A New Geochemical Model of the Yangbajing Geothermal Field, Tibet

Abstract

Drilling and geochemistry data show the Yangbajing Geothermal Field consists of two reservoirs, a shallow one at depths less than 450 m with temperature of 150-165oC, and a deep one at depths of 950 to 1336m with temperatures as high as 251oC. Waters from both reservoirs are sodium chloride and have the same B/CI ratio, implying a common origin. Quartz and chalcedony geothermometers are applicable to the deep and shallow reservoirs respectively. Hydrogen and oxygen isotopes suggest both thermal waters are of meteoric origin. The recharge area is the south slope of the Nyainquentanglha Range located northwest or the field.

Carbon dioxide is the main non-condensable gas component in the field. Helium isotopic ratios and a He-Ar-N2 diagram indicate a predominately crustal origin of the gases. Carbon dioxide is inferred to be derived from partial melting of the Nyainquentanglha core complex.

A new geochemical model shows heat and gases from molten crust in the Nyainquentanglhas core complex at a depth of 5 km rising up to a 250oC deep reservoir in fractured Himalayan granite. Thermal fluid rises from this deep reservoir to a cooler and shallower reservoir primarily in Quaternary alluvium. The thermal fluid is modified by degassing, oxidation, mixing, and precipitation processes prior to reaching the surface.