| Title | Insights Into Geothermal Systems from U-Pb Dating of Zircons in Hydrothermally Altered Rocks: the New Zealand Experience |
|---|---|
| Authors | C.J.N. WILSON, G. BIGNALL, I. CHAMBEFORT, J.W. COLE, B.L.A. CHARLIER, A.A. EASTWOOD, D.M. GRAVLEY, T.R. IRELAND, S.D. MILICICH, B. PEZARO, M.D. ROSENBERG, F. SEPULVEDA, J.L. WOODEN |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Taupo Volcanic Zone, zircon, U-Pb dating, geothermal systems |
| Abstract | A key element in optimal exploration and utilisation of geothermal systems is an accurate picture of their geological and structural history. The understanding of field architecture and strategies for exploration and reservoir management depend to a large extent on quantifying the location and duration of magmatic and tectonic events through geochronology. At several developed geothermal fields in New Zealand, constraints on these events have been achieved by U-Pb dating of magmatic zircon in rocks penetrated by geothermal drillholes using secondary ion mass spectrometry (SIMS) techniques. As well as underpinning magmatic and structural chronologies, the data have constrained the likely timing of heat and fluid contributions, the creation and disruption of permeability, and provided, verified or disproved stratigraphic correlations. New Zealand’s high temperature ( more than 250 degrees C fluid) geothermal systems lie within the Taupo Volcanic Zone (TVZ), a rifting arc system active since ~2 Ma. Most systems are located in the central TVZ, and are characterised by the products of prolific rhyolitic and subordinate andesitic volcanism which unconformably overlie a faulted Mesozoic metasedimentary (greywacke) basement. Tectonic controls on geothermal system location and hydrology linked to caldera boundaries and fault systems of TVZ are relatively well understood on a regional scale. Many system-scale studies have also provided insights through integration of geological and geophysical datasets, but until recently these have lacked a quantitative time scale. Provided that primary magmatic zircon is present in the rock (i.e. an intermediate or silicic protolith) and that the quality of the sample is constrained (core=good; cuttings = challenging), time scales can be generated and integrated into such studies. We highlight four examples here. At Mangakino, age data served to demonstrate that the rocks in the lowest 2/3 (~1.8 km) of the drilled stratigraphy were the intracaldera products of a linked pair of eruptions at ~1.0 Ma, indicating that the basement greywacke target was at unreachable depths. At Kawerau, age data have led to a complete revision of the geological and structural history, leading to creation of a new geological model for field management. At Ngatamariki, the age data have demonstrated the minimum age for the earliest andesites ( more than 1.9 Ma) and the onset of TVZ rifting and silicic volcanism in its present-day position, and provided reliable intrusion ages of the Ngatamariki intrusive complex. At Wairakei/Tauhara, the age data have demonstrated a major hiatus in activity from ~0.95 to ~0.35 Ma, and the extraordinarily rapid subsidence and infilling rates (~1 cm/yr) associated with rocks in the production areas. Use of age data in combination with detailed petrographic observations has proved to be critical in understanding field geology in all four cases (and others). |