Convergent margin Ni-Cu-PGE-Cr ore systems: U-Pb petrochronology and environments of Cu-PGE versus Cr-PGE mineralization in Alaskan-type intrusions

Abstract

Magmatic Ni-Cu-PGE deposits hosted by ultramafic-mafic intrusions in convergent margin or supra-subduction/ post-subduction tectonic settings are becoming an increasingly important global resource. In the northern Cordillera, this class of intrusions is restricted to the accreted arc terranes of the continental margin of North America where the Alaskan-type subclass is particularly prevalent. This report describes recent research on the Tulameen and Polaris Alaskan-type intrusions in British Columbia that contain orthomagmatic Ni-Cu-PGE mineralization. Preliminary U-Pb geochronological results and trace element analyses of zircon from the Tulameen intrusion are interpreted using an integrated petrochronological approach to distinguish crystallization from post-crystallization events and to constrain the duration of magmatism. Magmatic Cu-PGE mineralization (predominantly chalcopyrite-bornite) in the Tulameen intrusion is hosted by hornblende-magnetite-bearing ultramafic rocks near its periphery. Preliminary results suggest that emplacement and crystallization of the Tulameen intrusion occurred ca. 204-205 Ma (latest Triassic), coincident with a 6 million year period centred on 205 Ma that produced the most important porphyry Cu-Au deposits in British Columbia. The Cu-PGE mineralization constitutes a unique deposit type for Alaskan-type intrusions and has some attributes in common with stratiform or 'reef-style' mineralization in tholeiitic layered intrusions. Magmatic Ni-Cu-PGE sulphides (mainly pyrrhotite, chalcopyrite, minor pentlandite) in the Early Jurassic (ca. 186 Ma) Polaris intrusion are hosted by the more evolved ultramafic and mafic rocks. Remobilized ultramafic cumulates form a distinctive component of both intrusions: plastically deformed and dismembered olivine-chromite cumulates are widely distributed in dunite, and what are interpreted as synmagmatic avalanche deposits are well exposed at Tulameen. The remobilization of pre-existing cumulate sequences is rationalized in terms of an open-system magma recharge model that accounts for some of the key textural features exhibited by ultramafic rocks in Alaskan-type intrusions.