THE GEOCHEMISTRY OF MINE-WASTE PORE-WATER AFFECTED BY THE COMBINED DISPOSAL OF NATROJAROSITE AND BASE-METAL SULFIDE TAILINGS AT KIDD CREEK, TIMMINS, ONTARIO

Since 1985 natrojarosite residue (NaFe3(SO4)(2)(OH)(6)) from the zinc refinery at Kidd Creek near Timmins, Ontario, has been disposed of with mine tailings containing up to 25 wt.% pyrite. Pore-water geochemical data have been collected from depth profiles through the vadose and saturated zones in the tailings. Three zones are defined by the concentrations of metals, SO4, and HCO3. In the deepest zone the concentrations of most metals and anions are below detection and the concentrations of Fe, Mg, Na, K, Zn, Mn, HCO3, and SO4 are low similar to those in the mill discharge water. Higher concentrations of Na, K, Fe, Mg, Mn, Zn, Pb, As, SO4, and HCO3 occur within an intermediate zone. These increased concentrations are attributed to natrojarosite dissolution. A surficial zone affected by sulphide oxidation contains high concentrations of metals and SO4. High concentrations of Cd, Co, Cu, Ni, Cr, and AI in this zone characterize the effects of sulphide oxidation on pore-water composition as distinct from the effects of natrojarosite dissolution. The H+ released by Fe3+ hydrolysis following natrojarosite dissolution is neutralized by carbonate-mineral dissolution that reduces the acid-neutralization capacity of the tailings and releases HCO3. Natrojarosite codisposal with tailings also causes deterioration of the pore-water quality because the dissolution of natrojarosite releases Na, K, Fe, Ph, and SO4 to the pore water.