Post-Variscan hydrothermal vein mineralization, Taunus, Rhenish Massif (Germany): Constraints from stable and radiogenic isotope data

摘要

Post-Variscan hydrothermal base-metal mineralization of the Taunus ore district, SE Rhenish Massif (Germany), has been studied through combination of stable (S, C, 0) and radiogenic (Pb) isotope geochemistry. Based on field and textural observations, five hydrothermal mineralization types can be distinguished. These are (1) tetrahedrite-tennantite bearing quartz-ankerite veins, (2) quartz veins with Pb-Zn-Cu ores, (3) giant quartz veins, (4) metasomatic dolomite in Devonian reef complexes, and (5) calcite-(quartz) mineralization in Devonian reefs. The δ~(18)0_(v-s,ow) quartz values of base-metal veins are in the range of 18.0-21.5‰, whereas those of giant quartz veins have lower values of 15.9-18.6‰. This difference reflects the higher fluid fluxes and smaller extent of rock-buffering for the giant quartz veins. Hydrothermal carbonates from the tetrahedrite and Pb-Zn-Cu veins have variable but distinctly negative δ~(13)C_(V-PDB)-pdb values. They can be explained by contributions from fluids that had picked up low δ~(13)C_(V-PDB) carbon via oxidation of organic matter and from fluids that interacted with Devonian reef carbonate having positive δ~(13)C_(V-PDB). Metasomatic dolomite has positive δ~(13)C_(V-PDB) values that closely reflect those of the precursor limestone. By contrast, carbonates of calcite-(quartz) mineralization have negative δ~(13)C_(V-PDB) values which are negatively correlated with the δ~(18)O values. This pattern is explained by fluid mixing processes where contributions from descending cooler fluids with rather low salinity were dominant. The isotope data suggest that tetrahedrite veins, Pb-Zn-Cu veins, and giant quartz veins formed from fluid mixing involving two end-members with contrasting chemical features. This is supported by fluid inclusion data (Adeyemi, 1982) that show repeated alternation between two different types of fluid inclusions, which are hotter intermediate- to high-salinity NaCl-CaCl_2 fluids and cooler low-salinity NaCl-dominated fluids. The metal-rich saline fluids were likely generated at the boundary between the pre-Devonian basement and the overlying Devonian-Carboniferous nappe pile. Fault activation resulted in strong fluid focusing and upward migration of large volumes of hot Na-Ca brines, which mixed with cooler and more dilute fluids at shallower crustal levels. Variable contributions from both fluid types, local fluid fluxes, temperature variations, and variations in pH and oxidation state have then controlled the vein mineralogy and metal inventory.