55 EVOLUTION OF GEOTHERMAL SYSTEM Roger C. Henneberger Geothermal Institute, University of Auckland GEOLOGIC SETTING and petrographic study of hydrothermal The Ohakuri field, cut by the Waikato 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 Wairakei i n the Taupo Volcanic Zone (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 Waikato River within cavities left by the leaching 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