A MICROTEXTURAL STUDY OF SELECTED MACROCRYSTALLINE GOLD SAMPLES FROM NEVADA

Understanding the microtexture of macrocrystalline gold can provide insight into both modes of mineralization and into challenges of extraction from ore. Crystalline gold samples from Excelsior Mountain, Eugene Mountain, and Round Mountain in Nevada were analyzed using scanning electron microscopy (SEM) to gain a better understanding of crystal habits and textures present in these samples. SEM results suggest multiple generations of textures are present. Among the crystal shapes present include reverse hopper crystals and crystals with apparent hexagonal morphology, some showing the (111) faces. One of the dominant textures present is a step-like texture often showing hexagonal or triangular morphology. Another dominant texture is massive and slightly granular texture that sometimes occurs immediately adjacent to the step texture. Small discrete veins of relatively pure gold have also been observed. Energy dispersive spectroscopy (EDS) analysis indicates the majority of the gold contains significant silver content and some sample material may be electrum. A distinct separate generation of gold also occurs as euhedral to subhedral single crystals or crystal clusters and is often relatively pure, containing little silver. Textural and EDS data indicate that multiple generations of gold or gold-silver alloy exist, and data is interpreted such that the sequence of mineralization transitioned from granular texture to step texture, with the relatively pure gold crystals and veins forming afterwards.

Gold often occurs with appreciable amounts of quartz. One of the goals of this study is to gain a better understanding of the interface between these two minerals and its implications for the extraction of gold from the quartz and surrounding rock. In these samples gold occurs embedded in the matrix of quartz, as well as on the surface of the quartz. Further work will involve transmission electron microscopy analysis utilizing focused ion beam milling to further investigate these samples.