Isotopic evidence for multi-stage base metal enrichment in the Kupferschiefer from the Sangerhausen Basin, Germany

Thirteen samples of a 0.58-m thick succession of Kupferschiefer and Zechstein Limestone from the Sangerhausen Basin, Germany have been investigated by various geochemical methods, in order to clarify the mechanism and timing of base metal mineralization. The bottom section of the profile is characterized by Cu concentrations of up to 20 wt.%. The different processes leading to high-grade Cu mineralization are distinguished by contrasting carbon–sulfur–iron elemental relationships, Hydrogen Indices (HI) of organic matter obtained by Rock–Eval pyrolysis, and stable isotopic composition of organic matter, carbonates and sulfides. The isotopic data show the expected correlation with various facies-related parameters (Corg, C/S and HI) only in those parts of the profile which are not enriched in Cu and suggest a precipitation of metal sulfides (mostly pyrite) by bacterial sulfate reduction (BSR). During diagenesis, the pyrite was replaced by Cu–Fe-sulfides (PR=pyrite replacement), as indicated by microscopy and by Fe–Corg–S relationships in the bottom part of the Kupferschiefer strata. Upon PR, the primary sulfur isotopic composition is inherited. Based on average sulfur content of non-mineralized Kupferschiefer and mass-balance calculations, an additional source of reduced sulfur is required only when more than 8 wt.% of copper has been precipitated. In the high-grade mineralized section (>8 wt.% Cu), δ13C and δ18O values of carbonates are shifted up to 4‰ towards lower values, compared with those Kupferschiefer sections not enriched in Cu. An opposite shift to isotopically heavier carbon is observed in the kerogen and soluble organic matter (bitumen). δ34S values of sulfide vary in most parts of the profile between −44.7‰ and −35.2‰ (Canyon Diabolo Troilite (CDT)). Higher δ34S values up to −8.4‰ are obtained from Cu-sulfides of high-grade mineralized samples from the base of the Kupferschiefer strata. The results of carbon and sulfur isotopic analyses from this part of the Kupferschiefer profile provide evidence for oxidative degradation of organic matter and its participation in metal sulfide precipitation through thermochemical sulfate reduction (TSR).