The sulphur isotope record of sulphides in paleoproterozoic successions, North-West Scotland

Sulphide minerals in the Palaeoproterozoic Loch Maree Group at Kerry Road, Gairloch, North-West Scotland, have a near-zero sulphur isotope composition of magmatic origin. New isotopic measurements in the region show a wider range of compositions. Other samples from the Gairloch region have compositions up to 15‰ heavier than the ore samples in the Kerry Road VMS deposit, indicating that there was a source of sulphur additional to magmatic sulphur. Graphitic schists and marbles from several other localities contain pyrite with relatively heavy compositions typical of diagenetic sulphides elsewhere in the North Atlantic region, which suggests that the additional source of sulphur was from the ambient seawater. The magmatic signature is however recorded in South Harris and Scardroy, which combined with the occurrence of rust zones in Palaeoproterozoic successions suggests that the region merits further exploration.