Carbon and oxygen isotope composition of Pb-, Cu- and Bi-carbonates of the Schwarzwald mining district: Carbon sources, first data on bismutite and the discovery of an oxidation zone formed by ascending thermal water

Abstract

The carbon and oxygen isotope composition of cerussite, bismutite, malachite and azurite samples from the Schwarzwald mining district, SW Germany, was analyzed in order to evaluate carbon sources and conditions of formation. We deliberately chose samples showing various textures and coming from deposits at various altitudes and geological settings. δ13C values vary from +1.0‰ to −21.0‰ VPDB, cerussite, bismutite, malachite and azurite δ18O values show a variation from +11.2‰ to +17.9‰, +16.8‰ to +21.0‰, +23.0‰ to +28.1‰ and +26.3‰ to +30.2‰ VSMOW2, respectively. In accordance with earlier studies, the δ13C values indicate carbon partially derived from biogenic (vegetation-respired or from heterotrophic degradation of organic matter) soil CO2 combined with carbon from an isotopically heavier reservoir (e.g. atmospheric CO2, carbonate rocks, hydrothermal calcite from the ore vein) in varying proportions.We present the first dataset of bismutite carbon and oxygen isotope compositions. These data suggest that carbon isotope fractionation of bismutite is similar to cerussite, which is probably related to the similar atomic mass of Pb and Bi. However, a considerable shift to higher values in oxygen isotopes for bismutite compared to cerussite cannot be explained here and could be related to different formation temperatures, a different phosphoric-acid fractionation factor (PAFF) during analysis or variable fluid sources. We show that the PAFFs of malachite, azurite and bismutite have the same temperature dependence as the PAFF of calcite between 25 and 90°C.Earlier studies report δ18O values for heavy metal carbonates that correlate with values of modern meteoric waters and surface temperature. Most samples from the present study exhibit this correlation as well, but also more details of the supergene processes: calculated oxygen isotope values of water in equilibrium with cerussite, malachite and azurite overlap with the range that can be expected for precipitation in the study area at ambient temperatures. In the case of the Haus Baden mine near Badenweiler, calculated temperatures of formation of cerussites are considerably higher, even taking extremely light water into consideration. For a δ18O value of water of −10.0‰, temperatures of 13–31°C are calculated, which is up to 20°C above the mean annual air temperature at the locality. As this mine is situated on the Rhinegraben boundary fault and only about 800m south of the present-day Badenweiler thermal spring (28°C), the influence of thermal water plausibly explains the calculated high formation temperatures. Hence, in this case, the oxidizing fluids were ascending, while typically supergene fluids are descending. This discovery opens a new possibility for generating isotope variability in an oxidized ore deposit and it shows that oxygen isotope studies on secondary heavy metal carbonates are a powerful tool to reveal the source of oxidizing fluids and the direction of paleofluid flow.