AN ELECTRON MICROSCOPY INVESTIGATION OF GOLD AND ASSOCIATED MINERALS FROM ROUND MOUNTAIN, NEVADA

Much remains unknown regarding the details of gold mineralization in epithermal systems. A refined understanding of the interface between gold and associated minerals, as well as the microtextures and nanotextures present in epithermal gold could provide insight into the mechanism of gold concentration in ore and the dominant growth mechanisms of gold. In addition, a study conducted at this scale may help explain variations in extraction efficiencies using the cyanide heap leach method. Macrocrystalline gold samples from Round Mountain, Nevada were analyzed in this study using field emission scanning electron microscopy (FESEM) and focused ion beam milling (FIB) assisted transmission electron microscopy (TEM). The results suggest that the two dimensional nucleation and spread growth mechanism are dominant and that gold and silver nanoparticles and nanoparticle aggregates are common and have played an important role in the formation of this deposit by acting as seeds to promote gold growth. These nanoparticles were also observed as inclusions in other phases such adularia, quartz, titanium oxide, and illite-sericite. It appears that there is a gap at the interface between gold and other mineral phases that could provide heightened accessibility of the CN^- solution during leaching. Furthermore, nanoscale elemental mapping indicates that metals such as Fe, Cu, and Ag appear to be present in the surface of gold at the interface and may hinder dissolution of gold during cyanide leaching by causing competitive consumption or through leaching at different rates leading to decreased recovery efficiencies. The results of this study provide new constraints on low sulphidation epithermal gold mineralization at Round Mountain, Nevada and serve as a comparison for other similar deposits.