ABSTRACT

Anhydrite occurs as a representative hydrothermal mineral in the upflow zone of the Kakkonda geothermal field, northeast Japan. Gas analysis and microthermometry of fluid inclusions and sulfur isotope measurements were performed for these anhydrites in order to discuss origin of the reservoir fluids and precipitation mechanism of the anhydrite. Fluid inclusions are classified into two-phase, vapor-rich and liquid-rich inclusions, and polyphase inclusions comprising liquid, vapor and solids. The vapor-rich inclusions coexist with the liquid-rich inclusions in most samples, indicating that the fluid inclusions were trapped under boiling conditions. Bulk gas analyses of the liquid-rich inclusions show that the main non-condensable gas component is CO₂ (0.14-2.0 mol %) with subordinate amounts of N₂, CH₄ and Ar. The two-phase liquid-rich inclusions homogenize to liquid phase at temperatures between 222 and 380°C, and have salinities mostly from 0 to 29 wt.% NaCl + CaCl₂ equivalent. The polyphase inclusions homogenize to liquid phase at temperatures between 302 and >480°C after dissolving halite between 104 and 220°C on heating, and have salinities from 28 to 35 wt.% NaCl + CaCl₂ equivalent. In the deep reservoir, the salinities vary over a large range at homogenization temperatures of approximately 320 to 360°C. The δ³⁴S values of anhydrites ranging mostly from +21.6 to +24.2‰ suggest that sulfur in the anhydrite from the Kakkonda field is derived from marine anhydrite. These data indicate that the hypersaline fluid was produced by exsolution from a residual magma at the time of final crystallization of the Kakkonda granite, and moderate to low salinity fluids formed by dilution of the hypersaline fluid with the heated meteoric water and boiling of the dilute fluids. The precipitation of anhydrite might be explained by three mechanisms of a simple cooling of the hypersaline fluid, boiling of the reservoir fluid and pressure drop of the non-boiling reservoir fluid.