Arsenic speciation in pyrite and secondary weathering phases, Mother Lode Gold District, Tuolumne County, California

Abstract

Arsenian pyrite, formed during Cretaceous gold mineralization, is the primary source of As along the Melones fault zone in the southern Mother Lode Gold District of California. Mine tailings and associated weathering products from partially submerged inactive gold mines at Don Pedro Reservoir, on the Tuolumne River, contain H20±1300 ppm As. The highest concentrations are in weathering crusts from the Clio mine and nearby outcrops which contain goethite or jarosite. As is concentrated up to 2150 ppm in the ®ne-grained (<63 mm) fraction of these Fe-rich weathering products.Individual pyrite grains in albite-chlorite schists of the Clio mine tailings contain an average of 1.2 wt.% As. Pyrite grains are coarsely zoned, with local As concentrations ranging from H0 to 5 wt.%. Electron microprobe, transmission electron microscope, and extended X-ray absorption ®ne-structure spectroscopy (EXAFS) analyses indicate that As substitutes for S in pyrite and is not present as inclusions of arsenopyrite or other As-bearing phases. Comparison with simulated EXAFS spectra demonstrates that As atoms are locally clustered in the pyrite lattice and that the unit cell of arsenian pyrite is expanded by H2.6% relative to pure pyrite. During weathering, clustered substitution of As into pyrite may be responsible for accelerating oxidation, hydrolysis, and dissolution of arsenian pyrite relative to pure pyrite in weathered tailings. Arsenic K-edge EXAFS analysis of the ®ne-grained Ferich weathering products are consistent with corner-sharing between As(V) tetrahedra and Fe(III)-octahedra. Determinations of nearest-neighbor distances and atomic identities, generated from least-squares ®tting algorithms to spectral data, indicate that arsenate tetrahedra are sorbed on goethite mineral surfaces but substitute for SO 4 in jarosite. Erosional transport of As-bearing goethite and jarosite to Don Pedro Reservoir increases the potential for As mobility and bioavailability by desorption or dissolution. Both the substrate minerals and dissolved As species are expected to respond to seasonal changes in lake chemistry caused by thermal strati®cation and turnover within the monomictic Don Pedro Reservoir. Arsenic is predicted to be most bioavailable and toxic in the reservoir's summer hypolimnion. 7

EXAFS and XANES study of arsenic in contaminated soil

“…Arsenic K-edge XANES was used to probe directly the average As valence in the soil sample, by its effect on the energy position of the As K-edge. 2,7,9,11 With increasing oxidation state, the K-edge is shifted to higher energies. For a given type of ligand, a linear relation between the edge shift and the valence state was established.…”

“…Best-fit values of the structural parameters, listed in Table 1, are in good agreement with previous EXAFS results for scorodite. 3,7 The contribution of oxygen atoms in the second coordination shell was found to be negligibly small owing to a large static disorder (large Debye-Waller factors) and therefore excluded from the model.…”

Geological and anthropogenic factors influencing mercury speciation in mine wastes: an EXAFS spectroscopy study

“…The EXAFS-based speciation procedure for Hg is similar to that previously developed for and applied to other metal(loid)s such as arsenic (1,2) , lead (3)(4)(5) , and zinc (6,7) . It involves the comparison of an EXAFS spectrum collected from a Hg-bearing sample with a spectral library of model compounds previously generated from an assortment of Hg minerals and sorbed species (Figure 3).…”

Selenium and Other Trace Element Mobility in Waste Products and Weathered Sediments at Parys Mountain Copper Mine, Anglesey, UK

“…Oxidation of exposed pyrite can liberate trace elements [55][56][57][58][59] from the rapidly exhumed bedrock ore. The initial liberation and enrichment of all waste and natural deposits resulted from active pyrite oxidation, leaching, and release of trace elements, sulphates, and other heavy metals [60][61][62].…”