Magma and Ore Petrogenesis in the Talnotry Intrusion, SW Scotland

Abstract

Samples of sulfide mineralization from the lower Paleozoic Talnotry intrusion, SW Scotland, are examined to elucidate ore formation processes. Whereas magmatic Ni-Cu(-PGE) (platinum-group element) mineralization is well known from layered intrusions and the contact zones of high flux magmatic systems in anorogenic settings, rocks formed from small volume and relatively alkaline magmatism like Talnotry in orogenic regimes represent an unconventional setting for such ores. This study aims to characterize the PGE tenor and distribution of base-metal sulfide in the main part of the Talnotry ore body: the pyrrhotite-chalcopyrite assemblage. The Talnotry body itself has the mineralogical and geochemical characteristics of an appinite: despite secondary alteration, the primary magmatic mineral assemblage of biotite and amphibole (magnesio-hornblende and tschermakite) yields magmatic crystallization temperatures up to ∼855 °C and is similar to that of other appinites exposed throughout the Caledonian orogen. Our new major and trace element mineral chemical data reveal that the Talnotry sulfides are relatively depleted in some metals (e.g., Sb, Te, As) compared to typical magmatic sulfides and have PGE contents that are below detection limit (∼0.05 ppm) for all analyses. The relatively enriched bulk PGE contents of Talnotry ores are therefore mainly controlled by a rich and diverse assemblage of platinum-group minerals (PGM). Sulfide S/Se points to a crustal sulfur contribution for the base-metal sulfide mineralization. Although other mineralized appinites have not been identified in abundance yet, one other example (at Sron Garbh in NE Scotland) also exhibits relatively PGE-depleted sulfides and an abundance of PGM. Taken together, our new observations suggest that Talnotry fits well into a potential suite of PGE-mineralized alkaline/appinitic intrusions in southern Scotland. These intrusions are generally accepted to have formed at a late stage in the Caledonian orogenic cycle and to have derived from enriched continental lithospheric or juvenile mantle source(s), suggesting that the parental melts to Talnotry and other appinites in the region had relatively high PGE contents.