Chemical Characteristics of Geothermal Fluids in the Subduction Mélange: Insights from the Juisui Geothermal Field, Taiwan

The Juisui Geothermal Field, located in eastern Taiwan, is renowned for its hot springs, which are spread along the foothills and riverbanks. Covering only about 12 km², the area features a diverse range of springs, each with distinct characteristics, reflecting its location along a subduction mélange.

Around 15 to 10 Ma, turbidites at the edge of the Eurasian continent were affected by the rifting of the South China Sea. As a result, these rocks were subducted eastward, descending to depths of 40 to 60 km beneath the Philippine Sea Plate. The mantle wedge and oceanic plate’s basic/ultrabasic rocks mixed with sediments and experienced temperatures of up to 550 ℃ and pressures of 17 kBar (Tsai et al., 2013; Baziotis et al., 2017). These rocks were then rapidly uplifted to the surface due to orogeny. As a result, the host rocks in this area are not only quartz-mica schist, but also include serpentinite, meta-gabbro, epidote amphibolite, and Glaucophane schist. Shallow drilling can reach extremely high temperatures— for instance, the JS-5G well reaches 180℃ at just 190 m deep, and the highest temperature recorded was 204 ℃ at 1,428m in the CPC-JS-2 well.

This study analyzed 40 meteoric fluid samples to map the regional meteoric water line, and collected 77 spring and geothermal well  samples during different seasons. The salinity of the hot spring water in the Juisui area ranges from 0.44 psu to 9 psu, while total dissolved solids (TDS) range from less than 520 mg/L to 6,550 mg/L. Bicarbonate concentrations range from 400 mg/L to 6,500 mg/L. Hydrogen isotope analysis suggests that the recharge for the springs may come from mountains at elevations of 830 to 1,100 m. These meteoric waters undergo different types of circulation: shallow circulation is heated by the geothermal gradient, which leads to lower concentrations and places the water on the meteoric water line. Deep circulation hot spring, characterized by Na-K ratios, falls between the greywacke line and the 1:1 line, indicating the mixing of deep brine or metamorphic water, and is far removed from the seawater 27:1. A few hot spring samples have especially high bicarbonate concentrations, indicating past flow through marble. Even when sampling from the same well (GSMMA-RS-1) at different times, there’s significant variation in salinity (ranging from 0.58 psu to 6.89 psu), which highlights the strong heterogeneity of the fluid properties— a key feature of the subduction Mélange. A positive oxygen isotopic shift suggests that deep circulation fluids have undergone water-rock interactions with various surrounding rocks in high-temperature environments.

In terms of mineral saturation, the Saturation Index for both calcite and aragonite in the region is either saturated or near saturation. This suggests that scaling could be an issue in future development, and strategies to prevent it should be considered.