Mormon Bar Mining District, Mariposa County, California, USAi

Regional Level Types
Mormon Bar Mining District	Mining District
Mariposa County	County
California	State
USA	Country

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).

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Link to USGS MRDS:	10310652

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