To explore fault-bound hydrothermal systems, knowledge about fluid conduits and the (isotope-) geochemical composition of the geothermal water is necessary. At the hydrothermal vein deposit of the former Schönbrunn fluorspar mine, fluid conduits in the depth between the 293 and the 533 m level were outcropped. Attempts at utilization for deep geothermal energy failed in 1997, because the mine was flooded. The thermal water outflow of the most active fluid conduit, outcropped at the 533 m level was 1 m³ min^-1 and the temperature was 35,7°C. The fluid conduit is controlled by the intersection of a fault bundle of several NNW-SSE and NW-SE striking faults. The horizontal length of the intersection area is about 400 m. A geothermal anomaly exists at the segment of the intersection. The quartz-fluorite bearing mineralization (primary) was redeposited by carbonate-sulfide mineralization with depth.

Research on unpublished water and gas samples (96 and 115 data sets, respectively) from the so called “spa” at the 453 m level of the Schönbrunn mine was carried out by time-series measurements (weekly sampling rate) to investigate 1) composition and origin of the thermal water, 2) reservoir temperature via geothermometry, and 3) potential hydro- and gas-chemical effects of regional swarm seismicity. We revaluated the results and collated them with new findings of the Neumühle thermal water (Prause et al., session 4b) in order to develop deep geothermal energy projects at crystalline rock areas in Western Saxony.

The KTB site near Windischeschenbach is one of the best characterised locations of continental upper crust worldwide. The superdeep boreholes represent a unique in situ underground research laboratory in Europe, allowing research into the sustainable utilisation of the crystalline subsurface for the energy and heat transition.

The GEOREAL experiment aimed at hydraulically stimulating the fractured crystalline metamorphic basement rocks at 4 km depth and to minimise the potential risk of induced seismicity by real-time control of injection parameters. The KTB infrastructure allowed to inject 600 m³ of water into the 4 km deep KTB pilot hole, while the main borehole (only 200 m distant at the surface) was used for seismic monitoring.

Fluid injection took place during 6-15 November 2023 through the stuck GEOREAL packer pressurising the open borehole section at the depth interval of 3.85–4 km. Flow rates were variable (10– 220 l/min). No induced microseismic events were detected. Pressure data, monitored at the well head of the pilot and main borehole, was analysed in conjunction with recordings from a year-long injection experiment into the same formation performed in 2004/5. The hydraulic parameters show similar values as previously obtained. The observations provide more detail on the hydraulic connection at depth between both boreholes. The GEOREAL experiment had to be stopped prematurely due to a leak in the casing cement. Further activities at the KTB deep crustal lab are underway, substantiating the potential of petrothermal research at the KTB deep crustal lab after 30 years of operation.

The E4Geo Project aims to explore fault-bound hydrothermal systems associated with the hidden granite pluton of Eichigt-Schönbrunn in the Vogtland region of SW Saxony, Germany, for potential use as deep geothermal reservoirs. Evidence for enhanced geothermal gradients in this region has existed for several decades, but attempts at utilization for energy production and heating have thus far been limited.

We aim to constrain 1) composition of the thermal water, 2) reservoir temperature, 3) the effects of water-rock interaction, 4) fluid residence times and recharge rates, 5) contributions of radioactive decay to the local heat source, and 6) approximate heat flux. For this purpose, we present new geochemical data from time-series measurements performed on local water samples from regional thermal springs at Neumühle/Vogtland, which are contrasted with samples from the nearby Erzgebirge (Georgsquelle/Wiesenbad, Silbertherme/Warmbad) and Fichtelgebirge (Siebenquell/Weissenstadt) regions.

Geochemical time-series measurements of major cations and anions allow us to estimate reservoir temperatures via geothermometry and to characterize the chemical nature of the thermal waters, including potential scaling effects that may occur during operation of a geothermal plant. Potential hydrochemical effects of rainfall and regional swarm seismicity are likewise examined. Stable isotope analyses (δ¹⁸O and δ²H) of water samples are utilized to decipher the origin of thermal waters and to evaluate potential mixing with meteoric components. Lastly, specific activities of nuclides from the ²³⁸U and ²³²Th decay series are applied with ⁴⁰K in preliminary heat flux estimates and identification of ground water types, while residence times are approximated from ¹⁴C and ³H.

The valley of Geysers, is a multi-habitat environment for microorganisms, ranging from moderate to extreme environmental conditions within short distances. The aim of the present study is to characterize and compare the microbial communities in variable habitats: thermal water pools, outflow transect from a geyser to the Geysernaya river, river bed samples and a recently reactivated geyser in the river buried in landslides in 2007 and 2014. The study combines organic geochemical and geomicrobiological approaches with hydrochemical background data.

The different microbial habitats exhibit a wide range of different cell membrane biomarkers. Hot habitats are dominated by markers for archaea with lower abundances of bacterial biomass, indicating the presence of extremophiles in these habitats. Along the transect from a geyser to the river, the microbial community is dominated by bacteria with specific markers pointing to photosynthetic microbs. The river bed samples show only small amounts of bacterial biomass. A closer look at the taxonomy of the microbial communities of the different locations supports the clear difference between the communities influenced by hot and temperate water. Along the transect, the two sample at the upper end of the outflow (closer to the geyser, 66-84°C water temperature) show many extremophilic archaeal and also bacterial taxa, while the lower samples (around 32-39°C temperature) are characterized by mesophilic bacterial taxa with even some cyanobacteria. Initial result indicates the existence of unknown species within the communities. Overall, the microbial communities show a high spatial variability determined by the thermal conditions of the different life habitats.