TEMPORAL VARIATIONS IN THE FLOW AND CHEMISTRY OF MINE-WATER DRAINING FROM A PORTAL OF THE MYRA MINE, VANCOUVER ISLAND, BRITISH COLUMBIA

The Myra mine formerly produced Zn and Cu concentrates from a Zn-rich, Kuroko-type, volcanogenic massive sulfide deposit located in the mountainous interior of Vancouver Island. The climate at the site is classified as “Marine West Coast”, with annual precipitation exceeding 2200 mm. Groundwater from a losing stream on the mountainside above the mine follows preferential, fracture-controlled pathways into the upper workings before draining through the 10-level portal. With a view toward mine decommissioning, portal discharge rate was monitored continuously over a seventeen-month period during which 46 water samples were also collected. Effluent chemistry, dominated by Ca, HCO3 and SO4 , exhibits moderate to high total base metal concentrations and near-neutral pH. Carbonatization, mainly of mafic rocks in the hangingwall, provides significant acid neutralizing potential. Metal concentrations, in both dissolved and particulate phases, vary seasonally, with smaller spikes associated with summer storm events, and a main peak associated with flushing of the workings during heavy autumn rains. Aqueous speciation modeling suggests that Fe and Al concentrations are controlled by the solubilities of hydrous ferric oxides and microcrystalline gibbsite, respectively. Concentrations of Zn, Cu and Cd appear to be controlled mainly by adsorption rather than by the solubilities of mineral phases. A comparison of precipitate concentrations observed in portal effluent and predictions from mass balance (inverse) modeling suggests that less than 5% of precipitated Fe and Al hydroxides are transported from the mine. On the other hand, amounts of adsorbed Cu, Zn and Cd measured in the effluent are only slightly lower than predicted values. This suggests that the fraction of (probably finer) Fe particulates discharging from the portal accounts for most of the adsorbed Cu, Zn and Cd predicted by mass balance modeling. Metal loadings in effluent are explained by the oxidation of 3830 kg of pyrite, 600 kg of sphalerite and 190 kg of chalcopyrite, annually. Near-neutral drainage conditions are maintained by the reaction of about 19800 kg of calcite, annually.