PROGRESS IN UNDERSTANDING GOLD MINERALIZATION AT ROUND MOUNTAIN GOLD MINE, NEVADA FROM FIELD EMISSION SCANNING ELECTRON MICROSCOPY

The Round Mountain Gold Mine in Nevada is developed in a complex epithermal system where the primary host rock is an altered rhyolitic tuff. There are several different types of ore at Round Mountain which vary in lithology and mineralogy. The type 4 ore deposit host lithology is comprised of Paleozoic siliciclastics and carbonates, and the ore grade mined is highly variable. The type 4 ore is unusual in that it contains both disseminated and macrocrystalline gold. Previous microscopy studies on gold from Round Mountain may have used samples that had been rinsed in acids which would have likely removed the majority of paragenesis minerals. Understanding mineral paragenesis may provide insight into the mechanisms and timing of gold mineralization and provide insight as to why macrocrystalline gold occurs. New knowledge obtained aims to add to what is known about gold mineralization and ore formation. This knowledge can then be used to aid in exploration and refinement processes. Samples of both macrocrystalline gold and disseminated gold within bulk ore were acquired from the type 4 ore that are known to be unmodified. These samples were investigated to determine the associated sequence of mineralization using field emission scanning electron microscopy (FESEM), coupled with energy dispersive spectroscopy (EDS). This method was used to conduct textural as well as chemical compositional analysis. Preliminary data indicate that paragenetic minerals are present and differ between the macrocrystalline gold and disseminated gold within the bulk ore. The paragenetic minerals associated with macrocrystalline gold are dominantly illite, illite-smectite, and apatite. Paragenetic minerals associated with the disseminated gold within the bulk ore are dominantly feldspar, quartz, clay minerals, pyrite and calcite. Gold microtextures are highly variable. Data suggests that gold morphology is due to primary mineralization possibly followed by either mobilization of primary gold and perhaps other intergrowth minerals, or secondary gold growth. Gold mineralization is then followed by secondary growth of the paragenesis minerals. FESEM investigation suggests a distinct sequence for gold mineralization in the type 4 ore that is complex and differs between the macrocrystalline gold and disseminated gold.