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Convergent margin Ni-Cu-PGE deposits : geology, geochronology, and geochemistry of the Giant Mascot magmatic sulphide deposit, Hope, British Columbia Manor, Matthew John


The Giant Mascot Ni-Cu-PGE deposit remains British Columbia’s only past-producing nickel mine (1958-1974) with ~4.2 Mt of ore grading 0.77% Ni, 0.34% Cu, minor Co, Ag, and Au, and unreported platinum group elements (PGE). The deposit is part of a new class of ‘convergent margin’ Ni-Cu-PGE sulphide deposits containing orthopyroxene and magmatic hornblende. The ultramafic-mafic intrusions that host these deposits have relatively small footprints, generally less than ~10 km2 (e.g., Portneuf-Mauricie Domain, Québec; Huangshandong, China; Aguablanca, Spain), and they are becoming increasingly important economic resources globally. Zircon was successfully separated from feldspathic ultramafic rocks and yield a weighted ²⁰⁶Pb/²³⁸U age of crystallization for the Giant Mascot ultramafic intrusion of ca. 93 Ma (CA-TIMS, n=8), thus constraining the age of mineralization and distinguishing it as one of the world’s youngest Ni deposits. The Giant Mascot intrusion is a crudely elliptical, 4×3 km plug composed of ultramafic arc cumulates (olivine-orthopyroxene, hornblende-clinopyroxene) that intruded the Late Cretaceous Spuzzum pluton. Sub-vertical pipe-like, lensoid and tabular bodies (n=28) host orthomagmatic Ni-Cu-PGE mineralization as disseminated, net-textured, semi-massive, and massive ores consisting of pyrrhotite, pentlandite, chalcopyrite, minor pyrite, troilite, and Pt-Pd-Ni bismuthotellurides. The sulphides have high tenors (3-14 wt% Ni, 0.1-17.1 wt% Cu, 84 ppb-5 ppm total PGE) and distinct iridium-group PGE concentrations that represent varying stages of monosulphide solid solution fractionation and subsequent metal enrichment of two magma types forming the Western and Eastern mineralized zones. Sulphur isotopes (n=34) for sulphides in ultramafic rocks reveal δ³⁴S values (-3.4 to -1.3‰) lighter than typical mantle values and overlap with analyses from locally pyritiferous Settler schist (-5.4 to -1.2‰). Sulphide saturation in the Giant Mascot parental magma(s) was triggered in response to 1) reduction of an oxidized, mantle-derived arc magma, 2) addition of external sulphur and silica by assimilation of Settler schist and Spuzzum diorites, and 4) fractional crystallization. The presence of high-tenor sulphides indicates that orogenic Ni-Cu-PGE deposits may be of greater significance to future exploration globally than previously assumed.

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