Identification of hydrothermal alteration related to mineralisation using epidote mineral chemistry

The sorosilicate mineral epidote is a commonly occurring and easily recognisable mineral found within the alteration halo surrounding mineralised hydrothermal deposits that are hosted in volcano-sedimentary and igneous terranes. Epidote is also a product of greenschist and amphibolite facies regional metamorphism. As a result, in ancient arc-volcanic terranes it can be unclear whether epidote encountered during regional exploration has formed via metamorphic or magmatic-hydrothermal processes. In this study, we compare mineral chemical analyses of epidote from regional metamorphic terranes with epidote sampled from the hydrothermal alteration halos surrounding porphyry and skarn mineralisation in the Baguio district, Philippines. The results are subsequently used to test the ‘fertility’ of epidote-bearing samples collected from the Aconcagua river valley of central Chile. It was determined that there are measurable differences in trace and rare earth element chemistry that can be used to effectively discriminate between epidote formed from regional metamorphism and hydrothermal alteration related to mineralisation. The ability to geochemically distinguish between these two styles of epidote has substantial implications for mineral exploration in regionally metamorphosed terranes, as well as for situations where porphyry-style mineralisation is not exposed at surface.