La Grange District

Site Name

Primary: La Grange District

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Commodity

Primary: Gold
Secondary: Silver
Secondary: Platinum

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Location

State: California
County: Stanislaus
District: La Grange District

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Land Status

Not available

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: District
Operation Category: Producer
Deposit Type: Stream placer
Operation Type: Surface-Underground
Discovery Year: 1850
Years of Production:
Organization:
Significant: Y
Deposit Size: M

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Physiography

Not available

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Mineral Deposit Model

Model Name: Placer Au-PGE

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Orebody

Form: Irregular

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Structure

Not available

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Alterations

Not available

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Rocks

Name: Sand and Gravel
Role: Host
Age Type: Host Rock
Age Young: Quaternary

Name: Sand and Gravel
Role: Host
Age Type: Host Rock
Age Young: Tertiary

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Analytical Data

Not available

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Materials

Ore: Gold
Ore: Platinum
Gangue: Quartz

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Comments

Comment (Location): Location selected for latitude and longitude is the Mile 45 mark on the Tuolumne River on the USGS 7.5-minute Cooperstown quadrangle. This location represents approximately the half-way distance of the historic dredging activity along the river.

Comment (Geology): REGIONAL GEOLOGY The La Grange District is situated within the Great Valley geologic province, which is here represented by the San Joaquin Valley. It is adjacent to the westernmost edge of the Sierra Nevada geologic province. The Great Valley province is characterized by Cenozoic sedimentary and volcanic rocks, while the Sierra Nevada province is characterized by complex lithologies and structures that were assembled through various plate-tectonic processes. In this region, the Sierra Nevada province is composed of belts of Paleozoic-Mesozoic metamorphic complexes that are intruded by various Mesozoic plutons. Together, they compose the basement of the province. This basement is overlain at higher elevations by erosional remnants of Cenozoic volcanic and sedimentary rocks, including gravels. Most of these various lithologies contain gold in places. Structurally, the metamorphic rocks and some of the plutonic rocks have been deformed by folding and faulting. The major fault zones typically trend northerly or northwesterly, although in places intrusion of the younger plutons has deformed some of the zones so as to assume other trends as well. In contrast, the overlying Cenozoic rocks are relatively undeformed. The rocks of the Great Valley province overlie the basement of the Sierra Nevada where it extends westward underneath the San Joaquin Valley. LOCAL GEOLOGY The La Grange District is associated with both modern alluvial deposits of the main drainage of the Tuolumne River where it discharges into the San Joaquin Valley after passing through the Sierra Nevada basement terranes and with older Tertiary alluvial deposits. The materials in these deposits have been derived by erosion of the various basement and Cenozoic rocks at higher elevations. In places, the basement rocks contain gold within quartz veins and altered rock, while the Cenozoic deposits contain placer gold derived by erosion of these older basement rocks. Erosion of both the gold-bearing basement rocks and the older Cenozoic rocks provided the gold that was eventually deposited in the modern and Tertiary placer deposits along and adjacent to the Tuolumne River in the La Grange District. The dredging field south of La Grange appears to have exploited older alluvial channel deposits that may be both Quaternary and Tertiary in age. Geologic mapping of this area (Marchand and others, 1981) shows the presence of younger Quaternary alluvial deposits as well as older Tertiary sedimentary and volcanic deposits (Ione Formation, Valley Springs Formation, and Mehrten Formation). Gold-bearing gravels were likely present in both the Quaternary deposits and the Ione Formation.

Comment (Identification): This district, which was primarily a dredge field, includes a 9-mile segment of the modern Tuolumne River between the towns of La Grange and Waterford. It also includes an area of older abandoned river channels about two miles south of La Grange.

Comment (Development): As published information about this district is sparse, it is uncertain exactly what year mining activity began in this district. Nonetheless, some surface placer mining and hydraulicking were done in La Grange area during the gold rush. The dominant production from the district came from dredging in the 20th century. This method of mining began in 1907 and continued intermittently until the 1950?s. Amalgamation was used in the recovery process. Gold is still being produced as a by-product of aggregate mining along the Tuolumne River.

Comment (Economic Factors): Koschmann and Bergendahl (1968) reported about 108,000 ounces of gold produced from this district between !932 and 1959. Clark (1970) estimated that total production of gold for this district was approximately $13 million.

Comment (Deposit): In the main dredging field in and adjacent to the Tuolumne River, gold was recovered from Quaternary alluvial gravels and flood-plain and terrace deposits. The gravels are medium to coarse, loosely consolidated, and vary from about 10 to 40 feet in depth. Bedrock below the gravels is volcanic rock. In the field south of La Grange, the ore material was very hard and overburden was up to 90 feet thick in places. Charles (1947) described the ore material as a Pleistocene river channel, but accompanying descriptions of overlying volcanic material suggest that at least some of the material was Tertiary in age, possibly Ione Formation. Platinum is also present in the alluvial deposits.

Comment (Workings): The district is composed mainly of one large dredge field about nine miles long and 1/2 mile wide along the Tuolumne River. The main dredging operation (La Grange Gold Dredging Company) reportedly processed over 70 million cubic yards of material during its activity in this field. A small dredge field is about two mile south of La Grange. Minor workings from surface placering and hydraulicking processes in the 1800?s were also present locally in the district. Vast deposits of dredge tailings still remain in this district.

Comment (Commodity): Commodity Info: Gold recovered was fine, but of good grade. Reported average yields during the 1920?s were about $0.12 per cubic yard. Platinum was about 2% of the total value recovered from dredging in the main field.

Comment (Commodity): Ore Materials: Native gold, native platinum

Comment (Commodity): Gangue Materials: Metamorphic rock, igneous rock, quartz (all as clasts)

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References

Reference (Deposit): Wagner, D.L., Bortugno, E.J., and McJunkin, R.D., 1990, Geologic map of the San Francisco-San Jose Quadrangle, California: California Department of Conservation, Division of Mines and Geology Regional Geologic Map Series, Map No. 5A, scale 1:250,000.

Reference (Deposit): Charles, A., 1947, Mines and mineral resources of Stanislaus County: California Journal of Mines and Geology, v. 43, no. 2, p. 92-94.

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 85-86.

Reference (Deposit): Higgins, C.T. and Dupras, D.L., 1993, Mineral land classification of Stanislaus County, California: California Division of Mines and Geology Special Report 173, 174 p.

Reference (Deposit): Koschmann, A.H., and Bergendahl, M.H., 1968, Principal gold-producing districts of the United States: U. S. Geological Survey Professional Paper 610, 283 p.

Reference (Deposit): Laizure, C.M., 1925, Stanislaus County: California State Mining Bureau 21st Report of the State Mineralogist, v. 21, no. 2, p. 200-222.

Reference (Deposit): Laizure, C.M., 1935, Current mining activities in the San Francisco District with special reference to gold: California Journal of Mines and Geology, v. 31, no. 1, p. 48.

Reference (Deposit): Marchand, D.E., Bartow, J.A. and Shipley, S., 1981, Preliminary geologic maps showing Cenozoic deposits of the Cooperstown and La Grange quadrangles, Stanislaus and Tuolumne Counties, California: U.S. Geological Survey Open-File Report 81-1049, scale 1:24,000.

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California Gold

"Where to Find Gold in California" looks at the density of modern placer mining claims along with historical gold mining locations and mining district descriptions to determine areas of high gold discovery potential in California. Read more: Where to Find Gold in California.