Geochemical characteristics of silica scales precipitated from the geothermal fluid at the Onuma geothermal power plant in Japan

抄録

The scale precipitation is a major issue at geothermal power plants as it reduces the production rate of geothermal energy. The scale precipitates at different physical and chemical conditions in each geothermal power plant as a result from a fluid–rock interaction for the specific conditions at each plant. Thus, it is important to understand the chemical characteristics and precipitation process of the scale from geothermal fluid. The information on the process of transportation of metals and its precipitation from hydrothermal fluid in general will be useful to understand the formation of hydrothermal ore deposit. In this study, we have examined the chemical characteristics of silica scaling from the Onuma geothermal power plant at Akita Prefecture, Japan. The scale consists of mainly amorphous silica and trace amounts of smectite, kaolinite, and euhedral pyrite. Chemical composition of silica scale indicates that Fe content scale shows positive correlation with Pb, Cu, and REE. These elements probably incorporate into pyrite in silica scale. The texture of pyrite suggests that pyrite is possible to crystalize prior to the growth of amorphous silica. Silica scale gradually changes its chemical composition from the production well toward the reinjection well. Concentrations of SiO₂, Fe₂O₃, MgO, and MnO in silica scale significantly decrease toward to the reinjection well from the production well, and those of Al₂O₃, LOI, and alkali and alkali earth elements (Na₂O, K₂O, and CaO) increase toward to the reinjection well. Most of trace elements including REE in silica scale also significantly decrease toward to the reinjection well, and furthermore HREE decreases more extensively than LREE though alkali and alkali earth elements (Be, Rb, Sr, Cs, and Ba) increase toward to the reinjection well. The change of element concentration in silica scales can be utilized to understand the physical and chemical conditions in the pipes at the geothermal power plant.