Characterisation and Stratigraphic Correlation of the Andesites Encountered in the Wairakei Geothermal Field

Abstract

Active volcanism in the Central Taupo Volcanic Zone (TVZ) during the past ~1.8 Ma has changed from predominantly andesitic to silicic composition, with widespread rhyolitic volcanism dominating the Central TVZ by ~0.34 Ma. There are few surficial outcrops of andesites in the Central TVZ, although buried andesites have been encountered by drillholes at several TVZ geothermal fields, including 30 geothermal wells in the Wairakei Geothermal Field. Extrusive lavas of the Waiora Valley Andesite (WVA) occur at relatively shallow depth (i.e., less than 1 km depth) in the Western Borefield (WBF) at Wairakei, with deeper andesites (i.e., more than 1.5 mRL) in drillholes to the northwest and southeast of the WBF. In thin section, the WVA is distinguished from the deep andesites by two plagioclase populations (dominated by large, sieved plagioclase), and large plagioclase-pyroxeneamphibole glomerocrysts. In contrast, the deep andesites include rare amphibole, and plagioclase-only glomerocrysts within their mineral assemblage. In geochemical composition, the WVA is also distinct from the two deep andesites analysed, and points to separate magmatic sources for the WVA and the deeper Wairakei andesites. This has important implications for the timing and source of geothermal heat input to the Wairakei geothermal systems and evolution of the Central TVZ. Further work is required to constrain the emplacement mechanism(s) and timing of the deep Wairakei andesites, and their relationship both to each other, and to the nearby Spa Road andesite and Tauhara dacite.