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Abstract

Among the Cu-Ni-PGE occurrences hosted by the footwall units of the Sudbury Igneous Complex, low-sulfide systems possess the lowest sulfide content while being significantly enriched in Pd and Pt. Although the contribution of hydrothermal processes to the formation of this type of mineralization has already been recognized, no detailed studies have previously focused on the mineralogy and zonation of mineralization-related silicate assemblages and their distinction from regional processes postdating ore formation. Here, the results of detailed alteration mapping carried out on two recently discovered low-sulfide occurrences in the footwall of the Wisner area in the North Range of the Sudbury Igneous Complex are described to address these questions.

In both areas, disseminated sulfides, S-shaped sulfide veins, and extensional silicate veins trending northwest-southeast to north-northwest−south-southeast formed from hydrothermal activity driven by the heat of the Sudbury Igneous Complex. Hydrothermal vein assemblages in mafic to intermediate host rocks are dominated by actinolite whereas epidote and quartz predominate in granitic host rocks. Compositional similarity (high Ni, low K, and relatively low Mg contents) of actinolite rims of vein-filling amphiboles and actinolite in ore-bearing assemblages, coupled with anomalous PGE contents of amphibole veins and their spatial proximity to mineralized zones, suggest a direct genetic relationship between silicate veining and low-sulfide mineralization. Shear-type epidote veining postdates the Sudbury Igneous Complex-related hydrothermal alteration and slightly redistributes metals from the low-sulfide footwall ores on a local scale. Both Sudbury Igneous Complex-related and regional hydrothermal assemblages of the Wisner area show characteristics identical to similar occurrences elsewhere in the Sudbury structure.

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