Physico-chemical and environmental controls on siliceous sinter formation at the high-altitude El Tatio geothermal field, Chile

El Tatio geothermal field is located 4270 m above sea level in the Altiplano, northern Chile. Siliceous sinter deposits from El Tatio were studied to understand the influence of water chemistry and the extreme climatic conditions on their textures and mineralogy. The results of this study show that the mineralogy of El Tatio sinters include of opal-A and accessory minerals, such as halite, gypsum and cahnite (Ca4B2As2O12∙4H2O), which precipitate by full evaporation of high arsenic, boron and calcium thermal waters. El Tatio sinters show a high degree of structural disorder, probably linked to cation incorporation into the silica structure and/or the occurrence of micro- to nano-scale accessory minerals. The high content of cations in the thermal waters is strongly tied to relatively high silica precipitation rates considering silica concentration in water (147-285 mg/l SiO2). Precipitation rate reach 2.5 kg/m2 per year based on in situ precipitation experiments. The particular environmental conditions of this high-altitude geothermal area that produce high water cooling rate and high evaporation rate, may also be responsible for the fast silica precipitation. Low environmental temperatures create freezing-related sinter textures (i.e., silica platelets and micro columns/ridges). Silicified microbial filaments are also characteristic of El Tatio sinters, and they are indicative of water temperature and hydrodynamic conditions at the moment of sinter formation. However, sinter textural interpretation in a high-altitude Andean context must be done carefully as specific relationships between microbial and hydrodynamic textures are found.