Origin of petroleum and mercury in the Culver-Baer cinnabar deposit, Mayacmas District, California

Abstract

The Culver-Baer mercury deposit (Mayacmas district) is located within the strike-slip Mercuryville fault zone at the margin of the Geysers geothermal area where serpentine + magnetite protolith is altered to silica-carbonate rock. Silica-carbonate rock is silica rich at the center of the deposit and consists of the assemblage quartz + magnesite + pyrite. This assemblage is cut by numerous quartz-chalcedony veins and breccia veins composed of quartz + chalcedony + or - cinnabar + or - petroleum + or - millerite + or - barite. The cinnabar, which is chemically pure, and the petroleum, which consists predominantly of polycyclic aromatic hydrocarbons, occur preferentially along the hanging walls of the breccia veins. Outward from the deposit center there is a zonation to carbonate-rich silica-carbonate rock which consists of magnesite + or - quartz + or - pyrite. At the deposit margin itself, magnesite + or - magnetite is the dominant stable assemblage. All rock samples reveal a consistent temporal evolution from magnesite veinlets to silica veinlets.Geochemical modeling indicates that the mineralogical zonation from serpentinite to silica-rich silica-carbonate rock in the center of the deposit can arise from the interaction of serpentinite with a CO 2 -rich, H 2 O-CO 2 fluid (PCO 2 > or =30 bars at 200 degrees C). Petrographic and analytical data suggest that other than CO 2 , the only components added to the system during alteration and mineralization were H 2 S, Hg, and petroleum. Isotopic and biological marker data support the hypothesis that all of these components may have been derived from local sedimentary rocks in response to heating and transported to the site of deposition in a gas phase. The association of petroleum and cinnabar, particularly along the hanging walls of breccias, is most likely due to a common source and transport in a relatively buoyant fluid rather than due to a direct chemical link. Because there is no evidence for the introduction of any other components, these data suggest that the Culver-Baer deposit may be a fossil condensate zone akin to the model of White et al. (1971).