Abstract—This paper discusses patterns that are observable in the alteration of effusive rocks that were discharged by Baranskii Volcano (central Iturup Island) under the action of sulfate chloride as well as acidic and ultra-acidic water (in the Kipyashchaya Rechka thermal brook). We acquired data on changes in the chemical and mineralogic composition of the rocks, structural features, porosity, and petrophysical properties. The dynamics of leaching and the leaching phase in a flowing acidic (ultra-acidic) geothermal environment are described. We note that the mechanism that is responsible for hydrogen sulfate leaching of rocks at the ground surface may be largely analogous to the generation of secondary quartzites (mono-quartzites) in the zones of ascending acidic gas flows above small gabbro–diorite and diorite intrusions.