European journal of mineralogy vol:6 issue:6 pages:985-994
In the upper Visean limestones of the Campine Basin, the volumetrically most important vein type is represented by non-ferroan blocky calcites. In the overlying Namurian shales and sandstones this cement phase is absent and ankerite and quartz veins dominate. Both carbonate cements are one of the earliest fracture-filling cements in the upper Visean and Namurian. The oxygen isotopic composition of the non-ferroan (40-200 ppm Fe) calcite veins lies between - 16.6 parts per thousand and - 8.7 parts per thousand PDB. The delta(13) C values vary between 3.1 parts per thousand and + 2.4 parts per thousand PDB. Stable isotope analyses of the ankerites (6.2-12.5 % Fe) give delta(18)O values between - 13.6 parts per thousand and - 10.8 parts per thousand PDB and delta(13)C values around 0 parts per thousand PDB. Fluid inclusion microthermometry of the non-ferroan calcites and the first ankerite and quartz generation indicate precipitation of both cements in the same temperature range (45 degrees-93 degrees C) from a fluid with a broad range in salinity (9-26 eq. wt% CaCl2). The estimated oxygen isotopic composition of the fluid from which the ankerites precipitated is 6.6 parts per thousand SMOW, indicative for meteoric water. The blocky calcites probably formed from a ''marine-derived'' fluid which mixed with meteoric water (delta(18)O between - 4 %o and + 1 parts per thousand SMOW).