Sedimentation sequence of a high-temperature silica-rich hot spring: evidence from isothermal evaporation experiments and from petrology and mineralogy of sinters

High-temperature SiO2-rich springs are one of the most indicative manifestations of deep geothermal activities. Although they often represent the valuable geothermal potentials and significances of the areas, their high SiO2 contents usually bring some non-negligible problems when we exploit the geothermal resources, especially the silica scales. To understand the sedimentation sequences of SiO2-rich hot springs and to expose its potential relationships with natural sinter deposits and silica scales in geothermal plants, this study make a couple of analyses of the water and solid samples collected from a natural SiO2-rich hot spring in Tengchong (i.e. Dagunguo spring) and isothermal evaporation experiments, including water chemistry analyses, X-ray diffraction, thin-section observation, and electron microscopy analyses. The modern sinters in Dagunguo spring are amorphous SiO2 which is largely formed of tight siliceous layers and some amorphous SiO2 spheres. Some silicified microorganisms, elemental sulfur, and clay are also found. The Dagunguo spring water is Na–Cl–HCO3-type SiO2-rich water, with a pH of about 8 and low dissolved concentrations of Al3+, Fe3+ and Mg2+. Isothermal evaporation (T = 84 °C) resulted in a sequence of precipitated minerals: amorphous SiO2 → halite → sodium sesquicarbonate dihydrate (trona) → potassium chloride (sylvine), accompanied by calcite, hydrous sodium sulfate (mirabilite) and sodium bicarbonate (baking soda). The amorphous SiO2 produced in the evaporation experiments exhibited different shapes and structures than the spherical opal-A formed by hot spring water precipitation under natural conditions. The study of precipitation sequence and minerals of hot spring water provides insight into the scaling mechanism of silica-rich geothermal water and anti-scaling measures that can be applied in the process of exploitation and utilization of geothermal resources.