Fractionation of the platinum-group elements and gold in some komatiites of the Abitibi greenstone belt, northern Ontario

The noble element patterns from four nickel sulfide deposits in komatiites of the Abitibi greenstone belt exhibit various degrees of fractionation (Pd/Ir ratios for Dundonald, Texmont, Hart, and Alexo are 10, 5, 12, and 116, respectively). The noble element patterns could have been affected by: metamorphism (including hydrothermal alteration), contamination of the magma by a sulfide-rich sediment, crystal fractionation of monosulfide solid solution, solubility of Os-Ir alloys and RuS (sub 2) , and timing of sulfur saturation.Noble element patterns of samples from the deposits (Dundonald and Alexo) of the lowest metamorphic grade (prehnite-pumpellyite facies) are smooth and exhibit no or very small Au and Pt anomalies. However, some samples from Texmont (mid-greenschist facies metamorphism) contain Pt and Au anomalies. Further, all samples from the Hart deposit, which has experienced carbonate alteration, have large negative Pt and Au anomalies.Contamination of the komatiites with "Fe sulfide-rich" sediments found in their vicinity will not change the noble element patterns appreciably. A 10 percent contamination of the komatiites will change the Pd/Ir ratio of the sulfides from approximately 5 to 7.5.The massive sulfide at the base of the Alexo deposit has a Pd/Ir ratio of 44, the overlying net-textured ore has a Pd/Ir ratio of 171. This change in Pd/Ir ratios could be due to the fractionation of monosulfide solid solution from the sulfide liquid.The difference in noble element fractionation between the four deposits does not appear to be related to differences in the lithophile element composition of the silicate liquid from which the sulfides separated, since all four deposits have a similar lithophile chemistry. It is proposed that the differences in the fractionation of the noble element between the deposits is the combined result of the timing of sulfur saturation and the lower solubility of Os-Ir and RuS (sub 2) in the silicate melt compared to Rh, Pt, and Pd.