JURASSIC INTRUSION-RELATED OR EOCENE CARLIN-STYLE MINERALIZATION, WHAT BROUGHT THE GOLD TO BALD MOUNTAIN?

Nevada produces over 80% of the gold in the U.S., making it the fourth largest gold producer in the world. Most of the gold produced in Nevada comes from Carlin-type deposits, believed by most to be Eocene in age. Despite this apparent temporal constraint, the three largest Carlin-type districts are all spatially associated with Mesozoic plutons that are interpreted as pre-ore. In particular, the Bald Mountain district in Eastern Nevada, has production/reserves of >3Moz. It shares characteristics with Carlin-type deposits but also has many characteristics that suggest mineralization is more likely genetically related to a Jurassic intrusive center on which the district is centered.

The RBM deposit in the Bald Mountain Mining district appears to be an igneous end-member of the system, with ~80% of the ore hosted in Jurassic intrusions. Preliminary results based on petrography, core logging, and mapping of the RBM open pit indicate a spatial relationship between hydrothermal breccias and gold mineralization, as well as a zone of potassic alteration along and outboard from ore-bearing structures. These data have lead to the hypothesis that ore-bearing volatile rich fluids rapidly ascended from a porphyry center at depth along a pre-existing structural fabric brecciating host rocks and depositing gold. Breccias made up of abundant vein fragments that are veined by quartz and late pyrite indicate multiple generations of brecciation and veining. In addition, petrography and scanning electron microscopy have identified a late arsenian pyrite phase that forms rims on early non-arsenian pyrite cores, a characteristic common to Carlin-type deposits. Detailed microprobe analyses will be used to identify the nature of gold mineralization and its relationship to specific pyrite phases. Further work including U-Th/He dating on apatite, U-Th/He dating on zircon, Ar/Ar dating on muscovite, and U/Pb dating on zircon will test for the presence of a post-Jurassic hydrothermal event that could have introduced or remobilized gold within the system.