Aqueous and mineralogical sulfur speciation in legacy tailings with variable redox conditions
Matias Silva-Caceres, Adriana Guatame-García, Bas Vriens
Abstract
Sulfide mineral reactivity and dissolved sulfur (S) speciation in tailings systems remain poorly constrained across redox gradients, especially in heterogeneous materials. We characterized the S-bearing mineralogy, prevailing geochemical conditions, and dissolved, intermediate (incompletely oxidized) S speciation, at three legacy tailings in Ontario and two in Nova Scotia, Canada. Tailings in Cobalt, Ontario, contained up to 3.5 wt-% sulfide minerals, mainly authigenic chalcopyrite and cobaltite/gersdorfitte. Sulfate (up to 537 ppm) dominated dissolved S in surface and porewaters from these sites, but significant fractions of thiosulfate and tetrathionate, at >40% of total S, could be detected in tailings porewaters. Tailings at the Goldenville and Montague sites in Nova Scotia contained up to 7.5 wt-% sulfide minerals, mainly pyrite and arsenopyrite, but contained smaller fractions of intermediate S (<15% on average, mostly thiosulfate and sulfite). Across all sites, overlying surface waters contained both lower total S levels, as well as lower intermediate-S fractions, than tailings porewaters. Redox conditions appeared to exert a dominant control on aqueous S speciation, and correlations of intermediate S with dissolved Fe levels and tailings mineralogy suggested the latter presented lesser influences on S dynamics. This work demonstrates that incomplete S oxidation and resultant intermediate S species play an important role for S mobility in sub-oxic tailings systems and at legacy mine sites and should therefore be considered in long-term reactivity and water quality predictions.