Sulfide Oxidation and Mobilization of Arsenic in the Ketza River Mine Tailings

Abstract

Mineralogical compositions of the exposed tailings and those below a water cover are similar with quartz, goethite, calcite, dolomite, muscovite, clinochlore, scorodite, ferric arsenate, Ca–Fe arsenates and lepidocrocite being the dominant minerals. Arsenopyrite, pyrite and jarosite are present in minor abundances. Arsenopyrite and pyrite occur as relict particles embedded in goethite and arsenate minerals indicating that both are oxidized. Powder X-ray diffraction results point to very little variation with depth of the dominant arsenate minerals. Bulk X-ray absorption spectroscopy (XAFS) resolved the changes and variation with depth of the arsenic-bearing minerals in both the exposed and water-covered tailings. Speciation of As in arsenopyrite and pyrite and their oxidation products were examined by collecting micro-XAFS spectra from about 2x2 µm spots along several traverses intersecting the sulfide-oxide boundaries in an exposed tailings sample. These spectra show that As originally occurring as As-1 in arsenopyrite and pyrite is transformed to As+5 in the reaction rims. No reduced As species were detected at the particle rims suggesting that oxidation of As+3 to As+5 species in the solution was rapid and irreversible.