An infrared spectral reflectance study of hydrothermal alteration minerals from the Te Mihi sector of the Wairakei geothermal system, New Zealand
✍ Scribed by Kai Yang; Jonathan F. Huntington; Patrick R.L. Browne; Chi Ma
- Publisher
- Elsevier Science
- Year
- 2000
- Tongue
- English
- Weight
- 420 KB
- Volume
- 29
- Category
- Article
- ISSN
- 0375-6505
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✦ Synopsis
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