Abstract

The small pyrite-barite deposits at Aasen are situated in a 7-km-long mineralized zone near the top of ca. 1.8-b.y.-old acid volcanics in the Skellefte field in northeastern Sweden. Compared to other deposits in this province, Aasen is exceptional in its barite content, its relatively low-grade deformation, and the extreme stratiform and stratabound nature of the mineralization. In all other aspects, it is indistinguishable from the numerous other volcanogenic pyritic ores in the province.The pyrite-sulfur is isotopically very variable along the mineralized zone (--14.8 to +8.4ppm delta 34 S), but it is systematically lighter than coexisting barite-sulfur (delta = 11.7 to 17.1ppm). This contrasts with the pyrite-sulfur from other Skellefte field deposits which is remarkably homogeneous (+3 to --3ppm delta 34 S) with a strong modal value of +1 per mil delta 34 S.The pyrite-barite sulfur isotope relationship at Aasen is consistent with a Rayleigh distillation equation for sulfate reduction in a closed system, with a fractionation factor of 1.014 and an initial sulfate with +1 per mil delta 34 S. The data imply that low temperature, biologic sulfate reduction occurred ca. 1.8 b.y. ago. The coincidence between the isotopic composition of the initial Aasen sulfate and the bulk of the volcanic sulfide in the Skellefte province suggests that the sulfate was derived through oxidation of volcanic sulfur. Subsequent reaction of these partially reduced sulfate solutions with subsurface metalliferous brines in sea-floor depressions produced the Aasen ore concentrations.

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