2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 330-9
Presentation Time: 3:10 PM

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


MANOR, Matthew J., Pacific Centre for Isotopic and Geochemical Research - Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, 2207 Main Mall, Room 2020 - Earth Sciences Building, Vancouver, BC V6T 1Z4, Canada, SCOATES, James S., Pacific Centre for Isotopic and Geochemical Research - Earth, Ocean and Atmospheric Sciences, University of British Columbia, 2207 Main Mall, Room 2020 - Earth Sciences Building, Vancouver, BC V6T-1Z4, Canada, NIXON, Graham T., BC Geological Survey, Ministry of Energy and Mines, P.O. Box 9320, Stn Prov Govt, Victoria, BC V8W9N3, Canada and AMES, Doreen E., Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada, mmanor@eos.ubc.ca

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 Fo89-79 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.