Alto Mine

Site Name

Primary: Alto Mine

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: California
County: Calaveras
District: Alto District

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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

Owner Name: Various private owners

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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: Surface-Underground
Discovery Year: 1886
Years of Production:
Organization:
Significant: Y
Deposit Size: S

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

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Alterations

Alteration Type: L
Alteration Text: Hydrothermal pyritization of greenstone

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Rocks

Name: Greenstone
Role: Host
Age Type: Host Rock
Age Young: Late Jurassic

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: Slate
Ore: Gold
Ore: Pyrite
Gangue: Quartz

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Comments

Comment (Commodity): Commodity Info: Low -grade ore averaged $1.20 per ton. Some quartz stringers contained up to $10 per ton. Sulfides, primarily auriferous pyrite, comprised about 2 percent of the ore. Concentrates usually averaged $35 - $40 per ton.

Comment (Commodity): Ore Materials: Free-milling gold in quartz and disseminated auriferous pyrite in greenstone

Comment (Commodity): Gangue Materials: Quartz, slate, greenstone

Comment (Deposit): The Alto Mine produced over $1 million from dike-like low-grade bodies of mineralized greenstone striking N 50? W and dipping steeply to the northeast. Gold occurred as free gold in quartz stringers and in fine-grained disseminated auriferous pyrite within greenstone. Average ore grades during the early operations was about $1.20 per ton. Ore from the quartz stringers, however, contained up to $10 per ton.

Comment (Geology): REGIONAL GEOLOGY The Alto Mine area 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, gold deposits occur in the West Belt, the Mother Lode Belt, and the East Belt. The Mother Lode Belt is responsible for most of the gold produced in Calaveras County. There has also been substantial gold produced from the West Belt and East Belt (Clark and Carlson, 1956). 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 Spring slate, and Gopher Ridge volcanics. The Bear Mountains fault zone separates the melange from the Copper Hill volcanics. The Mother Lode Belt 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, 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 that traverse El Dorado County southward through Tuolumne County. Mother Lode veins are characteristically enclosed in Mariposa Formation slate with associated greenstone. The 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, and sericite. 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.

Comment (Geology): 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 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.

Comment (Location): Location selected for latitude and longitude is the Alto Mine shaft symbol on the USGS 7.5 minute New Melones Dam quadrangle

Comment (Development): 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. The total value of early production is estimated to be about $1 million at the old prevailing gold prices (Clark and Lydon, 1962). Reported production for Glamis Gold's operations remains confidential.

Comment (Economic Factors): Very little information is available regarding the production or gold values of the Alto Mine. The value of total production is estimated to be about $1 million at the old prevailing gold prices (Clark and Lydon, 1962).

Comment (Workings): The Alto Mine was developed by glory hole and underground mining methods. Clark and Lydon (1962) reported that the mine had two glory holes. The main glory hole was on the west vein and was about 500 feet long by 250 feet wide and as much as 100 feet deep. Four hundred feet east of the glory hole was a 450-foot vertical shaft on the east vein with levels at 100, 200, 300, and 400 feet. A smaller glory hole was about 500 feet to the southeast. A crosscut on the 100-foot level extends west to the main glory hole and a drift on this level extends southeast to the small glory hole. The most productive portion of the east vein was on the 300-foot level where considerable stoping was done. From the shaft on this level drifts extend 400 feet to the northwest and southeast (Clark and Lydon, 1962). 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.

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References

Reference (Deposit): Clark. W.B., and Lydon, P.A., 1962, Mines and mineral resources of Calaveras County, California: California Division of Mines and Geology County Report No. 2, p. 37, 40.

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): Miller, C. N., 1940, Unpublished letter report on Alto Mine for Smith Manufacturing Company, 2 p.

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 24.

Reference (Deposit): Clark, W.B. and Carlson, D.W., 1956, Mines and mineral resources of El Dorado County: California Division of Mines, California Journal of Mines and Geology, v. 52, p. 413-429.

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California Gold

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