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2 GRC Transactions, Vol. 35, 2011 Geothermal Fluid Evolution at Rotokawa: Hydrothermal Alteration Indicators L. Price 1, T. S. Powell 2, and L. Atkinson 3 1 Mighty River Power, Rotorua, New Zealand 2 Powell Geosciences Ltd, Rotorua, New Zealand 3 Xstrata, Mt Isa, Queensland, Australia Keywords Chalcedony, hematite, anhydrite, bladed calcite, sulphur, hydrothermal eruption, boiling, fluid inclusions, microthermometry Abstract The Rotokawa geothermal system is up to 20,000 years old as evidenced by hydrothermal eruptions occurring after this time and up to 3,700 years ago. Recent drilling has provided an opportunity to examine hydrothermal mineral alteration that may have recorded magma degassing subsequent to the systems inception. The current reservoir fluids are near-neutral, non-saline and between 320 and 330 C. They contain unusually high H 2 S concentrations compared to other Taupo Volcanic Zone geothermal systems. Paragenetic sequences in veins from the recently recovered cores below 1850 m indicate that propylitic type quartz ± chlorite ± epidote deposition has been interrupted by intermittent SO 2 - bearing saline fluids, CO2 enriched fluids and open-system boiling at temperatures >285 C. Propylitic veins have been overgrown and cross-cut by anhydrite veins and infillings (not related to supergene alteration) which have in turn been replaced or infilled by intergrown bladed calcite, chalcedony and hematite. It is suggested that magma degassing events could have initiated these fluid changes and overpressured the reservoir, resulting in deep-focal depth hydrothermal eruptions and decompressive boiling. Fluid changes driven by degassing are suggested by the presence of hydrothermal alteration and vein calcite while current reservoir fluids are undersaturated with respect to calcite. Up to 2.6 million tonnes of native sulphur has been deposited in the near surface as recently as 6060 ± 60 years ago. This is calculated to be greater than the current flux of H 2 S could deposit in the elapsed time, also suggesting significant changes to reservoir fluid chemistry. Over pressures in the reservoir released by hydrothermal eruptions and associated decompressive boiling would have resulted in vapour losses, oversaturating the reservoir fluid with silica resulting in chalcedony precipitating at temperatures >225 C; and undersaturated the fluid with respect to hydrogen and hydrogen sulphide, precipitating hematite with bladed calcite from the dissolution of pyrite or from chloride and bisulphide complexes in solution. Introduction The Rotokawa geothermal field is located in the Taupo Volcanic Zone (TVZ) in the central part of the North Island of New Figure 1. Location of the Rotokawa geothermal field, New Zealand. 977

3 Zealand (Figure 1). The TVZ is an extensional system with rifting in a NW-SE direction. The resulting extensional basin, up to 3 km deep at Rotokawa, is confined within Jurassic to Cretaceous age greywacke basement. Volcanic activity commenced in the TVZ around 2 Ma with the onset of calc-alkaline andesitic lava, joined by voluminous rhyolitic activity 1.6 Ma (Houghton et al., 1995); the rhyolitic deposits consequently filling the greywacke basin. The geothermal reservoir is predominantly hosted within the Rotokawa andesite. Preferential permeability in the reservoir appears to be associated with a series of sub-surface, northeast-southwest faults. These offset the greywacke basement and overlying Rotokawa andesite by up to 400 m and are older than 320 ka. A total of thirty one geothermal wells have been drilled at Rotokawa between 1965 and Thirteen of these have been drilled in the last four years. The wells are completed to depths between 2500 and 3000 m (Figure 2). The 34 MW Rotokawa power station has been in use since 1997 and was supplemented in March 2010 by the 134 MW Nga Awa Purua station. Reservoir Geochemistry RK16 RK18 RK27 RK27 L2 RK17 RK18 L2 RK32 RK28 RK26 RK8 NAP RK13 RK15 RK33 RK9 RK12 RK11 RK6 RK30 L1/L2 RK5 RK14 RKA RK25 RK29 RK21 RK22 RK20 RK24 RK23 RK ,000 Meters Figure 2. Rotokawa geothermal well locations with Rotokawa (RKA) and Nga Awa Purua (NAP) power stations (2011). The Rotokawa geothermal reservoir is high-temperature and liquid-dominated. The deep reservoir fluid is a near-neutral with a present chloride content of around 650 mg/kg at temperatures between 320 C to 330 C and is under-saturated with respect to calcite. Early investigations of the natural state of the geothermal field classified Rotokawa as an arc-type on the basis of the theoretical 15% magmatic water content, elevated CO 2 and relatively low salinity (Giggenbach, 1995). CO2 is the dominant gas, although H 2 S in the total discharge at 0.05 wt% is unusually high (Reyes et al., 2003). Thermal Surface Features Surface hydrothermal activity is prominent in the southern part of the field where steaming ground and fumarolic activity are widespread. Acid sulphate springs and collapse craters are common. Mixed chloride-sulphate springs occur along the northern side of Lake Rotokawa and partially contribute to the acid lake water (ph 2.3). Springs along the Waikato River that dissects the field, discharge neutral chloride-bicarbonate waters. There are no geysers or extensive silica terraces within the field. Extensive hydrothermal breccias are found at the surface in the southern area of the field. Approximately 2.6 million tonnes of native sulphur have been deposited in bedded lacustrine sediments under and proximal to Lake Rotokawa. These near surface sulphur deposits (upper 20 m) have been calculated to be larger than the current H 2 S flux could deposit in the time available, based on the age of the host sediments (Krupp and Seward, 1987). Hydrothermal Eruptions Up to thirteen large, prehistoric hydrothermal eruptions have occurred in the Lake Rotokawa area. The craters (up to 1.5 km diameter) have been completely filled and can only be located by mapping the distribution of clast size in the hydrothermal eruption breccia layers that cover an area of around 12 km 2. Individual eruption deposits can be up to 11m thick and cumulative sections exceed 40 m thickness. The craters formed by the hydrothermal eruptions are deduced to be aligned along a northeast striking structural feature, consistent with the regional faulting orientation (Collar and Brown, 1985). The eruptions took place between 22,700 and 3,700 years ago based on 14 C dating of interbedded rhyolitic tephras of regional extent and a tree buried by one eruption event (Collar and Browne, 1985). Many breccia clasts were hydrothermally altered and hydraulically brecciated prior to eruption. Rhyolite clasts in two of the breccia units dated between 3440 ± 80 and 6060 ± 60 years correlate with rhyolite flows encountered in geothermal wells at depths below 450 m and up to 850 m below ground level (Collar and Browne 1985). Occasional lumps of sulphur are intercalated with the 6060 year old breccia but no juvenile magmatic material has been observed in the deposits (Browne and Lawless, 2001). These observations mean the focal depths of some of the Rotokawa hydrothermal eruptions were relatively deep compared to other eruptions in New Zealand geothermal fields (Browne and Lawless, 2001). Hydrothermal Alteration Hydrothermal alteration at Rotokawa has been recognised as extensive, shallow acid alteration generally to levels <200 m below ground surface caused by interaction with steam heated acid sulphate waters, overlying 978

4 Price, et al. propylitic-style hydrothermal mineral assemblages (of quartz, wairakite, epidote, clinozoisite, illite, chlorite, adularia, albite and calcite) indicating deposition from near neutral ph, chloride waters (Rae, 2007). Bladed Calcite, Chalcedony and Hematite Rotokawa andesite core recovered from recently drilled wells RK18L2 and RK27L2 contain, bladed calcite intergrown with chalcedony ± hematite in veins and amygdales. Recent drilling has facilitated further study of the propyliticrk18l2 core (2219 and 2221 m) is composed of moderately style alteration mineral assemblages enabling interpretation of the to strongly altered porphyritic (plagioclase, clinopyroxene and evolution of the geothermal system fluid composition, temperature biotite) andesitic lava. The plagioclase phenocrysts are weakly and pressure changes. altered to calcite and albite. Pyroxene phenocrysts are completely The most notable recent observations have been the occuraltered to chlorite and calcite. Biotite phenocrysts are completely rence of altered to minor actinolite-tremolite, green hydrothermal biotite, chalcedony associated with bladed calcite and hematite at leuxocene and calcite. The groundmass is partially altered to drilled depths from 1480 to 2326 m, and calcite, chlorite, albite, leucoxene, pyrite and quartz (GNS, 2010). anhydrite at depths of 1853 m. The andesite is cross-cut by <5 mm wide veins lined with chlorite and filled with calcite and chalcedony (Photo 1). Some calcite crystals have a bladed habit and are intergrown with possible illite (GNS, 2010). RK27L2 core (2120 and 2126 m) is composed of moderately to weakly altered porphyritic (plagioclase and pyroxene) andesitic lava. Plagioclase phenocrysts are weakly altered to calcite, albite, adularia, leucoxene and rare epidote. Pyroxene pseudomorphs consist of actinolite-tremolite, biotite, calcite and chlorite. The fine-grained groundmass has been partially altered to iron oxides (GNS, 2010). ComPhoto 1. Photomicrograph of a calcite (Cc) chalcedony (Chal) vein (C) cross-polarised light, (D) plane mon amygdales are filled with secondary polarised light, RK18L2, 2220 m (GNS, 2010). minerals, mostly intergrowths of chlorite and quartz. Some amygdales are filled with chalcedony ± chlorite ± calcite + hematite ± pyrite with chalcedony enclosing bladed calcite crystals (Photo 2). Anhydrite Photo 2. Photomicrographs of an amygdale lined with quartz (Qz), chlorite-quartz (Chl-Qz), chlorite (Chl) and filled with chalcedony (Chal). Chalcedony includes bladed calcite (Cc) crystals (C) cross-polarised light and (D) plane-polarised light. (E) Amygdale filled with chalcedony (Chal), chlorite (Chl) and hematite (Hm) cross-polarised light. (F) Plane-polarised light, RK27L m (GNS, 2010). 979 RK27 cores (1850 and 2147 m) were examined in thin section. The rock is a strongly to intensely altered porphyritic (plagioclase and pyroxene) andesitic volcaniclastic breccia supported by an andesitic hydrothermal breccia matrix in places. Plagioclase phenocrysts are completely altered to albite, calcite, fine-grained quartz with rare epidote and anhydrite. Pyroxenes are completely replaced by chlorite and leucoxene. The clast groundmass is strongly altered but retains preserved igneous flow textures (aligned plagioclase laths). The breccia matrix contains crystal fragments of plagioclase, pyroxene and ilmenite and angular andesite clasts in a microcrystalline framework of hydrothermal minerals. Chlorite, albite, leucoxene and pyrite alteration of the matrix is intense with dusty hematite and notable rare chal-

5 Photo 3. Photomicrograph of a vein filled with calcite (cc) that encloses relict anhydrite (anh) crystals almost completely replaced by calcite. Cross-polarised light, RK m, (GNS, 2009). copyrite and bornite (GNS, 2010). At 1853 m the andesite breccia is cut by a network of very thin (<0.6 mm wide) epidote lined veins filled with chlorite. These are cross-cut by irregular anhydrite veins replaced by calcite (Photo 3). Fine, chalcopyrite crystals are a rare component of some of the calcite veins along with epidote. Microthermometry Fluid inclusion microthermometry was used to determine homogenisation (T h ) and melting temperatures (T m ) of calcite and bladed calcite-hosted fluid inclusions from the cores (GNS consultancy reports). The inclusions are all two-phase at room temperature and show a wide range of vapour bubble volumes (20 to 50%). The bladed habit of the calcite and the different vapour bubble volumes are indicative of deposition under boiling conditions. Upon heating, all inclusions homogenized to a liquid. Daughter salts, minerals and CO2-clathrates were absent, implying the trapped fluids have salinities <23.3 eq. wt.% NaCl (Roedder, 1984) and dissolved gas concentrations in the trapped fluids <3.5 wt.% CO 2 (Hedenquist and Henley, 1985). In the absence of CO2-clathrates, CO 2 in solution can depress the ice melting temperature to -1.5 C (Hedenquist and Henley, 1985), resulting in false apparent salinities ranging between eq. wt.% NaCl. Results of the microthermometry (Figure 3) indicate: RK18L2 inclusions are filled with a dilute liquid ( 1.4 eq. wt.% NaCl) that homogenized between 319 and 329 C. T m values ranged between -0.8 and -0.3 C. The homogenization temperatures are below the boiling point for pure water temperature for equivalent depth plotting close to the Chalcedony boiling point for depth curve of a 3.5 wt. % CO 2 solution. RK27L2 primary inclusions are filled with a dilute liquid ( eq. wt.% NaCl) that homogenized between 285 and 334 C. T m values ranged between -4.1 and -1.4 C. The homogenization temperatures are below the boiling point for pure water temperature for equivalent depth plotting close to the boiling point for depth curve of a 3.7 wt. % CO 2 solution. RK27 inclusions are filled with a dilute liquid (2 3 eq. wt.% NaCl) that homogenized between 300 and 352 C. T m values ranged between -1.8 and -1.3 C. Two primary fluid inclusions have distinctly lower T m values of C indicating the presence of a higher salinity fluid containing eq. wt. % NaCl. They are petrographically indistinct from the other primary inclusions. The homogenization temperatures plot close to the boiling point for pure water temperature for equivalent depth. Discussion Chalcedony is commonly the stable silica polymorph at temperatures <190 C (White and Hedenquist, 1990). At higher temperatures, quartz is the stable polymorph or controlling phase (Ellis and Mahon, 1977). The deposition of amorphous silica (later converted to chalcedony) at temperatures >225 C has been reported where extreme decompressive boiling and evaporation has oversaturated the fluid with respect to quartz (Moore et al., 2008). Chalcedony re-crystallised from amorphous silica generally exhibits relict colliform and botryoidal textures. Alteration mineralogy associated or intergrown with chalcedony in the Rotokawa cores indicates near neutral ph fluids with temperatures >190 C. This includes biotite+actinolite-tremolite Salinity (eq.wt.% NaCl) T h ( C) RK27L2 (2125 m) RK18L2 (2219 m) RK27 (1850 m) Figure 3. Bivariant plot showing measured homogenisation temperatures (T h ) and equivalent wt. % NaCl salinities. 980

6 indicating temperatures >300 C (Bird et al., 1984); bladed calcitehosted fluid inclusion homogenisation temperatures >285 C; vein epidote indicating temperatures >250 C (Browne and Ellis, 1970), and current well fluid temperatures >300 C. Hematite Hematite has been observed associated with bladed calcite, chalcedony and pyrite in the Karaha-Telage Bodas system (Moore, et al, 2008). Rapid release of hydrogen and hydrogen sulphide gases from hydrothermal liquid during boiling provides a possible geochemical mechanism for its formation. When a vapour phase is present, both hydrogen and hydrogen sulphide partition strongly to the vapour phase. In addition, the concentrations of these two gases are controlled by temperature-dependent fluid-fluid and fluidmineral geochemical reactions. Rapid boiling would deplete these gases in remaining liquid, stimulating reactions to replenish them. The fastest reaction to replenish hydrogen is the water breakdown reaction (Giggenbach 1980): H 2 O = H 2 + ½ O 2 Driving the reaction to the right to replenish hydrogen in the liquid phase would result in the production of surplus free oxygen. The most readily available reaction to replenish hydrogen sulphide would be simple dissolution of pyrite: FeS + 2H + = Fe +2 + H 2 S Driving this reaction to the right forms H 2 S, consuming free hydrogen ions resulting in an increase in fluid ph. In a simplified way, then, it can be shown that the necessary ingredients for hematite precipitation can result from boiling and vapour separation in the presence of pyrite. Iron present in the fluids as chloride and bisulphide complexes would also be available for hematite formation. Anhydrite In hydrothermal environments, anhydrite precipitates either from shallow-derived, acid-sulphate fluids or from deep-derived high temperature (>300 C) SO 2 -bearing, saline magmatic fluids (>20 eq. wt. % NaCl) which are common in porphyry copper deposits. Mixing between magmatic and meteoric derived fluids could explain the relatively saline fluids ( wt. % NaCl) in the RK27 calcite-hosted fluid inclusions (GNS, 2009). Deposition of anhydrite from mixed magmatic-meteoric fluids in well RK27 is supported by the presence of chalcopyrite and bornite in the breccia matrix and the lack of alteration of these minerals to chalcocite and covellite from downward percolating acid-sulphate fluids (supergene alteration). Conclusions The Rotokawa geothermal system is up to 20,000 years old as evidenced by hydrothermal eruptions occurring after this time and up to 3,700 years ago. The reservoir rocks record propylitic-style hydrothermal alteration indicating deposition from near neutral ph, chloride waters. The current reservoir fluids are near-neutral at temperatures between 320 and 330 C and contain 650 mg/kg of chloride. Paragenetic sequences in veins from recently recovered cores indicate the propylitic type quartz ± chlorite ± epidote deposition has been interrupted by intermittent SO 2 -bearing saline fluids, CO2 enriched fluids and open-system boiling at temperatures >285 C. Propylitic veins have been overgrown and cross-cut by anhydrite veins and infillings which have in turn been replaced or infilled by intergrown bladed calcite, chalcedony and hematite. It is suggested that magma degassing events subsequent to the geothermal systems inception initiated these fluid changes and overpressured the reservoir, resulting in deep-focal depth hydrothermal eruptions and decompressive boiling. Fluid changes driven by degassing are represented by the deposition of anhydrite not related to supergene alteration; and the volume of native sulphur (deposited at the near surface as recently as 6060 years ago) and calcite, both in excess of current reservoir fluid concentrations. Over pressures in the reservoir from magma degassing, released by hydrothermal eruptions and associated decompressive boiling, has produced vapour losses that have oversaturated the reservoir fluid with silica resulting in chalcedony precipitating at temperatures >225 C; and undersaturated the fluid with respect to hydrogen and hydrogen sulphide, precipitating hematite with bladed calcite from the dissolution of pyrite or from chloride and bisulphide complexes in solution. Acknowledgments The authors would like to acknowledge the significant contribution to this work by the geologists at GNS Wairakei, for many years of high quality and conscientious well site geology, petrographic descriptions and fluid inclusion analyses of RJV well samples, without which this work would not have been possible. References Bird, D.K., Schiffman, P., Elders, W.A., Williams, A.E., McDowell, S.D Calc-silicate mineralisation in active geothermal systems. Economic Geology, 79, Browne, P.R.L., Ellis, A.J The Ohaaki-Broadlands hydrothermal area, New Zealand: mineralogy and associated geochemistry. American Journal of Sciences, 269, Browne, P.R.L Hydrothermal alteration in active geothermal fields. Annual Review of Earth and Planetary Science: p Browne, P.R.L. and Lawless, J.V Characteristics of hydrothermal eruptions, with examples from New Zealand and elsewhere. Earth-Science Reviews 52: Collar, R.J. and Browne, P.R.L Hydrothermal eruptions at the Rotokawa Geothermal Field, Taupo Volcanic Zone, New Zealand. Proc. 7 th Geothermal Workshop, Ellis, A.J and Mahon, W.A.J., Chemistry and geothermal systems. Academic Press, New York. Giggenbach, W. F., 1980, Geothermal gas equilibria; Geochem Cosmochem Acta, V. 44, pp Giggenbach, W.F Variations in the chemical and isotopic compositions of fluids discharged from the Taupo Volcanic Zone: a review. Journal Volcanology andgeothermal Research 68, Hedenquist, J.W., and Henley, R.W The importance of CO 2 on freezing point measurements of fluid inclusions: evidence from active geothermal systems and implications for epithermal ore deposition. Economic Geology, 80, p

7 Houghton, B.F., Wilson, C.J.N, McWilliams, M.O., Lanphere, M.A., Weaver, S.D., Briggs, R.M. and Pringle, M.S Chronology and dynamics of a large scale silicic magmatic system: Central Taupo Volcanic Zone, New Zealand. Geology, 23, Krupp, R.E. and Seward, T.M The Rotokawa geothermal system, New Zealand; an active epithermal gold-depositing environment. Economic Geology; August 1987; v.82; no. 5; p Moore, J.N., Allis, R.G., Nemcok, M., Powell, T.S., Bruton, C.J., Wannamaker, P.E., Raharjo, I.B. and Norman, D.I The evolution of volcano-hosted geothermal systems based on deep wells from Karaha- Telaga Bodas, Indonesia. American Journal of Science, Vol. 308, p Rae, A Rotokawa Geology and Geophysics, GNS Science Consultancy Report 2001/83. Rae, A.J.; McCoy-West, A.; Ramirez, L.E Geology of Production Wells RK26 and RK27, Rotokawa Geothermal Field, GNS Science Consultancy Report 2009/ p. Ramirez, L.E.; Hitchcock, D Geology of Production Well RK27L2, Rotokawa Geothermal Field, GNS Science Consultancy Report 2010/ p. Ramirez, L.E.; McCoy-West, A.J.; Rae, A.J.; McNamara, D Geology of Production Well RK18L2, Rotokawa Geothermal Field, GNS Science Consultancy Report 2010/31. 37p. Reyes A. G., Trompetter W. J., Britten K. & Searle J.,2003. Mineral deposits in the Rotokawa geothermal pipelines, New Zealand. J. Volcanol. Geotherm. Res., 119: Roedder, E Fluid inclusions (Ribbe, P.H. ed.). Reviews in Mineralogy vol. 12. Mineralogical Society of America. White, N.C., Hedenquist, J.W Epithermal environments and styles of mineralisation: variations and their causes, and guidelines for exploration, in Hedenquist, J.W., White, N.C., Siddeley, G. (eds), Epithermal Gold Mineralisation of the Circum-Pacific: Geology, Geochemistry, Origin and Exploration, II: Journal of Geochemical Exploration, 36,