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Dobbas Mine (Dobbas group), Flagstaff Hill, Salmon Falls District (Flagstaff Hill area), El Dorado Co., California, USA

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A former Cr occurrence/mine located in the E½ sec. 16, W½ sec. 15 & sec. 22, T11N, R8E, MDM, about 3.65 km N of Flagstaff Hill and 1.1 km (0.7 mile) ENE of Shirttail Peak (Rattlesnake Bar District), on private land. Operated by D. J. Dobbas. There was considerable activity during WWI. The property was prospected in 1941 and some ore was produced. Some production also occurred during the period 1942 and 1944. MRDS database stated accuracy for this location is 100 meters.

The geographic coordinates presented in 3 of the 4 USGS MRDS files are in error stating a longitude value of 120 degree. The correct value is 121 degrees.

Mineralization is hosted in dunite and serpentinite. The ore body forms irregular pods and lenses with a width of 243.84 meters and a length of 1,609.3 meters. The deposit is located near the northern end of an elongated N-S-trending ultramafic mass approximately 5 miles long and 1 mile wide, which forms Flagstaff Hill. Most of the ore bodies trend N top NW, roughly parallel to the eastern margin of the ultramafic mass, and lie from 200 to 1,000 feet W of the contact. Local alteration includes much of the ultramafic rock altered to talc-chlorite or talc-serpentine. Associated rocks include pyroxenite, harzburgite, and diorite. Local rocks include ultramafic rocks, chiefly Mesozoic, unit 2 (Western Sierra Nevada and Klamath Mountains) and/or Mesozoic granitic rocks, unit 3 (Sierra Nevada, Death Valley area, Northern Mojave Desert and Transverse Ranges).

Workings include surface and underground openings. The Dobbas Mine workings are divided into 5 groups, A through E, which extend approximately a mile along a ridge N from Flagstaff Hill. Area A consists of 3 open cuts and an adit. The underground workings are inaccessible. Approximately 1 ton of "high-grade" ore is piled on the dump. Area B includes 6 old pits and 1 new pit. Ore in the new pit strikes N30W and dips steeply to the E. Area C consists of 10 pits or cuts, 2 adits and 1 shaft. Layers of magnesiochromite strike N45W. Area D is worked by a number of trenches. The ore strikes N20W and dips 75E. Area E consists of an open cut and a number of exploratory trenches. The ore strikes N60W and dips 60N.

Local geologic structures include the ore in area "C" cut by vertical, EW-striking faults.

Production data are found in: California Division of Mines and Geology Bulletin 134: 110.

Analytical data results: The ore ranges from 6 to 30% Cr2O3, CR:FE ratio = 0.67. The ore has a very high Fe content. Large reserves may be of little economic value because of the high Fe content.

Mineral List



6 entries listed. 4 valid minerals.

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References

Cater, F.W., Jr., G.A. Rynearson & D.H. Dow (1951), Chromite deposits of El Dorado County, California: California Division Mines Bulletin 134, part III, Chapter 4: 146-148, Pls. 8 & 9.

Clark, Wm. B. & D.W. Carlson (1956), Mines and mineral resources of El Dorado County, California: California Journal of Mines and Geology (Report 52): 52(4): 385, 386, 467.

Stinson, Melvin Clarence (1975) California Division of Mining and Geology Mineral Property Report (unpublished).

Pemberton, H. Earl (1983), Minerals of California; Van Nostrand Reinholt Press: 156 (map 4-7).

USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10043721, 10109201, 10110533 & 10211385.

U.S. Bureau of Mines, Minerals Availability System (MAS) file #0060170007.

 
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