55

EVOLUTION OF GEOTHERMAL SYSTEM

Roger C. Henneberger

Geothermal Institute, University of

GEOLOGIC SETTING

and petrographic study of hydrothermal The Ohakuri field, cut by the River, alteration at the now-extinct Ohakuri geothermal occurs in the western part of the Taupo Volcanic field has revealed a long history of activity which Zone, within the Maroa Volcanic Centre (Fig, 1). varied in nature lacation. Early flow of Hydrothermal activity took place within an area in neutral to alkaline chloride- type water through which the dominant rocks are a sequence of rocks now exposed was followed by extensive near- rhyolitic pyroclastics, informally termed the surface flow of acid- sulphate water, with minor Ohakuri Ignimbrite. The sequence consists of at hot- spring discharges, which fdrmed scattered least 150 m of nearly flat- lying, porous, pumice- sinters, comprising the last activity. High per- rich, massive, nonwelded ignimbrite flows, with meability of the porous host rocks facilitated minor cross- stratified ignimbrites and thin fluid movement, and regional hydrology may have units. The age of the sequence is unknown, but its significantly controlled flow patterns, whereas control by tectonic fractures appears to have been minor. in rock sealed by silicifica- tion was sustained by hydrofracturing which resulted in veining and local brecciation. Fluid temperatures probably did not exceed and were largely less than at the levels observed. The distribution of hydrothermal features suggests that upflow zones migrated northward and westward with time, and that the now-active Atiamuri geothermal system is probably descended from the fossil Ohakuri field.

INTRODUCTION

Hydrothermally tered rocks and hot spring deposits are exposed over an area of at least 15 near Lake Ohakuri, 25 north of i n the (Fig. 1). These hydro- thermal features are the remnants of a now-extinct geothermal system which was similar to active thermal areas i n the region. Variable erosion of the altered rocks affords an opportunity to study the upper 100 to 150 m of such a system i n much more spatial detail than is possible in an active system.

Mapping of surface geology and alteration types, along with petrographic investigation of I samples, allows reconstruction of a number of characteristics of the system, particularly its fluid composition and temperature and their variations through time, controls on fluid move- ment, and the duration of activity. This study shows that Ohakuri evolved over a long period, with significant changes i n the nature and location of Figure 1: Geology of Ohakuri and vicinity, modi- activity before the near- surface movement of fied after Grindley (1959). Young geothermal waters ceased. tephras (542,000 yrs) not shown. Rectangle indicates area of this study (Fig. 2). 56

Henneberger relationship to other volcanic units in the area HYDROTHERMAL ALTERATION indicates an age in the range of several hundred thousand to one million years Grindley, 1959; Hydrothermal fluids produced several types of Cole, 1979). Up to 10 m of fine- grained ash beds alteration in tne tuffaceous rocks now exposed. rest on the eroded surface of the Ohakuri Ignim- The different types can be distinguished in the brite in a small area on the north shore of the field and correlate with petrographic Waikat o River. distinctions determined by optical and X-ray Diffraction methods. Most of the alteration is A number of terrace-forming fluvial and pervasive, with alteration intensity and mineralogy lacustrine deposits flank the reflecting remaining constant over distances of up to 10 m or fluctuating river levels and periodic inundation by more. However, variations over smaller distances lakes since the inception of hydrothermal activity. (to as little as several are frequently At least five terraces are present, and or more observed where the original rock was inhomogeneous, of the older deposits have been altered by geother- in thinly bedded pyroclastics with varying mal water in the same manner as the ignimbrites. grain size distribution o r pumice content. The area is mantled by tephras which postdate Veining, which is common only i n intensely altered the hydrothermal features now exposed. The rock, has produced similar small- scale variations of these, the Rotoehu Ash, imposes a minimum age of which a r e visible as alteration haloes. Transi- 42,000 years on the bulk of the hydrothermal tions between types are usually gradual, occurring activity (Vucetich and Pullar, 1969). over distances of at least 10 m, but are locally sharp, particularly where later alteration over- Ohakuri lies on the southwest extension of the prints an earlier event. northeast- trending Ngakuru Graben, and also within the northwest-trending Maroa Graben proposed by Table 1 summarizes the characteristics of the Modriniak and Studt (1959). Tectonic deformation various alteration types. The most intense in the area is weak, but minor NE-trending faults alteration converted the tuffs to a quartz + have been identified, and several adularia + sericite (illite and similar sheet lineaments seen on aerial photographs appear to silicates) pyrite assemblage, with minor amounts correspond to zones of more concentrated altera- of chlorite, clinoptilolite and leucoxene. Quartz, tion. The setting of Ohakuri is similar to those the most abundant hydrothermal mineral, replaces of the Orakeikorako and Atiamuri thermal areas, virtually all vitric components and often fills both of which also flank the within cavities left by the of primary grains. the Maroa Volcanic Centre and the inferred Maroa Plagioclase is replaced by adularia, with minor Graben. amounts of sericite and rare apatite, and frequently is partially leached.

TABLE 1: Summary of hydrothermal alteration types at Ohakuri

Alteration Type Quartz - Adularia Zeolite Kaolinite Hot Spring Deposits

Alteration Minerals Quartz -- Cristobalite Adularia Illite - - Chlorite Clinoptilolite Mord e n i te Mon tmorillonit e - - Kaolinite Alunite Pyrite ---- Hematite ------. ------e - - - - Leucoxene Apatite

Veining Common; l o c a l l y R a r e Sparse but Rare intense locally intense

Probable Water Neutral - alkaline Neutral - alkaline Acid - sulphate ? Mixed chloride chloride chloride and sulphate? meteoric?)

Deduced Water 100 - 200 50 - 100 50 - 100 -100 Temperature 57

Henneberger

0

33°C

Figure 2: Distribution of alteration types, Ohakuri area. Kaolinite alteration is locally discontinuous. Temperatures of wells (depths 40 - 60 were measured February 1982.

Veins of quartz and quartz + pyrite which vary Quartz- adularia alteration forms a central i n thickness fom microscopic to several cm are zone i n the southern portion of the area (Fig. 2) common i n zones o f quartz- adularia alteration. which grades both upward and outward into a zone of Zoning within veins, and crosscutting relationships less intense zeolite- type alteration (Table 1). between veins, indicate that a number of distinct This latter type is characterized by weaker silici- episodes of quartz and quartz + pyrite precipita- fication, little to no veining, and less dense tion took place. As many as four such episodes can rocks. Mordenite, montmorillonite and cristobalite be recognized i n single outcrops. Successive a r e the most abundant hydrothermal minerals, and generations veins follow a progression from plagioclase is fresh or only partially replaced by extremely thin mm) fillings with broad haloes sericite + adularia. The zeolite zone grades into to thick cm) veins without haloes. Many o f t h e fresh rock, i n places through a narrow (10 - 50 m thickest veins contain angular clasts of wallrock, wide) zone i n which the only alteration is leaching and appear to be gradational with hydrothermal of pyroxene and amphibole crystals. breccias present at several locations. These breccias consist of angular and rounded clasts in a Zeolite and quartz- adularia alteration types quartz pyrite matrix, and range from about 1 m to extend from Ohakuri dam at least 2.5 km to the several tens of meters i n thickness. The breccias towards Atiamuri. Though lack of exposure prohi- and most veins are thought to be the product of bits tracing the zones further, the trend of the violent hydrofracturing with subsequent infilling mapped alteration indicates that the active by mineral precipitation Grindley and Browne, Atiamuri system is contiguous with, and very likely 1976). descended from, the Ohakuri system. Such a 58

by resistivity mapped and possibly extending farther north and anomaly-patterns (Dawson et al, 1981) and by the west (Fig. A period of erosion presence of warm water wells at the western margin followed near- surface flow of acid- sulphate of the Ohakuri area (Fig. 2). waters, cohdensed from steam and oxidized produced by boiling of chloride water at depth, A third type of alteration, dominated by which altered fresh and previously altered rocks to kaolinite plus variable amounts of c kaolinite type. The main zone for the deep hematite and local alunite (Table affected both chloride water was north of that for the earlier fresh rocks and rocks previously altnred to zeolite phase, with shallow fluids flowing southward toward and quartz- adularia types. of kaolinite type the Waikato River, then following approximately its are bleached white and mainly converted to kaolin- present course (Pig. 3B). ite or kaolinite + cristobalite. been leached away, and the rock usually has a low The presence of widespread acid water at density. silicification, however, is intense levels previously dominated by chloride water and infilling of leach cavities by cristobalite has ‘suggests that a drop i n the water table took place occurred. Cristobalite veins, in places containing between the two alteration episodes. Such a change alunite and pyrite, are present in some zones of could be due to local tectonic uplift, or to a stronger The mineralogy and change in regional hydrology. While tectonic bleached of the rock are characteristic movement is a possibility, the abundant lacustrine effects of alteration by acid- sulphate waters, deposits near Ohakuri, and throughout the Taupo usually produced by condensation of steam (plus Volcanic Zone, are evidence of the repeated oxidation of boiled from deeper chloride formation and draining of large lakes in the waters (Ellis and Mahon, 1977). region, which could cause significant water table fluctuations. Alteration to kaolinite type is only found north of the Waikato River (Fig. 2), and lenses of The last activity at Ohakuri consisted of unaltered rock are present within its zone of minor hot spring discharges which produced small occurrence. The most intense kaolinization occurs sinters and weakly silicified various surface in the north- central part of the zone, diminishing deposits. This activity took place in the sharply northward and more gradually to the south. northwestern part of the area, reflecting the This alteration locally overprints quartz- adularia migration of the system towards and zeolite types, and has affected younger Atiamuri. Although surface activity at Ohakuri is pyroclastics which lie on the eroded surface of now absent, the continued flow of hot water at rocks previously altered to quartz- adularia type. depth is not precluded. Additionally, the indica- These relationships suggest that most, if not all, tions of episodic activity in the past suggest that of the observed acid- sulphate alteration postdates the resumption of near-surface and hot-spring the activity that produced the other types, activity is not impassible. possibly following an erosional interval. FLUID PROPERTIES Several types of local hot spring deposits are present north of the river, being most abundant in The approximate temperature and composition the northwestern part of the area (Fig. 2). These the waters that produced the different alteration include laminated and massive siliceous sinters, types can be inferred from the mineral assemblages weakly silicified fluvial deposits, and silicified present in each type. Minerals temper- breccias containing fragments of sinter and other ature indicators include the silica polymorphs, surficial material. Relative ages of the hot epidote, sheet silicates (montmorillonite, illite spring deposits are not always known, and their and o t h e r s ) , and z e o l i t e s ; common temperature formation may have been partially concurrent with ranges are summarized by Browne (1978). The kaolinite alteration. However, stratigraphic absence of minerals such as epidote and relationships suggest that the waning stages of suggests that temperatures or fluids causing surface activity took the form of small hot springs quartz- adularia alteration probably did not exceed depositing sinters and causing weak silicification a t the depths exposed, while waters precipi- at shallow levels. tating cristobalite in the zeolite zone were probably cooler than (Table 1). The PHASES OF ACTIVITY abundance of cristobalite and rarity of quartz in kholinite- altered rocks and surficial deposits Field relationships indicate that the area of similarly indicates that the associated fluids exposed hydrothermal features a t Ohakuri is a were seldom hotter than inactive part of a geothermal system which includes the present Atiamuri geothermal area. Movement of The neutral to alkaline chloride waters of hydrothermal fluids through the Ohakuri Ignimbrite the early phase of activity were rich in and may have begun several thousand years ago, and and the absence of hydrothermal calcite, probably continued intermittently until surface common i n most geothermal systems, indicates a low activity ceased over 40,000 years ago. The dissolved content. Temporal changes fluid earliest phase of activity presumably consisted of chemistry are reflected in the episodic precipita- upward and lateral flow of neutral to alkaline tion of iron sulphide in quartz veins. Progressive chloride waters, which produced the zones of mineral changes observed i n the transition from quartz- adularia and zeolite alteration now exposed quartz- adularia to zeolite rock and on to fresh over much of t h e southern portion of the area rock probably reflect changes in fluid temperature 59

composition due to mixing of thermal with activity. Permeability was maintained as local meteoric water, and perhaps different degrees of fluid overpressure due to sealing caused rock-water interaction as well. turing and the formation of breccias. The fractures produced served as fluid channels until The complexity of near- surface alteration and precipitation of vein minerals again led to hot- spring deposits associated with the later sealing, causing the hydrofracturing cycle to phases of activity appears to reflect local repeat. The geothermal system was thus self- variations in fluid chemistry. Such variations may sustaining through multiple episodes of self- have been produced by mixing of different sealing. spring waters, possibly of both acid- sulphate and chloride types. CONCLUSIONS

CONTROLS ON FLUID MOVEMENT Hydrothermal alteration at Ohakuri has recorded the evolution of a geothermal system that Flow of hot water through the Ohakuri Ignim- underwent significant changes during its lifetime. brite was initially facilitated by high primary Similar changes have been documented i n a number permeability. Such permeability is indicated by of active systems, including Matsukawa, Japan the pervasive nature of rock alteration, and by the (Sumi, Roosevelt Hot Springs, U.S.A. abundance of adularia as a hydrothermal feldspar (Parry et al, and Orakeikorako (Lloyd, (Browne, 1978). There is little evidence for 1972). These changes reflect the complex, significant local control of flow by dynamic nature of the interactions between induced fractures such as faults, though the geothermal waters, heat sources, host rocks, general location of upflow zones may have been tectonic activity and regional hydrology that controlled by such features. constitute a geothermal system.

Fluid movement was apparently not constrained ACKNOWLEDGEMENTS by any lithologic barriers such as an impermeable cap rock, so near- surface flow may have been This work was undertaken as part of an strongly influenced by the regional hydrologic thesis study at the University of Auckland and was regime. The sharp transition to unaltered rock at funded by a Fulbright study grant. I wish to thank the eastern margin of the quartz- adularia and Dr. P.R.L. Browne, Bruce Christenson and Glenn zeolite zones, compared with a much more gradual White for their critical reviews of the manuscript. change to the east and northeast, may be the result of a dominant northwesterly groundwater flow. Such flow would cause rapid dilution and cooling of ascending thermal water over a narrow zone in the east, whereas dilution would occur over a broader zone at the other margins.

Precipitation of quartz and other minerals caused a progressive reduction of permeability in the quartz- adularia zone during the first phase of

Overprint Acid Hot Springs, Hot Springs? Waikato Zone Fumaroles

......

Chloride Brecciation Water Chloride Water

Quartz-Adularia . . . . Zeolite Kaolinite .. . . Alteration .... Alteration Alteration

Figure 3: Schematic cross section of the Ohakuri geothermal system during early (A) and later phases of activity. 60

Henneberger

REFERENCES LLOYD, E.F., 1972, Geology and hot springs of Orakeikorako: Geological Survey BROWNE, P.R.L., 1978, Hydrothermal alteration in Bulletin 85, 164 p. active geothermal fields: Annual Reviews of Earth and Planetary Science 6, 229-250. N. a n d STUDT, F.E., 1959, Geological structure and volcanism in the Taupo-Tarawera COLE, J.W., 1979, Structure, petrology and genesis New Zealand Journal of Geology and of Cenozoic volcanism, Taupo Volcanic Zone, Geophysics 2, 654-684. New Zealand - 'a review: New Zealand Journal of Geology and Geophysics PARRY, W.T., BALLANTYNE, J.M., BRYANT, N.L. and DEDOLPH, R.E., 1980, Geochemistry of hydro- DAWSON, G.B., RAYNER, H.H., and HUNT, T.M., 1981, thermal alteration at the Roosevelt Hot Reconnaissance geophysical measurements at Springs thermal area, Utah: Geochimica et Atiamuri geothermal area: New Zealand D.S.I.R. Cosmochimica Acta 44, 95-102. Geophysics Division Report 169, 37 p. K., 1968, Structural control and time ELLIS, A.J. and W.A.J., 1977, Chemistry and sequence of rock alteration in the Matsukawa Geothermal Systems: Academic Press, New York, geothermal area, with special referecnce to 392 p. those at Wairakei: Journal of Japan Geothermal Energy Association 17, 80-92. GRINDLEY, G.W., 1959, Sheet N85 Waiotapu, Geologic Map o f New Zealand New Zealand WCETICH, G.C. a n d PULLAR, W.A., 1969, Stratigraphy D.S.I.R., Wellington. and chronology of late Pleistocene ash beds of central , New Zealand: New Zealand GRINDLEY, G.W. and BROWNE, P.R.L., 1976, Structural Journal Geology and Geophysics 12, 784-837. and hydrological factors controlling the permeability of some hot-water geothermal fields: Proceedings of the 2nd U.N. Symposium on Development and Use of Geothermal Resources, San Francisco, 1975, 377-386.