Abstract

Mississippi Valley-type deposits are widespread in Carboniferous strata in eastern Belgium. Five successive Fe-rich, fracture-filling calcite generations have been recognized in the veins containing the Pb-Zn mineralization of Bleiberg. Fluid-inclusion evidence indicates that all the calcites formed from fluids with salinities between 16.0 and 23.1 equivalent wt% NaCl. The trapping temperature of the fluid inclusions decreases from ∼125 °C in the first two calcite generations to ≤50 °C in the last two vein cements. The δ13C values of the calcites vary between -0.1‰ and -8.3‰ relative to PDB (Peedee belemnite). The oxygen isotopic composition of the ambient fluids, calculated from the trapping temperature and the isotopic composition of the calcites, varies between -5.2‰ and +7.6‰ relative to SMOW (standard mean ocean water). The low δ13C values of the calcites are explained by the contribution of 12C from CO2 released during the oxidation of organic matter in the upper Carboniferous shales and the coupled reduction of sulfate in the mineralizing brine. The highly variable oxygen isotopic composition of the ambient fluids, the low δ18C value of the water from which one calcite cement precipitated, and the intense water-rock interaction necessary to leach metals indicate that the original fluids had a low δ18C, and that those fluids became enriched in 18O by water-rock interaction. Waters with such low δ18C, values must have had a meteoric origin. Flow of these waters into the deeper subsurface was likely gravity driven and took place from the uplifted parts of the Variscan orogen toward the foreland basin.

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