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Alto Mine

Past Producer in Calaveras county in California, United States with commodities Gold, Silver
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Sections on this page
  1. Identification information
  2. Geographic coordinates
  3. Site location context
  4. Geographic areas
  5. Public Land Survey System information
  6. Commodities
  7. Materials information
  8. Alteration
  9. Mineral occurrence model information
  10. Host and associated rocks
  11. Nearby scientific data
  12. Geologic structures
  13. Ore body information
  14. Controls for ore emplacement
  15. Economic information about the deposit and operations
  16. Mining district
  17. Land status
  18. Ownership information
  19. Bibliographic references
  20. General comments
  21. Reporter information

Geologic information

Identification information

Deposit ID 10310583
Record type Site
Current site name Alto Mine

Geographic coordinates

Geographic coordinates: -120.60654, 37.89457 (WGS84)
Elevation 120
Location accuracy 100(meters)
Relative position The Alto Mine lies six miles southeast of Copperopolis, CA

Site location context

Political divisions (FIPS codes)

Calaveras(county)

California(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

New Melones Dam(quadrangle 1:24,000 scale)

Oakdale(quadrangle 1:100,000 scale)

San Jose(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

Upper Stanislaus(hydrologic unit)

San Joaquin(hydrologic accounting unit)

San Joaquin(hydrologic subregion)

California(hydrologic region)

Geographic areas

Country State County
United States California Calaveras

Public Land Survey System information

Meridian Township Range Section Fraction State
Mount Diablo 001N 012E 36 SE/4 California

Comments on the location information

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

Commodities

Commodity Importance
Gold Primary
Silver Secondary

Comments on the commodity information

  • 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.
  • Ore Materials: Free-milling gold in quartz and disseminated auriferous pyrite in greenstone
  • Gangue Materials: Quartz, slate, greenstone

Materials information

Materials Type of material
Gold Ore
Pyrite Ore
Slate Ore
Quartz Gangue

Alteration

  • (Local) Hydrothermal pyritization of greenstone

Mineral occurrence model information

Model code 273
USGS model code 36a
Deposit model name Low-sulfide Au-quartz vein
Mark3 model number 27

Host and associated rocks

  • Host or associated Host
    Rock type Metamorphic Rock > Metasedimentary Rock > Slate
    Rock unit name Salt Springs slate
    Stratigraphic age (youngest) Late Jurassic
  • Host or associated Host
    Rock type Metamorphic Rock > Metavolcanic Rock > Mafic Metamorphic Rock > Greenstone
    Rock unit name Salt Springs slate
    Stratigraphic age (youngest) Late Jurassic

Nearby scientific data

(1) -120.60654, 37.89457
ListMap

Economic information

Geologic structures

Type of structure Regional
Structure description Bear Mountains fault zone, Melones fault zone

Ore body information

  • General form Tabular, pinch and swell

Controls for ore emplacement

  • Free gold within mesothermal gold-quartz stringers

Comments on the geologic information

  • 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.
  • 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.

Economic information about the deposit and operations

Operation type Surface-Underground
Development status Past Producer
Commodity type Metallic
Deposit size Small
Significant Yes
Discovery year 1886

Mining district

District name Alto District

Land status

Ownership category Private
Area name Calaveras County Planning Dept.

Ownership information

  • Type Owner
    Owner Various private owners

Comments on the workings information

  • 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.

Comments on other 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).

Comments on 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.

Reference information

Bibliographic references

  • Deposit

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

  • 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.

  • 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.

  • 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.

  • Deposit

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

General comments

Subject category Comment text
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.

Reporter information

Type Date Name Affiliation Comment
Reporter 17-APR-2006 Downey, Cameron (Higgins, Chris, T.) California Geological Survey CGS (Formerly CDMG)
Editor 01-SEP-2007 Schruben, Paul G. U.S. Geological Survey Converted from S&A FileMaker format to Oracle. Edit checks on rocks, units, and ages with Geolex search, and other fields.

Beyond USGS

Supplemental information added by qvyshift.com. Not part of the original USGS MRDS record.

External references

Maps centered on this site

Authoritative California resources

These are landing pages for further research — the state agencies don't currently expose per-mine deep links.