SCANNING ELECTRON MICROSCOPY PROVIDES NEW INSIGHTS INTO ORIGINS OF FINELY DISSEMINATED GOLD ORES IN ROUND MOUNTAIN, NEVADA

Gold is an important economic resource both in the U.S. and globally. In 2013 alone gold mining contributed $171 billion to the global economy and employed approximately 3 million people (World Gold Council, 2015). In the US, most gold production comes from the heap leaching of disseminated ore deposits such as those found in Round Mountain, Nevada. While the current technology of cyanide heap leaching is well refined and more environmentally friendly than the techniques of the past, extraction efficiencies have stagnated. At Round Mountain efficiencies are at about 75% which means that current refractory gold of this operation can be conservatively valued at over $3 billion. A major contributor to low extraction efficiencies is an incomplete understanding of the nature of finely disseminated gold ores. Little is known about the ore’s bulk chemistry, microtextures, or deposition. For this study, samples of Oligocene age poorly-welded tuff, known to contain finely disseminated gold (Type 2 ore), were collected from the Round Mountain mine. The samples were analyzed using scanning electron microscopy (SEM) to determine the mineral textures and phases. The dominant minerals present in the samples as determined by EDS are adularia, illite, and quartz. Disseminated particles of sulfides and rare earth phosphates generally smaller than 20 µm were also commonly observed. Micrometer scale cubic pyrite crystal was observed to be overgrown by a network of flakey clay minerals, indicating that the pyrite crystalized before the clay. In the same sample, a 20 µm silver flake observed in backscatter electron mode is discernibly continuous with the surrounding matrix of quartz and clay minerals. It can therefore be determined that the silver formed in place. The textures observed of these minerals provide critical insight into the nature of ore with which cyanide solution interacts and may lead to the development of more efficient extraction methods.