Trace elements in sulfides and release to porewater from sulfide oxidation in a historical waste-rock pile, Ontario, Canada

Porewater chemistry and sulfide mineral characteristics were evaluated in a waste-rock pile at the Detour Lake gold mine, Ontario, Canada, that had weathered in situ for more than 30 years. Geochemical characterization suggests that the waste rock would be potentially acid generating. Porewater extracted from the waste rock ranged in pH from 7.05 to 8.35, suggesting on-going acid neutralization by carbonate-mineral dissolution. Of the trace elements, nickel occurs at the highest concentration in pyrrhotite (median of 2738 ppm in historical samples), the associated alteration rims, and in the porewater samples (median of 140 mu g L-1). Nickel:cobalt and nickel:iron ratios in the sulfide grains and alteration rims, together with the porewater data, suggest no preferential retention of nickel over cobalt, and that sorption sites were saturated. X-ray absorption near edge structure (XANES) spectroscopy measurements at the iron-, sulfur- and nickel-edges also indicate trace element associations within the sulfide grains and alteration rims, and the presence of oxidized species of iron and sulfur.