Geology, sulphide geochemistry and geochronology of the Romsås Ni-Cu-Co mineralisation, Norway: Implications for ore formation and regional prospectivity

Abstract

The Romsås intrusion, located in the southern portion of the Idefjorden lithotectonic unit, is part of a cluster of small to medium-sized mafic intrusive bodies which host NiCu sulphide mineralisation, referred to as the Indre Østfold metallogenic area. The Romsås is an almost 1 km-long intrusion with rounded shape, comprising undifferentiated quartz-norite with local orbicular texture. The assemblage, comprising cumulus orthopyroxene and plagioclase, is partly recrystallised and altered, thus orthopyroxene is partly replaced by amphibole, whereas plagioclase is locally replaced by epidote. Locally, garnet, amphibole, plagioclase and magnetite define a granoblastic texture as product of amphibolite-facies metamorphism. Several sulphide-rich lenses, normally up to 5 m wide, occur within the Romsås intrusion and display a sharp contact with the host noritic rocks. The textures vary from disseminated, net-textured and massive sulphides. The sulphide assemblage is homogeneous within the deposit and consists mainly of pyrrhotite, followed by pentlandite, and minor chalcopyrite, with variable alteration of pentlandite.This contribution provides the first systematic characterisation of the Romsås Ni-Cu-Co mineralisation and constraints for its formation and geodynamic setting. We have measured the concentrations of S, PGE, TABS+ (Te, As, Bi, Sb and Se) and other chalcophile elements in both whole rock and sulphide minerals from different ore textures, and UPb and Hf isotope systematics in zircon. The Ni and Cu concentrations in 100% sulphides are around 2% and 1%, respectively. The PGE concentrations in all sulphide-bearing samples are low, generally below 0.3 ppm, with Pt and Pd displaying a broad positive correlation and Pt/Pd ratios varying from 1 to 2. Different ore types are modelled to have formed from a mantle-derived parental magma with higher Cu/Pd ratios and lower Pd contents than mantle values, interpreted as the result of 5% prior crystallisation under sulphide-saturated conditions. Negative mantle-normalised Nb anomalies relative to Th, As and Sb suggest around 10% of assimilation of a component similar to lower crust by the parental magmas. Moreover, S/Se ratios greater than mantle values likely reflect the combined effect of Se depletion during early sulphide removal and external S addition upon crustal assimilation. The Romsås Ni-Cu-Co mineralisation likely formed as a result of sulphide segregation in response to external S addition during several magma pulses. The composition of the sulphide ores can be modelled by an R-factor of around 500. A significant budget of the chalcophile elements is not hosted by sulphide minerals and must be thus accounted for other discrete phases, such as platinum-group minerals. Nevertheless, trace elements distribution in sulphides supports only local remobilisation of chalcophile elements and records compositions comparable to sulphides from pristine NiCu deposits worldwide. Sulphide alteration may have led to exsolution of a parcel of chalcophile elements from sulphide lattice. A UPb zircon crystallisation age of 1551 ± 10 Ma, εHft values between ca. 3.5 and 6, and regional geological considerations suggest that the intrusion formed as part of an arc or back-arc rift setting in the Idefjorden lithotectonic unit. The Romsås sulphide mineralisation represents another example of magmatic sulphide deposit formed at an arc-related geodynamic setting, normally unconventional for the formation of this type of deposit. Therefore, our findings support that arc-related settings in Idefjorden lithotectonic unit, southern Norway, should also be regarded as potential for the formation of Ni-Cu-PGE deposits.