Greenschist facies regional and contact metamorphism of the Thalanga volcanic-hosted massive sulfide deposit (northern Queensland, Australia)

Abstract

The conditions of two separate greenschist facies metamorphic events have been reconstructed at the syn-genetic Thalanga massive sulfide deposit (northern Queensland, Australia) using scarce garnet–biotite- and garnet–chlorite-bearing samples from the felsic volcano-sedimentary host-rock succession. Regional metamorphism (peak temperature: 440 to 480 °C) generated a foliation defined by aligned chlorite, muscovite and biotite, and rare, euhedral spessartine–almandine(-grossular)-rich garnets. The emplacement of a post-tectonic diorite pluton was associated with contact metamorphism and scarce anhedral spessartine–almandine-rich garnet poikiloblasts crystallized within a ~650-m wide aureole at temperatures of ~500 °C. Both metamorphic stages occurred under pressure conditions of ≤3.5 kbar. Our study documents a useful method for geothermobarometric reconstruction of metamorphosed, syn-genetic ore deposits where calculations based on sulfide assemblages failed to produce unequivocal constraints. Furthermore, we show that hydrothermally altered footwall rhyolite have higher \( {\rm X}_{{\rm Mg}}^{{\rm chlorite}} \;{\rm and}\;{\rm X}_{{\rm Mg}}^{{\rm biotite}} \) than unaltered rocks, which may help to identify fossil, metamorphosed, hydrothermal systems associated with massive sulfide deposits similar to Thalanga.