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Surface hydrothermal alteration and evolution of the Te Kopia Thermal Area, New Zealand
Geothermics Vol. 23, No. 5/6, pp. 645-658, 1994 CNR Elsevier Science Ltd Printed in Great Britain 0375-6505/94 $7.00 + 0.00
Pergamon
0375-6505(94)00040-9
SURFACE HYDROTHERMAL ALTERATION AND EVOLUTION OF THE TE KOPIA THERMAL AREA, NEW ZEALAND G B I G N A L L and P R L B R O W N E
Geothermal Institute and Geology Department, The University of Auckland, P 0 Box 92019, Auckland, New Zealand Abstract - The Te Kopia Thermal Area has surface manifestations that extend over an area of about 3 x 1.2 km along the Paeroa Fault Zone. Steaming ground, fi!maroles, mud and acid sulphate pools are present on both the upthrown and downthrown blocks of the Paeroa Fault, but near-neutral pH chloride-bicarbonate springs discharge up to 2 km west of the fault trace. The host rocks comprise mainly Quaternary ignimbrites that dip gently (-7*) eastwards but are vertically displaced by the fault by several hundred metres. Hydrothermal alteration is widespread, and the present thermal activity is now producing kaolinite, alunite, silica residue, hematite and cristobalite. This assemblage occurs both in the otherwise unaltered iguimbrites, as well as overprinting the products of earlier hydrothermal activity. Evidence for changes that have occurred in the nature and extent of thermal activity at Te Kopia include: (1) The widespread overprint ofa kaolinltealunlte-cristobalite assemblage upon hydrothermal minerals (including adnlaria, quartz and mordeuite) produced by alkali-chloride, bicarbonate-chloride, and heated groundwaters. (2) The occurrence of silica sinter which deposited from alkali chloride springs that discharged about 3000 years ago. This is now a steam zone. (3) Areas of cold ground that were once hot, as demonstrated by the presence of hydrothermal minerals. (4) The presence of eubedral quartz crystals at the surface on the upthrown fault block. Fluid inclusions in these crystals from two places homogenise at 188 +15°C and 196 :I:II°C. The trapped fluids have apparent salinities between 0.2 and 0.4 wt % NaCI equivalent. The quartz crystals thus grew at depths of at least 120 m below the water table. Vertical movements along the Paeroa Fault totalling at least 300 m have uplifted these crystals to their present positions. The surface geology and alteration provide evidence that thermal activity at Te Kopia has been long lived, possibly as long at 120 000 years. However, the hydrology of the system and its thermal regime have changed greatly during its lifetime, mainly in response to movements along the Paeroa Fault. KEYWORDS Hydrothermal alteration; fluid inclusion geothermetry; alteration over-printing; geothermal hydrology. INTRODUCTION Geothermal systems serve as natural laboratories where fluid/rock interactions occur. However, we still know little about the timespans of geothermal systems and how they change during their lifetimes. We can recognise some of the changes in extinct systems, such as in epithermal ore deposits, by deducing the chronology of cross-cutting veins and the textural relations of hydrothermal minerals. The Te Kopia Geothermal System is a large, still vigorously active geothermal system whose thermal history can be partly deduced from its hydrothermal alteration and geology. In addition, it offers a rare opportunity to learn about the temporal relationships between the Quaternary ignimbrites that host the geothermal system, faulting, hydrothermal alteration, fluid-rock interactions, and events such as landsliding and hydrothermal eruptions. In this paper, however, we describe the Te Kopia Geothermal System, mainly in terms of its evolution, based upon a study of its surficial hydrothermal alteration. 645
646
G. Bignall and P. R. L. Browne
The Te Kopia Geothermal Field is located on the scarp and at the base of the Paeroa Fault (Fig. 1). It lies about mid way between Orakeikorako and Waikite, both presently-active thermal areas. Hochstetter (1864) recorded almost continuous thermal activity along the Paeroa Fault between these two places, although this is not the situation now. Bignali (1994), however, showed that there is likely to be a single parent thermal fluid for both the Orakeikorako and Te Kopia geothermal systems. Several geophysical surveys have examined the likelihood of a hydrological connection between the Te Kopia and Waikite thermal areas. These include a controlled source Audio-Magnetic Telluric Survey, DC resistivity measurements, and an airborne video thermal infrared survey (Bromley, 1992). These surveys extended earlier resistivity soundings and regional traversing measurements made by Macdonald (1965) and a low-level aeromagnetic survey (Merchant, 1990). I
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The Ngatamariki-Orakeikorako-Te Kopia-Waikite area showing the roads, geothermal fields and locations of the four Orakeikorako and two Te Kopia drillholes; modified after Lloyd (1972).
Hydrothermal Alteration and Evolution, Te Kopia Thermal Area
647
Hochstetter (1864), Grange (1937), and Healy (1952, 1974) briefly described some of the thermal activity at Te Kopia. Grange (1937) also gave analyses of several springs and pools, and Mahon (1965a) and Sheppard and Klyen (1992) also report water and gas compositions. Two wells were drilled at Te Kopia (Fig. 1) in 1965-66 to depths of 944.9 m (TK-1) and 1250.4 m (TK-2); the subsurface stratigraphy of the field has been reported by Grindley (1965) and Bignall (1991, 1994), and its fluid chemistry by Mahon (1965b).
Geology of the Te Kopia Thermal Area The geology of the Te Kopia area is greatly influenced by its location near the margin of the Maroa Volcanic Centre, from which much of the pyroclastic material at the surface and encountered in drillholes probably derived. The host rocks at Te Kopia mainly comprise three southeast-dipping ignimbrite sheets. These consist of non-welded rhyolitic pumice lapilli tuffs that correlate, from top to bottom, with the Paeroa, Te Weta and Te Kopia ignimbrites, also exposed on the scarp north of the thermal area and encountered in the Te Kopia drillholes (Steiner, 1977; Bignall, 1994). Stratigraphic and structural studies in the Te Kopia-Waikite areas suggest that the eruption of the Paeroa Ignimbrite and subsequent collapse of the Maroa Volcanic Centre was followed by a period of uplift which occurred along west and northwesterly-striking faults. A later phase of normal faulting continues still. The Paeroa Fault is normal, strikes about 040 °, and is downthrown to the northwest. The maximum scarp height on the Paeroa Fault is 480 m; however, at Te Kopia it is only 220 m above the valley floor. The fault has a minimum vertical offset of 450 m, with downthrow to the northwest as indicated by topography, although Keall (1987) estimated that the Paeroa Ignimbrite has been downfaulted by 600 m. The eastern block is tilted about 7 ° to the southeast. Although the Paeroa Fault is the dominant structural feature in the Te Kopia area, several northwest-striking lineaments are also visible on aerial photographs. A combined structure and alteration map of the main thermal area (Fig. 2) shows the faults and lineaments. Their intersections may provide higher permeability for ascending fluids, and the locations of thermal features coincide with some fault intersections. Martin (1961) showed that Kaingaroa Ignimbrite occurs on both sides of the Paeroa Fault north of Te Kopia, which indicates that faulting has occurred since its emplacement about 240 thousand years ago (Nairn et al., 1994). In places, the Paeroa Fault Scarp is stepped and forms a composite scarp (Wallace, 1977), suggesting multiple sites of displacement. Nairn and Hull (1986) also found increasing vertical separation of Taupo Pumice (AD 186), Rotoma Ash ( - 9 0 0 0 BP) and Earthquake Flat Breccia (<42 ka) across the Paeroa Fault Zone. This indicates that faulting was still active 1800 years ago. Assuming that movement on the Paeroa Fault began at about 240 ka (Nairn et al., 1994), then the average rate of displacement across the fault zone is 2.5 m/1000 years. Hydrothermal eruption breccias occur in the vicinity of the Northern Lakes up to 250 m from their inferred craters. They are typically poorly sorted and contain subangular to angular blocks of hydrothermally altered tuff, ignimbrite and rhyolite lava supported by a fine-grained, clay-rich ash matrix. Ignimbrite clasts, similar in appearance to Paeroa Ignimbrite, are present and indicate that rocks were erupted from depths of at least 200 m. At least two eruption breccias younger than 22,600 years also occur in road cuttings interbedded with landslide breccias, themselves containing hydrothermally-altered rock clasts. Eruptions and landsliding were probably initiated by movements on the Paeroa Fault. Landslide deposits both pre- and post-date the Taupo volcanic eruption of 186 AD.
648
G. Bignall and P. R. L. Browne
KEY Thermal area Altered ground Hyckothermal eruption
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Structural features and distribution of thermal manifestations in the Te Kopia area. Fault traces and other lineaments were identified from aerial photographs taken in 1991 (SN C9160, A/4 and A/5). Spring numbers shown are the same as those used by Sheppard and Klyen (1992).
Present-day thermal activity at Te Kopia The most obvious thermal manifestations at Te Kopia are the extensive areas of altered and steaming ground that occurs along 2.5 km of the Paeroa Fault scarp (Fig. 3). Many places on the scarp, however, are barren, with grey to white clays and red-brown iron oxide staining. Numerous fumaroles occur at the base and on the crest of the scarp. Another type of thermal manifestation occurs in two "pond" areas, the "Northern Lakes" and the "Central Lakes" (Fig. 4). The ponds are shallow, perched steam-heated acid-sulphate waters with negligible surface discharge that occur in sub-circular depressions at the foot of the scarp. Most have chloride contents from < 2 to 40 mg/kg and bicarbonate contents from below detection to 19 mg/kg. Sulphate contents range from 21 to 1130 mg/kg (Table 1). Barren ground around the pools (and on the scarp) is characterised by a pale grey-coloured, "crusty" relict pumiceous silica residue (Fig. 5a). The warm (45°C to 60°C) pools are bubbling, kaolin lined, and commonly are covered by a film of kaolin. Dry collapse holes, mudholes (some containing mud volcanoes, Fig. 5b), small pools of acid-sulphate water
Hydrothermal Alteration and Evolution, Te Kopia Thermal Area
649
Fig. 3: Paeroa Fault Scarp at Te Kopia (Central Lakes area). Note extensive barren ground (kaolinitealunite alteration) and stunted vegetation. Warm acid sulphate pools are located at the foot of the scarp, behind the trees. and minor fumaroles, are interspersed among the lakelets. In the "Northern Lakes" area, a mud geyser, first reported by Hochstetter (1864), also occurs. The acid-sulphate springs (Fig. 5c) originate from steam heating of groundwaters where ascending HzS oxidises to H2SO, in the vadose zone. A sub-circular depression visible in the field and on aerial photographs (Fig. 2) is believed to have been the site of a hydrothermal eruption, since eruption breccias occur on its western margin. Several small ( < 10 m diameter), crater-like features, some hosting mud pools, occurring in the "Northern Lakes" area may also be hydrothermal eruption craters, although the breccia deposits nearby are difficult to distinguish from landslide and slump deposits. A third type of feature at Te Kopia consists of several neutral pH, - 45 °C, springs which discharge about 2 km northwest of the main thermal area (e.g., Murphy's Farm Spring, Table 1). Their flow rates differ (up to several l/s) but are difficult to measure since some springs occur in steep stream beds.
Hydrothermal alteration Altered rocks exposed over an area of - 5 km 2 provide evidence for the changing character of thermal activity at the Te Kopia field. Detailed mapping and examination of surface samples using transmitted light microscopy and X-ray diffractometry enabled four alteration types to be distinguished, each a product of a specific hydrothermal environment. They are: (a) kaolinite-alunite-cristobalite; (b) opal silicification; (c) weak clay-mordenite; (d) quartz-adularia-illite; (e) silicification.
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Hydrothermal Alteration and Evolution, Te Kopia Thermal Area
651
Clockwise from top left: Fig. 5a. Barren ground on the Paeroa Fault Scarp, characterised by pale grey, "crusty", pumiceous silica residue. Fig. 5b. Mud volcanoes, Central Lakes area. Fig. 5c. Acid-sulphate spring (pH 2.4, 94°C) on the crest of the Paeroa Fault Scarp. (a) Kaolinite-alunite-cristobalite A kaolinite dominated ( + opaline silica, alunite, cristobalite, jarosite, hematite, native sulphur) assemblage is widespread at Te Kopia. This type of alteration is a characteristic product in shallow zones of acidsulphate steam condensate where H2S, separated from a deeper chloride water, oxidises to H2SO, in the vadose zone. The kaolinite-altered rocks vary considerably in the amounts of amorphous silica that they contain. Matrix ash and pumice are mostly converted to a fine-grained kaolinite + opal. Primary plagioclase, hornblende and pyroxene phenocrysts have been readily leached, commonly leaving fine films or honeycombs of opaline silica, with cavities refilled with kaolinite + cristobalite + alunite. Hematite is commonly disseminated as tiny red-black grains; there is also a pervasive limonite staining and the iron oxides give the rocks a red-brown colour. Pyrite is rare but occurs as relict, disseminated grains towards the base of the fault scarp in a zone of silicified rock. Numerous planar surfaces on the scarp are coated by an opaline silica. The silica here has been leached from the host ignimbrites by the acid-sulphate waters. These waters then mixed with surface rainwater and re-deposited the silica at lower elevations. Some kaolinite-altered ignimbrites exposed on the scarp contain veins of opal and, rarely, opal + alunite. The veins are up to 4 cm wide and show a pronounced laminar morphology that may result from hydraulic fracturing events similar to those which occurred at the fossil Ohakuri hydrothermal system (Henneberger & Browne, 1988). The veins are irregular in width, discontinuous, and seldom cut pumice or other rock fragments. Many of the widest veins appear to follow older structures such as joints, although some occupy younger, randomly-orientated fractures. Some of the chalcedonic opaline vein material contains swirl-like textures that may have resulted from deformation while the silica was a gel.
G. Bignall and P. R. L. Browne
652
Table 1: Representative water analyses from Te Kopia
SPR1NO No. DATfl
Flow (l/s) T (°C) pH (18°C) Li+ Na+ K÷ Rb + Cs + Mg 2+ Ca 2+ StO 2 B NH 3 FCl" SO42" HCO3"(T) Fe
'45' 1926128
'81/186' 2616181
Southern Pool
ltrg¢ gray pool
middle pool
smlll pool
ted pool
weJt pool
2.7
Nil 89 2.1
Nil 62 2.9
<0.1 76 2.1
Nil 54 2.5
Ntl 48 2.5
0.04 7 9.5 0.04 <0.01 0.39 0.75 145
0.05 7 5.8 0.02 <0.01 0.73 2.2 99 <3 0.4 <0.01 5 115
<0.1 2 13 0.03 0.37 0.08 4.3 304 10 6.3 0,5 4 580
<0.1 14 25 0.13 0.19 0.08 7.3 376 <3 7.3 0.4 7 293
86.0 54.0
1.0 14.3 277.4 4.2 113.5 315.2
4 0.17 <2 1130
'81/190' 2616181
'8212' 1911182
'82/4' 19/1/82
11,2
q'K2' 1312/92
'TK4' 1312/92
'6997/8' 513/92
'6997112' 513/92
TK1 1965
Murpy'l Firm spdag|
hot spring
Drillhole diichar2c
Nil 49 2.4
7.5
4.1
241 9.5
0.38 27.4 18.3
0.36 27.3 20.3
0.41 86.0 3.4
0.12 47.0 30.4
3.1 254 40.2
0.099 0.93 5.6 265 0.39 1.6 1,5 33.6 312
0.096 0.91 5.5 263 0.37 1,5 1.6 32.3 318
7.3
7.2
0.096 0.40 3.7 140 0.90 0.08 5.7 89 18 56 <0.1
0.215 1.2 9.0 277 0.42 1.8 2,6 40 132 19 2.2
north pool
1.0 546 3.6 0.i 17 301 64 70
Atomic and molecular proportions Na/K Na/LI CI/B C1/F HCO3/CI C1/SO42"
2.7
1.3 52.8
2.1 42.3 >0.5
0.3 >6.0 0.1 4.3
1.0 >42.3 >0.7 9.4
2.5 21.8 26.3 12.0
2.3 22.9 26.6 10.8
0.1
0.0
0.1
0.3
0.3
<6.3 1.0
7.6 63.3 30.2 8.4 0.37 13.4
2.6 118.2 29.1 8.2 0.28 0.8
10.7 24,7 25.5 9.5 0.14 12.9
Data from Grange (1937), Mahon(1965a,b), DSIRChemistry Division records, and Sheppard and Klyen (1992). Drillhole TK-2 was neverdischarged. The spring locations, where known, are shown on Figure 2.
Kaolinite alteration is also intense at the foot of the scarp and around the perched, acid-sulphate pools. A kaolinite film floats on some pools. Lateral and vertical gradations of alteration intensity indicate that the acid-sulphate water followed variable flow paths, apparently independent of any structural control. (b) Opal silicification Small areas of decomposing opaline sinter occur where kaolinite is now forming. These sinters, up to 40 cm thick, are composed of layered white or jaspery silica. Some have incorporated ash and plant material. The most extensive sinter, near the "Central Lakes" (Fig. 4), overlies a kaolinite-altered tuff sequence and is inferred to have formed the lip of an alkali chloride or mixed alkali chloride-acid sulphate pool. Carbon 14 date on wood enveloped by the sinter at one such site gave an age of 3026 + 43 years BP (Fig. 6). (c) Weak clay-mordenite A zone of low-to-medium intensity alteration, characterised by poorly crystalline smectite and rare mordenite replacing primary crystals and pumice fragments, occurs south of the main thermal area. This is now an area without thermal expression that grades outwards into unaltered rocks.
Hydrothermal Alteration and Evolution, Te Kopia Thermal Area
653
Most plagioclases present throughout this zone are fresh although, where alteration intensity is highest, smectite has started to develop along cleavage and fracture planes. Kaolinite-alunite-cristobalite-type alteration overprints the weak clay-mordenite alteration zone near the top of the Paeroa Fault scarp, whereas relict quartz-adularia-illite alteration is present near its base. The weak clay-mordenite type alteration is interpreted as having formed where hot alkali-chloride water mixed with groundwater, although it may also have formed where conductively heated groundwaters reacted with the host rocks, as occurred at Ohakuri (Henneberger & Browne, 1988). The zone of transition between the quartz-adularia-illite and weak claymordenite types of alteration is narrow, as no intermediate alteration types suggestive of progressive waterrock interactions were found. (d) Quartz-adularia-illite The quartz-adularia-illite zone is the highest alteration rank in exposures at Te Kopia, although it is now almost completely overprinted by kaolinite-alunite-cristobalite alteration. The zone terminates south of the main thermal area with scattered outcrops of (relict) illite-bearing rocks. The vertical zonation from weak clay to quartz-adularia-illite alteration observed in driilcores suggests that the southern (cold) area may be underlain by the quartz-adularia-illite altered rocks. Plagioclase phenocrysts in this zone appear to have been initially replaced by adularia and illite; the latter forms small patches in the feldspar but has subsequently been mostly altered to kaolinite -t- alunite and cristobalite. Hornblende and pyroxene grains have been converted to chlorite and leached later and/or replaced by kaolinite. Pumice clasts have typically recrystallised into a mozaic of subhedral quartz, although most clasts show characteristic kaolinite alteration. (e) Silicification Ignimbrite at the foot of the Paeroa Fault scarp has been cemented in places with quartz and minor pyrite. Silicification intensity is slight to moderate with the rock matrix typically converted to a fine-grained mozaic of quartz.
Fig. 6:
Wood enveloped in sinter. The sample was found in a landslide deposit at the foot of the Paeroa Fault Scarp, Central Lakes area, Te Kopia. Sample is about 14 cm long.
654
G. Bignall and P. R. L. Browne
EVOLUTION OF THE TE KOPIA GEOTHERMAL FIELD Evidence for changes occurring in the hydrology of the Te Kopia geothermal system are: Overprint alteration textures
The dominant alteration mineralogy produced by the deep chloride fluid seen in drillcores recovered from below - 400 m depth in TK- 1 and below - 650 m in TK-2 consists of varying proportions of quartz, albite, adularia, illite, chlorite, calcite, pyrite, pyrrhotite, zeolites and epidote (Bignall, 1994).
Clockwise from left: Fig. 7a. Block in landslide deposit on the Paeroa Fault Scarp, containing prismatic quartz crystals hosting fluid inclusions. Fig. 7b. Relict "stockwork" quartz veins in a block on the Paeroa Fault Scarp. Fig. 7c. Relict "stockwork" with quartz veins near top of fault scarp at Te Kopia. Quartz-adularia-illite alteration present in rocks now at the surface is therefore interpreted to have formed from water-rock interactions within the geothermal reservoir. In many exposures, kaolinite-bearing assemblages have overprinted earlier quartz-adularia-illite type alteration. This overprinting is commonly complete, with pumice clasts and secondary minerals formed earlier from neutral pH water being replaced by kaolinite. The most strongly overprinted rocks have been converted to a puggy quartz + kaolinite + cristobalite + alunite assemblage. Landsliding on the Paeroa Fault
Tephra layers as old as 22 600 years separating landslide deposits indicate that movements on the Paeroa Fault have likely triggered large landslides in and outside the thermal area. These deposits contain clasts of different ignimbrites still exposed on the scarp, although the alteration mineralogy (including quartz and adularia) of some of the clasts was produced by alkali-chloride waters that do not now discharge there.
Hydrothermal Alteration and Evolution, Te Kopia Thermal Area
655
A clast in one such deposit has a planar surface with abundant euhedral quartz crystals which contain fluid inclusions (Fig. 7a), while "bladed quartz" also occurs. The latter is interpreted to result from boiling causing bladed calcite to deposit which was then replaced by quartz as cooling occurred.
Fluid inclusion measurements Quartz crystals containing measurable fluid inclusions occur at two localities. A slump/landslide deposit 180 ___ 5 m above the base of the Paeroa Fault, 600 m east of TK-1, contains blocks of tuff up to 15 cm diameter. These host abundant hydrothermal quartz crystals up to 8 mm long, which contain numerous fluid inclusions (Fig. 7). Abundant euhedral quartz crystals, up to 35 mm long, occur on an upstanding ridge 900 m northeast of TK1 at an elevation of 145 + 5 m above the valley floor. The crystals litter the ground (Fig. 7b), have collected in shallow depressions, and derive from a formerly extensive quartz vein system now being corroded here by acidic waters. The crystals host abundant fluid inclusions. Relict "stockwork" quartz veining is exposed at this locality (Fig. 7c). The veinlets are typically about a miilimetre wide and trend 130 ° (i.e., subparallel to the ridge), but lack measurable fluid inclusions. Most fluid inclusions appear to have formed along healed fractures and are thus secondary. Few of the host hydrothermal minerals have growth zones, and daughter minerals are absent. Inclusions range from 5 to 20/~m in length but are typically from 2 to 8 #m wide. Homogenisation temperatures (Th) were measured on 25 liquid-rich fluid inclusions in quartz from the location 600 m east of TK-1 (Fig. 2). No vapour inclusions were seen. Some inclusions are indeed primary, being isolated and up to 100 #m long. Some have negative crystal forms and do not occur in healed fractures. Their mean Th value is 188 + 15°C. The fluid inclusion data, when plotted on a boiling curve for pure water, indicate fluid entrapment at least 125 m below the water table, so the crystals are deduced as having been uplifted to their present elevation by movements on the Paeroa Fault totalling at least 300 m. Freezing measurements made on 9 inclusions gave Tm values from -0.2 to -0.1 °C (i.e., apparent salinities of 0.4 to 0.2 wt % NaC1 equivalent), similar to values for inclusions in subsurface samples (Bignall, 1994). Th measurements were also made on 14 liquid-rich inclusions in quartz from the sample locality northeast of TK-1 (Fig. 2). Vapour-rich inclusions occur here. These inclusions are up to 80/zm long and occur in zones; their mean Th is 196 + l l ° C . This indicates their entrapment at least 170 m below the water table, suggesting that they have been relatively uplifted along the Paeroa Fault by at least 315 m to reach their present elevation, now - 145 -I- 5 m above the base of the fault scarp.
Occurrence of silica sinter Several deposits of decomposing silica sinter occur in the vicinity of the acid-sulphate pools. Sinter is not now forming, however, so the system has clearly changed. These changes are consistent with a lowering of the water-table whereby neutral pH, alkali-chloride waters were replaced by steam and by acid-sulphate fluids.
Occurrence of cold altered ground Areas of cold ground occur to the south of the main thermal area. Tuff there contains a relict mordenitesmectite assemblage overprinted by kaolinite + alunite. This likely results from both cooling and a change in the pH of the altering fluid from near neutral to acidic.
G. Bignall and P. R. L. Browne
656
• MODEL FOR THE EVOLUTION OF THE TE KOPIA GEOTHERMAL FIELD The stratigraphic relationships, alteration mineralogy, textures and fluid inclusion results record several stages of thermal activity during the evolution of the Te Kopia Geothermal Field (Fig. 8); many events have likely gone unrecorded. Differences in the types of surface alteration preserved from different stages of activity reflect changes in the hydrological conditions and the depths at which fluid-rock interactions occurred. The evolution of the Te Kopia Geothermal Field is deduced from the observations described earlier and by analogy with the alteration mineralogy of geothermal reservoirs explored elsewhere in the Taupe Volcanic Zone (Browne, 1978). The following sequence of events is inferred. W
E initiation of NE trending faults
hot springs?
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Development of hydcothermal system at "re Kepis along E-W (snd younger NE-SW) trending normal faults. The active hydrothermsl system pre~ates recent movement on the NE trending Paeroa Fault. which exposed 320ks Paerce IgeimbrRe. W'
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Changes In the surface expression accomDankad movement on the Paeroa Fault - developrnent of kaolinite-style alteration above the local wster table. overprinting of neutral pH type alteration assemblages. W
over~lntlng kaolinite alteration Murphys' spring
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t
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Z ~ / ~ ~ k~lowenng wet ertal~e
".~#
upf~aulted __
Now
Lowerl~lg of the water treble wtth contklued fault movement. Io woduce the preset| surface e x l 0 r ~ o n of the hydrothermal system. Movernent on the Paeroe Fault hss triggered recent hydrothermal eruptions.
Fig. 8: Inferred stages of hydrothermal activity at Te Kopia: 1. Early alkali-chloride dominant (c. 120 000 years ago). 2. Acid-sulphate dominant. 3. Present hydrology.
Hydrothermal Alteration and Evolution, Te Kopia Thermal A r e a
657
Alteration produced by waters o f alkali-chloride composition
Zones of relict quartz-adularia-illite and weak clay-mordenite alteration now at the surface formed within the geothermal reservoir during an early stage of thermal activity, when ascending near-neutral pH, alkalichloride water reacted with the host ignimbrites. A hot ( > 2 2 0 ° C ) fluid produced the high rank quartzadularia-illite alteration in the central part of the field. The peripheral and overlying weak clay (smectite)mordenite alteration zone formed by reaction of the rocks with either heated groundwater (as occurred at Ohakuri) or with water that resulted from the mixing of hot, upwelling alkali-chloride water with cool groundwater. The nature of surface activity at Te Kopia during this early stage is unknown, partly because of erosion (including landsliding), but mainly due to changes in the hydrology and structure of the system caused by movements on the Paeroa Fault. The age of the alteration produced by the alkali-chloride waters is uncertain. Keall (1987) estimated that the Paeroa Ignimbrite had been offset vertically by 600 m. Since this ignimbrite is now known to be 240 thousand years old (Nairn et al., 1994), then the average rate of displacement since its eruption is 2.5 m/1000 years. As the minimum vertical offset of the altered rocks is deduced to be 300 m, then the Te Kopia Geothermal System is at least 120 thousand years old. Alteration produced by acid-sulphate waters
Each movement on the Paeroa Fault was probably followed by an expansion of the kaolinite-style alteration as the thermal water-table descended in the upthrown block. The acid-sulphate waters altered fresh rocks and overprinted some with quartz-adularia-illite, silicification and weak clay-mordenite alteration, all of which formed during the first stage of activity. Rocks below the piezometric level, however, continued to interact with neutral pH, alkali-chloride fluids. The maximum age of the acid alteration at Te Kopia is unknown but it is very likely that some was contemporaneous with quartz-adularia-illite alteration occurring within the reservoir. The decomposing 3026 + 43 years BP opaline sinter indicates that chloride water was then discharging at the base of the fault scarp, concurrently with acid-sulphate alteration at higher elevations. Hot spring activity today continues several hundred metres west of the Paeroa Fault but scattered sinters and weakly silicified tuffs testify that neutral pH, alkali-chloride waters once discharged along the fault trace.
Acknowledgments - The Waikato Regional Council provided financial support to GB. We thank Dr J. McLeod for his assistance, and Mr Murphy, Te Kopia, for permission to investigate the thermal features on his property. Dr C.P. Wood and Dr J.W. Hedenquist gave very helpful reviews; we thank them both.
REFERENCES Bignall, G., 1991. Subsurface stratigraphy and structure of the Orakeikorako and Te Kopia geothermal systems, New Zealand. Proc. 13th NZ Geothermal Workshop, 199-205. Bignall, G., 1994. Thermal evolution and fluid-rock interactions in the Orakeikorako-Te Kopia Geothermal System, Taupo Volcanic Zone, New Zealand. Phi) Thesis. University of Auckland, NZ (unpubl). Bromley, C.J., 1992. Waikite-Te Kopia: The missing link? Proc. 14th IV'-ZGeothermal Workshop, 217-222. Browne, P.R.L., 1978. Hydrothermal alteration in active geothermal fields. Ann. Rev. Earth Planet. Science 6, 229-250. Grange, L.I., 1937. The Geology of the Rotorua-Taupo subdivision, Rotorua and Kaimanawa subdivisions. NZ Geol. Survey Bulletin 37. Grindley, G,W., 1965. Subsurface geology of the Orakeikorako and Te Kopia geothermal fields. N'Z Geol. Survey, Geol. Geothermal Report 3. Healy, J., 1952. Te Kopia hot springs. Unpublished DSIR Report, GS 23/2. Healy, J., 1974. Te Kopia Geothermal Field. N-ZGeol. Survey Report 38, Minerals of New Zealand, Part D, Section 3.14: Geothermal Resources. DSIR, Wellington. Henneberger, R.C. and P.R.L. Browne, 1988. Hydrothermal alteration and evolution of the Ohakuri hydrothermal system, Taupo Volcanic Zone, New Zealand. J. Volc. Geotherm, Res. 34, 211-231.
658
G. B i g n a l l and P. R. L. B r o w n e
Hochstetter, F. von, 1864. Geologie von Neu Seeland: Beitrage zur Geologie der Provinzen Auckland und Nelson. NovaraExped. Geol. Theil. 1 (1): 274 pp. (Translated and edited by C.A. Fleming, 1959. Government Printer, Wellington, 320 pp.) Keail, J.M., 1987. Volcanic stratigraphy and hydrothermal alteration along the Paeroa Fault. Unpublished MSc thesis, Victoria University, Wellington, NZ (tmpubl). Lloyd, E.F., 1972. Geology and hot springs of Orakeikorako. N~ZGeol. Survey Bulletin 85, DSIR, Wellington, NZ. Macdonald, W.J.P., 1965. A preliminary study of electrical soundings made at Orakei Korako and Te Kopia. Geothermal Circular WJP Mac.D3., Geophysics Division, DSIR, Wellington, NZ (unpubl.). Mahon, W.A.J., 1965a. A chemical survey of the Waikite, Pankohurea and Te Kopia thermal springs. Report CD 118/12 WAJM/25. Chemistry Division, DSIR, Wellington, NZ (unpubl.). Mahon, W,A.J., 1965b. The chemistry of fluids discharged from Hole 3, Orakeikorako (later changed to Hole 1, Te Kopia). Report CD 118/12 - WAJM/29. Chemistry Division, DSIR, Wellington, NZ (unpubl.). Martin, R.C., 1961. Stratigraphy and structural outline of the Tanpo Volcanic Zone. NZ J. Geol. Geophys. 4, 449-478. Merchant, R.J., 1990. Final report on PL312183, Te Weta. Open File Mining Company Report. IGNS No. MR1511. Inst. Geol. Nuclear Sciences, Lower Hutt, NZ. Nalm, I.A. and A.G. Hull, 1986. Post-1800 years B.P. displacement of the Paeroa Fault zone, Tanpo Volcanic Zone. NZ Geol. Survey Record 8, 135-142. Nairn, I.A., C.P. Wood, and R.A. Bailey, 1994. The Reporoa Caldera, Taupo Volcanic Zone: Source of the Kaingaroa Ignimbrites. Bull. Volcanol. (in press). Sheppard, D.S. and L.E. Klyen, 1992. The Te Kopia geothermal field 1979-1992. DSIR Chemistry Division Report CD9040. Steiner, A., 1977. The Wairakei geothermal area, North Island, New Zealand: its subsurface geology and hydrothermal rock alteration. ArZ Geol. Survey Bulletin 90, 136 pp. Wallace, R.E., 1977. Profiles and ages of young fault scarps, north-central Nevada. Geol. Soc. Amer. Bull. 88, 1267-1281.
Pergamon
0375-6505(94)00040-9
SURFACE HYDROTHERMAL ALTERATION AND EVOLUTION OF THE TE KOPIA THERMAL AREA, NEW ZEALAND G B I G N A L L and P R L B R O W N E
Geothermal Institute and Geology Department, The University of Auckland, P 0 Box 92019, Auckland, New Zealand Abstract - The Te Kopia Thermal Area has surface manifestations that extend over an area of about 3 x 1.2 km along the Paeroa Fault Zone. Steaming ground, fi!maroles, mud and acid sulphate pools are present on both the upthrown and downthrown blocks of the Paeroa Fault, but near-neutral pH chloride-bicarbonate springs discharge up to 2 km west of the fault trace. The host rocks comprise mainly Quaternary ignimbrites that dip gently (-7*) eastwards but are vertically displaced by the fault by several hundred metres. Hydrothermal alteration is widespread, and the present thermal activity is now producing kaolinite, alunite, silica residue, hematite and cristobalite. This assemblage occurs both in the otherwise unaltered iguimbrites, as well as overprinting the products of earlier hydrothermal activity. Evidence for changes that have occurred in the nature and extent of thermal activity at Te Kopia include: (1) The widespread overprint ofa kaolinltealunlte-cristobalite assemblage upon hydrothermal minerals (including adnlaria, quartz and mordeuite) produced by alkali-chloride, bicarbonate-chloride, and heated groundwaters. (2) The occurrence of silica sinter which deposited from alkali chloride springs that discharged about 3000 years ago. This is now a steam zone. (3) Areas of cold ground that were once hot, as demonstrated by the presence of hydrothermal minerals. (4) The presence of eubedral quartz crystals at the surface on the upthrown fault block. Fluid inclusions in these crystals from two places homogenise at 188 +15°C and 196 :I:II°C. The trapped fluids have apparent salinities between 0.2 and 0.4 wt % NaCI equivalent. The quartz crystals thus grew at depths of at least 120 m below the water table. Vertical movements along the Paeroa Fault totalling at least 300 m have uplifted these crystals to their present positions. The surface geology and alteration provide evidence that thermal activity at Te Kopia has been long lived, possibly as long at 120 000 years. However, the hydrology of the system and its thermal regime have changed greatly during its lifetime, mainly in response to movements along the Paeroa Fault. KEYWORDS Hydrothermal alteration; fluid inclusion geothermetry; alteration over-printing; geothermal hydrology. INTRODUCTION Geothermal systems serve as natural laboratories where fluid/rock interactions occur. However, we still know little about the timespans of geothermal systems and how they change during their lifetimes. We can recognise some of the changes in extinct systems, such as in epithermal ore deposits, by deducing the chronology of cross-cutting veins and the textural relations of hydrothermal minerals. The Te Kopia Geothermal System is a large, still vigorously active geothermal system whose thermal history can be partly deduced from its hydrothermal alteration and geology. In addition, it offers a rare opportunity to learn about the temporal relationships between the Quaternary ignimbrites that host the geothermal system, faulting, hydrothermal alteration, fluid-rock interactions, and events such as landsliding and hydrothermal eruptions. In this paper, however, we describe the Te Kopia Geothermal System, mainly in terms of its evolution, based upon a study of its surficial hydrothermal alteration. 645
646
G. Bignall and P. R. L. Browne
The Te Kopia Geothermal Field is located on the scarp and at the base of the Paeroa Fault (Fig. 1). It lies about mid way between Orakeikorako and Waikite, both presently-active thermal areas. Hochstetter (1864) recorded almost continuous thermal activity along the Paeroa Fault between these two places, although this is not the situation now. Bignali (1994), however, showed that there is likely to be a single parent thermal fluid for both the Orakeikorako and Te Kopia geothermal systems. Several geophysical surveys have examined the likelihood of a hydrological connection between the Te Kopia and Waikite thermal areas. These include a controlled source Audio-Magnetic Telluric Survey, DC resistivity measurements, and an airborne video thermal infrared survey (Bromley, 1992). These surveys extended earlier resistivity soundings and regional traversing measurements made by Macdonald (1965) and a low-level aeromagnetic survey (Merchant, 1990). I
I
SCALE
I
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:/
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~ 176~05'E
Fig. 1:
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..
u
1761"15'
The Ngatamariki-Orakeikorako-Te Kopia-Waikite area showing the roads, geothermal fields and locations of the four Orakeikorako and two Te Kopia drillholes; modified after Lloyd (1972).
Hydrothermal Alteration and Evolution, Te Kopia Thermal Area
647
Hochstetter (1864), Grange (1937), and Healy (1952, 1974) briefly described some of the thermal activity at Te Kopia. Grange (1937) also gave analyses of several springs and pools, and Mahon (1965a) and Sheppard and Klyen (1992) also report water and gas compositions. Two wells were drilled at Te Kopia (Fig. 1) in 1965-66 to depths of 944.9 m (TK-1) and 1250.4 m (TK-2); the subsurface stratigraphy of the field has been reported by Grindley (1965) and Bignall (1991, 1994), and its fluid chemistry by Mahon (1965b).
Geology of the Te Kopia Thermal Area The geology of the Te Kopia area is greatly influenced by its location near the margin of the Maroa Volcanic Centre, from which much of the pyroclastic material at the surface and encountered in drillholes probably derived. The host rocks at Te Kopia mainly comprise three southeast-dipping ignimbrite sheets. These consist of non-welded rhyolitic pumice lapilli tuffs that correlate, from top to bottom, with the Paeroa, Te Weta and Te Kopia ignimbrites, also exposed on the scarp north of the thermal area and encountered in the Te Kopia drillholes (Steiner, 1977; Bignall, 1994). Stratigraphic and structural studies in the Te Kopia-Waikite areas suggest that the eruption of the Paeroa Ignimbrite and subsequent collapse of the Maroa Volcanic Centre was followed by a period of uplift which occurred along west and northwesterly-striking faults. A later phase of normal faulting continues still. The Paeroa Fault is normal, strikes about 040 °, and is downthrown to the northwest. The maximum scarp height on the Paeroa Fault is 480 m; however, at Te Kopia it is only 220 m above the valley floor. The fault has a minimum vertical offset of 450 m, with downthrow to the northwest as indicated by topography, although Keall (1987) estimated that the Paeroa Ignimbrite has been downfaulted by 600 m. The eastern block is tilted about 7 ° to the southeast. Although the Paeroa Fault is the dominant structural feature in the Te Kopia area, several northwest-striking lineaments are also visible on aerial photographs. A combined structure and alteration map of the main thermal area (Fig. 2) shows the faults and lineaments. Their intersections may provide higher permeability for ascending fluids, and the locations of thermal features coincide with some fault intersections. Martin (1961) showed that Kaingaroa Ignimbrite occurs on both sides of the Paeroa Fault north of Te Kopia, which indicates that faulting has occurred since its emplacement about 240 thousand years ago (Nairn et al., 1994). In places, the Paeroa Fault Scarp is stepped and forms a composite scarp (Wallace, 1977), suggesting multiple sites of displacement. Nairn and Hull (1986) also found increasing vertical separation of Taupo Pumice (AD 186), Rotoma Ash ( - 9 0 0 0 BP) and Earthquake Flat Breccia (<42 ka) across the Paeroa Fault Zone. This indicates that faulting was still active 1800 years ago. Assuming that movement on the Paeroa Fault began at about 240 ka (Nairn et al., 1994), then the average rate of displacement across the fault zone is 2.5 m/1000 years. Hydrothermal eruption breccias occur in the vicinity of the Northern Lakes up to 250 m from their inferred craters. They are typically poorly sorted and contain subangular to angular blocks of hydrothermally altered tuff, ignimbrite and rhyolite lava supported by a fine-grained, clay-rich ash matrix. Ignimbrite clasts, similar in appearance to Paeroa Ignimbrite, are present and indicate that rocks were erupted from depths of at least 200 m. At least two eruption breccias younger than 22,600 years also occur in road cuttings interbedded with landslide breccias, themselves containing hydrothermally-altered rock clasts. Eruptions and landsliding were probably initiated by movements on the Paeroa Fault. Landslide deposits both pre- and post-date the Taupo volcanic eruption of 186 AD.
648
G. Bignall and P. R. L. Browne
KEY Thermal area Altered ground Hyckothermal eruption
\
/~.)/ \xMurphy's Farm
• ....""" . ~ . "':.> .
- \ Springs
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--
crater
Fault inferred . . . . Spring + pools (for water analyses see Table 1) x TK4 Geothermal well 0
2OO
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(6997/8)
(6997/12)
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-
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Fig. 2:
Structural features and distribution of thermal manifestations in the Te Kopia area. Fault traces and other lineaments were identified from aerial photographs taken in 1991 (SN C9160, A/4 and A/5). Spring numbers shown are the same as those used by Sheppard and Klyen (1992).
Present-day thermal activity at Te Kopia The most obvious thermal manifestations at Te Kopia are the extensive areas of altered and steaming ground that occurs along 2.5 km of the Paeroa Fault scarp (Fig. 3). Many places on the scarp, however, are barren, with grey to white clays and red-brown iron oxide staining. Numerous fumaroles occur at the base and on the crest of the scarp. Another type of thermal manifestation occurs in two "pond" areas, the "Northern Lakes" and the "Central Lakes" (Fig. 4). The ponds are shallow, perched steam-heated acid-sulphate waters with negligible surface discharge that occur in sub-circular depressions at the foot of the scarp. Most have chloride contents from < 2 to 40 mg/kg and bicarbonate contents from below detection to 19 mg/kg. Sulphate contents range from 21 to 1130 mg/kg (Table 1). Barren ground around the pools (and on the scarp) is characterised by a pale grey-coloured, "crusty" relict pumiceous silica residue (Fig. 5a). The warm (45°C to 60°C) pools are bubbling, kaolin lined, and commonly are covered by a film of kaolin. Dry collapse holes, mudholes (some containing mud volcanoes, Fig. 5b), small pools of acid-sulphate water
Hydrothermal Alteration and Evolution, Te Kopia Thermal Area
649
Fig. 3: Paeroa Fault Scarp at Te Kopia (Central Lakes area). Note extensive barren ground (kaolinitealunite alteration) and stunted vegetation. Warm acid sulphate pools are located at the foot of the scarp, behind the trees. and minor fumaroles, are interspersed among the lakelets. In the "Northern Lakes" area, a mud geyser, first reported by Hochstetter (1864), also occurs. The acid-sulphate springs (Fig. 5c) originate from steam heating of groundwaters where ascending HzS oxidises to H2SO, in the vadose zone. A sub-circular depression visible in the field and on aerial photographs (Fig. 2) is believed to have been the site of a hydrothermal eruption, since eruption breccias occur on its western margin. Several small ( < 10 m diameter), crater-like features, some hosting mud pools, occurring in the "Northern Lakes" area may also be hydrothermal eruption craters, although the breccia deposits nearby are difficult to distinguish from landslide and slump deposits. A third type of feature at Te Kopia consists of several neutral pH, - 45 °C, springs which discharge about 2 km northwest of the main thermal area (e.g., Murphy's Farm Spring, Table 1). Their flow rates differ (up to several l/s) but are difficult to measure since some springs occur in steep stream beds.
Hydrothermal alteration Altered rocks exposed over an area of - 5 km 2 provide evidence for the changing character of thermal activity at the Te Kopia field. Detailed mapping and examination of surface samples using transmitted light microscopy and X-ray diffractometry enabled four alteration types to be distinguished, each a product of a specific hydrothermal environment. They are: (a) kaolinite-alunite-cristobalite; (b) opal silicification; (c) weak clay-mordenite; (d) quartz-adularia-illite; (e) silicification.
0 [
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Fig. 4: Central Lakes area (left) and Northern Lakes area (right), Te Kopia (see Fig. 2).
KEY
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Hydrothermal Alteration and Evolution, Te Kopia Thermal Area
651
Clockwise from top left: Fig. 5a. Barren ground on the Paeroa Fault Scarp, characterised by pale grey, "crusty", pumiceous silica residue. Fig. 5b. Mud volcanoes, Central Lakes area. Fig. 5c. Acid-sulphate spring (pH 2.4, 94°C) on the crest of the Paeroa Fault Scarp. (a) Kaolinite-alunite-cristobalite A kaolinite dominated ( + opaline silica, alunite, cristobalite, jarosite, hematite, native sulphur) assemblage is widespread at Te Kopia. This type of alteration is a characteristic product in shallow zones of acidsulphate steam condensate where H2S, separated from a deeper chloride water, oxidises to H2SO, in the vadose zone. The kaolinite-altered rocks vary considerably in the amounts of amorphous silica that they contain. Matrix ash and pumice are mostly converted to a fine-grained kaolinite + opal. Primary plagioclase, hornblende and pyroxene phenocrysts have been readily leached, commonly leaving fine films or honeycombs of opaline silica, with cavities refilled with kaolinite + cristobalite + alunite. Hematite is commonly disseminated as tiny red-black grains; there is also a pervasive limonite staining and the iron oxides give the rocks a red-brown colour. Pyrite is rare but occurs as relict, disseminated grains towards the base of the fault scarp in a zone of silicified rock. Numerous planar surfaces on the scarp are coated by an opaline silica. The silica here has been leached from the host ignimbrites by the acid-sulphate waters. These waters then mixed with surface rainwater and re-deposited the silica at lower elevations. Some kaolinite-altered ignimbrites exposed on the scarp contain veins of opal and, rarely, opal + alunite. The veins are up to 4 cm wide and show a pronounced laminar morphology that may result from hydraulic fracturing events similar to those which occurred at the fossil Ohakuri hydrothermal system (Henneberger & Browne, 1988). The veins are irregular in width, discontinuous, and seldom cut pumice or other rock fragments. Many of the widest veins appear to follow older structures such as joints, although some occupy younger, randomly-orientated fractures. Some of the chalcedonic opaline vein material contains swirl-like textures that may have resulted from deformation while the silica was a gel.
G. Bignall and P. R. L. Browne
652
Table 1: Representative water analyses from Te Kopia
SPR1NO No. DATfl
Flow (l/s) T (°C) pH (18°C) Li+ Na+ K÷ Rb + Cs + Mg 2+ Ca 2+ StO 2 B NH 3 FCl" SO42" HCO3"(T) Fe
'45' 1926128
'81/186' 2616181
Southern Pool
ltrg¢ gray pool
middle pool
smlll pool
ted pool
weJt pool
2.7
Nil 89 2.1
Nil 62 2.9
<0.1 76 2.1
Nil 54 2.5
Ntl 48 2.5
0.04 7 9.5 0.04 <0.01 0.39 0.75 145
0.05 7 5.8 0.02 <0.01 0.73 2.2 99 <3 0.4 <0.01 5 115
<0.1 2 13 0.03 0.37 0.08 4.3 304 10 6.3 0,5 4 580
<0.1 14 25 0.13 0.19 0.08 7.3 376 <3 7.3 0.4 7 293
86.0 54.0
1.0 14.3 277.4 4.2 113.5 315.2
4 0.17 <2 1130
'81/190' 2616181
'8212' 1911182
'82/4' 19/1/82
11,2
q'K2' 1312/92
'TK4' 1312/92
'6997/8' 513/92
'6997112' 513/92
TK1 1965
Murpy'l Firm spdag|
hot spring
Drillhole diichar2c
Nil 49 2.4
7.5
4.1
241 9.5
0.38 27.4 18.3
0.36 27.3 20.3
0.41 86.0 3.4
0.12 47.0 30.4
3.1 254 40.2
0.099 0.93 5.6 265 0.39 1.6 1,5 33.6 312
0.096 0.91 5.5 263 0.37 1,5 1.6 32.3 318
7.3
7.2
0.096 0.40 3.7 140 0.90 0.08 5.7 89 18 56 <0.1
0.215 1.2 9.0 277 0.42 1.8 2,6 40 132 19 2.2
north pool
1.0 546 3.6 0.i 17 301 64 70
Atomic and molecular proportions Na/K Na/LI CI/B C1/F HCO3/CI C1/SO42"
2.7
1.3 52.8
2.1 42.3 >0.5
0.3 >6.0 0.1 4.3
1.0 >42.3 >0.7 9.4
2.5 21.8 26.3 12.0
2.3 22.9 26.6 10.8
0.1
0.0
0.1
0.3
0.3
<6.3 1.0
7.6 63.3 30.2 8.4 0.37 13.4
2.6 118.2 29.1 8.2 0.28 0.8
10.7 24,7 25.5 9.5 0.14 12.9
Data from Grange (1937), Mahon(1965a,b), DSIRChemistry Division records, and Sheppard and Klyen (1992). Drillhole TK-2 was neverdischarged. The spring locations, where known, are shown on Figure 2.
Kaolinite alteration is also intense at the foot of the scarp and around the perched, acid-sulphate pools. A kaolinite film floats on some pools. Lateral and vertical gradations of alteration intensity indicate that the acid-sulphate water followed variable flow paths, apparently independent of any structural control. (b) Opal silicification Small areas of decomposing opaline sinter occur where kaolinite is now forming. These sinters, up to 40 cm thick, are composed of layered white or jaspery silica. Some have incorporated ash and plant material. The most extensive sinter, near the "Central Lakes" (Fig. 4), overlies a kaolinite-altered tuff sequence and is inferred to have formed the lip of an alkali chloride or mixed alkali chloride-acid sulphate pool. Carbon 14 date on wood enveloped by the sinter at one such site gave an age of 3026 + 43 years BP (Fig. 6). (c) Weak clay-mordenite A zone of low-to-medium intensity alteration, characterised by poorly crystalline smectite and rare mordenite replacing primary crystals and pumice fragments, occurs south of the main thermal area. This is now an area without thermal expression that grades outwards into unaltered rocks.
Hydrothermal Alteration and Evolution, Te Kopia Thermal Area
653
Most plagioclases present throughout this zone are fresh although, where alteration intensity is highest, smectite has started to develop along cleavage and fracture planes. Kaolinite-alunite-cristobalite-type alteration overprints the weak clay-mordenite alteration zone near the top of the Paeroa Fault scarp, whereas relict quartz-adularia-illite alteration is present near its base. The weak clay-mordenite type alteration is interpreted as having formed where hot alkali-chloride water mixed with groundwater, although it may also have formed where conductively heated groundwaters reacted with the host rocks, as occurred at Ohakuri (Henneberger & Browne, 1988). The zone of transition between the quartz-adularia-illite and weak claymordenite types of alteration is narrow, as no intermediate alteration types suggestive of progressive waterrock interactions were found. (d) Quartz-adularia-illite The quartz-adularia-illite zone is the highest alteration rank in exposures at Te Kopia, although it is now almost completely overprinted by kaolinite-alunite-cristobalite alteration. The zone terminates south of the main thermal area with scattered outcrops of (relict) illite-bearing rocks. The vertical zonation from weak clay to quartz-adularia-illite alteration observed in driilcores suggests that the southern (cold) area may be underlain by the quartz-adularia-illite altered rocks. Plagioclase phenocrysts in this zone appear to have been initially replaced by adularia and illite; the latter forms small patches in the feldspar but has subsequently been mostly altered to kaolinite -t- alunite and cristobalite. Hornblende and pyroxene grains have been converted to chlorite and leached later and/or replaced by kaolinite. Pumice clasts have typically recrystallised into a mozaic of subhedral quartz, although most clasts show characteristic kaolinite alteration. (e) Silicification Ignimbrite at the foot of the Paeroa Fault scarp has been cemented in places with quartz and minor pyrite. Silicification intensity is slight to moderate with the rock matrix typically converted to a fine-grained mozaic of quartz.
Fig. 6:
Wood enveloped in sinter. The sample was found in a landslide deposit at the foot of the Paeroa Fault Scarp, Central Lakes area, Te Kopia. Sample is about 14 cm long.
654
G. Bignall and P. R. L. Browne
EVOLUTION OF THE TE KOPIA GEOTHERMAL FIELD Evidence for changes occurring in the hydrology of the Te Kopia geothermal system are: Overprint alteration textures
The dominant alteration mineralogy produced by the deep chloride fluid seen in drillcores recovered from below - 400 m depth in TK- 1 and below - 650 m in TK-2 consists of varying proportions of quartz, albite, adularia, illite, chlorite, calcite, pyrite, pyrrhotite, zeolites and epidote (Bignall, 1994).
Clockwise from left: Fig. 7a. Block in landslide deposit on the Paeroa Fault Scarp, containing prismatic quartz crystals hosting fluid inclusions. Fig. 7b. Relict "stockwork" quartz veins in a block on the Paeroa Fault Scarp. Fig. 7c. Relict "stockwork" with quartz veins near top of fault scarp at Te Kopia. Quartz-adularia-illite alteration present in rocks now at the surface is therefore interpreted to have formed from water-rock interactions within the geothermal reservoir. In many exposures, kaolinite-bearing assemblages have overprinted earlier quartz-adularia-illite type alteration. This overprinting is commonly complete, with pumice clasts and secondary minerals formed earlier from neutral pH water being replaced by kaolinite. The most strongly overprinted rocks have been converted to a puggy quartz + kaolinite + cristobalite + alunite assemblage. Landsliding on the Paeroa Fault
Tephra layers as old as 22 600 years separating landslide deposits indicate that movements on the Paeroa Fault have likely triggered large landslides in and outside the thermal area. These deposits contain clasts of different ignimbrites still exposed on the scarp, although the alteration mineralogy (including quartz and adularia) of some of the clasts was produced by alkali-chloride waters that do not now discharge there.
Hydrothermal Alteration and Evolution, Te Kopia Thermal Area
655
A clast in one such deposit has a planar surface with abundant euhedral quartz crystals which contain fluid inclusions (Fig. 7a), while "bladed quartz" also occurs. The latter is interpreted to result from boiling causing bladed calcite to deposit which was then replaced by quartz as cooling occurred.
Fluid inclusion measurements Quartz crystals containing measurable fluid inclusions occur at two localities. A slump/landslide deposit 180 ___ 5 m above the base of the Paeroa Fault, 600 m east of TK-1, contains blocks of tuff up to 15 cm diameter. These host abundant hydrothermal quartz crystals up to 8 mm long, which contain numerous fluid inclusions (Fig. 7). Abundant euhedral quartz crystals, up to 35 mm long, occur on an upstanding ridge 900 m northeast of TK1 at an elevation of 145 + 5 m above the valley floor. The crystals litter the ground (Fig. 7b), have collected in shallow depressions, and derive from a formerly extensive quartz vein system now being corroded here by acidic waters. The crystals host abundant fluid inclusions. Relict "stockwork" quartz veining is exposed at this locality (Fig. 7c). The veinlets are typically about a miilimetre wide and trend 130 ° (i.e., subparallel to the ridge), but lack measurable fluid inclusions. Most fluid inclusions appear to have formed along healed fractures and are thus secondary. Few of the host hydrothermal minerals have growth zones, and daughter minerals are absent. Inclusions range from 5 to 20/~m in length but are typically from 2 to 8 #m wide. Homogenisation temperatures (Th) were measured on 25 liquid-rich fluid inclusions in quartz from the location 600 m east of TK-1 (Fig. 2). No vapour inclusions were seen. Some inclusions are indeed primary, being isolated and up to 100 #m long. Some have negative crystal forms and do not occur in healed fractures. Their mean Th value is 188 + 15°C. The fluid inclusion data, when plotted on a boiling curve for pure water, indicate fluid entrapment at least 125 m below the water table, so the crystals are deduced as having been uplifted to their present elevation by movements on the Paeroa Fault totalling at least 300 m. Freezing measurements made on 9 inclusions gave Tm values from -0.2 to -0.1 °C (i.e., apparent salinities of 0.4 to 0.2 wt % NaC1 equivalent), similar to values for inclusions in subsurface samples (Bignall, 1994). Th measurements were also made on 14 liquid-rich inclusions in quartz from the sample locality northeast of TK-1 (Fig. 2). Vapour-rich inclusions occur here. These inclusions are up to 80/zm long and occur in zones; their mean Th is 196 + l l ° C . This indicates their entrapment at least 170 m below the water table, suggesting that they have been relatively uplifted along the Paeroa Fault by at least 315 m to reach their present elevation, now - 145 -I- 5 m above the base of the fault scarp.
Occurrence of silica sinter Several deposits of decomposing silica sinter occur in the vicinity of the acid-sulphate pools. Sinter is not now forming, however, so the system has clearly changed. These changes are consistent with a lowering of the water-table whereby neutral pH, alkali-chloride waters were replaced by steam and by acid-sulphate fluids.
Occurrence of cold altered ground Areas of cold ground occur to the south of the main thermal area. Tuff there contains a relict mordenitesmectite assemblage overprinted by kaolinite + alunite. This likely results from both cooling and a change in the pH of the altering fluid from near neutral to acidic.
G. Bignall and P. R. L. Browne
656
• MODEL FOR THE EVOLUTION OF THE TE KOPIA GEOTHERMAL FIELD The stratigraphic relationships, alteration mineralogy, textures and fluid inclusion results record several stages of thermal activity during the evolution of the Te Kopia Geothermal Field (Fig. 8); many events have likely gone unrecorded. Differences in the types of surface alteration preserved from different stages of activity reflect changes in the hydrological conditions and the depths at which fluid-rock interactions occurred. The evolution of the Te Kopia Geothermal Field is deduced from the observations described earlier and by analogy with the alteration mineralogy of geothermal reservoirs explored elsewhere in the Taupe Volcanic Zone (Browne, 1978). The following sequence of events is inferred. W
E initiation of NE trending faults
hot springs?
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meteoric ~ groundwater extent of / system / ? i
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Development of hydcothermal system at "re Kepis along E-W (snd younger NE-SW) trending normal faults. The active hydrothermsl system pre~ates recent movement on the NE trending Paeroa Fault. which exposed 320ks Paerce IgeimbrRe. W'
Paeroa Fault
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2
Changes In the surface expression accomDankad movement on the Paeroa Fault - developrnent of kaolinite-style alteration above the local wster table. overprinting of neutral pH type alteration assemblages. W
over~lntlng kaolinite alteration Murphys' spring
-/.....--.
perched s!. . . . ~ heated pools ~
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3
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Lowerl~lg of the water treble wtth contklued fault movement. Io woduce the preset| surface e x l 0 r ~ o n of the hydrothermal system. Movernent on the Paeroe Fault hss triggered recent hydrothermal eruptions.
Fig. 8: Inferred stages of hydrothermal activity at Te Kopia: 1. Early alkali-chloride dominant (c. 120 000 years ago). 2. Acid-sulphate dominant. 3. Present hydrology.
Hydrothermal Alteration and Evolution, Te Kopia Thermal A r e a
657
Alteration produced by waters o f alkali-chloride composition
Zones of relict quartz-adularia-illite and weak clay-mordenite alteration now at the surface formed within the geothermal reservoir during an early stage of thermal activity, when ascending near-neutral pH, alkalichloride water reacted with the host ignimbrites. A hot ( > 2 2 0 ° C ) fluid produced the high rank quartzadularia-illite alteration in the central part of the field. The peripheral and overlying weak clay (smectite)mordenite alteration zone formed by reaction of the rocks with either heated groundwater (as occurred at Ohakuri) or with water that resulted from the mixing of hot, upwelling alkali-chloride water with cool groundwater. The nature of surface activity at Te Kopia during this early stage is unknown, partly because of erosion (including landsliding), but mainly due to changes in the hydrology and structure of the system caused by movements on the Paeroa Fault. The age of the alteration produced by the alkali-chloride waters is uncertain. Keall (1987) estimated that the Paeroa Ignimbrite had been offset vertically by 600 m. Since this ignimbrite is now known to be 240 thousand years old (Nairn et al., 1994), then the average rate of displacement since its eruption is 2.5 m/1000 years. As the minimum vertical offset of the altered rocks is deduced to be 300 m, then the Te Kopia Geothermal System is at least 120 thousand years old. Alteration produced by acid-sulphate waters
Each movement on the Paeroa Fault was probably followed by an expansion of the kaolinite-style alteration as the thermal water-table descended in the upthrown block. The acid-sulphate waters altered fresh rocks and overprinted some with quartz-adularia-illite, silicification and weak clay-mordenite alteration, all of which formed during the first stage of activity. Rocks below the piezometric level, however, continued to interact with neutral pH, alkali-chloride fluids. The maximum age of the acid alteration at Te Kopia is unknown but it is very likely that some was contemporaneous with quartz-adularia-illite alteration occurring within the reservoir. The decomposing 3026 + 43 years BP opaline sinter indicates that chloride water was then discharging at the base of the fault scarp, concurrently with acid-sulphate alteration at higher elevations. Hot spring activity today continues several hundred metres west of the Paeroa Fault but scattered sinters and weakly silicified tuffs testify that neutral pH, alkali-chloride waters once discharged along the fault trace.
Acknowledgments - The Waikato Regional Council provided financial support to GB. We thank Dr J. McLeod for his assistance, and Mr Murphy, Te Kopia, for permission to investigate the thermal features on his property. Dr C.P. Wood and Dr J.W. Hedenquist gave very helpful reviews; we thank them both.
REFERENCES Bignall, G., 1991. Subsurface stratigraphy and structure of the Orakeikorako and Te Kopia geothermal systems, New Zealand. Proc. 13th NZ Geothermal Workshop, 199-205. Bignall, G., 1994. Thermal evolution and fluid-rock interactions in the Orakeikorako-Te Kopia Geothermal System, Taupo Volcanic Zone, New Zealand. Phi) Thesis. University of Auckland, NZ (unpubl). Bromley, C.J., 1992. Waikite-Te Kopia: The missing link? Proc. 14th IV'-ZGeothermal Workshop, 217-222. Browne, P.R.L., 1978. Hydrothermal alteration in active geothermal fields. Ann. Rev. Earth Planet. Science 6, 229-250. Grange, L.I., 1937. The Geology of the Rotorua-Taupo subdivision, Rotorua and Kaimanawa subdivisions. NZ Geol. Survey Bulletin 37. Grindley, G,W., 1965. Subsurface geology of the Orakeikorako and Te Kopia geothermal fields. N'Z Geol. Survey, Geol. Geothermal Report 3. Healy, J., 1952. Te Kopia hot springs. Unpublished DSIR Report, GS 23/2. Healy, J., 1974. Te Kopia Geothermal Field. N-ZGeol. Survey Report 38, Minerals of New Zealand, Part D, Section 3.14: Geothermal Resources. DSIR, Wellington. Henneberger, R.C. and P.R.L. Browne, 1988. Hydrothermal alteration and evolution of the Ohakuri hydrothermal system, Taupo Volcanic Zone, New Zealand. J. Volc. Geotherm, Res. 34, 211-231.
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G. B i g n a l l and P. R. L. B r o w n e
Hochstetter, F. von, 1864. Geologie von Neu Seeland: Beitrage zur Geologie der Provinzen Auckland und Nelson. NovaraExped. Geol. Theil. 1 (1): 274 pp. (Translated and edited by C.A. Fleming, 1959. Government Printer, Wellington, 320 pp.) Keail, J.M., 1987. Volcanic stratigraphy and hydrothermal alteration along the Paeroa Fault. Unpublished MSc thesis, Victoria University, Wellington, NZ (tmpubl). Lloyd, E.F., 1972. Geology and hot springs of Orakeikorako. N~ZGeol. Survey Bulletin 85, DSIR, Wellington, NZ. Macdonald, W.J.P., 1965. A preliminary study of electrical soundings made at Orakei Korako and Te Kopia. Geothermal Circular WJP Mac.D3., Geophysics Division, DSIR, Wellington, NZ (unpubl.). Mahon, W.A.J., 1965a. A chemical survey of the Waikite, Pankohurea and Te Kopia thermal springs. Report CD 118/12 WAJM/25. Chemistry Division, DSIR, Wellington, NZ (unpubl.). Mahon, W,A.J., 1965b. The chemistry of fluids discharged from Hole 3, Orakeikorako (later changed to Hole 1, Te Kopia). Report CD 118/12 - WAJM/29. Chemistry Division, DSIR, Wellington, NZ (unpubl.). Martin, R.C., 1961. Stratigraphy and structural outline of the Tanpo Volcanic Zone. NZ J. Geol. Geophys. 4, 449-478. Merchant, R.J., 1990. Final report on PL312183, Te Weta. Open File Mining Company Report. IGNS No. MR1511. Inst. Geol. Nuclear Sciences, Lower Hutt, NZ. Nalm, I.A. and A.G. Hull, 1986. Post-1800 years B.P. displacement of the Paeroa Fault zone, Tanpo Volcanic Zone. NZ Geol. Survey Record 8, 135-142. Nairn, I.A., C.P. Wood, and R.A. Bailey, 1994. The Reporoa Caldera, Taupo Volcanic Zone: Source of the Kaingaroa Ignimbrites. Bull. Volcanol. (in press). Sheppard, D.S. and L.E. Klyen, 1992. The Te Kopia geothermal field 1979-1992. DSIR Chemistry Division Report CD9040. Steiner, A., 1977. The Wairakei geothermal area, North Island, New Zealand: its subsurface geology and hydrothermal rock alteration. ArZ Geol. Survey Bulletin 90, 136 pp. Wallace, R.E., 1977. Profiles and ages of young fault scarps, north-central Nevada. Geol. Soc. Amer. Bull. 88, 1267-1281.