ARSENIC AND ANTIMONY IN MILDLY ALKALINE DRAINAGE FROM A LOW-SULPHIDE GOLD-QUARTZ VEIN DEPOSIT, BRALORNE, BRITISH COLUMBIA

The historic Bralorne gold mine is hosted by a Mesozoic orogenic-type deposit similar to those of the Mother Lode district. The gold is found in shear veins cutting diorite stocks. The veins are often associated with albitite dykes and thin ultramafic bodies. They consist of quartz, calcite and sericite with up to 3% pyrite, arsenopyrite and minor stibnite. Wall-rock alteration exhibits a mineralogical zonation characterized by an outer chlorite-epidote zone, an intermediate calcite- albite-sericite zone and an inner quartz-ankerite-fuchsite zone containing up to 5% sulphides adjacent to the veins. Under-explored gaps in the extensive workings of the mine are being probed through drilling and new adits in order to redevelop the property. One such adit, the Upper Peter, intersects three veins at shallow depth. Drainage from the adit was sampled at the portal over a fourteen-month period on a quasi-weekly basis, except during freezing conditions. Drainage flow rate and electrical conductivity were monitored continuously over the same period at a flume situated underground, upstream of the portal. The hydrograph and EC chemograph reveal a main groundwater recharge period during the spring freshet, with dilution of mine effluent, and a secondary recharge period in the fall with episodic flushing of accumulated soluble salts. Median TDS of the drainage is 468 mg/L. Except during discrete recharge events, pH and alkalinity are stable at 8.7 and 206 mg/L CaCO₃, respectively. Arsenic and antimony concentrations are as high as 1738 and 316 μg/L, respectively. Speciation calculations indicate that the Na-Ca-Mg-HCO₃-SO₄ drainage waters are in equilibrium with chalcedony, gibbsite, illite and serpentine. They are slightly super-saturated with respect to calcite and ferrihydrite, and under-saturated with respect to gypsum, albite and chlorite. Additional mine water analyses from a pool within the adit indicate significant removal of several species including As and Sb upstream of the portal sampling location. Mass balance calculations based on these analyses yield weathering rates of 10.3, 0.4 and 0.02 moles/d for pyrite, arsenopyrite and stibnite, respectively. It can be concluded that effluent loadings of As and Sb are considerably attenuated by mass accumulation within the workings.