Till geochemistry and mineralogy near Cu-Au and Cu-Mo porphyry deposits in British Columbia; indicators of buried mineralization

Abstract

Till sampling orientation surveys were completed near the Cu-Mo Gibraltar Mine, the Cu-Au Mount Polley Mine, and the Cu-Mo-Au Woodjam prospect with the objective of defining geochemical and mineralogical indicators of buried Cu porphyry mineralization. Mineralization does not outcrop at Woodjam and was in large part covered by glacial sediments at Gibraltar and Mount Polley Mine at pre-mining time. First, we present the distribution of three indicators at each deposit: 1) Cu concentrations in the clay-sized fraction (<0.002 mm), 2) chalcopyrite grain counts and, 3) epidote grain percentages in the 0.25-0.5 mm fraction (specific gravity >3.2). At each site, the indicators occur in greater concentrations (Cu) or in greater amounts (chalcopyrite and epidote grains) near and down-ice of mineralized zones compared to surrounding background regions devoid of known mineralization. In addition to these three indicators of mineralization, gold grains are more abundant near the Cu-Au porphyry deposit at Mount Polley compared to background levels. Therefore, gold grains are interpreted to be indicative of the Au mineralization. Distribution of these indicators in till are related to the presence of mineralization in bedrock and its dispersal by glacial transport, hence the importance of reconstructing ice-flow histories. Secondly, using till compositional data from a previous project (Mountain Pine Beetle), we test the region of two Late Triassic - Early Jurassic intrusions (Takomkane and Thuya batholiths) for their potential to host Cu porphyry mineralization. Two areas with elevated Cu concentrations, and high chalcopyrite and gold grain counts in till suggest potential for porphyry-style Cu-Au mineralization: 1) the Spout Lake pluton, approximately five kilometres west of the Takomkane batholith, and 2) the north end of the Thuya batholith.