Carbon spherules in Ni-Cu-PGE sulphide deposits in the Sudbury impact structure, Canada

摘要

The formation of Sudbury's economically important magmatic Ni-Cu-PGE sulphide deposits is attributed to superheating of crustal rocks, following meteorite impact at 1.85 Ga. Crustal contamination of magmatic sulphide deposits is thought to be important in the mineralization process and this paper documents the first occurrence of sedimentary carbon in the Sudbury Ni-Cu-PGE sulphides near the base of the Sudbury Igneous Complex (SIC). Spherules of graphitic and more disordered carbon inclusions were found in chalcopyrite crystals, suggesting that carbon has behaved as a chalcophilic element in the impact melt. Samples from the impact structure have been subjected to carbon isotopic analysis and Raman microspectroscopy in order to investigate the relationship between carbonaceous strata of the Huronian basement, the crater-fill impact breccias, and the mineral deposits associated with the Sudbury impact. Carbonaceous material occurs in the impact breccias of the Onaping Formation and this study presents stratigraphically constrained carbon isotopic analysis of the impact breccias. Sulphides from the hydrothermal Zn-Pb-Cu Vermilion-Errington deposits 1.5 km above the SIC were found to contain both carbon spherules and larger irregular carbonaceous flakes, suggesting a genetic link between the Ni-Cu-PGE deposits and the hydrothermal Zn-Pb-Cu mineralization. A shared source of carbon for the Vermilion-Errington mineralization and the cross-cutting anthraxolite veins 100m above the deposits in post-impact carbonaceous shales of the Onwatin Formation is indicated by the similarity in their Raman spectra. This is considered to be carbon remobilized by metamorphic hydrothermal fluids preferentially scavenging disordered carbon from within the Onaping Formation breccias. The occurrence of carbon in the Sudbury Ni-Cu-PGE massive sulphide deposits indicates that carbon from the target rocks played a role in the mineralization process, hitherto unrecognized.