Geochemical characteristics of silica scales from geothermal fluid at the Onuma geothermal power plant at the Hachimantai-Yakeyama area, northeast Japan

Abstract

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 montmorillonite, kaolinite, and euhedral pyrite. 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. 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.