Geology of the Cove mine, Lander County, Nevada, and a genetic model for the McCoy-Cove magmatic-hydrothermal system

The McCoy Au skarn and Cove Au-Ag deposits are located in the northern Fish Creek Mountains, Lander County, Nevada. Through 2001, large-scale open pit and associated underground mining at the two deposits produced 3.3 million ounces of Au and 108 million ounces of Ag. Most production was from Cove, making it the third largest Ag producer in the history of Nevada. This study focuses primarily on the geology and geochemistry of Cove, and secondarily on the relationships between Cove, McCoy, and other deposits in the Great Basin physiographic province of North America.; Cove is hosted by the middle to early late Triassic Augusta Mountain Formation, which consists of limestone with lesser dolostone and clastic units. Ore also is present locally in Eocene porphyritic granodiorite dikes and sills. The deposit comprises two distinct ore types: a central core of base metal vein-type (BMVT) ore and an outer aureole of relatively Ag-rich Carlin-style ore. BMVT ore consists of pyrite-sphalerite-galena-dominated, Au- and Ag-bearing veins, veinlets, stockworks, crustifications, and disseminations in clastic and carbonate strata, and locally in the intrusions. Carlin-style ore comprises disseminated Fe ± As sulfides with arsenian-argentiferous-auriferous components ± native Au-electrum in silty to sandy carbonate strata. BMVT ore has >50:1 Ag:Au ratios, and Carlin-style ore has Ag:Au ratios that decrease from ∼50:1 near the feeder faults to ∼1:1 in one of the more distal ore zones. Both types of ore are associated with decarbonatized, silicified, and sericitized/illitized rocks. New structural and age data for ∼fresh and altered intrusive rocks indicate that mineralization at Cove occurred during active extension at ∼39 Ma (40Ar-39Ar). Fluid inclusion and δD and δ18O data indicate that the mineralizing fluids were ∼250 to 370°C and magmatic in origin, and that boiling did not occur.; Cove is located approximately 1.6 kilometers northeast of McCoy. Earlier studies indicate that early Tertiary igneous activity in the McCoy mining district occurred in two pulses. The first pulse consisted of a relatively oxidized magnetite-series magma, and formed the central stock at McCoy and related dikes that extend to the Cove deposit. This pulse occurred at ∼41.5 Ma, and produced sub-economic skarn at McCoy. The second pulse consisted of a relatively reduced ilmenite-series magma, and produced economic skarn ore at McCoy. Adularia from a mineralized skarn assemblage at McCoy was dated at ∼39 Ma (K-Ar). The age data indicate that McCoy and Cove formed contemporaneously. (Abstract shortened by UMI.)...