Till geochemistry and mineralogy: vectoring towards Cu porphyry deposits in British Columbia, Canada

Regional till sampling was completed near four Cu porphyry mineralized zones in south-central British Columbia, Canada: Highland Valley Copper (Cu-Mo), Gibraltar (Cu-Mo), and Mount Polley (Cu-Au-Ag) deposits, and the Woodjam (Cu-Au-Mo) prospect. At all sites, Cu concentrations in the clay-sized fraction and chalcopyrite grains (0.25-0.5 mm; >3.2 specific gravity) are found in greater abundance in till near and down-ice from mineralized zones compared to surrounding background regions. At Mount Polley, the abundance of gold grains in till defines a dispersal train extending at least 3 km down-ice (SW and NW) from mineralization. At three sites out of four, epidote in till heavy mineral concentrates occurs in greater percentage near and down-ice from mineralized zones compared to background regions suggesting that this mineral could be an indicator of propylitic alteration associated with porphyry mineralization. The distribution pattern of Cu concentrations and chalcopyrite grains in till is controlled by the distribution of Cu-porphyry mineral occurrences in bedrock and the direction of ice-flow movements which prevailed during the last glaciation. By comparing study sites, there is a positive relationship between the areal extent of bedrock mineralization that was exposed to glacial erosion and the absolute values of Cu concentrations and chalcopyrite grain counts in till. In the Woodjam region where the till is thick (>10 m), eight samples with background Cu concentrations in the clay-sized fraction of till contain >4 grains of chalcopyrite/10 kg which is indicative of mineralization. This study demonstrates that a combination of till geochemistry and mineralogy is an efficient method for mineral exploration for Cu porphyry deposits covered by variable amounts of glacial sediments.