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Without appropriate environmental management controls and regulatory oversight, the mining and milling of gold (Au) can result in significant risks to the environment and human health. Orogenic lode Au deposits, hosted mainly by quartz-carbonate veins, occur in deformed greenstone and metasedimentary terranes around the world. These deposits are the main source of Au in Canada, and are presently a key focus for exploration and development. This Open File Report summarizes results from a three-year environmental geochemistry and mineralogy study of past-producing Au deposits in the Bridge River Mining District in British Columbia. From 2006 to 2008, samples of stream water, sediment, waste rock, tailings, and mine drainage were collected from the Bridge River District, including the Bralorne, King, and Pioneer Au mines near Bralorne, BC. Together, these three mines represent the largest historical Au producer in the Canadian Cordillera (> 4.15 M oz. between 1932 and 1971). Samples were also collected from several nearby antimony (Sb) and mercury (Hg) deposits, which may represent the epizonal portions of the Bralorne-Pioneer hydrothermal system. Baseline concentrations of As, Sb, and Hg in stream sediments from the Bridge River District range from 0.6-168 mg/kg, 0.1-8.3 mg/kg, and 9-1010 μg/kg, respectively. The distribution of these elements is strongly controlled by the original pattern of hydrothermal mineralization within the district. Historical tailings and sediments from underground mine workings have much higher concentrations of As (7.4-137,000 mg/kg; median, 3300 mg/kg), Sb (0.3-707 mg/kg; median, 12.6 mg/kg), and Hg (49->100,000 μg/kg; median, 1200 μg/kg). The highest Hg concentrations occur in tailings at former mill sites where Hg was used for Au amalgamation, and in stream sediments at an abandoned Hg mine. Baseline concentrations of As, Sb, and Hg in filtered stream waters throughout the district ranged from 0.1-4.5 μg/L, 0.03-0.43 μg/L, and 0.6-4.1 ng/L, respectively. Dissolved concentrations of As and Sb were typically much higher in waters draining from mine workings, whereas Hg concentrations were relatively low (<12 ng/L) in all waters sampled. Drainage from the main portal of the Bralorne Mine contains high concentrations of As (2250-2560 μg/L) derived from the flooded mine workings, and this study provides new information on the key processes controlling the composition of this mine effluent. The results of this research can be used to better understand the environmental characteristics of orogenic lode Au deposits, and help to minimize the environmental impacts associated with Au extraction.

Baseline geochemistry and controls on mine drainage composition in the Bridge River Mining District, British Columbia