Within the Lower Carboniferous limestones in the boreholes of Poederlee, Heibaart and Turnhout (Campine Basin, NE-Belgium), open veins partially cemented by calcite and dolomite are considered important with regard to the geothermal reservoir potential of these limestones. Based on a detailed petrographic analysis, stable carbon and oxygen as well as clumped Δ47 isotope analysis and U/Pb age dating, the formation conditions of these veins have been reconstructed and were placed in the geodynamic evolution of the sedimentary basin. All host rocks are recrystallized, and the open carbonate vein cements often possess a similar depleted δ 18 O PDB signature as their recrystallized host rock. The latter thus indicates vein cement precipitation was likely rock-buffered. The δ 13 C signature of the host rock often displays a marine signature, while in the veins two isotopic clusters can be differentiated. The first cluster groups veins with cleavage twins, and are characterized by slightly to moderately depleted δ 13 C, reflecting a mixture of marine carbon with carbon derived from organic matter. This group typically occurs in the open veins from the microbially dominated Poederlee and Heibaart buildups characterised by on-lapping organic-rich Namurian black shales. One of these veins formed at 216 ± 6 °C, indicating that it crystallised during the Sudetic – Asturian deformation stage of late Carboniferous age, while others formed between 110 and 130 °C, indicating that they could have been formed during the late Carboniferous burial stage or after the Sudetic – Asturian deformation and uplift. The second cluster groups veins that may, but often do not, contain cleavage twins. They show host-rock buffered δ 13 C signatures. These veins have a Cretaceous (late Aptian/early Albian) and/or Paleocene U/Pb age and formed at temperatures dominantly between 50 and 80 °C based on clumped isotopes.