THE MASSIVE STIBNITE VEINS OF THE FRENCH PALEOZOIC BASEMENT - A METALLOGENIC MARKER OF LATE VARISCAN BRITTLE EXTENSION

The stibnite ore deposits of the French Palaeozoic basement are spatially related to major Late Variscan strike-slip faults. They occur as small discontinuous veins mostly hosted in epizonal or catazonal siliciclastics. Stibnite crystallizes in the final stage of a polymetallic paragenesis from an antimony-bearing solution, which deposits a first discrete Fe-As assemblage at 300-400-degrees-C and 0.5-0.8 Kbar. Experimental data have shown that antimony solubility drastically decreases on cooling. Characterization of the Late Variscan antimony-bearing fluids enables us to predict the temperature range -270/150-degrees-C - under which stibnite was precipitated. Since the latter coincides with the fluid inclusion homogenization temperature range, the pressure at which stibnite crystallized can be estimated from vapour pressure data to have been around 0.1 Kbar. Extensional brittle shear zones, developed at the end of the Late Variscan orogeny (probably at the Westphalian/Stephanian boundary), drained the hydrothermal fluids near to the surface (c. 1000 m ?) reaching the critical P-T conditions for stibnite crystallization.