A FIELD EMISSION SCANNING ELECTRON MICROSCOPY (FESEM) INVESTIGATION OF THE TEXTURE AND MINERALOGY OF MACROCRYSTALLINE GOLD FROM TYPE 4 ORE FROM ROUND MOUNTAIN, NEVADA

New investigations in the areas of discovery, extraction, utilization, and mineralization of gold is extremely important in economic geology. Understanding the mineralization of primary gold, mobilization of gold grains, and formation of secondary gold is crucial to meeting world demands. One of the most productive gold mining sites in the world is Round Mountain, Nevada. The Round Mountain mine is developed in a complex epithermal system that has four different types of ore. An unusual aspect of this deposit is that it has both disseminated gold and macrocrystalline gold. Samples of macrocrystalline gold 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).

Previous microscopy studies on gold from Round Mountain may have used samples that had been rinsed in acids which would have likely removed paragenesis minerals. Textural analysis as well as compositional analysis using energy dispersive spectroscopy (EDS) was conducted in order to identify paragenetic minerals on the unmodified crystals. Paragenetic minerals identified are dominantly smectite and apatite. These minerals were found in depressions and apparent dissolution pockets across the entire surface of the gold. Various microtextures were present on the surface of the gold crystals. Gold textures included hopper steps and euhedral “ghost” crystals, regions where an unidentified crystal had dissolved. Smectite is primarily found as typical cornflake lamellae aggregates. Apatite was euhedral and tabular with irregular crystal faces. Data suggests that there was possible dissolution of primary gold and perhaps other intergrowth minerals, followed by secondary growth of the paragenesis minerals on the crystals. Fungal hyphae were found on the surface of the gold in association with the paragenesis minerals. The relationship between the gold and fungus is not yet fully understood. FESEM investigation indicates that mineralization history is more complex and shows distinct paragenesis sequence for macrocrystalline gold at Round Mountain.