UNRAVELING BULK-ROCK COMPOSITIONAL CONTROLS ON METAMORPHIC ASSEMBLAGES AT THE LARONDE-PENNA VOLCANOGENIC MASSIVE SULFIDE DEPOSIT

Volcanogenic massive sulfide (VMS) deposits are a significant source of base and precious metals as well as a number of critical elements. Some of the largest gold-rich VMS deposits are located in the Archean Abitibi greenstone belt of Ontario and Quebec, Canada. This includes the LaRonde-Penna deposit, which contains an endowment of 64.6 Mt of ore grading on average 3.85 g/t Au. The ore lenses and their host rocks have been affected by regional amphibolite facies metamorphism. The volcanic succession is highly deformed and now dips subvertically and youngs to the south. A range of different metamorphic mineral assemblages are present within the former hydrothermal alteration halo, with different assemblages occurring at different distances to the ore zones.

This study aimed to constrain the reason for the observed mineralogical zoning within the metamorphosed alteration halo. By taking samples from different locations with respect to ore, it could be shown that differences in the metamorphic mineral assemblages relate to variations in bulk-rock composition. This can be explained by variations in the degree of hydrothermal alteration that predates regional metamorphism. The intensity of fluid-rock interaction varied across the alteration halo and was most pronounced in proximity to the ore zones. Petrological modeling using Theriak-Domino was conducted to evaluate bulk rock compositional controls on the makeup of the metamorphic assemblages. The petrological modeling employed principles of thermodynamics and Gibb’s free energy minimization to construct phase diagrams. Based on comparison of the modeled mineral assemblages with those present in thin sections, the pressure-temperature conditions of metamorphism were constrained. The developed approach can be used in in-mine exploration as the distribution of metamorphic mineral assemblages that can be observed in core and can now be easily translated to chemical gradients.

The ongoing investigations at the LaRonde Penna deposit form part of a bigger study that aims to determine which metamorphic mineral assemblages develop at the expense of different alteration facies at different metamorphic grades. The research has implications to mineral exploration for VMS deposits in high-grade metamorphic terrains.