Nanoparticles typically have sizes between 10-6 and 10-9 m (<1 micron in size) and have different chemical and physical properties than the bulk mineral solids. We now understand that they are transitional between truly dissolved atoms or molecules and crystalline solids and may have special properties that can be exploited by new nanotechnologies. Previous research at Auburn University has shown that some metallic hydrothermal ores in the western United States formed by aggregation of metallic nanoparticles, particularly ones rich in such valuable metals as gold and silver. Thus the transport of solid nanoparticles of gold, silver and other metals appears to be a new but little-studied ore-formation process. Previously, researchers have assumed that metals were delivered to the site of their ultimate deposition in true solution, and this now appears not to be the case, at least in some situations. A present-day example of this process is the black smoke emanating from sea floor hydrothermal vents on the ocean floor, which is composed mainly of metal-sulfide nanoparticles entrained in supercritical hydrothermal fluids.
This NSF-supported grant will look at transport of metallic nanoparticles involved in the formation of shallow bonanza ('epithermal') vein deposits of gold and silver, which were the type of precious-metal ores commonly mined in the western US in the 1800s. These ores not only exhibit spectacular mineral textures formed as a result of aggregation of metallic nanoparticles, but preliminary isotopic (copper, lead, and sulfur) investigations have suggested that the nanoparticles that formed the ore mineral textures, appear to have come from much deeper down in the crust than the shallow setting where they were deposited. The hypothesis to be tested with this study is that basaltic magmas, formed from partial melting in the underlying mantle, may release metals in a sulfur-rich, low density phase akin to a 'vapor', which forms nanoparticles of metals, metalloids, and sulfur upon some degree of cooling. Thus the nanoparticles appear to lock in high temperature and primitive isotopic compositions similar to the source magma chambers, and therefore must be transported upward a considerable distance (a few km?) to form the shallow ores. If this can be substantiated by the planned new isotopic investigations of ores from Nevada and Idaho, this will be a new aspect of hydrothermal ore-forming processes that can guide how and where such valuable ores occur, both at a regional and district scale. Results have implications for better understanding metal release and transport from magmas, and perhaps will shed light on what are the sources of magma composition that lead to formation of such ores.
