Re-Os isotopic study of komatiitic volcanism and magmatic sulfide formation in the southern Abitibi greenstone belt, Ontario, Canada

We have investigated the Re-Os isotope geochemistry of 2.7-Ga metakomatiiti c flows and associated Ni-Cu sulfide deposits from Alexo, Texmont and Hart in the Abitibi greenstone belt of Ontario in order to refine the thermal er osion model and evaluate the superimposed effects of metamorphism and hydro thermal alteration on ore environments and non-ore environments. Although t he geochemical characteristics of these komatiites have led to the belief t hat these lavas were uncontaminated, radiogenic Os isotopic compositions (g amma Os = +20.2 and +33.2) obtained from well-preserved komatiites and oliv ine separates suggest that the Alexo flow has been contaminated by crust-de rived material. These data are compatible with the trace-element enrichment observed in melt inclusions trapped within olivine. Redistribution of Os a nd Re did occur at least at the mineral scale and most likely during the Gr envilie orogeny. Hydrothermal fluids were channeled along the contact betwe en the komatiites and their basement, and were responsible for the remobili zation of Re or Os (or both) within the sulfides at Alexo and Hart. Matrix and disseminated sulfides from Texmont are located within the pile of cumul ates and seem to have escaped this localized alteration. Although the Abiti bi sulfides have experienced various degrees of metamorphism (from prehnite -pumpellyite to low amphibolite facies), the initial Re-Os isotopic composi tion of the flows appears to have been preserved at the whole-rock scale. R e-Os isotopic heterogeneity of the Abitibi sulfides is best explained by va riable R-factor of the sulfides. Re and Os concentrations and Os isotopic h eterogeneity of the Abitibi sulfides are consistent with the current model of nickel sulfide formation, which implies that the assimilation of sulfidi c sedimentary rocks was the trigger for sulfide saturation.