Genesis, evolution, speciation and fluid-mineral equilibrium study of an unexplored geothermal area in Northeast Himalaya, India

Abstract

Arunachal Pradesh geothermal area, located in the Northeast Himalayan geothermal province is an unexplored geothermal area in India due to its rugged terrain and inaccessibility. In this study, a detailed geochemical characterization of the thermal waters is carried out to elucidate the source of the dissolved constituents and also to determine the subsurface reservoir temperature. The thermal waters are found to be nearly neutral to slightly alkaline (pH = 7.15 to 8.40) having highly variable EC values (EC ranges from 204 to 3600 µS/cm). Most of the thermal waters are mixed cation (Na, Ca)-HCO3-SO4 type whereas a few of them are of Na-HCO3 type. Silicate weathering and ion-exchange processes are found to be the two important mechanisms controlling the concentrations of the dissolved solutes in the thermal waters. The temperature dependant (20 to 198 °C) speciation analysis of the major ions (Na, K, Ca and Mg) shows that the composition of the reservoir fluid is dominated by free Na+ ions followed by K+, Ca2+ and Mg2+. The sulphate complex is found to be majorly dominated by both CaSO4 and MgSO4 whereas carbonate is mainly controlled by CaCO3 and CaHCO3+ species. Environmental stable isotope (δ18O, δ2H) data confirms the meteoric origin of these thermal waters. The isotope composition of the thermal waters in this region is found to be depleted compared to the non-thermal water samples indicating a high altitude precipitational recharge for these thermal waters. The altitude of the recharge estimated from the δ18O-altitude plot reveals that the precipitation occurring at the Gorichen Peak (altitude of 6858 m), the highest peak of the Arunachal Pradesh, is the main source of recharge for these thermal waters. Of different silica geothermometers, the quartz geothermometer gives the most conservative estimate (71 to 133 °C) of subsurface reservoir temperature. The quartz geothermometer gives a lower estimate of reservoir temperature due to mixing with non-thermal water. However multicomponent geothermometrical modelling results give a tightly constrained subsurface temperature i.e. 160±2 °C which may be taken as the probable reservoir temperature in this unexplored geothermal region. The study highlights the potential of integrated isotope-geochemical studies in expanding the geothermal energy utilization in India.