Abstract:The Coulon deposit is a volcanogenic massive sulphide (VMS) system in the James Bay area, Superior Craton, Québec, that was metamorphosed to amphibolite-facies conditions. The chemistry and mineralogy of the VMS-related alteration halo proximal to the mineralised sulphide lenses are investigated, using samples collected in the field and 5583 chemical analyses provided by Osisko Ltd. Alteration is quantified using mass balance and normative calculations, and the application and performance of these methods in an exploration context are investigated. In VMS systems, altered rocks proximal to the ore zones are characterised by multi-element metasomatism, which is best quantified by mass balance methods that have been successfully applied in the study area. However, mass balance calculations necessitate the documentation of a precursor, which is not always possible in an exploration context; therefore an alternative method (i.e. alteration indices) was also evaluated. In most VMS systems, proximal alteration is characterised by chlorite (chloritisation), muscovite (sericitisation), and quartz (silicification) while, at the Coulon deposit, altered rocks contain mostly cordierite, biotite, sillimanite, and quartz. Alteration indices were calculated using observed and normative minerals, and provide satisfactory results similar to those obtained with mass balance calculations. Using these results, recommendations are made to estimate the intensity of alteration in the core shack using the proportions of observed minerals.Alteration indices are sensitive to the composition of precursors and, due to high grade metamorphism, chloritisation and sericitisation are not precisely quantified. Recognising these limitations is essential to successful quantification of alteration in areas metamorphosed to high-grade conditions.
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