Lithological influence on the composition of vein cements in the Carboniferous of the Campine Basin (northern Belgium)
Muchez, Philippe; Hein, Ulrich F.; Broeck, Kristel Van Den; Vandecasteele, Carlo
European Journal of Mineralogy Volume 6 Number 6 (1994), p. 985 - 994
published: Nov 30, 1994
manuscript accepted: Jun 21, 1994
manuscript received: Sep 29, 1993
ArtNo. ESP147050606003, Price: 29.00 €
Abstract 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 ‰ and - 8.7 %c PDB. The δ13C values vary between - 3.1 ‰ and + 2.4 ‰ PDB. Stable isotope analyses of the ankerites (6.2-12.5 % Fe) give δ18O values between - 13.6 ‰ and - 10.8 ‰ PDB and δ13C values around 0 ‰ 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.-93.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 ‰ SMOW, indicative for meteoric water. The blocky calcites probably formed from a "marine-derived" fluid which mixed with meteoric water (δ18O between - 4 ‰ and + 1 ‰ SMOW). The similarity of the precipitation temperature and of the position in the paragenetic sequence suggest a contemporaneous formation period of both carbonate cements. Although the calcite precipitated from a mixed marine - meteoric water and the ankerites from a meteoric water, this difference can not explain the major contrast in the mineralogy and chemical composition of the cements. This difference, however, can be related to the different lithology of the host-rock units. High strontium concentrations are present in the non-ferroan calcites (230-550 ppm). They are interpreted as the result of the enrichment of the Visean pore-fluids in strontium due to the recrystallization of the Visean limestones and the preferential loss of strontium. The fracture-filling carbonates in the Namurian have very high manganese (0.6-1.9 %), iron (6.2-12.5 %) and magnesium (4.4-7.1 %) contents. These elements are thought to have been derived from the siliciclastic host-rock. The processes which could have caused the enrichment in the ambient fluids are diffusion and interaction of the fluid with the sediments.