1 Predicting pit water quality for the Highway Reward

Open pit mining of the Highway Reward copper-gold deposit has produced a final mining void with a diameter of 600m and a depth of 280m, which will be left to fill with ground and rain water once pumping of pit water ceases. A detailed geological assessment of the pit walls indicated that the wall surface area consists of 13 % sulfidic dacite, 32 % sulfidic rhyolite, 12 % sulfidic breccia, 10 % metasediments, 12 % alluvial sediments, and 17 % massive sulfides (pyrite-chalcopyrite-sphalerite). The massive sulphide occurs partly as backfill in disused pit areas. Several leaching tables were constructed to simulate weathering reactions and surface water runoff in the major pit wall units during a 200-day leaching experiment. While sulphide oxidation in the pitwalls will produce acid metal rich leachate, bicarbonate rich groundwater inflow during the dry season acts as a weak buffer. The kinetic test data have been combined with measured pit water chemistry data, measured surface and seasonal groundwater inflows, climatic data, and calculated pit lake surface area and volume to produce a high resolution hydrological and geochemical pit water model. The hydrological model was run for 250 years and it is suggested that the pit lake will reach its equilibrium depth of ~100m in approximately 90 years from the start of filling. The geochemical model was run for 20 years and the results indicate that the pit lake will have a pH <4 and moderate but increasing metal concentrations. After 20 years the pit lake will start to flood the uncapped sulfide waste rock in the disused pit area. From then onwards, continuous wetting and drying of the waste rock along the water line and the dissolution of accumulated acid generating salts will cause decreasing pH and increased metal concentrations in the pit lake. This process will continue indefinitely as the pit lake will not be able to completely flood the sulfide waste rock.