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

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 homogeneous delta(34)S (+ 1.4 parts per thousand to + 2.7 parts per thousand V-CDT), whereas vein-hosted sulphides and sulphosalts exhibit similar, but generally less positive to slightly negative delta(34)S (-0.6 parts per thousand to + 2.0 parts per thousand). Comparison of the observed ore mineral assemblages with calculated phase equilibria in the system Fe - As - S - O - H and isotopic fractionation as a function of temperature, oxygen fugacity and pH indicates that the vein-type mineralization was formed from relatively reduced and rather alkaline hydrothermal fluids. At these reduced conditions, fractionation of delta(34)S via changes of fO(2) is insignificant, and thus the isotopic signatures of the vein minerals directly reflect the composition of the sulphur source. We therefore conclude that the vein-type ore essentially inherited the sulphur isotope signature from the pre-existing massive sulphides via metamorphic remobilization at approximately 300 - 400 degrees C 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 delta(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-hydrothermal processes.