Major chemical constituents in thermal w aters from thirty nine wells drilled into deep aquifer of the central Kanto Plain were analyzed to discuss water-rock interaction to make chemical properties and flow system of deep fluid. We discussed, moreover, application of the silica geothermometers and water temperature to estimate deep fluid temperature in the field.
Water temperature is useful to estimate deep fluid temperature because estim ated recovery temperature is close upon water temperature more than silica geothermometer. The thermal waters from the impermeable Pre-Neogene basement rocks such as the Sanbagawa, Chichibu and Shimanto belts belong to Na-Cl and Na-HCO₃ types. The deep fluid in the basement rocks are probably flowing along faults developed in the Kanto mountains. On the other hand, the thermal waters in Neogene and Quaternary sedimentary rocks such as Kazusa and Miura Groups belong to mainly Na-Cl, subordinately Na-HCO₃ types. The two-dimensional distributions of seawater fraction in the thermal waters and water temperature indicate that the deep fluid seems to be formed by mixing of the deep seated fossil seawater heated to approximately 40 °C with the heated local meteoric water infiltrated from the Kanto, Ashio and Yamizo mountains. The deep fluids are supersaturated with Mg-montmorillonite and kaolinite, and these compositions are mainly controlled by the following mechanisms: (1) Mg-ion exchange of Na-montmorillonite (2) formation of kaolinite by weathering of plagioclase.

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