Chemical process and environment of hydrothermal alteration of acidic volcanic rocks in the Mitsuishi district, southwest Japan.

The Mitsuishi district, noted for the largest Roseki production in Japan, is generally characterized by the alteration zoning from center to margin; Quartz zone → Roseki ore (Pyrophyllite, Pyrophyllite-sericite and Sericite zones) → K-feldspar-sericite zone → Albite zone. They are overlain by Weakly propylitized zone. The filling temperatures of fluid inclusions in euhedral quartz crystals indicate the hydrothermal solution was about 300–350°C for the formation of Quartz zone. The chemical composition of the hydrothermal solution was estimated from salinity of the fluid inclusions, analytical results of altered rocks and minerals (feldspars and sericite), thermochemical calculations on the K-feldspar-albite and muscoviteparagonite solid solutions and equilibria in the Na2O-K2O-SiO2-Al2O3-H2O-HCl system, and mobility calculation on the water-rock interaction process. It is deduced that pH dramatically increased, while K and SiO2 concentrations decreased, and Na concentration remained relatively constant in the hydrothermal solution from center to margin of the alteration zones.