Studies of Hydrothermal Activity and Alteration of the Tokaanu-Waihi Area, Taupo Volcanic Zone, New Zealand

Abstract

The Tokaanu-Waihi area is situated on alluvial river deltas at the eastern base of the Kakaramea-Tihia and Pihanga andesite massifs. These massifs form a part of the Tongariro volcanic centre which is located at the south-western end of the Taupo Volcanic Zone. Volcanic rocks in the area include lava flows, tuff breccias, tuffs, lahar deposits, together with colluvium and alluvial deposits. The lavas are labradorite and labradorite – pyroxene andesites. They are typically porphyritic with phenocrysts of plagioclase, orthopyroxene and clinopyroxene in a groundmass of plagioclase, clinopyroxene, orthopyroxene microlites; iron oxide and occasional brown glass. The groundmass has a pilotaxitic texture. In the hydrothermally altered andesites, secondary minerals include Ca-montmorillonite, illite, illitemontmorillonite, kaolinite, chlorite, quartz, cristobalite, chalcedony, calcite, pyrite and zeolites. The zeolites are heulandite, mordenite, thomsonite, chabazite and laumontite. Groundmass was the first to alter followed by the phenocrysts of hyersthene, augite and plagioclase. Progressive alteration of groundmass and phenocrysts is as follows: groundmass → Ca-montmorillonite → chlorite hypersthene → Ca-montmorillonite → illite → chlorite augite → chlorite labradorite → Ca-montmorillonite → chlorite The rank of alteration is subdivided into three grades, which are defined as (i) low: Ca-montmorillonite - interstratified clays (ii) medium: Ca-montmorillonite, interstratified clays, illite and zeolites (iii) high: Ca-montmorillonite, illite, zeolites and chlorite. However, these ranks are gradational rather than with distinct boundaries. The intensity of alteration is divided as incipient, moderate and most intense. The increasing intensity of alteration is correlated with an increase in the permeability of the three zones in the Tokaanu tunnel section. Even in the most permeable zone, plagioclase is only incipiently altered. This may be explained by the ca-rich nature of the reacting hydrothermal fluid. This is evident from the abundant occurrence of vein minerals such as calcite, Ca-montmorillonite and Ca-zeolities in the tunnel section. During alteration, H2O (8 to 25%) has been added to the andesites and with increasing intensity of alteration, oxidation resulted in a decrease of FeO and an increase in Fe2O3. There was poor correlation between mineral abundancessuch as Ca-montmorillonite and Ca wt % of the altered andesites. The paragenetic seguence of the vein minerals is as follows: Ca-montmorillonite - calcite - zeolites - chlorite. A maximum temperature of 230°C is estimated for the Tokaanu tunnel section, on the basis of the occurrence of clay and zeolite in altered andesites. A similar temperature and a PCO2 of about 40 bars are estimated from zeolite stability fields. Although during the present experiments on the andesite CO2 –H2O system, the Tokaanu clay mineral assemblage is reproduced, the experimental temperatures and PCO2 could not be utilised as a means of correlation. This is because equilibrium was not reached in the present experiments. The clay - calcite - zeolite assemblage is very common in veins within tuff breccia zones of the tunnel section. CO2 has a significant role in the stability of zeolite - calcite association. In some veins first calcite and then zeolites formed due to disequilibrium precipitation; in others a second generation of calcite formed later to zeolites due to fresh influx of hydrothermal fluid. With falling temperatures, thomsonite formed first followed by laumontite and mordenite, because of the effect of their water contents. The later formed mordenite being a relatively high silica zeolite with respect to thomsonite, also suggests a fresh influx of hydrothermal fluid. The chloride and enthalpy relations of all Waihi waters as well as some of the Tokaanu waters are explained by simple mixing of the liquid phase of ascending thermal fluid with cold meteoric water. The temperature of the hot-water component in the mixed waters increases from < 200°C at Tokaanu to about 250°C at Waihi, with a simultaneous increase in the cold water fraction of about 0.6 at Tokaanu to about 0.76 at Waihi. A NW trending subsurface fracture zone controls the dilution of the ascending thermal fluid. There was steam loss prior to mixing. Subsurface boiling took place at lower temperatures of < 200°C in the case of the Tokaanu waters whereas for the Waihi waters it occurred at about 250°C. This factor together with (i) temperature dependent variation in the equilibrium constant of CO2 (steam) / CO2 (water), (ii) temperature dependent variation in the dissociation equilibrium constant for H2CO3 and HCO3- and (iii) the calcite solubility and the temperature dependent variation in Ca complexing, explains the relatively high HCO3, CO2 and Ca contents in Waihi waters compared to that in Tokaanu waters. The results from experimental work on the andesite – CO2 – H2O system indicate that SiO2 concentrations in mildly acid thermal waters are controlled by the solubility of cristobalite. At and above 250°C, with 20 bars of PCO2 in the system, β-cristobalite controls the SiO2 concentrations in the coexisting solution, whereas, with 10 bars of PCO2, α - cristobalite controls the SiO2 concentrations. At 200°C, both with 10 and 20 bars of PCO2, SiO2 concentrations in acid thermal waters are controlled by α-cristobalite. Na/K ratios in the coexisting solution in the andesite – CO2 – H2O system are controlled by the reaction: labradorite → Ca montmorillonite → illite. Hiqher PCO2 promotes and accelerates the alteration of Ca-montmorillonite to illite. The alteration trends identified are: labr → Ca-mont; hyp → Ca-mont; Ca-mont → interstr Ca-mont - illite → illite → kaolinite; illite → chlorite; aug → Fe-rich chlorite, quartz and α -cristobalite are the other coexisting secondary minerals. Activity diagrams indicate that kaolinite is the only stable mineral phase in the coexisting fluid and all other minerals formed as metastable phases. At constant PCO2 of 20 bars and with increase in temperature from 200 to 250°C, the rate of the reaction involving the alteration of Ca-montmorillonite to illite increases at a faster rate than that leading to illites formation from plagioclase; the rate of the reaction: hypersthene → Ca-montmorillonite increases at a faster rate than those of the reactions: Ca-montmorillonite → illite and Ca-montmorillonite → kaolinite. At constant temperature of 200°C, with increase in PCO2 from 10 to 20 bars, the reaction involving the transformation of Ca-montmorillonite to illite increases at a faster rate than that of the alteration of illite to kaolinite. With increasing PCO2 from 10 to 20 bars, the rate of the reaction : labradorite → Ca-montmorillonite is not affected at a constant temperature of 200°C. The reaction however, proceeds faster at a constant temperature of 300°C, with increase in PCO2 from 10 to 20 bars. A low estimate of the subsurface reservoir/aquifer temperature of about 180° - 200°C is indicated by the SiO 2 geothermometer for the Tokaanu – Waihi hydrothermal field. The Na-K geothermometer yielded a low temperature estimate of about 200°C for Tokaanu waters, whereas a temperature of 250° - 270°C is indicated for Waihi waters. However, a maximum subsurface reservoir/aquifer temperature of about 280 - 300°C, as indicated by the Na-K-Ca and Na-Li geothermometers, is considered highly likely for the Tokaanu - Waihi area. This estimate is also in close agreement with the estimate of about 315°C made on the basis of Cl vs enthalpy plots. The implications of the present experimental results to chemical geothermometry are: (i) The solubility curve of the appropriate variety (α or β) of cristobalite (and not that of quartz) should be applied as the SiO2 geothermometer to mildly acid thermal waters. (ii) In the andesite – CO2 –H2O system, the Na/K ratios in the acid thermal waters are controlled by the reaction: plagioclase → Ca-mont → illite. Therefore a different Na-K geothermometer can be developed for acid thermal waters. (iii) The log linear trend of the Na, K and Ca concentrations of the reacted fluid in the present experiments suggests that it is possible to develop a separate Na-K-Ca geothermometer for acid thermal waters. On the basis of the above results, a subsurface model for the Tokaanu - Waihi area is proposed in which the events involved during the ascent of the deep hydrothermal fluid and how it caused the alteration of andesites are discussed. A subsurface reservoir at a temperature of 280 - 300°C feeds both the Tokaanu and Waihi areas. The deep thermal fluid during their ascent to the surface cools adiabatically; by mixing with cold meteoric water and also by conduction. These processes result in boiling springs, warm springs of mixed waters and low temperature springs with poor discharges. The difference in the temperature of subsurface flashing for waihi and Tokaanu waters, together with the factors such as the temperature dependent variation of the equilibrium constant for CO2 (steam) /CO2 (water) lead to higher concentrations of HCO3 and CO2 of Waihi waters relative to those of Tokaanu waters. The separated liquid phase following subsurface boiling, mixes with cold meteoric water in a NW trending underground permeable zone. The resultant mixed water emerges as Waihi warm springs as well as some of the Tokaanu warm springs. Towards the south of the Tokaanu area (i.e. Tokaanu tunnel section) the steam and liquid phases mix with each other resulting in acid hydrothermal fluid with a maximum temperature of 230°C. This fluid caused the hydrothermal alteration of andesites leading to the formation of the clay, calcite, zeolites and silica minerals. In the veins calcite followed by zeolites formed due to disequilibrium precipitation. However, the second generation of calcite which formed later than zeolites is explained by a fresh influx of hydrothermal fluid. This is also supported by the fact that precipitation of high silica zeolites succeeded that of low silica zeolites.