, 300 & 400 feet with the 100 foot level having crosscut to the glory hole. Considerable stoping was done on the East vein.

Unpublished records in the files of the California Geological Survey indicate that, in addition to the shaft, the mine had three glory holes of the following dimensions: 1000 feet long x 500 feet wide x 200 feet deep; 600' x 200' x 300? deep; and 800? x 200? x 200? deep.

Production data are found in: Tucker, W. Burling (1915) & Clark, Wm. B. & P.A. Lydon (1962).

The value of total production was estimated at 1 million dollars at the period values. Glory hole mining occurred 1902 to 1907. Quartz ore was up to $10 (period values)/ton with sulfide concentrates, primarily auriferous pyrite, which comprised about 2% of the ore, at $35 to $40 per ton of Au (period values). Some high-grade ore was found. A 40-stamp mill, No. 3 Gates gyratory crusher, and 8 concentrating tables were installed. During the period from 1902 to 1907 the ore from the quartz veins averaged $1.20 pwer ton (period values).eraged $1.20 per ton. Some quartz stringers contained up to $10 per ton.

Other Databases

Link to USGS MRDS:	10028786

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References