ISOTOPIC EVIDENCE CALCITE FORMED FROM STEAM-HEATED WATERS AT THE OHAAKI, WAIRAKEI AND WAIOTAPU GEOTHERMAL SYSTEMS, NEW ZEALAND

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1 Proc 13th New Zealand Geothermal Workshop 1991 SOTOPC EVDENCE CALCTE FORMED FROM STEAM-HEATED WATERS AT THE OHAAK, WARAKE AND WAOTAPU GEOTHERMAL SYSTEMS, NEW ZEALAND STUART F. SMMONS Geothermal nstitute,the University of Auckland SUMMARY: Oxygen in hydrothermal calcite (and aragonite) from at C range from 4.2 to 18.5 per mil and indicate equilibration with shallow steam-heated waters. By contrast, oxygen isotopes in hydrothermal calcite at C range from 1. to 7.4 per mil and indicate equilibration with deeply derived chloride waters. sotopic data from Wairakei and Waiotapu show similar trends by which equilibration with steam-heated or chloride waters are distinguished. Such distinctions help understand the origin of hydrothermal calcite in both geothermal systems and hydrothermalore deposits. 1. NTRODUCTON Hydrothermal calcite is widespread as an alteration mineral in many New Zealand geothermal systems Browne, 1971;Steiner, 1977;Wood, but it also occurs as pipe scale in geothermal wells Tulloch, 1982). At temperatures C within reservoir rocks m depth), calcite occurrence is controlled by the amount of C2 dissolved in the deeply derived geothermal fluid; it precipitates through boiling due to C2 loss or reaction between C2 and calcium aluminosilicates Ellis, 197; Browne and Ellis, 197). The compositions of these calcites typically indicate equilibration with the deep chloride water 1975;Clayton and Steiner, 1975). Calcite at temperatures and shallow depths m), is more likely to be associated with steamheated waters, containing high concentrations of dissolved Simmonsand Browne, 199). These are groundwaters that dissolve C2 gas liberated from the boiling of chloride water deep in the system, with their distribution being restricted to shallow and marginal parts of the upflow zone as described, for example, at Broadlands-Ohaaki(Mahon et al., 198;Hedenquist and Stewart, 1985;Hedenquist, 199). Because these waters are a mixture of local meteoric water and absorbed steam, their compositions are as much as 2 per mil or more lighter than the chloride water (Giggenbach and Stewart, 1982; Hedenquist and Stewart, 1985). Therefore, the compositions of associated calcites should differ, depending on whether they precipitated from with) either steam-heated or chloride waters. n this paper, present evidence from Wairakei and that supports this hypothesis. 2. METHODS AND RESULTS As part of a much larger study of the hydrologic factors controlling the distribution and Occurrence of hydrothermal carbonates in the Broadlands-Ohaaki geothermal system, the compositions of eighteen calcite samples in cores, pipe scales and well fluid precipitates from were obtained commercially through the nstitute of Nuclear Sciences, DSR. additional sample of aragonite precipitated onto a stem from fluid discharged from Br-6 (Browne, 1973) was also analysed. The samples were selected to cover a wide range of temperatures to C) and depths ( to m) from both the centre and margins of the system. The host rocks consist of Recent felsic and pyroclastics (Browne and Ellis, 197). n addition, samples were chosen from regions where steam-heated and chloride waters occur (Hedenquist, 199). microprobe analysis indicates that the calcite and aragonite contain no more than 2 of and combined. The isotopic results are plotted in Figure 1, together with data from earlier studies (Esslingerand Savin, 1973; 1975;Absar, as a function of well (Lee Joe and O'Sullivan, 1986) or fluid inclusion homogenisation temperatures. Fluid inclusion data provide more reliable temperatures of calcite formation, but inclusions were only found in crystals that deposited into open space. Calcites which replace calcium aluminosilicateslack fluid inclusions, and instead in well temperatures are assumed to reflect equlibration most cases to C. Temperatures and calcite-water fractionation data (O'Neil et al., 1969) were to the compostion water assuming equilibrium fractionation (Fig. 1). The measured isotopic compositions of chloride and steamheated waters at Broadlands-Ohaaki are -4.5 and about -6.5 per mil, respectively (Fig. 2). Published calcite data (Clayton and Steiner, 1975; Hedenquist and Browne, 1989) are plotted as a function of temperaturein Figure BROADLANDS-OHAAK CALCTE Figure 1 shows that most calcites which formed at temperatures c have values close to that equilibrium with the deep chloride water. Exceptions

2 86 i i 6-surface 5 L Figure 1. Plot of values for calcite and other versus temperature for Broadlands-OW; data from Esslinger and Savin Blattner and this study. Stippled shows the values of some replacement calcites determined by Absar (1988). The -8.5, 4.5, -.5 and on curves are the per mil values of water in isotopic with calcite as a function of temperature, using calcite-water fractionation factors given by O Neil et al. Deep chloride water has an oxygen composition of 4.5 mil. The dashed curve shows the effect of boiling and multi-step steam separation on the isotopic composition of the parent liquid, becoming isotopically heavier with Calcites farmed at C are labelled (well-m) ,.. chloride water Figure 2. Oxygen and hydrogen isotope compositions of fluids at showing the isotopic differences between chloride,ground and steam heated (redrawn from and Stewart, 1985).

3 87 5 A) Wairakei 5 Waiotapu Figure 3. Plots of values for calcite versus temperature for A) (Clayton and Steiner, 1975) and B) Waiotapu (Hedenquist and Browne, 1989). Curves represent the composition of calcite in equilibrium with water of constant oxygen isotope composition as a function of temperatwe, using calcite-water fractionation factors given by O'Neil et al. (1969). The data are for samples from four wells and 225). The stipple pattern shows the approximate oxygen isotopic composition of calcite in equilibrium with deep chloride water atwairakei- Tauhara (Clayton and Steiner, 1975; Stewart, 1978). The of deep chloride water at Waiotapu is about.2 per mil (Hedenquist and Browne, and the dashed curve in B shows the effect of boiling and multi-step steam separation on the residual liquid Temperature calculated steam heated waters \ \ chloride water Figure 4. Depth (m)versus calculated composition of water determined using calcite-water fractionation factors (O'Neil et al., 1969) and calcite isotope data from Br-7. Also shown are the compositions of chloride and steam-heated waters.

4 88 exist in which data indicate equilibrium with isotopically heavier waters data plotting to the right of the -4.5 per mil curve, Fig. 1) that probably resulted from extensivelocal interaction between the chloride water and rock Ċalcites formed at temperatures C mostly have isotopic values indicating their equilibration with water having a value of -6 per mil or less, similar in composition to steam-heated waters (Fig. 2). Three exceptions exist in which appear to be in equilibrium with chloride water , 6-891, 21-11; Fig. and this best explains the datum based on comparison with the hydrologic model put forth by Hedenquist (199). The locations of and 149, however, coincide with the of waters, and therefore the calculated value of -4.5 per mil is more likely attributed to isotopic enrichment of steam-heated water from its interaction with country rock. This is not surprising given the reactive nature of both the steam-heated waters which contain up to.3 molal dissolved essentially carbonic acid (Hedenquist, 199) and the glassy host rocks (Browne and Ellis, 197). Water-rock interactions can obviously mask identification of a water's origin. Data from Br-7 is discussed further as an illustration of isotopic compositions of calcites showing distinct fluid origins. sotopedata on calcites from Br-7 comprise the most extensive data set for a single well but also show an extreme range of values down hole (Blattner, 1975). Well Br-7 is located on the southeastmargin of the field, and the presence of steam-heated waters shallower than 5 m is indicated by extreme corrosion of well casing and from isotopic and chemical evidence in the adjacent Br-16 well (Hedenquist and Stewart, 1985). Unfortunately, no isotopic measurements on water samples from Br-7 are available. Figure 4 shows the thermal profile and the calculated value of water in equilibrium with calcites using in temperature measurements. At depths m temperatures are about 5' C below boiling, and the calculated water composition is about -7.5 per mil, consistent with steam-heated water. The absence of quartz and adularia and the presence of clays (illite, smectite and mixed layer) with calcite in these shallow samples further indicates alteration produced by steam-heated waters (Hedenquist, Simmonsand Browne, 199). From 4 to m, the temperature increases sharply downhole, converging with the boiling curve. The calculated values for calcite from these depths range from -3. to 1. per mil and are much heavier than the parent chloride water. These results can again be attributed to water-rock interaction, although Blattner (1975) suggested instead that extensive boiling of rising chloride water was the cause of their enrichment; boiling of 7 to 8% of the liquid by Rayleigh distillation is required here to account for enrichment of 2 per mil from to -2.5 per mil). Below 8 m calculated values range -3.9 to -5.4 per mil indicative of calcite precipitation from chloride waters. The data from Br-7 demonstrate that calcite precipitates from two genetically distinct fluids,shallow steam-heatedand deep chloride. 4. WARAKE AND WAOTAPU CALCTE The values of deep chloride water at Wairakei- Tauhara ranges between -5.7 and -5. per mil (Stewart, 1978; Henley and Stewart, 1983). Figure 3A shows that most of the calcites formed C are close to equilibrium with this fluid (Clayton and Steiner, 1975). Calcite formed at C have much lighter values indicating equilibrium with a fluid of about -7. per mil or less, but Clayton and Steiner (1975) could not explain these data. Although isotopic compositions of subsurface steam-heated waters have not been measured, steam-heated waters do exist at Wairakei (Henley and Stewart, 1983; Stewart; and it is likely that isotopically light calcites precipitated from them. The deep chloride water at Waiotapu has a value of.2 per mil C), which boiled to 23' C is about.5 per mil, as interpreted from analyses of well fluids by and Browne (1989). They also used trends in fluid compositions and fluid mineral equilibria to deduce a value of -5.5 per mil for steam-heatedwater C), although direct isotopic analyses of these fluids exists. The data (Fig. 3B) indicate that allbut one sample are in equilibrium with an isotopically light fluid of -2.5 per mil or less, which suggests that most calcites precipitated from a mixture of chloride and steam-heated water with the latter comprising wt. (Hedenquist and Browne, 1989). Note that the Waiotapu calcites are platy, late-stage vug fillings, indicating they precipitated due to boiling (Simmons and Christenson, 5. MPLCATONS The preceding account attempts to provide an oxygen isotope framework for interpreting the origin of calcite in systems where water boils and steam condenses. The interpretation uses knowledge of the calcite formation temperatures in order to apply isotopic factors. n active systems, it may prove useful to determine the isotopic compositions of calcite where it occurs as a scale in wells. Calcite scaling from ascending chloride waters is due to boiling and C2 loss near the flash point 1989; Benoit, 1989). Because of its inverse solubility, calcite may also precipitate from a shallow steam-heated water, initially slightly undersaturated in calcite as appears to be the case at due to heatingand possibly boiling. Consequently, strategies for mitigating calcite scaling may differ depending on the origin of the fluid entering the well. To my knowledge, however, no example of calcite scaling has yet been attributed to the process. Knowledge of calcite origins is also relevant to understanding the genesis of epithermal ore deposits, extinct geothermal systems. Some occurrences of calcite are intimately associated with precious and base-metal mineralisation, whereas others are not. The distinction may relate to whether the calcite saturated fluid is a deeply derived chloride water, which transports gold and silver at Broadlands-Ohaaki (Browne, 1969; Brown, or a shallow derived steam-heated water devoid of metals.

5 89 6. ACKNOWLEDGEMENTS This work was sponsored by a post-doctoral fellowship from the Australian Minerals ndustry Research Association. thank Dr.Peter Blattner for his assistance in making isotopic determinations of calcite the figures. Specialgratitude is extended to Pat Browne for providing some of the samples analysed and "nitpicks" on this paper. 7. REFERENCES Absar, A. 1988, Oxygen isotope and hydrothermal alteration studies at Kawerau and geothermal fields, New Zealand: unpublished thesis, University of Auckland, 334 S., 1989, Deposition of calcium carbonate minerals from geothermal waters - theoretical considerations: Geothermics, v.18, No. Benoit, W.R., 1989, scaling characteristics in Dixie Valley, Nevada, geothermal wellbores: Geothermics, v.18, No. Blattner, P., 1975, Oxygen isotopic compositions of fissure-grown quartz, adularia, and calcite from Broadlands geothermal field, New Amer. Jour. v.275: Brown, 1986, Gold deposition from geothermal discharges in New Zealand Geol., v.81: Browne, P. R.L., 1969, Sulfide mineralization in a Broadlands geothermal hole, Volcanic Zone, New Econ. Geol., Browne, P. R. L., 1971, Petrological logs of Broadlands drillholes BR 1 to BR 25: Surv. Rept. 52. Browne, P.R.L., 1973, Aragonite deposited from Broadlands geothermal drillhole water: Geol. 4, Browne, P. R. L. and Ellis, A. J., 197, The Broadlands geothermal area, New Zealand: mineralogy and related geochemistry: Amer. Jour. v.269: 131. Clayton, R.N. and Steiner, A., 1975, Oxygen isotope studies of the geothermal system at Wairakei, New Zealand: Geochimica et Cosmochimica Acta, Ellis, A.J., 197, Quantitative interpretation of chemical characteristics of hydrothermal systems: Geothermics (special issue 2), v.2, 1, Esslinger, E. V., and Savin, S. M., 1973, Mineralogy and oxygen isotope geochemistry of the hydrothermally altered rocks of the Oh&-Broadlands, New Zealand geothermal area: Amer. Jour. v.273, p. Giggenbach, W.F. and Stewart, M.K., 1982, controlling the isotopic composition of steam and water discharges steam vents and steam-heated pools in geothermal v. 11: Hedenquist, J. W., 199, The thermal and geochemical structure of the geothermal system: Geothermics, v.19: Hedenquist, J. W. and Stewart, M.K.,1985, Natural steam-heated waters at Broadlands, New Zealand: their chemistry, distribution and corrosive nature: Geotherm. Res. Council Annual Meeting, Hedenquist,J. W. and Browne, P. R. L., 1989, The evolution of the Waiotapu geothermal system, New zealand, basedon the chemical and isotopic composition of its fluids, minerals and rocks: Geochimica Cosmochimica Acta, Henley, R. W. and Stewart, M. K.,1983, Chemical and isotopic changes in the hydrology of the Tauhara Geothermal Field due to exploitation at Wairakei: R. A. and OSullivan, M. J., 1986, A summary of temperature data for the Broad-lands geothermalfield: unpubl. School of Engineering Report 398, Univ. of Mahon, W. A. J., Klyen, L. E., and M.,198, Neutral hot waters in geothermal systems: Chinetsu, v. 17: O'Neil, J.R., Clayton, R.N. and Mayeda, T.K., 1969, Oxygen isotope in divalent metal Cosmochimica Acta, Stewart M.K., 1978, Stable isotopes in waters the Wairakei Geothermal Area, New zealand: in Stable sotopes in Earth Sciences (B.W. Robinson, ed.), New Zealand DSR Res. Bull., Simmons,S. F., and Browne, P. R. L., 199, A three dimensional model of the distribution of hydrothermal alteration minerals within the Ohaaki-Broadlands geothermal field: Proceedings 12th New Zealand Workshop: Simmons, S. F., and Christenson, B. W., 199, Platy calcite is an indicator of boiling in epithermal deposits: Evidence from New Zealand geothermal systems: GSA w. abstr., vol. 22, GSA Annual Mtg., p. A42. Tulloch, A. J., 1982, Mineralogical observations on carbonate scaling in geothermal wells at Kawerau and Broadlands: Proceedings 4th New Geothermal Workshop: Wood, C. P., 1983, Petrological logs of drillholes BR 26 to BR Broadlands geothermal field; N. Z. Geol. Surv. 18.