BCGS Geoscience Map 2015-01
Geology, geochronology and Ni-Cu-PGE orebodies of the Giant Mascot ultramafic intrusion, Hope, southwestern British Columbia
The Giant Mascot ores belong to a class of convergent margin Ni-Cu-PGE deposits that are becoming an increasingly important, yet under-explored, economic resource. Mineralization is generally hosted by small ultramafic-mafic intrusions containing hornblende and characteristic orthopyroxene. The orthopyroxene distinguishes these bodies from classical Alaskan-type intrusions in the supra-subduction zone environment. Geoscience Map 2015-01 summarizes new and historic data for the Giant Mascot Ni-Cu-PGE sulphide deposit, incorporating recent research by Manor (MSc thesis, UBC, 2014) and original work by Aho (1954, 1956).
The Giant Mascot mine (1958-1974), the only past-producer of nickel in British Columbia, yielded over 4 Mt of ore with an average grade of 0.77 wt % Ni and 0.34 wt % Cu. The sulphide ores are hosted by the Giant Mascot ultramafic intrusion, which forms an elliptical plug (~4 km2) that is crudely zoned from a core of dunite and peridotite through pyroxenite to a discontinuous rim of pegmatitic hornblendite-hornblende gabbro. The intrusion is hosted by amphibolite-grade, metasedimentary rocks of the Settler schist (Upper Triassic ) and the Spuzzum pluton (Late Cretaceous). New U-Pb zircon and 40Ar-39Ar determinations indicate a crystallization age of ca. 93 Ma (Late Cretaceous) for the ultramafic suite, and a slightly older but statistically distinct age (ca. 95 Ma) for Spuzzum diorite, consistent with observed contact relationships. These results confirm the age of mineralization and establish the Giant Mascot ores as the world’s youngest known magmatic Ni-Cu-PGE sulphide deposit.
The sulphide ores exhibit unambiguous magmatic textures and are mainly hosted in dunite, peridotite and pyroxenite. The multiple ore zones form small, discrete bodies with pipe-like, lensoid, and tabular morphologies, and enclose massive, net-textured and disseminated sulphides (mainly pyrrhotite, pentlandite and chalcopyrite). The sulphide ores have high tenors (3-14 wt % Ni, 0.1-17.1 wt % Cu, and 84 ppb to 5 g/t total PGE; recalculated to 100% sulphide), and distinct Ir-group PGE in the Western vs Eastern mineralized zones. Platinum-group minerals are mostly bismuthotellurides, exsolved during cooling and fractionation of base-metal sulphides. The geometry, internal characteristics, and contact relationships of the ore-bearing structures are consistent with the features exhibited by magmatic conduits. These narrow conduit systems pose a significant challenge to exploration, yet the presence of PGE-enriched, high-tenor sulphides underscores the potential for economic Ni-Cu-PGE deposits in convergent margin environments.
This project is part of the Targeted Geoscience Initiative 4 (TGI-4) program funded by Natural Resources Canada and administered by the Geological Survey of Canada in collaboration with the British Columbia Geological Survey.
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