GOLD-BEARING HOT-SPRING SYSTEMS OF THE NORTHERN COAST RANGES, CALIFORNIA

Gold-mercury mineralization in the Knoxville and Sulphur Creek districts of the Coast Ranges of northern California fits the emerging hot spring classification of epithermal ore deposits. Quartz and chalcedony veins, which show hydrofracture texture, are capped by subaerial sinter deposits. All the deposits appear to be very young. The McLaughlin deposit and the Sulphur Valley and Elgin deposits contain hydrocarbons in their ore. Gold-bearing quartz of the San Quentin ore shoot of the McLaughlin deposit is isotopically heavy (delta-18O = 24 parts per thousand) and appears to have formed near 150-degrees-C from isotopically heavy waters. Such waters are presumed to have moved both hydrocarbons and precious metals to the surface sinter of the deposit. Hot springs composed of isotopically heavy fluids issue today from the centers of the Sulphur Valley and Elgin deposits. The hot spring fluids carry small concentrations of hydrocarbons and transport gold, silver, and mercury to the surface. Ag/Au weight ratios of the precipitates formed by the hot springs are about 1:8. Gold grades of the hot spring precipitates range from 0.5 to more than 10 ppm; the mean value for 20 samples is 3.3 ppm. Measurements show that the waters carry 1 ppb gold in solution. Thermodynamic calculations indicate the gold is probably present as a bisulfide complex. The chemical and isotopic characteristics of the hot spring fluids appear to be similar to the fluids inferred to have formed the McLaughlin deposit. In these two gold districts, nonmeteoric waters are responsible for the formation of epithermal gold deposits. A sedimentary or metasedimentary source for the ore-forming fluids is indicated by chemical and isotopic data and is consistent with the local geology.