Mormon Bar District

  

Mine Info

State: California

County: Mariposa

Elevation:

Primary Mineral: Gold

Lat, Long: 37.4649, -119.94885



Mormon Bar District MRDS details

Site Name

Commodity

Location

Primary: "Mormon Bar District"
Primary: Gold
State: California
County: Mariposa
District: Mormon Bar District
 

Land Status

Holdings

Workings

Land ownership:
Administrative Organization:
Type:


Ownership

Production

Deposit

Record Type: District
Operation Category: Past Producer
Deposit Type: Stream placer
Plant Type:
Plant Subtype:
Operation Type: Surface
Mining Method:
Milling Method:
Year First Production:
Year Last Production:
Discovery Year: 1848
Discovery Method:
Years of Production:
Organization:
Significant: Y
Deposit Size: S


Physiography

Mineral Deposit Model

Orebody

Name:
Form: Irregular


Structure

Alterations

Rocks

Name: Sand and Gravel
Role: Host
Description:
Age Type: Host Rock
Age in Years:
Dating Method:
Material Analyzed:
Age Young: Quaternary
Age Old:


Analytical Data

Materials

Ore: Gold
Gangue: Quartz


Comments

Comment Type:
Identification
Comment The deposits of this placer-mining district are along Mariposa Creek about two to three miles southeast of the town of Mariposa. This district was a relatively small producer of gold; consequently, there is little information on it in the mining and geologic literature.
Comment Type:
Location
Comment Location selected for latitude and longitude is the approximate center of the dredge tailings west of the county fairgrounds on the USGS 7.5-minute Mariposa quadrangle. The township values are projected because the deposit is on a historic land grant.
Comment Type:
Workings
Comment The district was mined initially by standard hand-placering methods of the gold-rush period. Later, dragline dredging was used. This method was probably responsible for the approximately 1,000-yard-long and 100-yard-wide placer tailings shown at present-day Mormon Bar on the Mariposa 7.5-minute quadrangle. Dredging likely extended to the bedrock surface, which is probably very shallow along Mariposa Creek based on the reported average depth of six feet for the gravels.
Comment Type:
Commodity
Comment Gangue Materials: Metamorphic rock, igneous rock, quartz (all as clasts)
Comment Type:
Development
Comment The area was placer-mined during the 1850's and 1860's and, by 1870, the rich, easily worked placers were largely exhausted. The area was mined again in the 1930's by dredging; mining was made difficult by the large boulders and difficult bedrock conditions, including pinnacles of bedrock that projected above the water level, which caused problems in maneuvering the dredge. These conditions contributed to excessive costs for mining and maintenance. In 1938, the Trebor Corporation installed a dragline dredge and floating wash plant on Mariposa Creek at Mormon Bar. Mining was begun in September , and 1,600 cubic yards per day were processed until the available gravels under lease were worked out in early 1939. The dredge was then moved to another district. Clark (1970) reported that there was minor prospecting subsequent to the dredging activity.
Comment Type:
Economic Factors
Comment Clark (1970) estimated a production of about 75,000 ounces of gold for this district.
Comment Type:
Geology
Comment REGIONAL GEOLOGY The Mormon Bar District is situated in the foothills of the Sierra Nevada geologic province, which 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. LOCAL GEOLOGY The geology of the placer deposits in this district is typical of the southern Mother Lode region of the Sierra Nevada. The deposits are in Quaternary alluvium and companion bench gravels derived from erosion of adjacent and upstream bedrock that is composed of Paleozoic-Mesozoic metamorphic rocks and Mesozoic granitic rocks that intrude them. Gold-bearing quartz veins and associated hydrothermally altered wallrock within this bedrock complex are the dominant sources of the gold in the alluvium at Mormon Bar. The Tertiary gold-bearing gravel and volcanic deposits characteristic of the Sierra Nevada north of Mariposa County are largely absent in the county. The average thickness of the exploited gravels at Mormon Bar was only about six feet (Julihn and Horton, 1940).
Comment Type:
Deposit
Comment The deposits in this district consist of Quaternary stream-bed alluvium and likely some shallow bench gravels that lie only a few feet above the present high-water levels of Mariposa Creek (Bowen and Gray, 1957). The average depth of the mined gravels was about six feet. The deposits are characterized by many large boulders and difficult bedrock conditions, with many pinnacles of bedrock that commonly project well above the water level. The gold accumulated in these alluvial materials by mechanical accumulation from eroded materials derived from the surrounding gold-bearing bedrock. Bowen and Gray (1957) considered most of the placer deposits in the county to be largely worked out. Nonetheless, there is probably on-going minor accumulation of gold in the alluvium of Mariposa Creek during times of high water.
Comment Type:
Commodity
Comment Commodity Info: Julihn and Horton (1940) reported a recovery value of 22 cents per cubic yard.
Comment Type:
Commodity
Comment Ore Materials: Native gold


References

Reference Category:
Deposit
Pages:
Reference: Bowen, O.E., Jr. and Gray, C.H., Jr., 1957, Mines and mineral resources of Mariposa County, California: California Journal of Mines and Geology, v. 53, nos. 1-2, p. 35-343.
URL:
Reference Category:
Deposit
Pages:
Reference: Clark, W. B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 93.
URL:
Reference Category:
Deposit
Pages:
Reference: Julihn, C.E., and Horton, F.W., 1940, Mineral industries survey of the United States - Mines of the southern Mother Lode Region, Part II - Tuolumne and Mariposa counties: U.S. Bureau of Mines Bulletin 424, 179 p.
URL:
Reference Category:
Deposit
Pages:
Reference: 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.
URL:
Reference Category:
Deposit
Pages:
Reference: Krauskopf, K.B., 1985, Geologic map of the Mariposa quadrangle, Mariposa and Madera counties, California: U.S. Geological Survey Geologic Quadrangle Map GQ-1586, scale 1:62,500.
URL:
Reference Category:
Deposit
Pages:
Reference: Schweickert, R.A., Hanson, R.E., and Girty, G.H., 1999, Accretionary tectonics of the Western Sierra Nevada Metamorphic Belt in Wagner, D.L. and Graham, S.A., editors, Geologic field trips in northern California: California Division of Mines and Geology Special Publication 119, p. 33-79.
URL:
Reference Category:
Deposit
Pages:
Reference: Strand, R.G., 1967, Mariposa Sheet: California Division of Mines and Geology Geologic Map of California, scale 1:250,000.
URL:
Reference Category:
Deposit
Pages:
Reference: 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.
URL:

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A mine is a hole in the ground, owned by a liar.
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About The MRDS Mines Database

All mine locations were obtained from the USGS Mineral Resources Data System. The locations and other information in this database have not been verified for accuracy. It should be assumed that all mines are on private property.

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