An infrared spectral reflectance study of hydrothermal alteration minerals from 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 ®eld have been characterised in this study by using ®eld portable short-wave infrared spectroscopy. Observed variations in the spectral re¯ectance 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 ¯uid-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 signi®cant component below 150±200 m. Illite has an octahedral cation composition close to that of muscovite, with insigni®cant 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 0375-6505/00/$20.00 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserved. PII: S0 37 5 -6 50 5 (0 0) 00 0 04 -3 Geothermics 29 (2000) 377±392 * Corresponding author. Tel: +61-2-9490-8666; fax: +61-2-9490-8921. E-mail address: k.yang@dem.csiro.au (K. Yang). 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. 7 2000 CNR. Published by Elsevier Science Ltd. All rights reserve