Constraints of major ions and arsenic on the geological genesis of geothermal water: Insight from a comparison between Xiong'an and Yangbajain, two hydrothermal systems in China

Abstract Hydrochemical similarities are shared by the geothermal waters from Xiong'an, a typical non-magmatic hydrothermal system, and Yangbajain, a typical magmatic hydrothermal system. Both systems are primarily discharging chloride- and sodium-rich geothermal waters with similar TDS values. However, the hydrochemical investigations made in this study show that the chloride in the Xiong'an geothermal waters originates mainly from the mixing of deep saline waters, while that in the Yangbajain geothermal waters is basically of magmatic origin, like many other magmatic hydrothermal systems hosted by felsic rocks worldwide. In contrast to chloride, the concentrations of arsenic in the geothermal waters from Xiong'an and Yangbajain differ greatly. The higher arsenic concentrations of the Yangbajain geothermal waters are attributable to the much greater reservoir temperatures in Yangbajain, and possibly, the input of magmatic fluids to the Yangbajain reservoirs. The contents of arsenic in reservoir rocks, instead, don't seem to be an important factor controlling the geothermal arsenic concentrations. Leaching of arsenic from arsenic-rich sandstone or carbonate rocks at relatively low Xiong'an reservoir temperatures should be much less than that from arsenic-poor granitic rocks at higher Yangbajain reservoir temperatures. The results obtained in this study indicate that arsenic coupled with other hydrochemical constituents can be used as a useful tool to identify the geological geneses of geothermal waters.