PARAGENESIS AND DISTRIBUTION OF TRACE ELEMENTS, REES, AND STABLE ISOTOPES OF EPITHERMAL AU-AG VEINS, KEN SNYDER MINE, MIDAS, NEVADA
Comparison of chondrite-normalized REE patterns of Midas veins to those of host rocks indicates that most stages of veins do not exhibit patterns similar to those of host rocks. Stages I, II, IV, and V show similarities to coeval glassy rhyolite dikes and may point to a magmatic contribution to the veins of REEs and possibly precious metals. Stage III veins, which formed during widespread brecciation, contain higher concentrations of REEs, immobile elements, and patterns similar to those of host rocks due to lithic fragments. In Stage VI the patterns may be due to increased abundances of calcite.
Stable isotope data for carbonates suggest that veins were deposited from a parent fluid with a composition of d13C = -4, and d18O = -5. A decrease in d13C in calcite from Stage I to II may have resulted from a decrease in pH of the parent fluid as the system developed and the buffering capacity of wall rocks along the flow path decreased, or from an influx of a different parent fluid. The similarity in REE patterns for Stages I and II suggests that they were derived from the same fluid source, evidence of a constant parent fluid and change in pH.