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Mormon Bar Mining District, Mother Lode Belt, Mariposa Co., California, USAi
Regional Level Types
Mormon Bar Mining DistrictMining District
Mother Lode BeltBelt
Mariposa Co.County
CaliforniaState
USACountry

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Latitude & Longitude (WGS84):
37° 27' 52'' North , 119° 56' 58'' West
Latitude & Longitude (decimal):
Locality type:


A placer Au mining area located in sec. 25, T5S, R18E, MDM, along Mariposa Creek about two to three miles SE 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.

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.

Mineralization is a placer Au-PGE deposit (Deposit model code 119; USGS model code 39a; BC deposit profile C01. C02; deposit model name: Placer Au-PGE; Mark3 model number 54), hosted in Quaternary unconsolidated sand and gravel .

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).

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.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.

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

Julihn and Horton (1940) reported a recovery value of 22 cents per cubic yard.Commodity Info: Julihn and Horton (1940) reported a recovery value of 22 cents per cubic yard.

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.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).

Regions containing this locality

Pacific OceanOcean
North America PlateTectonic Plate

Select Mineral List Type

Standard Detailed Strunz Dana Chemical Elements

Mineral List

Mineral list contains entries from the region specified including sub-localities

3 valid minerals.

Detailed Mineral List:

Gold
Formula: Au
Magnesite
Formula: MgCO3
Reference: Laizure, Clyde McK (1928), Mariposa, Madera Counties: California Mining Bureau. Report 24: 148; Murdoch, Joseph & Robert W. Webb (1966), Minerals of California, Centennial Volume (1866-1966): California Division Mines & Geology Bulletin 189: 248; Pemberton, H. Earl (1983), Minerals of California; Van Nostrand Reinholt Press: 203.; USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10031263.
Quartz
Formula: SiO2
Reference: U.S. Geological Survey, 2005, Mineral Resources Data System: U.S. Geological Survey, Reston, Virginia.

List of minerals arranged by Strunz 10th Edition classification

Group 1 - Elements
Gold1.AA.05Au
Group 4 - Oxides and Hydroxides
Quartz4.DA.05SiO2
Group 5 - Nitrates and Carbonates
Magnesite5.AB.05MgCO3

List of minerals arranged by Dana 8th Edition classification

Group 1 - NATIVE ELEMENTS AND ALLOYS
Metals, other than the Platinum Group
Gold1.1.1.1Au
Group 14 - ANHYDROUS NORMAL CARBONATES
A(XO3)
Magnesite14.1.1.2MgCO3
Group 75 - TECTOSILICATES Si Tetrahedral Frameworks
Si Tetrahedral Frameworks - SiO2 with [4] coordinated Si
Quartz75.1.3.1SiO2

List of minerals for each chemical element

CCarbon
C MagnesiteMgCO3
OOxygen
O MagnesiteMgCO3
O QuartzSiO2
MgMagnesium
Mg MagnesiteMgCO3
SiSilicon
Si QuartzSiO2
AuGold
Au GoldAu

References

Sort by

Year (asc) Year (desc) Author (A-Z) Author (Z-A)
Bowen, O.E., Jr. & C.H. Gray, Jr. (1957), Mines and mineral deposits of Mariposa County, California: California Journal of Mines and Geology (Report 53): 53(1&2): 35-343.
Clark, Wm. B. (1970a) Gold districts of California: California Division Mines & Geology Bulletin 193: 93.
Julihn, C.E. and F.W. Horton (1940), Mines of the Southern Mother Lode Region, Part II - Tuolumne and Mariposa Counties; in: Mineral Industried Survey of the United States, California. U.S. Bureau of Mines, Bulletin 424: 179 pp.
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.
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.
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: 33-79.
Strand, R.G. (1967), Mariposa Sheet: California Division of Mines and Geology Geologic Map of California, scale 1:250,000.
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.
USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10310652.

USGS MRDS Record:10310652

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