Hafnium-neodymium constraints on source heterogeneity of the economic ultramafic-mafic Noril'sk-1 intrusion (Russia)

Abstract

The ultramafic–mafic Noril'sk-1 intrusion in Polar Siberia (Russia) hosts one of the world's major platinum-group-element (PGE)-CuNi sulphide deposits. In situ Hf-isotope analyses of zircon and baddeleyite, combined with whole-rock Nd-isotope results, identify three distinct clusters of HfNd isotope values restricted to different lithological units (e.g., gabbro-diorite, unmineralised layered sequence gabbros, and mineralised portions, represented by ultramafic and taxitic-textured rocks). Hf-isotope signatures of zircon and baddeleyite from unmineralized mafic rocks (εHf from 7.3±1.1 to 11.4±0.3) reflect the dominant role of mantle-derived magmas and suggest that a juvenile mantle was one of the main sources for the ultramafic–mafic Noril'sk-1 intrusion. The less radiogenic Hf-isotope values for zircons from mineralised rocks (εHf from 4.9±1.4 to 6.4±1.2) and gabbro-diorite (εHf−1.2±1.9) are indicative of involvement of distinct source components, possibly equivalent to a subcontinental lithospheric mantle and a continental crust, respectively. The significant range in the initial 176Hf/177Hf values of unmineralised rocks and mineralised lithologies indicates interaction of distinct magma sources during formation of the Noril'sk-1 intrusion. Our new findings, in conjunction with whole-rock Nd-isotope results, imply that economic intrusions hosting PGE–CuNi deposits of the Noril'sk area have a far more complex magmatic history than is commonly assumed.