Fluid-rock interaction processes in the Te Kopia geothermal field (New Zealand) revealed by SEM-CL imaging

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1 Geothermics 33 (2004) Fluid-rock interaction processes in the Te Kopia geothermal field (New Zealand) revealed by SEM-CL imaging Greg Bignall a,, Kotaro Sekine b, Noriyoshi Tsuchiya a a Graduate School of Environmental Studies, Tohoku University, Aoba 20, Aramaki, Aoba-ku, Sendai , Japan b Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai , Japan Received 23 July 2003; accepted 19 March 2004 Available online 24 May 2004 Abstract Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of hydrothermal quartz exposed by weathering in the Te Kopia geothermal field (New Zealand) has revealed a history of crystal growth, dissolution, overprinting and fracturing that cannot be detected using other observational techniques (e.g. transmitted or reflected light microscopy, back-scattered electron imaging or secondary electron imaging). The crystals initially grew as CL-dark quartz, at least 350 m below their present location on the Paeroa Fault scarp, in a neutral ph, 215 ± 10 C liquid reservoir (inferred from the analysis of primary liquid fluid inclusions: mean T h of 213 C; wt.% NaCl eq. ). Relict quartz adularia illite alteration occurs at the surface, in the vicinity of the quartz crystals, and in drillcores from the nearby TK-1 exploration well. Repeated movement on the Paeroa Fault uplifted pyroclastic rocks hosting the quartz crystals, but also provided pathways for pulses of hot fluids to move through the system. Quartz precipitation occurred at the edge of the crystals as the reservoir fluids cooled, as indicated by micron-scale alternating CL-dark/CL-bright quartz growth bands, which contain fluid inclusions with T h values of 210 ± 40 C. Pressure fluctuations were the likely cause of dissolution, marked by corroded crystal edges, with subsequent precipitation of quartz into open space. SEM-CL imaging shows that the quartz crystals contain healed fractures, which trapped low salinity fluids with T h values of 201 ± 6 C. Low-pH fluids in the near-surface Corresponding author. Present address: Wairakei Research Centre, Institute of Geological and Nuclear Sciences Limited, Private Bag 2000, Taupo, New Zealand. address: bignall@mail.kankyo.tohoku.ac.jp (G. Bignall) /$ CNR. Published by Elsevier Ltd. All rights reserved. doi: /j.geothermics

2 616 G. Bignall et al. / Geothermics 33 (2004) setting also rounded the quartz crystals, and coated them with kaolinite and CL-grey amorphous silica residue CNR. Published by Elsevier Ltd. All rights reserved. Keywords: Cathodoluminescence; SEM-CL; Quartz microtextures; Fluid inclusion geothermometry; Hydrothermal alteration; Te Kopia geothermal field; New Zealand 1. Introduction Mineral non-stoichiometry, poor crystallographic ordering, lattice defects and the incorporation of trace elements into a crystal structure can generate cathodoluminescence ( CL ) of varying wavelengths and intensity, which is useful for interpreting a variety of geological phenomena (Marshall, 1988; Ramseyer et al., 1988; Trukhin, 1994; Götze et al., 2001), including the nature of quartz depositional processes (water-rock interaction) in active geothermal settings (Bignall et al., 2004). CL is caused by the emission of photons in response to electron bombardment, and can be observed using a scanning electron microscope (SEM) equipped with a CL detector. Although the physical causes of mineral CL are complex, interpretation of quartz textures seen by SEM-CL can be very revealing, and may indicate microtextural features (e.g. cryptic alteration, overprinting, fracturing and structural evidence for crystallisation and/or deformation) in quartz, calcite, anhydrite and other common hydrothermal minerals, which are not visible using other observational tools (such as hand specimen examination, optical microscopy, or SEM imaging). The SEM-CL technique has enabled a range of geological problems to be resolved, such as sedimentary provenance (Seyedolali et al., 1997), the nature of diagenetic and pressure solution processes (Schieber et al., 2000), sandstone fracturing (Milliken and Laubach, 2000) and the character of crystallisation and deformation processes in granite (Müller et al., 2002; Sekine, 2003). SEM-CL has proven to be useful in hydrothermal alteration studies, with the potential to reveal a range of diagnostic depositional processes, including the relationship between electrum precipitation and specific fluid composition/quartz-precipitation events (Wilkinson et al., 1999), the role of temperature pressure change in (quartz) vein formation (Rusk and Reed, 2002; Batkhishig et al., submitted for publication), quartz vein chronology (Penniston-Dorland, 2001), and occurrence of CL-distinctive hydrothermal quartz overprinting igneous quartz (Sekine, 2003). We have used SEM-CL techniques, complemented by detailed fluid inclusion thermometry and optical microscopy, to examine fluid-rock interactions in the active Te Kopia geothermal system, 25 km NNE of Taupo, New Zealand (Fig. 1). This paper details part of a broad study to understand chemical processes affecting rock permeability in active hydrothermal systems (Tsuchiya et al., 2001; Hirano et al., 2002), the nature of quartz fracturing (Hashida et al., 2001), and utilisation of deep-seated geothermal reservoirs (Nakatsuka, 1998). Progressive uplift of parts of the Te Kopia geothermal system has affected the hydraulic regime in the field and exposed quartz adularia illite hydrothermal mineral assemblages, including large (several centimetres long) quartz crystals, which formed a few hundred metres deep in the geothermal system (Bignall and Browne, 1994; Bignall et al., 2002). Here, we focus on

3 G. Bignall et al. / Geothermics 33 (2004) Fig. 1. Location of the Te Kopia geothermal field, North Island, New Zealand. the quartz crystals, to demonstrate the effectiveness of SEM-CL imaging as a geothermal and mineral exploration tool, and show how the technique, complemented by analysis of fluid inclusions and alteration mineralogy, provides new insights into the hydrology and thermal evolution of the Te Kopia geothermal system. 2. Geology The Te Kopia geothermal field is one of about 20 high-temperature (>240 C) thermal areas in the Taupo Volcanic Zone (TVZ), with surface manifestations comprising altered and steaming ground, hydrothermal eruption craters and steam-heated ponds of mixed acid-sulfate water, steam condensate and rainwater (Bignall and Browne, 1994) that extend >2.5 km along the 220-m high scarp of the Paeroa Fault (Hochstetter, 1864; Grange, 1937; Healy, 1952, 1974). Bignall and Browne (1994), Browne et al. (1994) and Martin et al. (2000a) described the surface alteration at Te Kopia, with the latter study utilising palynological techniques to assess the age and significance of in situ silica sinter in the

4 618 G. Bignall et al. / Geothermics 33 (2004) thermal area. Martin et al. (2000a) suggest that the presence of silica sinter indicates that alkali-chloride water discharged from the hot pools (and/or geysers) as recently as 1800 BP. Grange (1937), Mahon (1965a) and Sheppard and Klyen (1992) published water analyses from springs and pools, and gas compositions from several fumaroles, whilst Martin et al. (2000b) discussed the movement of aluminium in the near-surface hydrologic environment. Several geophysical studies have examined the hydrological connection between Te Kopia and other nearby hot spring areas, using audio-magnetic telluric, dc resistivity measurements, and airborne video thermal infrared surveys (Macdonald, 1965; Merchant, 1990; Bromley, 1992). Soengkono (1999) used electrical resistivity and aeromagnetic interpretations, and detailed digital topographic data, to deduce the relationship between the extent of the thermal activity at Te Kopia and local geological structures. The Paeroa Fault is the dominant structural feature at Te Kopia, but Soengkono (1999) suggests that several NW-striking lineaments also occur, and that their intersections may provide permeable zones for reservoir fluids to ascend to the surface, or for near-surface fluids to move laterally across the system. The Paeroa Fault is normal, strikes about 40 and dips steeply to the west, with a minimum vertical offset of 450 m. Nairn and Hull (1986) indicate that the fault has offset ash layers from Recent volcanic eruptions in the TVZ during the last 1800 years. Nairn et al. (1994) infer that movement on the Paeroa Fault began about 240,000 years ago, so the average rate of displacement across the fault zone is 2.5 m/1000 years. The fault movement has influenced surface manifestations, hydrology and hydrothermal alteration within the Te Kopia geothermal system (Browne et al., 1994). Geothermal exploration drilling in the mid-1960s revealed that the subsurface geology at Te Kopia is comprised of hydrothermally altered, SE-dipping, non-welded Quaternary ignimbrite, consisting of rhyolitic pumice lapilli tuff, correlated with the Paeroa, Te Weta and Te Kopia Ignimbrites (Steiner, 1977; Bignall and Browne, 1994) and intercalated ash deposits. Recent (<22,600 years old) hydrothermal eruption breccia deposits near the steam-heated pools contain subangular blocks of hydrothermally altered tuff, Paeroa Ignimbrite and rhyolite, supported by a clay-rich ash matrix (Bignall and Browne, 1994). Pre- and post-taupo volcanic eruption (186 a.d.) landslides have occurred at Te Kopia, which were most likely initiated by movement on the Paeroa Fault. 3. SEM-CL imaging of surface quartz 3.1. Quartz crystals Subhedral quartz crystals occur at two localities on the Paeroa Fault scarp (Fig. 2): in a landslide deposit 600 m east of TK-1 (#TKS-10; 180 ± 5 m above the base of the Paeroa Fault) and on an upstanding ridge 900 m NE of TK-1 (#TKS-12; 145±5 m above the floor of the Te Kopia valley). The translucent quartz crystals occur as loose fragments (Fig. 3A C), with one end having a well-formed crystal shape, whilst the other end has a truncated surface where it may have been attached to (eroded) wall rock. The largest quartz fragments are 4.5 cm long, although most are 1 2 cm in length, with the variation in crystal size probably a consequence of the available space in which each crystal grew. It is most likely that the crystals nucleated randomly, with the c-axis oriented

5 G. Bignall et al. / Geothermics 33 (2004) Fig. 2. Locality map of the Te Kopia thermal area, showing the exploratory geothermal drillholes, extent of thermal manifestations (steam-heated pools, altered/steaming ground and relict silica sinter) and quartz crystal locations. perpendicular to the wall rock, but it is impossible to confirm the original orientation of the crystals. There is no obvious crystal bending in each quartz crystal/fragment. The quartz crystals have hand specimen characteristics consistent with growth into open space, and may be derived from an extensive quartz vein system (remnant stockwork quartz veins occur in the thermal area) in pyroclastic rocks hosting a quartz adularia illite assemblage. The neutral-ph alteration assemblage has been widely overprinted by an acid-alteration assemblage, comprising kaolinite, lesser opaline silica, alunite, cristobalite, jarosite, hematite, and native sulfur. Petrographic examination shows that the Te Kopia quartz crystals are fractured and partly covered by a crusty, amorphous silica residue and kaolinite coating. Transmitted light microscopy reveals no other distinguishing features.

6 620 G. Bignall et al. / Geothermics 33 (2004) Fig. 3. Photograph of quartz crystals collected from the Paeroa Fault scarp (at TKS-12; see Fig. 2), used for SEM-CL imaging and fluid inclusion microthermometry Methodology Several 3 4-cm long quartz crystals from TKS-12 were selected for SEM-CL analysis, with 100- m thick, double polished sections (which were also utilised for fluid inclusion

7 G. Bignall et al. / Geothermics 33 (2004) analysis) prepared for each sample: (i) cut through the centre of each crystal perpendicular to the long (c-) axis and (ii) cut parallel to the c-axis (Fig. 3B). The double polished sections were initially examined by optical microscopy, then carbon coated for SEM-CL observation using a SEM-EDX Hitachi-S2460N scanning electron microscope at the Graduate School of Environmental Studies, Tohoku University, which is equipped with an Oxford Mini-CL detector and photomultiplier. Many workers have studied the luminescent property of quartz and other minerals (e.g. Pagel et al., 2000; Götze et al., 2001), and revealed textures not observable by other techniques, including transmitted light, reflected light, backscattered electron (BSE), and secondary electron (SE) imaging. The SEM-CL method has a high spatial resolution and offers a wide range of beam currents and acceleration voltages, making it advantageous for observing minerals with weak luminescence (e.g. quartz). We analysed the quartz thick-sections at kv, with a beam current set at na, to obtain optimal contrast in observed luminosity. In SEM-CL, grey-scale ( CL-dark, CL-grey or CL-bright ) images are produced with apparent intensity of observed luminescence dependent on machine operating conditions, i.e. beam current, acceleration voltage, photomultiplier contrast and brightness Results of SEM-CL imaging SEM-CL imaging indicates that the Te Kopia quartz crystals have had a complex history (e.g. Fig. 4). Most of the quartz crystals have a low intensity luminescence core (CL-dark), which is inferred to have grown under stable physico-chemical conditions within the Te Kopia geothermal reservoir. The SEM-CL mosaic also reveals microtextural evidence of (i) concentric, alternating CL-dark/CL-bright growth bands; (ii) indications that crystals were partly dissolved (at crystal edges and along fractures); (iii) turbid growth zones (with anomalous CL patterns); (iv) late-stage CL-bright quartz that precipitated into open spaces; (v) fracturing (with healed, quartz-filled fractures having CL luminescence that differs from the host quartz); and (vi) coating by CL-grey amorphous silica residue. Electron back-scattered and transmitted light imaging of the Te Kopia quartz, however, does not reveal the generations of crystal growth, dissolution and fracturing evident in SEM-CL. Instead, these images appear optically continuous. Back-scattered electron and SEM-CL images of the same quartz thick-section, cut perpendicular to the c-crystal axis, are shown in Fig. 5. SEM-CL imaging has revealed a variety of microtextural features, indicative of multi-stage crystal growth at different temperature pressure conditions, which include the following: Growth zones SEM-CL imaging of the surface quartz crystals reveals areas of concentric growth zoning, up to 1.5 mm wide at the edge of some crystals, with alternating micron-scale bands of CL-dark and CL-bright quartz of variable intensity and width. Individual bands are typically 5 20 m thick, although CL-bright bands tend to be narrower than CL-dark bands (Fig. 4). In most samples, the patterns of concentric growth zones are defined by euhedral-shaped crystal rhombs, indicative of growth into open space, which point toward the end of the crystal. In places, the CL-dark quartz is enveloped by CL-bright quartz overgrowths, and elsewhere SEM-CL imaging reveals a fusion of two or more growth zone regions

8 622 G. Bignall et al. / Geothermics 33 (2004) Fig. 4. A SEM-CL mosaic of a quartz crystal from TKS-12, cut along c-axis. SEM-CL reveals distinct stages of quartz deposition with CL-dark luminescence, and also CL-dark/CL-bright growth zones. The crystals are partly dissolved, with CL-bright quartz sealing fractures and filling open spaces. CL-grey amorphous silica occurs on the surface of the quartz crystals. Inset: sketch of same crystal, highlighting the observed microtextural features. and/or crystals, with apparent overlapping of growth zone regions, which vary in CL intensity. The quartz crystals contain regions of intergrown CL-dark, -grey and -bright quartz, that form irregular patterns against the planar boundaries of CL-grey-zoned quartz, and occur interstitially between areas of CL-continuous quartz. These turbid growth zones are similar to those described by Penniston-Dorland (2001), and comprise anomalous, concentric and/or irregular CL patterns, although the mechanism of their formation is not known Quartz dissolution Textural relationships revealed by SEM-CL indicate that the Te Kopia quartz crystals precipitated in open spaces, but were later fractured and partially dissolved. The most obvious indication of quartz dissolution is the rounded CL-bright crystal edges and shallow indented crystal faces, which are not seen using optical microscopy, with irregular open

9 Fig. 5. Back-scattered electron mosaic (A) and SEM-CL image (B) of the same quartz crystal, from TKS-12 locality. Euhedral growth zones, containing measurable fluid inclusions (Table 1), are only revealed by SEM-CL imaging. The CL-dark quartz core is likely to have grown under stable reservoir conditions, while micron-scale CL-dark/CL-bright quartz precipitated later, coinciding with the uplift of the Paeroa Fault Block. G. Bignall et al. / Geothermics 33 (2004)

10 624 G. Bignall et al. / Geothermics 33 (2004) spaces or late (CL-bright) quartz filling areas between regions characterised by euhedral crystal growth Replacement Replacement/recrystallisation is most obvious where CL-bright quartz overprints CL-grey quartz. In the Te Kopia samples, recrystallisation is typically revealed where the boundary of CL-grey quartz continues across the CL-grey/CL-bright boundary into CL-bright quartz, with the original grain boundaries of the CL-grey quartz being preserved. The distribution of CL-bright quartz is controlled by alteration that occurred later than the CL-grey quartz. In other places, CL-grey quartz occurs at the edge of CL-bright quartz, which indicates recrystallisation along some grain boundaries Quartz microfracturing In all samples, healed microfractures irregularly cut host CL-dark or -bright quartz. The braided microfractures are filled with fluid-inclusion bearing, CL-bright quartz that post-dates all other quartz generation phases, except the crusty, crystal-covering amorphous silica residue. Contacts of host quartz with the fractures are sharp, even though many fractures are optically/crystallographically continuous with their host quartz. In some cases, fractures filled with CL-bright quartz crosscut and offset one another, indicating that physical fracturing is taking place rather than dissolution of quartz. 4. Fluid inclusion microthermometry 4.1. Previous studies Bignall and Browne (1994) published homogenisation (T h ) temperature and ice-melting (T m ) data for primary, liquid-rich fluid inclusions in quartz collected at TKS-10 (180 ± 5m above the base of the Paeroa Fault), but stated that no growth zones or vapour inclusions were visible by optical microscopy. These authors indicated the mean T h value for TKS-10 fluid inclusions to be 188 ± 15 C(Table 1), with freezing measurements giving T m values of 0.2 to 0.1 C (i.e. apparent salinities of wt.% NaCl eq. ), which is similar to data for fluid inclusions in subsurface quartz from drillhole TK-1 (Bignall, 1994). Bignall and Browne (1994) also made T h measurements of liquid-rich fluid inclusions in quartz from TKS-12 (145 ± 5 m above the base of the Paeroa Fault), which had a mean T h of 196 ± 11 C, and salinity of wt.% NaCl eq. Bignall and Browne (1994) plotted fluid inclusion data on a boiling curve for pure water, and inferred that fluid inclusions in TKS-10 formed at least 125 m below the water table. They suggested that the crystals had been uplifted >300 m by movement on the Paeroa Fault. They also suggested that fluids in quartz from TKS-12 were trapped at least 170 m below the water table, and that those crystals must have been uplifted >315 m to their present elevation New fluid inclusion data New 100- m thick fluid inclusion plates were prepared to verify whether there was any variation in the temperature and/or salinity of fluids trapped in the Te Kopia quartz

11 Table 1 Fluid inclusion microthermometric data for quartz crystal samples from Te Kopia (see Fig. 2 for sample locations) Sample Reference Location (texture) No. Incl. TKS-10 Bignall and Browne (1994) TKS-12 Bignall and Browne (1994) Type T h range (mean) ( C) T m range (mean) ( C) Salinity (wt.% NaCl eq. ) All sample 25 Liquid primary (188.6) 0.2 to 0.1 ( 0.1) All sample 14 Liquid primary (195.6) Not measured Thermal regime ( C) T.U. 12 core This study CL-dark crystal core 13 Liquid primary (213.6) ± 10 T.U. 12 growth zone This study CL-dark/CL-bright growth zone 16 Liquid primary (209.6) Not measured 210 ± 40 T.U. 12 fracture This study CL-bright quartz-filled fracture 5 Liquid secondary (201.1) Not measured 201 ± 6 Tohoku University analyses indicated by T.U.. G. Bignall et al. / Geothermics 33 (2004)

12 626 G. Bignall et al. / Geothermics 33 (2004) crystals either in growth zone regions, in continuous CL-dark quartz, late-stage CL-bright quartz or in healed fractures revealed by SEM-CL imaging. Fluid inclusion microthermometry was undertaken using the Linkam THMS 600 heating-freezing stage at Tohoku University, using chips cut from doubly polished plates (after Barker and Reynolds, 1984). The thermocouple for the heating-freezing stage is regularly calibrated, using metal melting (e.g. tin, C; lead, C) and the ice-melting temperature of pure water, with T h values corrected for small errors (<1 C), while T m measurements are accurate to ±0.1 C. Most fluid inclusions are liquid-rich, although co-existing liquid and vapour inclusions were identified (the latter are necked and were not analysed, as they produce erroneous T h data). No daughter minerals or other phases (e.g. clathrate) were observed. The majority of fluid inclusions are primary (some have negative crystal form) and occur in well-defined growth zones, whilst some secondary fluid inclusions occur in healed fractures. No fluid inclusions were observed in late-stage CL-bright quartz. Size is variable (up to 80 m long), but most are 5 18 m long. No thermometric data were obtained on fluid inclusions <2 m long. Inferred salinities are consistent with Bignall and Browne (1994), but the present study reveals a wide variation (>80 C) in T h values, reflecting the occurrence of fluid inclusions in growth zones, as isolated inclusions, or in healed microfractures (Table 1; Fig. 6). Fluid inclusions in the CL-dark core of quartz crystals from TKS-12 have T h values of C (T h(mean) C, C higher than previous T h values from TKS-12). Fluid inclusions in growth zones have a wide range of T h values, from 168 to 248 C, although T h(mean) (210 C) is similar to data from the core of the crystal. Secondary fluid inclusions in TKS-12 quartz have T h values ( C; mean C) similar to T h measurements from primary inclusions in growth zones and unzoned crystal core. By plotting fluid inclusion data on a boiling point for depth curve for pure water, it is inferred that initial crystal growth occurred at least 210 m below the water table, in a 215 ± 10 C liquid reservoir (i.e. at least 350 m below their present position on the Paeroa Fault scarp). Our results are consistent with the interpretation of Bignall and Browne (1994), although we suggest that quartz crystal growth initially occurred slightly deeper than indicated by the earlier study, whilst SEM-CL imaging reveals that the story is more complicated than previously described (e.g. Fig. 7). 5. Hydrothermal alteration New fluid inclusion evidence suggests that the Te Kopia quartz crystals initially formed at least m below the water table, and were subsequently uplifted >350 m to their present position. The inferred amount of uplift is a minimum, as the crystals probably derive from higher elevation on the Paeroa Fault scarp than they are now found, as their host pyroclastic rocks have eroded. Rocks on the scarp contain a relict quartz adularia illite assemblage (i.e. plagioclase phenocrysts in the pyroclastic rocks were initially replaced by adularia and illite, with the latter forming small patches in the felspar), but this has been overprinted by alteration products typical of acid-sulfate steam condensate, including kaolinite, alunite, cristobalite and silica residue.

13 G. Bignall et al. / Geothermics 33 (2004) Fig. 6. Histogram of homogenisation temperature (T h ) values for primary fluid inclusions in quartz, from (A) CL-dark crystal core and (B) CL-bright/CL-dark growth zones; and (C) secondary inclusions from CL-bright, quartz-filled fractures. It is likely that the surface quartz crystals initially grew in the presence of neutral-ph, alkali-chloride fluids, which produced a hydrothermal mineral assemblage that includes varying proportions of quartz, adularia, illite, chlorite, calcite and pyrite, lesser pyrrhotite, zeolites (mainly wairakite) and epidote (Bignall, 1994; Bignall et al., 2002). The quartz adularia illite assemblage is first observed in crystal tuff/ignimbrite from 442 m drilled depth (to the bottom of the well, at m drilled depth), and underlies a zone of quartz albite illite alteration (from 381 m drilled depth). An assemblage of smectite mordenite alteration in shallow and peripheral parts of the field is likely to have formed by the reaction of pyroclastic rocks with waters resulting from the mixing of upwelling alkali-chloride

14 628 G. Bignall et al. / Geothermics 33 (2004) Fig. 7. SEM-CL mosaic of Te Kopia quartz crystal, showing growth zones, and partly corroded CL-dark crystal edge, with CL-bright quartz overprinting (see also Fig. 8). water and cool groundwater/condensate. Secondary quartz is first observed in core samples from 274 m drilled depth, in lacustrine tuff (Bignall, 1994). Mahon (1965b) analysed fluid that discharged from TK-1 (the second well at Te Kopia, TK-2, was not discharged), which has been recalculated to deduce fluid-mineral equilibrium conditions in the Te Kopia reservoir (Bignall, 1994). The inferred composition of Te Kopia reservoir fluid plots close to the metastable extension of the albite adularia univariant curve at 250 C(Fig. 8), with the Na/K ratio of the fluid buffered by this assemblage, although petrographic evidence reveals that equilibrium shifted to the illite stability field. Boiling, indicated by bladed calcite in surface exposures, occurred in the Te Kopia system, although it may have been brief and probably self-limiting due to mineral deposition and heat loss. It is likely that boiling occurred as a consequence of a pressure decrease, related to faulting events, although it may have been initiated by the entry of gas into the system (lowering the

15 G. Bignall et al. / Geothermics 33 (2004) Fig. 8. Activity diagram for K +,Na + and H + aqueous species in equilibrium with aluminosilicates at 250 and 225 C (quartz saturation), bold and dashed line, respectively. The inferred reservoir condition at Te Kopia is based on TK-1 well discharge chemistry and measured temperatures, and plots close to the metastable extension of the albite adularia univariant drawn for 250 C. A change in the ph of reservoir fluids, or cooling, may result in the hydrothermal fluid being in equilibrium with (late stage or overprinting) illite. boiling temperature at a given depth; Sutton and McNabb, 1977) and/or heating. Changes in the ph of reservoir fluids (e.g. by fluid mixing), or cooling accompanying boiling, may account for the inferred shifts in mineral-fluid equilibria, from a situation where the hydrothermal fluid is in equilibrium with albite and adularia, into the illite stability field. 6. Interpretation of SEM-CL imaging Bignall and Browne (1994) suggested that quartz crystals exposed in eastern parts of the Te Kopia thermal area had been uplifted at least 300 m since they formed, based on fluid inclusion analysis, presence of relict quartz adularia illite alteration mineral assemblage in nearby surface rocks, and +600 m vertical displacement on the Paeroa Fault. Nairn et al. (1994) indicate that movement on the Paeroa Fault began about 240,000 years ago, while Bignall and Browne (1994) suggest that the Te Kopia geothermal system may have been active for as long as 120,000 years, so the hydrology of the system and its thermal regime would have changed greatly during this time. Our SEM-CL imaging suggests that quartz now found on the Paeroa Fault scarp initially grew as CL-dark crystals (with no zoning) into fluid-filled space, under stable hydrostatic and thermal conditions, in a 215 ± 10 C liquid reservoir. This is based on T h values for

16 630 G. Bignall et al. / Geothermics 33 (2004) fluid inclusions found in CL-dark quartz. The inferred thermal regime is consistent with present conditions in the Te Kopia geothermal system, as revealed by physical measurements in the nearby TK-1 exploration well (maximum of 238 Cat 550 m drilled depth ( 115 m relative to sea-level RSL), but 228 C at well bottom ( m RSL); and a maximum of 227 C at m drilled depth ( 10 m RSL) in TK-2), by the occurrence of the quartz adularia illite hydrothermal assemblage in drillcores from TK-1 and in rocks exposed at the surface, with illite being a late-stage alteration product. Repeated movement on the Paeroa Fault uplifted rocks that hosted the quartz crystals by >350 m, and provided pathways for pulses of fluids to move through the system, leading to precipitation of CL-bright quartz from the cooling reservoir fluids and/or incursion of cool condensate from the margin of the system (accounting for smectite mordenite alteration on the periphery of the field). SEM-CL imaging reveals that the edge of most quartz crystals is characterised by micron-scale quartz bands of varying CL intensity (containing fluid inclusions with T h values of 210 ± 40 C), which are likely to have precipitated during periods of crystal growth coinciding with fault uplift. The zoned CL pattern may result from cyclic trace-element abundances, corresponding to fluids whose composition oscillates, or from changes in the rate of precipitation, as the band widths vary, and/or from temperature/pressure pulses (i.e. different physical/chemical conditions that would also affect trace-element abundances in the zoned quartz). As well as crystal growth, we find SEM-CL evidence of partly dissolved CL-dark quartz (containing weakly defined euhedral growth zones) with subrounded or corroded crystal edges, indicative of dissolution by quartz-undersaturated fluid (e.g. Fig. 9). In places, open spaces created by quartz dissolution were later filled by CL-bright quartz. Quartz dissolution is likely caused by changes in fluid pressure and temperature. Chemical processes, such as an increase of fluid ph, is unlikely to have caused initial dissolution as the quartz crystals precipitated from hydrothermal fluids that also produced a neutral-ph hydrothermal mineral assemblage, including adularia, illite, and epidote. Pressure change has a strong effect on quartz solubility and is the most likely cause of the quartz dissolution and precipitation textures, as pressure fluctuations almost certainly occurred in the fault-controlled Te Kopia thermal area. The areas of turbid growth comprise anomalous, concentric and/or irregular CL patterns, which contrast with the planar boundaries of CL-dark-zoned quartz or occur interstitially between areas of CL-continuous quartz. Penniston-Dorland (2001) described turbid quartz in vein quartz in the Grasberg Igneous Complex (Indonesia) and concluded that a turbulent fluid may have created the complex zoning, although a change in the nature of the fluid, or interruption in the crystal growth, may be responsible. In places, overprinting/replacement by CL-bright quartz obscures textural relations in early-formed CL-dark quartz, although the original shape of the quartz crystal is preserved (Fig. 9). The change in luminescence intensity may result from diffusion of elements in response to chemical gradients between quartz and the hydrothermal fluid. Healed microfractures are filled with CL-bright quartz and post-date all other quartz phases. The physical fracturing that took place underlines the complex history of repeated microfracturing, and resealing (through quartz deposition) that occurred during crystal growth. The most likely explanation for microfracturing is that it is linked with regional faulting (perhaps as a minute strike-slip component associated with movement on the Paeroa

17 Fig. 9. SEM-CL image (left) and sketch (right) of corroded CL-dark quartz, with truncated growth zones, marked by CL-bright quartz overprinting and coating by CL-grey amorphous silica. G. Bignall et al. / Geothermics 33 (2004)

18 632 G. Bignall et al. / Geothermics 33 (2004) Fault), or microfracturing is caused by protruding quartz crystals contacting other crystals. Whatever the cause of the microfracturing, quartz precipitation plugged most fractures, yet this may have produced a slight pressure increase, accounting for an increase in quartz solubility and resultant dissolution. 7. Conclusions Progressive regional faulting has uplifted parts of the Te Kopia geothermal system by at least 320 m, and influenced field hydrology, surface manifestations and hydrothermal alteration (Fig. 10). Rocks now exposed on the Paeroa Fault scarp contain relict quartz adularia illite assemblages, which have been overprinted by amorphous silica, kaolinite, cristobalite and/or alunite. Subhedral quartz crystals (up to 4.5 cm long) also occur at the surface, although they initially grew as much as 210 m below the water table. Electron back-scattered and transmitted light examination show that the quartz crystals are fractured and covered by crusty, amorphous silica, but these methods reveal no other distinguishing features, while SEM-CL imaging shows that the crystals have a complex history revealing much about the evolution of the Te Kopia geothermal system. SEM-CL imaging of the surface quartz crystals has provided evidence of multi-stage crystal growth, dissolution, recrystallisation and microfracturing. The crystal cores are composed of CL-dark quartz that are inferred to have grown within the Te Kopia geothermal reservoir, >350 m below their present location on the Paeroa Fault scarp, under stable physico-chemical conditions, as the quartz contains fluid inclusions with a narrow range of T h values, of 215 ± 10 C. The crystal edges contain micron-scale bands of CL-dark and CL-bright quartz, containing fluid inclusions with T h values of 210 ± 40 C, and are inferred to have precipitated during a later period of crystal growth, coinciding with uplift on the Paeroa Fault. Repeated fault movement provided pathways for pulses of hot reservoir fluids to move through the system, leading to precipitation of CL-bright quartz from cooling reservoir fluids and/or inflow of cool condensate. SEM-CL evidence of fractures in the quartz underlines a complex history of repeated microfracturing and resealing (through quartz deposition) during crystal growth. As well as growth textures, we find CL-dark quartz is partly dissolved, with corroded crystal edges, as a result of pressure fluctuations related to faulting, and changes in quartz solubility, with subsequent overprinting by CL-bright quartz. Low-pH fluids in the near-surface setting have also acted to round the quartz crystals, and coat them with CL-grey amorphous silica residue. SEM-CL imaging is an effective tool for understanding mineral-fluid interactions in active and fossil (mineralised) hydrothermal systems, and a useful aid in geothermal resource development. SEM-CL may reveal microtextures (growth zoning, dissolution, replacement/overgrowth and fracturing) in minerals that are not observable by optical microscopy, back-scattered electron imaging or secondary electron imaging. SEM-CL images, therefore, combined with trace-element analyses (e.g. Sekine, 2003) and fluid inclusion data can assist in resource development by (i) helping to decipher the thermal and/or chemical evolution of a geothermal system (e.g. the correlation of fluid inclusion populations, and their relative chronology, with specific hydrothermal events); (ii) revealing chronologic/physical relations

19 G. Bignall et al. / Geothermics 33 (2004) Fig. 10. Inferred stages of hydrothermal activity at Te Kopia (after Bignall and Browne, 1994), indicated by the nature of thermal manifestations, hydrothermal alteration (at the surface and in cores from TK-1 and TK-2), and SEM-CL imaging/fluid inclusion thermometry of quartz from the Paeroa Fault scarp.

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