Thioarsenic Species in the High-Temperature Hot Springs from the Rehai Geothermal Field (Tengchong) and Their Geochemical Geneses

Rehai, located in the Tengchong volcanic region of Yunnan Province, is a typical magma-heated hydrothermal system in mainland China. Taking Rehai as the study area, the thioarsenic species in hot springs were quantitatively determined for the first time and their geochemical geneses were identified. The sulfide-rich geothermal water samples were collected and immediately treated via rapid freezing technique in-situ and then detertnine the arsenic species in the lab, which is superior to the traditional methods of pretreatment and measurement of aqueous arsenic. The traditional acidification treatment of arsenic-bearing, sulfide-rich water sample inevitably results in the precipitation of tri-thioarsenate as the form of amorphous S-As compounds. Furthermore, the common use of anion-exchange column is not capable of fully collecting thioarsenic species and separating them from arsenate in-situ. Under the control of magmatic fluid input and intense fluid-rock interactions at high reservoir temperatures, the neutral to slightly alkaline hot springs discharged from the Rehai hydrothermal system are rich in sulfide and arsenic, facilitating the formation of thioarsenic species. Mono-thioarsenate, di-thioarsenate and tri-thioarsenate were detected in the Rehai hot springs with ratios to total arsenic up to 26.7%, 43.3% and 33.7%, respectively. In two sub-hydrothermal areas of Rehai, i.e. the Liuhuangtang area and the Zaotanghe area, the hot springs are distributed along the main N-S stretching faults and the secondary E-W stretching faults, respectively, and therefore the geothermal waters went through different cooling processes (adiabatic cooling and mixing with shallow cold groundwaters, respectively) prior to their discharge to the surface. Thus the hot springs from Liuhuangtang have relatively higher concentrations of sulfide and arsenic as well as various thioarsenic species.