ORIGIN AND MIGRATION OF PALAEOFLUIDS IN THE UPPER VISEAN OF THE CAMPINE BASIN, NORTHERN BELGIUM

Upper Visean limestones in the Campine Basin of northern Belgium are i ntensively fractured. The largest and most common fractures are cement ed by non-ferroan, dull brown-orange luminescent blocky calcite. First melting temperatures of fluid inclusions in these calcites are around - 57-degrees-C, suggesting that precipitation of the cements occurred from NaCl-CaCl2-MgCl2 fluids. The final melting temperatures (T(m)ice ) are between - 5 and - 33-degrees-C. The broad range in the T(m)ice d ata can be explained by the mixing of high salinity fluids with meteor ic waters, but other hypotheses may also be valid. Homogenization temp eratures from blocky calcite cements in the shelf limestones are inter preted to have formed between 45 and 75-degrees-C. In carbonates which were deposited close to and at the shelf margin, precipitation temper atures were possibly in the range 70-85-degrees-C and 72-93-degrees-C, respectively. On the shelf, the calcites have a deltaO-18 around - 9. 3 parts per thousand PDB and they are interpreted to have grown in a f luid with a deltaO-18 between - 3.5 and + 1.0 parts per thousand. SMOW . At the shelf margin, blocky calcites (deltaO-18 almost-equal-to - 13 .5 parts per thousand PDB) could have precipitated from a fluid with a deltaO-18 between - 4.0 and - 1.1 parts per thousand SMOW. The highes t oxygen isotopic compositions are comparable to those of Late Carboni ferous marine fluids (deltaO-18 = - 1 parts per thousand SMOW). The lo west values are more positive than a previously reported composition f or Carboniferous meteoric waters (deltaO-18 = - 7 parts per thousand S MOW). Precipitation is likely to have occurred in marine-derived fluid s, which mixed with meteoric waters sourced from near the Brabant Mass if Fluids with a similar negative oxygen isotopic composition and high salinity are actually present in Palaeozoic formations. The higher te mperature range in the limestones near the shelf margin is explained b y the upward migration of fluids from the 'basinal' area along fractur es and faults into the shelf.