A new look at the metal trap in the Kupferschiefer district using sulfur isotope microanalyses of pyrite and chalcopyrite

Abstract The European Kupferschiefer Province contains multiple sedimentary rock hosted copper (SRC) deposits and has been mined for many centuries. The formation of the SRC 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. The depositional paleoredox conditions for the Kupferschiefer (T1) organic rich mudstone unit have been reconstructed using whole rock analyses of redox-sensitive trace elements (RSTE; e.g., Mo, U, V). The strong enrichments in RSTE and high Mo/U ratios suggest that euxinic conditions prevailed in the water column in this part of the basin. Reflected light and scanning electron microscope (SEM) petrography help constrain key paragenetic relationships and sulfur isotope microanalyses of pyrite and chalcopyrite (secondary ion mass spectrometry; SIMS) have been used to reconstruct conditions of sulfide formation in the main host units. Notably, extensive pseudomorphic replacement of carbonate cement and feldspar clasts by ore stage sulfides is evidence that hydrothermal activity post-dated the formation of diagenetic phases and major compaction. Nevertheless, strongly negative δ 34 S values in pyrite (–58.3‰ to − 48.3‰) and chalcopyrite (–44.8‰ to − 34.5‰) indicate that reduced sulfur was derived from open system organoclastic sulfate reduction (OSR) in all units. The petrographic evidence of host rock replacement means the isotopic evidence of open system sulfate reduction cannot be explained by sulfide precipitation from early diagenetic pore fluids; rather, it could indicate (i) that bacteriogenic sulfur was derived from the replacement of diagenetic pyrite, or (ii) an external source of bacteriogenic sulfur, perhaps in the form of a low temperature sulfur rich brine. A review of compiled data from the Kupferschiefer district suggests that a combination of both of these sources could have contributed to the metal trap.