A sulphur isotopic study of the Bleikvassli Zn-Pb-Cu deposit,Nordland, northern Norway |
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| Authors: | H Skauli A J Boyce A E Fallick |
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| Institution: | (1) Department of Geology, University of Oslo, Blindem, P.O. Box 1047, N-0316 Oslo, Norway;(2) Scottish Universities Research and Reactor Centre, Isotope Geology Unit, G75 0QU East Kilbride, Glasgow, Scotland;(3) Present address: Saga Petroleum, P.O. Box 490, N-1301 Sandvika, Norway |
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| Abstract: | 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  34S, from 0.3 to 4.5% (x = 2.4 ± 1.2 , 1  , n = 26). The lowest 34S values (0.3–2.3) were found in sulphide veins in the footwall and vent proximal stratiform ore. More distal pyritic Zn-Pb ore has heavier average 34S values (up to 4.5). The ore sulphides were deposited from a hydrothermal solution with 34S about 2 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 34S from -12 to-10, 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. |
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