Moores Flat Mining District, Nevada County, California, USAi

Regional Level Types
Moores Flat Mining District	Mining District
Nevada County	County
California	State
USA	Country

Location: A former Au-Ag-Pt mining area, the Moore's Flat district is located in secs. 15, 16 & 21, T18N, R10E, MDM, in north-central Nevada County about 15 miles NE of Nevada City. It was discovered in 1851. It is both a lode and placer district and includes the "diggings" at Moore's Flat, Oreleans Flat, Woolsey Flat, Snow Point, and Snow Tent. The Alleghany district adjoins the Moore's Flat district on the NE. Moore's Flat was named for H. M. Moore who built a store there in 1851. Shortly thereafter, Moore's Flat became an important hydraulic mining town, with large-scale operations at Moore's Flat, Orleans Flat, Woolsey Flat, Snow Point, and Snow Tent. By 1880, the town had grown to a population of 500, but 1884 brought the Sawyer Decision, which effectively put an end to large-scale hydraulic mining in the northern Sierra Nevada. The district and town lived on while the emphasis changed from hydraulic mining to drift mining and the lode mines. The only significant drifting was done at Snow Point. The mines began to fail in 1895, and the town of Moore's Flat was abandoned. Location selected by the USGS for latitude and longitude is the Moores Flat site on the USGS 7/12-minute Alleghany quadrangle.

Geology: A number of gravel deposits were accumulated in a west-southwest-trending Tertiary channel of the Yuba River that continues W and SW into the North Bloomfield district. At Moore's Flat, Lindgren (1911, p. 141) estimated, 26 million cubic yards were removed and 15 million remained. The gravels are quartz-rich and in places more than 100 feet thick. Hydraulic mining here during the 1880s had reported gold recoveries of 11 to 15 cents per cubic yard. The gravels are capped by andesite on the south side of the district. Bedrock consists of amphibolite, slate, and serpentine. The gold-quartz veins usually are narrow and contain small but often rich pockets.

In the Moore's Flat District, basement rocks include slate and argillite of the Calaveras Complex and ultramafic rocks, amphibolite, and serpentinite of the Feather River Peridotite Belt. Bedrock is overlain by thick Eocene auriferous gravels that are in turn overlain by Oligocene to Pliocene Valley Springs and Mehrten Formation rocks on San Juan Ridge in the south part of the district. Elsewhere in the district, the Valley Springs and Mehrten rocks have been stripped by erosion exposing the underlying gravels.

The district straddles the junction of the Central Belt and the Feather River Peridotite Belt. Consequently, several faults of the Melones Fault Zone cut the basement rock in a north-south direction. The Goodyears Creek Fault, which trends north-south through the center of the district separates Calaveras Complex rocks from the ultramafic rock complex to the east.

Basal Eocene Auriferous Gravels:

The Moore's Flat District contains several separate hydraulic workings including those at Moore's Flat, Orleans Flat, Woolsey Flat, Snow Point. and Snow Tent. The auriferous gravels were deposited by a southwesterly flowing branch of the ancestral Yuba River. This branch continued southwest from the vicinity of Snow Point and was responsible for depositing the famous placer deposits at Malakoff Diggings in the adjacent North Bloomfield District.

Similar to neighboring Tertiary gravel deposits, the deposits can be divided lithologically and texturally into a lower and upper unit. The lower unit, or blue lead of the early miners, rests directly on bedrock, and contains the richest ores. These deeper gravels are well-cemented and quartz-rich. The lower gravels are generally immature and composed of bluish-black slate and phyllite of the Calaveras Complex, weathered igneous rocks, and quartz. Lower gravels are more than 100 feet thick in places.

The upper gravels form the majority of the gravel deposits. Upper gravels are much finer, with clasts seldom larger than pebble size and characterized by an abundance of quartz sand and clay and silt beds. Large-scale cross-bedding and cut-and-fill features are common. Upper gravels are typically much leaner than the lower gravels. Upper unit gravels reach a thickness of about 200 feet at Woolsey Flat.

The average value of the undifferentiated upper and lower gravels is reported to have been $0.19 per yard and $0.25 at Snow Point and Woolsey Flat respectively.

At Moore's Flat, two bodies of gravel are exposed. The eastern deposit rests on amphibolite bedrock, while the western exposure rests on slate. The undifferentiated gravels vary from 100 - 130 feet thick. Of particular note are the large boulders ranging up to 10 feet in diameter in the hydraulic pit at Moore's Flat. These granitic boulders are thought to have been transported at least 8 miles, the distance to the nearest upstream exposure of similar lithology.

Smaller areas of exposed gravel occur at Orleans Flat and Snow Point. The gravel bank is 135 feet high at Snow Point, with the lower gravel unit being only 15 feet thick.

Regional geologic structures include the Ramshorn Fault, Goodyears Creek Fault and the Melones Fault Zone. Local structures include the Goodyears Creek Fault Zone and the Melones Fault Zone.

Ore Deposits: Mineral occurrence model information: Model code: 119; USGS model code: 39a; BC deposit profile: C01. C02; Deposit model name: Placer Au-PGE; Mark3 model number: 54. Deposits are hosted in unconsolidated Tertiary sand and gravels. Ore bodies are irregular in shape. Controls for ore emplacement included mechanical accumulation on irregular bedrock riffles and within river- and stream-channel lag gravels, bars, and point bar deposits. Local rocks include Tertiary pyroclastic and volcanic mudflow deposits, unit 9 (Cascade Range).

Commodity Information: Ore materials include native gold which is fine to coarse and nuggets (.900 fine). Gangue materials include quartz and metamorphic gravels. Accessory minerals include magnetite, ilmenite, zircon, pyrite, amphibole, epidote, chlorite, and siderite. Average values of the undifferentiated upper and lower gravels at Snow Point and at Woolsey Flat and Orleans Flat are reported to have been $0.19/cubic yard and $0.25/cubic yard, respectively (@ $35/ounce Au).

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Link to USGS MRDS:	10310651

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References