| Title | Rhyolite of the Tauhara geothermal field: insights and correlations from geochemistry |
|---|---|
| Authors | M.P. Simpson, M.D. Rosenberg, M.F. Gazley, F. Sepulveda, N. de Groot, S.A. Alcaraz, L.B. Carson |
| Year | 2023 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Rhyolite, portable X-ray fluorescence, pXRF, geochemistry, Zr, Y, Ti, geothermal, Tauhara, Wairakei. |
| Abstract | Since 1965, numerous subsurface rhyolite lavas have been drilled at the Tauhara Geothermal Field, Taupo Volcanic Zone. These, and additional rhyolite lavas, were intercepted in 12 geothermal wells drilled from 2019–2022 for the Contact Energy Tauhara II (174 MWe) power development project. Some rhyolites had already been characterised and formally named (e.g., Racetrack Rhyolite), but others were only affiliated to broad groups based on phenocryst type and abundance. To validate stratigraphic correlations, portable X-ray fluorescence (pXRF) analysis was trialled during the drilling campaign to geochemically ‘fingerprint’ the rhyolites by their immobile element concentrations (i.e., elements unaffected by hydrothermal alteration). Of the elements robustly quantified by pXRF, only Zr, Y, and Ti were useful to distinguish the rhyolites. Since concentrations of Zr, Y, and Ti overlap it is typically not possible to identify a given unit exclusively based on chemistry with few exceptions (e.g., Tauhara Moana rhyolite). However, by evaluating pXRF geochemical data combined with spatial and stratigraphic information, plus mineralogy, this technique was able to confirm previous correlations for several rhyolites. It also distinguished rhyolite lavas that are not the same despite their spatial proximity and similar mineralogy (e.g., Racetrack and Kauri rhyolites) resulting in new correlations. |