Mineral Precipitation from the Saline Geothermal Water of the Fushime Area, Kyushu, Japan: the Precipitation Process of Anhydrite

Abstract

Fluids in the Fushime geothermal system, Kyushu, Japan, are high temperature (exceeds 300?C) and saline, which most likely originated from seawater-rock interaction. Anhydrite(scale) has been found in a specific zone of a production well where sulfate-rich low temperature water is assumed to mix with ascending high temperature sulfate-depleted water. To investigate the precipitation process of anhydrite, chemical compositions of the mixing end members were estimated and changes of chemical speciation with the mixing and boiling processes were calculated. The results show that the mixed waters are supersaturated with anhydrite only above 240*C, and this temperature range agrees with homogenization temperatures of fluid inclusions trapped in anhydrite(scale) sampled from the well. This agreement indicates that the mixing model used here is reasonable in explanation of the precipitation process of anhydrite in the well .