Hydrothermal karst cavities in a Devonian carbonate reservoir analogue (Rhenish Massif, Germany): Implications for geothermal energy potential

Deep geothermal reservoirs can provide renewable energy for electricity and heat generation [1]. In the Rhine-Ruhr area of western Germany, where Europe's largest district heating network is located, up to 1,300 m thick carbonates of Devonian age are available in ≥ 4,000 m depth [2, 3]. Near the surface, these rocks contain karst cavities, which host some of Germany´s longest caves [4, 5]. At reservoir depth, abundant karstification may significantly increase the geothermal reservoir potential [6] (Figure 1). In the Munich area (southern Germany) deep-seated karstified Upper Jurassic rocks are widely used for the city´s district heating network [7]. Most deep-seated geothermal systems are located at a depth of 1,000 m or more at temperatures over 60 °C [8]. Therefore, it is important to assess deep-seated karstified structures elsewhere to assess their geothermal reservoir potential. In the Rhine-Ruhr area, these could have the potential to be used as an alternative renewable energy resource. This study aims at the geological characterization of deep-seated (hydrothermal) karst cavities in Steltenberg Quarry (western Germany) where Middle/Upper Devonian carbonates (Massenkalk limestone) are present in the vicinity of two regional fault zones [9, 10]. We applied state of the art petrographical, geochemical, palaeothermometrical methods, and U-Pb dating. Here we present the first U-Pb age data of deep-seated hydrothermal karst precipitates in Germany, which formed at the Permian-Triassic boundary (252.4 ± 8.6 My, Table 1). The U-Pb age data of calcite cement veins (LMC 8), which were cutting the near-surface karst cavities before they got dissolved, points to an Oligocene maximum age (30.0 ± 2.81 My) of the karst cavities.