Evolution of palaeofluids at the Variscan thrust front in eastern Belgium

The geochemical evolution of the fluids migrating at the Variscan thrust fr ont in eastern Belgium has been investigated by a petrographic, mineralogic al and geochemical study of ankerite, quartz and ferroan calcite veins host ed by lower Devonian rocks. Three vein generations have been recognized. Th e first generation consists of quartz, chlorite and ankerite filling pre- t o early Variscan extensional fractures. The second generation is present as shear veins of Variscan age, and contains quartz, chlorite and ferroan cal cite. The third generation consists of ankerite filling post-Variscan fract ures. 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 siliciclasti c sediments (delta(18)O = -15.6 to -11.1 parts per thousand PDB and delta(1 3)C = -13.4 to -10.2 parts per thousand PDB). This suggests precipitation o f the carbonate veins from a fluid which was at mast only partly isotopical ly buffered by the calcite nodules in the host rock. The calculated oxygen isotopic composition of the ambient fluid from which the calcite veins form ed 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 zo ne to the south of the area studied. Taking into account the microthermomet ric and stable-isotope data, and the regional geological setting, the fluid most likely originated from metamorphic rocks and interacted with the lowe r Devonian along its migration path. This Is In agreement with numerical si mulations of the palaeofluid and especially the palaeotempera-ture field, w hich is based on chlorite geothermometry and vitrinite reflectance data. Th e second fluid type occurs as secondary inclusions in the shear veins and a s fluid inclusions of unknown origin in post-Variscan ankerite veins. There fore, it has a post Variscan age. The inclusions are characterized by a hig h salinity (18.6-22.9 eq. wt.% CaCl2). The composition of the fluid is simi lar to that which caused the development of Mississippi Valley-type Pb-Zn d eposits in Belgium.