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Mormon Bar District

Past Producer in Mariposa county in California, United States with commodity Gold
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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. Mineral occurrence model information
  9. Host and associated rocks
  10. Nearby scientific data
  11. Ore body information
  12. Controls for ore emplacement
  13. Economic information about the deposit and operations
  14. Mining district
  15. Bibliographic references
  16. General comments
  17. Reporter information

Geologic information

Identification information

Deposit ID 10310652
Record type District
Current site name Mormon Bar District

Comments on the site identification

  • 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.

Geographic coordinates

Geographic coordinates: -119.94983, 37.46483 (WGS84)
Location accuracy 100(meters)
Relative position The district is about two miles south-southeast of the town of Mariposa.

Site location context

Political divisions (FIPS codes)

Mariposa(county)

California(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

Mariposa(quadrangle 1:24,000 scale)

Shaver Lake(quadrangle 1:100,000 scale)

Mariposa(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

Fresno River(hydrologic unit)

San Joaquin(hydrologic accounting unit)

San Joaquin(hydrologic subregion)

California(hydrologic region)

Geographic areas

Country State County
United States California Mariposa

Public Land Survey System information

Meridian Township Range Section Fraction State
Mount Diablo 005S 018E 25 California

Comments on the location information

  • 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.

Commodities

Commodity Importance
Gold Primary

Comments on the commodity information

  • Commodity Info: Julihn and Horton (1940) reported a recovery value of 22 cents per cubic yard.
  • Ore Materials: Native gold
  • Gangue Materials: Metamorphic rock, igneous rock, quartz (all as clasts)

Materials information

Materials Type of material
Gold Ore
Quartz Gangue

Mineral occurrence model information

Model code 119
USGS model code 39a
Deposit model name Placer Au-PGE
Mark3 model number 54

Host and associated rocks

  • Host or associated Host
    Rock type Unconsolidated Deposit > Sand and Gravel
    Stratigraphic age (youngest) Quaternary

Nearby scientific data

(1) -119.94983, 37.46483
ListMap

Economic information

Ore body information

  • General form Irregular

Controls for ore emplacement

  • Mechanical accumulation within stream-channel lag gravels, bars, and point-bar deposits.

Comments on the geologic information

  • 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).

Economic information about the deposit and operations

Operation type Surface
Development status Past Producer
Commodity type Metallic
Deposit size Small
Significant Yes
Discovery year 1848

Mining district

District name Mormon Bar District

Comments on the workings information

  • 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.

Comments on other economic factors

  • Clark (1970) estimated a production of about 75,000 ounces of gold for this district.

Comments on development

  • 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.

Reference information

Bibliographic references

  • Deposit

    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.

  • Deposit

    Clark, W. B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 93.

  • Deposit

    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.

  • 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.

  • Deposit

    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.

  • Deposit

    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.

  • Deposit

    Strand, R.G., 1967, Mariposa Sheet: California Division of Mines and Geology Geologic Map of California, scale 1:250,000.

  • 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.

General comments

Subject category Comment text
Deposit 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.

Reporter information

Type Date Name Affiliation Comment
Reporter 19-FEB-2007 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.

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Authoritative California resources

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