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1 Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada
2 Consortium for Advanced Radiation Sources, University of Chicago at Brookhaven National Laboratory, Upton, New York 11973, USA
3 Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada
4 Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
E-mail address: jamieson{at}geol.queensu.ca
Understanding the solid-phase speciation of arsenic in soils and sediments is important in evaluations of the potential mobility of arsenic and of its bio-availability in the environment. This is especially true in mine-influenced environments, where arsenic commonly is present at concentrations two and three orders of magnitude above quality criteria for soils and sediments. Arsenicbearing particulates dispersed through hydraulic transport or aerosol emissions can represent a persistent source of contamination in sediments and soils adjacent to past mining and metallurgical operations. The stability and mobility of arsenic associated with these phases depend on the chemical form and oxidation state of the arsenic and the interaction with post-depositional geochemical conditions. The Giant mine in Yellowknife, Northwest Territories, roasted arsenic-bearing gold ore from 1949 to 1999. The roasting process decomposed arsenic-bearing sulfides (pyrite and arsenopyrite) to produce a calcine containing fine (generally <50 µm) arsenic-bearing iron oxides. We have applied synchrotron As K-edge micro X-ray Absorption Near-Edge Structure (µXANES) and µXRD as part of a grain-by-grain mineralogical approach for the direct determination of the host mineralogy and oxidation state of As in these roaster-derived iron oxides. The grain-scale approach has resolved potential ambiguities that would have existed had only bulk XANES and XRD methods been applied. Using combined optical microscopy, electron microprobe and µXRD, we have determined that the roaster-iron oxides are nanocrystalline grains of maghemite containing <0.5 to 7 wt.% As. Some of these arsenic-bearing nanocrystalline grains are a mixture of maghemite and hematite. All roaster iron oxides, including those present in 50-year-old tailings, contain mixtures of As5+ and As3+. The persistence of As3+ in roaster-derived maghemite in shallow subareal (oxidized) shoreline tailings for over 50 years suggests that the arsenic is relatively stable under these conditions, even though As3+ is a reduced form of arsenic, and maghemite is normally considered a metastable phase.
Keywords: arsenic speciation, maghemite, hematite, micro-XANES, micro-XRD, gold ore, ore roasting, Giant mine, Northwest Territories.
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