Petrology, geochemistry, and isotopic composition of magmatic sulphide occurrences in the Montagnais Sill Complex, northern Québec, Canada

• Main Author: Smith, William Daniel
• Format: Thesis
• Language: English
• Published: 2021
• Subjects: Canada; Cape Smith; montagnais
• Online Access: https://orca.cardiff.ac.uk/id/eprint/146047/; https://orca.cardiff.ac.uk/id/eprint/146047/1/2021smithwphd.pdf; https://orca.cardiff.ac.uk/id/eprint/146047/2/smithwphd.pdf

The Labrador Trough of northern Québec is a site of ongoing exploration for magmatic Ni-Cu-PGE (platinum-group element) sulphide deposits. The region is considered favourable for such deposits because it is host to voluminous mafic-ultramafic intrusive and extrusive rocks of the Montagnais Sill Complex (MSC) that are located proximal to trans-crustal faults and sulphidebearing country rocks. In addition, the MSC is the easternmost segment of the ∼ 1.88 Ga Circum-Superior large igneous province, which hosts world-class magmatic Ni-Cu-PGE deposits in the Thompson and Cape Smith belts. However, only a number of sub-economic magmatic sulphide occurrences have been identified in the MSC. In the present study, the newly discovered Idefix PGE-Cu and Huckleberry Cu-Ni-(PGE) prospects are used as case studies to evaluate ore-forming processes operating in the Labrador Trough as well as the prospectivity of the region regarding magmatic sulphide deposits. The Idefix PGE-Cu prospect represents a > 400-m-thick stack of aphyric gabbro sills (∼ 8- 14 wt.% MgO, ∼ 1-8 Cr/V) that contain disseminated sulphides (0.2-0.4 g/t Pd+Pt+Au, 0.1- 0.5% Cu and < 0.1% Ni over a thickness of 16-34 m) in its centre and globular sulphides at its base. Both sulphidic horizons are spatially associated with stratiform gabbroic pegmatites, all of which can be traced intermittently for ∼ 7 km along strike. Primary silicate, oxide, and sulphide minerals have undergone moderate to extensive alteration that prevented mineral chemical analysis. It was determined from whole-rock and PGE geochemistry that the sills share a common transitional-MORB source and that the extracted magma underwent significant fractionation before emplacement in the upper crust. Chalophile metals and sulphur show good positive interelement correlations, suggesting that the distribution of metals is primarily controlled by sulphides (pyrrhotite > chalcopyrite > pentlandite ≫ sulpharsenides) and partially controlled by tellurides and electrum. Whole-rock compositions ...