Contrasting origin of palaeofluids in a strike-slip fault system

Calcite cements in a dextral strike-slip fault system in the Variscan Front Complex (Belgium) have been investigated to determine the origin of the fluids migrating along the fault system. Generation of non-ferroan and ferroan calcites occurs along the fault plane. A right stepover between two faults is occupied by a breccia. It formed due to repeated hydraulic implosions within a dilational jog. Ferroan calcites cement the breccia. The general geological setting and the microthermometric data (Tt ≤ 50°C) indicate that calcite precipitation along the faults took place at shallow depth. However, the geochemistry and origin of the fluids from which the calcites along the fault plane and in the breccia precipitated were different. The non-ferroan calcites along the fault plane precipitated from a highly saline fluid (18.1 to 25.3 eq. wt% CaCl2) at a temperature around 50°C. Secondary inclusions in trails in these calcites demonstrate that similar highly saline fluids reached temperatures of at least 130°C. The latter migrated from a depth of over 4 km, implying the fluid was tapped from the Lower Palaeozic siliciclastic basement. The ferroan calcites along the strike-slip fault plane also formed from highly saline fluids, but which interacted with Upper Palaeozoic strata before migration to shallow depth. In contrast, primary fluid inclusions in the ferroan calcites in the breccia indicate cementation occurred from a low salinity fluid (1.7 to 8.6 eq. wt% NaCl) at a temperature of 50°C or less. This fluid likely originated from shallow depth. The contrasting origin of the fluids indicate different flow regimes have been intersected by the faults during their strike-slip reactivation.