Geothermometry and mineral equilibria of thermal waters from the Guanzhong basin, China

Abstract

This report describes the regional geotectonic and geothermal features to present a conceptual model of the thermal activity in the Guanzhong basin, China, and reviews the most important methods of geothermal sampling and chemical analysis used in China. Silica and cation geothermometers are applied to predict possible subsurface temperatures using chemical analytical data calibrated by chemical equilibrium calculations with the chemical speciation programme WATCH. In order to further confirm the reliability of the geothermometer temperatures, a great effort is made to study the equilibrium state of thermal waters by means of log Q/K diagram, Na-K-Mg triangular diagram and by studying the equilibrium state of some species at the chalcedony and quartz reference temperatures. Finally, the possible subsurface temperatures are predicted in the scattered thermally anomalous areas after a careful consideration of the equilibrium state of all the water samples. The studies of aqueous chemistry in the Guanzhong basin show that the use of geothermometry is conditional. In other words, physical processes like conductive cooling, mixing with cold water and boiling during upflow may affect possible water-rock equilibria in the deep reservoir, and this can result in unreliable temperature predictions. Furthermore, it is very useful to employ different geothermometers and methods for the studies of equilibrium state to check each and to exclude the unreliable samples.