STABLE ISOTOPE RELATIONS IN EPITHERMAL AU-AG DEPOSITS

Oxygen isotope analyses were made of hydrothermal minerals from 22 Tertiary epithermal Au-Ag vein deposits, located mainly in the Great Basin, Nevada. The majority of samples have relatively light δ18O values indicating a meteoric water component to the ore fluid, however δ18O values of samples from a few deposits are relatively heavy. Deuterium analyses of fluid inclusions in quartz and adularia from 15 deposits, including those with 18O-rich minerals, are uniformly isotopically light with a δD range of -90 to -139 per mil, essentially identical to the range of δD observed for modern spring waters in the Great Basin. The deuterium measurements unambiguously demonstrate the dominance and probable exclusivity of meteoric water in the hydrothermal fluids of such deposits. Knowledge of the age, location, and isotopic composition of meteoric waters allows possible reconstruction of ancient topographies and climates. These initial measurements indicate that such reconstructions are quite feasible for the Great Basin in the Tertiary. The results of isotopic analysis of a sample of quartz in bonanza silver ore from the 1,200-foot level of the Con Virginia mine of the Comstock Lode district are unique in this study. The δD of inclusion water from this sample is -68.5 per mil, implying that approximately 80 percent of the water in this fluid was magmatically derived.