Metamorphic ore remobilization in the Hällefors district, Bergslagen, Sweden: constraints from mineralogical and small-scale sulphur isotope studies

Abstract

The marble- and metavolcanic-hosted Pb-Zn- (Ag-Sb-As) deposits of the Hallefors district, located in the Palaeoproterozoic Bergslagen ore province, south central Sweden, comprise both stratabound sulphides and discordant, Ag-rich sulphide-sulphosalt veins. The complex sulphide-sulphosalt assemblages of the Alfrida- Jan Olof mines at Hallefors were investigated by a combination of ore microscopy, electron-microprobe analysis, and in situ laser sulphur isotope analysis. The massive ore is characterized by positive and homoge- neous d 34 S (+1.4& to +2.7& V-CDT), whereas vein- hosted sulphides and sulphosalts exhibit similar, but generally less positive to slightly negative d 34 S( 0.6& to +2.0&). Comparison of the observed ore mineral assemblages with calculated phase equilibria in the sys- tem Fe-As-S-O-H and isotopic fractionation as a function of temperature, oxygen fugacity and pH indi- cates that the vein-type mineralization was formed from relatively reduced and rather alkaline hydrothermal fluids. At these reduced conditions, fractionation of d 34 S via changes of fO2 is insignificant, and thus the isotopic signatures of the vein minerals directly reflect the com- position of the sulphur source. We therefore conclude that the vein-type ore essentially inherited the sulphur isotope signature from the pre-existing massive sulp- hides via metamorphic remobilization at approximately 300-400C and 2-3 kbar. Scales of remobilization observable are on the order of about 5 mm to 30 cm. Overall, the sulphide-sulphosalt assemblages from the Alfrida-Jan Olof mines exhibit d 34 S values which are comparable to a majority of metasupracrustal-hosted deposits in the Bergslagen province, thereby suggesting a common origin from ca. 1.90-1.88 Ga volcanic-hydro- thermal processes.