Aspects of the Chronology, Structure and Thermal History of the Kawerau Geothermal Field

Abstract

<p>The development and management of high-temperature geothermal resources for electrical power generation requires accurate knowledge of the local geological conditions, particularly where they impact on the hydrology of the resource. This study is an integrated programme of work designed to develop new perspectives on the geological and structural framework of the Kawerau geothermal resource as a sound basis for field management. Although the geological approaches and techniques utilised in this study have previously been used, their application to an integrated study of a geothermal system in New Zealand has not been previously undertaken. Correlating volcanic and sedimentary stratigraphy in geothermal areas in New Zealand can be challenging due to similarities in lithology and the destruction of distinctive chemical, mineralogical and textural characteristic by hydrothermal alteration. A means to overcoming these issues is to utilise dating to correlate the stratigraphy. Zircons are resistant to the effects of typical hydrothermal conditions and were dated using SIMS techniques (SHRIMP-RG) to retrieve U–Pb ages on zircons. These age data were then used to correlate units across the field, in part aided by correlations to material that had previously been dated from fresh rock by ⁴⁰Ar/³⁹Ar techniques, and used to redefine the stratigraphic framework for the area. [...] Although previously inferred to be a long–lived system, the modern Kawerau Geothermal Field is a Holocene entity reflecting the rejuvenation of magmatic heat flux associated with Putauaki volcano superimposed on an area of multiple reactivated fault structures, sporadic magmatism and variable rates of subsidence. This study documents past patterns of fluid flow, temperatures and chemistry, and inferred permeability within the field. Using textural relationships in selected samples, the relative timing and patterns of hydrothermal alteration, and fluid flows can be established. These textural relationships are then calibrated against fluid inclusion palaeotemperature measurements and isotope data and related to temperatures and compositions of past fluids. Short–lived heat sources beneath the field resulted from local magma intrusions, and are responsible for the 0.36 Ma and 0.138 Ma rhyolites and Holocene eruptive activity of Putauaki andesite–dacite volcano. The Putauaki activity is inferred to be responsible for the thermal and alteration characteristics of the modern system.</p>