Hydrochemical and isotopic characteristics and genesis of geothermal fluids in the Maojingba geothermal field, northern Chengde city

Abstract

The temperature of geothermal water exposed on the surface in the Maojingba area, northern Chengde city, can reach up to 98.7 °C, mostly occurring in the Jurassic coarse-grained diorite with well-developed fractures and high \begin{document}${\mathrm{SO}}_4^{2-} $\end{document} content. However, few studies on the recharge, evolution, and genetic mechanism of the geothermal fluids in this area. In order to understand the genesis of the geothermal system in the bedrock mountain area and evaluate its development and utilization potential, this study, based on the regional geothermal geological survey, tested and analyzed the hydrochemical composition of different water bodies, groundwater dating isotopes (3H and 14C), sulfur and oxygen isotopes in sulfate (δ34S-\begin{document}${\mathrm{SO}}_4^{2-} $\end{document} and δ18O-\begin{document}${\mathrm{SO}}_4^{2-} $\end{document}), carbon isotopes (δ13C-\begin{document}${\mathrm{HCO}}_3^- $\end{document}) and strontium isotope (87Sr and 86Sr) to explore the fluid genesis of the Maojingba geothermal system. The results show that: (1) the chemical type of geothermal water in Maojingba area is mainly \begin{document}$ {\mathrm{SO}}_4-$\end{document}Na type. The dissolution of silicate mineral and cation exchange promote the enrichment of Na+, K+and SiO2 in the geothermal water. The source of \begin{document}${\mathrm{SO}}_4^{2-} $\end{document} in water is speculated to be H2S oxidation from deep reduction environment or the reaction between high-temperature geothermal water and sulfur, rather than the dissolution of sulfate minerals. (2) The average ratio of n( 87Sr)/n(86Sr) in the geothermal water is 0.7092, close to the ratio of marine carbonate rock, indicating that marine carbonate rock may occur in the deep geothermal reservoir. (3) The geothermal water in the study area is old groundwater with poor circulation and renewal ability, based the groundwater age of 11.9−14.9 ka from 14C. The recharge of the geothermal water is from precipitation in the surrounding mountains, with elevation of1532−1632 m. (4) The deep geothermal reservoir temperature of the geothermal system is 142−144 °C, and the highest temperature is located in the northern part of the geothermal field. The research results are of great significance for the sustainable development and utilization of geothermal resources in the mountainous areas of northern Hebei Province.

Keywords

• maojingba geothermal field
• fluid circulation
• age of geothermal water
• sulfur
• thermal storage temperature