MERCURY DEPOSITS OF NORTHERN CALIFORNIA

Mercury (Hg), a once vital economic asset of California’s mining industry, is associated with the Coast Ranges of California. We studied three mercury deposits along a five-mile section of the Mayacamas Mountains, located within the central Coast Ranges (CA) to determine how host rock lithology correlated with mercury mineralization. We found that Hg-mineralization was associated with contacts between the Franciscan Assemblage and the Great Valley Sequence for the ore deposits associated with the Mayacamas Mountains. The deposits themselves consists of an orange, veined rock consisting of altered serpentinite. X-ray diffraction (XRD) analyses reveal that the veins consist of quartz and chalcedony (microcrystalline quartz). Compositions of mineral phases were collected using electron dispersive spectroscopy (EDS) analyses were conducted using a Hitachi scanning electron microscope (SEM) at Sonoma State University. Hg-mineralization occurs in the forms of both cinnabar (HgS) and montroydite (HgO). All sites additionally included quartz and caledony (SiO₂ polymorphs), and magnesite (MgCO₃) as rock forming/accessory minerals within the host rock. Serpentinite minerals such as chrysotile, lizardite, and and antigorite all Mg₃(Si₂O₅)(OH)₄ are present in unaltered serpentinites. Also found within the field area was pecoraite Ni₃(Si₂O₅)(OH)₄, a Ni-enriched alteration product of serpentinite, idrialite (C₂₂H₁₄), as well as trace petroleum. SEM data from these rocks show an elevated carbon level as well as small concentrations of several metals including aluminum, calcium, chromium, copper, and sodium, none of which are seen in the surrounding serpentine units. Analysis of sites via fieldwork, geologic maps, and mineral chemistry has shown that many ore bodies lie on contacts between serpentine and sandstone units that are often associated with faults. The hydrocarbon mineral idrialite and the presence of magnesite suggest that these deposits are the result of intrusion of heated CO₂ rich fluids.