PORPHYRY INDICATOR MINERALS AND GEOCHEMISTRY OF SUBGLACIAL TILLS OF THE CANADIAN CORDILLERA

Till orientation surveys at two porphyry mines (Gibraltar and Mount Polley) and one developed prospect (Woodjam) in south central British Columbia have defined geochemical and mineralogical indicators of porphyry mineralization in subglacial till. Detrital dispersal can be defined with Cu and Au enrichment in the clay- (<0.002 mm) and silt plus clay-sized (<0.063 mm) fractions of till using either partial (aqua regia ICP-MS) or near total (lithium metaborate/tetraborate fusion ICP-ES and MS) determinations. Characteristic porphyry ore (chalcopyrite, gold) and alteration (epidote, tourmaline, jarosite) minerals were recovered from the same till samples in the 0.25-0.50 mm size and >2.8-3.2 and >3.2 s.g. density fractions. These data show that: 1) these indicators are more abundant near mineralization than in surrounding regions; 2) there is variability in the absolute abundance of these indicators between study sites; 3) at a given study site, mineral grain counts within a dispersal train are in significant contrast with those in surrounding regions; and 4) elevated ore mineral grain counts are commonly coincident with elevated commodity elements values. The reconstructed ice-flow history of south-central British Columbia includes two dominant phases of ice movement. A west to southwest ice-flow event from the Cariboo Mountains towards Fraser River occurred at the onset of the Late Wisconsinan glaciation which was followed by a northwest ice-flow event, likely during the glacial maximum, related to the formation of an ice divide around 52° north latitude. Indicators of porphyry mineralization have been dispersed from <1 and at least up to 5 km in both directions of ice flow resulting in fan-shaped dispersal patterns. While commodity element values in till can be associated with many styles of mineralization, a specific mineral assemblage can potentially be linked to a specific deposit type. A combination of till mineralogy and geochemistry provides the best opportunity for success in exploring for porphyry deposits in glaciated terrain.