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UBC Theses and Dissertations

Origin of Cu-PGE-rich sulphide mineralization in the DJ/DB zone of the Turnagain Alaskan-type intrusion, British Columbia Jackson-Brown, Sarah


The Early Jurassic (>188-185 Ma) Turnagain ultramafic-mafic body, a composite Alaskan-type intrusion in the Northern Cordillera of northern British Columbia, hosts a significant nickel-cobalt resource (Horsetrail zone, 1842 Mt @ 0.21 wt. % Ni and 0.013 wt. % Co), and minor copper-platinum group element (Cu-PGE) mineralization. The 24 km² Turnagain intrusion comprises four temporally, spatially, and chemically distinct ultramafic-mafic phases that include dunite, wehrlite, clinopyroxenite, hornblendite and diorite. The 1.5 x 2 km DJ/DB zone, an area of Cu-PGE enrichment that was discovered through soil geochemistry and drilling of a previously under-explored area of the intrusion, is located 2.5 km northwest of the nickel resource. Clinopyroxenites and hornblendites with minor wehrlite are the major rock types of the DJ/DB zone. Orthomagmatic sulphide mineralization ranges from predominantly disseminated sulphides (<5 vol. %) to rare massive ores, and contains chalcopyrite and pyrrhotite, with minor pyrite and pentlandite, and trace arsenides (nickeline), arsenic-antimony sulphides (cobaltite, gersdorffite), and platinum group minerals (PGM; sperrylite, sudburyite, Pd-melonite, testibiopalladite). Sulphide and PGM mineralization underwent minor remobilization related to post-magmatic hydrothermal alteration. The DJ/DB zone represents the products of a magmatic event that is younger than the magmatism that produced the Ni-Co-rich mineralization. Sulphide saturation of Mg-rich arc parent magmas through assimilation of sulphur and carbon from pyrite- and graphite-bearing metasedimentary rocks surrounding the intrusion lead to the formation of the DJ/DB zone following extensive olivine fractionation (i.e., Ni depletion). The distinct nature of Cu-PGE mineralization in the DJ/DB zone indicates that Ni-Cu-PGE sulphides in Alaskan-type intrusions may have more significant exploration potential in convergent margins than previously considered.

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