FORMATION OF DISSEMINATED EPITHERMAL GOLD ORE AT ROUND MOUNTAIN, NEVADA

The Round Mountain gold deposit is a world-class bulk-minable low-sulfidation volcanic-hosted epithermal deposit with 20 million contained ounces of gold. The majority of mineralization is disseminated and occurs as multiple overlapping lenticular and WNW-trending oblong, podiform zones concentrating mostly at the top and bottom of the poorly-welded tuff unit of the Round Mountain tuff, which had high porosity and permeability due to pre-ore vapor-phase alteration. Research was aimed at better understanding the formation of the disseminated ore by looking at alteration and mineralization at all scales (microscopic, drill-fan and mine-wide) to determine how permeability and the nature of fluid pathways changed with time. Detailed logging, petrography, whole-rock geochemical assays, and hyperspectral data indicate disseminated mineralization is associated with strong potassic alteration expressed by replacement of plagioclase by adularia and illite and the metasomatic enrichment of potassium and depletion of sodium and calcium at the drill-fan to mine-wide scales. Hyperspectral analyses of a drill fan reveal multiple overprinting stages of concentric alteration assemblages consisting of inner quartz-adularia, middle illite and outer smectite zones within the mineralized lower poorly-welded tuff. Gold and silver are closely associated with quartz-adularia. At the mine-wide scale, several major upwelling zones are apparent in 3D models of gold assays and mass change models of major oxide elements derived from whole-rock geochemical assays. Upwelling zones are largely controlled by a WNW structural fabric in underlying Paleozoic meta-sedimentary rock. The ascent of the thermally buoyant hydrothermal fluids was impeded by the overlying densely welded tuff, causing ponding within the poorly welded tuff. Porosity and permeability decreased with each overprinting hydrothermal event, and fluid pathways appear to constrict with each subsequent event, focusing subsequent fluid flow below previously altered zones along the base of the poorly-welded tuff. Later, higher-grade tubular or vein-like structures cross-cut the disseminated zones, eventually culminating in late cross-cutting quartz-adularia veins with coarse native gold, which only account for a few percent of the total gold.