An infrared spectral reflectance study of hydrothermal alteration mineralsfrom the Te Mihi sector of the Wairakei geothermal system, New Zealand

Hydrothermal alteration minerals and their zoning with depth in the Te Mihi sector of the Wairakei geothermal field have been characterised in this st udy by using field portable short-wave infrared spectroscopy. Observed vari ations in the spectral reflectance of hydrothermally altered volcanic and s edimentary rocks indicate a downward gradation in the dominant alteration m inerals from mordenite, smectite, illite, to illite + chlorite (epidote). T he variations result from temperature and permeability controlled fluid-roc k interactions. Mordenite is abundant in the shallowest samples. Dioctahedr al, aluminous smectites (discrete phase or as mixed-layer species) are comm on 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 predomina te over montmorillonite. Illite, including the illite component in mixed-la yer illite/smectite, becomes a significant component below 150-200 m. Illit e has an octahedral cation composition close to that of muscovite, with ins ignificant amounts of Fe and Mg. Chlorite is nearly undetectable in the upp er 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 t he lower alteration sequence is Fe-rich, probably with the Mg# values aroun d 0.2-0.4. Epidote is abundant locally at depth. (C) 2000 CNR Published by Elsevier Science Ltd. All rights reserved.