| Arsenian pyrite, formed during Cretaceous gold mineralization, is the primary source of arsenic along the Melones fault zone in the southern Mother Lode Gold District of California. This study describes arsenic concentration and speciation in rocks and waters of three geologically similar but hydrologically distinct settings along a 15-km section of the Melones Fault Zone. These are the Clio mine, located on the shoreline of Don Pedro Reservoir; tailings of the Shawmut mine, submerged beneath the reservoir, and the Harvard open pit at the Jamestown Mine, in which a lake is developing.; Clustered substitution of arsenic in pyrite may be responsible for accelerating its oxidation, hydrolysis, and dissolution. Oxidation produces arsenic-bearing goethite, jarosite, and magnesiocopiapite, causing temporary attenuation of arsenic during summer when these minerals accumulate. EXAFS analysis indicates that arsenate tetrahedra are sorbed on goethite mineral surfaces but substitute for sulfate in jarosite. Rapid dissemination of these arsenic bearing minerals into aqueous environments is caused by annual winter storms. At the Harvard mine pit lake, filtered (0.45 μm) arsenic concentrations, dominated by inorganic As(V) species, have fluctuated between 700 and 1000 μg/L over three years.; A series of As-jarosites in the system K2O-Fe2O 3-As2O5-H2SO4-H2O were synthesized at 95° C and 1 bar. Arsenic was incorporated into all samples at higher AsT (defined as 100*As/(As+S)) than in starting solutions. Jarosite precipitated from starting solutions with up to 20% As T; its unit cell expands with increasing arsenic. Starting fluids with 33% and 50% AsT produced poorly crystalline scorodite. An X-ray amorphous material produced in the syntheses exhibits short-range order and is similar to scorodite over domains on the order of 10–14 Å. These polyhedral iron arsenate complexes poisoned against jarosite formation in synthetic samples with high arsenic concentrations.; Arsenic concentration, speciation, and distribution in the Sierra Nevada foothills depend on many factors including lithologic sources of arsenic, climatic influences on weathering of host minerals, and geochemical characteristics of waters with which source and secondary minerals react. As the population of the Mother Lode region grows, it is important to consider these effects during development of land and groundwater resources. |
