Subaerial sulfate mineral formation related to acid aerosols at the Zhenzhu Spring, Tengchong, China

Abstract

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 δ34S values of the subaerial sulfate minerals at the Zhenzhu Spring varied subtly from –0.33 to 1.88‰ and were almost consistent with the δ34S values of local H2S (–2.6 to 0.6‰) and dissolved SO42−(–0.2 to 5.8‰), while the δ18O values (–8.94 to 20.1‰) were between that of the spring waters (–10.19 to –6.7‰) and atmospheric O2(~23.88‰). 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 δ18O. 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.

Uranium enrichment associated with geothermal alteration: An example from Rehai, Tengchong volcanic area of China

“…For the hot spring water at RHGF, a departure from the meteoric water line and positive δ 18 O‐shift (Figure 3) may be related to enhanced water‐rock interactions (Du et al, 2005; Zhang et al, 2016) and/or evaporation due to varying temperatures (Hou et al, 2013). The spring water at Zhenzhu Spring displays the highest values of δ 18 O and δD (Table 1), probably because (a) water evaporation is the strongest here, given that water temperature at Zhenzhu Spring is measured to be 96 °C and acid‐fog is present (Wen et al, 2020), and (b) the water‐rock interaction is enhanced here due to alteration by sulfuric acid (Luo et al, 2019).…”

“…Given that the hot spring water originated from meteoric water (Figure 3), the enriched REE + Y content (0.3–4.1 ppb) of the hot spring water could only be derived from the surrounding host rocks through water‐rock interactions. The extremely high REE + Y concentration of spring water at Zhenzhu Spring (39 ppb) is probably related to the acidic property of the spring water (Guo, Nordstrom, & McCleskey, 2014; Guo & Wang, 2012; Luo et al, 2019; Zhang et al, 2008), because the mobility of REEs is elevated with low pH (Lewis, Palmer, Sturchio, & Kemp, 1997; Michard, 1989; Wood & Shannon, 2003; Zhang et al, 2016). Similarly, for the acidic hot spring waters from the Tamagawa geothermal area of Japan, for example, the REE concentrations vary from 148 to 462 ppb, which may result from the interaction between the hot spring water containing sulfuric acid and the rocks (Sanada, Takamatsu, & Yoshiike, 2006).…”

“…The sinters collected from the fault surface show similar fabrics to the sinters from Dagunguo (Peng & Jones, 2012), and the constituent amorphous silica was demonstrated to originate from spring water (Wen et al, 2020). The sulphates precipitated at Zhenzhu Spring and Dagunguo may be related to acid‐fog deposition and subsequent evaporation (Luo et al, 2019; Wen et al, 2020). For these hot spring deposits (sinters and sulphates), the distinctly negative Eu anomalies (Figure 5a) indicate their precipitation from hot spring water under reduced conditions.…”

“…Spring is measured to be 96 C and acid-fog is present (Wen et al, 2020), and (b) the water-rock interaction is enhanced here due to alteration by sulfuric acid (Luo et al, 2019).…”

“…Hot springs (water temperature from 42 to >96°C), geysers, fumaroles, and steaming ground are concentrated along the Sulfur Pond–Cucumber Gully Fault (also called Liuhuangtang–Huangguaqing Fault) of north‐south trending (Figure 1c). Sinters and sulfates often occur in the hot springs (Luo et al, 2019; Wen, Xu, Li, You, & Luo, 2020; Zhu & Tong, 1987). The Neogene clastic rocks surrounding the Sulfur Pond–Cucumber Gully Fault were altered by the geothermal fluid into three zones, including silicification–alunite–kaolinite inner zone, kaolinite–illite–smectite middle zone, and chlorite–illite–kaolinite outer zone (Xiao et al, 2017; Zhu & Tong, 1987).…”