Te-rich protolith for PGE mineralisation in NE Scotland

The geological factors controlling the mineralisation of basic layered Ni-Cu-PGE intrusive deposits are often complex and debated at length. Many models of Ni-Cu deposit formation rely on assimilation of carbonaceous continental crust to reach sulphide saturation for ore formation, rather than through fractionation of the melt alone. The Ordovician ‘Newer Basic Intrusions’ in NE Scotland have historically been targeted for their Ni, Cu and minor precious metal enrichments (up to 700 ppb Au+Pt+Pd). The distribution of PGEs within these intrusions is variable, with the highest concentrations associated with sulphide-rich graphitic pyroxenites. The host rocks for these enriched intrusions are the sulphidic pelites and diamictites of the Dalradian Argyll Group. Sulphides within these strata are often enriched in tellurium, selenium and gold relative to average crustal values and there is considerable evidence for assimilation of this country rock within the ultrabasic intrusions. We propose here an ore formation model of Dalradian country rock assimilation, sulphide saturation and semi-metal enrichment of the intrusive melt. Tellurium enrichment within the melt resulted in localised PGE-telluride mineral formation, associated with graphitic xenoliths. It is possible that more extensive PGE mineralisation exists in similar settings in NE Scotland and globally.