The Puga geothermal field is situated in proximity of southern area of ITSZ, marking the convergence point of the Indian and Asian plates, both of which are significant contributors to the Himalayan orogenic processes (Shanker et al., 1976; Virdi et al., 1977). Puga, located at an altitude of ~ 4400 m above mean sea level, is known for numerous geothermal springs, sporadically scattered across the entire valley with surface temperature up to 84°C (boiling point of water at that altitude) with splendid manifestations of sulfur and borax deposits (Mathur, 2004).
The Puga geothermal manifestation is a part of southern tectonic belt which is actively bounded by several protruding faults: Kiagar Tso fault in the West, Zildat fault in the East, and Mahe fault in the North (Shanker et al., 1976). The Northern tectonic section is comprised of sedimentary rocks belonging to Indus group while in Southern section of the valley, rocks of Proterozoic age, encompassing paragneiss, schist, quartzite and associated phyllite, limestone and eclogite, which form the base for the fossiliferous Tethyan sediments are exposed still further to the southern section of the valley (Absar, 1981; Dutta et al., 2023). Hard reconsolidated hydrothermal breccia is extended upto several depths forming the basement rock of Tso Morari Gneissic Complex (TMGC) which is extended by Polokong La granite in the further west (Harinarayana et al., 2004; Azeez and Harinarayana, 2007) (Fig. 1).
Geochemistry and Geothermometry
Thermal fluids of Puga are either relatively dilute Na − Cl type or mixed ion type (transition zone where no pair of cation and anion exceeds 50% of total concentration), with abundance of HCO3− and Cl− ions having total dissolved solids ~ 2400 mg/L (Dutta et al., 2023). It is a living hydrothermal system with consistent abundance of conservative species in thermal waters like Cl, B, Li, F, and Cs which provide an unambiguous evidence of a single reservoir source of these thermal discharges. Comparing with some global hydrothermal systems, like Steamboat springs of Nevada-USA, Yangbajing of China, El Tatio of northern Chile, the trend of rare alkali enrichment (RAEs) follows the sequence where either Li > Rb > Cs or Rb > Li > Cs. Puga is the only known geothermal fluid with RAE sequence Cs > Li > Rb (Shanker et al., 1999a; Dutta et al., 2023). Thermal fluids have enrichment of B (~ 140 mg/L), F (~ 13 mg/L), Li (~ 6.8 mg/L), Cs (~ 8 mg/L) with molal ratio of Na/K < 10 which suggests Na+ is depleted in thermal waters through action ion-exchange process or dilution with shallow groundwaters. The molal Na/Cl ratio is > 1 in all the thermal waters of Puga which suggests that high temperature fluids are repeatedly interacting with crystalline or volcanic rocks present in the ancient reservoir. The molal sum of ionic ratio, (Ca2+ + Mg2+) / (Na+ + K+), ranges between 0.03 to 0.06 which suggests active reverse ion exchange process and lower water flow course over carbonate rocks, or alkaline earths in hot fluids are exchanged with alkalis of aquifer rocks.
As the Na-K and K-Mg equilibration reactions are temperature dependent, cation geothermometers are formulated based on the ion-exchange reactions (Giggenbach, 1986; Fournier and Truesdell, 1973). Geothermometers play a crucial role in assessing emerging fields and tracking the water dynamics of geothermal systems. The thermal waters fall in immature water domain, surviving various stages of partial equilibrium phase depending on extent of water-rock interaction. As the hot fluids are discharged through geyseric manifestation with enrichment of Cl− ion, they retain deep seated reservoir information to a greater extent and such waters when used for reservoir temperature estimation, furnish temperature around 230 − 260°C, as per Na-K Giggenbach, Na-K Fournier and Na-K-Ca geothermometers. The reservoir temperature estimated from quartz geothermometry falls between 160 − 190°C which may be taken as the minimum temperature of reservoir as silica concentration in hot fluids is highly prone to dilution, boiling and precipitation. Taking fast equilibrating K-Mg geoindicators at shallow level and low temperature into consideration, reservoir temperature estimated in Puga is ~ 160°C which is in concordance with the reservoir temperature estimated from silica thermometry. This indicates that latest equilibration reactions have taken place in the reservoir at shallow level. Early studies by Shanker et al. (1999b) have shown by considering reservoir temperature of ~ 255°C before steam loss and ~ 200°C of shallow reservoir that reservoir bears steam fraction of 0.2 with Cl content of minimum 375 mg/l before steam loss and 15% cold water blended with 85% of hot fluid component before thermal discharge on surface. The upsurge of fluids is preceded by conductive cooling at post-dilution.