Post-Variscan hydrothermal vein mineralization,Taunus, Rhenish Massif (Germany): Constraints from stable and radiogenic isotope data |
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| Institution: | 1. Institute of Geology and Geochemistry of Petroleum and Coal, Energy and Mineral Resources Group (EMR), Lochnerstr. 4-20, RWTH Aachen University, D-52062 Aachen, Germany;2. Geological Survey of North Rhine-Westphalia, De-Greiff-Str. 195, D-47803 Krefeld, Germany;1. Department of Earth and Atmospheric Sciences, Saint Louis University, 205 O''Neil Hall, 3642 Lindell Blvd, Saint Louis, MO, 63108, USA;2. Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, WA, 98195, USA;3. Department of Geology and Geophysics, Texas A & M University, College Station, TX 77843, USA;4. Department of Geology, Utah State University, Logan, UT, 84322-4505, USA;1. Institute of Geosciences, Friedrich-Schiller University, Burgweg 11, D-07749 Jena, Germany;2. Department of Mineralogy and Petrology, National Museum, Cirkusová 1740, CZ-19300 Praha 9, Czech Republic;3. Department of Mineralogy and Petrology, Comenius University, Mlynská dolina G, SK-842 15 Bratislava, Slovakia;4. Department of Geochemistry, Comenius University, Mlynská dolina G, SK-842 15 Bratislava, Slovakia;5. Department of Mineralogy, Leibniz University, Callinstrasse 3, D-30167 Hannover, Germany;6. Department of Geosciences, Goethe University, Altenhöferallee 1, D-60438 Frankfurt am Main, Germany;7. Department of Geological Sciences and Geological Engineering, Queen''s University, Miller Hall, 36 Union Street, Kingston K7L 3N6, Ontario, Canada;8. Slovak Academy of Sciences, ?umbierska 1, SK-974 01 Banská Bystrica, Slovakia |
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| Abstract: | Post-Variscan hydrothermal base-metal mineralization of the Taunus ore district, SE Rhenish Massif (Germany), has been studied through combination of stable (S, C, O) 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 δ18OV-SMOW 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 δ13CV-PDB values. They can be explained by contributions from fluids that had picked up low δ13CV-PDB carbon via oxidation of organic matter and from fluids that interacted with Devonian reef carbonate having positive δ13CV-PDB. Metasomatic dolomite has positive δ13CV-PDB values that closely reflect those of the precursor limestone. By contrast, carbonates of calcite–(quartz) mineralization have negative δ13CV-PDB values which are negatively correlated with the δ18O 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–CaCl2 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. |
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