Differential settling and geochemical evolution of tailings’ surface water at the Central Manitoba Gold Mine

Differential settling of sulfides and carbonates in the mill discharge, 70 a ago, has resulted in lateral zoning across the tailings at the Central Manitoba Gold Mine site. Potentially acid-generating minerals within the tailings with a negative net neutralizing potential (NNP) were deposited close to the discharge point, while remote areas accumulated less-dense carbonate-rich sediments. Subsequent erosion and redeposition of the tailings may have modified the original segregation, but have not destroyed the original zonation. In the regions more than 300 m from the discharge site, there is still sufficient calcite to neutralize acid generated from the oxidation of sulphides and the oxidized tailings have pH values of 7–8. In the areas close to the discharge point, the tailings generate more acid than can be neutralized by the dissolution of carbonates and the pH of the oxidized tailings drops to 3. The tailings deposit contains two ponds, the barren Blue Pond located near the discharge point and the Green Pond (which contains horsetail plants) 100 m north of the mill site. The Blue Pond is impacted by acidic drainage from carbonate-poor, sulphide-rich tailings, and characterized by a pH of 4.4 ± 0.1. Further from the water source, the Green Pond is surrounded by tailings with higher carbonate than sulphide content, giving a positive NNP and a pH of 7.4 ± 0.4. Aluminum and Cu are dissolved from the acidic oxidized tailings, precipitated in slimes in streams and then washed into the Blue Pond, where dissolution of Al(OH)3(s), jurbanite and basaluminite maintain the constant pH.