Synthesis of geochemical techniques to identify the origin and multistage evolution of saline water in a complex geothermal system

¹ Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, 100029, Beijing, China
² Institute of Earth Sciences, Chinese Academy of Sciences, 100029, Beijing, China
³ University of Chinese Academy of Sciences, 100049, Beijing, China

^* Corresponding author: z.pang@mail.iggcas.ac.cn

Abstract

Elucidating brine origin and evolution is a fundamental but not easy task especially for coastal geothermal systems with possible marine constituents and multistage evolution, as subsequently physical, chemical and biological alteration processes may mask the original and early-stage signatures. Here chemical and isotopic characteristics of water (D and ¹⁸O) and dissolved constituents (¹³C, ¹⁴C, ¹¹B and ⁸⁷Sr/⁸⁶Sr) have been utilized to investigate the source and multistage evolution of the Jimo coastal geothermal system in eastern China, with dramatic differences of geochemical characteristics observed within a 0.2 km² area. Results show that geothermal water is derived from paleo-meteoric water and has undergone a 3-stages evolution that involves: (1) Dissolution of marine halite and potash salts in the deep reservoir; (2) Water-rock reactions especially cation exchange produces a Cl-Na-Ca type water as deep geothermal water upwells along the fault zone; (3) A minor (<0.3%) addition of fossil seawater to the shallow aquifer that produces Cl-Na type waters in the west, whereas sulfide oxidation and dissolution of aluminosilicate and carbonates in the east produces Cl-Na-Ca type waters. The methodology utilized in this study offers a means of examining other similar complex geochemical systems having a multistage evolution.