Seasonal Attenuation and Release of Arsenic by Pyrite Weathering Products, Southern Mother Lode Gold District, California

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9902859 Bird This project focuses on the geochemical cycle of arsenic released to the environment by weathering of arsenian pyrite at an open pit gold mine in the Mother Lode Mining District of California. Arsenic is concentrated by coprecipitation and sorption on pyrite weathering products (iron sulfates and oxyhydroxides). These secondary phases are destabilized by seasonally changing environmental conditions and by physical transport into the alkaline lake environment of the open pit mine. They are only temporary sinks for arsenic, with the potential to release arsenic to aquatic environments via desorption or dissolution. We approach this study through a series of field and laboratory observations/experiments and theoretical models, including: (1) field and laboratory analysis of arsenic distribution in primary minerals of the pit mine; the distribution and speciation of arsenic in outcrop weathering products; and arsenic concentration and speciation in the mine pit lake, springs, and groundwater; (2) laboratory experiments to evaluate arsenic uptake and speciation in jarosite, a major product of pyrite oxidation; and (3) geochemical modeling of pit lake evolution. Results will improve our understanding of the controls on arsenic attenuation and release by secondary minerals, linking seasonal field and analytical observations of weathering mineralogy with pit lake water quality. Evaluation of arsenic bioavailability requires knowledge of its oxidation state and bonding environment. Our study will address both solid-phase (by X-ray absorption spectroscopy) and aqueous arsenic speciation, enabling us to make predictions about arsenic release from secondary minerals which will be applicable to other pyrite-bearing environments such as coalfields, mine and mill tailings, and low-grade mineralized outcrops.