Pt-190-Os-186 and Re-187-Os-187 systematics of the Sudbury Igneous Complex, Ontario

We measured by negative thermal ionization mass spectrometry (NTIMS) Re, Os and 0s-186/Os-188 and Os-187/Os-188 in 26 samples of 18 Ni-Cu sulfide ores from the Falconbridge, McCreedy West, and Strathcona mines at Sudbury, Ontario. At McCreedy West and Falconbridge, the isochron Re-Os ages are 1835 +/- 70 Ma and 1827 +/- 340 Ma, and the initial Os-817/Os-186 ratios 0.514 +/- 0.019 and 0.550 +/- 0.024, respectively. The ages agree with the canonical value of 1850 +/- 1 Ma for the Sudbury Igneous Complex (SIC). For Hangingwall and Deep Zone ores at Strathcona, the age of 1780 +/- 7 Ma may reflect resetting by dyke activity. The high initial Os-187/Os-188 of 0.934 +/- 0.005 in these ores is distinct from those at McCreedy West and Falconbridge. Strathcona Deep Copper Zone ores have highly radiogenic Os giving a mean model age of 1883 +/- 54 Ma that is similar to ages at McCreedy West and Falconbridge, but distinct from other Strathcona sulfides. Initial Os-186/Os-188 in two Strathcona ores with low Pt-190/Os-188 average 0.119 826 +/- 0.000 009 (n = 3) and 0.119 827 +/- 0.000 004 (n = 3), respectively, with a grand mean of 0.119 827 +/- 0.000 003. This ratio may be slightly lower than the chondritic value at that time. Similar ores at Falconbridge and McCreedy West show more scatter, averaging 0.119 855 +/- 0.000 008 (n = 6) and 0.119 867 +/- 0.000 020 (n = 3), respectively. These values are substantially suprachondritic. The Re-Os isotope systematics of Sudbury ores are clearly of crustal origin and may be derived from a binary mixture of Superior Province and Huronian metasedimentary rocks, with Strathcona, Falconbridge, and McCreedy West ores containing, respectively, 55, 16, and 12 percent of Os from Superior sediments. The suprachondritic Os-186/Os-188 at McCreedy West and Falconbridge may be due to admixture of Archean or Paleozoic mafic rocks with Pt-190/0s-188 roughly 0.1. No trace of the asteroid that produced the Sudbury Structure has been reported. At the Whistle mine S-poor olivine melanorite inclusions with high Ni and Os and low Os-187/Os-188 may contain the signature of a magmatically fractionated asteroidal core contributing 1 to 2.5 percent metal. The S-poor melanorite Ni and Os data are equally well explained by admixture of roughly 40 percent mantle peridotite, however. (Author)