HYDROCHEMISTRY AS A TOOL FOR INVESTIGATING GEOTHERMAL ENERGY SOURCES IN THE HIMALAYAS

Field parameters, hydrochemistry, and environmental isotopes were used to investigate the origin, subsurface history and reservoir temperatures of major hot springs in western Himalayas. With this work I am extending the Himalayan Geothermal Belt (HGB) westward into north Pakistan. The hot springs lie in a belt bounded by Main Mantle Thrust (MMT) in the south and Main Karakoram Thrust (MKT) in the north in Pakistani Himalayas. The spring discharges range from 50 to 1200 liters per minute with orifice temperature range of 51 to 93^oC. Most of the thermal waters are meteoric in origin recharged from precipitation at higher altitude, which undergoes δ¹⁸O shift of about 1‰ due to rock-water interaction. The wide range of tritium concentrations shows that the shallow fresh groundwater is mixing with the thermal water in different proportions with a residence time in excess of 50 years. Different geothermometers give reservoir temperature in the range of 105-260^oC. The isochemical-mixing model based on the Na-K and quartz geothermometers estimates 227^oC temperature. Hot-end reservoir temperature estimates computed using various chemical geothermometers ranged from a low of 105^oC calculated from chalcedony and 155^oC from quartz solubility curves, while the Mg-Li geothermometer indicated the hot-end temperature of 128^oC. Much higher estimates of about 260^oC were obtained comparing mixing diagrams for a conservative solute, e.g. Cl- with the enthalpy (temperatures). δ¹⁸O(SO₄-H₂O) geothermometer indicates equilibrium temperatures from 130 to 185^oC. Rapid uplift and erosion of the Himalayas have brought hot rocks to near surface, where contact with meteoric water produces hot springs clustering along the major fault-lines. Using the geothermal gradient of 0.026 °C/m, permits inference of a 2000-m-deep circulation of meteoric water. In many locations along the HGB, the geothermal anomaly is high enough for generation of electricity. Thailand producing 300 kWe (from 117^oC water). Yangbajang (Tibet), generates 25 MWe, providing Lhasa with about 40% of its electricity. The studied geothermal fields can thus be developed as an alternative source of energy in that energy-stricken region.