Isotopic studies of two early Proterozoic sulphide ores in the Bergslagen district, south-central Sweden

Abstract: Two sulphide ores in Beraslaqen (Svärdsio and Ammeberq), south-central Sweden, have been examined for their isotope characteristics in order to elucidate their qenetic oriqin. The obtained isotopic data point to a common volcanic-exhalative mode of formation in aqreement with recent qeochemical data. Ore elements were mainly leached from the volcanics, closely after their crystallization. The banded impreqnation ore in Ammeberq is interpreted as a distal on-surface deposit emplaced in a basin where reducinq conditions prevailed, while the Svärdsjö massive, complex ore corresponds to a proximal subsurface deposit. Despite their common origin several dissimilarities exist with respect to sulphur isotopes between the two deposits, while ore lead data and carbon and oxyqen isotope ratios show no major variations. The reason for this is probably that ore lead was derived principally from one source (the volcanic basement), while sulphur probably oriqinated from more than one source (various proportions of leached maqmatic sulphides and H2S derived by reduction of sea water sulphate). It is believed that the measured 6 3'4S values in principal reflect the conditions at the primary ore formation and thus mirror the specific processes takinq place when the brine(s) ascended towards the sea floor, thouqh secondary processes have affected the sulphur isotope distribution and particularly the 6 13C and 6 ls0 values of the ore-associated carbonates. Differences in the type of hydrothermal convection system in Svärdsjö and Ammeberq may also have been a decisive factor for the type of ore formed, the former deposit suqqested to be formed from a hiqh-power discharqe system and the latter from a low-power steady-state system. The ore lead isotopic composition is almost identical in Svards 10 and Ammeberq (and are in fact very similar all over the ore province of Berqslaqen), a feature explained by the fact that the volcanic basement at the time of ore formation was isotopically homoqeneous. Ore lead model ages are too low in comparison with the accepted time of ore formation, which is probably due to a U-Th enrichment of the volcanite-forminq maqmas.