Arsenates, which correspond to the majority of known arsenic (As)-bearing minerals, control the mobilization of As in contaminated soils, sediments, and fluvial environments as well as in tailings and mine waste piles. Additionally, arsenate-bearing Fe-(hydr)oxides are of particular significance for the control of As mobility, as they are among the most thermodynamically stable minerals under near-neutral to alkaline pH conditions. However, in the surficial (upper 30 cm) alkaline mine tailings at the Cobalt Mining Camp in Northeastern Ontario, Canada, these phases only occur in trace amounts. This study attempts to understand this unusual mineralogical feature through an investigation of the relationships between nano- and macroscale mineralogical and geochemical features at two tailings sites (A and B) at the Cobalt Mining Camp. Sixty samples from two depth profiles (0–30 cm; i.e., one sample per centimeter) were collected at the two sites, analyzed for their major and minor chemical elements, and characterized for their mineralogical composition at the nano- to centimeter scale. The tailings material at both sites is predominantly composed of minerals of the amphibole, chlorite, and feldspar groups, as well as carbonates (calcite and dolomite). Minor phases are Co-Fe-Ni-Zn-sulfarsenides and -arsenates. The tailings material at site B contains, on average, higher concentrations of As, Co, Sb, and Zn and lower concentrations of Fe than the material at site A. Secondary (scanning electron microscope) and transmission electron microscopy studies indicate that the alteration of primary sulfarsenides to secondary arsenates may proceed in the following sequence: (1) the formation of Fe-hydroxide and -arsenate mineral surface coatings on sulfarsenides; (2) the downward mobilization of Co-Ni-Zn-arsenate and (FeOHCO3)aq species; (3) replacement of earlier-formed scorodite by Co-Ni-Zn-arsenates; (4) the precipitation of Co-Ni-Zn-arsenates on the surfaces of silicates; and (5) neoformation of Fe-rich hydroxy-interlayered minerals at greater depth, partly replacing earlier-formed Co-Ni-Zn-arsenates. These processes result in layers enriched in As, Co, Sb, and Zn (increase in Co#) and enriched and depleted in Fe (increase and decrease in Fe#) in tailings material at both sites. The TEM studies further indicate that Co-Ni-Zn-arsenates precipitate initially as nanoparticles on the surface of scorodite and detrital silicates and subsequently coarsen through Oswald ripening. The mineralogical-geochemical features depicted in this study provide a better understanding of the geochemical behavior of Co, Fe, and As in alkaline tailings and may assist in the interpretation of mineral-microbial community associations and the development of effective bioleaching strategies for the strategic element cobalt.

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