Paragenesis and tectonic significance of base and precious metal occurrences along the San Andreas fault at Point Delgada, California

Studies of sulfide vein mineralization at Point Delgada, California, suggest that the mineralizing process was intimately associated with the San Andreas fault system and/or plate-margin tectonism. The deposits occur along north-south to northeast-southwest-trending tension faults that intersect and cross a major northwest-trending fault previously considered to be a major trace of the San Andreas fault. Galena and sphalerite are concentrated near the intersection of the principal mineralized vein with the fault, suggesting that brecciated rocks at this intersection acted as a major conduit for hydrothermal circulation. Fluid inclusion data show that the sulfide mineralization occurred at a depth of at least 400 m, probably at a temperature of about 250 degrees C. Mineralization fluids were in the salinity range of 10.5 to 5.2 equivalent weight percent NaCl. Sodium and potassium were depleted from the mineralizing fluids in the early stages of mineralization, and traces of argentian tetrahedrite and cassiterite in the fluid inclusions probably account for some of the anomalous values of silver and tin in galena and sphalerite. The age of mineralization, based on K-Ar dating of adularia, is 13.8 + or - 0.4 m.y. This age suggests that the purported trace of the San Andreas fault crossed by the sulfide veins has been largely inactive since middle Miocene time. Lead isotope and geochemical data suggest that base and precious metals in the sulfide veins were derived largely from arc-related sedimentary rocks. The heat source for hydrothermal circulation may have been associated with intrusion of asthenosphere into a triangular slab window opened southeast of the northward-propagating Mendocino triple junction shortly before 13.8 m.y. ago.