IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1A1_1E-O4

Room A3

Date/Time: July 21 9:30-9:45

U-Pb dating of zircon as an aid in interpretation of geothermal systems: a case study
from the Kawerau Geothermal Field, New Zealand
Sarah D Milicich1 , Colin JN Wilson1 , Greg Bignall2 , Ben Pezaro3 , John Clark3
1

SGEES, Victoria University of Wellington, Wellington, New Zealand, 2 GNS Science, Wairakei
Research Centre, Taupo, New Zealand, 3 Mighty River Power Ltd, Auckland, New ZealandE-mail: s.milicich@gns.cri.nz

Exploration and development of geothermal resources for electrical power generation and direct heat use requires
an understanding of the geology that hosts the geothermal system. Key to this understanding is being able to dateand correlate marker horizons to link permeable formations, constrain displacement history of faults and time thepresence of local heat sources.

The Kawerau Geothermal Field is the most northeasterly of high-temperature geothermal systems in theTaupo Volcanic Zone (TVZ, New Zealand), an active Quaternary arc locus of rhyolitic volcanism and thermaloutput. The reservoir rocks are a 1 km thick pile of lava, pyroclastic rocks and sediments, supplied with fluid
circulating through faulted Mesozoic basement greywacke. Here, where correlation solely by petrographic andtextural observations was ambiguous due to hydrothermal alteration, we show the value of U-Pb age information oninterpreting geothermal systems. U-Pb age determinations on zircons from extensively altered lithologies provide
estimates of crystallisation and eruption ages that allow depositional and faulting histories of rocks hosting thegeothermal system to be reconstructed. These histories may be linked to evidence for episodic local magmatism
beneath the field and to changes in regional tectonic stress patterns.

Prior to the 0.32 Ma Matahina ignimbrite (a regional marker plane), deposition of ignimbrites occurred in
short-lived episodes around 1.45, 1.0, and 0.55-0.6 Ma, separated by thin sediment-dominated intervals whichaccumulated at average rates of 0.06 mm/yr. The ignimbrites represent marker horizons from other volcanic
centres and do not reflect the presence of local magmatic heat sources. Net subsidence rates inferred from
depths to these marker planes do not reflect the present-day situation. Modern rates of subsidence (2+/-1 mm/yr)
associated with TVZ rifting processes can have been active for no more than 50,000 years in the area of the field,
based on elevation differences of the Matahina ignimbrite top surface.

Bodies of coherent rhyolite (i.e., lava or intrusive material) occur at multiple intervals in the volcanic/sedimentary
cover and basement greywacke. Age dating resolves these multiple bodies as either domes or intrusions: twopetrographically contrasting magma types with associated tuffs were emplaced as domes and sills at 0.36+/-0.03
Ma, and a third type at 0.138+/-0.007 Ma as dikes, and domes exposed at surface. Heat sources beneath thefield resulted from these local magma intrusions, together with that feeding the Holocene eruptive activity of
Putauaki andesite/dacite volcano. Of these, the last is responsible for the thermal and alteration characteristicsof the modern field. We infer that only at long-spaced intervals is there evidence of magma at shallow enoughcrustal levels beneath the field to generate vigorous hydrothermal activity (as at the present day); at other times noKawerau geothermal system’ existed.
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