Gas isotope geochemistry of Tangquan hot spring system in Nanjing (China)

Tangquan hot springs is one of the three well-known geothermal systems in Nanjing, Jiangsu Province, eastern China. Previous studies have shown its high flow rate and ideal water chemistry favorable for utilization. Here we further examine the potential of the hot springs using gas geochemistry in this paper. 35 water samples and 16 gas samples were collected for chemical isotopic analyses. The geothermal gases are found to be primarily atmospheric in origin, containing high nitrogen, oxygen and carbon dioxide gases. Carbon isotope shows that the origin of CO₂ and CH₄ are thermal metamorphic in the upper crust. The isotope ratio of helium is 3.47–4.73 Ra, indicating that mantle He accounts for 44–60% in total of geothermal gases. These findings furnish the grounds to infer geothermal potential at the deeper part of the geothermal system. The immediate geothermal reservoir has a temperature of 90 °C, but the temperature of deeper part of the system could reach 150–205 °C, using the carbon gas isotope geothermometer. These findings show a promising future of deep geothermal resources in the region.