Tungsten Anomaly of the High-Temperature Hot Springs in the Daggyai Hydrothermal Area, Tibet, China

Extremely high concentrations of tungsten was detected in the hot springs discharged from the Daggyai hydrothermal area, Tibet, and their tungsten to molybdenum ratios are much higher than common natural waters as well. A hydrogeochemical study of typical Daggyai hot springs was carried out, based on which it was found that the tungsten concentrations of the neutral springs are significantly higher than those of the acid springs. The neutral hot springs at Daggyai were formed via the adiabatic cooling or conductive cooling of the parent geothermal fluids ascending from the deep levels to the surface, and the tungsten in these waters is primarily from the contribution of magmatic fluids. In contrast, the slightly acid hot springs are basically a mixture of the steam-heated acid waters and neutral geothermal waters, and the dilution of tungsten-depleted steam-heated waters lowered the tungsten concentrations of these springs to various degrees. In geothermal systems, tungsten behaves generally like chloride, a typical conservative component, and is difficult to precipitate from geothermal water or to be adsorbed by reservoir rocks. However, molybdenum is prone to precipitate from sulfide-rich Daggyai geothermal waters as the form of molybdenite, resulting in the high tungsten to molybdenum ratios of the Daggyai hot springs. Moreover, although sulfide exists in the Daggyai geothermal waters, the major species of tungsten is tungstate instead of thiotungstates. The formation of trace thiotungstates has little effects on the tungsten-involved hydrogeochemical processes.