Acid hot springs discharged from the Rehai hydrothermal system of the Tengchong volcanic area (China): formed via magmatic fluid absorption or geothermal steam heating?

Abstract

Across mainland China, Tengchong is geothermally unique because both magmatic-hydrothermal systems and late Cenozoic volcanic activities have been observed there. Rehai, located in the Tengchong volcanic area, is the only hydrothermal system in mainland China discharging acid springs with very low pH values. Two types of acid geothermal waters were identified: one is sulfate waters with low chloride concentrations and pH levels as low as around 2; the other has slightly higher chloride concentrations and is marked with diversified hydrochemical types. Both types of acid springs at Rehai are immature waters and far from being equilibrated with a thermodynamically stable mineral assemblage comprising albite, K–feldspar, muscovite, and clinochlore characteristic for felsic rock-hosted reservoirs. In contrast to those neutral chloride springs at Rehai that are formed via boiling, conductive cooling or mixing with cold groundwaters of a parent geothermal liquid (PGL), the sulfate-rich acid waters discharged from the Diretiyan area, the Dagunguo area, and the Shizitou Spring of Rehai are essentially locally perched groundwaters heated by H2S-rich steam separated from deep geothermal fluid. The acidity of these waters originates from the oxidation of H2S in near-surface environment. In contrast, the acid waters with higher chloride concentrations are the results of mixing between steam-heated waters and neutral chloride waters. The geochemical geneses of the Rehai geothermal springs inferred based on stable hydrogen and oxygen isotopes are generally identical. Unlike active volcanic areas across the world, there is a lack of acid waters at Rehai formed through partial neutralization of magmatic fluid, possibly owing to the comparatively deep-seated magma chamber there (no less than 7 km deep).