Nanoscale Mineralogy and Geochemistry of Arsenian Pyrite in Ore Deposits

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Arsenic-rich (arsenian) pyrite is the most common source of arsenic (As) that is released during weathering to contaminate water and soils, and it also contains significant amounts of other trace elements including Ag, Au, Bi, Cd, Co, Cu, Hg, Ni, Pb, Sb, Se, Te, Tl, and Zn. Many of these elements are of environmental concern and Au and Ag are economically important. Better understanding of the factors that control the substitution of these trace elements into arsenian pyrite is needed if we are to understand the processes that release elements into the environment and trap them in ore deposits. The project to be undertaken will clarify these factors by observation of the nanoscale mineralogy and geochemistry of material from a wide range of geologic environments. Previous work by the group has shown that the limit of solid solution for Au in arsenian pyrite from Carlin-type Au deposits is defined by a linear equation linking the concentrations of gold and arsenic. In the new project, the team will test whether arsenian pyrite in other types of ore deposits (epithermal, mesothermal, skarn, porphyry copper) also hosts Au and has similar As-Au solid-solution relations. They will also test whether other trace elements show similar solubility limits in arsenian pyrite and whether these elements occupy nanodomains when their concentrations exceed the solubility limit. Finally, they will test if compositional zones in arsenian pyrites are formed both by growth of crystal layers and by diffusion of adsorbed trace elements, whether true growth zones can be correlated from sample to sample, and whether they can be used to trace fluid pathways through individual deposits. Nanoscale observations will be carried out by high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM), which is a Z-contrast imaging technique at the near-atomic scale. Micrometer-scale analyses of arsenian pyrite will be made by secondary ion mass spectrometric (SIMS)methods for concentrations of S, Fe, Co, Ni, Cu, Se, Ag, Cd, Sb, Te, Au,and Hg, and by synchrotron X-ray fluorescence (SXRF) for oxidation states of the elements. The research will train students in methods to characterize minerals containing trace amounts of elements of economic and environmental importance and it will provide much-needed interlaboratory comparisonsof trace element analyses by SIMS and laser-ablation, inductively-coupled mass spectrometry (LA-ICP-MS) methods. It will also clarify processes by which As, Hg, Sb, Te and other toxic elements are dispersed into soils and groundwater and identify geological factors that complicate the recovery of Au, Ag and other economically important metals from hydrothermal ores, which are the main source of these metals to our industrial society. Finally, it will provide information on natural processes that form nanoparticles, which will aid in development of methods to produce synthetic nanoparticles.