BIBLIOGRAPHIC REFERENCE

Bignall, G.; Milicich S. D. 2012. Kawerau Geothermal Field: Geological Framework, GNS Science Report 2012/33. 35 p.

G. Bignall GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3350, New Zealand. [email protected] S.D. Milicich GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3350, New Zealand. [email protected]

© Institute of Geological and Nuclear Sciences Limited, 2012 ISSN 1177-2425 ISBN 978-1-972192-20-7

CONTENTS

ABSTRACT ...... II KEYWORDS ...... V 1.0 INTRODUCTION ...... 1 2.0 REGIONAL GEOLOGICAL SETTING ...... 1 3.0 KAWERAU SURFACE GEOLOGY ...... 3 4.0 KAWERAU SUBSURFACE GEOLOGY ...... 3 5.0 STRUCTURE OF THE KAWERAU GEOTHERMAL FIELD ...... 10 6.0 STRUCTURAL - STRATIGRAPHIC CONTROLS ON PERMEABILITY ...... 12 6.1 Stratigraphic permeability ...... 13 6.2 Structural permeability ...... 13 7.0 HYDROTHERMAL ALTERATION ...... 14 8.0 GEOLOGICAL HAZARDS ...... 15 9.0 GEOLOGICAL MODEL OF THE KAWERAU GEOTHERMAL FIELD ...... 16 10.0 REFERENCES ...... 18

FIGURES

Figure 1. Location of Kawerau Geothermal Field, showing mapped faults, inferred caldera boundaries (Gravley et al., 2007) and outcropping Mesozoic greywacke...... 2 Figure 2. Kawerau Geothermal Field, showing geothermal well locations and well tracks ...... 5 Figure 3. Schematic geological cross-section of the Kawerau Geothermal Field, showing the relationship of the main stratigraphic units ...... 10 Figure 4. Aerial photograph of the Kawerau area, showing inferred fault locations...... 11 Figure 5. Conceptual model of the Kawerau Geothermal Field ...... 17

TABLES

Table 1. Generalised stratigraphy of the Kawerau Geothermal Field...... 7

APPENDICES

Appendix 1 Geothermal (Geoscience) Glossary ...... 24

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ABSTRACT

The Kawerau Geothermal Field is one of the longest-lived active geothermal systems in the Taupo Volcanic Zone (TVZ) and is comparable in size to a number of other New Zealand geothermal fields developed for electrical power generation. Geological information on the Kawerau Geothermal Field has markedly increased over the last 30 years, and there is now a clearer understanding of the field compared with the state of knowledge when exploration drilling began in the 1950s.

The conceptual model of the Kawerau geothermal system points to deep reservoir fluid moving towards the surface (and laterally) via NE-trending normal faults (and NW-trending cross-faults) and fractures of high local permeability, which provide drilling targets within otherwise relatively impermeable ignimbrite, andesite and basement greywacke. The faults and fractures pass into overlying pyroclastic units and lacustrine sediments that infill the downfaulted Whakatane Graben, where hot water spreads laterally (northwards) along subhorizontal permeable volcanic and sedimentary formations. Intra-formation sediments, welded ignimbrite and hydrothermal eruption breccias are important aquicludes that separate near-surface aquifers from deeper reservoir fluids.

Geological Setting: • Geothermal activity at Kawerau occurs at the southern end of the Whakatane Graben (an area of active extension, faulting and regional subsidence), where the NE-striking active rift of the TVZ intersects N-trending strike-slip faults of the North Island Shear Belt.

• During the last ~1 million years, Mesozoic greywacke basement within the Whakatane Graben has downfaulted to 1-2 km below sea level, with the resultant NE-trending fault- bounded structural depression infilled by a sequence of locally-erupted Quaternary rhyolite, dacite and andesite lavas, ash-flow (ignimbrite) and lacustrine sediments.

• Hydrothermal eruption breccias have been inferred in the Kawerau stratigraphy, overlain by Matahina Formation ignimbrite, which point to geothermal activity in the Kawerau area for possibly >320,000 years.

• The Whakatane Graben is an area of active extension and faulting. Geodetic surveys reveal an average spreading rate of ~7-8 mm/year across the structural depression.

Kawerau Surface Geology: • The Kawerau area is covered by debris from recent Mt. Tarawera eruptions; deposits from Tarawera River flooding, and hydrothermal eruption breccias (associated with ~15,000 to 9,000 year-old hydrothermal events).

• Putauaki (Mt. Edgecumbe) is a multiple vent dacite-andesite volcano, near the southern boundary of the Field, with a youngest dated eruption of 2,400 years B.P. The main cone is <5,000 years old, which is much younger than the age of the geothermal system.

Kawerau Stratigraphy:

• The stratigraphy of the Kawerau Geothermal Field has been established from the study of core and cuttings from about 60 deep exploration, production and injection geothermal wells (KA1-50 and PK1-8) and several shallow monitor holes (KAM1-11).

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• Rock units in the near surface, or outcropping at the surface, include: Recent Alluvium, Putauaki Volcanics, Hydrothermal Eruption Breccias, Unconsolidated Pyroclastics (commonly referred to as “Rotoiti Breccia”) and Onepu Formation (rhyodacite domes).

• To ~500 m depth, the field stratigraphy comprises welded ash-flow and pumice lapilli tuff of the 320,000 year old Matahina Formation ignimbrite; unwelded pumice-rhyolite lapilli and intercalated water-laid(?) tuff of the Tahuna Formation (including units previously logged as Huka Group sediments, Tahuna Breccia and Onepu Ash), and buried brecciated to massive rhyolite domes of the Caxton Formation.

• With increasing depth, the stratigraphy at Kawerau comprises Karaponga Formation (tuffaceous sandstone, siltstone and partly welded crystal-vitric tuff), Onerahi Formation (muddy breccias, tuffaceous sandstone and siltstone), Kawerau Andesite (lava and breccias), Raepahu Formation (welded crystal-lithic tuff/breccia), Tasman Formation (breccia, sandstone and siltstone), Te Teko Formation (lenticulitic ignimbrite, crystal-lithic tuff and breccia) and Tamurenui Subgroup (including terrestrial sedimentary units of the Rotoroa Formation, and greywacke conglomerate/gravels of the Waikora Formation).

• Interbedded sandstone and argillite, hardened by low grade regional metamorphism (Torlesse greywacke basement), has been drilled in 29 wells from Kawerau, from about -670 mRL on the upfaulted south-eastern margin of the Whakatane Graben (e.g. from - 681 mRL in KA26, -667 mRL in KA29) and at greater depth towards the north-western margin (e.g. -1078 mRL in KA21, -1110 mRL in KA28 and -1285 mRL in KA48).

Structure of the Kawerau Geothermal Field:

• The greywacke basement at Kawerau is step-faulted on NE-trending normal faults, downthrown to the north-west. Combined with cross-cutting NW-trending faults this has produced a series of NE-plunging fault blocks. Some faults have been intersected by drillholes, which show basement faulting mainly occurred before eruption of the Kawerau Andesite, since inferred displacements decrease upward along the fault planes.

• The surface locations of many faults are not known, as young sediments mask their traces. The Rotoitipaku Fault (~2 km NW of Kawerau) reveals at least 5 major, normal displacement events during the last 8,500 years, with slip rates of 1-11 mm/year, and evidence movement may have been be associated with hydrothermal eruption events.

• The Edgecumbe Fault ruptured during the 1987 Edgecumbe Earthquake (with 0.4 m downthrow to its NW-side). This highlights the potential for movement on existing faults within the geothermal system, which could enhance permeability within the field.

Structural and Stratigraphic Controls on Permeability:

• The production area at Kawerau is characterised by several permeable, formation- hosted reservoirs, and low permeability units (e.g. lacustrine sediments of the Tahuna Formation) that separate near-surface waters from deep reservoir fluids. Lithological permeability is mainly confined to fractured rock units (e.g., Kawerau Andesite), and some sedimentary units within the Tamurenui Subgroup (including Waikora Formation).

• Welded ignimbrite can form barriers restricting vertical fluid flow, although unwelded layers also exist (e.g. in Matahina Formation), which allow lateral fluid flow. Fracture

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permeability at Kawerau is hosted by competent volcanics (e.g. Caxton Formation and Kawerau Andesite lavas), ignimbrite (Raepahu and Te Teko Formation) and greywacke basement. Well drilling reveal large-scale fracture networks and/or faulting in these units that facilitate fluid movement, and drilling targets with potential for high productivity.

• Whilst much of the present production at Kawerau derives from fractured greywacke, historical success in locating permeability in the basement has been mixed. Few wells drilled vertically into the greywacke basement have intersected faults, although the strategy of siting deviated wells to intersect inferred fault structures has increased the likelihood of intersecting productive (fracture) zones.

• A NW-trending fault inferred by the alignment of vents on Putauaki volcano may provide an important path for hot water, with intersections of NW cross-faults facilitating movement of reservoir fluids from a major upflow zone in the SE-part of the field.

• A seismic gap in earthquake epicentres at Putauaki following the 1987 Edgecumbe Earthquake may reflect reservoir rocks too hot and ductile to sustain brittle fracturing. It may explain why the productive area is located at the boundary between largely impermeable hot rock (close to Putauaki volcano) and surrounding (cooler) brittle rock.

Hydrothermal Alteration:

• Hydrothermal minerals at Kawerau are typically in equilibrium with the fluids from which they deposited, with a near-surface acid alteration zone within the shallow system, passing to a ‘typical’ neutral-pH hydrothermal assemblage (variously of epidote, quartz, calcite and illite, indicative of temperatures commonly exceeding 240 oC).

• A ~100 m-thick surficial blanket of lower pH-type alteration affects shallow levels of the Kawerau geothermal system, and is characterised by minerals (such as native sulphur)

consistent with condensation of steam (containing H2S) above a boiling zone.

• Thermal conditions inferred from the occurrence of high temperature minerals are generally consistent with measured well temperatures. Most of the producing wells at Kawerau are close to their boiling point for depth condition and this is reflected in the nature and intensity of their hydrothermal alteration mineralogy, and is consistent with inferences from reservoir engineering (pressure and temperature) assessments.

• Hydrothermal alteration mineralogy does indicate cooling in parts of the field, over a geological timescale, with epidote in wells where measured temperatures are lower than 240 oC (e.g. in KA22, KA29). As well as apparent cooling, however, there are indications of heating, which may have caused past hydrothermal eruption events in some areas.

Geological model of the Kawerau Geothermal Field:

• The Kawerau geothermal system occupies a structural depression, in which a thick sequence of volcanic lavas and volcaniclastics overlie (Torlesse) basement greywacke.

• Drillhole data defines the main borefield, and the field boundary is inferred from resistivity surveys and gravity data. Resistivity data, combined with the inferred location of the main heat and mass outflow, is consistent with deeply-sourced thermal fluids flowing from the southern (near Putauaki) towards the northern part of the Field.

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• Reservoir rocks in the Kawerau geothermal system comprise locally derived volcanic lavas (e.g. Kawerau Andesite, Onepu and Caxton Formation rhyolites), and distally- sourced silicic pyroclastic rocks (e.g. Raepahu and Te Teko ignimbrites), and sedimentary units (Rotoroa and Waikora Formations) which unconformably overlie greywacke basement (encountered in wells from ~900 to ~1350 m drilled depth). Lacustrine sediments, and welded rock within some ignimbrite units, act as aquicludes and separate the volcanics into discrete aquifers.

• The hydrological model points to deep-sourced fluid moving upwards (and laterally), via widely spaced, steeply dipping, NE-trending normal faults (and cross-cutting NW- trending faults) and/or fractures of high local permeability, within otherwise impermeable rock. Faults and fractures in basement greywacke pass into overlying units, where the hot water spreads laterally into permeable zones along subhorizontal volcanic and sedimentary layers.

• The main upflow of deep-sourced fluids occur in the southern part of the geothermal field. Only part of the geothermal upflow reaches shallow aquifers and the ground surface (e.g. in the Rotoitipaku - Umupokapoka area), with most of the natural surface outflow occurring via seepage to the Tarawera River.

• A subsurface outflow has been inferred to extend >3 km north of the resistivity boundary, with thermal fluids mixing with groundwaters in the Tarawera-Rangitaiki flood plain. Progressive dilution and cooling occurs with water flow to the north.

• The existence of buried rhyolite complexes is consistent with underlying local magmatic heat sources, which produced rhyolites of the Onepu and Caxton Formations. Whilst the present deep heat source for the Kawerau Geothermal Field is inferred to occur in the vicinity of Putauaki volcano, geothermal drilling has indicated good permeability in the upper ~0.5 to 1 km of the basement greywacke and overlying volcanics further north, where it is controlled by the location of active faults and fractures.

KEYWORDS

Geological Framework, stratigraphy, structure, U-Pb age dating, Kawerau Geothermal Field.

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1.0 INTRODUCTION

This report details the geology of the Kawerau Geothermal Field. The report provides geological information in support of a 2011 resource consent application for the take and injection of 45,000 tonnes per day of geothermal fluid from the Kawerau geothermal system by Ngati Tuwharetoa Geothermal Assets (NTGA). At present, fluid extracted from existing resource consent holders (i.e., Mighty River Power Ltd., NTGA, and Geothermal Development Ltd.) totals an annual daily average of 94,000 tonnes, with a maximum daily extraction rate of 113,560 tonnes.

A good understanding of the stratigraphy and structure of the Kawerau Geothermal Field, as well as controls on permeability and hydrothermal alteration assist in development of the conceptual model of the system. Geological data for the Kawerau Geothermal Field has increased over the last 30 years, with the understanding of the field stratigraphy and structure much clearer than when geothermal drilling began in the early 1950s. The Department of Scientific and Industrial Research and the Ministry of Works carried out initial scientific surveys and shallow drilling in 1951/52 to investigate the geothermal potential of the area for power production and process heating. Since that time, more than 60 geothermal wells have been drilled in the Kawerau area.

Early descriptions of the geology were provided by MacDonald and Muffler (1972; up to the drilling of well KA17), Healy (1974; to KA19), Nairn (1977; to KA25), Nairn (1986; to KA35) and Allis et al. (1995); for KA36, KA37). Overviews of the subsurface geology and structure have been published by Grindley (1986), Nairn (1986), Christenson (1987), Allis et al. (1995) and Milicich et al. (2011). Petrological logs for the Kawerau wells have been presented by Browne (1978), Tulloch (1990) and Milicich et al. (2010), with additional geological notes in Nairn and Solia (1980), Browne (1979), Nairn and Beanland (1989), and GNS Science well reports for the period from 2005 to 2010 (i.e. KA43 to KA48, KA50 and PK5 to PK8).

This report supersedes GNS Science Consultancy Report 2010/118 (Bignall, 2011) following a request in December 2011 for revision and additions to the report by NTGA, and takes account of comprehensive re-logging and review of available geological information, some recent U-Pb dating, and geological framework modelling of the Kawerau geothermal system.

A glossary of geothermal geology terms is provided in Appendix 1.

2.0 REGIONAL GEOLOGICAL SETTING

This section outlines the regional geological setting of the Kawerau Geothermal Field. Regional scale studies are routinely undertaken at reconnaissance and pre-drilling stages of geothermal development to provide information on possible heat sources, controls on permeability, and insights on possible subsurface stratigraphy and structure in a prospect area, and to support decisions regarding subsequent/proposed exploration.

The Kawerau Geothermal Field lies in the northern Taupo Volcanic Zone (TVZ; Figure 1). Geothermal activity occurs at the southern end of the Whakatane Graben, in a zone where the NE-striking active rift of the TVZ intersects N-trending strike-slip faults of the North Island Shear Belt (Nairn and Beanland 1989). The topographic expression of the NE-trending fault- bounded structural depression is evident east of Kawerau, where 320,000 year old Matahina Formation ignimbrite is exposed at the ground surface.

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A thick sequence of locally-erupted rhyolite, dacite and andesite lavas, as well as ash-flows (ignimbrite), tuff and other pyroclastic units, mostly derived from the Okataina Volcanic Centre, overlie greywacke basement within the Whakatane Graben. Lacustrine sediments, unconsolidated ash fall and breccias, some of local hydrothermal origin, are interbedded with the volcanic units. The occurrence of inferred hydrothermal eruption breccias in Kawerau drillholes, overlain by Matahina Formation ignimbrite, point to geothermal activity occurring in the Kawerau area for more than 320,000 years.

Southwest of Kawerau, the TVZ is transected by NNE-trending, late Quaternary faults of the 20 km-wide Taupo Fault Belt. Few faults are mapped northeast of the Okataina Volcanic Centre, although some faults are inferred west and north of Kawerau. Despite the paucity of surface fault traces, the Whakatane Graben is an area of active extension and faulting.

Figure 1. Location of Kawerau Geothermal Field, showing mapped faults (GNS Science active fault database), inferred caldera boundaries (Gravley et al., 2007) and outcropping Mesozoic greywacke (grey shading). NISB (inset) is the North Island Shear Belt. The Taupo Volcanic Zone (TVZ) is divided into segments dominated by andesite-dacite (A) and rhyolite (R) volcanism.

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A review of geodetic surveys from 1929 to 1976 show an extensional spreading rate across the Whakatane Graben of ~7-8 mm/yr (Sissons, 1979), with the 1987 Edgecumbe Earthquake near Kawerau accompanied by ~1 m of horizontal extension and associated subsidence.

The main structural features in the Kawerau area are steeply dipping, NE-trending normal faults, which are downthrown to the west. NW-trending cross-faults are strongly inferred by greywacke displacements in Kawerau drillholes, as well as by the alignment of surface thermal manifestations and vents on Putauaki (Mt Edgecumbe) volcano. Whilst the inferred age of the faults is variable, they have resulted in a step-wise displacement of the basement surface across the borefield of ~620 m.

3.0 KAWERAU SURFACE GEOLOGY

Putauaki (Mt. Edgecumbe) is a Holocene, multiple vent dacite-andesite volcano, located near the southern boundary of the Kawerau Geothermal Field. It has a youngest dated eruption of 2,400 years B.P. (Carroll et al., 1997). The main cone is <5,000 years old, which is much younger than the inferred lifetime of the Kawerau geothermal system (Browne, 1979). The Onepu domes (Onepu Formation) on the western margin of the field are older rhyodacite rocks, which erupted about 138,000 to 150,000 years ago, and are contemporaneous with intrusive rocks in several Kawerau geothermal drillholes (Milicich et al., submitted).

Breccias to the east of Kawerau are associated with ~15,000 to 9,000 year old hydrothermal eruptions (Nairn and Solia 1980). The breccias contain mineralogical evidence of high temperatures at the time of their eruptions, although shallow drillhole temperatures in the area are now cool.

There are no surface fault traces mapped within the western part of the Kawerau Geothermal Field, as the area is covered by debris from past Mt. Tarawera eruptions (Hodgson and Nairn, 2000), and sediments deposited by the 1904 A.D. floods that passed down the Tarawera River. The surface trace of the Onepu Fault, which was displaced during the 1987 Edgecumbe Earthquake, is evident ~1 km from the NW margin of the field.

4.0 KAWERAU SUBSURFACE GEOLOGY

Differentiation of units encountered by drilling at Kawerau is based on descriptions from Grindley (1986), Christenson (1987), Nairn (1977, 1981, 1982 and 1986) and Allis et al. (1995), and is vital for establishing the geological framework and structure of the geothermal system, and identifying possible fluid pathways. The data was summarised by Bignall and Harvey (2005), but is updated in this report based on re-interpretation of the borehole logs and some U-Pb age dating (Milicich et al., 2011; submitted. The locations of Kawerau geothermal wells are shown in Figure 2.

The revised stratigraphic succession, which has also resulted in the renaming of some units, has been incorporated into a 3D geological model of the field, which is maintained by Mighty River Power Ltd. Unfortunately, no images from the 3D model are available for this report. The stratigraphy is summarised in Table 1 and Figure 3, and described in following sections in the order the units are encountered by Kawerau geothermal wells. Some units do not extend across the entire field.

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Peat Deposits: Healy et al. (1964) and Nairn (1977) described the distribution of peat deposits in the SE part of the Kawerau Geothermal Field. The peat may be the product of small swamps following the post-Kaharoa (800 year B.P.) flood (Hodgson and Nairn, 2005).

Recent Alluvium: This unit includes sands and gravels deposited on the Tarawera River flood plain during the last 60,000 years, as well as older silicified alluvial deposits below 50 m depth. The youngest deposits range from unconsolidated 1904 A.D. flood deposits (Allis et al., 1995) to boulders, gravels and sand associated with the 1350 A.D. collapse of a natural dam at the eastern end of Lake Tarawera (Hodgson and Nairn, 2000). Some pumice alluvium is inferred to derive from the 800 year B.P. Kaharoa eruption, which obscures structural features over much of the Kawerau Geothermal Field (Allis et al., 1995).

Mt Edgecumbe (Putauaki) Volcanics: Mt Edgecumbe (Putauaki) volcanics have been dated at 8,350 to 2,400 years old (Carroll et al., 1997), which is younger than dated hydrothermal eruption deposits in the proximity of Putauaki volcano (Grindley, 1986; Bignall and Harvey, 2005). Dacitic lavas form the base and western part of Putauaki, which formed about 6,000 years ago, with high-SiO2 andesite lavas comprising the main cone of ~4,000 years age. Bailey and Carr (1994) describe cryptodomes on the north and north-eastern flanks of Putauaki that intruded into, and altered, the overlying Matahina Formation ignimbrite. No Putauaki eruptives are recognised in Kawerau drillholes.

Hydrothermal Eruption Breccias: Surficial hydrothermal breccia deposits were mapped by Nairn and Wiradiradja (1980) in southern and southeastern parts of the Kawerau Geothermal Field. At least 7 eruption centres have been identified (Christenson, 1987). The breccias contain clasts of hydrothermally altered Matahina Formation ignimbrite and Onepu Formation rhyolite, and indicate vent depths to at least -116 mRL. The eruption breccias are interbedded with pyroclastic sequences, and have been dated at 9,000 to 14,500 years old (Grindley, 1986). Bromley (2002) suggested a localised high-resistivity layer beneath the eruption craters could be a shallow intrusive, which provided the driving force for the hydrothermal eruptions.

Unconsolidated Pyroclastics: There are up to 16 non-welded ash and pumice pyroclastic flow deposits at Kawerau (Nairn and Wiradiradja, 1980; Nairn, 1986; Nairn 1989). These include the 61 ka Rotoiti Formation and 34 ka Mangaone Formation (Leonard et al., 2010; Nairn, 2002), with interbedded hydrothermal eruption breccias, pumice fall deposits and paleosols (Jurado-Chichay and Walker, 2000). Grindley (1986) correlated unwelded breccia and pumiceous tuff (which thicken from ~20 m in the KA34 area to ~100 m in KA32, and underlie alluvium and lahar deposits) with ignimbrite of the Rotoiti Formation.

Onepu Formation: Surficial rhyodacite lava domes also occur to the northwest of Kawerau. The lavas are flow banded, with plagioclase, minor quartz, amphibole, pyroxene, biotite and magnetite. The lavas pre-date Rotoiti Formation (61 ka; Wilson et al., 2007; Cole et al., 2010), but post-date Matahina Formation ignimbrite. Grindley (1986) suggested the rhyodacites represent the youngest domes of the Onepu Rhyolite dome complex, and recent U-Pb zircon dating of Onepu Formation samples reveal an age of 0.15 ± 0.01 Ma (Milicich et al., submitted), which is similar to a 40Ar/39Ar age determination on plagioclase from dome material of 138 ± 7 ka by M. Lanphere and B. Houghton (cited by Christenson, 1997). No age data is available for the unidentified pyroclastics that immediately overlie the domes, so the upper age limit of the surface Onepu domes is unknown. A dike encountered in KA30 is likely part of a feeder system for the dome complex.

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Figure 2. Kawerau Geothermal Field, showing geothermal well locations and well tracks (projected to surface). For clarity, Kawerau well labels lack their “KA” prefix. KAM monitor wells are not shown.

Matahina Formation (comprising Matahina Formation ignimbrite): The 320 ka Matahina ignimbrite is inferred to have erupted from the Haroharo Caldera in the Okataina Volcanic Centre (Nairn, 1981; Bailey and Carr, 1994; Leonard et al., 2010), and covers ~2000 km2 across the northern TVZ. The ignimbrite consists of a multiple-flow, compound cooling unit, with three ash-flow members that vary laterally in degree of welding. In Kawerau drillholes, the upper member is partially to non-welded (Grindley, 1986), and grades into poorly sorted ash and pumice lapilli tuff that is increasingly welded with depth, and a poorly welded basal (airfall) tephra member (Bailey and Carr, 1994).

The Matahina ignimbrite crops out in the Tarawera Valley, and was first described at Kawerau in KA21 (Nairn, 1977). Its extent has been revised, and it is now recognised in Kawerau wells where it had previously been described as Whakamana Breccia or Rotoiti Breccia (e.g. KA3, KA16-19, KAM1-4). As a result, Matahina ignimbrite is now known in geothermal wells across the field (Milicich et al., 2011). The Matahina ignimbrite consists of poorly sorted pumice lapilli, with cuspate glass shards, crystals (plagioclase, embayed quartz, hornblende and pyroxene) and lithics (rhyolite lava, obsidian, siltstone, sandstone and tuff). The matrix is eutaxitic and pumice fiamme are common, whilst devitrification has

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led to the development of spherulitic textures. Unit thickness is likely controlled by the Onepu Formation, with the ignimbrite thickening to the east (~200 m thick in KA28) where the rhyodacite is absent.

Tahuna Formation: The Tahuna Formation includes all sedimentary units and intercalated tuff between the Matahina Formation and Karaponga Formation, including units previously logged as Huka Group, Tahuna Breccia and Onepu Ash. The Huka Group (Grindley, 1959) comprises the Waiora and Huka Falls Formations (the latter the generic name for all lacustrine units of the TVZ), and was used previously for all sedimentary intervals intersected by Kawerau wells. At Wairakei, the Waiora Formation includes all volcaniclastic and sedimentary deposits underlying Huka Falls Formation and overlying the Wairakei Ignimbrite (a ~320-340 ka Whakamaru Group ignimbrite). Tahuna Formation sediments underlie 320 ka Matahina ignimbrite, so they should not be regarded as Huka Group (c.f. Grindley, 1965).

The Tahuna Formation is inferred to be predominantly terrestrial, with siltstones and crystal- rich sandstones dominating the sedimentary sequence. Other lithologies include pumiceous tuff, crystal-rich lithic sandstones and siltstones (some carbonaceous, with variable proportions of pyroclastic and diatomaceous material, whilst other siltstones have an inferred marine origin; Browne, 1978). The type section for Tahuna Formation is KA21 where it is 92 m thick, whilst it is 385 m thick in KA22 where pumiceous tuff and sandstones occur with breccia of possible hydrothermal eruptive origin. The mean U-Pb zircon age of tuff collected from near the top of the formation is 0.44 ± 0.02 Ma (Milicich et al., submitted), which is contemporaneous in age and stratigraphic position to the Caxton Formation rhyolite lavas.

Browne (1979) described “Upper Tahuna Breccia” as a lapilli tuff and hydrothermal eruption deposit. The breccia is >200 m thick in KA24 and KA32 (compared to 8 m for the thickest surficial hydrothermal eruption deposit; Nairn and Wiradiradja, 1980), and ~100 m thick in KA28. Grindley (1986) suggested the breccia may have a phreatomagmatic origin, associated with emplacement of rhyolite into lacustrine sediments.

Lithological variations in sedimentary horizons of the Tahuna Formation are gradational, and correlation between wells is hampered by lateral variability in thickness (up to 360 m). At Kawerau, the sediments form a relatively impermeable aquiclude at ~400 m depth, between the Caxton Formation aquifer and underlying Kawerau Andesite or ignimbrite.

Caxton Formation: Rhyolite bodies have been intersected by many Kawerau drillholes (e.g. in northern (e.g. KA32) and southwest (KA23, KA30) parts of the field), with coherent rhyolite below Matahina Formation ignimbrite (previously known as Onepu Rhyolite; Grindley, 1986) being up to 450 m thick. Deeper bodies were described by Grindley (1986) as rhyolite lava and called Caxton Rhyolite, and were inferred to have extruded from multiple vents to form a large rhyolite complex interbedded with breccia, ignimbrite and fluviatile sediments.

Recent U-Pb dating has revealed the Onepu and Caxton Rhyolites are of similar age (Milicich et al., submitted) with overlapping age determinations that has prompted the two units to be collectively assigned to the Caxton Formation. Athough Christenson (1987) suggested Caxton Rhyolite to be chemically distinct from the Onepu Rhyolites, U-Pb zircon age dating (Milicich et al., submitted) points to the Caxton Rhyolites being a series of intrusions that are probable feeders to Onepu Rhyolite lavas.

Across the field, intrusive and extrusive members of the Caxton Formation are represented, with Milicich et al. (2011) identifying a “crystal-poor” variant (~5 vol. %, with quartz,

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plagioclase, biotite and amphibole), and a moderately “crystal-rich” variant (with ~15 vol.% fractured quartz, plagioclase and apatite) that occurs only in wells in the NW part of the field. Best estimates for eruption ages based on U-Pb zircon age dating are identical, at 0.36 ± 0.03 Ma for both rhyolite types (Milicich et al., submitted).

Karaponga Formation (new name): Most well logs at Kawerau describe the occurrence of partly welded, crystal-vitric “Rangitaiki-type Ignimbrite” that is lithologically similar to ignimbrite exposed around the Okataina Volcanic Centre. The Rangitaiki Ignimbrite is a correlative to the regionally extensive, 320-340 ka Whakamaru Group ignimbrites (Grindley, 1986), which extend ~13,000 km2 across the TVZ (Brown et al., 1998).

Browne (1978) and Tulloch (1990) divided “Rangitaiki Ignimbrite” at Kawerau into subunits, on the basis of welding, and crystal and lithic contents. Recent U-Pb dating, however, along with new trace and rare earth element (REE) data, has provided unequivocal evidence that the “Rangitaiki Ignimbrite” is unrelated to the 320-340 ka Whakamaru Group ignimbrites (Milicich et al., submitted), and instead is one of a series of older ignimbrites.

Table 1. Generalised stratigraphy of the Kawerau Geothermal Field.

Formation Lithology Thickness (m)

Recent alluvium Peat deposits; sands and gravels; unconsolidated pyroclastics 10 – 50 (including Whakamana Breccia, Rotoiti Breccia)

Hydrothermal eruption breccia Hydrothermal eruption deposits (14,000 and 9,000 yr BP) 1 – 4 , < 10

Unconsolidated pyroclastics Unwelded pumiceous pyroclastic flows and airfall tuffs 0 – 90

Onepu Formation Surficial rhyodacite (domes) & porphyritic (crystal-rich) intrusion ~ 200

Matahina Formation Partly welded grey-brown lenticulite and vitric tuff 10 – 410

Tahuna Formation Crystal-rich sandstone, siltstone, muddy lithic-breccia and 0 – 360 unwelded pumice-rhyolite lapilli tuff

Caxton Formation Buried spherulitic/banded quartz-plagioclase rhyolite domes, 0 – 450 and associated intrusions

Karaponga Formation Partly welded, crystal-lithic lapilli tuff 0 – 180

Onerahi Formation Tuffaceous to muddy breccias, coarse tuffaceous sandstone 0 – 85

Kawerau Andesite Augite-plagioclase andesite lava, breccias and tuff 0 – 300

Raepahu Formation Partly welded crystal-vitric tuff (lithic poor) and lithic-rich tuff 0 – 165

Tasman Formation Muddy breccia, sandstone and siltstone 0 – 25

Te Teko Formation Partly welded grey crystal-vitric tuff 0 – 255

Rotoroa Formation Tuffaceous sandstone, poorly sorted crystal and vitric, water- 0 – 200 laid tuff and sandstone

Waikora Formation Greywacke pebble conglomerate, and minor intercalated tuff 0 – 450

Greywacke basement Weathered, sheared greywacke and argillite -

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The Karaponga Formation is a new name for material previously attributed to “Rangitaiki Ignimbrite”, and comprises volcanic units with a cumulative thickness of up to 180 m. These include crystal- and lithic-rich tuff (e.g. in KA48) and tuff (e.g. in KA17 and KA21), possibly contemporaneous to the “quartz-biotite tuff” of Nairn (2002), that derive from eruption events between 0.51 and 0.59 Ma ± 0.02 Ma. Zircon U-Pb age data and textures from crystal-rich tuff match zircon ages extracted from crystal-rich, quartz and biotite-bearing pumices in fluvial outwash deposits exposed on the Bay of Plenty Coast (Hikuroa et al., 2006), which are correlated with quartz-biotite ignimbrite (Nairn, 2002; Cole et al., 2010) that have been 40Ar- 39Ar dated at 0.557 ± 0.003 Ma (Leonard et al., 2010).

Onerahi Formation: At Kawerau, tuffaceous to muddy breccias, coarse tuffaceous sandstone and siltstone intervals occurring sporadically below Karaponga Formation were previously included in the Huka Falls Formation (Grindley, 1986; Allis et al., 1995). Consideration of stratigraphic age relationships however, exclude these sedimentary units from the Huka Group, and they are renamed as Onerahi Formation.

Kawerau Andesite: The Kawerau Andesite consists of upper and lower lava flow members, and an andesitic tuff (Bogie, 1981). It thickens toward an inferred vent in the northwestern part of the field (Allis et al., 1995) where magnetic surveys (Macdonald and Muffler, 1972) point to a possible eruption centre, although no inferred feeder structures have been intercepted by geothermal drillholes. The dense, fractured andesite is highly permeable, particularly where it is cut by faults, and a production aquifer that was a target for early exploration drilling. Many wells produce from the Kawerau Andesite (Allis et al., 1995), and maximum temperatures in several wells occur in these zones (Healy, 1974).

The andesite is thicker and deeper to the northwest and thins to the southeast (e.g. ~200 m thick in the KA43/KA44 area, ~40 to 60 m thick in the PK4/PK5 area, and <50 m in KA21 and KA27). The Kawerau Andesite underlies Onepu Formation at ~600 to 1000 m depth, and commonly overlies ignimbrite of the Raepahu Formation (or Te Teko Formation, in the absence of Raepahu Formation), although in KA35 it occurs above and below Raepahu Formation. Andesitic tuff occurs at the same stratigraphic level as Kawerau Andesite in southern parts of the field (i.e. KA23, KA25, KA26, KA29, KA34, KA41, KA42, PK6 and PK7), but it is absent close to the inferred source of the Caxton Rhyolite (i.e. KA28, KA30, KA31 and KA32). Browne (1978) correlated friable red-brown material (“andesitic tuff”?) in KA23, KA25, KA26 and KA34 with the Kawerau Andesite, although the inference is discounted here, as the intervals lie within Waikora and Rotoroa Formations and are too old to be related to the Kawerau Andesite.

Raepahu Formation: The Raepahu Formation was defined by Edbrooke et al. (2005), and includes Rocky Hill Ignimbrite (Martin 1961), Kidnappers fall deposit and ignimbrite (Wilson et al., 1995; Cooper et al., 2012), and inferred equivalents (e.g. Potaka Tephra). The partially welded, pumice- and crystal-rich Rocky Hill Ignimbrite was dated at 1.00 Ma by 40Ar/39Ar techniques (Houghton et al., 1995). It has a partially welded base, capped by densely welded ignimbrite (Wilson 1986), and a microfractured upper part likely resulting from rapid cooling. The Kidnappers deposits consist of bedded phreatoplinian ash and crystal-rich, non-welded ignimbrite, and originate from a ~1.02 Ma eruption (Wilson et al., 1995), likely from the Mangakino Volcanic Centre.

At Kawerau, the Raepahu Formation consists of a series of ignimbrites underlying Kawerau Andesite and Onerahi Formation. The youngest of the Raepahu Formation ignimbrites at

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Kawerau is lithic-poor and has a U-Pb age of ~1.0 Ma (Milicich et al., submitted), and has been correlated (on a basis of its comparable U-Pb zircon age spectra; Wilson et al., 2008) with Kidnappers ignimbrite. Whilst the Raepahu Formation occurs across the Kawerau Geothermal Field, the Kidnappers ignimbrite has limited areal extent (i.e. in KA23, KA45, KA46 and KA30), and was previously identified as Rangitaiki Ignimbrite, as they both contain biotite and large, embayed and bipyramidal quartz.

Tasman Formation: The Tasman Formation comprises a sequence of sedimentary units that overlie volcaniclastic and sedimentary rocks of the Te Teko Formation. It includes muddy breccia, sandstone and siltstone, and is also represented by a <20 m-thick interval of reddish brown siltstone/paleosol. It includes sediments previously logged by Browne (1978) and Grindley (1986) as Huka Group, and lithic-lapilli tuff, breccia and interbedded weakly stratified crystal tuff of the Tasman Breccia. The breccia contains rhyolite lava, pumice, ignimbrite, rare greywacke, basalt and granophyre clasts, and crystal fragments. As a result of U-Pb dating, and respective stratigraphic positions, sedimentary intervals between the Raepahu Formation and underlying Te Teko Formation can no longer be regarded as Huka Group. Browne (1978) suggested the Tasman Breccia may be a hydrothermal eruption breccia that preceded volcanic activity from the Okataina Volcanic Centre.

Te Teko Formation: Volcaniclastic and sedimentary units stratigraphically below the Tasman Formation and/or the Raepahu Formation, and above the Tamurenui Subgroup, have recently been renamed as Te Teko Formation (Milicich et al., 2011). The formation includes deposits previously assigned by Browne (1978) and Grindley (1986) to Pyroclastic unit A & B, Tasman Breccia, Ignimbrite A and B, Te Teko Ignimbrite, Rangitaiki-type Ignimbrite and Opunoke Ignimbrite. A palaeosol and/or sediments (possibly Tasman Formation) immediately overlie the Te Teko Formation (beneath ignimbrite-dominated Raepahu Formation), whilst a distinctive weathering horizon characterises its basal unit.

At Kawerau, Te Teko Formation is dominated by a partially-welded ignimbrite (possibly a compound cooling unit, based on variable degrees of welding) with a zircon age of 1.46 ± 0.01 Ma (Milicich et al., submitted). The ignimbrite has common embayed quartz, plagioclase, minor biotite, and altered ferromagnesian minerals in a vitroclastic matrix. Lithics are common, and include tuff, greywacke, tuffaceous sandstone, andesite and rhyolite lava (some spherulitic). Pumice clasts are common, with many having flattened textures indicative of compaction. In well KA23, two ignimbrites are present, separated by 25 m of sediments (or palaeosol). The upper ignimbrite has an estimated age of 1.34 ± 0.04 Ma (Milicich et al., submitted), which contrasts to the 1.46 ± 0.01 Ma age of dated ignimbrite from KA23, KA45 and KA46.

Tamurenui Subgroup (incl. Rotoroa Formation and Waikora Formation): All sedimentary and volcaniclastic strata underlying the Te Teko Formation and overlying the greywacke basement has been renamed as Tamurenui Subgroup (new name), and includes sandstone, siltstone, greywacke conglomerate and tuff that have previously been defined as Waikora Formation. The term Waikora Formation is used throughout the TVZ for greywacke pebble conglomerates (Gravley et al., 2006), and includes units at Kawerau where basement faulting has produced accommodation basins into which greywacke gravels accumulated (up to ~450 m thick in KA44). There are sedimentary (predominantly siltstone) intervals above the basement (e.g. in KA23), so use of the name Waikora Formation can be misleading.

The term Waikora Formation is retained for intervals dominated by greywacke conglomerate, but all other sedimentary intervals, including minor pyroclastic deposits, have been renamed

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Rotoroa Formation. The greywacke conglomerate/gravels thin to the southwest, and are coeval with Rotoroa Formation, which consists of fine-grained sediments (sands, silts). Plant remains are locally present (e.g. KA34), indicative of terrestrial deposition during early periods of subsidence of the Whakatane Graben. Two lithologically distinct, unwelded ignimbrites from the Rotoroa Formation with ages of 2.38 ± 0.05 Ma and 2.17 ± 0.05 Ma (from KA25 and KA34 respectively), predate all known TVZ surficial eruptive units or units penetrated in TVZ geothermal fields (Wilson et al., 2010).

Basement greywacke: The greywacke basement is inferred to be part of the Waioeka petrofacies of the Torlesse Terrane (Adams et al., 2009; Leonard et al., 2010; Mortimer, 1995; 2004). It is exposed in the Raungaehe Range to the west of Kawerau. The dipping basement surface can be attributed to progressive down-faulting and possible northwest tilting. Deformed Mesozoic greywacke has been drilled in 29 geothermal wells at Kawerau, from -667 mRL in KA29 on the upfaulted eastern margin of the Whakatane Graben, to -1239 mRL in KA17 on the western margin of the field (Wood, 1996; Wood and Braithwaite, 1999; Wood et al., 2001; Braithwaite et al., 2002). The greywacke comprises medium-grained litharenite, with lenses of carbonaceous argillite that have been regionally metamorphosed to a prehnite-pumpellyite metamorphic facies (Mortimer, 1995; Wood et al., 2001).

Figure 3. Schematic (representative) geological cross-section of the Kawerau Geothermal Field, showing the relationship of the main stratigraphic units (KA = Kawerau; PK = Putauaki).

5.0 STRUCTURE OF THE KAWERAU GEOTHERMAL FIELD

Information on the structure of the Kawerau Geothermal Field, which is reviewed in this section, is important for constructing the geological framework model of the field, and has provided geothermal exploration geologists and subsequent developers with insights on possible controls on fluid flow in the active geothermal system.

Inferences of the deep structure at Kawerau are based in part on studies of the NE-trending Whakatane Graben (Nairn and Beanland, 1989), with its regional NW-SE extension (up to 8 mm/yr), and strain rate of ~10-13 sec-1 across the northern TVZ (Darby and Meertens, 1995). Interpretation of Kawerau drillhole stratigraphy indicates Mesozoic greywacke

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basement is step-faulted on NE-trending normal faults (Figure 4; coincident with the regional structural fabric), downthrown to the northwest, which produced a series of blocks that step down to the north-northeast.

A regional structural influence on the Kawerau geothermal system is the North Island Shear Belt (Figure 1; Beanland, 1995), which includes the north-striking Waiohau Fault (evident ~12 km east of Kawerau). Bailey and Carr (1994) suggested movement occurred on the fault when the Matahina Ignimbrite was deposited. Beanland (1995) indicated the fault had mainly normal movement with ~10% dextral strike-slip at its northerly extent. Studt (1958) used gravity models to estimate ~1 km displacement on the Waiohau Fault.

Several structural features occur in the vicinity of the Kawerau Geothermal Field (Figure 4). Nairn (1982) and Nairn and Beanland (1989) proposed a fault with an easterly strike, northwest of the Waiwera Fault, which has been named the Tahuna Fault (SKM 2003). The history of displacement on the NE-SW trending Rotoitipaku Fault (~2 km NW of the Kawerau Geothermal Field) was investigated by Berryman et al. (1998), who identified 5 major normal displacement events during the last 8,500 years (with slip rates of 1-11 mm/year), as well as evidence of movement that may have been associated with hydrothermal eruption events.

Figure 4. Aerial photograph of the Kawerau area, showing inferred fault locations (i.e. dominant NE-SW trending normal faults, downthrown to NW, and NNW-SSE trending cross-faults), from Bignall (2007; after SKM 2003).

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Structural interpretations have been proposed within the Kawerau Geothermal Field, based on air photograph interpretation (SKM, 2003) and drillhole stratigraphic correlations (including Grindley, 1986; Christenson, 1987; Nairn and Beanland, 1989; Allis et al., 1995; Stevens and Koorey, 1996; Wood et al., 2001), as shown in Figure 4. The locations of many faults are not precisely known, as sediments mask recent fault traces. No surface traces are evident within the drilled part of the Kawerau Geothermal Field. However, a fault step is inferred towards the southern side of the field, which corresponds to a fault intersection encountered by KA29. In the past, a fault has been correlated with fluid production from KA24 (initially producing 340 t/hr), KA30 (500 t/h) and KA35 (660 t/h; Grindley, 1986), with another fault postulated in the deepened part of KA17 (Nairn, 1982).

Wood et al. (2001) contoured the depth to the greywacke basement and postulated easterly striking faults cut the Kawerau area. Allis et al. (1995) suggested basement displacements occurred mainly before the eruption of the Kawerau Andesite, as apparent displacements decrease upwards along inferred fault planes. As well as NE-SW trending faults, there is also evidence for NW-trending cross faults. Vents on Putauaki are strongly aligned, with their extrapolation to the northwest passing close to the southern-most surface manifestations and southwestern resistivity boundary of the field (Christenson, 1987; Stevens and Koorey, 1996). There is also evidence from apparent stratigraphic displacements for a buried fault, with an inferred WNW trend (downthrown to the north), in the northern part of the Kawerau Geothermal Field (Grindley, 1986), extending from the KA21 / KA27 area towards KA17, with a hydrological connection between the two areas indicated by interference tests. The Edgecumbe Fault ruptured ~1 km outside the northwestern margin of the Kawerau Geothermal field during the 1987 Edgecumbe Earthquake (Hodges et al., 1991; Beanland et al., 1989), and demonstrates the potential for movement on active faults within the geothermal system.

6.0 STRUCTURAL - STRATIGRAPHIC CONTROLS ON PERMEABILITY

The nature of reservoir permeability within the Kawerau geothermal system is important to understand, and is described here, both for locating permeability that may provide fluid for high productivity wells, as well as to recognise how the field has evolved, and how it may change under future production. Permeable zones identified by drilling and well testing at Kawerau have provided invaluable information about the hydrothermal system, whilst the temperature and pressure distribution and the way the reservoir has changed during production provides insights into the permeability structure of the field.

The hydrological model of the Kawerau geothermal system envisages fluid moving upwards (and laterally) via widely spaced active faults (and/or fracture zones) of high local permeability (based on interference testing), within largely impermeable Torlesse basement greywacke. Indeed, the intersection of basement faults and fractures has been inferred in a number of Kawerau wells (including KA21 and KA27). Extension of the fractures into the overlying sub-horizontal pyroclastic units, lavas and sediments facilitates lateral spreading of the geothermal fluid. Mixing occurs at shallow levels, as cooler waters enter the field along gently-dipping cover strata, producing temperature reversals in marginal wells and lower temperatures in shallow production aquifers. The southern part of the Kawerau Geothermal Field contains wells with measured temperatures up to 320°C (e.g., in KA34). Whilst recent wells have been successful in finding permeability (e.g., KA41, KA42, KA45A, KA46), not all wells drilled in the southern part of the system have encountered production permeability.

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The Kawerau Geothermal Field lacks extensive intraformational aquifers, and drillhole production relies on intersecting permeable faults and/or fractures in impermeable ignimbrite, rhyolite, andesite (e.g., Kawerau Andesite is lava of low intrinsic permeability, but with fractures that facilitate good lateral permeability) and greywacke basement. The volcanic units tend to be permeable only where they are hard (i.e., brittle) enough to support open fractures (e.g., in welded ignimbrites, or massive rhyolite lavas).

6.1 Stratigraphic permeability

The production area at Kawerau is characterised by a number of permeable, formation- hosted aquifers, separated by low permeability formations, including lacustrine sediments of the Tahuna Formation and welded ignimbrite of the Matahina Ignimbrite (Figure 3). Good permeability within the shallow part of the production area is likely to be associated with enhanced stratigraphic permeability, related to primary porosity (e.g., within breccia units).

Whilst welded ignimbrite sheets can form barriers that restrict vertical fluid movement, unwelded layers also exist within some pyroclastic units at Kawerau, and these layers allow lateral fluid flow. The Matahina Ignimbrite has a highly welded central member and is commonly inferred to be impermeable, it also has high porosity upper and lower members with good horizontal permeability, that act as a groundwater aquifer.

In the deep Kawerau geothermal reservoir, lithological permeability is mainly confined to fractured lithologies, such as the Kawerau Andesite (e.g., in KA17, KA19, KA35, KA43) and the Caxton Formation (e.g., in KA19, KA24, KA28, KA30). Sedimentary units within the Tamurenui Subgroup, occurring above basement greywacke (including Waikora Formation), have also shown useful production across the Kawerau Geothermal Field.

6.2 Structural permeability

Fracture permeability within the Kawerau Geothermal Field is hosted by competent volcanic units (e.g., Caxton Formation, Kawerau Andesite) and greywacke basement. Well drilling points to the development of large-scale fracture networks and/or faulting in these units, which facilitate vertical and horizontal fluid movement, and present targets for production drilling. Much of the present production derives from fractured greywacke, but success in locating this permeability has been mixed.

Whilst a fault block structure for the Kawerau Geothermal Field has long been inferred, early wells drilled vertically into basement in the south-eastern part of the field (e.g. KA25, KA26, and KA34) did not intersect faults. It has only been with the drilling of deviated wells KA41, KA42, KA45A and KA46 that geothermal production has been proven in that part of the Field.

The relationship between fracture permeability and production at Kawerau is illustrated by past drilling results from KA3, KA35 and KA37. Well KA3 was vertically drilled through unfaulted Kawerau Andesite on the upthrown side of a major fault and produced <50 t/h, whereas KA35 was deviated into the fault zone and produced >600 t/h from fractured andesite. Well KA37 was drilled as a deviated well within greywacke basement for a horizontal distance of ~300 m. Until the drilling of KA37, it was believed nearby KA8 was close to a major deep upflow, as it discharged fluid with high downhole chloride composition. Well KA37 showed that wells produced from a KA36-KA37-KA21 oriented fault zone.

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A NW-trending fault inferred by the alignment of vents on Putauaki volcano may also provide an important fluid pathway at Kawerau, with its intersections with NE-trending faults facilitating movement of reservoir fluids to the current production borefield. Robinson (1989) identified a seismic gap in earthquake epicentres at Kawerau-Putauaki, from after-shocks of the 1987 Edgecumbe earthquake, which he suggested may be due to the reservoir rocks being too hot (i.e. ductile) to sustain brittle failure. Robinson (1989) argued that the rock behaviour might explain why some Kawerau-Putauaki wells were not productive, despite tapping high temperatures, whilst productive wells occurred at the boundary between largely impermeable hot (ductile) rock (i.e. close to Putauaki volcano) and surrounding brittle rock.

7.0 HYDROTHERMAL ALTERATION

The Kawerau Geothermal Field is inferred to be one of the longest-lived hydrothermal areas in New Zealand (Browne, 1978), and the fluid-rock interaction data provide insight into its evolution. Hydrothermal mineral assemblages at Kawerau are generally in equilibrium with the fluids from which they deposited, with alteration mineralogy indicating the presence of a near-surface acid alteration zone within the shallow geothermal system, which passes to a ‘typical’ neutral-pH hydrothermal assemblage with increasing depth.

Acid alteration forms a ~100 m-thick surficial “blanket” at Kawerau, affecting shallow levels of the system. Near surface ash and alluvium are typically unaltered, although minerals such as native sulphur, consistent with condensation of steam containing H2S above a boiling zone, have been identified (Nairn and Wiradiradja, 1980). The influence of cool meteoric fluids extends to >300 m depth, as much as 750 m deep in some south-eastern wells (e.g., KA25, KA26 and KA29) and 900 m depth in KA23. It is important to recognise the presence of low temperature mineral assemblages (e.g., halloysite, cristobalite, kaolinite, smectite, tridymite) at levels deeper than high temperature mineral indicators (e.g., wairakite or epidote), as it may indicate the presence of cool, acidic(?) waters that might need to be cased off.

Temperature and/or chemical changes within a geothermal reservoir may be indicated by the co-existence of non-equilibrium mineral assemblages. At Kawerau, ‘neutral’ and ‘low pH’ mineral assemblages are reported together, or in close proximity, as in KA5, KA16, KA21 and KA25. In KA21, cristobalite, tridymite and alunite (characteristic of acid fluids) occur with smectite and albite (indicative of neutral conditions), but the occurrence of low pH mineral indicators is difficult to explain, as there are no surficial acid fluids in the KA21 area and there is little driving force for the ingress of near-surface acid waters.

Christenson (1987) described the occurrence of chalcedony and epidote in the same vein in KA10, and reflected on the chalcedony being indicative of <190°C fluid conditions whereas the latter first appears in Kawerau wells where downhole temperatures are 250-290°C (although it is recognised chalcedony may not necessarily indicate lower temperatures, as it sometimes occurs as a result of rapid depressurisation in boiling zones). High- and low temperature mineral assemblages also occur in KA16 and KA23, with the low temperature minerals overprinting neutral pH, high temperature mineral assemblages. For the most part, thermal conditions inferred from the occurrence of high temperature mineral assemblages are consistent with measured well temperatures, with the overprinting assemblage reflecting localised cool downflows.

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Most of the producing wells at Kawerau are close to their boiling point for depth condition, and this is reflected in the nature of their hydrothermal alteration mineralogy. Illite clay occurs from ~300 to 500 m drilled depth, which is consistent in most wells with boiling point for depth conditions. In some wells, however, the mineralogy indicate the isotherms are depressed; e.g. interlayered clays persist to ~500 m depth in KA25, with hydrated halloysite, cristobalite and kaolinite (indicative of <160°C, and somewhat acidic fluids) at ~746 m depth, with the first appearance of illite at ~820 m depth (indicative of >210°C). Although KA25 encountered high temperatures at depth, it is likely the upper 800 m in this part of the field has always been relatively cool, due to the influx of cool near surface waters via channels created by past hydrothermal eruption events (Nairn and Wiradiradja, 1980).

Hydrothermal alteration mineralogy indicates cooling in parts of the field over a geological timescale, with epidote in some wells where measured temperatures are lower than 240°C

(e.g. KA22, 480 m, Tmeas = 210°C; KA29, 487 m, Tmeas =120°C; KA34, 739 m, Tmeas =195°C). The presence of disequilibrium alteration assemblages in clasts in Tahuna Formation breccias also points to thermal change in the hydrothermal system. As well as cooling in some areas, there are indications of heating, such as in the south-east part of the geothermal field, which may have caused (geologically recent) hydrothermal eruptions in that area.

8.0 GEOLOGICAL HAZARDS

There are a number of geological hazards with potential to impinge on field development and management at Kawerau; they are:

Earthquakes: The Whakatane Graben area is tectonically active, as shown by the occurrence of the 1987 Edgecumbe Earthquake. Even though Berryman and Beanland (1988) suggested there is a return period of ~1000 years for major earthquakes in the area, geothermal field development needs to take into account another major earthquake may occur within the lifetime of the geothermal programme. The effect of the 1987 Edgecumbe Earthquake was to cause ~0.3 m of subsidence, although no fault ruptures occurred in the vicinity of the geothermal production area.

Volcanic eruption: There are three volcanic hazards in Kawerau area, from: (a) local eruptions, i.e. from Putauaki (Mt Edgecumbe) volcano; (b) eruptions from the Okataina Volcanic Centre; and (c) regional-scale eruption events related to volcanic activity in the TVZ.

Nairn (1995) assessed the hazard posed by Putauaki volcano, and indicated eruptions are likely to reoccur, although the eruptive history is too short to accurately predict its future behaviour. It is likely future eruptions will be of similar magnitude to past events, and may trigger hydrothermal eruptions. An eruption from Putauaki would likely involve intermittent ash falls and likely have only a small effect on the area, although ballistic impacts could affect resource utilisation more seriously. Only in the worst case, involving an extensive lava flow and/or pyroclastic surge, would a Putauaki volcanic eruption cause widespread damage to electricity generation facilities (and/or the paper mill) in the Kawerau area.

The effect of a volcanic eruption from the Okataina Volcanic Centre is difficult to quantify. Volcanic events from Okataina could result in minor ash fall, with negligible effect, whilst a major volcanic eruption could impact the plant and structures in the area. The possibility of a major ignimbrite eruption over the period of the project is unlikely. Regional eruptions from

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the TVZ would have similar effects to an eruption from the Okataina Volcanic Centre. The probability of a regional eruption substantially affecting electricity power generation at Kawerau is likely small. If a major ignimbrite eruption did occur from the TVZ, and inflicted severe damage in the central North Island, then this would impact on the Kawerau development, whether or not the Kawerau area was directly affected.

Flooding: Flooding has been a notable occurrence at Kawerau, with several historical events from the Tarawera River; notably in 1904 A.D. due to natural dam failure at the outlet of Lake Tarawera, and in ~1350 A.D. following volcanic eruptions from Mt. Tarawera (Hodgson and Nairn, 2000). It is possible that further dam building and flooding might occur in the future, and damage to infrastructure might occur.

Hydrothermal eruptions: There is a long inferred history of hydrothermal eruptions within the Kawerau Geothermal Field, with possible hydrothermal events at ~280,000 years and 330,000 years ago. In addition, hydrothermal eruption breccias are exposed at the ground surface and have been dated by tephrachronology at between 9,000 and 14,500 years, as well as other undated hydrothermal eruption features described by Christenson (1987, 1997). It is possible hydrothermal eruptions may occur in the Kawerau thermal area in the future.

9.0 GEOLOGICAL MODEL OF THE KAWERAU GEOTHERMAL FIELD

The geological model of the Kawerau Geothermal Field is based on drillhole stratigraphy (logging), insights from recent U-Pb dating of units, and published (regional) information of the actively subsiding, NE-trending Whakatane Graben. In the Kawerau area, basement Torlesse greywacke is step-faulted on dominant NE-trending normal faults, downthrown to the NW, which combined with cross-cutting NW-trending faults has produced a series of fault blocks that plunge to the north-east (coincident with the regional structural fabric).

Kawerau occupies a structural depression, in which a thick sequence of volcanic lava and volcaniclastic units overlie older basement greywacke, with fluid recharge occurring via faults in the impermeable basement. The total area of the Kawerau Geothermal Field, based on data from resistivity surveys is ~20-35 km2, with the productive area covering ~10 km2.

Insights from exploration and production well drilling at Kawerau point to deep-sourced reservoir fluids in the southern part of the geothermal system (towards Putauaki volcano) moving upwards (and laterally) through basement greywacke, via widely spaced, steeply dipping, NE-trending normal faults (and cross-cutting NW-trending faults) and/or fractures of high local permeability, within otherwise largely impermeable rock (Figure 5). Faults and fractures in the basement greywacke pass into overlying units, where hot water spreads laterally into permeable zones along subhorizontal volcanic and sedimentary layers. Reservoir rocks in the Kawerau geothermal system comprise locally derived volcanic lavas (e.g., Kawerau Andesite, Caxton Formation) and distally-sourced silicic pyroclastic rocks (e.g., ignimbrites of the Karaponga, Raepahu and Te Teko Formations), which overlie greywacke basement. Lacustrine sediments and welded intervals within some ignimbrite units act as aquicludes and separate the volcanics into discrete aquifers.

The existence of buried rhyolite complexes is consistent with local rhyolite magmatic heat sources that have underlain the Kawerau area for >350,000 years, and produced the Onepu and Caxton Formation rhyolites. Whilst Putauaki volcano is younger than the Kawerau

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geothermal system its surface development does not appear to have affected the system. Geothermal exploration and production drilling points to Putauaki being the heat source and upflow area for the Kawerau geothermal system, with reservoir permeability controlled by the location of active faults and fracture networks. Faults are present in the south, but these are inactive and possibly sealed by hydrothermal mineral deposition.

Figure 5 shows a SE-NW profile through the Kawerau geothermal system, with inferred isotherms defined by measured well temperatures and reservoir permeability hosted in the top 0.5 to 1 km of the basement greywacke (and overlying volcanics). Drillhole permeability data helps to define the extent of the reservoir, with the field boundary inferred from resistivity and gravity surveys. Resistivity data, combined with the inferred location of the main heat and mass outflow, is consistent with deeply-sourced thermal fluids flowing from the south towards the northern part of the field. A subsurface outflow is inferred to extend to the north, with progressive dilution and cooling of the geothermal fluid as it mixes with groundwaters in the Tarawera-Rangitaiki flood plain.

Figure 5. Conceptual model of the Kawerau Geothermal Field (from Bignall 2011, after Holt, 2007 and Christenson, 1997).

Only part of the deep geothermal upflow reaches the surface within the present Kawerau Geothermal Field, with most of the natural surface outflow occuring via seepage to the Tarawera River. Thermal inversions have been identified in basement greywacke in some wells towards the north-western extremity of the system (e.g., KA24, KA27 and KA32).

The present production borefield occurs in the central part of the field, in an area where there is relatively high permeability over a wide range of depths (with the top of the reservoir at about -400 mRL), where the geothermal fluid rises into shallow aquifers and towards the ground surface, via a number of closely-spaced normal faults. Development effects have impacted on the reservoir fluids, and include temperatures reducing in some wells, particularly related to the shallow aquifers, as well as precipitation of calcite in wells and/or adjacent feedzones and associated degassing of aquifer fluids, although these effects are not discussed further in this geology review of the Kawerau Geothermal Field.

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10.0 REFERENCES

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Carroll, L. D., Gamble, J. A., Houghton, B. F., Thordarson, T., Higham, T. F. G. 1997. A radiocarbon age determination for Mount Edgecumbe (Putauaki) volcano, Bay of Plenty, New Zealand. New Zealand Journal of Geology and Geophysics, 40: 559-562. Christenson, B. W. 1987. Fluid-mineral equilibria in the Kawerau hydrothermal system, Taupo Volcanic Zone, New Zealand. Thesis (Ph.D.). University of Auckland, 1987. 452 p. Christenson, B. W. 1997. Kawerau geothermal field: geochemical structure of the reservoir and its response to exploitation. Geothermal Resources Council Transactions, 21: 17-24. Cole, J.W., Spinks, K.D., Deering, C.D., Nairn, I.A., Leonard, G.S. 2010. Volcanic and structural evolution of the Okataina Volcanic Centre; dominantly silicic volcanism associated with the Taupo Rift, New Zealand. Journal of Volcanology and Geothermal Research 190, 123-135. Cooper, G.F., Wilson, C.J.N., Millet, M.-A., Baker, J.A., Smith, E.G.C., 2012. Systematic tapping of independent magma chambers during the 1 Ma Kidnappers supereruption. Earth and Planetary Science Letters, 313-314: 23-33. Darby, D. J., Meertens, C.M. 1995. Terrestrial and GPS measurements of deformation across the Taupo back arc and Hikurangi forearc regions in New Zealand. Journal of Geophysical Research, 100: 8221-8232. Edbrooke, S.W. (compiler), 2005. Geology of the Waikato area: scale 1:250,000. Institute of Geological & Nuclear Sciences 1:250,000 geological map 4. Institute of Geological & Nuclear Sciences Limited, Lower Hutt, New Zealand. Gravely, D.M., Wilson, C.J., Rosenberg, M.D., Leonard, G.S. 2006. The nature and age of Ohakuri Formation and Ohakuri Group rocks in surface exposures and geothermal drillhole sequences in the central Taupo Volcanic Zone, New Zealand. New Zealand Journal of Geology and Geophysics 49, 305-308. Gravley, D.M., Wilson, C.J.N., Leonard, G.S. and Cole, J.W. 2007. Double trouble: Paired ignimbrite eruptions and collateral subsidence in the Taupo Volcanic Zone, New Zealand. Geological Society of America Bulletin 119: 18-30. Grindley, G.W. 1959. Sheet N85 Waiotapu, “Geological Map of New Zealand 1:63,360”. New Zealand Department of Scientific and Industrial Research, Wellington, New Zealand. Grindley, G.W. 1965. The geology, structure and exploitation of the Wairakei Geothermal field, Taupo, New Zealand. New Zealand Geological Survey Bulletin 75. Grindley, G.W. 1986. Subsurface geology and structure of the Kawerau geothermal field. The Kawerau geothermal field: contributions from the 1982 seminar and other recent scientific investigations: 49-65. DSIR Geothermal report 10. Healy, J., Schofield, J. C. Thompson, B. N. 1964. Sheet 5 Rotorua (1st ed.). “Geological Map of New Zealand 1:250,000”. DSIR, Wellington, New Zealand. Healy, J. 1974. Kawerau geothermal field. Minerals of New Zealand: part D, geothermal: Section 3.3. New Zealand Geological Survey report 38D. Hikuroa, D.C.H., Gravley, D.M., Wilson, C.J.N., Browne, P.R.L., Olsen, A.W. 2006. Recent stratigraphic studies at Matata: implications for Kawerau geothermal field modelling and subsurface interpretation. Proceedings, 28th New Zealand Geothermal Workshop, Auckland, New Zealand, 16-17 November 2006, 6 pp (CD-ROM).

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Hodges, S., O'Brien, C., O'Shaughnessy, B., Wilson, A. 1991. Rangitaiki Plains groundwater resource evaluation. Bay of Plenty Regional Council technical publication 2: 1 v. Hodgson, K., Nairn, I. A. 2000. The catastrophic ~1350AD post-eruption flood from Lake Tarawera, New Zealand. Bay of Plenty Regional Council resource planning publication 2000/01: 61 p. Hodgson, K.A., Nairn, I.A., 2005. The c. AD 1315 syn-eruption and AD 1904 post-eruption breakout floods from Lake Tarawera, Haroharo caldera, North Island, New Zealand. New Zealand Journal of Geology and Geophysics, 48: 491-506. Holt, R.J., 2007. Numerical Model of the Kawerau Geothermal Reservoir. Report Submitted to Mighty River Power Limited, June 18, 2007. Houghton, B.F., Wilson, C.J.N., McWilliams, M.O., Lanphere, M.A., Weaver, S.D., Briggs, R.M., Pringle, M.S., 1995. Chronology and dynamics of a large silicic magmatic system: central Taupo Volcanic Zone, New Zealand. Geology, 23: 13-16. Jurado-Chichay, Z., Walker, G. P. L., 2000. Stratigraphy and dispersal of the Mangaone Subgroup pyroclastic deposits, Okataina Volcanic Centre, New Zealand. Journal of Volcanology and Geothermal Research 104: 319-383. Leonard, G.S., Begg, J.G. and Wilson, C.J.N. (compilers), 2010. Geology of the Rotorua area. Scale 1:250,000. GNS Science geological map 5, 102 p. + 1 folded map. GNS Science, Lower Hutt, New Zealand. MacDonald, W.J.P., Muffler, L.J.P. 1972. Recent geophysical exploration of the Kawerau Geothermal Field, North Island, New Zealand. New Zealand Journal of Geology and Geophysics, 15(3): 303-317. Martin, R.C., 1961. Stratigraphy and structural outline of the Taupo Volcanic Zone. New Zealand Journal of Geology and Geophysics 4: 449-478. Milicich, S.D., Fruetsch, F., Ramirez, L.E., Rae, A.J., Alcaraz, S.A., Kallenberg, B., McCoy- West, A.J., Bignall, G. 2010. Stratigraphic correlation study of the Kawerau Geothermal Field. GNS Science Consultancy Report 2010/23. Confidential report to Mighty River Power Limited. Milicich, S.D., Wilson, C.J.N., Bignall, G., Pezaro, B., Charlier, B.L.A., Wooden, J.L., Ireland, T.R., 2011. Buried rhyolite in the Kawerau Geothermal Field, Taupo Volcanic Zone, New Zealand: sources of a rejuvenated geothermal system. Proceedings of the New Zealand Geothermal Workshop 2011: University of Auckland. Milicich, S.D., Wilson, C.J.N., Bignall, G., Pezaro, B., Charlier, B.L.A., Wooden, J.L., Ireland, T.R. (submitted). U-Pb dating of zircon in hydrothermally altered rocks of the Kawerau Geothermal Field, Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research. Mortimer, N. 1995. Origin of the Torlesse Terrane and coeval rocks, North Island, New Zealand. International Geology Review, 36: 891-910. Mortimer, N., 2004. New Zealand's geological foundations. Gondwana Research, 7: 261-272. Nairn, I. A. 1977. Geology of Kawerau geothermal field : preliminary report on new data. Geothermal circular IN/1: 1 v.

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Nairn, I. A. 1981. Some studies of the geology, volcanic history and geothermal resources of the Okataina volcanic centre, Taupo Volcanic Zone, New Zealand. Thesis (Ph.D.) - Victoria University of Wellington, 1981: 371 p. Nairn, I. A. 1982. Geology of Kawerau geothermal field (MK11) : results of drilling, 1977- 1982. Geothermal circular IAN/4: 23 p. Nairn, I. A. 1986. Geology of Kawerau geothermal field - results of drilling, 1977-present. The Kawerau geothermal field: contributions from the 1982 seminar and other recent scientific investigations: 23-47. DSIR Geothermal report 10. Nairn, I. A. 1989. Sheet V16AC – Mt. Tarawera. Geological map of New Zealand 1:50,000 Map and Notes. Department of Scientific and Industrial Research, Wellington. Nairn, I.A. 1995. The probability and likely effects of a future eruption at Mt Edgecumbe (Putauaki). Report prepared for Bay of Plenty Regional Council. Nairn, I.A., 2002. Geology of the Okataina Volcanic Centre : sheets part U15, part U16, part V15 & part V16, scale 1:50,000. Institute of Geological & Nuclear Sciences geological map. Institute of Geological & Nuclear Sciences, Lower Hutt, 156 pp. Nairn, I. A., Beanland, S. 1989. Geological setting of the 1987 Edgecumbe earthquake, New Zealand. New Zealand Journal of Geology and Geophysics, 32(1): 1-13. Nairn, I. A., Solia, W. 1980. Late Quaternary hydrothermal explosion breccias at Kawerau Geothermal Field, New Zealand. Bulletin of Volcanology, 43 (1): 1-13. Nairn, I. A., Wiradiradja, S. 1980. Late Quaternary hydrothermal explosion breccias at Kawerau geothermal field, New Zealand. Bulletin Volcanologique, 43(1): 1-13. Robinson, R. 1989. Aftershocks of the 1987 Edgecumbe earthquake, New Zealand: seismological and structural studies using portable seismographs in the epicentral region. New Zealand Journal of Geology and Geophysics, 32:61-72. Sissons, B. A. 1979. The horizontal kinematics of the North Island of New Zealand. Thesis (Ph.D.) – Victoria University of Wellington, 1979.: 117 p. SKM 2003. Putauaki Geothermal Project. Resource Review and Exploration Well Strategy. Report to Mighty River Power Limited. October 2003. Stevens, L., Koorey, K. J. 1996. Interpretation of interference effects in three production wells in the Kawerau Geothermal Field, New Zealand. Proceedings of the 21st Workshop on Geothermal Reservoir Engineering: 427-432: Stanford Geothermal Program, Stanford University. Studt, F. E. 1958. Geophysical reconnaissance at Kawerau, New Zealand. New Zealand Journal of Geology and Geophysics, 1(2): 219-246. Tulloch, A. J. 1990. Petrological logs of drillholes KA 27-35 and KA 10, 17 (deepened), Kawerau Geothermal Field. DSIR Geology and Geophysics contract report 1990/11:66 p. Wilson, C.J.N., 1986. Reconnaissance stratigraphy and volcanology of ignimbrites from Mangakino volcano. In: I.E.M. Smith (Ed.), Late Cenozoic Volcanism in New Zealand. Royal Society of New Zealand Bulletin 23, 179-193. Wilson, C.J.N., Houghton, B.F., McWilliams, M.O., Lanphere, M.A., Weaver, S.D., Briggs, R.M. 1995. Volcanic and structural evolution of Taupo Volcanic Zone, New Zealand: a review. Journal of Volcanology and Geothermal Research 68, 1-28.

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Wilson, C.J.N., Rhoades, D.A., Lanphere, M.A., Calvert, A.T., Houghton, B.F., Weaver, S.D., Cole, J.W., 2007. A multiple-approach radiometric age estimate for the Rotoiti and Earthquake Flat eruptions, New Zealand, with implications for the MIS 4/3 boundary. Quaternary Science Reviews 26, 1861-1870. Wilson, C.J.N., Charlier, B.L.A., Fagan, C.J., Spinks, K.D., Gravley, D.M., Simmons, S.F., Browne, P.R.L., 2008. U-Pb dating of zircon in hydrothermally altered rocks as a correlation tool: application to the Mangakino geothermal field, New Zealand. Journal of Volcanology and Geothermal Research, 176: 191-198. Wilson, C.J.N., Charlier, B.L.A., Rowland, J.V., Browne, P.R.L., 2010. U-Pb dating of zircon in subsurface, hydrothermally altered pyroclastic deposits and implications for subsidence in a magmatically active rift: Taupo Volcanic Zone, New Zealand. Journal of Volcanology and Geothermal Research, 191: 69-78. Wood, C. P. 1996. Basement geology and structure of TVZ geothermal fields, New Zealand. Proceedings of the 18th New Zealand Geothermal Workshop: 157-162: Geothermal Institute, University of Auckland. Wood, C. P., Brathwaite, R. L. 1999. The basement at Kawerau geothermal field. Proceedings of the 21st New Zealand Geothermal Workshop: 101-106: Geothermal Institute, University of Auckland. Wood, C. P., Brathwaite, R. L., Rosenberg, M. D. 2001. Basement structure, lithology and permeability at Kawerau and Ohaaki geothermal fields, New Zealand. Geothermics, 30(4): 461-481.

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APPENDICES

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APPENDIX 1 GEOTHERMAL (GEOSCIENCE) GLOSSARY

Adiabatic (a thermodynamic process) taking place without loss or gain of heat

Andesite fine grained, intermediate composition (52–66 wt% SiO2) volcanic igneous rock (sometimes with a porphyritic texture, of large crystals in a fine grained crystal groundmass) Anhedral grains devoid of crystal faces Aquiclude a layer of rock, sediment, or soil through which ground water cannot flow Aquitard An aquitard is a zone within the earth that restricts the flow of groundwater from one aquifer to another. An aquitard can sometimes, if completely impermeable, be called an aquiclude or aquifuge Aquifer a permeable, underground rock formation that stores and transmits groundwater (and geothermal reservoir fluids) in sufficient quantity to supply wells Argillite mudstone, hardened by incipient low grade regional metamorphism Aseismic (area or region) free of earthquakes Basement oldest rocks in a given area, commonly a complex of metamorphic and igneous rocks that underlie younger sedimentary or volcaniclastic formations (in the Taupo Volcanic Zone, this is Mesozoic greywacke) Breccia rock consisting (typically) of angular fragments embedded in a finer matrix, produced either by sedimentary processes, hydrothermal or volcanic activity (e.g. “brecciated” or broken fragments of a lava flow, coarse-grained pyroclastic deposit, or hydrothermal eruption) Brittle description of material that breaks abruptly when its elastic limit is reached (opposite of ductile) Calcite common mineral, consisting of crystallized calcium carbonate, occurring naturally in geothermal systems (also constituent of limestone etc), as well as forming as a scale/precipitate in geothermal wells Carapace (thick) enveloping cover Carbonaceous relating to, or consisting of carbon; or containing carbonaceous material Clast a rock fragment or grain resulting from the breakdown of larger rocks Cristobalite common silica mineral (low temperature polymorph of quartz), naturally forms in hydrothermal systems where it is regarded as indicative of low (cool) temperature alteration (typically <120 oC in NZ geothermal systems

Dacite silica-rich igneous, volcanic rock – high wt% SiO2 (~60-70% wt % SiO2) composition, albeit intermediate between andesite and rhyolite Degassing removal, or loss, of gas (e.g. from magma body as it rises to shallow depth in the crust) Diatomaceous containing diatoms (microscopic unicellular alga, having a cell wall impregnated with silica) or their fossil remains Dip angle by which a rock layer / strata or other planar feature deviates from the horizontal plane (perpendicular to the strike of the rock unit) Drawdown lowering of the watertable and/or fluid pressure in a geothermal reservoir as a consequence of fluid abstraction Dyke a roughly planar body of intrusive igneous rock that has discordant contacts (i.e. cuts across bedding or foliation) with the surrounding rock

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Enthalpy a thermodynamic quantity equal to the internal energy of a system, measures amount of available stored energy in a substance for extraction Epidote common hydrothermal (igneous and metamorphic) mineral: hydrous calcium aluminium iron silicate, typically regarded as precipitating from >240 oC fluid, and therefore a mineral indicator of “high-temperature” reservoir conditions Equilibrium stable state of a system - once equilibrium is reached, no further net change occurs in the chemical or physical state of materials in the system, or in their proportions without some external interference Euhedral minerals entirely bounded by crystal faces Fault a planar or gently curved fracture in the Earth’s crust, across which there has been relative displacement Feedzone zone of fluid inflow into a well or geothermal system Fumarole (small) vent in the ground from which volcanic gases and steam (heated geothermal fluids) emerge – a local, discrete manifestation, rather than an “area” of steaming ground Geothermometer solute geothermometers based on mineral solubility (silica) and exchange reactions (Na-K; Na-K-Ca etc); gas (e.g. CO2-H2S-H2-CH4 empirical geothermometer) and isotope (based on temperature dependent isotopic fractionation) to estimate the temperature of the reservoir fluid Geyser hot spring that forcibly ejects (near-boiling) hot water and steam into the air at a regular interval Graben downfaulted structural depression (valley or trough) and zone of (regional-scale) subsidence, bounded by large-scale faults and blocks of uplifted rock (a tensional feature) Greywacke poorly sorted, fine-grained (lithic-rich) sandstones, interbedded with argillite (or mudstone), hardened by incipient low grade regional metamorphism Habit the size and shape of the crystals in a crystalline solid Heat flow the rate at which heat escapes at the Earth’s surface, related to the nature of the surface rocks and rate at which heat is supplied from below Halloysite clay-like mineral, occurring in soft amorphous masses - can occur as a product of low temperature hydrothermal alteration or surface weathering of aluminosilicate minerals (such as feldspar) Holocene recent geological epoch, beginning about 10,000 years ago Hydrothermal rare geological event, caused by the expansion of gases and liquids from Eruption below the earth’s surface as they depressurise to atmospheric conditions, causing the expulsion of water, steam and rocks (forming a breccia deposit, typically of hydrothermally altered material) Ignimbrite volcanic rocks formed as a result of deposition of ash flows at high temperature, consisting of layers of tuff (including pumice, crystals and rock fragments) that may have been be so hot when that the fragments weld together when it was deposited (forming an impermeable rock layer) Illite hydrous aluminous silicate clay mineral, formed by hydrothermal alteration or weathering, typically of aluminium-rich minerals. Common mineral in hydrothermal setting, regarded as indicative of precipitation from >220 oC, near neutral pH to weakly acidic fluid Isotherms (on geological cross-section), contour/line of equal temperature, e.g. 250 oC

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Isotope one of two or more forms of the same (chemical) element who vary by having a different number of neutrons Kaolinite clay mineral is formed by the weathering of feldspar, and in hydrothermal systems typically indicative of low temperature (<160 oC), commonly somewhat acidic fluid, although can occur over a wide range of conditions Lacustrine formed or deposited in lakes (e.g. Huka Falls Formation) Lapilli rock fragments between 2 and 64 mm in diameter ejected from a volcano during an explosive eruption (means "little stones" in Italian). Lapilli may consist of many different types of tephra, including scoria and pumice Lava term given to magma when it reaches the Earth’s surface Lenticulitic descriptive term, elongate, flattened and/or stretched (pumice) fragments giving a pyroclastic rock a streaky appearance Lineation linear arrangement of features (e.g. springs, eruption vents, surface displacements) Lithic fragment of rock Lithology systematic description of rocks, in terms of their mineral composition and abundance, and texture Magma molten rock found beneath the Earth’s crust from which igneous rocks are formed (may contain solid particles and gases) Massive rock rock that is little or not broken by joints, cracks, foliation or bedding, and tending to show a homogeneous appearance Mesozoic geological period, from ~249 million years to 65 million years B.P, which includes the Triassic (245-208 Million Years Ago), Jurassic (208-146 Million Years Ago), and Cretaceous (146-65 Million Years Ago) periods Metamorphosed (rock) to have undergone, or caused to have undergone, a considerable change (metamorphosis) from its original structure and composition, due to (increased) heat and/or pressure Meteoric water water derived from atmospheric sources (rainwater, snow, etc) Methylene Blue Methylene Blue (MeB) is an organic dye that shows high selectivity for adsorption by smectite clays. This selectivity permits it to be used to estimate, on a semi- quantitative basis, the amount of smectite present in alteration mineral assemblages. Paleosol ancient soil layer Permeability measures the resistance of rock formations to fluid flow (primary (e.g. lithological or formational), or secondary (e.g. fracture) permeability). Permeabilities are measured in Darcies or milli-Darcies Petrology study of the composition, origin structure and formation or rocks Phreatomagmatic (violent) volcanic eruption (of mud and debris) caused by the expansion of steam formed when magma comes in contact with a confined groundwater (or “wet” lacustrine sediments) Porosity measures the capacity of the rock to store water, and is measured as a fraction (percentage) of the total volume of the rock or rock unit (i.e. not occupied by mineral or rock fragments) Porphyritic large crystals (phenocrysts) enclosed in a fine grained matrix (groundmass) Porphyry igneous rock containing abundant, coarse-sized crystals (phenocrysts)

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Pumice form of volcanic glass, usually high-silica composition and low density, which commonly has a texture characterised by vesicles (a cavity formerly occupied by a bubble of magmatic gases), and resembling a sponge Pyroclastic fragmental volcanic material blown into the atmosphere (ash fall or flow (i.e. ignimbrite) deposits) by explosive volcanic activity, from volcanoes whose magma is a viscous type (e.g. rhyolite, as in TVZ) Quaternary geological period that comprise the Pleistocene and Holocene epochs (2 million years B.P. to present) Resistivity measures electrical resistance of the rock and water. Hot rock and water can often be identified by very low resistivities although low resistivity does not necessarily indicate high temperatures Rhyodacite volcanic rock, equivalent of monzogranite, distinguished from rhyolite on the basis of a somewhat greater proportion of plagioclase crystals (on a plagioclase - alkali feldspar – quartz (mineral) ternary differentiation plot)

Rhyolite fine-grained volcanic or extrusive equivalent of granite, with >68wt% SiO2 - variously banded, massive, glassy or flow-textured appearance Rift zone elongated trough, bounded by faults, producing a tectonic depression (synonymous with graben) Seismic of, subject to, or caused by an earthquake or earth vibration

Silicic (igneous) rock containing more than two-thirds SiO2 by weight, usually as quartz or feldspar (e.g. granite, rhyolite) Sill horizontal, tabular intrusion, with concordant contact (i.e. following the bedding of the country rock) Smectite clay mineral, part of a group of phyllosilicates that contain water trapped between their silicate sheets - Common in hydrothermal systems, where it is produced from near-neutral pH waters, typically of <150 oC Solubility the mass of a substance that can be dissolved in a certain amount of solvent (i.e. a medium, usually a liquid) if chemical equilibrium is attained Stratigraphy the study of the composition, relative position etc of rock strata, in order to determine their geological history Steady state a steady state is one in which pressures, temperatures and flows do not change with time. That is the system is stable and unchanging. Subhedral partly bounded by crystal faces Subsidence a gradual, large or local-scale (primarily) vertical movement (mm/year), where an area of the crust, near surface strata or ground-surface sinks without appreciable deformation Determined by precise levelling. Tephrochronology geochronological technique based on the dating of layers of volcanic ash (c.f. tephra) Transmissivity the transmissivity, T, is a measure of how much water an aquifer can transmit horizontally. Tuff a fragmental rock consisting of the smaller kinds of volcanic detritus, as ash or cinder, usually more or less stratified. Also called volcanic tuff Unconformity surface that separates two strata (rock units), representing an interval of time in which deposition stopped, erosion removed some sediments and rock, and then deposition resumed

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Vadose Zone region in the ground between the surface and the watertable in which pores are not filled with water Vein deposit of minerals within a rock fracture or joint (e.g. mineral deposition from fluids in the active hydrothermal (geothermal) setting) Vitric having the characteristics or appearance of glass Wairakite member of the zeolite-group of minerals, (hydrated calcium-aluminium silicate), common in hydrothermal systems (first identified at Wairakei). Typically regarded o as having precipitated from >200 C, low CO2 fluid Welded tuff pyroclastic tuff that was so hot when deposited that constituent crystal and rock fragments were weld together into a solid unit (typically impermeable, except where a fracture network was developed on cooling) XRD X-ray diffraction (XRD) is a versatile, non-destructive technique that reveals detailed information about the chemical composition and crystallographic structure of materials.

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Principal Location Other Locations

1 Fairway Drive Dunedin Research Centre Wairakei Research Centre National Isotope Centre Avalon 764 Cumberland Street 114 Karetoto Road 30 Gracefield Road PO Box 30368 Private Bag 1930 Wairakei PO Box 31312 Lower Hutt Dunedin Private Bag 2000, Taupo Lower Hutt New Zealand New Zealand New Zealand New Zealand T +64-4-570 1444 T +64-3-477 4050 T +64-7-374 8211 T +64-4-570 1444 www.gns.cri.nz F +64-4-570 4600 F +64-3-477 5232 F +64-7-374 8199 F +64-4-570 4657