Formation and Evolution Mechanisms of Geothermal Waters Influenced by Fault Zones and Ancient Lithology in the Yunkai Uplift, Southern China

Geothermal systems play a crucial role in understanding Earth’s heat dynamics. The Yunkai Uplift in southern China exemplifies a geothermally rich region characterized by ancient lithologies and high heat flow. This study investigates the geochemical characteristics of geothermal waters in the Yunkai Uplift. Both geothermal and non-thermal water samples were collected along the Xinyi–Lianjiang (XL) Fault Zone and the Cenxi–Luchuan (CL) Fault Zone flanking the core of the Yunkai Mountains. Analytical techniques were applied to examine major ions, trace elements, and dissolved CO2 and H2, as well as isotopic characteristics of O, H, Sr, C, and He in water samples, allowing for an investigation of geothermal reservoir temperatures, circulation depths, and mixing processes. The findings indicate that most geothermal waters are influenced by water–rock interactions primarily dominated by granites. The region’s diverse lithologies, change from ancient Caledonian granites and medium–high-grade metamorphic rocks in the central hinterland (XL Fault Zone) to low-grade metamorphic rocks and sedimentary rocks in the western margin (CL Fault Zone). The chemical compositions of geothermal waters are influenced through mixing contacts between diverse rocks of varying ages, leading to distinct geochemical characteristics. Notably, δ13CCO2 values reveal that while some samples exhibit significant contributions from metamorphic CO2 sources, others are characterized by organic CO2 origins. Regional heat flow results from the upwelling of mantle magma, supplemented by radioactive heat generated from crustal granites. Isotopic evidence from δ2H and δ18O indicates that the geothermal waters originate from atmospheric sources, recharged by precipitation in the northern Yunkai Mountains. After infiltrating to specific depths, meteoric waters are heated to temperatures ranging from about 76.4 °C to 178.5 °C before ascending through the XL and CL Fault Zones under buoyancy forces. During their upward migration, geothermal waters undergo significant mixing with cold groundwater (54–92%) in shallow strata. As part of the western boundary of the Yunkai Uplift, the CL Fault Zone may extend deeper into the crust or even interact with the upper mantle but exhibits weaker hydrothermal activities than the XL Fault Zone. The XL Fault Zone, however, is enriched with highly heat-generating granites, is subjected more to both the thermal and mechanical influences of upwelling mantle magma, resulting in a higher heat flow and tension effect, and is more conducive to the formation of geothermal waters. Our findings underscore the role of geotectonic processes, lithological variation, and fault zone activity in shaping the genesis and evolution of geothermal waters in the Yunkai Uplift.