Re-Os isotope geochemistry of the Palaeoproterozoic Sakatti Cu-Ni-PGE sulphide deposit in northern Finland

Abstract

The Sakatti Cu-Ni-PGE sulphide deposit is located in the Central Lapland greenstone belt in northern Finland. It comprises three mineralised olivine cumulate units called the main, north-east and south-west bodies. Sulphides form massive, semi-massive, disseminated and vein-type ores in these ultramafic rocks. The ore mineral assemblage is typical for magmatic sulphide deposits, including chalcopyrite, pyrrhotite, pentlandite, pyrite and magnetite. We present Re-Os concentration and isotope data for whole-rock olivine cumulate samples and mineral separates (chromite, olivine) as well as for massive sulphides and associated oxides. Chromite separates contain 2.49–115.4 ppb Os and 0.03–2.85 ppb Re while olivine separates have 3.13–42.1 ppb Os and 0.43–3.89 ppb Re. Their host olivine cumulates show Os and Re concentrations of 1.17–23.7 ppb and 0.04–0.93 ppb, respectively. The sulphide and magnetite separates show a considerable variation and overlap in the Os and Re concentrations, with the total range being 0.69–393.5 ppb for Os and 4.44–246.9 ppb for Re. Among the sulphide separates, the highest Re and Os concentrations were obtained for pyrrhotite and pyrite, respectively. On average, Re/Os ratios increase in the following order: chromite, whole-rock olivine cumulate, olivine, pyrite, pentlandite, chalcopyrite, pyrrhotite, and magnetite. Excluding the most disturbed samples, 30 analyses yielded a Re-Os isochron age of 2063 ± 35 Ma for the Sakatti deposit, being similar to the U-Pb zircon age of 2058 ± 4 Ma determined for the Kevitsa Ni-Cu sulphide ore-bearing intrusion, which is located ~20 km NE of Sakatti. Re-Os dating of massive sulphide samples yields variable Re-Os isochron ages from 1852 ± 200 Ma to 2045 ± 300 Ma, with all sulphide samples together giving an age of 1928 ± 16 Ma. This indicates resetting and open-system behaviour of some sulphide samples, potentially related to Svecofennian orogenic events. In spite of this resetting, the low Re/Os ratios and high Os concentrations of chromite and several sulphide separates allow us to determine their initial γOs values, which turned out to be very close to chondritic for chromite and moderately radiogenic (γOs from +21 to +56) for sulphides, indicating that sulphide saturation took place after chromite crystallisation.