Relationship between metal enrichment and organic composition in Kupferschiefer hosting structure-controlled mineralization from Oberkatz Schwelle, Germany

Base metal mineralization is observed in some Kupferschiefer localities whereRote Fäule (oxidized front of hematite-bearing fluids) is not present, and where contemporaneous Mesozoic to Tertiary tectonism controlled the (vein-type) metal enrichment processes after diagenesis (structure-controlled mineralization). One of these localities is the Spessart-Rhön-Treischfelder-Oberkatz Schwellen region in the Hessian Depression, Germany. In order to clarify the mechanism of base metal precipitation under these particular conditions, a closely-sampled Kupferschiefer sequence from the Oberkatz Schwelle was investigated using organic and inorganic geochemical methods. The structure-controlled mineralization is enriched in As, Mo and U in the bottom part of the Kupferschiefer and in the Weissliegendes sandstones. Further elements such as Cu, Ag, Co, Sb, Zn and Pb show a redox-controlled pattern of zonation in the Kupferschiefer section. These elements were precipitated or redistributed in the final stage of Kupferschiefer mineralization. Participation of organic matter in base metal deposition is suggested by the depletion of H equivalents in the organic matter. A similar phenomenon has been observed inRote Fäule-controlled base metal enrichment in the North-Sudetic Syncline of SW Poland. However, observed differences in the composition of extractable organic matter might be due to the oxidizing potential of the migrating metal-bearing fluids. In the North-Sudetic Syncline, the ascending fluids were more oxidizing compared with the Oberkatz Schwellen area. Additional S content and depletion of Corg are observed in the bottom section of the profile. The H Index (HI) values from Rock-Eval analyses provide further evidence for the depletion of H equivalents in the organic matter. Reconstruction of the thermal regime by the measurement of vitrinite reflectance and determination of molecular rank parameters indicate a maximum temperature in the Kupferschiefer of around 150°C.