An assessment of acid rock drainage potential of waste rock and implications for long term weathering of the North Dump at Island Copper Mine, Port Hardy, B.C.

Island Copper Mine, owned by BHP Minerals Canada Ltd., is located at the north end of Vancouver Island, British Columbia. The mine, a copper-molybdenum porphyry deposit and one of Canada’s largest open low grade copper mines, commenced operation in 1971 and with current reserves, mining is expected to continue until late 1995. Over the years of operation, approximately 616 million tonnes of waste rock has been placed both on land (76 million tonnes) and into Rupert Inlet (540 million tonnes), an adjacent marine fjord. Acid rock drainage, the term used for contaminated drainage resulting from oxidation of certain sulfide minerals, was first detected in the open pit area in 1982, and from the on land waste rock dumps in 1985. Currently, all drainage from the mine area is directed to a water management pond for recycling to the concentrator and discharge when all provincial effluent standards are met. In comparison with other Canadian mines with acid rock drainage, effluent from Island Copper’s on land dump has relatively low concentrations of contaminants. The objective of this study was to assist Island Copper in long term prediction of water quality emanating from the North dump, the largest on land dump at the mine. The study involved characterization of both the waste rock dump as a whole, and of the various waste rock types comprising the dump. Waste rock dump characterization entailed examination of existing data coupled with field measurements. Results indicate that there is sufficient oxygen in almost all areas of the on land dumps for pyrite oxidation. There is also evidence of convective transport of air through the dump. In addition, elevated temperatures, indicative of increased sulfide oxidation rates, have been documented in several of the drill holes through the waste rock dumps. The limited historical monitoring of one dump indicates that temperatures have increased over the last five years, but may be stabilizing in the low 20° C range. Waste rock characterization involved geological, mineralogical, geochemical and physical assessment, as well as kinetic testing of samples from eight selected sites on the North dump. From this work, three categories of waste rock were derived: i) type I rocks, which are interpreted to have sufficient excess alkalinity to do some degree of buffering on infiltrating acidic drainage, ii) type II rocks, which although possibly generating alkaline leachate at present, are not regarded to have sufficient excess alkalinity to adequately buffer infiltrating acidic drainage, but are not expected to significantly contribute to net acidity of the waste rock dump; and iii) type III rocks, which are presently producing leachate with high net acidity. Type III rocks are of variable lithology, strongly hydrothermally altered, and contain elevated levels of fine grained disseminated pyrite. The dumps or dump areas documented to be producing acidic effluent contain at least 14 percent of type III and 21 percent of type II material. Comparison of leachate quality from laboratory kinetic tests and waste rock dump effluent indicates that the kinetic tests significantly overestimate actual contaminant loads, and only net acid load and molar calcium to sulfate can be confidently scaled from laboratory to field conditions. The model derived for prediction of future effluent quality indicates that when dump temperatures stabilize, effluent quality will also stabilize and contaminant concentrations will gradually decrease over time.Applied Science, Faculty ofMining Engineering, Keevil Institute ofGraduat