The mineralogical fate of arsenic during weathering of\ud sulfides in gold-quartz veins: a microbeam analytical study

Thesis (M.S., Geology)--California State University, Sacramento, 2012.Mine waste piles within the historic gold mining site, Empire Mine State Historic Park (EMSHP) in Grass Valley, California, contain various amounts of arsenic and are the current subject of remedial investigations to characterize the arsenic present. In this study, electron microprobe, QEMSCAN (Quantitative Evaluation of Minerals by SCANning electron microscopy), and X-ray absorption spectroscopy (XAS) were used collectively to locate and identify the mineralogical composition of primary and secondary arsenic-bearing minerals at EMSHP. Primary arsenic-bearing minerals identified include the following sulfoarsenides: arsenian pyrite (Fe(S,As)2), arsenopyrite (FeAsS), and cobaltite ((Co,Fe)AsS). Subaerial weathering of these primary sulfoarsenide minerals within mine waste piles has led to oxidation of As(-I) to As(V), allowing for the formation of several arsenic-bearing secondary minerals including the hydrous ferric oxides (HFO) ferrihydrite (5Fe2O3???9H2O) and goethite (FeOOH), scorodite (FeAsO4???2H2O), and various other hydrous ferric arsenates (HFA) and Ca-Fe arsenates. Some of the secondary oxide and arsenate minerals contained more arsenic on a weight basis than the primary sulfide minerals, up to a maximum of 48.1 wt. % arsenic compared to a maximum 44.8 wt. % arsenic in primary minerals. This trend of higher concentrations of arsenic in the secondary minerals than in the primary minerals may be caused by multiple factors, including preferential weathering of arsenic-rich regions in zoned arsenian pyrite, weathering of higher arsenic arsenopyrite, and incorporation of arsenate in HFO and HFA by adsorption or coprecipitation. According to other studies, secondary minerals such as arsenic-bearing Fe-oxides and Ca-Fe arsenates are more soluble in the human gut than the primary As-bearing sulfide minerals, leading to higher bioaccessibility and bioavailability. Results from studies conducted in this thesis may have implications for improving the understanding of arsenic bioaccessibility of mine waste within the EMSHP, and possibly at other historic gold mine sites in California and elsewhere that have similar mine waste undergoing subaerial weathering involving oxidation of arsenic-bearing primary sulfoarsenide minerals and formation of secondary oxide and arsenate minerals.Geolog