Composition and origin of the Kupferschiefer bed

Karl Hans Wedepohl

Abstract


 

The Kupferschiefer of Upper Permian age sampled in Poland, Germany, the Netherlands and England has been investigated for its mineral, chemical and (S, Pb, O and C) isotopic composition. Rate of deposition of the sapropelic marly shale (rarely more than half a metre in thickness) was very low. This is indicated by high V/C, Mo/C and U/C ratios. Galena and sphalerite mineralization in the sediment, exceeding 0.5% metal, occurs over relatively large areas. Mineralization of chalcocite, bornite and chalcopyrite with more than 0.5% Cu is restricted to small regions which occur on top of voluminous Rotliegendes sediments. Economic grades for former and present copper mining in the latter areas are due to additional mineralization of the footwall (Weissliegendes) and/or hanging wall (Zechstein Limestone) of the Kupferschiefer bed. Late diagenetic remobilization of Cu into the hanging wall is usually indicated by adjacent zones of secondarily oxidized metal barren "Rote F¬ule". The isotopically very light sulphur (d34S -24 to -40%.) in the majority of the syngenetically, early and late diagenetically formed sulphides is explained as a product of bacterial sulphate reduction. Precipitation of sulphides from seawater does not cause high Cu concentrations or even moderate Pb concentrations in sapropelic sediment. Seawater of closed nearshore basins and diagenetic fluids received their high metal concentrations from the porous Rotliegendes sediments in the Kupferschiefer footwall. Because Zn-Pb-Cu sulphides and pyrite contain different and not equilibrated sulphur isotopes, the former sulphides cannot have generally replaced the latter mineral. Vertical and lateral zonation of Cu (plus Ag), Pb and Zn sulphides in this sequence is controlled by their solubility and by the rate of bacterial sulphide production. The lead isotopic composition of the Kupferschiefer from Germany is intermediate between that of galena from the Middle Devonian Rammelsberg ore and from Jurassic vein deposits of the Harz Mts. The majority of properties of the Kupferschiefer bed can be explained by a syngenetic (indicated by framboids) to diagenetic (and not epigenetic) mineralization with metals mainly derived from soaked Rotliegendes sediments.


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