Zircon U-Pb Dating of Hydrothermal Altered Rocks for a New Stratigraphic Framework at Kawerau Geothermal Field, Taupo Volcanic Zone, New Zealand

Abstract

The exploration and successful management of hightemperature geothermal systems for electrical power generation and direct heat usage require an understanding of its geological structure and permeability pathways within the system. Central to sound interpretation of the geological structure is dating and correlating key marker horizons in order to correlate laterally permeable formations and estimate the timing and rates of movements across faults. The correlation of Taupo Volcanic Zone (TVZ) hydrothermally altered rocks is challenging, due to similarities in the lithologies, and effects of hydrothermal alteration that alter distinctive chemical, mineralogical and textural characteristics. Although correlations can be made on the basis of petrographic characteristics, such relationships may be made ambiguous by overprinting alteration. Zircons from evolved (dacite to rhyolite) rocks can be dated by U-Pb techniques, supplying a spectrum of crystallisation ages for primary magmatic material in these rocks. The spectrum places a maximum age limit on the eruption(s), or can provide an estimate of the emplacement/cooling age for intrusive rocks. A combination of age data and geological studies then permits the structural history of geothermal field to be more confidently inferred. In this paper we report new U–Pb age data from a sequence of hydrothermally altered rocks sampled from wells in the Kawerau Geothermal Field. We show how model-age data from zircons can be used to constrain interpretations of stratigraphy where conventional petrographic information might be ambiguous. The U-Pb age data have proved decisive when used with petrographic observations in deciding on an overall stratigraphic sequence. Comparisons of zircon crystallisation spectra are used to demonstrate how these correlations can be extended to regional studies.