Bulk compositional controls on mineral assemblages in metamorphosed ore deposits: an example from the LaRonde-Penna gold-rich VHMS deposit, Quebec, Canada

The hydrothermal processes driving alteration during the formation of VHMS deposits, and therefore the overall geochemistry, mineralogy, and geometry of alteration halos, are reasonably well understood. However, less is known about the effects of metamorphism on altered rocks and the resulting metamorphic mineral assemblages that develop in different alteration zones. Exploration for VHMS deposits is typically targeted at the alteration halos, which can extend for considerable distances compared to the deposits themselves. However, in terranes of greenschist grade and above exploration can be complicated by the effects of metamorphism and deformation, which may have changed the original mineralogy and geometry of alteration zones. To address these issues, research targeted the link between pre-metamorphic alteration halo zonation and resulting mineral assemblages by integrating detailed characterization of mineral assemblage and geochemical zoning with phase equilibria modeling of footwall rocks to the LaRonde-Penna VHMS deposit. The work aimed to further our understanding of the bulk compositional controls on mineral assemblage development during metamorphism, and to test whether a phase diagram approach is a viable method to assist future exploration for VHMS deposits in metamorphosed terranes. The LaRonde-Penna Au-rich VHMS deposit (71 Mt of Au at 3.9 g/t), located in the southern Abitibi greenstone belt in Quebec, Canada, is a well-characterized metamorphosed VHMS deposit. Two transects through the altered rhyodacitic-rhyolitic footwall to the main (20N) ore lens were selected, with each transect preserving contrasting mineral assemblages that have previously been interpreted to reflect different alteration types. Within Transect 1, garnet-bearing, aluminosilicate-absent assemblages show an increase in size, abundance, and Mn content of garnet, inferred to reflect increased alteration intensity as the ore lens is approached. Alteration intensity in the footwall area of Transect 1 is characterized by increases in Mn, Fe, Mg, and K, and decreases in Si, Na, and locally Ca. In Transect 2, aluminosilicate-bearing assemblages with or without garnet and staurolite are developed in altered rocks, and assemblages show a broad increase in porphyroblast size and intensity of schistosity as the ore lens is approached. Alteration intensity in the footwall in Transect 2 is similarly characterized by increases in Fe, Mg, and K, and decreases in Si and Na. Mn is not enriched. The results from this study indicate that garnet stability is distinctly influenced by Mn content and relative proportions of Fe and Mg. In addition, with sufficient enrichment in rock Mn contents, garnet may be stabilized to temperatures as low as ~270°C (in altered metarhyolite), significantly lower than temperatures required to stabilize biotite-bearing assemblages. In addition, biotite-bearing assemblages are stabilized to lower temperatures in rocks with elevated Ti, and the persistence of chlorite to higher temperatures is likely controlled by rock…