Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources: The Ohakuri ignimbrite (Taupō Volcanic Zone, New Zealand)

Journal Pre-proof Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources: the Ohakuri ignimbrite (Taupo¯ Volcanic Zone, New Zealand) Michael J. Heap, Darren M. Gravley, Ben M. Kennedy, H. Albert Gilg, Elisabeth Bertolett, Shaun L.L. Barker

PII:

S0377-0273(19)30468-8

DOI:

https://doi.org/10.1016/j.jvolgeores.2019.106703

Reference:

VOLGEO 106703

To appear in: Received Date:

28 August 2019

Revised Date:

29 October 2019

Accepted Date:

30 October 2019

Please cite this article as: Heap MJ, Gravley DM, Kennedy BM, Gilg HA, Bertolett E, Barker SLL, Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources: the Ohakuri ignimbrite (Taupo¯ Volcanic Zone, New Zealand), Journal of Volcanology and Geothermal Research (2019), doi: https://doi.org/10.1016/j.jvolgeores.2019.106703

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Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources: the Ohakuri ignimbrite (Taupō Volcanic Zone, New Zealand)

Michael J. Heap1*, Darren M. Gravley2, Ben M. Kennedy2, H. Albert Gilg3, Elisabeth

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Bertolett2, and Shaun L. L. Barker4

Géophysique Expérimentale, Institut de Physique de Globe de Strasbourg (UMR 7516

CNRS, Université de Strasbourg/EOST), Strasbourg, France 2

Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand

3

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Lehrstuhl für Ingenieurgeologie, Technische Universität München, Munich, Germany

4

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Centre for Ore Deposits and Earth Sciences (CODES), University of Tasmania, Australia

Highlights

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Silicification reduces matrix permeability by up to four orders of magnitude.

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*Corresponding author: M.J. Heap ([email protected])

Alteration to clay only reduces matrix permeability by up to an order of magnitude. Only silicification increases propensity for permeability enhancing fracture formation.

Silicified rock-masses (reservoir) are more permeable than those altered to clay (cap).

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 

We show how alteration creates geothermal and epithermal resources from a petrophysical standpoint.

Abstract

Hydrothermal fluids can alter the chemical and physical properties of the materials through which they pass and can therefore modify the efficiency of fluid circulation. The role of hydrothermal alteration in the development of geothermal and epithermal mineral resources, systems that require the efficient hydrothermal circulation provided by fracture networks, is investigated here from a petrophysical standpoint using samples collected from a well exposed and variably altered palaeo-hydrothermal system hosted in the Ohakuri ignimbrite deposit in the Taupō Volcanic Zone (New Zealand). Our new laboratory data show that, although quartz and adularia precipitation reduces matrix porosity and permeability, it

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increases the uniaxial compressive strength, Young’s modulus, and propensity for brittle

behaviour. The fractures formed in highly altered rocks containing quartz and adularia are also more planar than those formed in their less altered counterparts. All of these factors

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combine to enhance the likelihood that a silicified rock-mass will host permeability-

enhancing fractures. Indeed, the highly altered silicified rocks of the Ohakuri ignimbrite

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deposit are much more fractured than less altered outcrops. By contrast, smectite alteration at the margins of the hydrothermal system does not increase strength or Young’s modulus, or

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decrease permeability, and creates a relatively unfractured rock-mass. Using our new laboratory data, we provide permeability modelling that shows that the equivalent

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permeability of a silicified rock-mass will be higher than that of a less altered rock-mass or a rock-mass characterised by smectite alteration, the latter of which provides a lowpermeability cap required for an economically viable hydrothermal resource. Our new data show, using a petrophysical approach, how hydrothermal alteration can produce rock-masses

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that are both suitable for geothermal energy exploitation (high-permeability reservoir and low-permeability cap) and more likely to host high-grade epithermal mineral veins, such as gold and silver (localised fluid flow).

Key words: geothermal; epithermal; hydrothermal alteration; porosity; permeability; uniaxial compressive strength

1 Introduction Large-scale hydrothermal convection, driven by magmatic heat (e.g., Ingebritsen et al., 2010), can create economically viable geothermal and epithermal mineral resources (e.g., White and Hedenquist, 1990; Stimac et al., 2015). Geothermal energy exploitation is most efficient at high temperatures and high flow rates and requires a high-permeability reservoir with a low-permeability cap (e.g., Grant and Bixley, 2011; Stimac et al., 2015; Cumming, 2016). Although the permeability required for efficient fluid flow is often provided by fracture networks (e.g., Curewitz and Karson, 1997; Dezayes et al., 2010; Kushnir et al.,

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2018), high matrix porosities and permeabilities contribute to the long-term response of the

reservoir to fluid extraction and porous rocks provide high storage volumes for hydrothermal

fluids (e.g., Stimac et al., 2015). The rocks forming a geothermal system are altered as a result

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of the circulation of hydrothermal fluids and they can be broadly classified into three

alteration zones: argillic, transition, and propylitic (e.g., Stimac et al., 2015 and references

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therein). The argillic zone, dominated by minerals such as smectite and kaolinite, forms the low-permeability cap and the propylitic zone, where illite and adularia may dominate, forms

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the high-permeability reservoir (e.g., Stimac et al., 2015). Epithermal mineral resources can form in the shallow part (< 2 km) of geothermal systems where temperatures are less than 300

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°C (e.g., White and Hedenquist, 1990; Hedenquist et al., 2000; Simmons et al., 2002; Simmons and Rowland, 2012). Epithermal systems can be classified as either lowsulphidation (neutral pH, reduced) or high-sulphidation (acidic, oxidised) (e.g., Hedenquist and Lowenstern, 1994; Sillitoe and Hedenquist, 2003). High-grade epithermal mineral

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deposits typically form in high-flow (i.e. high-permeability) zones and, in particular, when the flow of hydrothermal fluids is focussed within fractures (e.g., Simmons and Rowland, 2012). Since volcanically active areas often host hydrothermal systems, many geothermal

resources, such as those within the Taupō Volcanic Zone of New Zealand (e.g., Browne et al. 1992; Bibby et al., 1995; Kissling and Weir, 2005) and Iceland (Arnórsson, 1995; Fridleifsson and Elders, 2005), and epithermal mineral resources (e.g., Sillitoe, 1985; Heald et al., 1987; Hedenquist et al., 2000; Simmons and Brown, 2007; Rowland and Simmons, 2012;

Simmons et al., 2016) are located within sequences of volcanic and volcaniclastic deposits (e.g., Stimac et al., 2015). It is well known that the circulation of hydrothermal fluids can result in mineral dissolution, replacement, and/or precipitation (e.g., Browne, 1978) that can lead to changes to the physical properties of volcanic rocks (Dobson et al., 2003; Sruoga et al., 2004; Pola et al., 2012; Wyering et al., 2014; Siratovich et al., 2014; Frolova et al., 2014; Heap et al., 2015a, Mayer et al., 2016; Heap et al., 2017a; Mordensky et al., 2018; Cant et al., 2018; Farquharson et al., 2019; Callahan et al., 2019; Heap et al., 2019a). Frolova et al. (2014), for example, showed that the matrix porosity and permeability of volcanic rocks from

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the Kuril-Kamchatka island arc (located in the northwest Pacific Ocean) either increased or

decreased depending on the nature of the host rock, the fluid type and composition, as well as the duration of rock-fluid interaction. Mordensky et al. (2018) found that the matrix

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permeability of andesite breccia affected by argillic alteration at Mt. Ruapehu (Taupō

Volcanic Zone, New Zealand) decreased due to pore- and crack-filling mineral precipitation.

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Heap et al. (2017a) showed that alteration associated with the precipitation of pore- and crack-filling alunite reduced the matrix porosity and permeability of tuff from Whakaari

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volcano (Taupō Volcanic Zone, New Zealand). Callahan et al. (2019) found that silicification of granodiorite within the Dixie Valley – Stillwater fault zone (USA) significantly increased

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uniaxial compressive strength, fracture toughness, and Young’s modulus. Silicification of volcaniclastic sandstone from the Yellowstone geothermal system (USA) was also found to reduce porosity and permeability (Dobson et al., 2003). However, the matrix porosity and permeability of rocks from the Solfatara region of Campi Flegrei (Italy) was found to increase

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following alteration with sulphur-rich hydrothermal fluids (Mayer et al., 2016). Farquharson et al. (2019) also found that exposure to sulphuric acid increased the porosity and permeability of andesite from Mt. Ruapehu. It is clear from these studies that the evolution of physical properties as a function of alteration depends heavily on rock type, the initial microstructural (e.g., grain and pore size) and physical properties of the rock, and the nature (chemical composition, pH, and temperature) of the hydrothermal fluid.

The failure mode (brittle or ductile) of volcanic rock has been shown to depend on the initial porosity of the rock (e.g., Heap et al., 2015b). Low-porosity volcanic rock will respond in a brittle manner (shear fracture formation) to an applied stress, while high-porosity volcanic rock will behave in a ductile manner (driven by cataclastic pore collapse) (e.g., Kennedy et al., 2009; Zhu et al., 2011; Loiaza et al., 2012; Adelinet et al., 2013; Heap et al., 2015b; Zhu et al., 2016). Due to the porosity dependence of the failure mode of volcanic rocks, hydrothermal alteration that increases or decreases the porosity could therefore alter the failure mode (i.e. brittle or ductile) of volcanic rock. Indeed, Siratovich et al. (2016)

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showed that the mechanical behaviour of andesite from the Rotokawa geothermal field

(Taupō Volcanic Zone, New Zealand) under triaxial conditions differs due to alteration. A recent study by Mordensky et al. (2019) showed that, under the same pressure conditions,

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advanced argillic alteration altered the failure mode of andesite from Mt. Ruapehu from

brittle to ductile. Importantly, the mode of deformation controls the evolution of rock physical

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properties. The porosity and permeability of volcanic rock deforming in a ductile manner will decrease (e.g., Heap et al., 2015b; Farquharson et al., 2017), while brittle behaviour will result

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in porosity and permeability augmentation (e.g., Nara et al., 2011; Farquharson et al., 2016a; Heap and Kennedy, 2016; Wang et al., 2016; Pérez-Flores et al., 2017; Eggertsson et al.,

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2018).

To summarise, hydrothermal alteration can decrease or increase the matrix porosity

and permeability of a volcanic rock through pore- and crack-filling precipitation and mineral dissolution, respectively. Therefore, due to the porosity dependence of failure mode (e.g.,

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Heap et al., 2015b), hydrothermal alteration will also control the response of the rock to deformation (e.g., Mordensky et al., 2019). High-porosity materials will undergo reductions to porosity and permeability as a result of cataclastic pore collapse (i.e. ductile deformation), and low-porosity materials will respond to deformation by forming permeability and porosity enhancing fractures (i.e. brittle deformation) (e.g., Heap et al., 2015b). Hydrothermal alteration can therefore also transform the permeability of the rock-mass. As a result, hydrothermal alteration will likely influence the economic potential of geothermal and

epithermal mineral resources hosted in volcanic rocks. The importance of these processes to ore mineralisation and in shaping geothermal resources is well understood qualitatively (e.g., Cox, 2005; Rowland and Sibson, 2004; Rowland and Simmons, 2012). For example, Berger and Henley (2011) suggested that advanced argillic alteration and related silicification in volcanic areas in Peru and Chile lead to decreasing permeability and an increase in rock strength, promoting fracturing and focussing of magmatic fluids through those fractures. Similarly, Rowland and Sibson (2004) and Rowland and Simmons (2012) suggested that hydrothermal alteration in the Taupō Volcanic Zone of New Zealand reduces porosity and

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promotes embrittlement, providing the permeability and fluid flow channelisation required for geothermal and epithermal resources, respectively. However, the quantitative impact of

advanced argillic alteration and related silicification on permeability change and rock strength

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remains poorly constrained from a rock physical property perspective.

In order to assess the quantitative links between hydrothermal alteration and rock

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physical properties, we report herein on an experimental study designed to explore the influence of hydrothermal alteration on the rock matrix and rock-mass properties of a well

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exposed ignimbrite deposit in the Taupō Volcanic Zone – the Ohakuri ignimbrite – that has been variably altered by a palaeo-hydrothermal system. We use these data to assess how

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hydrothermal alteration can help shape geothermal and epithermal mineral resources. This study complements and builds upon more qualitative and conceptual approaches (cf. Rowland and Simmons (2012) and references therein).

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2 Materials and methods

The Ohakuri ignimbrite – the product of a large explosive eruption within the Taupō

Volcanic Zone (Figure 1a, inset; Wilson et al., 1995) about 240,000 years ago (Gravley et al., 2006, 2007; Bégué et al., 2014) – is variably altered due to hydrothermal activity postdating the Ohakuri eruption (Henneberger and Browne, 1988). The deposit, chemically and lithologically similar unwelded rhyolitic ignimbrite and air-fall tuffs, has been altered by two different environment types: alkali chloride water alteration, followed by surface to near-

surface alteration (Henneberger and Browne, 1988). This alteration has produced six distinct altered lithologies, as defined by Henneberger and Browne (1988): (1) weak clay, (2) mordenite, (3) quartz-adularia, (4) quartz silicification, (5) kaolinite, and (6) opal silicification. The hydrothermally altered rocks are now well exposed at the surface (due to erosion and excavations) and the deposit represents an ideal natural laboratory to better understand and quantify the role of hydrothermal alteration on the evolution of geothermal and epithermal mineral resources. Furthermore, the chemical alteration history and architecture of the system has already been characterised in detail by Henneberger and

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Browne (1988), thus providing the ideal context for an investigation into how permeability

evolves from a rock physical property perspective. For the purpose of this study, we collected samples of the unaltered and unlithified deposit (Figure 1a) and six variably altered blocks

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(dimensions of approximately 30 × 30 × 30 cm), including a block of the highly altered

2.1 Mineralogical characterisation

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material at the Ohakuri Dam on the Waikato River (Figure 1b).

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The mineral content of the seven materials collected was quantified using X-ray powder diffraction (XRPD) (Table 1). Powdered aliquots of each sample, containing an

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internal standard of 10 wt.% ZnO, were ground for 8 min with 10 ml of isopropyl alcohol in a McCrone Micronising Mill using agate cylinder elements. The XRPD analyses were performed on powder mounts using a PW 1800 X-ray diffractometer (CuKα, graphite monochromator, 10 mm automatic divergence slit, step-scan 0.02° with 2θ increments per

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second, counting time one second per increment, 30 mA, 40 kV). The phases in the whole rock powders were quantified using the Rietveld program BGMN (Bergmann et al., 1998). To identify the clay minerals, we also separated <2 µm fractions by gravitational settling and prepared oriented mounts that were X-rayed in an air-dried and ethylene-glycolated state. The unaltered deposit contains a high quantity of glass (86 wt.%) together with plagioclase (9 wt.%) and quartz (5 wt.%) (Table 1). Our six lithified blocks can be split into two groups based on their degree of alteration. The first group, hereafter termed “slightly

altered”, contains the blocks with a slightly lower glass content (70 wt.%) than the unaltered deposit (blocks 2B, TF2, and TF3; “mordenite alteration type” from Henneberger and Browne, 1988), as well as some alteration minerals (smectite and zeolites) (Table 1). The second “highly-altered” group contains blocks now devoid of glass (blocks 2A, 2C, and 2D) (Table 1). This second group can be further divided into rocks containing predominately smectite (block 2C; “mordenite alteration type” from Henneberger and Browne, 1988) and those that contain adularia and quartz (blocks 2A and 2D; “quartz-adularia alteration type”

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from Henneberger and Browne, 1988) (Table 1).

2.2 Textural characterisation

Backscattered scanning electron microscope (BSE) images of a slightly altered

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(sample 2B) and a highly altered sample (sample 2A), taken using a Tescan Vega 2 XMU

system, are shown in Figure 2. The slightly altered sample is characterised by a porous matrix

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comprising angular shards of glass and plagioclase that hosts larger (~0.5 mm) plagioclase and porous glassy fragments (Figure 2a). The porosity of the highly altered sample is

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noticeably reduced and primary angular fragments are much more difficult to distinguish (Figure 2b). Quartz crystals are observed to grow within pores of the highly altered sample

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(Figure 2c) and the once-porous glassy fragments now contain abundant precipitated quartz crystals (Figure 2d).

2.3 Porosity and permeability measurements

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Cylindrical samples (20 mm in diameter and precision-ground to between 30-40 mm

in length) were prepared from each of the six lithified blocks (examples shown in Figure 3). The prepared cylindrical samples, and an aliquot (~50 g) of the unlithified deposit, were first dried in a vacuum-oven at 40 °C for at least 48 h prior to experimentation. The connected porosity of each cylindrical sample was calculated using the bulk sample volume (determined using the sample dimensions; lengths are measured to a precision of ± 0.005 mm) and the connected skeletal volume of the sample determined using a helium

pycnometer (Micromeritics AccuPyc II 1340; volumes are measured to a precision of ± 0.00005 cm3). Total porosities were calculated using the bulk sample density (determined using the sample mass and dimensions, masses are measured to a precision of ± 0.0005 g) and the solid density (determined using the mass and volume of a powdered sample, the latter measured using the helium pycnometer). We measured the connected and total porosity for each of the cylindrical samples. However, only the total porosity could be measured for the unlithified deposit. To do so, we first measured the bulk density by weighing the material poured inside a container of known volume. The connected skeletal volume of this material

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was then measured using the helium pycnometer. The experimental error on the porosity measurements is <<1%.

Permeabilities were measured using a nitrogen gas permeameter (Farquharson et al.,

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2016b; Heap and Kennedy, 2016) under a confining pressure of 1 MPa. Our permeameter

setup comprises a “Quick Release Coreholder” pressure vessel from VINCI Technologies,

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EL-FLOW© mass flow meters from Bronkhorst, and a LEX1 high precision digital manometer from KELLER. Permeability was measured using the steady-state method (for

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high-permeability samples) or the pulse-decay method (for low-permeability samples). The precision of the measurements of length (length and radius), pressure, and volumetric flow

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rate required for the calculation of permeability are ± 0.005 mm, ± 5 Pa, and ± 8.33 × 10-11 m3/s, respectively. These measurements were corrected, when necessary, for the Forchheimer (Forchheimer, 1901) and Klinkenberg (Klinkenberg, 1941) corrections. More details on these methods of permeability determination can be found in Heap et al. (2017b). To measure the

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permeability of the unlithified deposit, we poured the ash and lapilli mixture (removing the centimetric lapillis) into a rubber jacket of known internal volume. Metal endcaps (5 mmthick containing a central hole and pore spreader plates) were placed on either side of the sample to prevent material from falling from the jacket. The permeability of the unlithified deposit was then measured using the steady-state method. We highlight that our technique to measure the permeability of the unlithified sample erased the original texture of the deposit, which may influence the representativity of our measurements. In an attempt to address this,

we measured the permeability of eight samples of the unlithified material in which we varied the lapilli content and porosity (i.e. packing) to provide a range of permeability for the unlithified material that, hopefully, encompasses the permeability of the deposit in situ. The experimental error on the permeability measurements is <1%.

2.4 Uniaxial and triaxial experiments Unconfined compressive strengths (UCS) were measured on samples of the six lithified blocks (20 mm in diameter and 40 mm in length) using a uniaxial load frame. We

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performed a total of 42 uniaxial experiments: 13 samples of 2A, six samples of 2B, five

samples of 2C, 13 samples of 2D, three samples of TF2, and two samples of TF3 (Figure 3;

Table 2). Oven-dry samples were deformed at an axial strain rate of 10-5 s-1 until macroscopic

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failure. A lubricating wax was placed on the end-faces of the samples to avoid problems with

friction between the sample and the pistons. During deformation, axial displacement and axial

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load were measured using a linear variable differential transducer (LVDT) and a load cell, respectively. The deformation of the pistons and endcaps was removed from the axial

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displacement signal. Axial displacement and axial load were converted to axial strain and axial stress using the sample dimensions. The Young’s modulus was calculated using the

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slope of the strain-stress curve within the linear elastic region (e.g., Heap et al., 2019b). Triaxial deformation experiments were performed on highly altered (sample 2A;

Figure 3) and slightly altered (sample 2B; Figure 3) samples (20 mm in diameter and 40 mm in length) to understand the influence of alteration intensity on failure mode (brittle or

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ductile). Prior to triaxial testing, the samples were vacuum-saturated with deionised water. The pore fluid pressure (10 MPa) and confining pressure (15 to 30 MPa) were applied and controlled by servo-controlled pressure intensifiers. We assume a simple effective pressure law where the effective pressure (Peff) is the confining pressure minus the pore fluid pressure. The samples were then deformed at an axial strain rate of 10-5 s-1. Measurements of axial displacement and axial load were measured using an LVDT and a load cell, respectively. The deformation of the pistons and endcaps was removed from the axial

displacement signal. Axial displacement and axial load were converted to axial strain and axial stress using the sample dimensions. We performed triaxial experiments on a slightly altered (sample 2B) and a highly altered (sample 2A) sample at a Peff of 5 MPa (i.e. a depth of about 300 m using a bulk density of 1500 kg/m3, see Table 2). We performed an additional experiment on a highly altered sample at a Peff of 20 MPa (i.e. a depth of about 1300 m). The experiments performed at a Peff of 5 MPa were designed to explore the mechanical behaviour of these materials under the reservoir pressure conditions (see the cross section in Henneberger and Browne, 1988). The experiment performed at a Peff of 20 MPa, a pressure

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condition that corresponds to slightly deeper than the reservoir, was designed to test the brittle limit of the highly altered sample.

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4 Results

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4.1 The influence of alteration on matrix porosity and permeability

The porosity of the blocks collected varies from ~0.25 to ~0.6 (Figure 4; Table 2).

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Our porosity data also show that there is essentially zero isolated porosity in the samples collected (Figure 4; Table 2). The permeability is an increasing function of porosity and

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varies from ~2 × 10-17 to ~5 × 10-13 m2 (Figure 5; Table 2). In detail, the porosity of our prepared unlithified samples varies from 0.51 to 0.57 (in agreement with estimations of the porosity of the original deposit of > 0.45; Rowland and Sibson, 2004), and their permeability ranges from 1.47 to 4.77 × 10-13 m2 (Figure 5; Table 2). The slightly altered silicified samples

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have porosities between 0.48 and 0.5 and permeabilities between ~3 × 10-15 and ~1.5 × 10-14 m2 (Figure 5; Table 2). The highly altered silicified samples have the lowest porosities (as low as 0.24) and the lowest permeabilities (as low as ~2 × 10-17 m2; Figure 5; Table 2). By contrast, the porosities and permeabilities of the highly altered clay-bearing samples are similar to the slightly altered samples (Figure 5; Table 2). Therefore, based on our data, (1) silicification is capable of reducing matrix porosity and permeability by up to a factor of two and by four orders of magnitude, respectively, and (2) clay alteration does not significantly

influence porosity and only reduces permeability by an order of magnitude (Figure 5; Table 2).

4.2 The influence of alteration on sample-scale strength and Young’s modulus Representative uniaxial stress-strain curves for each of the six lithified blocks (2A, 2B, 2C, 2D, TF2, and TF3; Figure 3) are provided as Figure 6a and the UCS values for all samples are plotted as a function of porosity in Figure 6b (data available in Table 2). Our data show that the UCS of the samples increases with decreasing porosity (Figure 6b). The UCS

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values for the slightly altered and clay-bearing highly altered samples are below 10 MPa (Figure 6; Table 2). However, the highly altered silicified samples have much higher

strengths (up to ~95 MPa) (Figure 6; Table 2). Our data therefore show that silicification

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increases strength by up to an order of magnitude (Figure 6; Table 2). By contrast, we find

that smectite alteration does not significantly influence sample strength (Figure 6; Table 2).

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The static Young’s moduli for all samples are plotted as a function of porosity in Figure 7 (data available in Table 2). The data show that the Young’s modulus increases with

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decreasing porosity (Figure 7; Table 2). The highly altered silicified samples have high Young’s moduli (up to ~20 GPa), whereas the slightly altered and clay-bearing highly altered

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samples have Young’s moduli below 3 GPa (Figure 7; Table 2). Our data therefore show that silicification increases the Young’s modulus by up to an order of magnitude, but that smectite alteration does not significantly influence Young’s modulus (Figure 7; Table 2).

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4.3 The influence of alteration on rock failure mode The strain-strain curves for the triaxial experiments performed on a highly altered

sample (sample 2A) and a slightly altered sample (sample 2B) are shown in Figure 8 (the effective pressure (Peff) is given next to each curve). The stress-strain curves for the experiments performed on the highly altered sample, at Peffs of 5 and 20 MPa (i.e. up to a depth of about 1300 m), are characteristically brittle: a peak stress followed by a stress drop (Figure 8) (e.g., Wong and Baud, 2012). Failure in these samples was manifest as

throughgoing shear fractures. By contrast, the stress-strain curve for the slightly altered sample deformed at a Peff of 5 MPa (i.e. a depth of about 300 m) does not show a peak stress or a stress drop (Figure 8) and deformation proceeded without strain localisation (i.e. no shear fracture formed in the sample), which represents characteristically ductile behaviour (e.g., Wong and Baud, 2012). Our data therefore suggest that silicification increases the tendency for brittle behaviour.

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5 Discussion

5.1 Physical properties of volcanic rocks

The data presented in this study (Figure 5) is in accordance with previous findings

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that show that low and high matrix porosity is often associated with low and high matrix

permeability, respectively (e.g., Sruoga et al., 2004; Mueller et al., 2005; Wright et al., 2009;

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Farquharson et al., 2015; Wadsworth et al., 2016). Our data also show that strength (Figure 6b) and Young’s modulus (Figure 7) is a decreasing function of porosity, as shown in

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previous studies (e.g., Heap et al., 2014; Schaefer et al., 2015; Coats et al., 2018; Harnett et al., 2019; Heap et al., 2019b). The aforementioned studies present data for largely unaltered

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volcanic materials, and differences in porosity are primarily the result of differences in the number density and volume of pores and microcracks. We document here how different alteration styles can influence the physical properties of volcanic deposits. We find that silicification (the precipitation of quartz and adularia; e.g., Figures 2c and 2d) reduces

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porosity that, in turn, decreases permeability (Figure 5) and increases strength (Figure 6b) and Young’s modulus (Figure 7). By contrast, our data show that alteration to clay (predominantly smectite) results in much smaller modifications to rock physical properties (Figures 5, 6, and 7). These new data therefore provide detailed information on which hydrothermal alteration processes exert greater influence on the physical properties of volcanic rocks. Importantly, because we target a single deposit with little or no spatial differences in composition and componentry (Henneberger and Browne, 1988), the physical

property modifications presented herein (Figures 5, 6, and 7) can be considered solely the result of hydrothermal alteration. Previous studies that document the influence of hydrothermal alteration on the physical properties of volcanic rocks have had to contend with strong variations in, for example, burial depth, lithology, and rock microstructure (e.g., Pola et al., 2012; Frolova et al., 2014; Wyering et al., 2014; Mordensky et al., 2018).

5.2 Upscaling permeability: the permeability of variably altered volcanic rock-masses The values of permeability presented in this study are representative of the sample

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lengthscale and therefore do not take large fractures into account, features that are known to dictate the permeability of a rock-mass. To better understand the influence of hydrothermal alteration on the development of a geothermal and epithermal mineral resource, we must

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upscale our laboratory measurements to the rock-mass scale.

Two samples of the highly altered (2A) and three samples of the slightly altered (2B)

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ignimbrite were prepared (20 mm in diameter and 20 mm in length) to provide the fracture permeabilities for permeability upscaling modelling (following the procedure outlined in

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Heap and Kennedy (2016)). The permeability of these samples was measured using the same procedure outlined above. The samples were then wrapped in electrical tape and loaded

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diametrically in compression in a servo-controlled uniaxial loadframe until the formation of a throughgoing tensile fracture. The samples were then carefully unloaded and their permeability was remeasured. The plane of the resulting throughgoing fracture was orientated parallel to the direction of flow in the permeability experiments.

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The permeability of the fracture, 𝑘𝑓 , can be determined using a two-dimensional

model that considers flow in parallel layers (the same model used to determine the permeability of fractures in Heap and Kennedy (2016), Farquharson et al. (2016b), Kushnir et al. (2018), and Heap et al., (2019a)):

𝑘𝑓 =

(𝐴 ∙ 𝑘𝑒 ) − (𝐴𝑖𝑛𝑡𝑎𝑐𝑡 ∙ 𝑘0 ) , 𝐴𝑓

(1)

where 𝐴 is the cross-sectional area of the sample, 𝑘𝑒 is the equivalent permeability (the permeability of the fractured sample), 𝐴𝑖𝑛𝑡𝑎𝑐𝑡 is the area of intact material, 𝑘0 is the intact permeability, and 𝐴𝑓 is the area of the fracture. If we consider that the fractures are 0.25 mm wide (a reasonable estimate based on measurements made on the fractured samples), then the average fracture permeabilities for the highly and slightly altered samples calculated using Equation (1) are ~5.5 × 10-11 and ~1.5 × 10-11 m2, respectively. These data highlight that a fracture in the highly altered rock is more permeable than a fracture in the slightly altered

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rock. This is because the fractures in the highly altered rock were straighter and smoother,

factors known to increase fracture permeability (e.g., Brown, 1987; Thompson and Brown,

1991; Zimmermann et al., 1992; Heap and Kennedy, 2016). Using these values of 𝑘𝑓 , we can

(𝑤𝑖𝑛𝑡𝑎𝑐𝑡 ∙ 𝑘0 ) + (𝑁𝑤𝑓 ∙ 𝑘𝑓 ) 𝑊

,

(2)

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𝑘𝑒 =

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parallel fractures using the following relation:

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model the equivalent permeability of a given rock-mass populated with 0.25 mm-wide

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where 𝑤𝑖𝑛𝑡𝑎𝑐𝑡 is the width of the intact rock, 𝑁 is the total number of fractures of width 𝑤𝑓 , and 𝑊 is the total width of the rock-mass considered (Heap and Kennedy, 2016). The equivalent permeability of the rock-mass parallel to the plane of the fractures can then be modelled as a function of fracture density (defined as 𝑁/𝑊 m-1) (where 𝑘0 is taken as 9.8 ×

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10-17 and 8.0 × 10-15 m2 for samples 2A and 2B, respectively; Table 2). The equivalent permeability of a highly and slightly altered rock-mass is modelled as

a function of fracture density in Figure 9. The modelling shows that the permeability of the highly altered rock-mass is higher than the slightly altered rock-mass above a fracture density of 1 m-1. We note that a 10-m transect of the highly altered outcrop at the Ohakuri Dam (Figure 1b) revealed a fracture density of 2.4 m-1, whereas very few (<< 1 m-1) were observed in the adjacent slightly altered outcrop. Therefore, despite its lower matrix permeability, the

modelled equivalent permeability of the highly altered silicified rock-mass is much higher than that of the slightly altered rock-mass. We conclude that not only is the intensity of alteration important in controlling rock-mass equivalent permeability, but also the type of alteration. Smectite alteration does not result in the decrease in porosity (Table 2) and increase in Young’s modulus (Figure 7) required to form macroscopic fractures – as observed in outcrops characterised by smectite alteration – and will therefore create rock-masses with a lower equivalent permeability than highly altered silicified rock-masses. We highlight that our modelling likely underestimates the equivalent permeability of

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the highly altered rock-mass. Our upscaling method populates the rock-mass with fractures that are 0.25 mm in width, while our field analysis revealed that the fracture widths on the

surface outcrop range from 0.1 to 20 cm. Although we could use the total fracture width in

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our modelling, we hesitate as (1) it is likely that wide-aperture fractures measured at the

surface will not persist at depth, (2) increasing fracture width likely leads to changes in flow

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inertia, and (3) the fracture permeabilities determined using our laboratory approach are influenced by grain-scale heterogeneities and perhaps underestimate the permeability of the

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wider fractures found in the field. Nevertheless, the presented modelling is simply designed to highlight potentially large differences in rock-mass permeability between the differently

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altered rock-masses.

5.3 Implications for geothermal and epithermal resources Magmatic-hydrothermal systems responsible for geothermal and epithermal

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resources, as found in the Taupō Volcanic Zone (e.g., Rowland and Simmons, 2012 and references therein), have a characteristic hydrothermal alteration and associated permeability structure. In particular, these geothermal and epithermal mineral resources are dependent on adequate fracture permeability and the development of a low-permeability cap that effectively maintains and modulates reservoir pressure. Our study demonstrates, from a petrophysical standpoint, how the Ohakuri geothermal system evolved from an unaltered ignimbrite with relatively high porosity and matrix permeability, but no fractures, to an assemblage of

variegated rock-masses with porosities and equivalent permeabilities that depend on the intensity and type of alteration. In areas of high upflow of alkali-chloride fluid, quartz and adularia will precipitate into the pore space of the host ignimbrite (e.g., in our samples 2A and 2D; Figures 2c and 2d), as previously documented by Henneberger and Browne (1988) for the shallow (< 200 m from the surface) discharge zone of the Ohakuri hydrothermal system (Figure 10). Our experimental data show that quartz and adularia precipitation can decrease matrix porosity and permeability by a factor of two and by two orders of magnitude, respectively (Figure 5).

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Although this may at first seem detrimental for geothermal and epithermal resource

development, which require efficient and localised fluid flow, we demonstrate experimentally that this is an important evolutionary step to increase the Young’s modulus of the rock

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(Figure 7) and therefore its propensity for brittle failure (Figure 8), i.e. increasing the

likelihood that the rock-mass will host permeability-enhancing fractures (as anticipated by,

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for example, Rowland and Sibson, 2004; Rowland and Simmons, 2012). Indeed, the highly altered rock-mass at the Ohakuri Dam contains many more fractures (represented by our

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sample 2A; Figure 1b) than the adjacent slightly altered rock-mass (represented by samples 2B, TF2, and TF3). Our simple permeability modelling suggests that the equivalent

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permeability of a fractured silicified rock-mass will be high as a result of the development of this fracture network (Figure 9). Increased fracture density is an important requirement for gold mineralisation, as demonstrated within the Ohakuri hydrothermal (epithermal) system which hosts a large (> 2 Moz) Au-Ag resource, albeit low grade (0.4 g/t; Grieve et al., 2006).

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Most importantly, gold mineralisation, including some high Au grade (up to 50 g/t) veins, occurs within the intensely fractured quartz-adularia alteration zone (Henneberger and Browne, 1988), consistent with the rock physical property data presented here. On the margins of the high upflow area, and where mixing with colder groundwater occurs (Henneberger and Browne, 1988), high-intensity alteration can occur, but instead of the host ignimbrite converting to quartz and adularia, the original glassy ignimbrite is completely altered to predominately smectite (e.g., our sample 2C; Figure 10). Our

experimental data show that smectite alteration does not significantly change porosity and only reduces the matrix permeability by an order of magnitude (Figure 5). Importantly, smectite alteration does not significantly increase the Young’s modulus of the material (Figure 7) and, as a result, rock-masses characterised by smectite alteration are unlikely to host permeability-enhancing fractures. Indeed, outcrops of the Ohakuri ignimbrite characterised by smectite alteration were observed during our site investigation to be relatively fracture-free. Our permeability modelling shows that a rock-mass with a high matrix permeability but a low fracture density will be characterised by a lower equivalent

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permeability than a rock-mass with a low matrix permeability and a high fracture density

(Figure 9). We suggest that rock-masses characterised by smectite alteration have the rock

properties characteristic of the low-permeability cap that is fundamental to the evolution of an

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economically viable hydrothermal resource (e.g., see Stimac et al., 2015; Cumming, 2016; Figure 10).

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Our experimental permeability and rock strength results are consistent with, for example, the field evidence and model presented for the evolution of permeability structure in

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high-sulfidation epithermal Au-Ag deposits by Berger and Henley (2011). In their model, near-surface smectite alteration acts as a low-permeability cap, while the formation of alunite

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and quartz as alteration minerals at greater depth caused a decrease in matrix permeability, and an increase in rock strength, thus controlling later fracturing and related Cu-Au ore deposition.

Our petrophysical study supports the hypothesis that (e.g., Rowland and Simmons,

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2012), at least in some systems, fractures and high permeability fluid flow pathways may form as a response to earlier hydrothermal alteration, rather than being the controlling factor on the location and distribution of hydrothermal alteration. Indeed, having sufficient longevity of hydrothermal activity to optimise the rock properties for fracturing may be a critical element of “ground preparation” that paves the way for subsequent mineralisation. Therefore, our study highlights that recognising the style, extent, and timing of hydrothermal alteration, factors that control the resultant physical properties of the rock and rock-mass, will

inform on whether a particular suite of hydrothermally altered rocks is capable of hosting a viable geothermal or mineral resource. We highlight that we have studied what could be considered, from a lithological point of view, as a relatively simple system. The formation of geothermal and epithermal resources in regions characterised by a structure and/or stratigraphy complex enough to influence the intensity and spatial distribution of alteration will likely evolve differently.

6 Conclusions

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Our study quantitatively explores how hydrothermal alteration can create a viable geothermal and epithermal mineral resource. To do so, we measured the rock physical

properties (porosity, permeability, Young’s modulus, and uniaxial compressive strength) of

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hydrothermal altered materials from a well exposed ignimbrite deposit – the Ohakuri

ignimbrite – that has been variably altered by a palaeo-hydrothermal system. These data show

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that silicification significantly reduces (by up to four orders of magnitude) matrix porosity and permeability (relative to the original deposit) and that clay alteration only reduces

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permeability by an order of magnitude. Although a large reduction in matrix permeability appears detrimental to the generation of a geothermal and epithermal mineral resource, we

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also show, using uniaxial and triaxial deformation experiments, that silicification increases the propensity for permeability-enhancing fracture formation. By contrast, materials altered to smectite will likely deform in a ductile manner under the reservoir conditions. Using a twodimensional model that considers flow in parallel layers, we upscaled our laboratory

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permeability measurements and conclude that the hydrothermal alteration of the Ohakuri ignimbrite produced both the fractured and permeable reservoir rock (silicification) and the relatively unfractured and less permeable cap (clay alteration) required for an economic geothermal resource (Figure 10). Further, a rock-mass characterised by a low matrix permeability and a high fracture density provides the localised fluid low required for highgrade epithermal mineral veins (Figure 10). Our petrophysical approach complements previous studies that have addressed this topic from a geological, structural, and geochemical

perspective (e.g., Henneberger and Browne, 1988; Rowland and Sibson, 2004; Rowland and Simmons, 2012).

Acknowledgements We acknowledge an Erskine Fellowship (University of Canterbury), Agence Nationale de la Recherche (ANR) grant CANTARE (ANR-15-CE06-0014-01), LABEX grant ANR-11-LABX-0050_G-EAU-THERMIE-PROFONDE, and New Zealand Ministry of Business and Innovation and Education grant “Energy Straight from Magma”. We thank the

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2017 Frontiers Abroad students, T. Reuschlé, R. Spiers, G. Morvan, C. Higgins, S. Pope, A. Kushnir, and J. Farquharson. The comments of two reviewers helped improve the clarity of

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this manuscript.

Data availability

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Author contributions

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The data collected for this manuscript are available in Tables 1 and 2.

M.H led the project and wrote the manuscript. M.H., D.G., B.K., and E.B. conducted

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the fieldwork and collected the samples. M.H. prepared the laboratory samples and measured the rock physical properties. A.G. performed the XRPD measurements and analysed the data. M.H. performed the SEM analyses, with help from E.B. All of the authors contributed to the

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interpretation of the data and the writing of the manuscript.

Conflict of interest We decalre no conflicts of interest.

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Siratovich, P. A., Heap, M. J., Villenueve, M. C., Cole, J. W., & Reuschlé, T. (2014). Physical property relationships of the Rotokawa Andesite, a significant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand. Geothermal Energy, 2(1), 10. Siratovich, P. A., Heap, M. J., Villeneuve, M. C., Cole, J. W., Kennedy, B. M., Davidson, J., & Reuschlé, T. (2016). Mechanical behaviour of the Rotokawa Andesites (New Zealand): Insight into permeability evolution and stress-induced behaviour in an actively utilised geothermal reservoir. Geothermics, 64, 163-179. Sruoga, P., Rubinstein, N., and Hinterwimmer, G., 2004, Porosity and permeability in volcanic rocks: a case study on the Serie Tobı́fera, South Patagonia, Argentina: Journal of Volcanology and Geothermal Research, v. 132, p. 31-43, doi: 10.1016/S0377-0273(03)00419-0. Stimac, J., Goff, F., & Goff, C. J. (2015). Intrusion-related geothermal systems. In The Encyclopedia of Volcanoes (pp. 799-822). Academic Press. Thompson, M. E., & Brown, S. R. (1991). The effect of anisotropic surface roughness on flow and transport in fractures. Journal of Geophysical Research: Solid Earth, 96(B13), 21923-21932. Wadsworth, F. B., Vasseur, J., Scheu, B., Kendrick, J. E., Lavallée, Y., and Dingwell, D. B., 2016, Universal scaling of fluid permeability during volcanic welding and sediment diagenesis: Geology, v. 44, p. 219-222, doi: 10.1130/G37559.1. Wang, G., Mitchell, T. M., Meredith, P. G., Nara, Y., & Wu, Z. (2016). Influence of gouge thickness and grain size on permeability of macrofractured basalt. Journal of Geophysical Research: Solid Earth, 121(12), 8472-8487. White, N. C., & Hedenquist, J. W. (1990). Epithermal environments and styles of mineralization: variations and their causes, and guidelines for exploration. Journal of Geochemical Exploration, 36(1-3), 445-474. Wilson, C. J. N., Houghton, B. F., McWilliams, M. O., Lanphere, M. A., Weaver, S. D., and Briggs, R. M., 1995, Volcanic and structural evolution of Taupo Volcanic Zone, New Zealand: a review: Journal of Volcanology and Geothermal Research, v. 68, p. 1-28, doi: 10.1016/03770273(95)00006-G. Wong, T. F., and Baud, P., 2012, The brittle-ductile transition in porous rock: A review: Journal of Structural Geology, v. 44, p. 25-53, doi: 10.1016/j.jsg.2012.07.010. Wright, H. M., Cashman, K. V., Gottesfeld, E. H., & Roberts, J. J. (2009). Pore structure of volcanic clasts: measurements of permeability and electrical conductivity. Earth and Planetary Science Letters, 280(1-4), 93-104. Wyering, L. D., Villeneuve, M. C., Wallis, I. C., Siratovich, P. A., Kennedy, B. M., Gravley, D. M., and Cant, J. L., 2014, Mechanical and physical properties of hydrothermally altered rocks, Taupo Volcanic Zone, New Zealand: Journal of Volcanology and Geothermal Research, v. 288, p. 76-93, doi: 10.1016/j.jvolgeores.2014.10.008. Zhu, W., Baud, P., Vinciguerra, S., & Wong, T. F. (2011). Micromechanics of brittle faulting and cataclastic flow in Alban Hills tuff. Journal of Geophysical Research: Solid Earth, 116(B6). Zhu, W., Baud, P., Vinciguerra, S., & Wong, T. F. (2016). Micromechanics of brittle faulting and cataclastic flow in Mount Etna basalt. Journal of Geophysical Research: Solid Earth, 121(6), 4268-4289. Zimmerman, R. W., Chen, D. W., & Cook, N. G. (1992). The effect of contact area on the permeability of fractures. Journal of Hydrology, 139(1-4), 79-96.

Figure captions

Figure 1. (a) Photograph of the unaltered deposit (OI1). Inset shows map of New Zealand showing the position of the Taupō Volcanic Zone (TVZ) and the Ohakuri caldera. (b) Photograph of the highly altered rock (2A) outcrop adjacent to the Ohakuri Dam on the

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Waikato River.

ro of -p re lP ur na Jo Figure 2. (a) Backscattered scanning electron microscope (BSE) image of the slightly altered sample 2B. (b) BSE image of the highly altered sample 2A. (c) BSE image showing pore-

filling quartz precipitation in highly altered sample 2A. (d) BSE image of highly altered

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sample 2A showing a once-porous fragment now filled with precipitated quartz crystals.

Figure 3. Photographs of the experimental samples. The reference for each sample, and the

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symbol used in Figures 4, 5, 6, and 7, are given below each image.

Figure 4. Total porosity as a function of connected porosity for the samples prepared from the six blocks collected. Corresponding sample photographs for the symbols are provided in

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Figure 3.

Figure 5. Matrix permeability as a function of connected porosity for the samples prepared from the six blocks collected, and the unlithified (unaltered) sample. Permeability measurements were collected under a confining pressure of 1 MPa. The total porosity

measured for the unlithified sample is assumed here to be equal to the connected porosity.

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Corresponding sample photographs for the symbols are provided in Figure 3.

Figure 6. (a) Representative uniaxial stress-strain curves for six blocks collected. (b) Uniaxial compressive strength as a function of connected porosity for the samples prepared from the six blocks collected. Corresponding sample photographs for the symbols are provided in Figure 3.

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Figure 7. Young’s modulus as a function of connected porosity for the samples prepared from the six blocks collected. Corresponding sample photographs for the symbols are

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provided in Figure 3.

Figure 8. Differential stress-strain curves for the three triaxial experiments performed for this study. Number next to each curve indicates the effective pressure (Peff) under which the

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experiment was performed.

Figure 9. Equivalent permeability modelled as a function of fracture density for a slightly altered and a highly altered rock-mass (see text for details). Dotted line indicates the

measured fracture density on an outcrop of the highly altered rock adjacent to the Ohakuri

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Dam on the Waikato River (see Figure 1b).

Figure 10. Schematic cross section of the Ohakuri hydrothermal system following early-stage alteration and the formation of a seal, i.e. prior to late-stage erosion and overprinting (redrawn from Henneberger and Browne, 1988). Our study highlights how changes in the petrophysical properties of the deposit, as a result of alteration, can create a fractured and permeable reservoir (shown in orange, similar to our sample 2A) and a relatively unfractured and less permeable cap (shown in grey, similar to our samples 2B and 2C).

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Table 1. Mineral content of the seven samples of Ohakuri ignimbrite used in this study (one unlithified (OI1) sample and seven lithified blocks), as determined by X-ray powder diffraction. Values are in wt.%. Unaltered

Highly

Slightly

Highly

Highly

Slightly

Slightly

deposit

altered

altered

altered

altered

altered

altered

(OI1)

deposit

deposit

deposit

deposit

deposit

deposit

(2A)

(2B)

(2C)

(2D)

(TF2)

(TF3)

86

-

68

-

-

70

73

Plagioclase

9

-

21

23

4

22

19

Adularia

-

53

-

-

58

-

-

Quartz

5

47

5

6

38

5

4

Cristobalite

-

-

-

29

-

-

-

Smectite

-

-

4

42

-

-

-

Mordenite

-

-

2

-

-

3

3

Opal-CT

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Clinoptilolite

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Table 2. Rock physical properties for the samples of Ohakuri ignimbrite measured in this study. UCS – uniaxial compressive strength. Isolated porosity

Matrix permeability (m2)

UCS (MPa)

Young’s modulus (GPa)

0.30 0.25 0.24 0.24 0.28 0.32 0.33 0.27 0.30 0.26 0.24 0.30 0.32 0.31 0.31 0.33 0.33 0.26 0.48 0.48 0.48 0.46 0.48 0.49 0.49 0.48 0.50 0.49 0.51 0.52 0.51 0.51 0.52 0.42 0.58 0.43 0.52 0.50 0.55 0.57 0.59 0.39 0.38 0.39 0.39 0.40

0.55 0.54 0.53 0.57 0.52 0.56 0.51 0.52 0.30 0.26 0.25 0.25 0.28 0.32 0.33 0.28 0.31 0.27 0.25 0.31 0.33 0.31 0.32 0.33 0.33 0.26 0.48 0.48 0.49 0.47 0.48 0.49 0.48 0.48 0.49 0.48 0.50 0.48 0.49 0.50 0.50 0.43 0.58 0.44 0.52 0.50 0.54 0.56 0.57 0.39 0.38 0.39 0.39 0.40

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

1.49 × 10-13 2.67 × 10-13 1.83 × 10-13 4.77 × 10-13 2.32 × 10-13 2.11 × 10-13 1.47 × 10-13 2.00 × 10-13 1.53 × 10-16 8.21 × 10-17 4.08 × 10-17 2.12 × 10-17 1.53 × 10-15 1.87 × 10-16 3.79 × 10-16 1.90 × 10-16 1.99 × 10-16 1.05 × 10-16 3.71 × 10-17 2.30 × 10-16 2.20 × 10-16 1.11 × 10-16 3.06 × 10-16 4.86 × 10-16 1.89 × 10-14 5.46 × 10-17 3.64 × 10-15 4.35 × 10-15 4.52 × 10-15 4.46 × 10-15 5.37 × 10-15 4.11 × 10-15 2.96 × 10-15 6.36 × 10-15 3.78 × 10-15 4.10 × 10-15 7.14 × 10-15 5.89 × 10-15 6.81 × 10-15 4.59 × 10-15 3.57 × 10-15 4.14 × 10-14 1.22 × 10-14 1.59 × 10-14 8.11 × 10-15 9.61 × 10-15 1.56 × 10-14 1.78 × 10-14 2.84 × 10-14 9.72 × 10-17 3.77 × 10-17 6.59 × 10-17 1.62 × 10-16 1.32 × 10-16

67.3 94.6 81.6 68.8 59.9 40.9 70.0 60.9 50.0 39.0 63.0 54.5 41.2 5.9 6.9 8.3 6.3 6.7 7.4 9.4 4.0 10.6 4.9 6.5 33.2 34.4 32.7 32.8

14.3 19.7 19.0 15.3 13.8 11.4 17.0 15.5 14.0 13.0 13.9 13.0 12.2 1.5 1.9 2.2 1.7 1.7 2.1 1.1 0.6 1.2 0.8 0.9 12.0 12.9 10.1 13.1

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Total porosity

-p

Unaltered Unaltered Unaltered Unaltered Unaltered Unaltered Unaltered Unaltered Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Slightly Slightly Slightly Slightly Slightly Slightly Slightly Slightly Slightly Slightly Slightly Slightly Slightly Slightly Slightly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly

Connected porosity

re

OI1_1 OI1_2 OI1_3 OI1_4 OI1_5 OI1_6 OI1_7 OI1_8 2A 1 2A 2 2A 3 2A 4 2A 5 2A 6 2A 7 2A 8 2A 9 2A 10 2A 11 2A 12 2A 13 2A 14 2A 15 2A 16 2A 17 2A 18 2B 1 2B 2 2B 3 2B 4 2B 5 2B 6 2B 7 2B 8 2B 9 2B 10 2B 11 2B 12 2B 13 2B 14 2B 15 2C 1 2C 2 2C 3 2C 4 2C 5 2C 6 2C 7 2C 8 2D 1 2D 2 2D 3 2D 4 2D 5

Dry bulk sample density (kg/m3) 998 1018 1040 967 1082 981 1094 1079 1852 1960 1985 1986 1898 1793 1767 1902 1829 1922 1984 1819 1769 1813 1798 1754 1759 1946 1246 1242 1240 1286 1246 1233 1244 1245 1220 1247 1214 1244 1221 1216 1206 1351 993 1317 1135 1181 1083 1039 1002 1576 1621 1594 1587 1568

lP

Alteration

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Sample

Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Highly Slightly Slightly Slightly Slightly Slightly

1597 1605 1567 1624 1503 1561 1516 1603 1582 1456 1606 1292 1250 1229 1241 1248

0.39 0.39 0.40 0.37 0.42 0.40 0.42 0.39 0.40 0.44 0.38 0.45 0.47 0.48 0.48 0.47

0.39 0.38 0.40 0.38 0.42 0.40 0.42 0.38 0.39 0.44 0.38 0.47 0.48 0.49 0.48 0.48

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.01 0.01 0.00 0.00

6.19 × 10-17 1.43 × 10-16 1.27 × 10-16 2.78 × 10-17 1.54 × 10-16 1.41 × 10-16 1.75 × 10-16 8.12 × 10-17 1.29 × 10-16 2.09 × 10-16 1.88 × 10-16 1.30 × 10-14 1.41 × 10-14 1.49 × 10-14 1.36 × 10-14 1.27 × 10-14

30.0 37.1 30.5 36.9 34.4 33.9 28.3 19.4 34.0 8.1 5.9 5.5 9.2 9.0

13.0 14.1 11.7 13.0 11.4 13.2 12.1 9.6 13.1 2.0 2.0 1.0 2.8 2.8

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2D 6 2D 7 2D 8 2D 9 2D 10 2D 11 2D 12 2D 13 2D 14 2D 15 2D 16 TF2 1 TF2 2 TF2 3 TF3 1 TF3 2