Proceedings 20th NZ Geothermal Workshop 1998 SURFACE ALTERATION BETWEEN ORAKEIKORAKO AND TE KOPIA THERMAL AREAS

J.P. P.R.L.

Institute, The University of Auckland, Auckland, NZ

SUMMARY: The surface alteration between the Te Kopia and Orakeikorako geothermal areas the thermal evolution of these systems. This study correlates and describes the distribution of active and fossil hydrothermal activity along the Paeroa Fault and the uplifted Paeroa Block between the two thermal areas. Alteration styles found, consist of acid alteration by heated waters, silicic and near neutral alteration by deep chloride waters, and acid overprinting of earlier neutral alteration. Previously unreported sinters and hydrothermal breccias are located between the two active areas showing that thermal activity was more extensiveprior to 22.7 Ka. The reduction in thermal activity was preceded by a drop in the water table which may have resulted from self sealing, or migration of the thermal areas controlled by the Paeroa Fault.

1.0 INTRODUCTION

The Orakeikorako and Te Kopia geothermal areas are situated in the central , along the Paeroa Range. They are situated within the about 270 km south of Auckland and 25 km northeast of Taupo (Figure 1). The Paeroa fault, with its spectacular scarp, is the dominant structural feature of the area; it strikes northeast bisecting Orakeikorako, Te Kopia and Waikite thermal areas.

In Ferdinand von Hochstetter visited the Orakeikorako-Te Kopia area. His map shows a continuous Iine of hotsprings at Orakeikorako that crossed the River from the west to east and extends along the base of the Paeroa Range. However he did not indicate just how far these springs to the northeast towards Te Figure 1. Map of the Taupo Volcanic Zone Kopia. In 1863, Hochstetter and Petermann (TVZ) indicating the location of Te Kopia and produced a regional geological map of the Orakeikorako with respect to other active Auckland Province showing the thermal activity at geothermal areas and volcanic centres. Te Kopia as being more extensive to the north east (perhaps as far as Waikite) and to the southwest 2.0 GEOLOGY than is presently observed. In 1870, the Hon. H. gave an account of numerous multi- 2.1 FAULT coloured decaying rocks along the Paeroa scarp, and of extensive fumarolic activity along the area. The western side of the Paeroa Range is marked by The objectives of this study are to determine the the dramatic scarp of the Paeroa Fault. The fault extent, and if possible, the timing of past thermal strikes and disappears to the northeast activity between Orakeikorako and Te Kopia beneath pumice breccias. To the southwest, the thermal areas. fault is covered by rhyolite lavas.

271 The upper part of the scarp is made up of the Fault and was described in detail by Wilson (1985). Paeroa Ignimbrite which overlies the Within the study area Taupo Ignimbrite consists of pumice breccias and the Te Kopia Ignimbrite. a tan to off-white layer of rounded pumice blocks, Southwest of the Te Kopia thermal area, the scarp averaging -10 cm in size, with a fine grained is less pronounced and comprises sandstones, rhyolitic ash matrix. Pieces of wood are breccias and siltstones of the Huka Group which common and vary from small fragments cm in overlie the Paeroa Ignimbrite (Grindley, 1959). size to large branches and small trunks up to -20 South-easterly dipping strata of the upthrown cm in width. The layer varies in thickness Paeroa Block are little affected by faulting with the -10 cm on ridges to 4 m in some topographic lows. exception of the Ngapouri Fault. The Ngapouri Taupo Pumice Alluvium was formed by Fault diverges and strikes cutting across the aggradation of rhyolitic air fall and pyroclastic Paeroa block and continues as the Rotomahana flow deposits of the 186 Taupo eruption. The Fault on the southern edge of the Horohoro Caldera Taupo Pumice Alluvium crops out at the foot of the (Wood, 1994). Grindley (1959, 1963) and Paeroa Fault on the floor. The Hedenquist (1983) suggest that geothermal features underlying Hinuera Formation consists of cross situated south of the Ngapouri Fault were located bedded rhyolitic sands and gravels. The Hinuera along NNE ,near parallel faults. Grindley Formation is thought to result from strong erosion stated that the low permeability along these faults during the last glaciation (Grindley, 1959) and is (as interpreted fiom exploration well discharges) dated palynologically by Schofield (1965) as dating may be caused by mineral precipitation. Wood between 12-16 Ka. (1994) noted the Huka Falls Fonnation siltstones prevent fluids dischargingat the surface over much Throughout most of the study area Oruanui Ash of the area. formation (otherwise known as the Kawakawa tephra) underlies Taupo Pumice Alluvium. This is The Paeroa Fault may tap a deep regional source of a fine grained rhyolitic ash carbon dated at 22.6 Ka thermal water (Grindley et al., 1994). (Wilson et al., 1988). This layer is an important Interpretation of three CSAMT sounding profiles marker within the and the study area. This I- (Bromley, 1992, 1993) suggest that the two orange ash layer, with its characteristic thermal areas are connected at depth, although this accretionary lapilli mantles even more of the study interpretation has been questioned (Bibby et al. area than does the Taupo Pumice, and obscures 1994). Electrical resistivity surveys by Bibby et al. many of the outcrops. The ash layer has (1994) imply that the Waikite springs the uniform thickness averaging 2 m. Throughout the outflow from a deep hydrothermal system to the area a thick sequence of pumice breccias and southeast that also supplies . This could lacustrine silts known as the Huka Group underlie mean that the Paeroa Fault channels westward, the Oruanui Ash. The Huka Group mainly consists moving geothennal water upwards fiom shallow of middle to late Pleistocene lake sediments that aquifers. formed within volcanic basins. In the southeast of the area, the flat lying Mihi Pumice Breccia 2.2 STRATIGRAPHY conformably overlies the Group (Grindley, 1959). The local stratigraphy based on surface mapping by Grindley Healy et al. Nairn The grey to quartzose Paeroa Ignimbrite was Houghton et al. and on the sequence first described by Grindley (1959) and forms the penetrated by the Waiotapu exploration wells is crest of the Paeroa Range in the Te Kopia thermal summarised in Table 1. Units that were area. Houghton et al. using methods encountered within the study area are described dated the Paeroa Ignimbrite at 0.33 0.01 Ma. below, and the distribution of units older than 22.7 The Paeroa Ignimbrite overlies the Te Weta Ka is shown in Figure 2. Ignimbrite as seen in Waiotapu wells (Steiner, 1963; Hedenquist, but the Te Weta In about 186 AD, the Taupo Volcanic centre Ignimbrite does not occur in the exploration wells experienced a catastrophic eruption (Wilson et al. at Te Kopia (MacKenzie, 1995). The Ohakuri 1980). This produced pyroclastic flow and air fall Formation unconformably underlies the Huka deposits that blanketed a large portion of the Group and consists of massive pumice breccias and central North Island. Taupo Ignimbrite mantles much of the study area on both sides of the Paeroa

272 soft ignimbrites. The position of the Ohakuri 1. Low intensity acid alteration: Formation within the ignimbrite sequence can be intensity acid sulphate alteration. Kaolinite deduced from its position in the Paeroa Fault scarp alunite cristobalite native sulphur pyrite where it underlies the Paeroa Ignimbrite but hematitie. overlies the Te Kopia Ignimbrite (Grindley, 1959). 2. High intensity acid alteration: Haparangi Rhyolites do not crop out within the Te intensity acid sulphate alteration. Kaolinite Kopia thermal area (MacKenzie, although alunite cristobalite native sulphur pyrite rhyolite occurs in the cores from wells at Te Kopia, hematite. between two Akatarewa Ignimbrite sheets. 3. neutral alteration: Rhyolites are more common to the west of the quartz illite-smectite chlorite adularia Orakeikorako thermal area within the Maroa montmorillonite illite pyrite. Volcanic Centre. 4. Acid overprint: Silica veining hosted by acid alteredrocks. The Te Kopia Ignimbrite crops out along the base 5. Sinter: Silica sinter indicating the of the Paeroa Fault and consists of three sheets of discharge of deep chloride waters at the surface. dark grey quartzose ignimbrites. The top sheet is separated from the bottom two by a layer of soft Two previously unreported sinters have been breccia containing large blocks of rhyolite lava and recognised in the area. Both sinters are covered by ignimbrite (Grindley, 1959). These clasts likely the Oruanui Ash showingthat they are at least 22.6 derive from the eruption vent wall (Keall, 1987). Ka. The sinter that crops out north of Houghton et using methods, dated Pukemoremore Rd (Figure 3) is soft and friable. the Te Kopia Ignimbrite as 0.34 0.01 Ma. Petrographic and SEM examinations show that it contained laminated stromatalitic mats and well 3.0 SURFACEALTERATION preserved filamentous microbes. The sinter comprises amorphous silica that gives an 3.1 peak that is between Opal-A and Opal-CT. For 200 m southeast of the sinter, along Pukemoremore The distribution of the surface alteration is shown road, high intensity acid alteration (kaolinite + in Figure 3. The mineralogy was determined by hematite) crops out beneath the Oruanui Ash. This thin section petrography and methods. For clay alteration leads into a zone of opal simplicity surface, alteration in the area has been silicification. A collapse breccia has been altered to divided into five classifications: opal + kaolinite + hematite. The opaline alteration is similar to the opaline layer described by Schoen et al. (1974) at Steam Boat Springs,Nevada. The

273 2. Local geology of the area between Orakeikorako and Te Kopia. The map is partly based on (1959). opaline sheet is formed at the water table as the Debris flows are also shown in Figure 3, because result of argillisation by descending acid fluid. The the landslide deposits may be related to sinter that is located near Rd is more hydrothermal activity that weakened the country vitreous and contains numerous fossil casts of plant rock and resulted in slope failure. material. There are at least three new hydrothermal breccias recognised in the area (see Figure 3) that 3.2DISCUSSION all underlie the Oruanui Ash. The hydrothermal breccia that crops out along Te Kopia Rd is on Except inside the presently active areas, altered average -30 cm thick. The breccia is unusual in the Oruanui Ash does not occur in the study area. This large variation of its clast lithology and alteration. indicates that the more widespread thermal activity The breccia contains clasts of silicified wood, silica had ceased before the 22.6 Ka Taupo eruption. sinter, surge deposits, silicified containing Whether the cessation of the widespread activity pyrite, acid altered tuffs, near neutral altered tuffs, and the 22.6 Taupo eruption are related is and clasts of other breccias. The hydrothexmal unclear. What is clear, however, is that there was a breccia that occurs south of Trig H, overlies acid change in hydrology prior to the cessation of altered Huka sediments and has a maximum activity. Silica veins hosted by soft acid-sulphate thickness of 2.5 m.. The breccia thickens to the altered ignimbrite indicate an acid overprint of west and therefore the eruption centre probably lies earlier near neutral alteration. to the west away from Te Kopia thermal area.

274 LEGEND Low intensity acid

acid alteration

Acid overprint

Debris flow

3. Outcrop map of surface alteration between Orakeikorakoand Te Kopia thermal areas.

There are two possible reasons for the change in 2. There was a drop in the water table, as indicated the widespread thermal activity. The system may by acid overprinting, that may have been related to have experienced reduced permeability due to movements of the Paeroa Fault. mineral deposition. (1994) noted the siltstones of the Huka Group have low permeability 3. The change in thermal activity was probably due and may act as an aquitard. Fluid must either travel to either self sealing or migration of thermal laterally along more permeable lithologies or activity along the Paeroa Fault. upwards through faults and joints. If these permeable pathways are by mineral 4. Reports by early European visitors to the area of deposition, the system will essentially be “self more widespread activity along the fault scarp are sealed”. The other possibility is that the thermal not consistent with field evidence. system has simply migrated with time. This migration would be controlled by the Paeroa Fault 5.0 ACKNOWLEDGMENTS as it is the dominant structural feature of the area. I thank the Epithermal Mineralisation Research 4.0 CONCLUSIONS Unit (EMRU), Environment Waikato and Assoc. Prof. Freeston for funding that made this study 1. Thermal activity in the area was widespread possible. prior to the 22.6 Taupo eruption.

275 6.0 REFERENCES Volcanic Zone. thesis, Geology Dept. University Victoria, (unpub.). Bibby, H.M.,Bennie, S.L., Stagpoole, V.M., Caldwell, T.G., 1994, Resistivity structure of the MacKenzie, K.M., 1995,Evolution of the Te Kopia Waimangu, Waiotapu, Waikite and geothennal system, Taupo Volcanic Zone, New geothermal areas, Geothemics vol. 23, No. Zealand. thesis, Geology Dept. University 1. Auckland, (unpub.).

Bromley, C.J., 1992, Waikite - Te Kopia the Meade, H., 1870, A ride through the disturbed missing link? Proc. 14th NZ Geothenn Workshop, districts of ; together with some 217-222. account of the South Sea Islands. John Murray, London, Bromley, C.J., 1993, Tensor CSAMT study of the fault zone between Waikite ad Te Kopia Nairn, 1973, Geology of the Waimangu geothermal fields. J. Geomag. Geoelectr. 45, 887- geothermal mapping project area, Central Volcanic 896. Region, New Zealand Rep. Geol. Survey, DSIR,Lower (unpubl.). Grindley, 1959, Geologic Map of New Zealand Sheet N85,Waiotapu. Geol. Schoen, R., White, Hemley, 1974, Survey, DSIR, Wellington,New Zealand. Argillization by descending acid at Steamboat Springs, Nevada. Clays and Clay Minerals 22, 1- Grindley, G.W., 1963, Geology and structure of 22. Waiotapu geothennal field. In: Waiotapu Geothermal Field. Bulletin DSIR,Wellington, Schofield, 1965, The Hinuera Formation and New Zealand; 10-25 associated Quaternary events. New Zealand Journal of Geology and Geophysics8,772-79 Grindley, G.W., T.C., Kohn, B.P., 1994, Stratigraphy,paeleomagnetism, geochronology and Steiner, 1963, The rocks penetrated by structure of silicic volcanic rocks, drillholes in the Waiotapu thermal area, and their Range Area, New Zealand. Geothennics Vol. 23, hydrothermal alteration. DSIR Bull. 155, 643- No. 646.

Healy ,J., Schofield, J.C., Thompson, B.N., 1964, Wilson, C.J.N.,1985, The Taupo Eruption, New Geologic Map of New Zealand Sheet 5, Zealand; The Taupo Ignimbrite. Phil. Trans R. , Geol. Survey, DSIR,Wellington,New SOC. A 314.299-310 Zealand. Wilson, C.J.N., N.N., Bradley, J., Hedenquist, J.W., 1983, Waiotapu, New Zealand: Walker, G.P.L., 1980, A new date for the Taupo The geochemical evolution and mineralisation of eruption, New Zealand. Nature, 288, an active hydrothermal system. thesis, 252-253. Geology Dept. University of Auckland, (unpubl.) . Wilson, V.R., Ward, A.P., 1988, A new age for the Oruanui (Wairakei) eruption, Hochstetter, F. von, 1864, von Neu- New Zealand. GeologicalMagazine 125,297-300. transl. C. A. Govt. Printer, Wellington,NZ, Wood, C.P., 1994, Aspects of the geology of Waimangu, Waiotapu, Waikite and Reporoa Houghton, B.F.,Wilson, C.J.N., McWilliams, M., Geothermal Systems, Taupo Volcanic Zone, New Lanphere, M.A., Weaver, S.D., Briggs, R.M., Zealand. Geothemics Vol.23, No. M.S.,1995, Chronology and dynamics of a large silicic magmatic system: central Taupo Volcanic Zone,New Zealand. Geology

Keall, J.M., 1988, Volcanology and ignimbrite stratigraphy along the Paeroa Fault, Taupo

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