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Department of Earth Sciences, Laurentian University, Volume MSc, p.139 (2012)


Hydrothermally altered volcanic rocks associated with the Amulet F deposit were metamorphosed due to emplacement of the post-volcanic Lac Dufault Pluton, resulting in four compositionally controlled isograds. These isograds are an expression of the changes in bulk rock composition related to the pre-metamorphic hydrothermal alteration. The style of alteration is a progressive replacement of the host lithofacies and is expressed by a gradational transition between metamorphic assemblages within each lithofacies, reflecting the strong dependence of metamorphic assemblages on the bulk-rock composition. There is a progressive depletion of alkaline elements and enrichment in magnesium and iron towards sulfide mineralization, which is reflected in the development of progressively more ferromagnesian-rich phases towards sulfide mineralization. <br/>The metamorphosed Amulet F deposit is the ideal location for evaluating metamorphic phase equilibria modeling. The framework that the detailed petrography and geochemistry provides is critical in establishing thermodynamic models that can be used to interpret metamorphosed hydrothermal alteration. Understanding the precursor lithofacies, structural controls on alteration processes, and bulk-rock changes means that thermodynamic models can be constructed to illustrate mineralogical variations as a function of both temperature and alteration intensity.<br/>The successful application of phase equilibria modeling to a natural alteration system requires an expanded chemical system that incorporates the enrichment and depletion of elements during progressive alteration. Typical carbonate-absent assemblages observed in VMS-related footwall alteration systems dictate that the NCKFMASHTO chemical system is the minimal system required to account for the dominant silicate and oxide assemblages observed. A critical assessment of quantitative phase equilibrium predictions for VMS-related footwall alteration systems in this chemical system using recent thermodynamic datasets and activity models has not been previously reported. This thesis addresses limitations of existing thermodynamic datasets and activity models to reproduce the observed mineral assemblages and compositions for the various metamorphic zones enveloping the Amulet F deposit. These limitations include: partitioning of sodium between amphibole and cordierite and reproducing the coexistence of anthophyllite and cummingtonite for specific bulk rock compositions.<br/>Generally, the phase equilibria predictions match petrographic observations. Modeling of individual rock compositions using a P - T pseudosection indicate peak metamorphic conditions of 2.5 to 4 kbar and 560 to 610°C for the Amulet F deposit. T - X pseudosections for various protoliths illustrate the mineral assemblages across the metamorphosed hydrothermally altered rocks, and can be used to understand the architecture of these metamorphosed alteration zones.