ULTRAMAFIC ARC CUMULATES IN THE CANADIAN CORDILLERA AND NI-SULPHIDE MINERALIZATION: THE LATE CRETACEOUS GIANT MASCOT NI-CU-PGE DEPOSIT, HOPE, BRITISH COLUMBIA

The Giant Mascot Ni-Cu-PGE deposit is part of a relatively new class of “orogenic” Ni-Cu-PGE sulphide deposits containing orthopyroxene and magmatic hornblende (e.g., Portneuf-Mauricie Domain, Québec; Huangshandong, China; Aguablanca, Spain). The ca. 93 Ma Giant Mascot intrusion is a crudely elliptical, 4×3 km plug composed of ultramafic arc cumulates (olivine-orthopyroxene, hornblende-clinopyroxene) that intruded the ca. 95 Ma Spuzzum pluton and was assembled in several hundred thousand years (CA-TIMS U-Pb zircon, n=9). Sub-vertical pipe-like, lensoid and tabular bodies (n=28) host orthomagmatic Ni-Cu-PGE mineralization with moderate grade (0.77% Ni, 0.34% Cu) and high tenors (3-14 wt% Ni, 0.1-17.1 wt% Cu, 84 ppb-5 g/t or ppm total PGE). Olivine compositions from barren and mineralized rocks range from Fo_89-79 and have highly variable Ni concentrations (335-3860 ppm). Modeling results indicate that sulphides include a variable mixture of 20-60% fractionated monosulphide solid solution (mss) and sulphide liquid. Net-textured sulphides have high Cu and low PGE concentrations with Cu/Pd=1.2×10⁵ and Ni/Cu=3.3, and they originated from a more fractionated mss (R-factor ~50-750). Disseminated sulphides, characterized by Cu/Pd=4.0×10⁴ and Ni/Cu=5.0, represent sulphide melt with upgraded metal contents due to greater silicate melt interaction (R-factor ~500-3000). Sulphur isotopes (n=34) for ultramafic rocks reveal a restricted range of slightly light δ³⁴S values (-3.4 to -1.3‰; mean=-2.3‰) relative to typical mantle values and overlap with analyses from locally pyritiferous Settler schist (-5.4 to -1.2‰; n=4). Sulphide saturation within the Giant Mascot ultramafic magmas occurred in response to a combination of 1) assimilation of the Spuzzum pluton and Si-addition, 2) fractional crystallization, 3) pre- and syn-emplacement mixing of Ni-poor and Ni-rich magmas, and 4) addition of small amounts of external sulphur by assimilation of Settler schist. Arc-related conduit systems that host ultramafic cumulates and favour the formation of Ni-Cu-PGE mineralization similar to that observed at Giant Mascot suggest orogenic deposits may be more abundant in the Canadian Cordillera than previously considered.