Intraformational fluid flow in the Thuringian Syncline (Germany) - Evidence from stable isotope data in vein mineralization of Upper Permian and Mesozoic sediments

Vein mineralization observed in boreholes, quarries and outcrops of Upper Permian and Mesozoic sediments in the Thuringian Syncline, Germany, was investigated for the first time by petrography, electron microprobe analysis and stable isotope analysis. The objective was a characterization of paleo-fluid systems within this basin. Veins are mostly restricted to NW-SE trending fault systems and comprise carbonates (calcite, dolomite), minor sulfates (gypsum/anhydrite, celestite, barite), and rarely sulfides. They are variably distributed within the Late Paleozoic to Mesozoic succession. Calcite vein mineralization occurs in almost all Triassic strata, though dominating in the Lower Muschelkalk subgroup (mu). Gypsum/anhydrite veins occur in the Middle Keuper subgroup (km), Middle Muschelkalk subgroup (mm), Middle and Upper Buntsandstein subgroup (sm, so) as well as in the Upper Permian (Zechstein). Sulfate and carbonate isotope analyses indicate that most vein mineralization dominantly crystallized from intraformational fluids within the Upper Permian and Triassic sediments. Crystallization from descending meteoric waters was observed especially for calcite veins in the Lower Muschelkalk, but also for gypsum/anhydrite veins in the Upper Buntsandstein. Mineralization from ascending, carbonate-pH-buffered, warm waters was restricted to individual parts of the southern, deep reaching main border fault zone of the Thuringian Syncline. Mixing and fault crossing fluid movements are subordinate within the Thuringian Syncline. We conclude that no large scale cross-formational fluid flow occurred in Thuringian Syncline over the last millions of years.