California Coastal Ranges chromite

Region West, Northwest
Mineral systems
Deposit types
Critical minerals
Other minerals

Information leading to the delineation of this focus area

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); Peterson (1984); Dow and Thayer (1946); Walker and Griggs (1953).
Identified resources Identified resources and historical production of chromium (chromite).
Production All chromite deposits 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). Northern Coast Ranges (pre 1946): ~56,000 tons of chromite (Dow and Thayer, 1946); Southern Coast Ranges (mostly from San Luis Obispo County): ~80,000 tons (Walker and Griggs, 1953).
Status Past mining; no current mining or exploration.
Estimated resources The chromite deposits of the Sierra Nevada foothills, the Klamath Mountain district, and the Coastal Ranges have about 150,000 ton of chromite in identified reserves (Thayer and Lipin, 1979). Resource estimates and production for mines and prospects pre- mid-1950s given in Dow and Thayer (1946) and Walker and Griggs (1953).
Geologic maps Wagner and others (1991), scale 1:250,000; Jennings and Strand (1958), scale 1:250,000; Jennings (1958), scale 1:250,000; Jennings and others (2010), scale 1:750,000; Wells and others (1946), multiple scales.
Geophysical data Mostly Rank 2 with minor Rank 1 aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Coastal Range Ophiolite ultramafic rocks.
Deposits El Divisadero (MRDS dep_id: 10109766), Bacon mine (MRDS dep_id: 10043410), Grey Eagle mine (MRDS dep_id: 10098580).
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, but most known deposits are small (see Dow and Thayer, 1946).
Cover thickness and description Many deposits are at the surface, but some are underground.
Authors Laurel G. Woodruff, Catherine Wesoloski, Greg Marquis.
New data needs Aeromagnetics, lidar, geochemistry, geologic mapping.
Geologic mapping and modeling needs Updated geologic mapping.
Geophysical survey and modeling needs High resolution, Rank 1, aeromagnetic surveys would be useful in delineating serpentinite and other ultramafics, but unclear if a new geophysical survey would be effective.
Digital elevation data needs Lidar partial coverage, mostly from wildfires; needs full coverage.