GSWA Geochat 116 Matthew Demmer on melting the lower crust and PGE deposits

Matt Demmer, a CCRF PhD candidate at The University of Western Australia, was invited to present at a GSWA GeoChat on recent thermodynamic modelling results. The results challenge some long-held thinking about crustal contamination, and the talk explores what this could mean when viewed from a systems perspective. Many of the world’s great platinum-group-element deposits, including the Bushveld Complex, Stillwater, the Great Dyke and WA’s Gonneville–Julimar, formed from magmas with an unusual chemical fingerprint. They carry more silica and light rare earth elements than mantle melts should. That has long been put down to the magma picking up upper-crustal rock on its way up. The new models point to contamination somewhere deeper, 30-45 km below the surface in the lower crust. Analysed from a systems perspective, the modelling challenges the idea that the lower crust acts as a passive filter and that, in fact, its rheology may be an overriding control on where and when mineralising systems form.