Mineralogical, microtextural and geochemical studies have been performed on altered volcanic products from active hydrothermal areas, on phreatic tephra from the 1995 activity and on fragments from older Castle Peak dome and the present (1995--97) dome. The mineral assemblages of the hydrothermal system (primarily silica-polymorphs) are typical of the upper alteration zone of a high sulfidation system. Volatile and mobile elements chemistry show no evidence of interaction between the hydrothermal system and erupting magmas. These results suggest that the on-going eruption conduits are isolated from the hydrothermal system due to precipitation of vapor transported silica which drastically reduces the magma degassing through country rocks. ¿ 1998 American Geophysical Union