Metal accumulation during and after deposition of the Kupferschiefer from the Sangerhausen Basin, Germany

A densely sampled profile (58 cm in thickness) composed of 13 samples of the Kupferschiefer and overlying Zechstein carbonates from the Sangerhausen Basin, Germany has been analysed by various geochemical and microscopic methods in order to clarify the mechanism of base metal accumulation. In this location, the Kupferschiefer is only slightly influenced by the hematite-bearing, oxidized fluids calledRote Fäule. The determination of facies-dependent parameters along the profile indicates that Kupferschiefer from the Sangerhausen Basin was largely deposited in a marine environment; only at the beginning of Kupferschiefer sedimentation did euxinic conditions prevail. The bottom part of the profile is significantly enriched in trace elements such as Cu, Ph, Zn, As, Co, Ag and U. The Cu concentration amounts to 19.88 wt.%. Post-depositional oxidation of the organic matter is observed only in the transition zone between the Kupferschiefer and the Zechstein conglomerate indicating the influence of ascending, oxidizing brines. Microscopic analyses show that only Fe sulfides form framboidal textures; Cu minerals are present along the total profile preferentially in fractures and as patchy structures composed of chalcocite, chalcopyrite and bornite. In the highly mineralized bottom section, Cu sulfides are associated with pyrobitumen, sparry calcite and arsenopyrite. Results from maturation studies of organic matter suggest that the maximum temperature affecting the Kupferschiefer was approximately 130°C. A 3-step-process of metal accumulation is proposed. During deposition of the sediment, framboidal pyrite and pyrite precursors were precipitated by bacterial SO42− reduction (BSR). During diagenesis the pyrite and pyrite precursors were largely replaced by mixed Cu/Fe minerals and by chalcocite (PR). In the section with very high Cu contents (> 8%) reduced sulfur from Fe-sulfides was not sufficient for precipitation of Cu and other trace metals from ascending solutions. In this part of the profile, thermochemical SO42− reduction (TSR) occurred after pyrite replacement as indicated by the presence of pyrobitumen and sparry calcite.