THE SOLID-PHASE CONTROLS ON THE MOBILITY OF HEAVY-METALS AT THE COPPER CLIFF TAILINGS AREA, SUDBURY, ONTARIO, CANADA

The Copper Cliff Tailings Disposal Area, located near Sudbury, Ontario , covers an area of approximately 2200 ha and constitutes more than 10 % of the total area of all mine tailings in Canada. The area has been utilized since 1936, receiving sulphide-containing tailings from the I nco Sudbury operations. Field measurements of pore-gas oxygen and carb on dioxide in the vadose zone indicate that sulphide oxidation has pro gressed to depths of 1.6 m to 1.7 m within the tailings. The oxidation of sulphide minerals within the vadose zone, and the accompanying dis solution of carbonate and aluminosilicate minerals within these tailin gs releases SO4, Fe(II) and other metals to the pore water. In the vad ose and saturated zones, concentrations of Fe and Ni exceed 10100 mg/l and 2210 mg/l, respectively. These high concentrations of dissolved m etals are attenuated by a series of precipitation, coprecipitation and adsorption reactions. The precipitation of secondary sulphate and hyd roxide phases also create hardpan layers at or near the oxidation fron t. Geochemical modelling of the pore-water chemistry suggests that pH- buffering reactions are occurring within the shallow oxidized zones, a nd that secondary-phase precipitation is occurring at or near the unde rlying hardpan and transition zones. Mineralogical study of the tailin gs confirmed the presence of jarosite, gypsum and goethite within the shallow tailings, suggesting that these phases are controlling the dis solved concentrations of Fe, SO4 and Ca. Extraction experiments conduc ted on the tailings solids indicate that the constituents contained in the water-soluble fraction of the shallow, weathered tailings are der ived from the original pore water and the dissolution of highly solubl e phases such as gypsum. The acid-leachable fraction of the weathered tailings accounts for up to 25% of the heavy metals, and the reducible fraction may contain up to 100% of the heavy metals within the shallo w, weathered tailings. Based on the pore water profiles and the geoche mistry of the tailings solids, a relative mobility scale of Fe = Mn = Ni = Co > Cd Zn > Cr = Pb > Cu can be determined. (C) 1998 Elsevier Sc ience B.V. AU rights reserved.