Metallogeny and tectonics of porphyry copper–molybdenum deposits in British Columbia

Abstract

Metallogenic studies in British Columbia are based on data for over 1200 deposits that include metals and ore minerals recorded, age and lithology of sedimentary and volcanic host rocks, and the nature of any related granitoid intrusions. Analysis of these data defines a bimodal source of metals in porphyry deposits. Cu and Mo are derived from the primary granitoid melt, and the Cu/Mo ratio reflects the composition of that melt. Pb, Zn, Ag, and Au occur in veins peripheral to porphyry intrusions, and the metal content of these veins is controlled by the lithology of their wall rocks.Petrogenetic constraints imply that granitoids are predominantly anatectic crustal melts; thus we use the distribution of porphyries with varying metal ratios to define a map of the lower crust, which then constrains a tectonic model. Two major volcanic–plutonic arcs (Coastal: Jurassic – early Tertiary; Takla: Triassic) were generated above two subduction zones. Anatexis of the associated underplates produced Cu porphyries. The Cu–Mo porphyries of the Intermontane Belt were produced by anatexis of mixed volcanic, volcaniclastic, and clastic sedimentary rocks deposited on a fragment of oceanic crust trapped between the two magmatic arcs in the Jurassic.