Genetic mechanisms of low-temperature geothermal water and its associated helium gas in Meixian County in the piedmont of Qinling Mountains

Meixian County in the piedmont of the Qinling Mountains boasts abundant geothermal water resources with highly abundant associated helium gas. However, the characteristics and origin of geothermal fluids in the study area have yet to be studied. By analyzing the chemical composition and isotopic geochemistry of geothermal fluids, combined with the analysis of regional structures and faults distribution, this study delved into the characteristics and genetic mechanisms of geothermal fluids in the study area. The results show that the geothermal water exhibits a salinity ranging from 638.96 to 1026.83 mg/L and a hydrochemical type of HCO₃-Na. The low-degree metamorphism suggests that the geothermal water is either unbalanced or partially balanced, with solutes primarily originating from the weathering of silicate rocks. The geothermal water shows δD and δ¹⁸O values ranging from −77.3‰ to −70.8‰ and from −10.9‰ to −9.8‰, respectively, both of which fell near the meteoric water line. Furthermore, the geothermal water features an average recharge elevation of approximately 1030 m, recharge water temperatures ranging between 4.9℃ and 7.0℃, and a maximum circulation depth of about 3200 m. The geothermal water-associated gas is dominated by nitrogen gas, exhibiting low hydrocarbon gas concentrations and highly abundant helium gas. The methane in the associated gas showed an average δ¹³C value of −52.8‰ and an average δD value of −247‰. Besides, the associated gas displayed low R/R_a ratios and high ⁴He/²⁰Ne ratios. This study reveals that meteoric water from the Qinling Mountains infiltrated downward into fault fractures and sandstone pores through deep-seated faults and their secondary faults, ultimately forming geothermal water via geothermal heating. The low-temperature and high-volume geothermal water in the study area are primarily due to the proximity and abundance of water sources, poor sealing, and rapid circulation at shallow depths. The helium in the associated gas is principally crust-derived and formed by the radioactive decay of uranium and thorium elements in crustal rocks. The results of this study provide critical geochemical evidence for the exploitation and utilization of geothermal resources in the study area.