Geochemical characterisation of hydrothermally altered ignimbrites in active geothermal fields from the central Taupo Volcanic Zone, New Zealand

Three subsurface ignimbrites at the active Orakeikorako geothermal field can be distinguished from one another despite their having been hydrothermally altered. They can also be recognised in drillcores from five other geothermal fields up to 18 km distant from Orakeikorako. “Fingerprinting” of the three Quaternary ignimbrites was attempted by comparing their petrographic, mineralogical and chemical compositions. Analyses of 43 samples shows that a combination of the following parameters can be used to characterise the ignimbrites: