MARCUS K. JOHNSTON* Ralph J. Roberts Center for Research in Economic Geology, University of Nevada - Reno

The McCoy mining district is located in the northern Fish Creek Mountains, Nevada. The district contains two related precious metals deposits, with contrasting ore styles reflecting the different physical and chemical environments of coeval mineralization. In terms of economy, the larger deposit is the Cove Mine, with pre-mining reserves of 3.6 million ounces Au and 165 million ounces Ag. The smaller deposit is the McCoy Mine, with pre-mining reserves of 880,000 ounces Au and 2.3 million ounces Ag.

The Cove deposit is hosted dominantly by the middle to early-late Triassic Augusta Mountain Formation, a post-Sonoma orogeny passive margin sequence. Three host units have been exposed by open-pit mining: 1) the early Ladinian Home Station Member, consisting of thickly bedded silty to sandy dolomitized limestone; 2) the late Ladinian Panther Canyon Member, consisting of a lower primary dolostone submember and an upper transitional submember that grades from basal microcrystalline limestone, through middle silty limestone and calcite-cemented sandstone, to upper conglomerate; and 3) the late Ladinian to early Karnian Smelser Pass Member, consisting of medium to thickly bedded limestone with lesser carbonate shale interbeds. Ore also occurs locally in a series of Eocene porphyritic granodiorite dikes and related sills, and the entire host package is unconformably overlain by the post-mineral Oligocene Caetano Tuff.

The Cove Mine is a hybrid deposit that displays characteristics of three different mineralizing systems. Two ore types are related to a single hypogene system, and consist of base metal vein-type (BMVT) and related Carlin-style ore. BMVT ore comprises pyrite+sphalerite+galena-dominated veins, stockworks, crustifications, and disseminations that are generally restricted to the central portion of the deposit. Carlin-style ore comprises an outboard aureole of disseminated Fe*As sulfides with arsenian-auriferous-argentiferous components. Associated sericite was dated at 39.4*0.23 Ma. The hypogene assemblages are overprinted by the third system, consisting of supergene leaching with minor Ag*Cu enrichment.

Cove is located 1.5 km northeast of the McCoy gold(-copper) deposit, a proximal skarn related to a comagmatic suite of Tertiary stocks and dikes, and monzonitic and lamprophyric dikes and sills. The intrusions comprise two groups related to mineralization. Early low grade skarn is related to magnetite-bearing intrusions. Late economic skarn is related to ilmenite-bearing intrusions with 3 alteration mineral suites: 1) biotite- and pyroxene-bearing assemblages; 2) andradrite-diopside skarn and epidote-bearing assemblages; and 3) quartz-pyrite*adularia (QP*A) and biotite- and chlorite-bearing retrograde assemblages. Most ore was associated with the QP*A and retrograde assemblages. Adularia ages
indicate ore deposition at 38.7*1.1 Ma.

Six lines of evidence support a district-scale zonation model for Cove and McCoy: 1) the deposits are proximal to one another; 2) they are localized along the same northeast-striking structural zone; 3) they share identical host rocks; 4) they are associated with similar intrusions; 5) sulfide d34S values for Cove BMVT and McCoy skarn ores share nearly identical ranges of 2.2 to 3.3 per mil; and 6) the economic ore systems for both indicate a single mineralization age of circa 39.4 Ma. In a district sense, McCoy and Cove can be considered as proximal and distal components, respectively, of a large magmatic-hydrothermal system. Similar zonations have been proposed for the Bingham-Barneys Canyon-Melco system in Utah, the Yauricocha District in Peru, and the Bau District in Malaysia. These relationships clearly offer support for earlier genetic models involving peripheral base metal and Carlin-style systems related to central porphyry stocks, and in the Great Basin, to late Eocene intrusions.

*Present address: Newmont Mining Corporation, P.O. Box 669, Carlin, NV, 89822-066