The Coulon deposit: quantifying alteration in volcanogenic massive sulphide systems modified by amphibolite-facies metamorphism

Abstract

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.

Quantifying Hydrothermal Alteration: A Review of Methods

“…Furthermore, such methods are interesting because they link field observations with chemical analyses. Normative methods are also an alternative to mineral counts even if, ideally, the calculation should be validated using thin sections [14]. Indeed, petrological studies provide more accurate estimates of mineral proportions, reveal the texture of minerals and veins, and provide insights into cross-cutting relationships (for overlapping alterations).…”

“…This metamorphosed VMS has altered and unaltered rocks that still display distinctive chemistries despite the high grade metamorphic event ( Figure 1a). The bulk-rock chemical changes induced by metamorphism are likely limited compared to the changes induced by the preceding hydrothermal event, and insights into the alteration process can still be gained from the analysis of major elements [14]. As most samples are positioned close to the upper right-hand corner of the diagram, chloritisation is likely intense (Figure 1b).…”

“…These indices are therefore ideal in poorly documented contexts. In cases of metamorphism, such as VMS metamorphosed to a high-grade condition [14], it is assumed that chemical changes induced by alteration processes are much greater than metamorphism-related modifications. This assumption is reasonable, as long as the investigated alteration process does not implicate a volatile element other than H 2 O (e.g., carbonatisation), and as long as migmatites are rare to absent.…”

“…T ratio is about 0.2 for a portion of the samples, and a ratio of 0.2 was applied to estimate FeO and Fe 2 O 3 for the six remaining samples. Normative minerals can be displayed in a circle diagram ( Figure 2b) similar to the one described by Mathieu et al [14]. To plot a sample in a circle with radius R and centre with Cartesian coordinates (Cx, Cy), x and y coordinates are calculated using Equations (12) and (13), and using factors F n and G n to control the location of the displayed minerals (M n ).…”

Origin of the Vanadiferous Serpentine–Magnetite Rocks of the Mt. Sorcerer Area, Lac Doré Layered Intrusion, Chibougamau, Québec

“…Such mass changes were likely induced by chloritization. By comparing with calculations performed on the Coulon VMS [62], the measured mass changes (Fe + Mg > 2-5 g per 100 g of precursor) correspond to moderate to intense chloritization.…”