Contrasting paleofluid systems in the continental basement: a fluid inclusion and stable isotope study of hydrothermal vein mineralization, Schwarzwald district, Germany

An integrated fluid inclusion and stable isotope study was carried out on hydrothermal veins (Sb-bearing quartz veins, metal-bearing fluorite-barite-quartz veins) from the Schwarzwald district, Germany. A total number of 106 Variscan (quartz veins related to Variscan orogenic processes) and post-Variscan deposits were studied by microthermometry, Raman spectroscopy, and stable isotope analysis. The fluid inclusions in Variscan quartz veins are of the H2O-NaCl-(KCl) type, have low salinities (0-10 wt.% eqv. NaCl) and high T-h values (150-350 degrees C). Oxygen isotope data for quartz range from +2.8 parts per thousand to +12.2 parts per thousand and calculated delta O-18(H2O) values of the fluid are between -12.5 parts per thousand and +4.4 parts per thousand. The delta D values of water extracted from fluid inclusions vary between -49 parts per thousand and +4 parts per thousand. The geological framework, fluid inclusion and stable isotope characteristics of the Variscan veins suggest an origin from regional metamorphic devolatilization processes. By contrast, the fluid inclusions in post-Variscan fluorite, calcite, barite, quartz, and sphalerite belong to the H2O-NaCl-CaCl2 type, have high salinities (22-25 wt.% eqv. NaCl) and lower T-h values of 90-200 degrees C. A low-salinity fluid (0-15 wt.% eqv. NaCl) was observed in late-stage fluorite, calcite, and quartz, which was trapped at similar temperatures. The delta O-18 values of quartz range between +11.1 parts per thousand and +20.9 parts per thousand, which translates into calculated delta O-18(H2O) values between -11.0 parts per thousand and +4.4 parts per thousand. This range is consistent with delta O-18(H2O) values of fluid inclusion water extracted from fluorite (-11.6 parts per thousand to +1.1 parts per thousand). The delta D values of directly measured fluid inclusion water range between -29 parts per thousand and -1 parts per thousand, -26 parts per thousand and -15 parts per thousand, and -63 parts per thousand and +9 parts per thousand for fluorite, quartz, and calcite, respectively. Calculations using the fluid inclusion and isotope data point to formation of the fluorite-barite-quartz veins under near-hydrostatic conditions. The delta O-18(H2O) and delta D data, particularly the observed wide range in delta D, indicate that the mineralization formed through large-scale mixing of a basement-derived saline NaCl-CaCl2 brine with meteoric water. Our comprehensive study provides evidence for two fundamentally different fluid systems in the crystalline basement. The Variscan fluid regime is dominated by fluids generated through metamorphic devolatilization and fluid expulsion driven by compressional nappe tectonics. The onset of post-Variscan extensional tectonics resulted in replacement of the orogenic fluid regime by fluids which have distinct compositional characteristics and are related to a change in the principal fluid sources and the general fluid flow patterns. This younger system shows remarkably persistent geochemical and isotopic features over a prolonged period of more than 100 Ma.