Mineralogical examination of basaltic rocks obtained during drilling of a successful 1,962-m-deep geothermal well (HGP-A) in the east rift zone of Kilauea Volcano, Hawaii, reveals three zones of hydrothermal alteration beneath a zone of unaltered lavas. Each alteration zone is characterized by the dominance of a particular mineral: zone 1, montmorillonite; zone 2, chlorite; zone 3, actinolite. Three zones of relative permeability can be tentatively identified on the basis of filled versus partly filled vesicles and fractures. Because the well has not returned to thermal equilibrium as of this writing, stability relations between secondary minerals and temperatures cannot be calculated with accuracy. However, the latest downhole temperatures measured at the boundaries of alteration zones, compared with similar data from high-temperature geothermal areas in Iceland, indicate that HGP-A temperatures are considerably higher than those encountered for the same alteration-zone boundaries in Icelandic wells. This indicates that the present-day thermal regime in HGP-A is relatively young and that the hydrothermal minerals probably have not reached equilibrium with existing conditions.