Occurrence and significance of anomalous chloride waters at the Orakei Korako geothermal field, Taupo Volcanic Zone, New Zealand

Geothermics 35 (2006) 211–220

Occurrence and significance of anomalous chloride waters at the Orakei Korako geothermal field, Taupo Volcanic Zone, New Zealand Patrick R.L. Browne a,b,∗ , Kerry A. Rodgers b,c a

Geothermal Institute, University of Auckland, Private Bag 92019, Auckland, New Zealand b Department of Geology, University of Auckland, Private Bag 92019, Auckland, New Zealand c Research Associate, Australian Museum, College Street, Sydney, NSW, Australia Received 14 October 2005; accepted 20 February 2006 Available online 3 April 2006

Abstract Water with Cl concentrations from 15 to almost 10,000 mg/kg, and molecular SO4 /Cl ratios ranging from 0.003 to 1.87, drips periodically from the roof of Ruatapu cave and a side chamber, Rahu Rahu, located in the Orakei Korako geothermal field, Taupo Volcanic Zone, New Zealand. Pools in the bottom of both Ruatapu and Rahu Rahu contain sulfate–chloride waters with pH values ranging from 2.5 to 3.0; their Cl contents have varied temporally from 120 to 240 mg/kg and their molecular SO4 /Cl ratios from 0.86 to 1.30. The Cl in the water dripping from the cave roof cannot come directly from the alkali chloride–bicarbonate water that circulates in the reservoir at Orakei Korako since the modern and historic piezometric surfaces are several meters below the cave roof. Nor does all the Cl in the cave pool waters derive from the reservoir fluid as the volume input required is incompatible with their Na and K contents. A more likely source for the Cl is one whereby rain water, percolating through the fractured rhyolitic country rock, dissolves Cl present either in glass shards or halite deposited in prehistoric times when trapped alkali chloride water boiled to dryness. Given that Cl in the cave pool waters is therefore supplied from a source above rather than, as previously assumed, below, the axiom that Cl present in acid sulfate–chloride ± bicarbonate waters is necessarily a signature of a deep water or magmatic input needs qualification. © 2006 CNR. Published by Elsevier Ltd. All rights reserved. Keywords: Thermal waters; Cave hydrology; Geothermal fluids; Orakei Korako; New Zealand

∗

Corresponding author. Tel.: +64 9 373 7599; fax: +64 9 373 7435. E-mail address: [email protected] (P.R.L. Browne).

0375-6505/$30.00 © 2006 CNR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.geothermics.2006.02.005

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1. Introduction The compositions of fluids that occur in active geothermal areas reflect several factors, including subsurface temperatures, the mineralogies of the rocks through which they flow, the processes that occur within their reservoirs, the duration of fluid/rock interactions, and the contributions made by any magmatic components. To a great extent these factors are simply a consequence of the hydrology of a geothermal field and its environs. The three most common types of fluid in and around most liquid-dominated geothermal fields (alkali chloride, bicarbonate and acid sulfate waters) are intimately related genetically, but their end members are compositionally quite distinct. Typically, the primary fluid within a geothermal reservoir is alkali chloride water of near-neutral pH. This ascends as a single phase until it reaches a depth where it boils and the gases dissolved within it, chiefly CO2 and H2 S, preferentially partition into the vapor phase and ascend with it. The CO2 and steam then either discharge at the surface or dissolve in perched groundwaters to form bicarbonate or CO2 -rich waters (Hedenquist and Stewart, 1985; Hedenquist, 1990). The separated H2 S is commonly oxidised to H2 SO4 near to, or at, the ground surface to yield acid sulfate waters. The process of boiling also causes the parent alkali chloride water to become slightly more saline since all Cl concentrates in the liquid phase. In effect, Cl is almost infinitely soluble in thermal waters, as its concentration is not controlled, in a practical manner, by the solubility of a particular mineral. Only when a body of thermal water boils or evaporates to near dryness would halite, or other halides, form. This happens only rarely as halite is uncommon in active geothermal fields, although it occasionally occurs as a daughter mineral in fluid inclusions, e.g. at the Ngatamariki field, New Zealand (Christenson et al., 1997) and at the Karaha field, Indonesia (Moore et al., 2000, 2004). Thermal waters in the Taupo Volcanic Zone (TVZ) of New Zealand have Cl contents of <2200 mg/kg (Ellis and Mahon, 1977) and the fluids trapped in most inclusions are of low salinity (Browne et al., 1976; Hedenquist and Henley, 1985; Hedenquist, 1990). The only exceptions known are rare sphalerite at Broadlands-Ohaaki, which hosts inclusions with apparent salinities of 5.7 to >20 wt.% NaCl equivalent (Simmons and Browne, 1997), and diorite at Ngatamariki, which has inclusions with apparent salinities of up to 41 wt.% NaCl (Christenson et al., 1997). Halite itself has only been reported on a surface of freshly broken core from a hole drilled at the Wairakei-Tauhara field (Kakimoto, 1983). Except in obviously volcanic systems, the exclusivity of Cl to waters that are essentially parent geothermal fluids has long been taken as the primary criterion to characterize them (e.g. Ellis and Mahon, 1977). Where thermal fluids of different types and genesis have mixed, their Cl contents have been used to estimate the proportion of the parent water present and also to help locate piezometric surfaces. For example, low (<10 mg/kg) concentrations of Cl in acid sulfate pools at the bottom of pits or craters have been taken to indicate that some alkali chloride water discharges into them (e.g. Hedenquist and Browne, 1989; Simmons and Browne, 1997). However, the occurrence of Cl-rich water in a cave in the Orakei Korako geothermal field (Rodgers et al., 2000) that could not have been sourced, at least directly, from a parental alkali chloride fluid, means that the assumption that all Cl in thermal waters derives directly from either a primary alkali chloride water or a magmatic source needs to be reconsidered. 2. Cave location and description The Orakei Korako geothermal field (latitude: 38◦ 28 S; longitude: 176◦ 09 E) is situated in the Taupo-Rotorua segment of the TVZ on the eastern margin of the Maroa Volcanic Centre,

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Fig. 1. Location of the Orakei Korako thermal area, showing site of Ruatapu cave and Artist’s Palette (after Rodgers et al., 2000). Topographic contours in meters above sea level.

26 km northeast of Taupo (Fig. 1). Surface manifestations include extensive silica sinter (Lloyd, 1972; Campbell et al., 2001) surrounding a large number of hot pools that discharge dilute alkali chloride–bicarbonate waters. Geysers and large areas of warm and steaming ground also occur. Ruatapu cave (Figs. 1 and 2) is located in the eastern portion of the active field, 250 m south of Artist’s Palette (Fig. 1), a prominent composite sinter apron that covers more than one hectare and has accumulated as a result of the discharge of the alkali chloride–bicarbonate waters (Lloyd, 1972). The cave descends orthogonally into the south side of a small rocky hillock of eroded Quaternary rhyolitic vitric tuff (Lloyd, 1972). The principal cavern extends a distance of ∼45 m beneath a large block at a slope of ∼30◦ to give a vertical drop of ∼23 m (Fig. 2). The block has been undercut by thermal activity or, at least, weakened by intense acidic steam condensate alteration. Throughout most of its length the cave maintains a crudely cylindrical cross-section, averaging ∼15 m across to encompass a minimum void space of 8000 m3 . The rhyolitic tuff that forms the cave walls and roof is bedded and extensively fractured. Many fracture surfaces are slab-like, planar and coated with secondary minerals (Cody, 1978; Rodgers et al., 2000). The floor now entirely comprises boulders and rubble, of unknown thickness, which have fallen from the roof and walls. The cave has a sulfurous smell but this is never strong (Rodgers et al., 2000).

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Fig. 2. Plan and cross-sections through Ruatapu cave, modified after Rodgers et al. (2000). In section A-A, the solid grey arrows signify assumed movement path of meteoric water in tuff and floor rubble; open grey arrows indicate steam movement in cave.

Ruatapu cave is remarkable in that at its base it contains Waiwhakaata (Fig. 2), a clear, warm (T = 43–48 ◦ C), acid (pH = 2.1–3.0) pool (∼12 m × 8 m, 1–2 m deep) that is perched ∼15 m above the level of the Artist’s Palette (Lloyd, 1972) where alkali chloride–bicarbonate springs discharge. A subsidiary chamber, Rahu Rahu (∼5 m × 15 m; Fig. 2) opens ∼5 m above Waiwhakaata in the northwestern wall of the main cave. This chamber hosts two ephemeral pools, one in a depression in the floor of its antechamber and the other in a pit near its end wall. When containing thermal water these discharge small amounts of steam into the cave, making its atmosphere very humid. Prior to the recent visitor traffic, the climate in the cave was more or less stable from day to day. Little rain enters directly into Ruatapu cave through its entrance, which is partly screened by tree ferns that also moderate the light and restrict air circulation. Infiltration of water along fractures may increase during or after heavy rain, but water throughflow is low. No specific spring source or drainage outlet has been identified within the main cave or its subsidiary chamber, although some water must seep down fractures and through clay-rich rubble on the floor of Ruatapu. 3. Cave waters 3.1. Drip water From time to time water drips from the roof of the cave in several places. This is most noticeable at the entrance to Rahu Rahu but drips fall elsewhere in this chamber and also in the main cave. The floor of Rahu Rahu is often wet and contains small depressions clearly produced by the cumulative impacts of individual drops over a long period. However, the rate of water drip varies temporally; at times water drips from 20 or more separate points but at other times none falls

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anywhere in the cave. In March 2000, at the end of a dry summer, for example, no water was dripping in the cave, but in winter 1999, a high rainfall season, the drip rate was the highest observed. The fastest drip rate seen at a single point was only a single drop falling about every 10 s so that the total volume of drip water is low (<1 l/h at most). Rahu Rahu chamber is typically hot and humid and on occasions steam ascends from the ephemeral pools in its floor. However, little of this steam condenses to contribute to the drips. 3.2. Pool waters Waiwhakaata is one of the few permanent acid sulfate pools known from the eastern portion of the Orakei Korako field, whose springs principally discharge dilute alkali chloride–bicarbonate water of near-neutral pH (Lloyd, 1972). Historic visitor accounts testify to the pool having varied in depth and Rodgers et al. (2000) demonstrated that a mineralised horizon, 1.5 m above the late January 1999 water surface, marks a former, enduring, high-water level. From February 1998–January 1999, the level of the water fluctuated by 200–300 mm. The highest level occurred in October, following a higher than usual rainfall, when the lowest temperature (43 ◦ C) and highest pH (3.0) of the pool during 1998 were recorded. Following very heavy rainfall in July 1998, however, no changes in the pool water level were evident. The rocks in the cave are not hot except where they are in contact with hot pool water. In January 1999 the temperature in the main cave, seven meters above Waiwhakaata, was 21 ◦ C and the relative humidity 80% (Rodgers et al., 2000). A small ephemeral, shallow, cooler pool occasionally develops after high rainfall adjacent to Waiwhakaata, within a shallow, rubble-lined depression beneath the southeastern wall. In January 1999 the temperature of this puddle was 28 ◦ C and its pH was 3.0, the latter the same as water in the main pool. Rodgers et al. (2000) summarised the historical records concerning the ephemeral pool in the antechamber of Rahu Rahu. Temperatures reportedly have varied from 36 to 69 ◦ C. On occasions this pool has been as deep as 2.1 m but over the last 5 years it has been empty for much of the time, except in January 1999 when it was ∼6 m × 3 m and 0.3 m deep. By 14 July 1999 the level of this pool had fallen and the floor was dry by November. In January 1999 the air temperature was 22 ◦ C and the relative humidity 86%. 4. Water compositions Table 1 lists the compositions of waters present within the cave. This includes waters from Rahu Rahu, Waiwhakaata, the adjacent ephemeral pool, as well as water dripping from the roofs of the main cavern and Rahu Rahu. For comparison, the composition of the pool close to Waiwhakaata on Artist’s Palette (Spring 826; Fig. 1), as measured on two occasions, is included, as is an analysis of water discharged from one of the deep Orakei Korako drillholes, OK2 (located about 2500 m NNE of the cave). 4.1. Drip water The two partial analyses (Nos 1 and 2 in Table 1) of cave drip water, collected 6 months apart, differ greatly in their Fe3+ , Cl− contents and most likely pH. However, their SiO2 , Al3+ , Mg2+ , Ca2+ , Mg2+ and even SO4 2− contents are similar. Iron varies by two orders of magnitude and the Cl content of the acid drip water collected in January 1999 (Analysis 1) is 640 times that of

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Table 1 Analyses of pooled and dripping water in Ruatapu cave, Orakei Korako thermal area 2

3

4

5

6

7

8

9

10

11

12

13

76 9.4 3.8 5.8 n.d. 5.9 1.4 – – – – 91 9864 – – – – 0.003 – –

62 8.2 0.028 4.0 2.8 4.1 1.2 – – – – 78 15.4 0.21 0.04 – 3.32 1.87 2.42

– – – – – – – – – – – 85 23 – – – – 1.37 – –

281.6 12.6 8.0 167.0 65.0 6.6 2.5 – – – 1.4 345.1 184.7 – – – 3.0 0.89 4.36 –

299 – – 197 47 2.8 0.80 0.51 0.42 2.8 0.66 444 190 6.0 – – 2.5 0.86 7.10 43

285 15.6 1.5 146 38 4.0 1.5 – – – – 439 130 5.3 0.41 – 2.5 1.25 6.53 45

274 14.0 3.4 151 38 2.6 1.0 – – – – 483 144 5.6 0.37 – 2.5 1.23 6.76 44

285 14.9 2.2 134 34 3.1 1.2 – – – – 430 123 5.0 0.34 – 3.0 1.29 6.70 28

365 42 8.0 259 65 8.6 3.7 – – – – 836 237 7.0 0.62 – 3.0 1.30 6.78 47

260 11.2 6.1 186 40 1.9 0.70 – – – – 438 181 6.8 0.42 – 2.6 0.87 7.90 56

325 –

332 –

350 54 1.1 0.3 – – 3.6 0.45 200 340 13 – 217 8.3 0.22 11.0 99

307 55 1.3 0.04 0.56 0.69 3.5 0.17 204 310 9.4 – 94 7.1 0.24 9.49 99

480 – – 550 54 <1.0 – – – 3.1 0.1 142 546 5.7 – 405 9.1 – 17.3 –

All concentrations are in mg/kg; –, not analyzed. 1, RP022, water dripping from roof, main cavern, January 1999; 2, 9901686, water dripping from roof of entrance to Rahu Rahu, 14 July 1999; 3, 2000795, water dripping from roof, Rahu Rahu, 20 November 1999; 4, “Acid pool in Alum Cave, Orakei Korako”, Grange (1937, pp.104–105); 5, “Waiwahkaatu [sic], Ruatapu (Aladdin’s) cave”, 24 June 1980, Sheppard and Lyon (1984, pp.331); 6, RP008, Waiwhakaata pool, January 1999; 7, 9901688, Waiwhakaata, 14 July 1999; 8, RP010, ephemeral pool adjacent to Waiwhakaata, beneath southeast wall, January 1999; 9, RP014, ephemeral pool in antechamber of subsidiary chamber, Rahu Rahu, January 1999; 10, 9901687, ephemeral pool in antechamber of subsidiary chamber, Rahu Rahu, 14 July 1999; 11, Spring 826, small pool, southeastern Artist’s Palette, 1960 (Lloyd, 1972); 12, Spring 826, small pool, southeastern Artist’s Palette, 24 June 1980 (Sheppard and Lyon, 1984); 13, Water discharged from deep drillhole OK2, June 1965 (Sheppard and Lyon, 1984, reporting data of W.A.J. Mahon).

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SiO2 Al Fe3+ Na K Ca Mg Rb Cs Li NH4 SO4 Cl F Br HCO3 pH SO4 /Cl Na/K T (◦ C)

1

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the drip water collected on 14 July (Analysis 2) of the same year. Indeed, the Cl concentration of the January 1999 acid drip water is about five times greater than the highest Cl content of any non-volcanic thermal water previously reported from the TVZ. Differences in H+ likely reflect the need to maintain charge balance. The molecular SO4 /Cl ratios of the drip water differ accordingly, from 0.003 to 1.87, i.e. they show a >500-fold difference. The Na+ and SO4 2− contents of the drip waters are both much lower than those of the pool waters but their Ca2+ and Mg2+ contents are similar. 4.2. Pool waters The pool waters (Analyses 4–10; Table 1) in the cave are of acid sulfate–chloride type whose compositions have varied only slightly in recent years. They derive their acidity from the oxidation of ascending H2 S (Rodgers et al., 2000). Sodium is the main cation (146–259 mg/kg) with subordinate potassium (34–65 mg/kg). Their aluminum contents are appreciable (11–42 mg/kg), reflecting the acid pHs (2.5–3.0) of the waters and their ability to leach the surrounding rhyolitic rocks (Rodgers et al., 2000). Iron is ≤8 mg/kg. The similar SO4 /Cl (1.25–1.30) and Na/K (6.5–6.8) ratios of waters in the pools in both caves (Analyses 6 and 9), including that adjacent to Waiwhakaata (Analysis 8), collected on the same day (January 1999), suggest that they are hydrologically connected. However, the different SO4 /Cl ratios of the waters collected from the Waiwhakaata (1.25) and Rahu Rahu (0.87) pools on 14 July 1999 (Analyses 2 and 10) show that these waters were not then mixed, possibly because the pool level in Rahu Rahu had dropped, and that mixing occurs only when its level is high. The temporal variations in compositions are likely due to the extent of evaporation, the amount of drip water added to the pools, and reactions between the water and secondary minerals (Rodgers et al., 2000). 5. Slurry drip Rodgers et al. (2000) reported that, adjacent to Waiwhakaata in July 1998 (winter) and again in October 1998 (spring), small patches of whitish-yellow, semi-thixotropic, acid slurry were found splattered over boulders and floor litter, where it had dripped from the roof of Ruatapu. Air-dried portions gave X-ray powder diffraction patterns of tamarugite (NaAl(SO4 )2 ·6H2 O) plus halite. A semiquantitative X-ray fluorescence analysis of the October 1998 sample yielded SiO2 2.7, Al2 O3 13.6, Fe2 O3 0.5, CaO 0.18, MgO 1.3, Na2 O 14.0, K2 O 0.6, SO3 39.2, Cl 5.7, and F 0.7 wt.%, with Mn 800, P 700, Cu 500, Zn 132, Br 88, Y 48, and Ba 147 ␮g/g. The authors concluded that the slurry had formed where infiltrating meteoric water had partially dissolved/decomposed surface efflorescences that occur on the walls and roof of the cave. 6. Discussion The most striking feature of the analyses (Table 1) is the variability in some components of the cave drip waters, ranging from the extremely high (∼10,000 mg/kg) Cl content of that collected in January 1999 to those with much lower, but still noteworthy, Cl contents, collected at other times. The occurrence of drops of slurry on the cave floor on two separate occasions that contained up to 5.7 wt.% Cl provides substance to the conclusion that Cl-rich waters drip from the cave roof from time to time. The waters present in Waiwhakaata and in the pool in Rahu Rahu also contain appreciable Cl (123–237 mg/kg).

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A conventional interpretation for the origin of the Cl in cave pool waters would have it derived from deep alkali chloride water. Many thermal areas within the TVZ and elsewhere, for example, contain pools with acid sulfate waters that have appreciable proportions of Cl. At Rotokawa the Cl component of these pools is indeed almost certainly derived from the ascending parent alkali chloride water (Ellis and Mahon, 1977). This was formerly also thought to be the case elsewhere, e.g. at Te Kopia and Waiotapu (Hedenquist and Browne, 1989; Bignall and Browne, 1994), but this assumption now needs to be reconsidered. The water discharged from borehole OK2 had a Cl content of about 546 mg/kg (Analysis 13; Table 1). Sheppard and Lyon (1984) estimated that, before phase separation occurred, the Cl value of the deep water was ∼430 mg/kg. A representative feature (Spring 826; Fig. 1 at an elevation of 319 m asl) on Artist’s Palette discharged water with 310–340 mg/kg Cl (Analyses 11 and 12; Table 1). If all the Cl (and Na) present in the cave pool waters comes from the same parent water as that in Spring 826, this would require it to contribute from 35 to 70% of their volumes. However, the molecular ratios of the ions in the waters are equivocal. The SO4 /Cl ratios of the water in that spring (0.22–0.24) contrasts with those of the cave drip water (0.003–1.87), the slurry drip (3.1) and the cave pool waters (0.86–1.30). The SO4 /Cl ratios of the cave pool waters are probably controlled by micro-conditions within the cave and depend on factors that include the volume and rate of water drip, the amount of rainfall, the plumbing within the cave roof and the amount of steam condensate that forms in and on the cave walls and then mixes back into the pool waters. The Cl/Na ion ratios of the modern cave pool waters are between 0.58 and 0.63 and are close to those of both the deep water (0.64; Analysis 13) and water in Spring 826 (0.62–0.65; Analyses 11 and 12). At first sight these similarities imply that the cave pool waters may indeed derive directly from the parent water in the geothermal reservoir but the similarities may simply result from the need for ion balance, and sodium ions are readily available in the surrounding rocks and drip water to do this. The Na/K ratios of the modern cave pool water (6.5–7.9) are less than in any other thermal waters at Orakei Korako; e.g. the Na/K ratios of waters in Spring 826 on Artist’s Palette are between 9.5 and 11.0 (Sheppard and Lyon, 1984). These differences, and the ∼15 m elevation difference between the cave pools and those on Artist’s Palette, imply that the cave pool waters contain not only deeply derived thermal water but also water with different Na/K ratios from elsewhere. The slurry drip with up to 5.7 wt.% Cl and the exceptionally high Cl concentration in the cave drip water of January 1999 show that Cl must move, at least locally, as an aqueous phase high above the piezometric surface in the Orakei Korako geothermal system. Also supporting this conclusion, Rodgers et al. (2000) reported 425–700 mg/kg Cl present in wall rocks surrounding Waiwhakaata, compared with only 250 ␮g/kg Cl present in the same rocks exposed outside the cave entrance. Given its composition, the cave drip water in Rahu Rahu is not entirely steam condensate nor, as shown earlier, can it be deeply derived alkali chloride water. The only alternative source is meteoric water that has reacted with minerals in rocks of the cavern roof as this water percolated down bedding planes and/or interconnected joints in the rhyolite tuff above the cave. While such an origin can account for the episodic dripping, too little monitoring has been done to determine whether drip rate is indeed directly related to seasonal variations in rainfall. The residence time of rainwater in the Ruatapu overburden is unknown. Such an occurrence and explanation raises questions as to the source of the Cl. There are two possible sources: (a) glass in the host rhyolite tuffs and (b) earlier formed halite. Rhyolitic tuffs in the TVZ, such as the Orakei Korako Tuff, commonly contain 1000–3000 mg/kg Cl,

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mainly hosted in their glass shards (Shane, 2000). Presumably, percolating meteoric water could leach this Cl, particularly where it becomes heated by conduction and/or ascending steam. This explanation is not entirely satisfactory, however, as the number of fluid flow paths is finite and the source of Cl (and other constituents) available to supply the drip water would soon become exhausted. An alternative explanation might lie in the prehistoric discharge of alkali chloride waters in the vicinity of Ruatapu cave entrance at elevations several metres higher than they do so now. Silica sinter and silicified rocks outcrop nearby, demonstrating the former presence of a liquid outflow. These waters would have pervaded and penetrated the fractured rhyolitic country rocks above and around the cave. Perhaps following the 10,000–16,000 BP hydrothermal eruption on the site where Artist’s Palette now occurs (Lloyd, 1972), the piezometric surface suddenly descended to leave pockets of alkali chloride water trapped behind in pores and fractures. Subsequently, more than 99% of this water evaporated to leave halite, and possibly other chloride salts, disseminated throughout the rock mass. These salts would have readily dissolved wherever the rocks became open to percolating rainwater. Simmons and Browne (1997) explained the occurrence of highly saline fluid inclusions in sphalerite at the Broadlands-Ohaaki field as having formed as a result of local boiling to dryness. A similar explanation was proposed by Moore et al. (2004) to explain the presence of halite-saturated inclusions in anhydrite high-salinity fluid inclusions in quartz from the Karaha-Telaga Bodas geothermal system in Java. Notwithstanding the provenance, the opportunity exists, therefore, for the Cl in the cave pools to be derived from descending, rather than ascending, waters. It is only the fortunate circumstance provided by the occurrence of Ruatapu cave in an active thermal area that allows this hydrological situation to be recognised. As this process may well occur in other geothermal fields, its occurrence at Orakei Korako draws attention to the dangers of assuming that the presence of Cl in acid sulfate waters, ipso facto, implies either a direct input from HCl gas or deeply derived alkali chloride waters. In other words, the Cl component of acid sulfate pool waters is not necessarily a signature of the presence of the deeply derived parent water or, therefore, the location of the piezometric surface. Acknowledgements Financial help was provided by the Auckland University Research Committee and Radiometer, New Zealand. Tim Boddy and Terry Spitz generously allowed access to Orakei Korako and Ruatapu and granted permission to collect water samples for analysis. The recent water samples were analysed at the Institute of Nuclear and Geological Sciences at Wairakei. Joe Moore, Rich Gunderson and Michael Adams offered valuable suggestions. Louise Cotterall kindly drafted the two figures. References Bignall, G., Browne, P.R.L., 1994. Surface hydrothermal alteration and evolution of the Te Kopia thermal area. Geothermics 26, 647–658. Browne, P.R.L., Roedder, E., Wodzicki, A., 1976. A comparison of past and present geothermal waters from a study of fluid inclusions, Broadlands geothermal field, New Zealand. Proceedings of International Water-Rock Interaction Symposium, Prague, pp. 140–149. Campbell, K.A., Sannazzaro, K., Rodgers, K.A., Herdianita, N.R., Browne, P.R.L., 2001. Sedimentary facies and mineralogy of the Late Pleistocene Umukuri silica sinter, Taupo Volcanic Zone, New Zealand. J. Sediment. Res. 71, 727–746.

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Christenson, B.W., Mroczek, E.K., Wood, C.P., Arehart, G.B., 1997. Magma ambient production environments: PTX constraints for paleo-fluids associated with the Ngatamariki diorite intrusion. Proceedings of the 19th Geothermal Workshop, Auckland, pp. 87–92. Cody, A.D., 1978. Ruatapu Cave, Orakei Korako. NZ. Speleolo. Bull. 6, 184–187. Ellis, A.J., Mahon, W.A.J., 1977. Chemistry and Geothermal Systems. Academic Press, New York, 392 p. Grange, L.I., 1937. The geology of the Rotorua-Taupo subdivision. NZ Geol. Surv. Bull. 37, 1–138. Hedenquist, J.W., 1990. The thermal and geochemical structure of the Broadlands-Ohaaki geothermal system. NZ. Geothermics 19, 151–185. Hedenquist, J.W., Browne, P.R.L., 1989. The evolution of the Waiotapu geothermal system, New Zealand, based on the chemical and isotopic composition of its fluids, minerals and rocks. Geochim. Cosmochim. Acta 53, 2235–2257. Hedenquist, J.W., Henley, R.W., 1985. The importance of CO2 on freezing point measurements of fluid inclusions: evidence from active geothermal systems and implications for epithermal ore deposition. Econ. Geol. 80, 1379–1406. Hedenquist, J.W., Stewart, M.K., 1985. Natural CO2 -rich steam heated waters in Broadlands-Ohaaki geothermal system, New Zealand: their chemistry, distribution and corrosive nature. Geotherm. Resour. Council Trans. 9 (Part II), 245–250. Kakimoto, P.K., 1983. Hydrothermal alteration and fluid-rock interaction in the TH3 and THMI drillholes, Tauhara geothermal field, New Zealand. Unpublished MSc Thesis, University of Auckland Library, Auckland. Lloyd, E.F., 1972. Geology and hot springs of Orakeikorako. NZ. Geol. Surv. Bull. 85, 1–164. Moore, J.N., Lutz, S.J., Renner, J.L., McCulloch, J., Petty, S., 2000. Evolution of volcanic-hosted vapor dominated system. Petrologic and geochemical data from corehole T-8, Karaha-Telaga Bodas, Indonesia. Geotherm. Resour. Council Trans. 23, 259–263. Moore, J.N., Christenson, B., Browne, P.R.L., Lutz, S.J., 2004. The mineralogic consequences and behavior of descending acid-sulfate waters: an example from the Karaha-Telaga Bodas geothermal system, Indonesia. Can. Mineral. 42, 1483–1499. Rodgers, K.A., Hamlin, K.A., Browne, P.R.L., Campbell, K.A., Martin, R., 2000. The steam condensate alteration mineralogy of Ruatapu cave, Orakei Korako geothermal field, Taupo Volcanic Zone, New Zealand. Mineral. Mag. 64 (1), 125–142. Shane, P., 2000. Tephrochronology: a New Zealand case study. Earth-Sci. Rev. 49, 223–259. Sheppard, D.S., Lyon, G.L., 1984. Geothermal fluid chemistry of the Orakeikorako field, New Zealand. J. Volcanol. Geotherm. Res. 22, 329–349. Simmons, S.F., Browne, P.R.L., 1997. Saline fluid inclusions in sphalerite from the Broadlands-Ohaaki geothermal system: a coincidental trapping of fluids being boiled towards dryness. Econ. Geol. 92, 485–489.