Analysis of surface karst phenomena in Devonian carbonates in North Rhine-Westphalia – context to geothermal exploration

The deep geothermal reservoir potential in North Rhine-Westphalia was assessed in Devonian carbonates (Massenkalk) in the Steltenberg Quarry in Western Germany. The karstification phenomena of the carbonates were recorded utilising drone images in the quarry. Rocks at the karst chutes were examined by polarisation- and cathodoluminescence microscopy regarding karstification phenomena and carbonate cements. X-ray diffraction and inductively coupled plasma analyses were carried out to gain mineralogical information about different carbonate phases in various samples. Carbon and oxygen isotopes were analysed to understand the formation conditions of the carbonates. The examined rocks at the karstification structures show that the host rock (Massenkalk) has average marine δ¹³C_mean values of 3.7 ‰ VPDB and -4.7 ‰ VPDB for δ¹⁸O_mean. At the karstification surfaces and leaching zones, the isotope analyses revealed values of meteoric overprinting in the form of increasingly more negative δ¹³C values down to -8.5 ‰ VPDB and -5.9 ‰ VPDB for δ¹⁸O. These changes in the formation conditions are strengthened by cathodoluminescence results. The narrowing of karstification observed by drone with increasing depth in the quarry walls and the siliciclastic sediments in these structures indicate near-surface karstification. These dissolution structures have presumably developed in cracks caused by tectonism. It can be assumed that karstification at greater depths is also possible due to fault associations, which could be caused not only by meteoric waters but deep hydrothermal CO2-bearing fluids