TECTONIC EVOLUTION OF THE BRITISH COLUMBIA CORDILLERA DURING MESOZOIC TIME AND CONSEQUENCES FOR THE GENESIS OF MAGMATIC-HYDROTHERMAL ORE DEPOSITS

The accretion of allochthonous terranes and oceanic island arcs to the western margin of ancestral North America characterized the Mesozoic era in British Columbia (BC). A wide variety of magmatic-hydrothermal mineral deposits including porphyry Cu-Mo-Au and associated precious metal epithermal systems, polymetallic skarns, and rare, intrusion-related, Au deposits were directly related to arc magmatism and the subduction of various Pacific Ocean plates. The majority of BC's Fe, Cu, and Au skarns developed during Early to Middle Jurassic oceanic island arc activity in Wrangellia, Quesnellia, and Stikinia. In contrast, nearly all the W skarns are related to a belt of Cretaceous plutons that intrude sedimentary rocks deposited either close to or on the ancestral North American continent. Intrusion-related Au deposits are rare in BC, which is expected given the unusually complex tectonic setting required for their formation. The porphyry deposits formed in two distinct periods of time: the Late Triassic to Middle Jurassic and Early Cretaceous. Alkalic porphyry Cu-Au deposits formed between about 182 and 212 Ma in intraoceanic volcanic arc assemblages in Quesnellia and Stikinia prior to accretion (?) of those terranes onto North America. Remarkable stratigraphic and metallogenic similarities between Quesnellia and Stikinia suggest that they may have formed as a continuous arc, but the history of events leading to their current tectonic configuration is not fully resolved. The more common calc-alkalic porphyry Cu-Mo-Au deposits also occur in Quesnellia and Stikinia in addition to Wrangellia, but are typically of larger tonnage and may have associated epithermal Au-Ag mineralization. In the Toodoggone district in northeastern Stikinia, Early Jurassic porphyry Au-Cu (Mo) deposits at Kemess South, Kemess North, and Pine occur with slightly younger high- and low-sulphidation epithermal Au-Ag deposits (i.e., Shasta, Baker, Lawyers, and Al). The complexity of plate configurations and terrane accretion in the Mesozoic is a mixed blessing for mineral exploration in BC. The location, type, and age of arc-related magmatic-hydrothermal mineral deposits are problematic, but this uncertainty also provides an opportunity for mineral discovery by explorationists with imagination and good geological intuition.