Geothermal evolution of deep parent fluid in Western Guangdong, China: evidence from water chemistry, stable isotopes and geothermometry

The groundwater flow patterns and temperature distribution of deep parent fluid in an area of Western Guangdong (China) have been reconstructed using chemical geothermometry and multicomponent mineral equilibrium (MME) based on water chemistry and stable isotopes. Thermal groundwater samples (three drill holes, nine springs) and groundwater samples of 'normal' temperature (one spring, eight wells) were collected and analyzed to characterize the studied hydrothermal system. The geothermal waters mainly contain the cation Na(+)followed by Ca(2+)and the anion HCO(3)(-)followed by Cl-. The stable isotope composition (delta D, delta O-18) indicates a meteoric origin for both geothermal water and 'normal' groundwater. The exchange temperature in the geothermal reservoir of Western Guangdong is estimated to be 162.6 degrees C using MME and K-Na-Ca geothermometry, while other chemical geothermometers (Na-K, K-Mg, silica and chalcedony) provide unsuitable results. The results indicate that the meteoric waters descend through the fissures and reach a maximum depth of about 2.3 km, where they are heated by a subjacent granitic body and become the deep parent fluid. The rise of deep geothermal fluid is controlled by thermodynamics, and the fluid is cooled by heat conduction and possibly mixing with shallow groundwater. Water-rock interaction affects the chemistry of the deep geothermal fluid, and mixing with shallow groundwater changes the fluid chemistry in the shallow subsurface. Seawater incursion is identified in the thermal groundwater, contributing to higher Na(+)and Cl(-)contents in the water. This circulation mechanism probably dominates most of the hydrothermal system in the coastal area of Western Guangdong.