| Basis for focus area |
Distribution of ultramafic rocks in ophiolite belts, MRDS records of chromite occurrences; outlines of Cr provinces in Thayer and Lipin (1979) and Peterson (1984). |
| Identified resources |
Identified resources and historical production of chromium (chromite). |
| Production |
Chromite production data are undoubtedly inaccurate, but it is estimated that a total of about 32,000 long tons chromite was produced from the northern Sierra Nevada (data end in 1949; Rynearson, 1953). All chromite deposits in the Sierra Nevada foothills are relatively small, with most containing less than 100 tons of ore, although several disseminated deposits contained over 100,000 tons (Thayer and Lipin, 1979). Calaveras and Almanor Counties: ~7,800 tons (Cater, 1948); Tuolomne and Mariposa Counties: ~7,000 tons (Cater, 1948); Tulare and Fresno Counties: ~30,000 tons produced (Rynearson, 1948); El Dorado County: ~50,000 tons (Cater and others, 1951); Northern Sierra: ~35,000 tons (Rynearson, 1953). |
| Status |
Past mining; no current mining or exploration. |
| Estimated resources |
Resource estimates and production for mines and prospects in El Dorado County are 600,000 tons of reserves (Cater and others, 1951). |
| Geologic maps |
Matthews and Burnett (1965), scale 1:250,000; Wagner and others (1981), scale 1:250,000; Wagner and others (1991), scale 1:250,000; Saucedo and Wagner (1992), scale 1:250,000; Jennings and others (2010), scale 1:750,000; Wells and others (1946), multiple scales; California Geological Survey Regional Geologic Maps and Geologic Atlas of California, scale 1:250:000. |
| Geophysical data |
Adequate Rank 2 aeromagnetic and aeroradiometric coverage. |
| Favorable rocks and structures |
Ultramafic rocks in the Sierra Nevada range. |
| Deposits |
Bear River chrome mine (MRDS dep_id: 10043540), West Chrome mine (MRDS dep_id: 10110839). |
| Evidence from mineral occurrences |
MRDS; Dow and Thayer (1946). |
| Geochemical evidence |
No data. |
| Geophysical evidence |
No single geophysical method gives unequivocal identification of buried massive pods of chromite. It is possible that a combination of gravity, magnetic, and seismic data, and complex resistivity, might be used to explore successfully if used in a systematic fashion. |
| Evidence from other sources |
Ultramafic intrusions are known to host podiform chromite bodies, although most known deposits are small (for example, Rynearson, 1953). |
| Cover thickness and description |
Many deposits at the surface, but some are underground. |
| Authors |
Laurel G. Woodruff, Catherine Wesoloski, Greg Marquis. |
| New data needs |
Updated geologic mapping, geochemistry, aeromagnetic and aeroradiometric surveys. |
| Geologic mapping and modeling needs |
Updated geologic mapping. |
| Geophysical survey and modeling needs |
High resolution, Rank 1, aeromagnetic surveys would be useful in delineating serpentine, ultramafics, but unclear if a new geophysical survey would be effective. |
| Digital elevation data needs |
Lidar partial coverage, mostly from wildfires; needs full coverage. |
