Volcanic evolution of the Huka Group at Wairakei-Tauhara Geothermal Field, Taupo Volcanic Zone, New Zealand

Abstract

Basin-hosted stratigraphy in volcanic arc settings reflects the interplay between ancient environments, volcanism, magmatism and tectonism. Lithostratigraphic variations within basins can be used to identify the location and timing of the processes contributing to their evolution. However, when deposits are hydrothermally altered, the use of many traditional analytical techniques for assessing their volcanic origin become impracticable, making analysis challenging. Examination then relies on an integrated mix of detailed macroscopic assessment and techniques utilising remaining stable magmatic phases. The Huka Group at Wairakei-Tauhara Geothermal Field (Wairakei-Tauhara) is primarily comprised of volcanic deposits preserving ~300 kyr of evolution in the Taupo Volcanic Zone (TVZ), New Zealand. Intensive geothermal well drilling in the field has identified the distribution and variation comprising its Waiora and Huka Falls Formations. The volcanic, structural and environmental history of the Huka Group, however, remains poorly understood. This thesis is concerned with identifying the stratigraphic and geothermal significance of the Huka Group from recent drill core samples at Wairakei-Tauhara. Drill core facies analysis confirm a spatially and temporally complex depositional history at the site. Deposits forming Waiora Formation were sourced from local explosive and effusive eruptions over ~100 kyrs within extensional basins hosting paleo-Lake Huka. Lacustrine and fluvial deposition prevailed for the following ~200 kyrs, as volcanism ceased, depositing the Huka Falls Formation. Frequent drilling of Huka Falls Formation has identified and thoroughly constrained facies variations of a local pyroclastic member, the Middle Huka Falls Formation. This eruption evolved as a series of water-supported, eruption-fed density currents from a sublacustrine vent in Tauhara transported beneath Lake Huka. Examined Huka Group core samples were hydrothermally altered and required the use of novel assessment techniques for comprehensive stratigraphic assessment. This alteration provided an opportunity to locally date the geothermal system within the Huka Group reservoir. Stratigraphic variations of resistant magmatic phenocrysts (feldspar) and immobile elements (Ti, Zr, V and Y) added new details of depositional processes and lithostratigraphy. Regional magmatic immobile element comparisons identified geochemical similarities within Huka Group ignimbrites that may have implications for the longevity and recurrence of caldera magma systems in TVZ. Geothermal activity in the Waiora Formation reservoir was dated using pristine hydrothermal adularia and 40Ar/39Ar dating methods. Results recognised a young phase of the system’s evolution (<30 ka) and the applicability of 40Ar/39Ar dating for use in geothermal chronology. Lastly, a conceptual evolutionary model for the Huka Group presents ~300 kyr of depositional processes, landscapes and structural events at Wairakei-Tauhara. The long-lived lacustrine setting is recognised to have been continually modified by episodic volcanism and gradual tectonism. Variations in Huka Group stratigraphy between the Wairakei and Tauhara Fields identify contemporaneous, but separate evolution of the underlying controlling horst (ridge) and graben (basin) structure. This study highlights the unique tectonic, magmatic, volcanic and sedimentary processes forming basins in the TVZ and improve our understanding on the geological evolution of geothermal systems. Techniques trialled in the study are demonstrated to be suitable for investigating altered volcanic materials and can be utilised elsewhere in the TVZ or other geothermal settings.