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

Michael Heap, Darren Gravley, Ben Kennedy, H. Albert Gilg, Elisabeth

Bertolett, Shaun L.L. Barker

► To cite this version:

Michael Heap, Darren Gravley, Ben Kennedy, H. Albert Gilg, Elisabeth Bertolett, et al.. Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources: The Ohakuri ignimbrite (Taupō Volcanic Zone, New Zealand). Journal of Volcanology and Geothermal Research, 2019, pp.106703. 10.1016/j.jvolgeores.2019.106703. hal-02376488

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1 Quantifying the role of hydrothermal alteration in creating

2 geothermal and epithermal mineral resources: the Ohakuri

3 ignimbrite (Taupō Volcanic Zone, New Zealand)

4

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

6 Bertolett², and Shaun L. L. Barker⁴

7

8 ¹Géophysique Expérimentale, Institut de Physique de Globe de Strasbourg (UMR 7516

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

10 ²Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand

11 ³Lehrstuhl für Ingenieurgeologie, Technische Universität München, Munich, Germany

12 ⁴Centre for Ore Deposits and Earth Sciences (CODES), University of Tasmania, Australia

13

14 *Corresponding author: M.J. Heap (heap@unistra.fr)

15

16 Abstract

17 Hydrothermal fluids can alter the chemical and physical properties of the materials
18 through which they pass and can therefore modify the efficiency of fluid circulation. The role
19 of hydrothermal alteration in the development of geothermal and epithermal mineral
20 resources, systems that require the efficient hydrothermal circulation provided by fracture
21 networks, is investigated here from a petrophysical standpoint using samples collected from a
22 well exposed and variably altered palaeo-hydrothermal system hosted in the Ohakuri
23 ignimbrite deposit in the Taupō Volcanic Zone (New Zealand). Our new laboratory data show
24 that, although quartz and adularia precipitation reduces matrix porosity and permeability, it
25 increases the uniaxial compressive strength, Young's modulus, and propensity for brittle
26 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 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 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 decrease permeability, and creates a relatively unfractured rock-mass. Using our new laboratory data, we provide permeability modelling that shows that the equivalent 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 low-permeability cap required for an economically viable hydrothermal resource. Our new data show, using a petrophysical approach, how hydrothermal alteration can produce rock-masses 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).

41

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

44

# 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., 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 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 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 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 °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 low-sulphidation (neutral pH, reduced) or high-sulphidation (acidic, oxidised) (e.g., Hedenquist and Lowenstern, 1994; Sillitoe and Hedenquist, 2003). High-grade epithermal mineral 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 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
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.
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
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
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
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)
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,
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 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 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., 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., 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 promotes embrittlement, providing the permeability and fluid flow channelisation required for geothermal and epithermal resources, respectively. However, the quantitative impact of
138 advanced argillic alteration and related silicification on permeability change and rock strength
139 remains poorly constrained from a rock physical property perspective.

140 In order to assess the quantitative links between hydrothermal alteration and rock
141 physical properties, we report herein on an experimental study designed to explore the
142 influence of hydrothermal alteration on the rock matrix and rock-mass properties of a well
143 exposed ignimbrite deposit in the Taupō Volcanic Zone – the Ohakuri ignimbrite – that has
144 been variably altered by a palaeo-hydrothermal system. We use these data to assess how
145 hydrothermal alteration can help shape geothermal and epithermal mineral resources. This
146 study complements and builds upon more qualitative and conceptual approaches (cf. Rowland
147 and Simmons (2012) and references therein).

148

149 **2 Materials and methods**

150 The Ohakuri ignimbrite – the product of a large explosive eruption within the Taupō
151 Volcanic Zone (Figure 1a, inset; Wilson et al., 1995) about 240,000 years ago (Gravley et al.,
152 2006, 2007; Bégué et al., 2014) – is variably altered due to hydrothermal activity postdating
153 the Ohakuri eruption (Henneberger and Browne, 1988). The deposit, chemically and
154 lithologically similar unwelded rhyolitic ignimbrite and air-fall tuffs, has been altered by two
155 different environment types: alkali chloride water alteration, followed by surface to near-
156 surface alteration (Henneberger and Browne, 1988). This alteration has produced six distinct
157 altered lithologies, as defined by Henneberger and Browne (1988): (1) weak clay, (2)
158 mordenite, (3) quartz-adularia, (4) quartz silicification, (5) kaolinite, and (6) opal
159 silicification. The hydrothermally altered rocks are now well exposed at the surface (due to
160 erosion and excavations) and the deposit represents an ideal natural laboratory to better
161 understand and quantify the role of hydrothermal alteration on the evolution of geothermal
162 and epithermal mineral resources. Furthermore, the chemical alteration history and
163 architecture of the system has already been characterised in detail by Henneberger and
164 Browne (1988), thus providing the ideal context for an investigation into how permeability
165 evolves from a rock physical property perspective. For the purpose of this study, we collected
samples of the unaltered and un lithified deposit (Figure 1a) and six variably altered blocks (dimensions of approximately 30 × 30 × 30 cm), including a block of the highly altered material at the Ohakuri Dam on the Waikato River (Figure 1b).

169

## 2.1 Mineralogical characterisation

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 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 20 increments per 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” from Henneberger and Browne, 1988) (Table 1).

192

## 2.2 Textural characterisation
Backscattered scanning electron microscope (BSE) images of a slightly altered (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 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 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 (Figure 2c) and the once-porous glassy fragments now contain abundant precipitated quartz crystals (Figure 2d).

## 2.3 Porosity and permeability measurements

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 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., 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, 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 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 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 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 mm-thick 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 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 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 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 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 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 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 condition that corresponds to slightly deeper than the reservoir, was designed to test the brittle limit of the highly altered sample.
278

279 **4 Results**

280

281 **4.1 The influence of alteration on matrix porosity and permeability**

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

283 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 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 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).

298

299 **4.2 The influence of alteration on sample-scale strength and Young's modulus**

300 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 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 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).

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 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 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).

## 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.

# 5 Discussion

## 5.1 Physical properties of volcanic rocks
The data presented in this study (Figure 5) is in accordance with previous findings 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; 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 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 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 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 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 upscale our laboratory measurements to the rock-mass scale.

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

The permeability of the fracture, $k_f$, 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)):

$$k_f = \frac{(A \cdot k_e) - (A_{\text{intact}} \cdot k_0)}{A_f}, \quad (1)$$

where A is the cross-sectional area of the sample, $k_e$ is the equivalent permeability (the permeability of the fractured sample), $A_{\text{intact}}$ is the area of intact material, $k_0$ is the intact permeability, and $A_f$ 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 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,
389 1991; Zimmermann et al., 1992; Heap and Kennedy, 2016). Using these values of $k_f$, we can model the equivalent permeability of a given rock-mass populated with 0.25 mm-wide parallel fractures using the following relation:

392

393

$$k_e = \frac{(w_{\text{intact}} \cdot k_0) + (N w_f \cdot k_f)}{W}, \quad (2)$$

394

395 where $w_{\text{intact}}$ is the width of the intact rock, $N$ is the total number of fractures of width $w_f$, 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 $N/W \text{ m}^{-1}$) (where $k_0$ is taken as $9.8 \times 10^{-17}$ and $8.0 \times 10^{-15} \text{ m}^2$ for samples 2A and 2B, respectively; Table 2).

400 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 $1 \text{ m}^{-1}$. We note that a 10-m transect of the highly altered outcrop at the Ohakuri Dam (Figure 1b) revealed a fracture density of $2.4 \text{ m}^{-1}$, whereas very few ($\ll 1 \text{ m}^{-1}$) 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.

401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 We highlight that our modelling likely underestimates the equivalent permeability of 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 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 inertia, and (3) the fracture permeabilities determined using our laboratory approach are influenced by grain-scale heterogeneities and perhaps underestimate the permeability of the 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 altered rock-masses.

424

## 5.3 Implications for geothermal and epithermal resources

Magmatic-hydrothermal systems responsible for geothermal and epithermal 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.

426

427

428

429

430

431

432

433

434

435

436

437

438

439

440

441

442

443

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

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
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
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,
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.

496

## 6 Conclusions

Our study quantitatively explores how hydrothermal alteration can create a viable
geothermal and epithermal mineral resource. To do so, we measured the rock physical
500 properties (porosity, permeability, Young's modulus, and uniaxial compressive strength) of
501 hydrothermal altered materials from a well exposed ignimbrite deposit – the Ohakuri
502 ignimbrite – that has been variably altered by a palaeo-hydrothermal system. These data show
503 that silicification significantly reduces (by up to four orders of magnitude) matrix porosity
504 and permeability (relative to the original deposit) and that clay alteration only reduces
505 permeability by an order of magnitude. Although a large reduction in matrix permeability
506 appears detrimental to the generation of a geothermal and epithermal mineral resource, we
507 also show, using uniaxial and triaxial deformation experiments, that silicification increases
508 the propensity for permeability-enhancing fracture formation. By contrast, materials altered to
509 smectite will likely deform in a ductile manner under the reservoir conditions. Using a two-
510 dimensional model that considers flow in parallel layers, we upscaled our laboratory
511 permeability measurements and conclude that the hydrothermal alteration of the Ohakuri
512 ignimbrite produced both the fractured and permeable reservoir rock (silicification) and the
513 relatively unfractured and less permeable cap (clay alteration) required for an economic
514 geothermal resource (Figure 10). Further, a rock-mass characterised by a low matrix
515 permeability and a high fracture density provides the localised fluid low required for high-
516 grade epithermal mineral veins (Figure 10). Our petrophysical approach complements
517 previous studies that have addressed this topic from a geological, structural, and geochemical
518 perspective (e.g., Henneberger and Browne, 1988; Rowland and Sibson, 2004; Rowland and
519 Simmons, 2012).

520

521 Acknowledgements

522 We acknowledge an Erskine Fellowship (University of Canterbury), Agence
523 Nationale de la Recherche (ANR) grant CANTARE (ANR-15-CE06-0014-01), LABEX grant
524 ANR-11-LABX-0050_G-EAU-THERMIE-PROFONDE, and New Zealand Ministry of
525 Business and Innovation and Education grant "Energy Straight from Magma". We thank the
526 2017 Frontiers Abroad students, T. Reuschlé, R. Spiers, G. Morvan, C. Higgins, S. Pope, A.
527 Kushnir, and J. Farquharson. The comments of two reviewers helped improve the clarity of
528 this manuscript.

529

530 **Data availability**

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

532

533 **Author contributions**

534 M.H led the project and wrote the manuscript. M.H., D.G., B.K., and E.B. conducted
535 the fieldwork and collected the samples. M.H. prepared the laboratory samples and measured
536 the rock physical properties. A.G. performed the XRPD measurements and analysed the data.

537 M.H. performed the SEM analyses, with help from E.B. All of the authors contributed to the
538 interpretation of the data and the writing of the manuscript.
539 **Figure captions**

540

541 **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 Waikato River.

545

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

550

551 **Figure 3.** Photographs of the experimental samples. The reference for each sample, and the symbol used in Figures 4, 5, 6, and 7, are given below each image.

553

554 **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 Figure 3.

557

558 **Figure 5.** Matrix permeability as a function of connected porosity for the samples prepared from the six blocks collected, and the un lithified (unaltered) sample. Permeability measurements were collected under a confining pressure of 1 MPa. The total porosity measured for the un lithified sample is assumed here to be equal to the connected porosity. Corresponding sample photographs for the symbols are provided in Figure 3.

563

564 **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
566 six blocks collected. Corresponding sample photographs for the symbols are provided in
567 Figure 3.

568
569 **Figure 7.** Young's modulus as a function of connected porosity for the samples prepared
570 from the six blocks collected. Corresponding sample photographs for the symbols are
571 provided in Figure 3.

572
573 **Figure 8.** Differential stress-strain curves for the three triaxial experiments performed for this
574 study. Number next to each curve indicates the effective pressure (Peff) under which the
575 experiment was performed.

576
577 **Figure 9.** Equivalent permeability modelled as a function of fracture density for a slightly
578 altered and a highly altered rock-mass (see text for details). Dotted line indicates the
579 measured fracture density on an outcrop of the highly altered rock adjacent to the Ohakuri
580 Dam on the Waikato River (see Figure 1b).

581
582 **Figure 10.** Schematic cross section of the Ohakuri hydrothermal system following early-stage
583 alteration and the formation of a seal, i.e. prior to late-stage erosion and overprinting (re-
584 drawn from Henneberger and Browne, 1988). Our study highlights how changes in the
585 petrophysical properties of the deposit, as a result of alteration, can create a fractured and
586 permeable reservoir (shown in orange, similar to our sample 2A) and a relatively unfractured
587 and less permeable cap (shown in grey, similar to our samples 2B and 2C).
**Table 1.** Mineral content of the seven samples of Ohakuri ignimbrite used in this study (one un lithified (OI1) sample and seven lithified blocks), as determined by X-ray powder diffraction. Values are in wt.%.

591

<table><thead><tr><th></th><th>Unaltered<br>deposit<br>(OI1)</th><th>Highly<br>altered<br>deposit<br>(2A)</th><th>Slightly<br>altered<br>deposit<br>(2B)</th><th>Highly<br>altered<br>deposit<br>(2C)</th><th>Highly<br>altered<br>deposit<br>(2D)</th><th>Slightly<br>altered<br>deposit<br>(TF2)</th><th>Slightly<br>altered<br>deposit<br>(TF3)</th></tr></thead><tbody><tr><td>Glass</td><td>86</td><td>-</td><td>68</td><td>-</td><td>-</td><td>70</td><td>73</td></tr><tr><td>Plagioclase</td><td>9</td><td>-</td><td>21</td><td>23</td><td>4</td><td>22</td><td>19</td></tr><tr><td>Adularia</td><td>-</td><td>53</td><td>-</td><td>-</td><td>58</td><td>-</td><td>-</td></tr><tr><td>Quartz</td><td>5</td><td>47</td><td>5</td><td>6</td><td>38</td><td>5</td><td>4</td></tr><tr><td>Cristobalite<br>Opal-CT</td><td>-</td><td>-</td><td>-</td><td>29</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Smectite</td><td>-</td><td>-</td><td>4</td><td>42</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Mordenite<br>Clinoptilolite</td><td>-</td><td>-</td><td>2</td><td>-</td><td>-</td><td>3</td><td>3</td></tr></tbody></table>

592
Table 2. Rock physical properties for the samples of Ohakuri ignimbrite measured in this

study. UCS – uniaxial compressive strength.

595

<table><thead><tr><th>Sample</th><th>Alteration</th><th>Dry bulk sample density (kg/m³)</th><th>Connected porosity</th><th>Total porosity</th><th>Isolated porosity</th><th>Matrix permeability (m²)</th><th>UCS (MPa)</th><th>Young's modulus (GPa)</th></tr></thead><tbody><tr><td>OI1_1</td><td>Unaltered</td><td>998</td><td>-</td><td>0.55</td><td>-</td><td>1.49 × 10<sup>-13</sup></td><td>-</td><td>-</td></tr><tr><td>OI1_2</td><td>Unaltered</td><td>1018</td><td>-</td><td>0.54</td><td>-</td><td>2.67 × 10<sup>-13</sup></td><td>-</td><td>-</td></tr><tr><td>OI1_3</td><td>Unaltered</td><td>1040</td><td>-</td><td>0.53</td><td>-</td><td>1.83 × 10<sup>-13</sup></td><td>-</td><td>-</td></tr><tr><td>OI1_4</td><td>Unaltered</td><td>967</td><td>-</td><td>0.57</td><td>-</td><td>4.77 × 10<sup>-13</sup></td><td>-</td><td>-</td></tr><tr><td>OI1_5</td><td>Unaltered</td><td>1082</td><td>-</td><td>0.52</td><td>-</td><td>2.32 × 10<sup>-13</sup></td><td>-</td><td>-</td></tr><tr><td>OI1_6</td><td>Unaltered</td><td>981</td><td>-</td><td>0.56</td><td>-</td><td>2.11 × 10<sup>-13</sup></td><td>-</td><td>-</td></tr><tr><td>OI1_7</td><td>Unaltered</td><td>1094</td><td>-</td><td>0.51</td><td>-</td><td>1.47 × 10<sup>-13</sup></td><td>-</td><td>-</td></tr><tr><td>OI1_8</td><td>Unaltered</td><td>1079</td><td>-</td><td>0.52</td><td>-</td><td>2.00 × 10<sup>-13</sup></td><td>-</td><td>-</td></tr><tr><td>2A 1</td><td>Highly</td><td>1852</td><td>0.30</td><td>0.30</td><td>0.00</td><td>1.53 × 10<sup>-16</sup></td><td>67.3</td><td>14.3</td></tr><tr><td>2A 2</td><td>Highly</td><td>1960</td><td>0.25</td><td>0.26</td><td>0.00</td><td>8.21 × 10<sup>-17</sup></td><td>94.6</td><td>19.7</td></tr><tr><td>2A 3</td><td>Highly</td><td>1985</td><td>0.24</td><td>0.25</td><td>0.00</td><td>4.08 × 10<sup>-17</sup></td><td>81.6</td><td>19.0</td></tr><tr><td>2A 4</td><td>Highly</td><td>1986</td><td>0.24</td><td>0.25</td><td>0.00</td><td>2.12 × 10<sup>-17</sup></td><td>-</td><td>-</td></tr><tr><td>2A 5</td><td>Highly</td><td>1898</td><td>0.28</td><td>0.28</td><td>0.00</td><td>1.53 × 10<sup>-15</sup></td><td>68.8</td><td>15.3</td></tr><tr><td>2A 6</td><td>Highly</td><td>1793</td><td>0.32</td><td>0.32</td><td>0.00</td><td>1.87 × 10<sup>-16</sup></td><td>59.9</td><td>13.8</td></tr><tr><td>2A 7</td><td>Highly</td><td>1767</td><td>0.33</td><td>0.33</td><td>0.00</td><td>3.79 × 10<sup>-16</sup></td><td>40.9</td><td>11.4</td></tr><tr><td>2A 8</td><td>Highly</td><td>1902</td><td>0.27</td><td>0.28</td><td>0.00</td><td>1.90 × 10<sup>-16</sup></td><td>70.0</td><td>17.0</td></tr><tr><td>2A 9</td><td>Highly</td><td>1829</td><td>0.30</td><td>0.31</td><td>0.01</td><td>1.99 × 10<sup>-16</sup></td><td>60.9</td><td>15.5</td></tr><tr><td>2A 10</td><td>Highly</td><td>1922</td><td>0.26</td><td>0.27</td><td>0.01</td><td>1.05 × 10<sup>-16</sup></td><td>-</td><td>-</td></tr><tr><td>2A 11</td><td>Highly</td><td>1984</td><td>0.24</td><td>0.25</td><td>0.01</td><td>3.71 × 10<sup>-17</sup></td><td>-</td><td>-</td></tr><tr><td>2A 12</td><td>Highly</td><td>1819</td><td>0.30</td><td>0.31</td><td>0.01</td><td>2.30 × 10<sup>-16</sup></td><td>50.0</td><td>14.0</td></tr><tr><td>2A 13</td><td>Highly</td><td>1769</td><td>0.32</td><td>0.33</td><td>0.01</td><td>2.20 × 10<sup>-16</sup></td><td>39.0</td><td>13.0</td></tr><tr><td>2A 14</td><td>Highly</td><td>1813</td><td>0.31</td><td>0.31</td><td>0.01</td><td>1.11 × 10<sup>-16</sup></td><td>63.0</td><td>13.9</td></tr><tr><td>2A 15</td><td>Highly</td><td>1798</td><td>0.31</td><td>0.32</td><td>0.01</td><td>3.06 × 10<sup>-16</sup></td><td>54.5</td><td>13.0</td></tr><tr><td>2A 16</td><td>Highly</td><td>1754</td><td>0.33</td><td>0.33</td><td>0.00</td><td>4.86 × 10<sup>-16</sup></td><td>41.2</td><td>12.2</td></tr><tr><td>2A 17</td><td>Highly</td><td>1759</td><td>0.33</td><td>0.33</td><td>0.00</td><td>1.89 × 10<sup>-14</sup></td><td>-</td><td>-</td></tr><tr><td>2A 18</td><td>Highly</td><td>1946</td><td>0.26</td><td>0.26</td><td>0.00</td><td>5.46 × 10<sup>-17</sup></td><td>-</td><td>-</td></tr><tr><td>2B 1</td><td>Slightly</td><td>1246</td><td>0.48</td><td>0.48</td><td>0.00</td><td>3.64 × 10<sup>-15</sup></td><td>-</td><td>-</td></tr><tr><td>2B 2</td><td>Slightly</td><td>1242</td><td>0.48</td><td>0.48</td><td>0.00</td><td>4.35 × 10<sup>-15</sup></td><td>5.9</td><td>1.5</td></tr><tr><td>2B 3</td><td>Slightly</td><td>1240</td><td>0.48</td><td>0.49</td><td>0.00</td><td>4.52 × 10<sup>-15</sup></td><td>6.9</td><td>1.9</td></tr><tr><td>2B 4</td><td>Slightly</td><td>1286</td><td>0.46</td><td>0.47</td><td>0.00</td><td>4.46 × 10<sup>-15</sup></td><td>8.3</td><td>2.2</td></tr><tr><td>2B 5</td><td>Slightly</td><td>1246</td><td>0.48</td><td>0.48</td><td>0.00</td><td>5.37 × 10<sup>-15</sup></td><td>6.3</td><td>1.7</td></tr><tr><td>2B 6</td><td>Slightly</td><td>1233</td><td>0.49</td><td>0.49</td><td>0.00</td><td>4.11 × 10<sup>-15</sup></td><td>6.7</td><td>1.7</td></tr><tr><td>2B 7</td><td>Slightly</td><td>1244</td><td>0.49</td><td>0.48</td><td>0.00</td><td>2.96 × 10<sup>-15</sup></td><td>7.4</td><td>2.1</td></tr><tr><td>2B 8</td><td>Slightly</td><td>1245</td><td>0.48</td><td>0.48</td><td>0.00</td><td>6.36 × 10<sup>-15</sup></td><td>-</td><td>-</td></tr><tr><td>2B 9</td><td>Slightly</td><td>1220</td><td>0.50</td><td>0.49</td><td>0.00</td><td>3.78 × 10<sup>-15</sup></td><td>-</td><td>-</td></tr><tr><td>2B 10</td><td>Slightly</td><td>1247</td><td>0.49</td><td>0.48</td><td>0.00</td><td>4.10 × 10<sup>-15</sup></td><td>-</td><td>-</td></tr><tr><td>2B 11</td><td>Slightly</td><td>1214</td><td>0.51</td><td>0.50</td><td>0.00</td><td>7.14 × 10<sup>-15</sup></td><td>-</td><td>-</td></tr><tr><td>2B 12</td><td>Slightly</td><td>1244</td><td>0.52</td><td>0.48</td><td>0.00</td><td>5.89 × 10<sup>-15</sup></td><td>-</td><td>-</td></tr><tr><td>2B 13</td><td>Slightly</td><td>1221</td><td>0.51</td><td>0.49</td><td>0.00</td><td>6.81 × 10<sup>-15</sup></td><td>-</td><td>-</td></tr><tr><td>2B 14</td><td>Slightly</td><td>1216</td><td>0.51</td><td>0.50</td><td>0.00</td><td>4.59 × 10<sup>-15</sup></td><td>-</td><td>-</td></tr><tr><td>2B 15</td><td>Slightly</td><td>1206</td><td>0.52</td><td>0.50</td><td>0.00</td><td>3.57 × 10<sup>-15</sup></td><td>-</td><td>-</td></tr><tr><td>2C 1</td><td>Highly</td><td>1351</td><td>0.42</td><td>0.43</td><td>0.01</td><td>4.14 × 10<sup>-14</sup></td><td>9.4</td><td>1.1</td></tr><tr><td>2C 2</td><td>Highly</td><td>993</td><td>0.58</td><td>0.58</td><td>0.00</td><td>1.22 × 10<sup>-14</sup></td><td>4.0</td><td>0.6</td></tr><tr><td>2C 3</td><td>Highly</td><td>1317</td><td>0.43</td><td>0.44</td><td>0.01</td><td>1.59 × 10<sup>-14</sup></td><td>10.6</td><td>1.2</td></tr><tr><td>2C 4</td><td>Highly</td><td>1135</td><td>0.52</td><td>0.52</td><td>0.00</td><td>8.11 × 10<sup>-15</sup></td><td>4.9</td><td>0.8</td></tr><tr><td>2C 5</td><td>Highly</td><td>1181</td><td>0.50</td><td>0.50</td><td>0.00</td><td>9.61 × 10<sup>-15</sup></td><td>6.5</td><td>0.9</td></tr><tr><td>2C 6</td><td>Highly</td><td>1083</td><td>0.55</td><td>0.54</td><td>0.00</td><td>1.56 × 10<sup>-14</sup></td><td>-</td><td>-</td></tr><tr><td>2C 7</td><td>Highly</td><td>1039</td><td>0.57</td><td>0.56</td><td>0.00</td><td>1.78 × 10<sup>-14</sup></td><td>-</td><td>-</td></tr><tr><td>2C 8</td><td>Highly</td><td>1002</td><td>0.59</td><td>0.57</td><td>0.00</td><td>2.84 × 10<sup>-14</sup></td><td>-</td><td>-</td></tr><tr><td>2D 1</td><td>Highly</td><td>1576</td><td>0.39</td><td>0.39</td><td>0.00</td><td>9.72 × 10<sup>-17</sup></td><td>33.2</td><td>12.0</td></tr><tr><td>2D 2</td><td>Highly</td><td>1621</td><td>0.38</td><td>0.38</td><td>0.00</td><td>3.77 × 10<sup>-17</sup></td><td>34.4</td><td>12.9</td></tr><tr><td>2D 3</td><td>Highly</td><td>1594</td><td>0.39</td><td>0.39</td><td>0.00</td><td>6.59 × 10<sup>-17</sup></td><td>-</td><td>-</td></tr></tbody></table>
<table><tr><td>2D 4</td><td>Highly</td><td>1587</td><td>0.39</td><td>0.39</td><td>0.00</td><td>1.62 x 10<sup>-16</sup></td><td>32.7</td><td>10.1</td></tr><tr><td>2D 5</td><td>Highly</td><td>1568</td><td>0.40</td><td>0.40</td><td>0.00</td><td>1.32 x 10<sup>-16</sup></td><td>32.8</td><td>13.1</td></tr><tr><td>2D 6</td><td>Highly</td><td>1597</td><td>0.39</td><td>0.39</td><td>0.00</td><td>6.19 x 10<sup>-17</sup></td><td>-</td><td>-</td></tr><tr><td>2D 7</td><td>Highly</td><td>1605</td><td>0.39</td><td>0.38</td><td>0.00</td><td>1.43 x 10<sup>-16</sup></td><td>-</td><td>-</td></tr><tr><td>2D 8</td><td>Highly</td><td>1567</td><td>0.40</td><td>0.40</td><td>0.00</td><td>1.27 x 10<sup>-16</sup></td><td>30.0</td><td>13.0</td></tr><tr><td>2D 9</td><td>Highly</td><td>1624</td><td>0.37</td><td>0.38</td><td>0.00</td><td>2.78 x 10<sup>-17</sup></td><td>37.1</td><td>14.1</td></tr><tr><td>2D 10</td><td>Highly</td><td>1503</td><td>0.42</td><td>0.42</td><td>0.00</td><td>1.54 x 10<sup>-16</sup></td><td>30.5</td><td>11.7</td></tr><tr><td>2D 11</td><td>Highly</td><td>1561</td><td>0.40</td><td>0.40</td><td>0.00</td><td>1.41 x 10<sup>-16</sup></td><td>36.9</td><td>13.0</td></tr><tr><td>2D 12</td><td>Highly</td><td>1516</td><td>0.42</td><td>0.42</td><td>0.00</td><td>1.75 x 10<sup>-16</sup></td><td>34.4</td><td>11.4</td></tr><tr><td>2D 13</td><td>Highly</td><td>1603</td><td>0.39</td><td>0.38</td><td>0.00</td><td>8.12 x 10<sup>-17</sup></td><td>33.9</td><td>13.2</td></tr><tr><td>2D 14</td><td>Highly</td><td>1582</td><td>0.40</td><td>0.39</td><td>0.00</td><td>1.29 x 10<sup>-16</sup></td><td>28.3</td><td>12.1</td></tr><tr><td>2D 15</td><td>Highly</td><td>1456</td><td>0.44</td><td>0.44</td><td>0.00</td><td>2.09 x 10<sup>-16</sup></td><td>19.4</td><td>9.6</td></tr><tr><td>2D 16</td><td>Highly</td><td>1606</td><td>0.38</td><td>0.38</td><td>0.00</td><td>1.88 x 10<sup>-16</sup></td><td>34.0</td><td>13.1</td></tr><tr><td>TF2 1</td><td>Slightly</td><td>1292</td><td>0.45</td><td>0.47</td><td>0.02</td><td>1.30 x 10<sup>-14</sup></td><td>8.1</td><td>2.0</td></tr><tr><td>TF2 2</td><td>Slightly</td><td>1250</td><td>0.47</td><td>0.48</td><td>0.01</td><td>1.41 x 10<sup>-14</sup></td><td>5.9</td><td>2.0</td></tr><tr><td>TF2 3</td><td>Slightly</td><td>1229</td><td>0.48</td><td>0.49</td><td>0.01</td><td>1.49 x 10<sup>-14</sup></td><td>5.5</td><td>1.0</td></tr><tr><td>TF3 1</td><td>Slightly</td><td>1241</td><td>0.48</td><td>0.48</td><td>0.00</td><td>1.36 x 10<sup>-14</sup></td><td>9.2</td><td>2.8</td></tr><tr><td>TF3 2</td><td>Slightly</td><td>1248</td><td>0.47</td><td>0.48</td><td>0.00</td><td>1.27 x 10<sup>-14</sup></td><td>9.0</td><td>2.8</td></tr></table>
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<figure><img src="image_2.png" /></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2, Heap et al., JVGR</figcaption></figure>
<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4, Heap et al., JVGR</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5, Heap et al., JVGR</figcaption></figure>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>Figure 6; Heap et al., JVGR</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 7, Heap et al., JVGR</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 8, Heap et al., JVGR</figcaption></figure>
<figure><img src="image_11.png" /></figure>
<figure><img src="image_12.png" /></figure>