IntroductionFOR MORE than 25 years, the association between specific rhyolite suites and volcanogenic massive sulfide (VMS) deposits was promoted as an effective tool for exploration. Thurston (1981) and Campbell et al. (1982) first identified this relationship using Archean VMS deposits in the Canadian Shield. Lesher et al. (1986) and Barrie et al. (1993) subsequently developed formal classifications for felsic volcanic rocks based on trace element concentrations and suggested that certain types of rhyolites were more prospective for VMS deposits than others. Recently, Hart et al. (2004) revitalized the concept by proposing a conceptual petrogenetic model to account for the association of specific rhyolite geochemical signatures with VMS deposits. The classification geochemically discriminates rhyolites as FI, FII, FIIIa, or FIIIb types. The general conclusions drawn from Lesher et al. (1986), Lentz (1998), and Hart et al. (2004) are that Archean VMS deposits are hosted mainly by FIII rhyolites, whereas most post-Archean VMS deposits are hosted predominantly by FII rhyolites, and FI rhyolites are unfavorable for VMS mineralization. The link between VMS formation (hydrothermal activity) and rhyolite petrogenesis invokes a particular geodynamic setting and, more specifically, the depth of rhyolite generation (Lentz 1998; Hart et al., 2004).Although exceptions and limitations were recognized, the concept was rapidly adopted by exploration companies, and rhyolites with the most potential were commonly selected without any consideration for the mineral potential of other AbstractThe relationship between rhyolite geochemistry and volcanogenic massive sulfide (VMS) mineralization has been proposed as an exploration tool to discriminate prospective felsic volcanic centers. The most widely used classification discriminates between four types of rhyolite: FI, FII, FIIIa, and FIIIb. The FI rhyolites are calcalkaline, with strongly fractionated REE patterns and strongly negative Ta and Nb anomalies. They are usually considered barren, unless associated with FII or FIII felsic volcanic rocks. The FII rhyolites are calc-alkaline to transitional with moderately fractionated REE patterns and moderate Ta and Nb anomalies. They range from barren to having a high potential to host VMS mineralization. The FIIIa and FIIIb rhyolites are tholeiitic and show weakly fractionated REE patterns and weak to absent Nb and Ta anomalies. They have the highest potential to host VMS mineralization. The FIIIb rhyolites are high-temperature rhyolites with flat REE patterns and no Ta or Nb anomalies.The Abitibi greenstone belt, especially in Quebec, is well known for its abundant and diverse VMS deposits. Representative samples of VMS-associated rhyolites within and outside of mining districts, including the classic Noranda VMS district, were analyzed for major and trace elements to validate their proposed favorability for hosting VMS deposits. Results indicate that all of the rhyolite types are prospective, but mineralization may differ from...

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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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