RE-OS ISOTOPIC AND TRACE ELEMENT STUDY OF KOMATIITIC VOLCANISM AND IMPLICATIONS FOR MAGMATIC SULPHIDE FORMATION IN THE SOUTHERN ABITIBI GREENSTONE BELT

*Department of Earth Sciences, Monash University, Australia ; ylahaye@earth.monash.edu.au,

The formation of Archaean nickel sulphide ore deposits is dependent on the host komatiite lava attaining sulphide saturation. Because komatiites are thought to be sulphide-undersaturated on eruption, crustal contamination of turbulently flowing, high temperature komatiites via thermal erosion of sulphidic sediments has been proposed as a mechanism by which sulphide saturation was achieved. In order to refine the ground melting model and evaluate the superimposed effects of metamorphism and hydrothermal alteration on ore environments and non-ore environments, we have investigated the Re-Os isotope geochemistry of 2.7 Ga komatiitic flows from Alexo, Texmont and Hart in the Abitibi greenstone belt (Canada). Serpentinised and/or carbonated silicate whole rocks and ore samples from Alexo and Hart yield heterogeous and radiogenic initial Os isotopic compositions (+21 to +143) in comparison to the chondritic initial defined by the Texmont ores (+0.4 to +3.5) and komatiite lavas and ores from Western Australia (Foster et al, 1996). Based on a Re-Os internal isochron and correlations in Re, Re/Os and initial Os isotopic composition, these results indicate at least two stages of Re mobility, probably soon after emplacement (2.7 Ga) and most certainly during the Grenville orogeny (1.2 Ga). Magmatic olivine from the Alexo flow preserves a primary Os isotopic and trace element geochemical signature. These crystals yield a radiogenic initial Os isotopic composition, similar to the least altered olivine cumulate (+19.3). Trace element analyses of melt inclusions within these magmatic olivine crystals by laser ablation HR-ICP-MS are characterised by a slight enrichment in alkali and alkaline earth elements in comparison with similar but uncontaminated melt inclusions from Belingwe (South Africa). In conclusion, this study indicates a low level of crustal contamination (<5% in Alexo) or a lack of contamination (Texmont) during the genesis and emplacement of the Abitibi komatiites. Bulk crustal assimilation does not appear to be necessary for the magma to reach sulphide saturation and to generate a nickel sulphide ore deposit.