GEOENVIRONMENTAL CHARACTERISTICS OF OROGENIC LODE GOLD DEPOSITS IN THE BRIDGE RIVER MINING DISTRICT, BRITISH COLUMBIA

Without appropriate environmental management controls and regulatory guidelines, the mining and milling of gold 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 the focus of considerable exploration and development.

From 2006 to 2008, samples of stream water, sediment, waste rock, tailings, and mine drainage were collected around the past-producing Bralorne, King, and Pioneer Au mines in British Columbia. 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 Sb and Hg deposits in the Bridge River Mining District, which may represent the epizonal portions of the Bralorne-Pioneer gold system. Background concentrations of As, Hg, and Sb in stream sediments range from 3–82 mg/kg, 9–2700 µg/kg, and 0.16–2.0 mg/kg, respectively. In contrast, historical tailings and rock flour associated with recent mining have much higher concentrations of As (220–13,000 mg/kg), Hg (49–29,000 µg/kg), and Sb (2–700 mg/kg). The highest Hg concentrations occur in tailings at former mill sites where Hg was used for gold amalgamation, and in stream sediments at an abandoned Hg mine. Background concentrations of As, Hg, and Sb in filtered stream waters throughout the district ranged from <0.1–2.6 µg/L, 0.56–3.3 ng/L, and 0.03–0.43 µg/L, respectively. Dissolved concentrations of As and Sb were typically much higher in waters draining from mine workings, whereas the concentrations of Hg were relatively low (<12 ng/L) in all waters sampled. Drainage from the main portal of the Bralorne Mine contains high concentrations of As (~2300 µg/L) associated with passive drainage from the flooded mine workings.

The results of this study are being used to develop a geoenvironmental model for Canadian orogenic lode gold deposits. This model will assist industry and regulators to understand the key environmental characteristics of this type of ore deposit, and will help to minimize the environmental impacts associated with past, present, and future gold extraction.