Fluid geochemistry of the Cuopu high temperature geothermal system in the eastern Himalayan syntaxis with implication on its genesis

Abstract

High-temperature geothermal fluids dissolve constituents pertinent to water-rock interaction and magmatic volatile absorption, resulting in high total dissolved solid (TDS) values. However, this study focuses on the hydrochemical evolution of the low-salinity HCO3–Na type high-temperature geothermal fluid in Cuopu, eastern Himalayas. The geothermal water is recharged by local precipitation and glacier water from surrounding mountains. The TDS values are below 834 mg/L and the constituents are mainly products of the water-carbon dioxide-rock interactions lacking magmatic volatile dissolved in the fluid. The geothermal water reaches an almost complete chemical equilibrium with the feldspar or plagioclase-enriched reservoir rock in a reducing condition. The reservoir temperature is between 175 °C–200 °C, while the temperature could reach up to 400 °C in the deep crustas indicated by the carbon isotopic exchange equilibrium between CO2 and CH4. The infiltrated glacier water was heated during its circulation within the hot thickened crust and continued dissolving the crustal metamorphic gas, such as radiogenic helium and limestone metamorphic CO2, until the junction of the two sets of faults provided an ascending channel for it. Upon rising along the conduit, the geothermal water mixed with cold groundwater to different degrees. Approximately 0.015 mol/L CO2 escaped from the geothermal fluid when it scattered as bubbling hot springs on the surface of the anisotropic porous Quaternary sediments. Such kind ofhydrochemical evolution of lowsalinity alkaline HCO3–Na type water represents a typical formation mechanism of the high-temperature geothermal systems along the Himalayas.