Boiling, Degassing, and Mixing of Fluids in the Trans-Himalayan Geothermal Systems, India

Geothermal fluids, during ascent, are subjected to various secondary processes which alter their chemistry from reservoir to surface discharge. We have characterized and quantified various secondary processes such as degassing, boiling, and mixing in this study for the Trans-Himalayan geothermal fluids. The geochemical facies of thermal waters are found to be Na–Cl for Puga, mixed type (Na–Cl–SO4–HCO3 ) for Chumathang and Panamik, and Na–HCO3 type for Changlung having neutral to moderately alkaline pH (6.7–8.9) along with TDS ranging from 525 mg/l to 2931 mg/l. The geothermal reservoir temperature cal- culated from the Na–K cation geothermometry varies between 170–260◦C. The reser- voir fluids, constructed by adding liquid and steam phases compositions, exhibit near- neutral pH (6.47–7.03) with lower TDS than surface discharges (∼561–1811 mg/l). The surface geothermal water is found to be the resultant mixture of three end-members (non- thermal water, boiled reservoir water, and condensed steam) based on which a ternary mixing model has been developed. The SiO2–temperature relation indicates undersatu- ration with respect to amorphous silica but equilibrium to supersaturation with respect to chalcedony and quartz. This pattern implies boiling-driven fluid evolution, kinetically con- trolled silica scaling, and progressively more mature reservoir waters from Panamik and Puga geothermal area. Geochemical modeling result shows that all geothermal fluids have >40% of boiled deep fluids, traced with conservative component, Cl. The fraction of con- densed steam is highest in Changlung (∼ 11–34%) and lowest in Chumathang (∼ 13%) and Panamik (∼2–19%) with Puga lying in the intermediate stage (∼8–24%) (traced with reactive component, dissolved CO2 ). All thermal waters exhibit mixing with local meteoric waters, with cold-water contributions ranging from 14–42%. Overall, these results highlight the role of boiling and mixing phenomenon shaping the fluid composition of the Trans- Himalayan geothermal systems.