TRANSMISSION ELECTRON MICROSCOPY (TEM) OF TYPE 2 GOLD ORE AT ROUND MOUNTAIN, NEVADA REVEALS COMPLEXITIES IN MINERALIZATION AND POINTS TO CAUSES FOR EXTRACTION IN EFFICIENCY

Round Mountain is an epithermal gold deposit that is located in the Great Smoky Valley at the base of the Toquima Mountain Range in Nevada. Macrocrystalline gold mineralization includes electrum in conjunction with quartz, iron oxides, pyrite, and adularia in dominantly vein morphologies in Paleozoic sedimentary rocks (Type 4 ore) with extensive disseminated deposits (Type 2 ore) in the Toquima range ash flow tuffs, dated at approximately 26 Ma. Round Mountain is an open-pit operated now by Kinross Gold Corporation. Company information indicates gold production first began at Round Mountain in 1906, and peaked in 2003 at 785,300 oz./yr. Production currently is at approximately 325,000 oz/ yr and the mine has an average heap leach extraction recovery of 75-80%. The main ore body mined for heap leach extraction is the Type 2 (Toquima range ash flow tuffs). This lithology is dominated by illite-sericite mineralization, however there is little detailed information regarding the mineralogy of this unit. Unknowns include variation in the phyllosilicates and the nature of the texture of gold mineralization in the bulk ore. Recent investigations of macrocrystalline gold from the stratigraphically lower Type 4 ore suggest complex nanoparticle aggregation mechanism to form macrocrystalline gold. Whether or not this is specific to the process of macrocrystalline gold mineralization or is pervasive into the Type 2 ore is unclear. TEM investigation of grain mount samples indicates that the illite-sericite minerals are relatively homogenous both in texture and chemical compositions. Limited examples of potential gold nanoparticles in association with illite-sericite minerals have been observed. These are analogous to better examples observed in illite overgrown on macrocrystalline gold in the type 4. Additionally Fe-rich nanoparticles have been observed associated with the illite in the Type 2. Preliminary results indicate mineralization of gold in the Type 2 involves nanoparticle gold, but lacks a fundamental aggregation process that occurs in the Type 4 below. TEM results also point to variation and impurities which may impede extraction processes such as nanoparticles of Fe. Further work will seek to identify exact textural relationships between nanoparticle gold and illite-sericite.