Petrology of the Yerington Batholith, Nevada; evidence for evolution of porphyry copper ore fluids

Abstract

The Jurassic Yerington batholith, western Nevada, is a composite pluton that contains several centers of porphyry copper mineralization and is exposed in structural cross section at paleodepths ranging from 0 to 8 km. Within these exposures the McLeod Hill quartz monzodiorite, Bear quartz monzonite, and Luhr Hill granite form successive intrusions that are in turn volumetrically smaller ( approximately 75, 19, and 6 vol %, respectively), more deeply emplaced (tops at 65-km 3 Luhr Hill granite. The low Cu content ( approximately 10 ppm) of the granite would allow extraction of approximately 50 ppm Cu from it during crystallization. These data suggest an orthomagmatic model in which both high- and low-density saline aqueous fluids formed during exsolution of water from the crystallizing Luhr Hill granite; and from which salts, Cu, Fe, and S strongly partitioned into the high-density fluid. Low-density fluids may have risen from the magma as a vapor plume. Mineralization occurred when fluid overpressures caused fracturing of wall rock and upward intrusion of granite porphyry dikes and high-density saline ore fluids. Both the f (sub O 2 ) path during subsolidus cooling and the deposition of most S as chalcopyrite-pyrite in ore suggest that reduction of magmatic S to sulfide may have been a mechanism for oxidation of magmatic ferromagnesian silicate and Fe-Ti oxide minerals.