| Basis for focus area |
Distribution of polymetallic base metal and Mn deposits in MRDS, Mn provinces outlined in Tooker and Cannon (1980) and Peterson (1984). Permissive tract PC11 for Massive sulfide, Besshi-type deposits polygons extracted and merged from the 1995 National Assessment (U.S. Geological Survey National Mineral Resource Assessment Team, 2002). |
| Identified resources |
Historical production of copper, gold, manganese, silver, and zinc. |
| Production |
~175 locations in California total have produced 263,000 short tons Mn pre-1957 (Davis, 1957); historical mines produced anywhere from 1,000 to 20,000 tons Mn (Davis, 1957). The Island Mountain deposit produced 120,000 t of pyrrhotite-rich ore averaging 3.3% Cu, 33 g/t Ag, and 2 g/t Au. |
| Status |
Past mining; no current mining or exploration. |
| Estimated resources |
Unknown. |
| 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. |
| Geophysical data |
Adequate Rank 2 with minor Rank 1 aeromagnetic and aeroradiometric coverage. |
| Favorable rocks and structures |
Crescent Formation (middle and lower Eocene), mainly marine, subalkaline, pillow-dominated basaltic rocks with interbeds of basaltic tuff, chert, red argillite limestone, and siltstone. Chert-greenstone complexes (Franciscan Formation) hosts multiple small stratiform Mn ore deposits. |
| Deposits |
Island Mountain (MRDS dep_id: 10072353); Blue Jay mine (MRDS dep_id: 10109404). |
| Evidence from mineral occurrences |
MRDS. |
| Geochemical evidence |
Manganese-chert deposits are enriched in Mn, Fe, Si, Cu, Ni, Zn, and Ba, with occasional enrichment of As, Co, V, and Hg to the maximum contents of 200 ppm As, 500 ppm Co, 2,000 ppm V, and 3.5 ppm Hg (Silberman and others, 1994). |
| Geophysical evidence |
Unknown. |
| Evidence from other sources |
Unknown. |
| Comments |
Manganese deposits of the 'Franciscan type' are generally thin, elliptical bodies that are concordant with bedding. Ore is composed of psilomelane, pyrolusite, rhodochrosite, and hausmannite as well as other Mn oxides and silicates. Very low estimate for undiscovered VMS deposits (U.S. Geological Survey National Mineral Resource Assessment Team, 2002). |
| Cover thickness and description |
Most deposits exposed at the surface and show evidence of surface oxidation. |
| Authors |
Laurel G. Woodruff, Catherine Wesoloski, David Reioux, Allen K. Andersen, Deborah A. Briggs, Ben Parrish (ed.). |
| New data needs |
Aeromagnetics, lidar, geochemistry, geologic mapping. |
| Geologic mapping and modeling needs |
Additional 1:24,000 scale mapping over a larger area. |
| Geophysical survey and modeling needs |
High-resolution, Rank 1 aeromagnetic and aeroradiometric surveys. |
| Digital elevation data needs |
Current QL2 Lidar coverage in areas with some QL1 locally. Need more complete Lidar in Humboldt and Del Norte counties. |
