Relationship between magmatism and mineralization in the RBM gold deposit, White Pine County, Nevada

The RBM deposit in the Bald Mountain mining district is a breccia-hosted intrusion-related gold deposit. Petrography, core logging, and mapping of the RBM open pit are used to characterize the deposit in detail and define its relationship to magmatism. Statistical analysis of trace elements from drill core and surface samples from in and around the RBM pit are combined with detailed petrography and microprobe analyses to identify the mineralogical and geochemical signatures of distinct mineralization events. These data define an early mineralization event characterized by potassic alteration, skarn, and hornfels formation that is overprinted by sericitic alteration. Minor gold along with molybdenite, some copper, and base-metal skarn mineralization occurred during this early event. A second mineralization event, which accounts for the bulk of the gold in the deposit is characterized by advanced argillic alteration, free gold, and abundant pyrite in the matrix of magmatic-hydrothermal breccias. This event has a distinct geochemical signature characterized by gold, bismuth, tin, and, to a lesser extent, tellurium. Subsequent mineralization events include late arsenic-rich marcasite mineralization associated with further argillic alteration, and late carbonate-pyrite veins, neither of which appear to be associated with significant gold deposition. Four U/Pb dates on intrusive phases, including one that appears to post-date gold mineralization, define the intrusive system as Jurassic in age. Apatites and zircons were dated using the (U-Th)/He system, and phlogopite was dated using the 40Ar/39Ar system in an attempt to define the thermal history and the age of hydrothermal alteration in the deposit. In addition to the Jurassic hydrothermal activity, these data, along with previously published apatite fission track dates from the district, indicate a later Miocene hydrothermal event that appears to have been a low-temperature (<~170° C), short-duration hydrothermal event with little to no associated gold mineralization.