Hydrothermal alteration minerals and their zoning with depth in the Te Mihi sector of the Wairakei geothermal field have been characterised in this study by using field portable short-wave infrared spectroscopy. Observed variations in the spectral reflectance of hydrothermally altered volcanic and sedimentary rocks indicate a downward gradation in the dominant alteration minerals from mordenite, smectite, illite, to illite + chlorite (epidote). The variations result from temperature and permeability controlled fluid-rock interactions. Mordenite is abundant in the shallowest samples. Dioctahedral, aluminous smectites (discrete phase or as mixed-layer species) are common in the upper part of the sequence, but decrease in abundance below 200 m. In the smectite-rich zone, beidellite is common and locally may predominate over montmorillonite. Illite, including the illite component in mixed-layer illite/smectite, becomes a significant component below 150-200 m. Illite has an octahedral cation composition close to that of muscovite, with insignificant amounts of Fe and Mg. Chlorite is nearly undetectable in the upper part (0-200 m) of the altered sequence, and may become a minor component below 200 m. Chlorite is abundant only in andesitic rocks. Chlorite from the lower alteration sequence is Fe-rich, probably with the Mg# values around 0.2-0.4. Epidote is abundant locally at depth. (C) 2000 CNR