Stable isotopic variations of clay minerals: A key to the understanding of Kupferschiefer-type mineralization, Germany

Abstract

Isotopic compositions of silicate minerals and carbonates of selected Kupferschiefer samples from the Hessian Depression and from the Lower Rhine Basin have been studied in detail. In the area of the Richelsdorf ore district, stable isotope analyses of the Cu-mineralized sections in the Kupferschiefer show that the metal enrichment processes are associated with significant changes in the oxygen and hydrogen isotope composition of the clay minerals. The isotopic compositions of the < 2-μ-size fractions reflect a decrease of approximately 4%. in δ 18O (18.4 to 13.7%.) and an increase of nearly 50%. in δD (−74 to −25%.) with decreasing distance from the “Rote Fäule” zone. This isotopic zonation in the Kupferschiefer shale is explained by fluid-rock interaction with ascending, oxidizing, hypersaline solutions, which transported high amounts of base metals from the Lower Permian red beds into the Kupferschiefer horizon acting as a geochemical trap. The estimated isotopic composition of the involved water ( δ 18O between +3.0 and +6.5%.; δD between −10 and +3%.) supports the characterization of the ore-bearing solutions as basinal brines. Internal oxygen isotope fractionation of illite from samples next to “Rote Fäule” corresponds to a temperature of approximately 130°C, which is suggested as the temperature of metal precipitation. In the Lower Rhine Basin, the metal content does not exceed values commonly observed in black shales. Enhanced base metal (Pb, Zn) mineralization is observed only in the bottom section of the Kupferschiefer. The results of the oxygen and hydrogen isotope analyses of the selected core samples provide no evidence for correlation between isotopic composition of the silicate minerals and the extent of lead and zinc sulphide precipitation. Futhermore, they do not point towards isotopic exchange reactions between clay minerals within the Kupferschiefer shale and fluids out of isotopic equilibrium with the investigated mineral phases. However, it is shown that the lack of detectable interaction between the rock and external fluids, as reflected by these data, in general coincides with the proposed concept that the base metals Pb and Zn as well as Ba have been coprecipitated from Carboniferous formation waters.