Silica accumulation rates for siliceous sinter at Orakei Korako geothermal field, Taupo Volcanic Zone, New Zealand

Abstract

The rate of silica accumulation on glass slides over 895 days (2 years 5 months and 12 days) was determined using in-situ experiments in the discharge channel of Map of Australia hot pool, located at Orakei Korako, Taupo Volcanic Zone (TVZ), New Zealand. The discharge channel consisted of constantly flowing alkali chloride water with a temperature of 75 °C, pH of 7, 269 ppm silica, a flow rate of 1.4 l/s and mass flow of 1.36 kg/s. One slide was placed parallel to the flow, while another slide was perpendicular to the flow direction. Amorphous silica (opal-A spheres) deposited on both slides to form abiotic, sub-aerial, lily-pad textures which grew on basal sub-aqueous sinter structures. Lily-pad sinter forms by capillary motion of the water reaching and wetting the sub-aerial sinter surface, where the silica accretes to previously formed sinter surfaces parallel to the pool water level. In our experiment, silica deposition was greater in the horizontal direction than the vertical direction. Silica accumulation was up to four times greater on the front face of each slide. The maximum vertical and horizontal growth rates of sinter observed in our experiment were 3.9 and 7.6 mm/year, respectively. The observation that very little sinter grows below the water line is consistent with most of the sinter precipitating as a result of cycles of wetting, evaporation and drying. Quantitative calculations show that the observed growth rates are within the maximum amount of sinter expected to be deposited by evaporation. These measurements provide a constraint on the mass flow and time required to form siliceous sinter deposits of a known thickness, at extinct hot spring sites. Better constraining siliceous sinter growth rates improves our understanding and reconstruction of paleo-hydrological hot spring conditions.