Geochemistry of thermal fluids and the genesis of granite-hosted Huangshadong geothermal system, Southeast China

Abstract

The geochemistry of the thermal waters in the granite-hosted Huangshadong geothermal system was studied to characterize the hydrogeology of the system and potential geothermal energy prospects. In recent years, interest in geothermal energy utilization as an alternative and non-fossil fuel energy source has grown in China. The thermal waters have temperatures of 48–99 °C at the surface and are classified as of HCO3-Na type with low total dissolved solids (TDS) content (192–727 mg/L) and neutral pH (7.12–8.15). Based on the isotope (δ2H, δ18O, 87Sr/86Sr) and chemical (Li, Cl, and Sr) compositions, the thermal waters were sourced from local meteoric water, followed by water-rock interaction with the granite host rocks and mixing with non-thermal water upon water ascent to the surface. Applying geothermometry and mixing models, the chemical composition (SiO2, B, Li, Na, K, Rb, Ca, Mg, Sr, Al, F, Cl, Br, HCO3, and SO4) of deep thermal water was assessed and the reservoir temperature was predicted as approximately 150–165 °C. The corresponding thermal water elemental composition was in equilibrium with common secondary minerals. Based on noble gas isotope systematics (3He, 4He, and 20Ne), He is a crustal source (>95%) being accompanied by radiogenic heat, which is presumably the principal source of induced heat observed in the geothermal systems. Other volatiles (CO2, N2, and Ar) were also of crustal and/or atmospheric origin. Based on chemical and isotopic systematics, it was concluded that the Huangshadong geothermal field is a medium-enthalpy geothermal system with thermal water circulated to a depth of approximately 3.5 km, corresponding to a power density of 4.4 MW/km2. Such geothermal systems are potentially suitable for deep drilling (>2–3 km), followed by energy production and the direct use of thermal water. These findings reveal the genesis mechanism of hydrothermal systems in the granite-hosted area of Southeast China and indicate that high-enthalpy geothermal fluids can hardly be expected within a drilling depth of 4 km.