The role of organic matter during copper enrichment in Kupferschiefer from the Sangerhausen basin, Germany

In a previous study (Sun, Y.-Z., Püttmann, W., 1997, Metal accumulation during and after deposition of the Kupferschiefer from Niederröblingen, Sangerhausen basin, Germany. Applied Geochemistry 12, 577–592) Cu enrichments of up to 20% in the Kupferschiefer have been reported from the Sangerhausen basin, Germany. Petrological and geochemical analyses have shown that Cu was precipitated by two different processes: pyrite replacement (PR) and thermochemical sulfate reduction (TSR). In the present study, the composition of the organic matter has been studied in a profile from the Sangerhausen basin in order to estimate the amount of Cu precipitated by PR and by TSR. Analyses of the soluble organic matter, by GC and GC/MS, and of kerogen by Rock-Eval pyrolysis, indicated that PR is responsible for Cu precipitation with enrichments of less than 8%. Organic matter is not involved in PR but is required as hydrogen donor in the thermochemical formation of H2S through sulfate reduction. In a sample from the bottom section of the Kupferschiefer, with 19.88% Cu, the degree of hydrogen depletion in the organic matter allows one to assess the amount of Cu precipitated by TSR to be approximately 12% (60% of the total Cu). Saturated and aromatic hydrocarbons, kerogen, and possibly methane served as hydrogen donors for TSR. The results argue for stepwise precipitation of Cu sulfides during diagenesis of the shale. First, Cu is precipitated during pyrite replacement, and when the reduced S stored in the sediment is used up, additional H2S is generated by TSR.