Geochemical constraints on the origin and migration of palaeofluids at the northern margin of the Variscan foreland, southern Belgium

Abstract

Three major fracture types in the Dinantian of the Namur syncline at the northern margin of the Variscan foreland in southern Belgium, have been investigated by fluid inclusion and stable isotope analysis. The oldest and volumetrically most important fracture type is characterized by conjugated and sigmoidal en-echelon calcite veins which formed during the Variscan folding. These veins, and the surrounding limestones, have both a similar dull brown-orange luminescence and stable isotopic composition ( δ 18 O = −11ℵ. to −8‰ PDB and δ 13 C = 0ℵ. to + 3‰ PDB ). This indicates precipitation of the calcite cement from a fluid buffered by the rock. In the area studied, only a limited amount of fluids was expelled through the Dinantian during the main phase of Variscan compressional tectonism. Fractures filled with ferroan calcites cross-cut the Variscan .folds. Fluid inclusion and stable isotopic evidence indicates that the calcites precipitated at 40°–60°C from a saline fluid (9.2 to 23.2 eq. wt% NaCl) with an estimated δ 18O composition between −2.0‰ and −0.6ℵ. SMOW. The most likely origin of these high-salinity fluids with a relatively low-oxygen isotopic composition is a gravity-driven meteoric water which underwent an intense water-rock interaction. The topographic relief created by the Variscan tectonism could have allowed groundwater to penetrate into the deeper subsurface and to migrate towards the margin of the foreland basin. Non-ferroan, fracture-filling calcites formed later during the Mesozoic or Tertiary. Low-salinity meteoric waters and high-salinity CaCl 2NaCl brines with a temperature around 50°C migrated through this fracture system. Along these fractures, meteoric water migrated down into the subsurface and the brines could have flowed upward from the basement.