GRC Transactions, Vol. 38, 2014 Overprinting Hydrothermal Systems in the Taupo Volcanic Zone Sarah D. Milicich1, Isabelle Chambefort2, Greg Bignall2, and John Clark3 1GNS Science, Lower Hutt, New Zealand 2GNS Science, Wairakei Research Centre, Taupo, New Zealand 3Mighty River Power Ltd, Rotoru, New Zealand [email protected] Keywords and evolution. Numerical modelling (e.g., Kissling and Weir, New Zealand, Taupo Volcanic Zone, Kawerau, geothermal 2005; Kissling et al., 2009) suggest TVZ geothermal systems system, longevity, heat source, stable isotopes, fluid inclusions have lifetimes similar to that of the TVZ (i.e., ~2 Myr), with the heat fed from depths of ~8 - 10 km (i.e., below the inferred brittle- ductile transition: Bryan et al., 1999). These studies suggest the Abstract hydrothermal systems are fixed in their geographic position, but are not positioned above a localised heat source. Rather, heat driv- Numerical reservoir modelling point to geothermal systems ing the hydrothermal systems results from deep ‘under-plating’ in the Taupo Volcanic Zone (TVZ, New Zealand) having life- of magma. Recent work undertaken at the Kawerau geothermal times similar to that of the TVZ itself, with heat fed from below system (Fig. 1), where age data can be used to pin the geological the brittle-ductile transition in the TVZ crust (~8-10 km). The history, suggests that the lifetime of individual high-temperature models suggest the geothermal systems are relatively fixed in hydrothermal systems are characterised by episodic thermal geographic position, but are not positioned above a localised events, impacting on the order of tens of thousands of years, heat source. Recent work however, suggests localised magmatic interspersed with periods of quiescence or relative inactivity. intrusions playing a major role in providing heat to TVZ high-temperature hydrothermal systems, with individual hydrothermal systems active for tens of thousands of years. The study of hydrothermal processes and source fluids provides an indication of the evolution of heat source(s) associated with a geothermal system. Traditional petrological techniques, combined with hydrothermal alteration studies, stable isotopic tracers and geochronology can resolve the nature and composition of the fluids involved in the hy- drothermal processes and how these might change through time. By mapping hydrothermal mineral occurrences and textural relationships at Kawerau geothermal system, we provide new insights into its thermal and chemical evolution and evidence of distinct hydrothermal events, including inferred input of magmatic-derived fluids and associated with thermal input, over the lifetime of the system. 1. Introduction Drilling of high-temperature geothermal systems Figure 1. Left: Locality map for the Taupo Volcanic Zone (TVZ) with the Kawerau geothermal in the TVZ for electricity generation has provided the system. Right: location of wells in the Kawerau geothermal system, with wells sampled for opportunity to investigate their geological structure stable isotopes highlighted. 511 Milicich, et al. A combination of age data, alteration mineral petrography and Of these identified episodes of local magmatism, only those geochemical techniques provide evidence of geographical coin- associated with the latest Quaternary Putauaki and the 0.43–0.36 cident, though temporally distinct hydrothermal systems. For the Ma Tahuna/Caxton formations have left clear influence on the purpose of this paper, we follow the definition of Bates and Jackson geothermal system in the form of hydrothermal eruption breccias, (1987), whereby “geothermal” pertains to the heat of the interior of hydrothermal alteration and magmatic fluids. The surficial Onepu the earth, and “hydrothermal” refers to the action of hot water and Formation domes and their feeder dikes are inferred to represent products of this action, such as mineral precipitation. Consequently, a single isolated event, based on age data (Milicich et al., 2013), whilst acknowledging likely differences in opinion amongst the and thus may not have been accompanied by any significant or geothermal science community, in this paper we will consider a sustained thermal pulse at the surface. It is also unlikely, that the “hydrothermal system” as an individual convecting hydrothermal magmatism from which the Kawerau Andesite was derived fuelled cell, isolated in time and related to a specific heat source. a geothermal system, as andesite composite cones of similar di- mensions in the TVZ tend to not have large or long-enough lived 2. Setting magmatic sources at shallow depths to fuel geothermal systems (e.g. Rolles Peak, Pihanga). The TVZ (Fig. 1) is the southern, continental termination of However, periods of thermal activity at Kawerau could be the Tonga-Kermadec arc, and is associated with the oblique west- related to magma not yet encountered by drilling or to the emplace- ward subduction of the Pacific plate under the Australian plate. ment of magmatic intrusions at depths too deep to be encountered The TVZ has evolved over the last 2 M.y. and for the last ~350 by drilling that have never had any related surface expression. For ka has been a structurally and magmatically segmented rifting arc system (Rowland and Sibson, 2004; Rowland et al., 2010). The central portion of the TVZ is dominated by vigorous rhyolitic caldera volcanism, and containing most of the high-temperature geothermal systems, whilst the northern and southern segments are dominated by andesite-dacite composite-cone volcanism with localised geothermal systems (Bibby et al., 1995; Wilson et al., 1995). The Kawerau geothermal system (Fig. 1) is the most north- eastern of the active high-temperature geothermal systems in the TVZ (Bibby et al., 1995; Rowland and Sibson, 2004; Kissling and Weir, 2005; Rowland and Simmons, 2012). Kawerau occurs in an area where normal faulting of the TVZ rift interacts with the domi- nantly strike-slip faulting of the North Island Shear Belt (Begg and Mouslopoulou, 2010; Villamor et al., 2011). The Kawerau geothermal system is hosted in Mesozoic greywacke basement, overlain by a ~1 km sequence of volcanic and volcano-sedimentary rocks. Although many of the primary volcanic units are large ignimbrites and tuffs sourced from outside of the Kawerau area, there have been several periods during the geological record when magmatism occurred directly beneath Kawerau. 3. Key Factors Constraining the Evolution of Kawerau Geothermal System 3.1 Proximal Magmatism There have been at least four periods in the geological record when magma has been present beneath the Kawerau area (Milic- ich et al., 2013a), which provide direct evidence for distinct heat sources beneath the evolving geothermal system (Fig. 2). These are: (i) the Kawerau Andesite, with bracketing ages of 0.60 and 1.0 Ma (Milicich et al., 2013b); (ii) buried rhyolite lavas and intrusions, plus the slightly older locally source tuffs of the Ta- huna Formation, dated at least 0.36 and 0.43 Ma (Milicich et al., 2013b); (iii) domes and dikes of the 150 ka rhyo-dacitic Onepu Formation (Milicich et al., 2013b); and (iv) andesite-dacite domes of Mt Putauaki, with ages between 2400 and 8350 years (Carrol et Figure 2. Representative stratigraphic column from the Kawerau geother- al., 1997), which have been inferred to be the current heat source mal system. Arrows indicate periods where a magmatic source has been of the hydrothermal system (Bignall, 2010). directly beneath the area. 512 Milicich, et al. example, the feeder network that connects the Onepu Formation 3.3 Stable Isotopes intrusives with the surface domes has not been encountered despite Oxygen stable isotopes were measured in calcite and vein the large amount of drilling carried out in the area. quartz in samples from across the Kawerau geothermal system (Table 1). Taking a temperature range (±10 ºC) from primary 3.2 Hydrothermal Eruption Breccia fluid inclusions, where available (Milicich, 2013), or interpreted The presence of hydrothermal eruption breccia in the natural state well temperature (MRP, 2013) as representative of the geological record at Kawerau provides direct evidence of a thermal conditions of isotopic equilibration, enabled the isotopic convecting hydrothermal system that facilitate heated fluids ratio of the fluids in equilibrium with the hydrothermal quartz and moving towards the surface. Two periods of hydrothermal erup- calcite to be calculated (Table 1 and Fig. 3; using the fractionation tion activity have been recognised at Kawerau. The oldest is factors from Zheng (1993) and O’Neil et al. (1969) respectively). recognised in well KA25 from core recovered below the 0.36 The fluid in equilibrium with hydrothermal quartz and calcite (Fig. Ma Caxton Formation rhyolite dome. This is recognised in thin 3) are typically in range of modified meteoric water, implying section where a greywacke fragment within a breccia is cut by that at the time of crystallisation conditions were similar to the wairakite and prehnite veins, in an alteration assemblage that modern geothermal system. In contrast, some samples, primarily is not in equilibrium with reservoir conditions at this depth. A from wells in the south of the system, show enrichment in 18O second period of hydrothermal eruptive activity is represented (Fig. 3), which can indicate an input of magmatic derived oxygen. by surface breccias mapped by Nairn and 18 13 18 13 Table 1. Measured δ OVSMOW and δ CVPDB plus δ OH2O and δ CH2CO2 calculated in equilibrium Wiradiradja