Geochemical Characteristics and Formation Mechanism of Neogene Geothermal Fluids in the southern part of Jiyuan-Kaifeng Depression

With the long-term unsustainable exploitation, the medium to low temperature geothermal water in the plain areas of Henan Province have experienced a water table decline and resource wastage. Given the influence of complex tectonic units, investigating spatial differences in hydrochemical characteristics and the underlying formation mechanisms is of great importance for the sustainable development and utilization of geothermal water resources. This study focuses on the Jiyuan-Kaifeng Depression and systematically analyzes the hydrochemical and hydrogen-oxygen stable isotope compositions of geothermal water using multivariate statistical analysis, Pearson correlation, Positive Matrix Factorization (PMF), and the silica geothermometer. The results indicate that the geothermal water in the study area primarily originates from atmospheric precipitation in mountainous regions, with the circulation depth of 1 360.9~2 545.1 m and reservoir temperature ranging from 57.6 to 94.1 °C. The variation range of the circulation depth and reservoir temperature of the geothermal water in the Minghuazhen Formation is greater than that in the Guantao Formation. During deep circulation along faults or fractures, prolonged water-rock interactions, combined with geothermal heating and deep geothermal water, have led to the formation of HCO₃-Na-type geothermal water with hydrochemical compositions that exhibit significant, regular differentiation in both horizontal and vertical directions. The hydrochemical compositions of the geothermal water are jointly influenced by the weathering of carbonates, evaporites, and silicates, with silicate weathering and cation exchange playing dominant roles in the formation and evolution of its chemical components. These findings provide a scientific basis for the efficient and rational development and utilization of geothermal resources in the Jiyuan-Kaifeng Depression and the broader plain regions of Henan Province.