Geochemical Controls on Waste-rock Dump Seepage Chemistry at Several Porphyry Mines in the Canadian Cordilleran

In 2003, Red Chris Development Company started collecting baseline data to support its application for environmental approvals to mine the Red Chris porphyry copper-gold deposit located in northwestern British Columbia, Canada. An extensive geochemical testing program revealed that the majority of the waste rock has the potential to generate acid. However, the presence of significant quantities of carbonate minerals indicated that most of the rock would probably take several decades to generate acid. This precluded determination of which rock components would leach under acidic conditions using conventional laboratory methods. This information was required to predict future water chemistry and potential requirements for water treatment. As an alternative, waste-rock seepage chemistry data from six operating or recently closed porphyry mines in British Columbia were compiled and evaluated to investigate common hydrogeochemical features which could be used to predict water chemistry at Red Chris. The resulting database contains dissolved ion chemistry spanning the full range of pH conditions. The database indicated that dissolution of aluminum minerals such as biotite and chlorite exert a strong consistent pH control which can be explained by the solubility of basic aluminum sulfates and hydroxide. Likewise, iron concentrations were similar at all sites and appear to follow the solubility of ferric hydroxide. Dissolved concentrations of copper, zinc, cadmium, and molybdenum in waters were related to pH but reflected the variable presence of minerals such as chalcopyrite, sphalerite (zinc, cadmium) and molybdenite (molybdenum). In conclusion, concentrations of major ions affecting drainage acidity (sulfate, aluminum, and iron) are very similar at different sites, but concentrations of other elements depend on the abundance of their sulfide minerals. Additional