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Spatial and Temporal Relationships between Carlin-style Gold and Polymetallic Mineralization at the Deep Cove Gold-Silver Deposit, Lander County, Nevada
AuthorBonner, Wilson M.
AdvisorMuntean, John L.
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Cove is a late Eocene previously mined Au-Ag deposit in the Battle Mountain district of Lander County, Nevada that contains both Carlin-style and polymetallic mineralization. It has been interpreted as a distal disseminated deposit approximately 1.6 km northeast of the Eocene McCoy Au skarn deposit and adjacent late Eocene quartz monzonite Brown stock. Previous geologic mapping, stable isotope data, and geochronology have suggested the mineralization at Cove is genetically associated with an Eocene magmatic-hydrothermal system also responsible for the skarn mineralization at McCoy (Johnston et al., 2008). Exploration drilling by Premier Gold Mines, Ltd since 2012 at Cove has further defined the Carlin-style Helen and CSD deposits and discovered the Carlin-style Gap zones and the 2201 polymetallic zone. The mineralization at the Helen and Gap zones is hosted in Middle Triassic silty limestones overlying dolostones. All three deposits occur along the axis of the northwest trending Cove anticline at depths of 350 – 550 m below surface. Carlin-style mineralization in the CSD, Gap, and Helen zones is controlled by intersections of NE- (and NS-) striking faults and dikes and the NW trending Cove anticline. The ore fluids propagated out from these faults and dike contacts along the axis of the anticline by taking advantage of stratigraphic contacts and pre-mineral mafic sills. The polymetallic mineralization at the 2201 zone, occurs beneath the CSD zone, approximately 600 - 900 m below surface. It is characterized by mostly pyrite-sphalerite-galena carbonate veins and disseminations with lesser but higher-grade precious and base metals in pyrite-sphalerite-galena-quartz-siderite-(chalcopyrite-arsenopyrite-pyrrhotite-electrum) veins hosted in Early to Middle Triassic conglomerates and carbonate sandstones. At the contact of the siliciclastics overlying limestone, there manto-style replacement sulfides bodies locally formed as a result of a strong pH gradient. Although all Carlin-style ore zones display similar associated decarbonatization and silicification alteration, the pyrite textures and geochemistry varies among zones based on distance from the polymetallic minerals. Arsenian pyrite in the Helen zone is more commonly very fine-grained, anhedral (“fuzzy”), and lacks well-defined rims whereas the arsenian pyrites in the CSD zone are fine-grained, often euhedral to subhedral, and display well defined As- (Ag-)bearing rims. Ag was first detected in Carlin-style pyrite rims at the CSD zone using energy dispersive spectroscopy (EDS) on a scanning electron microscope (SEM) and later confirmed with an electron microprobe. The results of the microprobe work show Ag can exist in Carlin-style rims up to 1.5 wt %. The Carlin-style mineralization at Cove has variable Ag/Au ratios that average ~1.5 across the deposit and generally decrease along the Cove anticline to the northwest from the CSD zone averaging ~12 to the Helen zone averaging ~0.5. Beneath the CSD zone, we observe Carlin-style mineralization overprinting preexisting manto-style polymetallic mineralization in the basal Favret limestone. This study describes the mineralization at Cove including pyrite geochemistry and district-scale geochemical trends, reports new geochronology for the district, and places Cove along a continuum between Carlin-type gold deposits and distal disseminated sedimentary rock-hosted deposits.