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Alpha-Omega Mines, Washington District (Omega District), Nevada Co., California, USA

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Latitude & Longitude (WGS84): 39° 20' 12'' North , 120° 45' 32'' West
Latitude & Longitude (decimal): 39.33669, -120.75893
 
These data are extracted from a USGS MRDS file that was constructed to cover the 2 major mines in the Washington District under a single file.

Location: The Alpha and Omega mines are the two primary hydraulic placer mines (Au-Ag-Pt) that comprise the Washington mining district in east-central Nevada County. They are located in secs. 16, 17 & 18, T17N, R11E, MDM, approximately 18 miles NE of Nevada City, in a National Forest area. The Alpha "Diggings" produced over $2 million (period values). Discovered after 1850. Production figures are unavailable for the Omega diggings, a mile to the E. The location selected by the USGS for latitude and longitude is the approximate center of the larger "Omega Diggings" on the USGS 7-1/2 minute Washington quadrangle.

History: While mining in the Washington District began in 1848-1849 as miners recovered considerable gold from placers of the Middle Yuba River, hydraulic mining of the Alpha and Omega deposits commenced in the mid-1850s. The mines were worked extensively until the mid-1880s when the Sawyer Decision put an end to large-scale hydraulic mining in the Sierra Nevada. Prior to that time, the Omega Mine was hydraulicked using three monitors. Water was obtained from a 9-mile flume and company ditches, which provided 5,000 miner's inches from the South Fork of the Yuba River, and another ditch, which brought 1,200 miner's inches from Diamond Creek. Tailings were channeled through a 3,000-foot bedrock tunnel and discharged into Scotchman Creek, which drained into the South Fork of the Yuba River. Later, Chinese miners reworked the old hydraulic tailings. Sometime before 1914, a restraining dam had been constructed in Scotchman Creek to contain the tailings. The Omega gravels were intermittently worked as late as 1914.

Commodity information: The average values for the undifferentiated gravels at the Omega workings were estimated to be $0.23 Au/cubic yard (at $35 ounce for Au). Ore materials: native gold comprised of fine- and coarse-grained gold and nuggets (.900 fine). Gangue materials: quartz and metamorphic gravels; accessory minerals include magnetite, ilmenite, zircon, pyrite, amphibole, epidote, chlorite, and siderite.

Mineralogy/Geology: Mineralization is a placer deposit (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), hosted in Tertiary sand and gravel. The ore body is irregular in form. 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: Paleozoic marine rocks, undivided, unit 4 (Western Sierra Nevada).

The basement beneath the Alpha and Omega mines consists of slate, schist, and quartzite of the Paleozoic Shoo Fly Complex. The Melones Fault Zone is just west of the Alpha Mine and separates the Shoo Fly Complex from the Feather River Peridotite Belt. Basal Eocene Auriferous Gravels The Alpha and Omega mines produced from Tertiary channel gravels that were part of an important branch of the ancestral Yuba River, which flowed westward to the auriferous gravel deposits at Relief Hill in the North Bloomfield District six miles to the west. The gravels rest directly on basement, into which the ancient river incised its channel. The bottom of the channel lies 1000 feet above the level of the present day South Fork of the Yuba River, which is nearby. Rocks of the Valley Springs and Mehrten Formations, which were originally deposited over the gravels, have been stripped away by erosion exposing the gravels. Consistent with most Tertiary gravel deposits in neighboring districts, the deposits can be divided lithologically and texturally into lower and upper units. The lower unit, or blue lead of the early miners, rests directly on bedrock and contains the richest ores. At the Omega workings, several hundred acres of auriferous gravel were exposed with a maximum exposed thickness of 175 feet. The deposits are composed of 150 feet of lower auriferous gravel covered by 6 feet of clay, which is overlain by another 20 feet of auriferous gravel that carries fine gold. In the lower gravels, the bulk of the material is less than 6 inches in diameter; some large granite boulders rest on bedrock. Compared to most other Tertiary gravel deposits where the finer upper gravels are the thicker unit, the extraordinarily thin unit at the Omega deposits suggests extensive Pliocene and later erosion. Omega gravels were known for their consistent yield of between 13 ? and 23 cents per yard. At the Alpha Workings, about 75 acres of auriferous gravels were preserved. The lower gravels are mainly quartz, quartzite, and hard conglomerate. Some quartz boulders up to 5 feet in diameter rest on bedrock. The gravel banks were 90 feet high including a 20-foot thick clay bed at the top.

Regional geologic structures include the Melones Fault Zone and the Foresthill Fault. Local structures include the Melones Fault Zone.

Production data: Jarman (1927) estimated that at Omega, 13 million cubic yards were mined and yielded an average 13-? cents per cubic yard. Lindgren (1911) estimated that 40 million cubic yards remained.

Mineral List



10 entries listed. 9 valid minerals.

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References

Lindgren, Waldemar (1900), Description of the Colfax sheet, California: USGS Geological Atlas, Colfax folio (Folio No. 66), 10 pp.

Lindgren, Waldemar (1911), The Tertiary gravels of the Sierra Nevada of California: USGS Professional Paper 73, 226 pp.: 139-141.

MacBoyle, Errol (1919), Mines and mineral resources of Nevada County: California Journal of Mines and Geology, California Mining Bureau (Report 16): 16: 59-63.

Jarman, Arthur (1927), Report of the Hydraulic Mining Commission upon the feasibility of the resumption of hydraulic mining in California, Omega Mine: California Division of Mines 23rd Report of the State Mineralogist (Report 23): 111-113.

Clark, L. D. (1960), Foothills fault system, western Sierra Nevada, California: Geological Society of America Bulletin: 71: 483-496.

Clark, Wm. B. (1966), Gold, in Mineral resources of California: California Division of Mines and Geology Bulletin 191: 179-185.

Clark, Wm. B. (1970a) Gold districts of California: California Division Mines & Geology Bulletin 193: 128.

Yeend, W.E. (1974) Gold-bearing gravel of the ancestral Yuba River, Sierra Nevada, California. USGS Professional Paper 772, 44 pp.

Saucedo, G. J. and Wagner, D. L. (1992), Geologic map of the Chico Quadrangle, California: California Department of Conservation, Division of Mines and Geology Regional Geologic Map Series, Map No. 7A, scale 1:250,000.

USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10310582.

USGS quadrangle Washington, California, 7½ minute topo map.

California Geological Survey Mineral Resources files, Sacramento, California, file No. 339-5934

 
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