Pre-emplacement Re–Os ages from ultramafic inclusions in the sublayer of the Sudbury Igneous Complex, Ontario

Re–Os isotope analyses have been performed on a suite of 14 melanorite inclusions from the Sublayer of the Sudbury Igneous Complex (SIC) at Whistle Mine, 30 km NNE of Sudbury, Ontario, Canada. Six S-poor olivine melanorites have relatively unradiogenic present-day 187Os/188Os ratios of 0.16–1.2 and define an imprecise age of 1.97 Ga. Their 187Os/188Os(1.85) ratios of 0.107–0.237 are compatible with derivation either from the mantle at the time of SIC formation 1.85 Ga ago, or from pre-existing crustal rocks with low Re/Os ratios and unradiogenic Os isotope compositions. In contrast, the relatively S-rich inclusions define much older ages of 2.46 Ga (melanorites) and 2.67 Ga (olivine melanorites), while 187Os/188Os(1.85) ratios of 0.41–1.80 unequivocally demonstrate that their Os was largely derived from pre-existing crustal lithologies. The Os isotope compositions and S abundances of the inclusions are consistent with mixing at 1.85 Ga between an unradiogenic silicate fraction defined by the S-poor suite and a radiogenic sulphide-rich component with Re/Os ratios similar to those of pre-existing crustal lithologies. Attempts to evaluate the source of the Ni, Cu, S and PGE in the SIC through mass-balance calculations are not definitive because of the difficulties in estimating accurately both present-day and pre-SIC crustal abundances. However, crude estimates suggest that the overall levels of Ni and Cu in the pre-impact crust were probably sufficient to account for the budget of these elements in the present-day SIC, but do not rule out the possibility of significant late-Archaean/early-Proterozoic sulphide mineralisation in the pre-existing SIC target rocks.