Folsom District

Site Name

Primary: Folsom District

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Commodity

Primary: Gold
Secondary: Silver
Secondary: Platinum

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Location

State: California
County: Sacramento
District: Folsom 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: 1849
Years of Production:
Organization:
Significant: Y
Deposit Size: L

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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

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Analytical Data

Not available

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Materials

Ore: Gold
Ore: Platinum
Ore: Electrum
Gangue: Quartz

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Comments

Comment (Geology): REGIONAL GEOLOGY The Folsom District is situated within the Great Valley geologic province, which is here represented by the Sacramento 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 Folsom District is associated with Cenozoic alluvial deposits of the main drainage of the American River where it discharges into the Sacramento Valley after passing through the Sierra Nevada basement terranes. 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 Cenozoic placer deposits along the American River at Folsom and farther downstream. Platinum was probably derived from erosion of ultramafic rocks and serpentinite in the Sierra Nevada basement.

Comment (Identification): The Folsom District is one of the two largest dredging fields in California. It extends along the American River from the city of Folsom southwestward for about 10 miles to the community of Rancho Cordova.

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

Comment (Deposit): Gold was recovered from Tertiary and Quaternary alluvial gravels and flood-plain and terrace deposits in and adjacent to the American River. The gold-bearing gravels are found chiefly near the bases of certain Plio-Pleistocene terraces and near the contact of the Laguna Formation with the underlying Mehrten Formation (Carlson, 1955). These settings have been exploited mainly south of the river. Platinum is also present in the alluvial deposits.

Comment (Development): The region around the city of Folsom was extensively placer-mined during the gold rush and continued through the 1890?s. Carlson (1955) reported some hydraulicking and drifting along older Cenozoic channels in the Folsom area before 1900, with drift mining continuing into the early 1900?s. Dredging began in a minor way in 1894, but became a major activity after 1898 and continued until 1962. The dominant gold production in the district was from dredging. Production of platinum as a by-product of dredging began in 1905 (Carlson, 1955). Amalgamation was used in the recovery process. Platinum was recovered as a by-product of dredging since 1905. Gold is still being recovered as a by-product from aggregate mining in similar geologic materials adjacent to this district.

Comment (Commodity): Commodity Info: Recoveries of gold ranged from about $0.10 to $0.20 per cubic yard at $35/oz. Carlson (1955). Carlson (1955) reported that platinum recovery from dredging operations in California ranged from 1/3 to one ounce for every 50 ounces of gold. Logan (1919) reported that about 150 ounces of platinum were produced from dredging annually during 1914-1916.

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

Comment (Location): Location selected for latitude and longitude is the intersection of Buffalo Creek with Nimbus Road south of US Highway 50 on the USGS 7.5-minute Buffalo Creek quadrangle. This location approximates the center of the regional dredging area.

Comment (Workings): Early workings in this district consisted of those from surface-placer methods, hydraulicking, and drift mining. The area mined by dredging is about 10 miles long in a southwest direction and up to seven miles wide. Digging depths ranged from about 30 to 110 feet. Approximately one billion cubic yards of gravel were dredged by the Natomas Company alone. Vast deposits of dredge tailings still remain in this district.

Comment (Economic Factors): Koschmann and Bergendahl (1968) reported gold produced from this district was at least 3 million ounces. Clark (1970) estimated that total production of gold for this district was approximately $125 million.

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References

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 47-48.

Reference (Deposit): Carlson, D.W., 1955, Mines and mineral resources of Sacramento County, California: California Journal of Mines and Geology, v. 51, no. 2, p. 134-143.

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): Lindgren, W., 1911, Tertiary gravels of the Sierra Nevada: U.S. Geological Survey Professional Paper 73, p. 222.

Reference (Deposit): Logan, C.A., 1919, Platinum and allied metals in California: California State Mining Bureau Bulletin 85, 120 p.

Reference (Deposit): Wagner, D.L. and others, 1981, Geologic map of the Sacramento Quadrangle, California: California Department of Conservation, Division of Mines and Geology Regional Geologic Map Series, Map No. 1A, scale 1:250,000.

Reference (Deposit): Winston, W.B., 1910, Gold dredging in California: California State Mining Bureau Bulletin 57, p. 175-204.

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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.