Geochemistry of Buried Rhyolite Lavas, Western Steamfield, Wairakei Geothermal Field, Taupo Volcanic Zone, New Zealand

Abstract

Fractured rhyolite lava domes and flows provide geothermal production and injection targets at 500 – 1500 m depth in the Wairakei geothermal field (New Zealand). Differentiating between the rhyolites will resolve an important part of the Wairakei stratigraphy, that is of regional significance for the geologic history of the Taupo Volcanic Zone, and may also have applications for geothermal well targeting. To date, several buried rhyolite bodies have been intersected by drilling. Karapiti (IIa, IIb and III) Group rhyolites are stratigraphically separated by pumiceous pyroclastics and sediments of the Wairoa Formation from recently discovered, deeper Poihipi (I, II) Rhyolites. All rhyolites are spatially clustered in the western part of the geothermal field beneath the Te Mihi sector and were erupted during the time span of the Waiora Formation, which post-dates the ~330 ka Wairakei Ignimbrite (Whakamaru Group). The Karapiti and Poihipi rhyolites are calc-alkaline with homogenous major and trace element compositions. At Wairakei, there is a general increase of hydrothermal alteration intensity with depth where K, Na and Ca gain is geochemically observed and attributed to adularia, albite, calcite, illite, epidote and wairakite precipitation. The REE distribution patterns, especially the magnitude of the Eu anomaly, can distinguish Karapiti from Poihipi rhyolites (Eu/Eu*= 0.21-0.34 for Karapiti; Eu/Eu*= 0.17-0.19 for Poihipi). Such differences can be explained by variable plagioclase ± amphibole crystal fractionation. Age determination of each rhyolite group, using zircon U/Pb isotope systematics, will provide valuable insights into the temporal and chemical evolution of rhyolitic magmatic activity at Wairakei that occurred post-eruption of the Whakamaru Group ignimbrites, but prior to deposition of the pre-26.5 ka Huka Falls Formation lake deposits.