Subaerial Sulfate Mineral Formation Related To Acid Aerosols At The Zhenzhu Spring, Tengchong, China

The Zhenzhu Spring, located in the Tengchong volcanic field, Yunnan, China, is an acid hot spring with high SO42- concentrations and intense acid aerosol generation. In order to understand the formation mechanism of sulfate minerals at the Zhenzhu Spring and provide a better insight into the sulfur isotope geochemistry of the associated Rehai hydrothermal system, we investigated the spring water hydrochemistry, mineralogy and major-element geochemistry of sulfate minerals at the Zhenzhu Spring together with the sulfur-oxygen isotope geochemistry of sulfur-containing materials at the Rehai geothermal field and compared the isotope results with those in other steam-heated environments. Subaerial minerals include a wide variety of sulfate minerals (gypsum, alunogen, pickeringite, tamarugite, magnesiovoltaite and a minor Mg-S-O phase) and amorphous SiO2. The delta S-34 values of the subaerial sulfate minerals at the Zhenzhu Spring varied subtly from -0.33 to 1.88 parts per thousand and were almost consistent with the delta S-34 values of local H2S (-2.6 to 0.6 parts per thousand) and dissolved SO42- (-0.2 to 5.8 parts per thousand), while the delta O-18 values (-8.94 to 20.1 parts per thousand) were between that of the spring waters (-10.19 to -6.7 parts per thousand) and atmospheric O-2 (similar to 23.88 parts per thousand). The results suggest that most of the sulfate minerals are derived from the oxidation of H2S, similar to many sulfate minerals from modern steam-heated environments. However, the rapid environmental change (different ratio of atmospheric and water oxygen) at the Zhenzhu Spring accounts for the large variation of delta O-18. The formation of subaerial sulfate minerals around the Zhenzhu Spring is related to acid aerosols (vapour and acid water droplets). The intense activity of spring water around vents supply the aerosol with H2SO4 (H2S oxidation and acid water droplets formed by bubble bursting) and few cations. Deposition of the acid sulfate aerosol forms the acid condensate, which attacks the underlying rocks and releases many cations and anions to form subaerial sulfate minerals at the Zhenzhu Spring.