Short-wave infrared (SWIR) reflectance spectrometric characterisation of clays from geothermal systems of the Taupō Volcanic Zone, New Zealand

Many hydrothermal minerals including clays show zoned distributions in geothermal systems that are useful for inferring formation temperatures, fluid composition and the geohydrologic structure (i.e. upflow, outflow, influx zones) of the geothermal reservoir at the time these minerals formed. This study investigates the use of short-wave infrared (SWIR) reflectance spectroscopy to identify clay minerals in geothermal systems of the Taupō Volcanic Zone (TVZ). The study also recognises and documents that volcanic glass, a common constituent in unaltered and weakly altered volcanic rocks and reworked equivalents, and amorphous silica also have a spectral signature that overlap with some clay minerals (i.e. smectite). The spectral distinctions between smectite, mixed-layered illite-smectite and illite have been determined by calibrations of the calculated SWIR H2O/Al-OH depth ratio against clay-separate X-ray diffraction (XRD) mineral identification and interlayering quantification. In general, a hull quotient corrected H2O/Al-OH depth ratio of <0.76 and>0.96 broadly correspond to smectite and illite, respectively; however, there is overlap. In the case of mixed-layered illite-smectite it is not possible to quantify the amount of interlayering apart from making a generalisation that it is either smectite-rich or illite-rich. Care needs to be taken when evaluating the H2O/Al-OH depth ratio since it can be perturbed by spectrally overlapping kaolinite, volcanic glass or silica. Nonetheless, in the case examples illustrated here for four geothermal wells, the H2O/Al-OH depth ratio value was successfully used to determine clay rank that closely parallels those determined by clay-separate XRD analyses.