Microthermometric evidence for the formation of Ca-rich hypersaline brine and CO2-rich fluid in the Mori geothermal reservoir, Japan

Fluid inclusions in hydrothermal quartz and anhydrite samples from the Mori geothermal field, southwest Hokkaido, have been studied microthermometrica lly using heating/freezing stages. Based on homogenization temperatures of fluid inclusions, salinities after correction of effect of freezing-point d epression of CO2 on ice melting temperatures of fluid inclusions, and previ ous geochemical data by Yoshida (1991), we discuss the geneses of various t ypes of inclusion fluids in the Mori geothermal reservoir. Fluid inclusions are divided into four types based on phase relations at ro om temperature and on cooling: (A) liquid-rich, low- to moderate-salinity, (B) vapor-rich, (C) vapor- and CO2-rich, low-salinity (less than or equal t o 3.8 wt % NaCl eq.) and (D) polyphase. Ca-rich hypersaline (CaCl2/NaCl = 2 .2 to 2.9 and 29 to >42 wt % NaCl+CaCl2 eq.). Type D inclusions invariably coexist with type B inclusions. The compositions of fluid inclusions and th e relationship between total salinities and homogenization temperatures of fluid inclusions reveal that the geneses of various types of the inclusion fluids in the Mori geothermal system are summarized as follows. At the earl iest stage of fluid evolution in the system, Ca-rich hypersaline brines had been produced with vapor and CO2-rich low-salinity fluids during phase sep aration due to boiling of an initial homogeneous hot seawater conductively heated by an unknown deep-seated magmatic heat source. The heat source migh t be related to volcanic activity which formed the Nigorikawa caldera in th e deeper pans. Afterwards, low-salinity aqueous fluids derived from the mix ing of seawater and meteoric water have been dominant in the geothermal res ervoir since the recent natural state stage before: operation of the Mori g eothermal power plant in 1982. Fluid pressure has dropped approximately sev eral tens bars throughout fluid evolution in the system.