Ni-Cu-PGE Deposits in the Pacific Nickel Complex, Southwestern BC; A Profile for Magmatic Ni-Cu-PGE Mineralization in a Transpressional Magmatic Arc

The Giant Mascot mine remains British Columbia’s only historic nickel-copper producer, yielding over 26,500 tonnes of Ni, more than 13,000 tonnes of Cu and more than 140 tonnes of Co from 4,319,976 tonnes of ore processed. Significant values of platinum group elements (PGEs) have also been reported. The economic sulphide mineralization is both massive and disseminated. It is hosted by steeply dipping pipes of varying but distinct ultramafic lithology within the Pacific Nickel Complex, proximal to pipe-like apophyses of diorite of the Spuzzum Intrusion. Previous workers noted that the mineralized pipes lie crudely parallel to a regional, northerly-dipping linear fabric. The Pacific Nickel Complex is an assemblage of undeformed to weakly deformed ultramafic rocks that are spatially associated with gabbronorite and with the Spuzzum Intrusion. Both intrusions cut rocks of the Cascade terrane at the accreted eastern margin of Wrangellia. Pyroxenitic rocks with sulphide mineralization and microscopic and macroscopic textures identical to those at Giant Mascot occur as far north as Fir Creek, 35 km from the mine. These rocks are interpreted as a single petrologic province with a common magmatic heritage. The pyroxenites are cumulates with a two-stage history of crystallization. In the first stage, spinel, olivine and clinopyroxene crystallized; spinel compositions overlap those of Noril’sk rift-related intrusions which host a world-class Ni-Cu-PGE deposit. The first-stage phenocrysts are overgrown by a thin rim of clear orthopyroxene and enclosed by anhedral oikocrysts of orthopyroxene, intergrown with clinopyroxene and magmatic pyrrhotite, pentlandite and chalcopyrite. The latest phase of magmatic o r deu te r i c ac t iv i ty compr i sed rep lacement o f orthopyroxene by hornblende. The Pacific Nickel Complex intrudes banded paragneisses containing kyanite/sillimanite, staurolite and garnet with a metamorphic closure age of 96+6/-3 to 91.5±2 Ma. The Spuzzum suite of intermediate pyroxene and amphibole-phyric intrusive rocks that cut the Pacific Nickel Complex have a closure age between 96 and 93 Ma, i n d i c a t i n g t h a t t h e t w o i n t r u s i o n s a r e penecontemporaneous with the Barrovian stage of metamorphism. Inclusions of paragneiss within Pacific Nickel Complex pyroxenite are partially or completely assimilated, contaminating the pyroxenite with felsic material. Sulphide mineralization increases next to metamorphic xenoliths. A continuous range of textures and modal compositions exists between the Pacific Nickel pyroxenites, “feldspathic pyroxenites” in the Cogburn Creek area and pyroxene or uralitic hornblende-phyric “gabbronorite” and “diorite”. Although the chemical compositions of the intrusive rocks in the Pacific Nickel and Spuzzum intrusions overlap and define a general calc-alkaline trend, their incompatible element ratios preclude generation of the more felsic rocks by crystal fractionation of phases observed in the Pacific Nickel cumulates. The near-contemporaneity of the Pacific Nickel and Spuzzum intrusions and the textural evidence of assimilation strongly suggest that the more felsic rocks in either suite were generated by contamination of an ultramafic cumulate mush with material assimilated from the gneissic wallrocks. We propose a model for this deposit type that is based upon the model for the Aguablanca Ni-Cu-PGE deposit in Spain. According to the model, a primary Pacific Nickel magma was generated in the mantle by decompression melting facilitated by an extensional window created by regional transpressive tectonics. This magma ponded at intermediate levels in the crust and began fractional crystallization of spinel, olivine and clinopyroxene. Both the intercumulate and supernatent fluids gained substantial amounts of sulphur from assimilated pyritic paragneiss and schist, supersaturating the system in sulphur and triggering nucleation of a sulphide liquid; this process was coeval with second-stage crystallization of orthopyroxene oikocrysts. The Spuzzum intrusive suite may represent a felsic component of this hybrid system. The transpressive stress field persisted and permitted injection of both the mineralized pipes and apophyses of felsic magma. This hypothesis allows for exploitation of linear zones of weakness in the transpressive stress field, either by mechanical (sulphide-silicate crystal mush) or high-temperature hydrothermal emplacement of sulphide mineralization.