I describe here a trip I took to an interesting mercury mine approximately two hours from the San Francisco Bay area.

The Contact mine, located near the Geysers geothermal field, happens to be on Federal land and is accessible to collecting. It is just off of Pine Flat Road, a narrow, paved road that winds through the mountains northeast of Healdsburg. The Mindat map pages for the Contact mine show the locations of the old workings (none of which I saw), but the small mine dump is located a short distance to the east, closer to (and downhill of) Pine Flat Road. Access to the area is only possible from the southwest, as I found Pine Flat Road to be closed to the east. This photograph shows a view, from Pine Flat Road, of the trail leading to the mine dump:

This trail passes a nice outcrop of serpentine (may it forever remain our state rock). The mine dump (or at least what I saw of it) is rather small:

Banded vein material (in what I assume is silica-carbonate rock, an alteration product of serpentine) is abundant on this dump, and much of it contains cinnabar.

The gangue material appears to consist of fine (chalcedony?) and coarse (comb quartz) grained silica and a tan colored carbonate. Some of the vein material smells of petroleum when it is cracked open. A variety of hydrocarbons are present in the California Coast Range silica-carbonate mercury deposits.