Short Wavelength Infrared (SWIR) Spectral Characterisation of Smectite, Illite-Smectite and Illite for Geothermal Fields of the Taupo Volcanic Zone, New Zealand

Abstract

The occurrence and distribution of smectite, interstratified illite-smectite and illite in geothermal fields can be used to infer formation temperatures and the geohydrologic structure (i.e. up flow, outflow, influx zones) of the geothermal reservoir when these minerals formed. This study has qualified the spectral distinction between smectite, interstratified illite-smectite and illite by short wave infrared reflectance (SWIR) spectroscopy for cuttings from various geothermal fields of the Taupo Volcanic Zone by comparison with the results from clay separate X-ray diffraction (XRD) analyses. These clays, which have overlapping absorption features, are distinguished by their H2O / AlOH ratios calculated from both reflectance (raw) and hull quotient corrected spectra. The latter shows less scatter and overlap. In general, samples with a hull quotient corrected H2O / AlOH ratios of more than 0.96 and less than 0.76 correspond to illite and smectite, respectively. Values between 0.76 and 0.96 encompass most interstratified illite-smectite, although some illite-smectite with a high percentage of illite (i.e., more than 80% illite) or conversely a high percentage of smectite (i.e., more than 80% smectite) can plot within the illite or smectite fields, respectively. This overlap reflects the limitations of SWIR in detecting small amounts of either interstratified smectite or illite in illite-smectite. Nonetheless, despite some overlap, this SWIR spectral parameter in conjunction with careful examination of spectral profiles can be used to rapidly distinguish these clays types and their distribution in geothermal fields.