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 incl usions from the Sublayer of the Sudbury Igneous Complex (SIC) at Whistle Mi ne, 30 km NNE of Sudbury, Ontario, Canada. Six S-poor olivine melanorites h ave relatively unradiogenic present day Os-187/Os-188 ratios of 0.16-1.2 an d define an imprecise age of 1.97 Ga. Their Os-187/Os-188((1.85)) ratios of 0.107-0.237 are compatible with derivation either from the mantle at the t ime of SIC formation 1.85 Ga ago, or from pre-existing crustal rocks with l ow 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 1.67 Ga (olivine melanorites), while Os-187/Os-188((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 acid S abunda nces of the inclusions are consistent with mixing at 1.85 Ga between an unr adiogenic silicate fraction defined by the S-poor suite and a radiogenic su lphide-rich component with Re/Os ratios similar to those of pre-existing cr ustal 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-SIG cru stal abundances. However, crude estimates suggest that the overall levels o f Ni and Cu in the pre-impact crust were probably sufficient to account for the budget of these elements in the present-day SIG, but do not rule out t he possibility of significant late-Archaean/early-Proterozoic sulphide mine ralisation in the pre-existing SIC target rocks. (C) 2000 Elsevier Science B.V. All rights reserved.