The geochemical evolution of the fluids migrating at the Variscan thrust front in eastern Belgium has been investigated by a petrographic, mineralogical and geochemical study of ankerite, quartz and ferroan calcite veins hosted by lower Devonian rocks. Three vein generations have been recognized. The first generation consists of quartz, chlorite and ankerite filling pre- to early Variscan extensional fractures. The second generation is present as shear veins of Variscan age, and contains quartz, chlorite and ferroan calcite. The third generation consists of ankerite filling post-Variscan fractures. The oxygen and carbon isotopic composition of the two ankerite phases and of the ferroan calcites are respectively between -16.4 and -11.4 parts per thousand PDB between -17.8 and -1.7 parts per thousand PDB. This range is greater than that of calcite nodules in the lower Devonian siliciclastic sediments (delta(18)O = -15.6 to -11.1 parts per thousand PDB and delta(13)C = -13.4 to -10.2 parts per thousand PDB). This suggests precipitation of the carbonate veins from a fluid which was at mast only partly isotopically buffered by the calcite nodules in the host rock. The calculated oxygen isotopic composition of the ambient fluid from which the calcite veins formed is between +7.8 and +10.0 parts per thousand SMOW. Two main fluid types have been recognized in fluid inclusions in the quartz and carbonates. The first fluid type is present as secondary fluid inclusions in the first and second vein generations. The fluid has a salinity of 0.5-7.2 eq. wt.% NaCl and a high, but variable, homogenization temperature (Th = 124-188 degrees C). Two origins can be proposed for this fluid. It could have been expelled from the lower Devonian or could have been derived from the metamorphic zone to the south of the area studied. Taking into account the microthermometric and stable-isotope data, and the regional geological setting, the fluid most likely originated from metamorphic rocks and interacted with the lower Devonian along its migration path. This Is In agreement with numerical simulations of the palaeofluid and especially the palaeotempera-ture field, which is based on chlorite geothermometry and vitrinite reflectance data. The second fluid type occurs as secondary inclusions in the shear veins and as fluid inclusions of unknown origin in post-Variscan ankerite veins. Therefore, it has a post Variscan age. The inclusions are characterized by a high salinity (18.6-22.9 eq. wt.% CaCl2). The composition of the fluid is similar to that which caused the development of Mississippi Valley-type Pb-Zn deposits in Belgium.