Magmatic fluid input explaining the geochemical anomaly of very high arsenic in some southern Tibetan geothermal waters

Tibet is characterized by extremely high terrestrial heat flow and widely distributed hydrothermal systems, among which many are discharging geothermal waters with extremely high arsenic concentrations from over 10 mg/L to up to 126 mg/L. The distribution of these high arsenic waters is basically in accordance with the location of the Indian continental subduction zone. A detailed analysis indicates that host rock leaching alone cannot explain the observed arsenic anomaly. Instead, substantial contribution of arsenic from the underlying magma chambers is postulated, which are likely mantle-derived intrusions that have been severely contaminated by the very thick crustal material below southern Tibet, possibly including deep-seated arsenic-rich sedimentary rocks. Alternatively, the partial melts at crustal depths reflected by the detected low-velocity and/or high-conductivity anomalies in southern Tibet, serving as the magmatic heat source of the high‑arsenic hydrothermal systems, may also form indirectly under the high-temperature conditions generated by mantle upwelling or due to crustal thickening. Low sulfide concentrations further promote high dissolved arsenic concentrations with little formation of thioarsenates and no precipitation of limiting arsenic-sulfide phases. In contrast, mantle magmas less affected by crustal contamination, serving as the heat source of many hydrothermal systems in rift zones and hot spots worldwide where the crust is thinner, are releasing magmatic fluids lower in arsenic and higher in mantle sulfur species. Overall, the existence of a magmatic heat source alone does not ensure the formation of geothermal waters with very high arsenic concentrations, and geothermal waters from sedimentary rock-hosted systems are neither necessarily arsenic-rich. The most critical factor controlling the arsenic concentration of geothermal water discharging from a magmatic hydrothermal system is the geological genesis of the magma fluid and its chemical composition.