The origin of the Kupferschiefer mineralization (eastern Germany): constraints from petrography and analysis of stable sulfur isotopes of pyrite and chalcopyrite

The European Kupferschiefer Province contains multiple sediment-hosted stratiform copper (SSC) deposits and has been mined for many centuries. The mineralized rocks of the Kupferschiefer are stratabound and hosted by a stratigraphic succession of Late Permian terrestrial sandstones (Rotliegend Formation) and transgressive marine mudstones and limestones (Zechstein Formation). The formation of the Kupferschiefer deposits has been described by various genetic models, which primarily differ in terms of the timing of ore stage sulfide formation relative to host rock deposition (i.e. syn-genetic vs. epigenetic). In this study, samples from two drill cores in the Spremberg-Graustein Kupferschiefer deposit (eastern Germany) are described. Reflected light and scanning electron microscope (SEM) petrography has been used to determine key paragenetic relationships and in situ sulfur isotope values of pyrite and chalcopyrite (secondary ion mass spectrometry; SIMS) have been generated to determine pathways of sulfide formation. The extensive replacement of carbonate and feldspar by ore stage sulfides provides evidence that hydrothermal activity post-dated the formation of diagenetic phases in all units. The highly negative δ³⁴S values of pyrite (–41.9‰ to –35.7‰) and chalcopyrite (–38.9‰ to –34.5‰) indicate that reduced sulfur was generated via open system organoclastic sulfate reduction (OSR). The indistinguishable δ³⁴S_chalcopyrite values preserved in the Rotliegend sandstones and Zechstein mudstones suggest a common origin of sulfides in distinct lithologies. To reconcile the petrographic evidence of host rock replacement with the isotopic evidence of open system sulfate reduction requires an external source of bacteriogenic sulfur, most likely in the form of a low temperature sulfur rich brine. The infiltration of low temperature brines transporting bacteriogenic sulfur is a key feature of genetic models in other sediment hosted systems (e.g., Irish Zn Ore Field). If this model is applied to the Kupferschiefer district, exploration programs should target subbasins with evidence of low temperature brine circulation from marginal sedimentary facies.