THE EARLY DIAGENESIS OF SUBMERGED SULFIDE-RICH MINE TAILINGS IN ANDERSON LAKE, MANITOBA

The oxidation of sulphide-rich mine tailings stored on land by acidoph ilic bacteria is frequently associated with severe environmental degra dation, as metal-rich acid drainage from the tailings contaminates gro undwater and surface runoff. Permanent storage of tailings underwater may prove to be more chemically benign if oxidation is inhibited suffi ciently. To assess this possibility, the chemistry of interstitial wat ers and the associated solid phases in tailings and organic-rich (20-3 0 wt.% C) natural sediments in Anderson Lake, Manitoba, is examined. T he lake has been used as a receiving basin for sulphide-rich tailings since 1979, and is contaminated with metals as a result of the input o f acid mine drainage along the north shore and the discharge of mill p rocess water with the tailings. Three cores were collected and process ed under nitrogen to extract interstitial waters. At all sites, the oc currence of high concentrations of dissolved iron in shallow pore wate rs indicates that the deposits are anoxic within several millimetres o f the sediment-water interface. Despite high dissolved metal inventori es in the lake, the concentrations of Zn, Cu, Pb, and Cd decrease abru ptly with depth in the pore waters. This cannot be due to non-steady-s tate effects and indicates unequivocally that both the tailings and na tural sediments are acting as sinks for metals, rather than as sources . Dissolved sulphate data and the widespread occurrence of framboidal pyrite in surface sediments suggest that the metals are being precipit ated as sulphide phases at shallow depths.