What do Re-Os ages of sulfide minerals at amphibolite facies mean: resolving syngenetic vs metamorphic ages in an Archean VMS deposit

Establishing the age of mineral deposits and their host rock sequences is fundamental to mineral exploration, as it constrains the temporal evolution of prospective geological environments and the geodynamic processes responsible for ore formation. This is particularly critical for syn-genetic stratiform systems such as volcanogenic massive sulfide (VMS) deposits, which are typically localized along favorable stratigraphic horizons within volcanic belts and linked to episodes of regional extension and magmatism. In Archean cratons, however, VMS deposits commonly experience multiple post-ore deformation, metamorphic, and hydrothermal events that may obscure primary isotopic signatures and result in metal remobilization, complicating efforts to constrain the timing of syn-genetic mineralization.

In this study, we apply an integrated geochronological approach including U–Pb zircon and titanite, Lu–Hf garnet, Re–Os sulfide and molybdenite, and Pb–Pb galena dating to constrain the age of syn-genetic mineralization in an Archean VMS deposit metamorphosed to amphibolite facies. Our results demonstrate that the Re–Os isotopic system in syn-genetic pyrite can be preserved through high-grade metamorphism and yields ages consistent with U–Pb zircon ages of the felsic host rocks, providing a robust means to directly date VMS mineralization in highly metamorphosed Archean terranes. In contrast, Re–Os ages obtained from pyrrhotite record prograde metamorphism and align with Lu–Hf garnet ages, indicating their utility for constraining metamorphic overprinting and metal remobilization events.

We further show that Re–Os dating of bulk massive sulfide ore dominated by both pyrite and pyrrhotite produces mixed, geologically meaningless ages, rendering this approach unreliable where extensive pyrrhotite formation has occurred via pyrite desulfidation. Although molybdenite within the footwall stratigraphy yields robust Re–Os ages despite amphibolite-facies metamorphism, these ages reflect late granitoid emplacement and regional metamorphism rather than syn-genetic VMS mineralization. Collectively, our findings provide new constraints on the timing of metamorphosed VMS deposits and have significant implications for regional exploration strategies, particularly within the Yilgarn Craton.