A SULFUR ISOTOPIC STUDY OF THE BLEIKVASSLI ZN-PB-CU DEPOSIT, NORDLAND, NORTHERN NORWAY

The Bleikvassli Zn-Pb-Cu deposit occurs in the Uppermost Allochthon in the Caledonides of northern Norway. The orebody is enclosed in amphibolite-facies schists and gneisses, underlain by amphibolites, and it has been classified as a sediment-hosted massive sulphide (SEDEX) deposit. The stratiform ore is dominantly pyritic, with a basal layer of pyrrhotitic ore. Sulphide veins occur in the footwall. The orebody generally has a limited range of deltaS-34, from 0.3 to 4.5 parts per thousand (x = 2.4 +/- 1.2 parts per thousand, 1 sigma, n = 26). The lowest deltaS-34 values (0.3-2.3 parts per thousand) were found in sulphide veins in the footwall and vent proximal stratiform ore. More distal pyritic Zn-Pb ore has heavier average deltaS-34 values (up to 4.5 parts per thousand). The ore sulphides were deposited from a hydrothermal solution with deltaS-34 about 2 parts per thousand, perhaps with the incorporation of a minor portion of sulphide from the ambient seawater. The hydrothermal solution probably acquired most of its sulphide from the underlying mixed lithology; notably basaltic rocks. Sulphide produced by thermochemical reduction of seawater in the deep conduit system may also have been incorporated. Bacteriogenic sulphide is not likely as a major source of ore sulphur in the massive ore. Sulphide incorporated in distal pyrite, which have deltaS-34 from -12 to -10 parts per thousand, could have formed either by oxidation of the hydrothermal sulphide, or by bacterial reduction of seawater sulphate in the depositional environment. Exchange of sulphur isotopes probably took place only on a localized scale during Caledonian metamorphism, the bulk sulphur isotopic composition of the ore being preserved in a hand specimen scale.