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Abstract:

The Ni-Cu-(PGE) deposits of the Thompson nickel belt in the Circum-Superior boundary zone of northern Manitoba define the second largest Ni-Cu-(PGE) mining camp in Canada and one of the premiere Ni-Cu-(PGE) camps of the world. Despite a complex deformation and metamorphic history, the deposits in the Thompson nickel belt exhibit many fundamental characteristics similar to those of other major magmatic Ni-Cu-(PGE) districts: they are hosted by or associated with ultramafic intrusions that appear to represent dynamic feeders, the ores occur at or near the bases of the intrusions, and there is evidence for incorporation of significant amounts of sulfur from the Ospwagan Group metasedimentary country rocks. However, they differ from most other deposits of this type in being metamorphosed to much higher grades, in being much more complexly deformed, and in being mobilized to much greater degrees into the country rocks. The ultramafic intrusions are generally lensoid in shape, reflecting the effects of superimposed deformation on the enclosing metasedimentary rocks, range in composition from komatiitic dunite to komatiitic pyroxenite, are variably serpentinized, and are interpreted to represent a series of sills and low-angle dikes that intruded and interacted with the Ospwagan Group metasedimentary rocks. High Fo contents in relict igneous olivine (as much as Fo92) indicate a low Mg komatiitic parental magma with 22 to 24 percent MgO. Mineralization occurs as type II disseminated sulfides within the ultramafic rocks (e.g., William Lake), as type V tectonically modified massive sulfides within or adjacent to the ultramafic bodies (e.g., Pipe and Birchtree), and as type IV magmatically and metamorphically mobilized sulfides within metasedimentary rocks of the Ospwagan Formation (e.g., Thompson). Intrusions occur at all almost all levels within the Ospwagan Group, but mineralized intrusions are localized exclusively within the lower and middle parts of the Pipe Formation, which contains abundant sulfide-facies iron formation. Density-driven magma emplacement models indicate that the Ospwagan metasedimentary rocks were likely partially lithified prior to magma emplacement and the absence of significant thermal aureoles suggests that they were being metamorphosed. Stratigraphic correlations between ultramafic intrusions, S-rich rocks of the Pipe Formation, and Ni-Cu-(PGE) sulfide mineralization, together with nonmantle δ34S values and S/Se ratios in the ores and nonmantle Th/Yb and Th/Nb ratios in the host rocks, collectively suggest that the mineralization formed by incorporation of S-rich sedimentary rocks by high-temperature komatiitic magmas. Postore deformation and metamorphism have significantly modified the primary characteristics of many of the Thompson nickel belt ore deposits, mobilizing Cu, Au, and Pt. The best exploration tools appear to be aeromagnetic surveys to identify serpentinized ultramafic bodies, which are the heat and metal sources; stratigraphic studies to recognize appropriate levels of the Pipe Member of the Ospwagan Group, which is the S source; lithogeochemical studies to identify the most magnesian and most contaminated host units; which provide the evidence of magma-sediment interaction; and recognition of areas of anomalous Cu, Au, and Pt dispersion halos.<br/>