The style and intensity of hydrothermal alteration in active geothermal systems reflect the thermal regime and chemical composition of waters resulting from hydrothermal fluid-mineral interactions. This study uses an experimental approach to deduce dynamic mineral formation mechanisms in the active geothermal setting, to understand the interaction processes that produced Na-rich hydrothermal alteration zones in the East Hachimantai Geothermal Field (Japan). Hydrothermal experiments were conducted under saturated vapour pressure conditions, at 75 - 250 °C, using pyroclastic rock samples. The hydrothermal (laboratory) alteration of Tamagawa Welded Tuff did not proceed according to expected stoichiometric constraints, with respect to observed mineral formation and whole rock chemical compositions, and there is a great disparity in mineral dissolution rates and incorporation of elements into new-formed minerals. Mass transport coefficients and enrichment of certain major element in the reacted solutions resulting from hydrothermal fluid/rock interaction also varied, which lead us to interpret a new diffusion - advection -reaction rate equation that enables kinetic and mass transport parameters to be quantitatively estimated.