On the processes that formed Archaean Ni-Cu sulfide mineralisation in the deep continental crust, Thrym Complex, southeastern Greenland

Abstract

We report high-precision geochronological, structural and geochemical data from a series of mafic–ultramafic bands and lenses hosted in orthogneiss of the remote Archaean Thrym Complex of southeast Greenland. These rocks primarily comprise mafic granulite and ultramafic rocks, which locally host orthomagmatic Ni-Cu-PGE sulfide mineralisation that consists of pyrrhotite, pyrite, pentlandite and chalcopyrite. In situ SHRIMP analyses on zircon grains from a representative mafic granulite sample yield an age of 2859±6Ma, which is interpreted as the crystallisation age for the protolith of the mafic granulite. A younger 2744±9Ma zircon is interpreted to represent peak metamorphism associated with the Skjoldungen Orogeny. On the basis of zircon textural features, whole rock geochemistry and cross-cutting field relationships between the ultramafic rocks and the mafic granulite, it is possible to conclude that emplacement and crystallisation of the Ni-Cu-PGE mineralised peridotite and pyroxenite of the Thrym Complex took place at lower crustal levels (∼25–40km depth), synchronously with the granulite facies conditions that prevailed at the base of the Skjoldungen Orogen. Occurrences of Ni-Cu-PGE mineralisation in the high-grade metamorphic terrains of the Thrym Complex are extremely significant, because they shed light on ore forming processes that happen at the base of the continental crust. Therefore, outcomes from this study suggest that the current exploration search space for Ni-Cu-PGE sulfide mineralisation, which is largely constrained to upper and middle crustal settings, could be significantly expanded to exhumed deeper crustal level terrains of high metamorphic central orogenic complexes.