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

Abstract

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: Paeroa Ignim. Te Kopia Ignim. Akatarewa Ignim. Zr Yb 44.2–57.8 90.7–107.6 68.3–91.5 Zr Y 4.89–6.35 9.67–11.92 7.55–9.22 Yb Hf 0.48–0.63 0.33–0.39 0.39–0.47 La Lu 59.7–83.9 67.8–78.0 55.3–66.0 Nb Hf 1.61–2.20 1.18–1.63 1.37–1.69 P 20 5 (%) 0.013–0.040 0.060–0.075 0.070–0.15 Ta (ppm) 0.59–0.77 0.39–0.46 0.38–0.60 The Te Kopia and Akatarewa ignimbrites are also characterised by their flatter REE spidergrams, whereas the sanidine-bearing Paeroa Ignimbrite has a distinctive Eu depletion. These patterns are not clear, however, where alteration is most intense.The proportions of primary minerals present in the three ignimbrites are not diagnostic of them, and glass and their Fe-Ti oxides are very unstable in a geothermal environment. Plagioclase, hornblende and biotite were much more stable during hydrothermal alteration but only hornblende has compositions (with respect to Na, A1 iv and Ti) that are sufficiently different to distinguish between the Paeroa and Te Kopia ignimbrites.