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Proc. 10Th New Zealand Geothermal Workshop 1988 GEOPHYSICAL EXPLORATION for PROSPECTIVE GEOTHERMAL RESOURCES in the TARAWERA

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123 Proc. 10th New Zealand Geothermal Workshop 1988 GEOPHYSICAL EXPLORATION FOR PROSPECTIVE GEOTHERMAL RESOURCES IN THE TARAWERA FOREST C. J. BROMLEY2, J. J. BOTTOMLEY1, C.F. PEARSON2 1. FLETCHER CHALLENGE LTD 2. GEOPHYSICAL CONSULTANT (formerly KRTA LTD) ABSTRACT An integrated geophysical survey, involving GEOLOGY resistivity, gravity and magnetic measurements, was conducted early in 1987 over a 300km2 area Geological inferences suggest that the most within and north of the Tarawera Forest, to promising geothermal prospects are located along investigate potential geothermal resources for the north-eastern boundary of Haroharo Caldera, Fletcher Challenge Ltd. Two significant which lies within the Okataina Volcanic Centre. anomalies have been delineated. The first is the The Haroharo and Tarawera complexes have been Tikorangi resistivity and magnetic low, sporadically active for the last 250,000 years, associated with the north-eastern boundary of the most recently in 1886. This suggests that a magma Haroharo Caldera and centered south of the chamber still underlies the caldera (Nairn, 1981). Tikorangi solfataras. The second anomaly, which Important structural features of the caldera in- is situated within the Puhipuhi Basin, east of clude its margin and the Haroharo and Tarawera the caldera, is postulated to represent a vent lineations. As a result of multiple, shallow separate but cooling geothermal system, with dip slumping, associated with the caldera margin, implications of possible epithermal enhanced vertical permeability in the upper mineralisation. kilometre may be distributed over a broad zone. In addition, the underlying ring fault probably Interpretation of the gravity measurements has creates a deep zone of substantial permeability. led to identification of a north-east trending Recent vents form two broad parallel lineations graben passing through Lake Rotoma, and orientated at 50° which are probably associated intersecting the Haroharo Caldera near with deep-seated basement fractures. The Haroharo Tikorangi. combined interpretation Of all vent lineation intersects the caldera boundary, at available data resulted in a pre-drilling a position marked by Tikorangi, a small 5,000 year conceptual hydrological model for the Tikorangi old rhyolite dome. Within the caldera there are system and a target for future exploration very few mapped faults which is probably a drilling. function of the relatively young age of the surficial volcanics. However, outside the INTRODUCTION caldera, numerous north-east trending faults have This paper presents the results of a geophysical been mapped, and many of these may also pass investigation of prospective geothermal resources through the caldera, providing vertical in and near the Tarawera Forest (between Rotorua permeability beneath the surficial volcanics. and Kawerau) for Fletcher Challenge Ltd. Previous studies (Nairn, 1981, Yamada, 1985) The near surface geology of the Haroharo Complex provided encouraging evidence for the possible consists of a relatively thick sequence of recent existence of a geothermal system in the area rhyolite lavas (approximately 5,000 to 9,000 years between Lake Rotoehu, Lake Rotoma, and the old), volcanic breccias and pyroclastics. East of Tarawera River. Thermal manifestations, occur the caldera boundary, the approximately 200,000 along the shores of both lakes, at Tikorangi, and year old Haparangi rhyolites make up the within the Te Haehaenga Basin further south. The Maungawhakamana massif and Rere hills bordering Puhipuhi Basin, south of the Tarawera River, was Lake Rotoma. To the south, the Puhipuhi Basin added to the area of interest because of evidence marks the site of a small depression or caldera of intense alteration on Puhipuhi Hills, and the now largely filled with lacustrine sediments and existence of the neighbouring Waiaute warm ignimbrites which were uplifted and altered by the springs. emplacement of the Puhipuhi dacite volcano (160,000 years old). The Puhipuhi dacite is A number of previous geophysical surveys, highly brecciated and has been intensely altered conducted in this area, were used to guide the by acidic fluids to an assemblage consisting of an recent work. These included: an assessment of advanced argillic cap of alunite-cristobalite Okataina Volcanic Centre using gravity, seismic, (opal) and pyrite, underlain by kaolinite and resistivity and aero-magnetics (Rogan, 1980); a pyrite. Silica flooding is also common. This brief resistivity survey of the Rotoma-Tikorangi advanced argillic cap is typical of the surface area by students from the Geothermal Institute expression of shallow boiling in epithermal (Doens, 1985, Kohpina, 1985); and a sequence of environments, and it may be underlain by a detailed aero-magnetic surveys flown over the paleo-boiling zone containing mineralisation. Haroharo and Tarawera complexes (Salt, 1986). Gold and silver traces were reported by These are discussed in more detail by Hochstein prospectors in the 1920's in opalised pyritised et al. (1987). Late in 1987, another two quartz veins. The lack of significant surface Institure students (Ayala, Estrada) conducted thermal features around Puhipuhi suggests that follow-up resistivity and gravity work. this prospect is probably a waning hydrothermal system. The geophysical studies described here include a gravity survey of 208 stations, a resistivity traversing survey of 278 stations (with AB/2 The major thermal features of interest (see Figure spacings of 500m and 1000m), 40 deep resistivity 2) are located at Tikorangi (solfataras), Lakes soundings, and some additional magnetic property Rotoma and Rotoehu (Waitangi and Otei Springs), in measurements on surface rock samples. the Te Haehaenga Basin (Mangakotukutuku Spring), 124 Bromley et al. and west of Puhipuhi Hills (Waiaute Spring). Other nearby thermal features are located on the south eastern shore of Lake Okataina, Humphreys Bay of Lake Tarawera, and the Centre Basin of Lake Rotoiti. Descriptions of the major thermal manifestations are given by Nairn (1981) and Yamada (1985). During the present geophysical survey another thermal feature was discovered in a swamp along Waterfall Road next to the Tarawera River (Grid: N77 993029). An extensive area of C0? degassing is associated with warm chloride fluids within the swamp. A maximum temperature of 30°C was recorded in mud at about lm depth. Measurements of flow rate and chloride concentration of the Tarawera River have identified a substantial increase in total chloride flux (about 150 gm/s) entering the river between the Tarawera Falls and the Waterfall Road bridge. RESISTIVITY SURVEYS Previous resistivity measurements in the area include an early DSIR traversing survey along State Highway 30 and six Schlumberger soundings, some dipole-dipole measurements and magneto-telluric soundings by Rogan (1980) along the Tarawera River. Rogan's models are reasonably consistent with subsequent resistivity sounding interpretations, although there is some doubt over the existence of a 1 ohm-m layer at 3 km depth near Lake Tarawera, which is based on one noisy dipole-dipole reading. A deeply penetrating magneto-telluric sounding located beside the outlet of Lake Tarawera revealed apparent resistivities of less than 10 ohm-m at a period of 10 seconds, increasing to more than 100 ohm-m at 200 seconds. Again reliable interpretation is FIGURE 1: hindered by noisy data and three-dimensional Apparent Resistivity Contours AB/2 = 1000M effects, but a simple layered model of the the soundings and selected section lines are shown magneto-telluric curve suggests that if a 1 ohm-m in figure 2. Interfaces between the layers are layer exists at 2 to 3 km depth, then it is connected to portray subsurface variations in the probably only about 500m thick and is underlain by factors that have caused the resistivity much higher resistivities (in excess of 500 contrasts. In general, these factors are ohm-m). Geothermal Institute students Kohpina variations in porosity, temperature, saturating (1985) and Doens (1985) conducted a total of 10 fluid mineralisation and intensity of clay soundings and 18 traversing measurements between alteration. Together, these factors can provide Rotoma and the Tarawera River. These have been powerful indicators of the shallow hydrological reinterpreted in conjunction with the recent processes occurring in a geothermal system, resistivity survey. although care is necessary to avoid misinterpretations caused by combinations of The resulting apparent resistivity traversing alteration, porosity and fluid mineralisation contour maps reveal a complex zone of moderately contrasts without a corresponding change in low resistivities elongated in a north-south temperature. direction along the eastern caldera boundary (see Figure 1). The anomaly is bounded in most The first interface that appears in all the directions by resistivities in excess of 400 sections is closest to the surface, and probably ohm-m. The low resistivity zones form two represents the ground water table. It is marked anomalies: the Tikorangi-Rotoma-Te Haehaenga by a contrast in resistivity of approximately one anomaly and the Puhipuhi anomaly. The only zone order of magnitude from several thousand ohm-m of very low apparant resistivity (less than 10 (unsaturated porous pyroclastics, rhyolites etc) ohm-m) is a 1.2 km fan-shaped area north of the to several hundred ohm-m (interpreted to be porous Tikorangi solfataras suggesting the existence of a volcanics saturated with fresh ground
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