GRC Transactions, Vol. 38, 2014

Overprinting Hydrothermal Systems in the

Sarah D. Milicich1, Isabelle Chambefort2, Greg Bignall2, and John Clark3 1GNS Science, Lower Hutt, 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, , 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.

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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 (1980), which were deposited with Kawerau quartz and calcite samples at tabulated temperature ranges. around ~16 ka and 9 ka (constrained by tephra ages from Lowe et al., 2013). Depth 13 18 18 13 Well Mineral δ CVPDB δ OVSMOW Temperature δ OH2O δ CH2CO2 (mRF) (‰) (‰) range (ºC) (‰) (‰) 3.3 Hydrothermal Alteration Hydrothermal alteration at Kawerau KA21 262 quartz 8 220 to 240 -2.5 to -1.5 manifests as a series of overprinting thermal- KA21 475 quartz 6.5 250 to 270 -2.5 to -1.6 chemical events, with wairakite-prehnite alteration overprinted in the south of the KA21ST 765 quartz 5.5 270 to 290 -2.6 to -1.9 geothermal system by extensive calcite pre- calcite -8.4 2.3 270 to 290 -4.2 to -3.6 -6.8 to -6.5 cipitation. These earlier events are overprinted by the modern alteration assemblage charac- KA21 829 quartz 6.1 270 to 290 -2.0 to -1.3 terised by calcite, quartz, calc-silicates, illite calcite -7.4 3.3 280 to 300 -2.9 to -2.3 -5.6 to -5.4 and adularia (Absar, 1988). KA23 262 quartz 9.6 190 to 210 -2.7 to -1.4 For the most part, hydrothermal minerals calcite -5.8 6.7 190 to 220 -3.4 to -1.8 -5.7 to -5.1 at Kawerau are in equilibrium with the fluids from which they deposited, with a near-surface KA23 348 quartz 11 225 to 245 0.8 to 1.8 acid alteration zone within the shallow system, calcite -6.4 5.2 220 to 240 -3.3 to -2.5 -5.7 to -5.3 passing to a ‘typical’ neutral-pH hydrothermal KA26 543 calcite -7.9 9.6 250 to 270 2.3 to 3.0 -6.6 to -6.3 assemblage (variously of wairakite, epidote, quartz, calcite and illite, indicative of tempera- KA30 540 quartz 8.3 255 to 275 -0.4 to 0.4 tures exceeding 240 ºC; Bignall, 2010). There is calcite -6.0 5.7 260 to 280 -1.2 to -0.5 -5.3 to -5.0 evidence of apparent cooling in the north of the geothermal system, for example in well KA10, KA47 365 quartz 8.2 210 to 230 -2.8 to -1.7 the occurrence of chalcedony (typically, but not KA47 505 quartz 6.2 220 to 240 -4.3 to -3.3 universally indicative of <190 ºC conditions) after epidote (at Kawerau, generally at down- KA48 520 calcite -6.0 5.6 185 to 220 -4.8 to -3.0 -6.3 to -5.3 hole temperatures between 250-290 ºC) in the same vein (Christenson, 1987). High- and low KA48 775 calcite -3.0 10.8 230 to 250 2.7 to 3.5 -2.1 to -1.7 temperature mineral assemblages also occur KA48 840 calcite -7.0 3.7 230 to 250 -4.4 to -3.6 -6.1 to -5.7 in KA16 and KA23, with the low temperature minerals overprinting neutral pH, high tempera- ture mineral assemblages. The carbon isotope values of the fluid (dissolved CO2 – Overall fluid inclusion homogenisation temperatures ob- H2CO3, Table 1) in equilibrium with calcite were calculated tained by Christenson (1987) and Milicich (2013) agree well using the fractionation factors of Ohmoto and Rye (1979). 13 with temperatures presently recorded in bores. However, there δ CH2CO3 values range from -6.8 to -1.7 ‰ (Table 1). Most of 13 are distinct differences seen in the bottom of KA16, KA23 and the δ CH2CO3 values of CO2 in equilibrium Kawerau calcite lie KA24 in the north of the geothermal system. KA16 has seen a within the range of mantle–derived CO2 (e.g. magmatic) which decrease of about 18˚C, KA23 a decrease of 20˚C and KA24 a has δ13C values between -4 and -8 ‰ (Rye and Ohmoto, 1974; decrease of about 30˚C (283˚C to 250˚C) since the fluids were Pineau and Javoy, 1983), implying it has derived from deep trapped. degassing of magma.

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The current conceptual model of the system is consistent with deeply-sourced thermal fluids, with a heat source in the south related to Mt Putauaki, flowing from the south towards the northern (Bignall, 2010). This is indicated by the highest measured temperatures, pressures and discharge chloride concentrations at Kawerau occurring towards the southern part of the geothermal system, in the vicinity of Mt. Putauaki. The andesitic/dacitic Putauaki composite cone first erupted around 8 ka (Carroll et al., 1997), but earlier hydrothermal eruption breccias present at surface imply that magma was intruded to shallow depths as early as ~16 ka (Bromley, 2002; Nairn and Wiradiradja, 1980, constrained by ages from Lowe et al., 2013). It is inferred here that the calcite precipitation event in the south of the geothermal system is related to the initiation and growth of the Putauaki magmatic system. The relatively more positive O values associated with this calcite are consistent with degassing of the Putauaki magmatic system (Fig. 3). These inferences would imply that the modern Kawerau hydrothermal system was initiated around ~16 ka. With a cooling of the inferred deep magmatic body and a decrease in magmatic degassing, the fluid in the geothermal system would have transi- 18 Figure 3. δ18O values of the fluids calculated in equilibrium with quartz and tioned from the relatively more positive δ O value fluids in the calcite using measured temperature ranges (Table 1). The temperatures are calcite event to dilute, predominantly meteoric conditions similar estimated from primary fluid inclusions from Milicich (2013) and well tempera- to today (Fig. 4). tures from MRP (2013). The of values calculated (at Tqz) for modern Kawerau This work suggests the Kawerau geothermal system is made aquifer waters from Mroczek et al. (2001) and MRP (2013) are indicated, along with an average value for local meteoric water. up of two overprinted hydrothermal systems since approximately 400,000 years B.P. The state of the system between that driven 4. Discussion

Our work suggests there have been at least two temporally isolated but spatially over- printed hydrothermal “systems” at Kawerau. The earliest of these is most likely related to heat supplied by the magmatic events that gave rise to the Caxton Formation sills and extrusive domes (hereafter referred to as the Caxton magmatic system). The hydrothermal system associated with the Caxton magmatic system would have initiated around 400 ka, with its clearest evidence being the presence of a hy- drothermal eruption breccia stratigraphically beneath the Caxton Formation rhyolite dome (Fig. 4). It is inferred here this hydrothermal system is responsible for the earliest wairakite- prehnite alteration event, with the main upflow located in the north of the geothermal system. Overprinting of high-temperature alteration with low-temperature is a feature in the north of the geothermal system, supporting the notion of previously higher temperatures in this area. Evidence from oxygen isotope values indicates that fluids in this earlier hydrothermal system would have been meteoric–water dominated, Figure 4. Top: Early magmatic intrusion related to the Caxton magmatic system created thermal and similar to the present day. The lack of evi- instability and hydrothermal eruptions at surface ~400 ka. Initial growth of the Caxton Formation dence of magmatic fluids suggests the Caxton rhyolite domes and development of a convection plume at ~360 ka. Hydrothermal features would magmatic system was not actively degassing at have been present at surface. Bottom: Intrusion related to the Putauaki magmatic system created thermal instability and hydrothermal eruptions at surface ~16 to 9 ka. Since ~ 9 ka there has been shallow depth and was acting solely as a heat growth of the Putauaki dacite-andesite dome complex and the development of a hydrothermal source driving a convection plume with the convection plume. Hydrothermal features would have been present at surface since that time, waters of meteoric origin. reflecting the present day situation in the Kawerau geothermal system.

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by the earlier Caxton magmatic system and the current Putauaki Absar, A. and P., Blattner, 1985. Successive hydrothermal events as indicated driven system is not known. Given the volumes of magma repre- by oxygen isotope composition and petrography of greywacke basement rocks, Kawerau geothermal field, New Zealand. New Zealand Geothermal sented either by the Caxton Formation rhyolites themselves (~1 Workshop Proceedings; Auckland, New Zealand, p. 115-122. km3), or the total volume of magma inferred to have generated 3 Arehart, G.B., B.W. Christenson, C.P. Wood, K.A. Foland, and P.R.L. Browne, the rhyolites by assimilation/fractional crystallisation (~10 km ), 2002. Timing of volcanic, plutonic and geothermal activity at Ngatama- the Caxton magmatic system would be able to feed an active riki, New Zealand. Journal of Volcanology and Geothermal Research, geothermal system only for a period of the order of 102 to 104 v. 116, p.201–214. years (e.g. Cathles, 1977). Taking this and the lack of preserved Allis, R.G., 1997. The natural state and response to development of Kaw- record of surface hydrothermal activity in the geological record erau Geothermal Field, New Zealand. Geothermal Resources Council after the Caxton Formation (i.e. sinters, hydrothermal eruption Transactions, v. 21, p. 3-10. breccia), it is probable that geothermal activity waned or ceased Bates, R.L. and J.A. Jackson. Glossary of geology (third edition). Alexandria, between these events as heat was lost from the initial Caxton Virginia, American Geological Institute, 1987. magmatic system. Begg, J.G. and V. Mouslopoulou, 2010. Analysis of late Holocene faulting Other geothermal systems in the TVZ support the notion of within an active rift using lidar, Taupo Rift. New Zealand. Journal of temporally isolated, but overprinting hydrothermal systems. The Volcanology and Geothermal Research, v. 190, p. 152-167. Ngatamariki geothermal system has evidence of two hydrothermal Bibby, H.M., T.G. Caldwell, F.J. Davey, and T.H. Webb, 1995. Geophysical systems, a magmatic intrusive event, with a corresponding contact evidence on the structure of the Taupo Volcanic Zone and its hydrother- metamorphism/alteration halo where the fluid-rock interactions mal circulation. Journal of Volcanology and Geothermal Research, v. 68, p. 29–58. are inherently associated with the intrusion, and the modern meteoric-sourced convective hydrothermal system. Other extinct Bignall, G., 2010. Kawerau Geothermal Field: geological assessment for 2010 resource consent variation. GNS Science consultancy report 2010/118. hydrothermal systems, such as Ohakuri (active >160,000 years ago; Henneberger and Browne, 1988) or substantially cooling, Bromley, C.J., 2002. Putauaki (Kawerau) MT resistivity survey. Proceedings such as Mangakino (Fagan et al., 2006) supports the concept that of the 24th New Zealand Geothermal Workshop, p. 135-139. hydrothermal systems can wax, wane and be replenished. Bryan, C.J., S. Sherburn, H.M. Bibby, S.C. Bannister, and A.W. Hurst, 1999. Shallow seismicity of the central Taupo Volcanic Zone, New Zealand: its distribution and nature. New Zealand Journal of Geology and Geophys- 5. Summary ics, v. 42, p. 533–542. Carroll, L.D., J.A. Gamble, B.F. Houghton, T. Thordarson, and T.F.G. Higham, This work implies that over the course of the evolution of the 1997. A radiocarbon age determination for Mount Edgecumbe (Putauaki) TVZ multiple distinct hydrothermal systems can occur in the same volcano, , New Zealand. New Zealand Journal of Geology area, potentially triggered by proximal magmatic input. In contrast and Geophysics, v. 40, p. 559-562. to numerical modelling approaches (e.g. Kissling and Weir, 2005) Cathles, L., 1977. An analysis of the cooling of intrusives by ground-water there appears to important role for localised magmatic intrusions convection which includes boiling. Economic Geology, v. 72, 804-826. in fuelling TVZ geothermal activity. The results emphasise the Chambefort, I., B. Lewis, C.J.N. Wilson, A.J. Rae, G. Bignall, C. Coutts, impermanence of individual hydrothermal systems in some TVZ and T.R. Ireland, 2014. Stratigraphy and structure of the Ngatamariki geothermal systems. Without multiple ‘pulses’ of intruded magma geothermal system: new U-Pb geochronology and its implications for Taupo Volcanic Zone evolution. Journal of Volcanology and Geothermal the lifetime of an individual hydrothermal system, as linked to Research, v. 274, p. 51-70. a specific heat source, is likely to be brief (of the order of 102 to 104 years). However a geothermal system can be rejuvenated by Christenson, B.W., 1987. Fluid-Mineral Equilibria in the Kawerau Hydro- thermal System, Taupo Volcanic Zone, New Zealand. PhD (Geology) a ‘pulse’ of intruded magma, allowing multiple temporally unre- thesis, University of Auckland. lated hydrothermal systems to overprint in the same geographic Christenson, B., C. Wood, and G. Arehart, 1998. Shallow magmatic degas- location. sing: Processes and PTX constraints for paleo-fluids associated with the Ngatamariki diorite intrusion, New Zealand. Water-Rock Interaction, Acknowledgements v. 9, 435-438. Fagan, C., C. Wilson, K. Spinks, P. Browne, and S. Simmons, 2006. Stratigra- We acknowledge funding support for this project from Mighty phy, hydrothermal alteration and evolution of the Mangakino geothermal River Power Ltd. and their permission, along with that of Ngati system, Taupo Volcanic Zone, New Zealand, Proceedings 28th New Tuwharetoa Geothermal Assets Ltd., to publish these data. Thanks Zealand Geothermal Workshop. also go to A8D Trust, Putauaki Trust, Te Tahuna Putakuaki Trust Henneberger, R. and P. Browne, 1988. Hydrothermal alteration and evolution and other local Maori trusts for access to borehole core and cut- of the Ohakuri hydrothermal system, Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research, v. 34, p. 211-231. tings. IC, GB and SM also acknowledge funding support for this project from GNS Science CSA (Core Science Area) Geothermal Kissling, W., S. Ellis, F. Charpentier, and H. Bibby, 2009. Convective flows in a TVZ-like setting with a brittle/ductile transition. Transport in Porous Research Programme. Media, v. 77, p. 335–355. Kissling, W.M, and G.J. Weir, 2005. The spatial distribution of the geothermal fields in the Taupo Volcanic Zone, New Zealand. Journal of Volcanology References and Geothermal Research, v. 145, p. 136–150. Absar, A., 1988. Oxygen isotope and hydrothermal alteration studies at Lewis, B., C. 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