| Basis for focus area |
Three past producers (from north to south, Unnamed Location, Skinner Ranch, and Unnamed) are listed as flake (Robinson and others, 2017), Multipart focus area composed of graphite mineral sites surrounded by a 5 km buffer. MRDS includes the same three past producers, one prospect, and several occurrences. Associated with Franciscan Complex (KJfs in Jennings and others (1977) or metamorphic rocks in Horton and others (2017). |
| Identified resources |
Historical production of graphite. |
| Production |
Unknown. |
| Status |
Past mining; no current mining. |
| Estimated resources |
Unknown. |
| Geologic maps |
Wagner and Gutierrez (2010), scale 1:100,000; Graymer and others (2006), scale 1:275,000; Jennings and others (1977), scale 1:750,000. |
| Geophysical data |
Mostly adequate aeromagnetic coverage. Mostly inadequate radiometric coverage. Fair gravity coverage. |
| Favorable rocks and structures |
Franciscan Formation. |
| Deposits |
Skinner Ranch (MRDS dep_id; 10077828), Black Magic graphite (MRDS dep_id: 10238330). |
| Evidence from mineral occurrences |
MRDS; Hamilton (1915); Robinson and others (2017). |
| Geochemical evidence |
None. |
| Geophysical evidence |
Some magnetic anomalies in the San Francisco Bay Area are related to ophiolite and volcanic rocks. |
| Evidence from other sources |
Unknown. |
| Comments |
Graphite from the Skinner Ranch deposit (opened in 1894) was shipped to San Francisco for paint manufacture; the output was valued at $9,000. |
| Cover thickness and description |
On or near Franciscan Complex rocks and undivided pre-Cenozoic metasedimentary and metavolcanic rocks (Jennings and others, 1977). |
| Authors |
David A. Ponce. |
| New data needs |
Geologic mapping, geophysics, lidar. |
| Geologic mapping and modeling needs |
Detailed geologic mapping. |
| Geophysical survey and modeling needs |
Aeromagnetic, gravity, and aeroradiometric data. Graphite is unusually highly conductive and magnetotelluric surveys may be useful. |
| Digital elevation data needs |
Lidar complete over part of the scattered focus area; not available in the rest. |
