SULFUR ISOTOPE CHARACTERISTICS OF METAMORPHOSED CU-(ZN) VOLCANOGENIC MASSIVE SULFIDE DEPOSITS IN THE NORWEGIAN CALEDONIDES

Sulphur isotope data are presented for 86 separated samples of pyrite and other sulphides and four anhydrites from within five massive sulph ide deposits in the Norwegian Caledonides, each of which was metamorph osed at different facies conditions during the Caledonian Orogen. Alth ough the isotopic compositions fall within a relatively limited range (delta(34)S ranging from -8 to +12 parts per thousand, with most sampl es clustered between -2 and +9 parts per thousand), each deposit is is otopically distinct. Variations between average delta(34)S values for the various deposits, and between sub-groups identified in the data fo r the Sulitjelma, Gjersvik and Lokken deposits are best explained by d ifferences in Eh, pH and other physico-chemical conditions during syng enetic formation and/or mildly differing sources of sulphur. Regional metamorphism did not lead to an extensive degree of homogenisation, an d it is proposed that primary patterns have been largely preserved. Li thostratigraphic control of isotope compositions is evident and relati ve enrichment in S-34 in disseminated (distal) and stockwork (feeder z one) ore facies is a feature of several deposits. Although the metamor phic episode was effectively a closed system as far as S isotopes are concerned, the structurally-controlled mobilisation of sulphides in so me parts of massive sulphide lenses at Sulitjelma has apparently led t o some considerable modification of S-isotope signatures, typified by pyrites with delta(34)S values of -8 to -4 parts per thousand. However , this would appear to have taken place on a highly localised basis. T he data shed new light on the origin of the abundant anhydrite at Suli tjelma, strongly suggesting that this mineral formed by oxidation of p re-existing sulphides subsequent to syngenetic deposition.