Abstract

The Giant Mascot Ni-Cu-PGE deposit is the only past-producing nickel mine (1958–1974) in the Canadian Cordillera of British Columbia, Canada, with ~4.2 Mt of ore grading 0.77% Ni and 0.34% Cu (Ni/Cu = 2.3). The deposit is part of an emerging class of convergent margin Ni-Cu-PGE sulfide deposits where the host rocks are characterized by abundant orthopyroxene and magmatic hornblende. The Giant Mascot intrusion is a crudely elliptical, 3- × 2-km plug composed of ultramafic arc cumulates (olivine-orthopyroxene, hornblende-clinopyroxene) that intruded the Late Cretaceous Spuzzum dioritic pluton. The ores are contained within subvertical pipe-like, lensoid, and tabular bodies (n = 28), interpreted to represent mineralized conduits and consist of pyrrhotite, pentlandite (+argentopentlandite), chalcopyrite, minor pyrite, troilite, and Pt-Pd-Ni bismuthotellurides. The sulfides have moderate tenors (3–14 wt % Ni, 0.1–17.1 wt % Cu, 84 ppb to 5 ppm total PGE), and variations in PGE concentrations distinguish two geographic mineralized zones (western and eastern). Sulfur isotopes for sulfides span a restricted range of δ34S values (−3.4 to −1.3‰) and overlap with analyses from locally pyritiferous Settler schist (−5.4 to −1.2‰). Olivine chemistry reveals a wide range of both Fo (80–89 mol %) and Ni contents (336–3,859 ppm) and provides evidence that Ni contents of olivine were locally upgraded during equilibration with sulfide liquid. Sulfide saturation in the Giant Mascot parental magma(s) was triggered in response to (1) reduction of an oxidized, mantle-derived arc magma, producing ores that formed at an oxidation state of ~ ΔQFM +1; (2) addition of external sulfur and silica by assimilation of Settler schist and Spuzzum diorites; and (3) fractional crystallization of olivine ± orthopyroxene. The presence of high-tenor sulfides in the Giant Mascot deposit indicates that orthomagmatic Ni-Cu-PGE deposits in convergent margin tectonic settings may be of significance to future global exploration.

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