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 Zechste in 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 characte rized by Cu concentrations of up to 20 wt.%. The different processes leadin g to high-grade Cu mineralization are distinguished by contrasting carbon-s ulfur-iron elemental relationships, Hydrogen Indices (HI) of organic matter obtained by Rock-Eval pyrolysis, and stable isotopic composition of organi c matter, carbonates and sulfides. The isotopic data show the expected corr elation with various facies-related parameters (C,,,, C/S and HI) only in t hose parts of the profile which are not enriched in Cu and suggest a precip itation of metal sulfides (mostly pyrite) by bacterial sulfate reduction (B SR). During diagenesis, the pyrite was replaced by Cu-Fe-sulfides (PR=pyrit e replacement), as indicated by microscopy and by Fe-C-org-S relationships in the bottom part of the Kupferschiefer strata, Upon PR, the primary sulfu r isotopic composition is inherited. Based on average sulfur content of non -mineralized Kupferschiefer and mass-balance calculations, an additional so urce 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), de lta C-13 and delta O-18 values of carbonates are shifted up to 4 parts per thousand towards lower values, compared with those Kupferschiefer sections not enriched in Cu, An opposite shift to isotopically heavier carbon is obs erved in the kerogen and soluble organic matter(bitumen). delta S-34 values of sulfide vary in most parts of the profile between -44.7 parts per thous and and -35.2 parts per thousand (Canyon Diabolo Troilite (CDT)). Higher de lta S-34 values up to -8.4 parts per thousand are obtained from Cu-sulfides of high-grade mineralized samples from the base of the Kupferschiefer stra ta. The results of carbon and sulfur isotopic analyses from this part of th e Kupferschiefer profile provide evidence for oxidative degradation of orga nic matter and its participation in metal sulfide precipitation through the rmochemical sulfate reduction (TSR). (C) 2001 Elsevier Science B.V. All rig hts reserved.