Geochemistry of chromite from the Alexo Komatiite, Dundonald Township : preliminary results from electron microprobe and laser ablation inductively coupled plasma mass spectrometric analyses

This project is part of a M.Sc. study based at Université du Québec à Chicoutimi (UQAC, Saguenay, Québec) under the supervision of Drs. P. Pagé, S.-J. Barnes (UQAC) and M.G. Houlé (GSC-Q) under the High-Mg Ultramafic to Mafic System subproject under the Targeted Geoscience Initiative IV (TGI-4) (see Houlé et al., Article 42, this volume). Komatiite-associated Ni-Cu-(PGE) sulfide deposits exhibit generally a relatively small footprint and are becoming more and more difficult to find especially within poorly exposed and extensively overburden covered Archean and Proterozoic greenstone belts. It has been proposed that lithogeochemistry can be useful to discriminate between rocks from mineralized and barren environments (e.g., Lesher et al., 2001). However, despite the relevance of these lithiogeochemical indicators that can be used as exploration guidelines, Barnes et al. (2004) suggests that overall, the most reliable indicators of the presence of nickel sulfide mineralization is by far the presence of nickel-enriched sulfides. Recently, PGE (Platinum Group Element) content of chromite and particularly the Ru appears to be a good indicator for the presence of Ni-Cu and PGE sulfide deposits and barren komatiitic systems (Locmelis et al., 2011; Pagé et al., 2012; Locmelis et al., 2012). It has been suggested from experimental studies on the PGE fractionation that Os, l¿Ir, Rh, and Ru are included preferentially in the chromite whereas Pd and Pt stayed behind in the magma (Righter et al., 2004). Furthermore, Locmelis et al. (2011) and Pagé et al. (2012) shown that chromite from komatiites are close to the only ones to exhibit concentration above the detection limit for Os, Ir, and Ru. Based on these, we postulated that chromites from unmineralized komatiites (i.e., no sulfides present) have high concentrations of Ru and chalcophile elements in general whereas chromites from mineralized komatiites (i.e., sulfides present) have low concentrations of Ru and other chalcohiles (Figure 46.1). In this contribution, we present the preliminary results of chromites analysis that come from mineralized and unmineralized komatiitic flows/sills at Alexo mine area in the Dundonald Township within the Abitibi greenstone belt in Ontario