Source and Evolution of Thermal Fluids in the Lushan Geothermal Field

Geochemically, the oxygen isotope values range from −7.3‰ to −10.7‰, and hydrogen isotope values range from −72.6‰ to −57.2‰. Most data plot along the meteoric water line, indicating a dominant meteoric origin, while a small number of samples deviate slightly from this line, possibly reflecting fluid fractionation associated with boiling. An integrated three-dimensional geothermal geological model was constructed using: (1) surface DEM data, (2) regional geological maps and cross-sections, (3) borehole core descriptions and lithologic logs, (4) 3-D MT data, and (5) well temperature measurements. The Lishan Fault, located on the western margin of the Lushan geothermal area, is a highly active fault that has created a favorable fracture network serving as conduits for meteoric water infiltration and as conditions for geothermal reservoir development. Combined with previously acquired MT profiles across central Taiwan, the data reveal a low-resistivity zone extending upward from depth in the southwestern region along the Lishan Fault and spreading eastward into the Lushan geothermal area. This indicates that the primary heat/fluid source of the Lushan geothermal system is derived from deep circulation originating in the southwestern subsurface of the region.

Veins in the Lushan geothermal area are dominated by quartz veins, with minor occurrences of calcite veins. Based on field occurrences, the veins can be classified into three successive stages: (1) quartz veins parallel to slaty cleavage with homogenization temperatures between 220 and 300 °C, and salinities ranging from 5.7 to 9.1 wt.%, (2) quartz veins cutting across slaty cleavage with temperatures mainly between 220 and 290 °C, with salinities of 4.0–8.0 wt.%, and (3) euhedral to subhedral crystals infilling fractures and pores, yielding homogenization temperatures mostly between 220 and 300 °C, with salinities of 3.1–9.7 wt.%, whereas calcite-hosted fluid inclusions show lower homogenization temperatures of 150–210 °C and salinities of 1.0–5.7 wt.%. Comparison of fluid inclusion temperatures indicates that similarly high homogenization temperatures were attained during all three stages. No clear correlation is observed between temperature and salinity, and the salinity distributions are comparable among different stages. These features suggest the presence of a stable brine source constrained by synclinal structures in the region. The fluids are inferred to originate from a persistent deep heat source beneath Chunyang, where they were heated at different depths before ascending and precipitating mineral veins during successive tectonic episodes.