Abstract

The McCoy Au-Ag skarn and Cove Au-Ag deposits are located in the northern Fish Creek Mountains, Lander County, Nevada. Through the end of mining in 2001, large-scale open-pit and associated underground production at the two deposits yielded 3.3 million ounces (Moz) of Au and 108 Moz of Ag. Most production was from Cove, making it the fourth-largest Ag producer in the history of Nevada.

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 polymetallic vein-type ore and an outer aureole of relatively Ag rich Carlin-style ore. Polymetallic veins consist 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, and auriferous components ± native Au-electrum in silty to sandy carbonate strata. Polymetallic vein-type ore has Ag/Au ratios of >50/1, 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 illitized rocks. New structural and age data for fresh and altered intrusive rocks indicate that mineralization at Cove occurred during active extension and magmatism at ~39 Ma (40Ar-39Ar). Fluid inclusion and δD and δ18O data for polymetallic vein-type ore indicate that the mineralizing fluids had temperatures of 250° to 370°C and were magmatic in origin.

Cove is located 1.6 km northeast of McCoy, where Early Tertiary igneous activity occurred in two pulses. The first pulse consisted of 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 subeconomic skarn at McCoy. The second pulse consisted of 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. The δ34S data for ore-stage sulfides from skarn ore at McCoy, and polymetallic vein-type and Carlin-style ore at Cove indicate a common source for S for the three types of ore. This source is interpreted to have been the pulse of ilmenite-series magma.

Although the volume of the Carlin-style ore at Cove is substantially larger than that of the polymetallic vein-type ore, the relatively high concentrations of base metals and clear associations with igneous activity have made Cove difficult to classify. The new data indicate that Cove is a telescoped system consisting of polymetallic vein-type and porphyry-related, distal disseminated ores. The zonation for the McCoy-Cove system is nearly identical to zonations described by other workers for Au-enriched porphyry systems. In this case, the porphyry center and skarn ore at McCoy comprise the proximal component, and polymetallic vein-type and Carlin-style ores at Cove are the intermediate and distal components, respectively. Economic concentrations of Au occur in all three zones, which are separated from one another by subeconomic concentrations of Au.

The Carlin-style orebodies at Cove share several important similarities with ores in classic Carlin-type deposits, including ore characteristics, associated alteration styles, and host lithologic units. The late Eocene age of the McCoy-Cove system is also a typical age for Carlin-type deposits in the Great Basin. The proposed genetic model for McCoy-Cove may have important implications for the exploration potential of intrusion-related and Carlin-type deposits worldwide.

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