IGNEOUS GEOCHRONOLOGY AND PETROGRAPHY OF THE CORTEZ HILLS CARLIN-TYPE GOLD DEPOSIT, CORTEZ, NEVADA

Carlin-type gold deposits have been mined continuously for about forty years and their reserves ensure that the state of Nevada will continue to be one of the world's leading gold producers. Since their discovery, numerous studies have attempted to elucidate the mechanisms by which the deposits form; a key goal is to develop better exploration models. General characteristics of Carlin-type deposits include: the age of mineralization is between 36 and 42 Ma; ore is found primarily in silty carbonaceous, pyritic host rocks; gold occurs as submicron particles in pyrite that forms small grains or overgrowths on pre-existing pyrite, or in oxidized rocks as native gold; gold was transported by and precipitated from low salinity aqueous fluids at temperatures on the order of 200ºC; and precipitation occurred at depths on the order of 1-3 km.

Oxygen isotope data from ore stage quartz, determined by cathodoluminescence, and oxygen and hydrogen isotope data from fluid inclusions within the quartz suggest that meteoric, magmatic, and/or metamorphic waters may have been involved in gold transport and precipitation. Paleodepth estimates, based on apatite fission track data, suggest that the temperature of the host rocks was not greater than approximately 100ºC at the time of mineralization. Thus, there is a need to invoke either an anomalously high geothermal gradient, or a deep, possibly magmatic, heat source. At the Cortez Hills deposit, located at the southern edge of the Cortez Mountains in Lander County, Nevada, there are abundant intrusive and extrusive igneous rocks which exhibit a variety of pre and post-mineralization contacts with Carlin-type mineralized rocks. This study aims to elucidate the potential relationship between magmatism and gold-ore-deposition at Cortez by bracketing mineralization using cross cutting relationships and by using geochronological analyses to establish a temporal link between magmatism and Carlin-type mineralization.