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Abstract

The Calistoga Mining District, is one of three districts in the northern California Coast Ranges, where epithermal precious metals deposits have been economically mined. The district produced over 1.5 million ounces of silver (with lesser amounts of gold, copper and lead) intermittently over a 76 year period. The Palisade and Silverado mines were the two producers of the district.

Precious metals enrichment is associated with northeast-striking, en echelon quartz + chalcedony + adularia vein systems, hosted by flows and pyroclastic rocks of die Tertiary Sonoma Volcanics. Basement rocks are part of the Mesozoic and early Cenozoic Franciscan Complex, composed largely of tectonized marine sediments and dismembered ophiolite. Silver dominates gold by 74:1, and occurs in various sulfides, sulfosalts and selenides. Gold occurs in its native state, but is rarely macroscopic. In addition, this system is highly enriched in Cu, Sb, Zn, Pb, As and Ba, and contains anomalous concentrations of Se, Cd, Hg, Te, Ga, Tl, Mo and Bi.

Veins formed at +200 m depths, along dilational segments of normeast-striking (sinistral) conjugate Reidel shears, associated with a zone of San Andreas-style dextral wrench faulting. The zone of alteration and vein propagation is restricted to a corridor 1.5 km wide by 14.5 km long. This corridor appears to be structurally related to a local dilatiuaal jog in a 305°-striking dextral shear zone. The orientation of the optimal plane of extension was approximately 196° (right hand rule), dipping 74°. Because the volcanic pile has deformed as a homogeneous medium, a comparison of 595 vein and shear joint attitudes, taken in the Silverado and Palisade vein systems, provides a means of determining finite strain ellipsoid axes for vein formation. Based on these data, the orientation of Z is calculated to be 202°, plunging 18° and X at 106°, plunging 16°. Because of scatter in the data, the above axes are assigned error limits of ±15°. These values do not take into account the possibility of post-mineral rotation.

Fluid inclusion geothermometry indicates that this was a boiling system, with temperatures averaging 212° and 249° for the Palisade and Silverado veins, respectively. The mineralizing fluids were low salinity (-1.0 wt. % NaCl equivalent), NaCl-dominated, and possibly enriched in CO2. They are believed to be evolved meteoric waters, chemically and isotopically similar to those of the Geysers and McLaughlin systems. Metals were presumably transported as bisulfide and (to a lesser extent) chloride complexes. Precipitation was triggered by CO2 and H2S partitioning in response to fault-induced increases in vertical permeability.

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