Siliceous-sulphate rock coatings at Zhenzhu Spring, Tengchong, China; the integrated product of acid-fog deposition, spring water capillary action, and dissolution

• Published in: Geological magazine 2020-02, Vol.157 (2), p.201-212
• Main Authors: Wen Huaguo, Wen Huaguo, Xu Wenli, Xu Wenli, Li Yi, Li Yi, You Yaxian, You Yaxian, Luo Xiaotong, Luo Xiaotong
• Format: Article
• Language: English
• Subjects: Acids; aerosols; Aggregates; Alum; Asia; Capillarity; Capillary water; Cellulose acetate; China; Climate change; Coatings; Computational fluid dynamics; Cooling; Crystals; Deposition; Dilution; Dissolution; Dissolving; electron microscopy data; encrustations; Evaporation; Far East; Fluids; Fog; framework silicates; geochemistry; ground water; Gypsum; Hot springs; hydrochemistry; Laminates; laminations; Mica; mica group; Mineralogy; Minerals; Muscovite; planar bedding structures; Pores; Precipitation; quartz; Rain; Rain water; Rainfall; Rainy season; rock, sediment, soil; Rocks; sed rocks, sediments; Sedimentary petrology; sedimentary structures; SEM data; sheet silicates; Silica; silica minerals; silicates; siliceous composition; Silicon dioxide; Spring; Spring (season); Spring water; springs; Sulfates; Tengchong; thermal waters; Volcanic fields; voltaite; Water temperature; water vapor; water-rock interaction; Wet season; X-ray diffraction data; Yunnan China
• ISSN: 0016-7568; 1469-5081
• DOI: 10.1017/S0016756819000542

Siliceous-sulphate rock coatings were observed at Zhenzhu Spring, an acid sulphate hot spring in the Tengchong volcanic field, China. These rock coatings are mainly formed of gypsum and amorphous silica. Some alum-(K), voltaite, α-quartz and muscovite were also found. Four different laminae are developed in the rock coatings: gypsum layer, tight siliceous layer, tabular siliceous layer and siliceous debris layer. The gypsum layer is located at the top of the rock coatings, while other siliceous layers appear below the gypsum layer. Geochemical modelling of the fluids was performed to identify the mechanisms responsible for the formation of gypsum and amorphous silica. The results indicated that the occurrence of gypsum is related to the acid-fog deposition and amorphous silica mainly originates from spring water. Fog deposition provided the rock coatings with abundant SO42- and Ca, and the subsequent complete evaporation of the condensed fluids produced gypsum. Seasonal climate change (especially variation in rainfall) determines the fluctuations of capillary action and dissolution. Rainfall events in the wet season led to periods of non-precipitating gypsum and promoted the capillary rise of the spring water. Slightly diluted capillary water (a small amount of rainwater) covered the rock coatings, formed a tight siliceous layer on the rock-coating surface and/or filled the pores among the gypsum crystals forming many tabular siliceous aggregates. Heavy rainfall (high dilution), however, resulted in non-precipitating amorphous silica and accelerated the gypsum dissolution, leaving tabular pores around tabular siliceous aggregates and forming a tabular siliceous layer.