Search

Numerous Variscan syntectonic calcite veins cross-cut Palaeozoic rocks in the Moravian Karst. A structural, petrographic and stable isotopic analysis of the calcite veins and a microthermometric study of fluid inclusions in these vein cements have been carried out to determine the origin of the Variscan fluids and their migration during burial and deformation. The isotopic parameters of white (older, more deformed) and rose (younger) calcites are: [.sup.87]Sr/[.sup.86]Sr is between 0.7078 and 0.7082 (white) and 0.7086 (rose), [[delta].sup.18]O is between +17.7 and +26.1 (white) and between +14.8 and +20.7 [per thousand] SMOW (rose), [[delta].sup.13]C ranges from +0.1 to +2.5 (white) and from -0.3 to +1.6 [per thousand] V-PDB (rose). The isotopic signatures point to precipitation in an older fluid system buffered by the host rock (white calcites) and to an open, younger fluid-dominated system (rose calcites). Parent fluids ([H.sub.2]O-NaCl system) had salinities between 0.35 and 17.25 eq. wt % NaCl. The pressure-corrected and confined homogenization temperatures suggest formation of the calcite veins from a fluid with a temperature between 120 and 170[degrees]C, a pressure of 300-880 bar at a depth between 2.1 and 3.2 km. The fluids were most likely confined to a particular sedimentary bed as a bed-scale fluid migration (white older calcite veins) or, later, to a pile of Palaeozoic sediments as a stratigraphically restricted fluid flow (rose younger calcite veins). The low temperatures and pressures during precipitation of calcites, which took place close to a peak of burial/deformation, confirm the distal position of the Moravian Karst region within the Variscan orogen. Keywords: Palaeozoic, calcite, veins, syntectonic processes, fluid dynamics.

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 [approximately] 125 [degrees] C in the first two calcite generations to [is less than or equal to] 50 [degrees] C in the last two vein cements. The [[Delta].sup.13]C values of the calcites vary between -0.1 [per thousand] and -8.3 [per thousand] 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 [per thousand] and +7.6 [per thousand] relative to SMOW (standard mean ocean water). The low [[Delta].sup.13]C values of the calcites are explained by the contribution of 12C from C[O.sub.2] 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 [[Delta].sup.18]O 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 [[Delta].sup.18]O, and that those fluids became enriched in 18O by water-rock interaction. Waters with such low [[Delta].sup.18]O 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.