Aqueous and mineralogical sulfur speciation in legacy tailings with variable redox conditions

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 (50 samples from 5 tailings sites.•Intermediate, non-sulfate S occurred in sub-oxic conditions at up to 73% of total S.•Low-Fe porewater had largest non-sulfate S fractions; oxic surface waters smallest.•Diagenetic sulfide precipitation likely decreased intermediate S mobility at depth.