Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

We have investigated the Re–Os isotope geochemistry of 2.7-Ga metakomatiitic 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 erosion model and evaluate the superimposed effects of metamorphism and hydrothermal alteration on ore environments and non-ore environments. Although the geochemical characteristics of these komatiites have led to the belief that these lavas were uncontaminated, radiogenic Os isotopic compositions (?Os = +20.2 and +33.2) obtained from well-preserved komatiites and olivine separates suggest that the Alexo flow has been contaminated by crust-derived material. These data are compatible with the trace-element enrichment observed in melt inclusions trapped within olivine. Redistribution of Os and Re did occur at least at the mineral scale and most likely during the Grenville orogeny. Hydrothermal fluids were channeled along the contact between the komatiites and their basement, and were responsible for the remobilization 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 cumulates and seem to have escaped this localized alteration. Although the Abitibi sulfides have experienced various degrees of metamorphism (from prehnite–pumpellyite to low amphibolite facies), the initial Re–Os isotopic composition of the flows appears to have been preserved at the whole-rock scale. Re–Os isotopic heterogeneity of the Abitibi sulfides is best explained by variable R-factor of the sulfides. Re and Os concentrations and Os isotopic heterogeneity of the Abitibi sulfides are consistent with the current model of nickel sulfide formation, which implies that the assimilation of sulfidic sedimentary rocks was the trigger for sulfide saturation.