The origin of the Kupferschiefer-type mineralization in the Richelsdorf Hills, Germany, as deduced from stable isotope and organic geochemical studies

Abstract

The Kupferschiefer horizon in the Richelsdorf section, Federal Republic of Germany, acted as a geochemical trap by accumulating metals from ascending brines. Chemical composition of the extracted bitumens reflects a drastic change in the degree of oxidation of the organic matter towards “Rote Fäule”. This alteration pattern corresponds with regular changes in the isotopic composition of organic carbon and carbonate, as well as with the base metal zonation around “Rote Fäule”. The enrichment of the organic material in 13C suggests the preferential release of isotopically light hydrogen equivalents through progressive degradation and oxidation of organic matter. The opposite tendency towards lower δ 13C and δ 18O values of carbonate minerals provides evidence for isotopic reequilibration between carbonate and isotopically light, oxidizing solutions. The occurrence of isotopically heavy organic carbon, together with low δ 13C and δ 18O values of carbonate suggests that 12C-rich CO 2, generated due to oxidation of organic material, may have been a source of carbonate. The results reveal a post-sedimentary formation of high-grade mineralization and associated “Rote Fäule” by ascending, oxidizing brines. They are in agreement with the results of mineralogical and geochemical studies, which provided evidence for metal precipitation from basinal brines, enriched in Na and sulphate S. The organic geochemical and isotopic data suggest that metal precipitation was caused by thermochemical sulphate reduction with organic matter acting as a reducing agent and proton donor.