Dissertations / Theses on the topic 'Taupō volcano'

Maroa Volcanic Centre (Maroa) is located within the older Whakamaru caldera, central Taupo Volcanic Zone, New Zealand. Dome lavas make up the majority of Maroa volume, with the large Maroa West and East Complexes (MWC and MEC, respectively) erupted mostly over a short 29 kyr period starting at 251 ± 17 ka. The five mappable Maroa pyroclastics deposits are discussed in detail. The Korotai (283 ± 11 ka), Atiamuri (229 ±12 ka), and Pukeahua (~229 -196 ka) pyroclastics are all s 1 km3 and erupted from (a) northern Maroa, (b) a vent below Mandarin Dome and (c) Pukeahua Dome Complex vents, respectively. The Putauaki (272 ± 10 ka) and Orakonui (256 ± 12 ka) pyroclastics total ~ 4 km3 from a petrologically and geographically very similar central Maroa source. The ~ 220 ka Mokai pyroclastics outcrop partly within Maroa but their source remains unclear, whereas the ~ 240 ka Ohakuri pyroclastics appear to have come from a caldera just north of Maroa. The ages of the Mamaku, Ohakuri and Mokai pyroclastics are equivocaL The Mamaku and Ohakuri pyroclastics appear to be older (~ 240 ka) than the age previously accepted for the Mamaku pyroclastics. Maroa lavas are all plagioclase-orthopyroxene bearing, commonly with lesser quartz. Hornblende +/- biotite are sometimes present and their presence is correlated with geochemical variation. All Maroa deposits are rhyolites (apart from two high-silica dacite analyses) and are peraluminous and calcic. They all have the trace element signatures of arc-related rocks typical of TVZ deposits. Maroa deposits fall geochemically into three magma types based on Rb and Sr content: M (Rb 80-123 ppm, Sr 65-88 ppm), T (Rb 80-113 ppm, Sr 100-175 ppm) and N (Rb 120-150 ppm, Sr 35- 100 ppm). The geochemical distinction of these types is also seen in the concentrations of most other elements. Based on the spatial, chronological and petrological similarities of the MWC/MEC and Pukeahua eastern magma associations (termed (1) and (2)) a further four magma associations are determined ((3) through (6)). These six associations account for almost all Maroa deposits. Two end-member models are proposed for the sources of each of the Maroa magma associations: (a) a single relatively shallow magma source feeding spatially clustered eruptions, and (b) a deeper source feeding multiple shallower offshoots over a wider area. Sources for the Maroa magma associations probably lie on a continuum between these two model end members. The distinction between Maroa and Taupo Volcanic Centres is somewhat arbitrary and is best considered to be the easting directly north of Ben Lomond, north of which most volcanism is older than 100 ka and M and N type, and south of which most volcanism is younger than 100 ka and T type. The remaining boundaries (north to include Ngautuku, west to include Mokauteure and east to include Whakapapa domes) are arbitrary, and include the farthest domes linked closely, spatially and magmatic ally, to the other Maroa domes. From 230 to 64 ka there was a hiatus in caldera-forming ignimbrite eruptions. Maroa and the Western Dome Belt (WDB) constitute the largest concentrated volume of eruptions (as relatively gentle lava extrusion) during this period. The rate of Maroa volcanism has decreased exponentially from a maximum prior to 200 ka. In contrast volcanism at Taupo and Okataina has increased from ~ 64 ka to present. The oldest Maroa dome (305 ± 17 ka) constrains the maximum rate of infilling of Whakamaru caldera as 39-17 km3/kyr. This highlights the extraordinarily fast rate of infilling common at silicic calderas and is in agreement with international case studies, except where post-collapse structural resurgence has continued for more than 100 kyr. The majority of caldera fill, representing voluminous eruption deposits in the first tens of thousands of years post collapse, is buried and only accessible via drilling. The WDB and Maroa are petrologically distinct from one another in terms of some or all of Rb, Sr, Ba and Zr content, despite eruption over a similar period. Magma sources for Maroa and the WDB may have been partly or wholly derived from the Whakamaru caldera magma system(s), but petrological distinctions among all three mean that Maroa and the WDB cannot be considered as simple magmatic resurgence of the Whakamaru caldera. Maroa's distinct Thorpe Rd Fault is in fact a fossil feature which hasn't been active in almost 200 kyr. In addition, the graben across Tuahu Dome was likely created by shallow blind diking. Several recent studies across TVZ show structural features with some associated dike intrusion/eruption. Such volcano tectonic interaction is rarely highlighted in TVZ but may be relatively common and lie on a continuum between dike-induced faulting and dikes following structural features. Although rates of volcanism are now low in Maroa magmatic intrusion appears to remain high. This raises the possibility of a causative link between faulting and volcanism in contrast to traditional views of volcanism controlled by rates of magmatic ascent. Probable future eruptions from Maroa are likely to be of similar scale (<0.1 km3 ) and frequency (every ~ 14,000 years) to most of those over the last 100 ka. Several towns lie in a range of zones of Maroa volcanic hazard from total destruction to possible ash fall. However, the probability of a future eruption is only ~ 0.6 % in an 80 year lifetime.

Waiotapu Ignimbrite (0.710 ± 0.06 Ma) is a predominantly densely welded, purple-grey coloured, pumice rich lenticulite, which is exposed on both eastern and western flanks of Taupo Volcanic Zone. The unit is uniform in terms of lithology and mineralogy over its entire extent and has been deposited as a single flow unit. The unit contains abundant pumice clasts which are often highly attenuated (aspect ratios of c.1 :30) and are evenly distributed throughout the deposit. Lithic fragments are rare, never exceeding 1% of total rock volume at an outcrop and no proximal facies, such as lithic lag breccias, have been identified. The deposit is densely welded to the base and only in more distal exposure does the ignimbrite become partially welded at the top of the deposit. Post-depositional devitrification is pervasive throughout the deposit, often destroying original vitroclastic texture in the matrix. Vapour phase alteration is extensive in welded and partially welded facies of the deposit. Pumices within Waiotapu Ignimbrite appear to have been derived from two distinct magma batches, with differing Rb concentrations, that originated along different fractionation trends. Type-A pumices have significantly lower Rb than the subordinate type-B pumices. The presence of the pumices may represent the simultaneous evisceration of two spatially discrete magma chambers or the type-B chamber may have been intruded into type-A body, the magmas subsequently mingling prior to, or during, the eruption. The source of Waiotapu Ignimbrite is poorly constrained, largely owing to the lack of meaningful maximum lithic data, and poor exposure of the unit. The distribution of the ignimbrite suggests that it was erupted from within Kapenga volcanic centre. If so the most proximal exposures of Waiotapu Ignimbrite are approximately 10km from the vent. Intensive and voluminous silicic volcanism, beginning with the eruption of the 0.33 Ma Whakamaru Group Ignimbrite eruptions, and extensive faulting within Kapenga volcanic centre will have obscured any intra-caldera Waiotapu Ignimbrite. The mechanism of eruption suggests that the source may not have been a caldera in the strictest sense, but instead a series of near linear fissures aligned with the trend of regional faulting. Waiotapu Ignimbrite was generated in one sustained eruption and produced an energetic and high temperature pyroclastic flow. The lack of any recognised preceding plinian deposit, coupled with the energetic nature and paucity of lithics suggests eruption by an unusual mechanism. The eruption most likely resulted from the large scale collapse of a caldera block into the underlying chamber resulting in high discharge rates, which were no conducive to the development of a convecting column, and minimal vent erosion, resulting in negligible entrainment of lithics. The density of welding and recrystallisation textures suggest that the flow retained heat to considerable distances which allowed the ignimbrite to weld densely to the base. The deposit was most likely progressively aggraded from the base, with material being supplied from an overriding particulate flow.

Ngauruhoe is an active basaltic andesite to andesite composite cone volcano at the southern end of the Tongariro volcanic complex, and most recently erupted in 1954-55 and 1974-75. These eruptions constructed the inner crater of Ngauruhoe, largely composed of 1954-55 deposits, which are the basis of this study. The inner crater stratigraphy, exposed on the southern wall, is divided into seven lithostratigraphic units (A to G), while the northern stratigraphy is obscured by the inward collapse of the crater rim. The units are, from oldest to youngest: Unit A, (17.5 m thick), a densely agglutinated spatter deposit with sharp clast outlines; Unit B, (11.2 m) a thick scoria lapilli deposit with local agglutination and scattered spatter bombs up to 1 m in length; Unit C, (6.4 m thick) a clastogenic lava deposit with lateral variations in agglutination; and Unit D, (10 m thick) a scoria lapilli with varying local agglutination. The overlying Unit E (15 cm thick) is a fine ash fallout bed that represents the final vulcanian phase of the 1954-55 eruption. Unit F is a series of six lapilli and ash beds that represent the early vulcanian episode of the 1974-75 eruption. The uppermost Unit G (averaging 10 m thick) is a densely agglutinated spatter deposit that represents the later strombolian phase of the 1974-75 eruption. Units A-D juvenile clasts are porphyritic, with phenocrysts of plagioclase, orthopyroxene, clinopyroxene, minor olivine, within a microlitic glassy groundmass. Quartzose and greywacke xenoliths are common in most units, and are derived from the underlying basement. The 1954-55 and 1974-75 eruptions are a product of a short-lived, continental arc medium-K calc-alkaline magma. The magma originated from the mantle, then filtered through the crust, undergoing assimilation and fractionation, and evolving to basaltic andesite and andesite compositions. The magma body stagnated in shallow reservoirs where it underwent further crustal assimilation and fractionation of plagioclase and olivine, and homogenisation through magma mixing. Prior to the 1954-55 eruption a more primitive magma body was incorporated into the melt. The melt homogenised and fed both the 1954-55 and 1974-75 eruptions, with a residence time of at least 20 years. The 1954-55 eruption produced alternating basaltic andesite and andesite strombolian activity and more intense fire fountaining, erupting scoria and spatter that built up the bulk of the inner crater. A period of relative quiescence allowed the formation of a cooled, solid cap rock that resulted in the accumulation of pressure due to volatile exsolution and bubble coalescence. The fracturing of the cap rock then resulted in a vulcanian eruption, depositing a thin layer of fine ash and ballistic blocks. The 1974-75 eruption commenced with the rupturing of the near-solid cap rock from the 1954-55 eruption in an explosive vulcanian blast, the result of decompressional volatile exsolution and bubble coalescence, and possible magma-water interaction. The eruption later changed to strombolian style, producing a clastogenic lava that partially flowed back into the crater.

Taupo Volcanic Zone (TVZ) is the largest active volcanic belt in New Zealand, and has erupted >10.000 km3 of dominantly rhyolitic magma during the last 1.6 m.y. This study concerns the field relations, volcanology and petrology of two post-Whakamaru (330 ka) - pre-Mamaku (140 ka) ignimbrites, informally named as the Pokai and Chimp ignimbrites, occurring in a ca. 360 km2 area SW and W from Rotorua in the north-western TVZ. The Pokai Ignimbrite has a minimum volume of ca. 33 km3 DRE, whereas the older Chimp Ignimbrite has a minimum volume of only ca. 5 km3 DRE. Of the two ignimbrites the younger Pokai Ignimbrite is better preserved and is thus the main emphasis in this thesis. The Chimp Ignimbrite is relatively pumice- and crystal-poor (1-2 vol. % phenocrysts), and the exposed flow units are relatively thin (4-6 m). A short plinian phase preceded the Chimp Ignimbrite, whereas the Pokai Ignimbrite is marked by a number of pre-ignimbrite air-fall pumice and ash layers. The Pokai Ignimbrite represents a multiple flow unit ignimbrite, with single flow units usually ranging from 6-30 m. Thick deposits (>20 m thick) are usually welded in the upper middle part of the deposit. Ground deposits, i.e. layer 1 deposits, are rare. Field evidence suggest that the Pokai Ignimbrite originated from the Kapenga Volcanic Centre, a multiple caldera structure in the northern central TVZ. Two pumice types occur in the Pokai Ignimbrite; a crystal-poor type (2-3 % phenocrysts) and a crystal-rich type (6-12 % phenocrysts). Plagioclase is the dominant phenocryst throughout, with minor amounts of orthopyroxene, Fe-Ti oxides and quartz, which occurs in ca. 30 % of the pumices. Hornblende and clinopyroxene are present occasionally. The Pokai Ignimbrite ranges from mildly to strongly peraluminous, whereas the Chimp Ignimbrite is mildly peraluminous, both coinciding with other TVZ rhyolitic ignimbrites, but clearly differing from the rhyolitic lavas which are usually metaluminous to only mildly peraluminous. Whereas most TVZ rhyolitic eruptives have been regarded as relatively homogeneous, the Pokai Ignimbrite shows significant geochemical variation. The magma chamber was compositionally zoned from crystal-poor, high silica, low Sr (77 % SiO2 , 50 ppm Sr) rhyolitic top to more crystal-rich, low silica, high Sr (70 % SiO2 , 130 ppm Sr) rhyolite at the deeper levels. Prior to the eruption vigorous mixing of magma from different levels occurred, producing different pumice types in the airfall deposits, and multiple phenocryst populations in single pumice clasts. As the eruption progressed successively deeper levels of the magma chamber were tapped, the last eruption products representing the less evolved, crystal-rich magma. Least squares and Rayleigh fractionation models indicate that the Pokai, and the Chimp magmas most probably generated by AFC from TVZ andesitic magmas contaminated by Mesozoic basement sediments.

The evolution of rhyolitic lava from effusion to cessation of activity is poorly understood. Recent lava dome eruptions at Unzen, Colima, Chaiten and Soufrière Hills have vastly increased our knowledge on the changes in behaviour of active domes. However, in ancient domes, little knowledge of the evolution of individual extrusion events exists. Instead, internal structures and facies variations can be used to assess the mechanisms of eruption. Rhyolitic magma rising in a conduit vesiculates and undergoes shear, such that lava erupting at the surface will be a mix of glass and sheared vesicles that form a permeable network, and with or without phenocryst or microlites. This foam will undergo compression from overburden in the shallow conduit and lava dome, forcing the vesicles to close and affecting the permeable network. High temperature, uniaxial compression experiments on crystal-rich and crystal-poor lavas have quantified the evolution of porosity and permeability in such environments. The deformation mechanisms involved in uniaxial deformation are viscous deformation and cracking. Crack production is controlled by strain rate and crystallinity, as strain is localised in crystals in crystal rich lavas. In crystal poor lavas, high strain rates result in long cracks that drastically increase permeability at low strain. Numerous and small cracks in crystal rich lavas allow the permeable network to remain open (although at a lower permeability than undeformed samples) while the porosity decreases. Flow bands result from shear movement within the conduit. Upon extrusion, these bands will become modified from movement of lava, and can therefore be used to reconstruct styles of eruption. Both Ngongotaha and Ruawahia domes, from Rotorua caldera and Okataina caldera complex (OCC) respectively, show complex flow banding that can be traced to elongated or aligned vents. The northernmost lobe at Ngongotaha exhibits a fan-like distribution of flow bands that are interpreted as resulting from an initial lava flow from a N – S trending fissure. This flow then transitioned into intrusion of obsidian sheets directly above the conduit, bound by wide breccia zones which show vertical movement of the sheets. Progressive intrusions then forced the sheets laterally, forming a sequence of sheets and breccia zones. At Ruawahia, the flow bands show two types of eruption; long flow lobes with ramp structures, and smaller spiny lobes which show vertical movement and possible spine extrusion. The difference is likely due to palaeotopography, as a large pyroclastic cone would have confined the small domes, while the flow lobes were unconfined and able to flow down slope. The vents at Ruawahia are aligned in a NE – SW orientation. Both domes are suggested to have formed from the intrusion of a dyke. The orientations of the alignment or elongation of vents at Ngongotaha and Ruawahia can be attributed to the overall regional structure of the Taupo Volcanic Zone (TVZ). At Ngongotaha, the N – S trending elongated vent is suggested to be controlled by a N – S trending caldera collapse structure at Rotorua caldera. The rest of the lobes at Ngongotaha, as well as other domes at Rotorua caldera, are controlled by the NE – SW trending extensional regional structure or a NW – SE trending basement structure. The collapse of Rotorua caldera, and geometry of the deformation margin, are related to the interplay of these structures. At Ruawahia, the NE – SW trending vent zone is parallel to the regional extension across the OCC, as shown by the orientation of intrusion of the 1886AD dyke through the Tarawera dome complex. The NE – SW trending regional structures observed at both Rotorua caldera and Okataina caldera complex are very similar to each other, but differ from extension within the Taupo rift to the south. Lava domes, such as Ngongotaha, that are controlled by this structure show that the ‘kink’ in the extension across Okataina caldera complex was active across Rotorua caldera during the collapse at 240 ka, and possibly earlier. This study shows the evolution of dyke-fed lava domes during eruption, and the control of regional structures in the location and timing of eruption. These findings improve our knowledge of the evolution of porosity and permeability in a compacting lava dome, as well as of the structures of Rotorua caldera, the longevity of volcanic activity at dormant calderas and the hazard potential of dyke-fed lava domes.

Basin-hosted stratigraphy in volcanic arc settings reflects the interplay between ancient environments, volcanism, magmatism and tectonism. Lithostratigraphic variations within basins can be used to identify the location and timing of the processes contributing to their evolution. However, when deposits are hydrothermally altered, the use of many traditional analytical techniques for assessing their volcanic origin become impracticable, making analysis challenging. Examination then relies on an integrated mix of detailed macroscopic assessment and techniques utilising remaining stable magmatic phases. The Huka Group at Wairakei-Tauhara Geothermal Field (Wairakei-Tauhara) is primarily comprised of volcanic deposits preserving ~300 kyr of evolution in the Taupo Volcanic Zone (TVZ), New Zealand. Intensive geothermal well drilling in the field has identified the distribution and variation comprising its Waiora and Huka Falls Formations. The volcanic, structural and environmental history of the Huka Group, however, remains poorly understood. This thesis is concerned with identifying the stratigraphic and geothermal significance of the Huka Group from recent drill core samples at Wairakei-Tauhara. Drill core facies analysis confirm a spatially and temporally complex depositional history at the site. Deposits forming Waiora Formation were sourced from local explosive and effusive eruptions over ~100 kyrs within extensional basins hosting paleo-Lake Huka. Lacustrine and fluvial deposition prevailed for the following ~200 kyrs, as volcanism ceased, depositing the Huka Falls Formation. Frequent drilling of Huka Falls Formation has identified and thoroughly constrained facies variations of a local pyroclastic member, the Middle Huka Falls Formation. This eruption evolved as a series of water-supported, eruption-fed density currents from a sublacustrine vent in Tauhara transported beneath Lake Huka. Examined Huka Group core samples were hydrothermally altered and required the use of novel assessment techniques for comprehensive stratigraphic assessment. This alteration provided an opportunity to locally date the geothermal system within the Huka Group reservoir. Stratigraphic variations of resistant magmatic phenocrysts (feldspar) and immobile elements (Ti, Zr, V and Y) added new details of depositional processes and lithostratigraphy. Regional magmatic immobile element comparisons identified geochemical similarities within Huka Group ignimbrites that may have implications for the longevity and recurrence of caldera magma systems in TVZ. Geothermal activity in the Waiora Formation reservoir was dated using pristine hydrothermal adularia and 40Ar/39Ar dating methods. Results recognised a young phase of the system’s evolution (<30 ka) and the applicability of 40Ar/39Ar dating for use in geothermal chronology. Lastly, a conceptual evolutionary model for the Huka Group presents ~300 kyr of depositional processes, landscapes and structural events at Wairakei-Tauhara. The long-lived lacustrine setting is recognised to have been continually modified by episodic volcanism and gradual tectonism. Variations in Huka Group stratigraphy between the Wairakei and Tauhara Fields identify contemporaneous, but separate evolution of the underlying controlling horst (ridge) and graben (basin) structure. This study highlights the unique tectonic, magmatic, volcanic and sedimentary processes forming basins in the TVZ and improve our understanding on the geological evolution of geothermal systems. Techniques trialled in the study are demonstrated to be suitable for investigating altered volcanic materials and can be utilised elsewhere in the TVZ or other geothermal settings.

The Taupe Volcanic Zone (TVZ) is a major Pliocene-Quaternary NNE-SSW orientated volcano-techonic complex, in central North Island, New Zealand. It is a region characterised by voluminous rhyolitic eruptions, high natural heat flow, intense shallow seismic activity and active NW-SE extension. The central portion of the TVZ is regarded as the most frequently active and productive silicic volcanic system on Earth, yet to date no direct evidence for the source for the magmatisim has been found. In February and December 2001, as part of the NIGHT (North Island GeopHysical Transect) experiment, a total of ten 500 kg land slots were fired into an NW-SE array that ran the width of central North Island, New Zealand. An additional passive array of broad-band and short-period instruments centred on the TVZ recorded local and teleseismic earthquakes for six and a half months. Forward and inverse modelling of this active and shallow (< 10 km) earthquake data shows low-velocity (2.0-3.5 km/s) volcanic sediments reaching a maximum thickness of 3 km beneath the central TVZ. Underlying these sediments to 16 km depth are velocities of 5.0-6.5 km/s, interpreted as quartzo-fieldspathic crust. East and west of the TVZ, these velocities are observed to depths of 30 and 23 km respectively. Beneath the TVZ, material with P-wave velocities of 6.9-7.3 km/s are observed to ~30 km depth and are interpreted as heavily intruded or underplated lower crust. Modelling of deep (> 40 km) earthquake events originating near the top of the subducting Pacific plate, reveals a low-velocity region (LVR) (Vp of 7.4-7.8 km/s) overlying a northwest dipping high-velocity structure that coincides with the Wadati-Benioff zone.

The Taupo Volcanic Zone (TVZ) is investigated to determine the interaction of regional structure and volcanism. A three-tiered approach is employed involving (i) analysis of rift geometry and segmentation in Modem TVZ(<300 ka) from remote sensing and digital topographic data; (ii) fault kinematic data collected along the length of TVZ; and (iii) combining new and existing volcanological data for TVZ. Modem TVZ is a NNE-SSW trending intra-arc rift zone, subject to dextral transtension, and characterised by a segmented axial rift zone with a number of offset and variably oriented rift segments. These segments are subject to varying degrees of extension, and a general correlation exists between the amount of extension and the volume and style of volcanism in each segment. Segments with the highest degrees of extension correspond to the Okataina and Taupo Caldera Complexes in the central rhyolitic zone of Modem TVZ, while segments with a higher degree of dextral transtension correspond to the volumetrically-subordinate andesitic extremities. The influence of the structural framework on the shape and formation of calderas in Modem TVZ has been inferred from remote sensing and ground-based structural analysis. Detailed analysis of caldera structure and geometry in Modem TVZ indicates that caldera evolution is largely a function of caldera location relative to the axial rift zone. Calderas peripheral to the rift are simple, single-event structures, while those located within the axial rift zone are multiple-event caldera complexes with geometries dictated by their coincidence with rift faulting. These results show that in Modem TVZ the type, volume, and spatial distribution of magmatic activity is strongly influenced by rift structure and kinematics. The inter-relationship between rift geometry and caldera-complex development is particularly clear at the intra-rift Okataina Caldera Complex (OCC). OCC is located at a step-over in the rift where local rotation of the extension direction accompanies the development of a major transfer zone. Three main collapse events are spatially concentrated in a zone of orthogonal extension within the transfer zone. The 28 x 22 km OCC is elongate parallel to the extension direction, with a complicated topographic margin largely controlled by regional faulting. Major embayments occur on each side of OCC where it is intersected by adjacent rift segments. These are contiguous with two intra-caldera dome complexes forming two overlapping linear vent zones, which transect the caldera complex. The development of volcanism at OCC records the progressive interaction between offset rift segments and the propagation of overlapping rift segment axes. As rift propagation proceeded, a diffuse zone of volcanism progressively concentrated in the centre of the transfer zone then divided into two spatially restricted eruptive centres as through-going faults became established. Field investigations at OCC reveal a major revision to the eruptive stratigraphy that has implications for the development of the caldera and for hazard assessment in northern TVZ. Kawerau Ignimbrite is a partially welded pumice-rich ignimbrite that fills Puhipuhi Basin on the eastern side of the caldera complex and forms a thick terrace in and around the Kawerau township area. Within Puhipuhi Basin it is ~100 m thick, exposed on clear-felled knolls and locally forms jointed bluffs in thickest sections where it is valley ponded. Originally mapped as Kaingaroa Ignimbrite, it was subsequently considered distinct and renamed Kawerau Ignimbrite by Beresford & Cole (2000) with an accepted age of 240 ka. In Puhipuhi basin the Kawerau Ignimbrite overlies both the ~280 ka Matahina and ~65 ka Rotoiti ignimbrites and also the older tephras of the 43-31 ka Mangaone Subgroup. Whole-rock and glass geochemistry tie the ignimbrite specifically to the 33 ka Unit I eruptive phase of the subgroup, vastly increasing the eruptive volume of that unit and implying caldera collapse in this recent phase of OCC activity. Two pumice compositions are identified, reflecting eruption of two distinct magma bodies. Vertical variation in the ignimbrite records rapid depletion of a subordinate dacitic magma such that pumices of this composition are rare beyond proximal exposures. Lithic and pumice size distribution data indicate a source within OCC to the west of Puhipuhi basin. The residual volume of the ignimbrite is <15 km3, but estimates of the original volume approach 50 km3 when intra-caldera volumes are considered. Kawerau Ignimbrite thus represents the largest eruption from OCC in the last 65 ka since the Rotoiti event, and is the youngest partially-welded ignimbrite in TVZ.

The 0.23 Ma Kaingaroa Ignimbrite is a composite multiple flow-unit ignimbrite erupted from Reporoa Caldera, Taupo Volcanic Zone (TVZ), New Zealand. The Kaingaroa Ignimbrite has a complex internal stratigraphy with a complex basal tephra sequence of intercalated fall, surge and flow deposits, and three ignimbrite units, with strikingly proximal to medial facies variation. Proximal facies deposits are dominated by coarse lithic breccias up to 45m thick which are interpreted as co-ignimbrite lag breccias. These lag breccias are-some of the thickest so far documented. Welding and thickness variations in the extensive Old Waiotapu Rd (OWR; kg1) and Webb ignimbrite unit (WIU; kg2) suggests gradual thickening away from source, interpreted to represent ponding in a shallow alluvial lowland or basin. A detailed lithic componentry study indicates changes in lithic diversity and abundance between stratigraphic units which mark changes in vent conditions, increasing depth of lithic provenance and hence inferred fragmentation level. Lithic fragments reveal aspects of the sub-caldera geology, which is dominated by an andesitic volcano with leuco-gabbroic subvolcanic roots, intercalated welded ignimbrites, rare low-grade metasedimentary basement and meta-rhyolites. Gabbros and meta-rhyolites suggest complex metasomatic and fumarolic processes adjacent to the Kaingaroa magma system. The presence of tourmaline-bearing meta-rhyolites and meta-ignimbrites and tourmalinite is the first documented occurrence of tourmaline and tourmalinite in TVZ. Four pumice types are defined on pumice chemistry and mineralogy. These pumices are interpreted to represent samples of a weakly continuously zoned magma chamber (70-75% SiO2), which was progressively tapped during the eruption. Trace element and rare earth element systematics are consistent with an origin of type A magma from a type D parent by minor fractionation of plagioclase, zircon, and trace contents of Fe/Ti oxides and orthopyroxene. An additional hornblende-, 2-pyroxene-phyric dacite pumice/bleb (69% SiO2) was sampled from the Tokiaminga sub-unit, but is mineralogically and compositionally different from Kaingaroa pumices. Post-caldera rhyolites are mineralogically and chemically variable, with broad similarities to Kaingaroa pumices. The Kaingaroa magma components show reverse isotopic zonation i.e. decreasing 87Sr/86Sr and increasing 143Nd/144Nd with differentiation, suggesting syn-eruptive mingling and evisceration of the multiple magma batches occurred during the climactic caldera collapse phase. The Kaingaroa Ignimbrite has been mis-correlated by previous workers with the Matahina, Mamaku, and Rangatira Point ignimbrites, and three new units described in this thesis; Kawerau ignimbrite, Wheao sheet, and the welded ignimbrite of Wairakei drill holes. It is clear that ignimbrite correlation is difficult in TVZ because of the poor exposure and the limited stratigraphic sections that document multiple units. The Kawerau ignimbrite remains an enigma, largely because of the anomalously high Zr, Hf and Zn contents, suggestive of a relationship to 'alkaline' rhyolites, and the presence of unusual magnesium poor manganoan fayalite of vapour-phase origin. Identification of these units and other intermediate size ignimbrite in the stratigraphic interval between Whakamaru-group, and Mamaku ignimbrites requires further careful documentation, but suggests a temporal clustering of ignimbrites sourced from throughout TVZ.

The Kawerau hydrothermal system lies at the northern end of the Taupo Volcanic Zone, on the some 20 km south of the Bay of Plenty. The system, which is thought to have been active for at least 200,000 years, is situated over an area which has been volcanically active through time. Relatively recent local magmatism is found in the 800 m high, 3000-10,000 year old Mt. Edgecumbe dacite massif and the 200 m high Onepu Dome complex which lie adjacent to and within, respectively, the present day resistivity anomaly. Shallow reservoir fluids show evidence of steam heating as expressed by elevated bicarbonate and/or sulphate contents and mildly to strongly acidic pH, whereas the deep fluids are dominantly alkaline at their respective temperatures. The calculated base fluid composition is comprised of 2.5 wt% CO$/sb2$ and ca. 890 mg/kg Cl at 310$/sp/circ$C. Fluid inclusion studies show a largely stable, boiling point thermal regime through time, whereas oxygen stable isotope studies on hydrothermal carbonates prove the existence of one or more pulses of isotopically heavy fluids into the reservoir at some time(s) in the past. Hydrothermal alteration associated with these isotopic anomalies indicate strongly oxidising conditions relative to both alteration elsewhere in the reservoir and the present day reservoir redox conditions. Collectively, the data suggest a magmatic source for these transient, isotopically heavy fluids. The present day system is ore forming, as evident from both metal rich scales formed in the production silencers of the geothermal wells and open fracture reservoir mineralogy. Stockwork environments in the deep reservoir are host to both base and precious metals, and evidence indicates that boiling is the main depositional mechanism for these ore phases.
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The central segment of the Taupo Volcanic Zone (TVZ) is one of the world’s most productive areas of silicic volcanism and geothermal activity. Rhyolites largely predominate the eruptive output in the central TVZ, with only minor basalts, andesites and dacites. The rhyolites show diversity in composition, and form a compositional continuum between two end-member types (R1 and R2), as suggested in previous studies. In this thesis I present results from a quartz- (and rare plagioclase-) hosted melt inclusions study, focussing on the volatile concentration (i.e. H2O, Cl, F, CO2) and their relative distribution between R1 and R2 rhyolites. The main objective is to add further constraints on the magmatic systems with regard to their contribution to the hydrothermal systems in the central TVZ. A comparative study between R1 and R2 melt inclusions show distinct volatile, fluid-mobile, and highly incompatible element compositions. Differences in the bulk volatile concentration of the parental magmas (i.e. basalts intruding the lower crust) are suggested to be at the origin of these volatile disparities. Further analysis on the volatile exsolution of R1 and R2 melts lead to the observation that the two rhyolite types exsolve a volatile phase at different stages in their magmatic history. From Cl and H2O concentrations, it is suggested that R1 magmas exsolve a vapour phase first, whereas R2 rhyolites more likely exsolve a hydrosaline fluid phase. These results have considerable implications for the magmatic contribution into the hydrothermal systems in the central TVZ, as differences in the composition of the resulting volatile phase may be expected. The hydrothermal systems in the central TVZ are subdivided into two groups based on their gas and fluid chemistry; and the current model suggests that there are two distinct contributions: a typical ‘arc’ system, with geochemical affinity with andesitic fluids, located along the eastern margin of the TVZ, and a typical ‘rift’ system, with geochemical affinity with rhyolitic/basaltic fluids, located along the central and/or western region of the TVZ. The addition of the new data on the rhyolitic melt inclusions, leads to a re-evaluation of the magmatic contribution into the hydrothermal systems, with a particular focus on B and Cl. The results indicate a more diverse variety of contributions to the meteoric water in the hydrothermal systems, and also show that the east-west distribution of ‘arc’ and ‘rift’ fluids is not a viable model for the central TVZ. This work emphasises that melt inclusion data and their volatile degassing history cannot be underestimated when characterising and quantifying the magmatic component in hydrothermal fluids. The melt inclusion data also provide further insight into the pre-eruptive magmatic plumbing systems and are particularly important from a hazard perspective. Included in the thesis is a detailed petrological analysis of rhyolite melt inclusions across the central TVZ and an interpretation that large silicic magma systems (in the TVZ) are typically comprised of multiple batches of magma emplaced at some of the shallowest depths on Earth. Tectonic activity is suggested to play an important role in triggering large caldera-forming eruptions as the evacuation of one magma batch could cause a regional-scale readjustment that is sufficient enough to trigger and allow simultaneous eruption of an adjacent melt batch.

Subsidence occurring at geothermal fields requires monitoring, analysis, and understanding of the mechanisms in order to ensure that it does not affect field operations. This study utilised a broad range of techniques including spatial analysis, three-dimensional modelling, and the comparison of samples of the cover sequences to investigate the subsidence at Kawerau Geothermal Field. Subsidence at Kawerau is of concern because the Tasman Pulp and Paper Mill is located within the geothermal field and utilises machinery with small alignment tolerances that are sensitive to ground deformation. A probabilistic hazard analysis of Kawerau was completed and maps created indicating the potential for subsidence in the future. Spatial analysis of benchmark re-levelling surveys revealed two types of subsidence features: 1) field wide subsidence and 2) subsidence anomalies. Field wide subsidence, currently covering ~17 km2, is driven by thermal contraction of reservoir deep formations and/or compaction of the reservoir due to effective stress increases related to pressure drawdown. Four local subsidence anomalies each covering 150 – 400 m2 are likely driven by varying shallow processes. Two of these features, termed Bowls B and D, south of the Kawerau Geothermal Ltd. power station are the main focus of this thesis along with an assessment of the Tasman Mill site, its potential to develop an anomaly, and the mechanism of subsidence currently occurring across it. Three-dimensional modelling of the cover sequences to 750 m below relative level was completed in Leapfrog Geo using well logs from Kawerau. Modelling revealed an anomalous thickness of Tahuna Formation below Bowls B and D, and relatively uniform thicknesses across the mill site of other shallow formations. The anomalous thickness of Tahuna Formation was hypothesised as being responsible for the presence of the subsidence bowls by being more compressible than the overlying Caxton Formation which is thicker across the mill site while the Tahuna Formation is thinner. Alternative hypotheses were explored by mapping the relative level of the Matahina ignimbrite, thickness of the Caxton Formation, and distribution of brecciation. To test the main hypothesis, samples of Tahuna and Caxton formations were collected from the Kawerau Core Shed and tested for their physical properties and relative compressibility. XRD and thin section analysis was also completed on the samples. Tahuna Formation was found to have more than three times the porosity of the Caxton Formation and have smectite clays present. Using a method developed for testing the relative compressibility of weak rock the Tahuna Formation was found to generally be twice as compressible and elastic as the Caxton Formation when saturated. Samples of Recent alluvium from the mill site were also tested for their physical properties and found not to have the potential to contribute to subsidence across the mill site. However further investigation is required to confirm the mechanisms of Bowls B and D. A hazard analysis of Kawerau Geothermal Field found that the field has a low annual probability of being impacted by volcanic and volcanogenic, earthquake, and flooding events. Probabilities are calculated based on the reoccurrence intervals for each event. A hazard map for subsidence at Kawerau is also developed and outlines four zones of risk. Infrastructure at risk based on trends of subsidence is also analysed for its susceptibility to subsidence and mitigation methods discussed. The overall conclusion is that the geological conditions beneath the mill site are unlikely to form a local subsidence anomaly, and the mill site is largely unaffected by the field wide subsidence bowl. Ground tilt values are within mill machinery tolerances, and based on current trends the spatial extent of subsidence anomalies will remain approximately the same into the future.

Sixteen percent of New Zealand’s power comes from geothermal sources which are primarily located within the Taupo Volcanic Zone (TVZ). The TVZ hosts twenty three geothermal fields, seven of which are currently utilised for power generation. Ngatamariki Geothermal Field is the latest geothermal power generation site in New Zealand, located approximately 15 km north of Taupo. This was the location of interest in this project, with testing performed on a range of materials to ascertain the physical properties and microstructure of reservoir rocks. The effect of burial diagenesis on the physical properties was also investigated. Samples of reservoir rocks were taken from the Tahorakuri Formation and Ngatamariki Intrusive Complex from a range of wells and depths (1354-3284 mbgl). The samples were divided into four broad lithologies: volcaniclastic lithic tuff, primary tuff, welded ignimbrite and tonalite. From the supplied samples twenty one small cylinders (~40-50mm x 20-25mm) were prepared and subjected to the following analyses: dual weight porosity, triple weight porosity, dry density, ultrasonic velocity (saturated and dry) and permeability (over a range of confining pressures). Thin sections impregnated with an epoxy fluorescent dye were created from offcuts of each cylinder and were analysed using polarised light microscopy and quantitative fluorescent light microstructural microscopy. The variety of physical testing allowed characterisation of the physical properties of reservoir rocks within the Ngatamariki Geothermal Field. Special attention was given to the petrological and mineralogical fabrics and their relation to porosity and matrix permeability. It was found that the pore structures (microfractures or vesicles) had a large influence on the physical properties. Microfractured samples were associated with low porosity and permeability, while the vesicular samples were associated with high porosity and permeability. The microfractured samples showed progressively lower permeability with increased confining pressure whereas samples with a vesicular microstructure showed little response to increased confining pressure. An overall trend of decreasing porosity and permeability with increasing density and sonic velocity was observed with depth, however large fluctuations with depth indicate this trend may be uncertain. The large variations correlate with changes in lithology suggest that the lithology is the primary control of the physical properties with burial diagenesis being a subsidiary factor. This project has established a relationship between the microstructure and permeability, with vesicular samples showing high permeability and little response increased confining pressure. The effects of burial diagenesis on the physical properties are subsidiary to the observed variations in lithology. The implications of these results suggest deep drilling in the Tahorakuri Formation may reveal unexploited porosity and permeability at depth.

This thesis employs crystal-specific techniques, combined with field observations, petrology, geochemistry and numerical modelling to reconstruct the magmatic system associated with the ~ 340 ka Whakamaru supereruption, New Zealand. Comparisons are drawn with the ~ 74 ka Youngest Toba Tuff (YTT) supereruption. Whakamaru Group Ignimbrites contain five pumice types, characterised by different mineralogies and crystal contents. Pumice petrography and geochemistry indicate that basaltic magma mixing occurred, possibly triggering eruption. Geothermobarometers suggest an eruption temperature of ~ 770°C and magma storage at ~ 5 km depth. High-resolution thermal records from Ti-in-quartz analysis indicate a thermal pulse of ~ 100°C prior to eruption. Diffusion timescales show multiple recharge events with the most significant event occurring ~ 35 y prior to eruption. Zircon U-Pb data show that most crystallisation occurred at ~ 400 ka, with antecrysts and xenocrysts incorporated. Zircon trace-element data suggest multiple recharge events and complex mixing over ~ 100 ky, consistent with an incrementally growing reservoir. Oxygen-isotope data illustrate that zircon, quartz and feldspar crystallised together in equilibrium, with isotopically homogenous magma sources feeding the reservoir over time. Whakamaru and YTT tephra thickness and grain-size data were used in ash dispersal modelling. Results indicate the YTT eruption had a ~ 35 km column height and erupted volumes of 1500 – 1900 km³, with deposition from a co-ignimbrite phase; whereas Whakamaru had a Plinian column ~ 45 km high with SE dispersal and a minimum volume of ~ 400 km³. The widespread dispersal of large volumes of fine ash from both eruptions would have had global environmental consequences. The data are integrated to reconstruct a new Whakamaru magma reservoir model. The complex crystal records indicate the system was characterised by long periods of incremental assembly, mixing, recycling of material, and reactivation during multiple recharge episodes which perturbed the system and primed the magma for eruption.

A continuum of rhyolite compositions has been observed throughout the Taupo Volcanic Zone (TVZ) over the past 550 kyr. reflecting changes in the ƒH2O, ƒO₂, and P-T conditions in a lower crustal 'hot-zone' (10-30 km) where these evolved melts are generated by crystal fractionation of successively intruded basaltic magmas. The rhyolite compositional continuum is bound by two distinct end-member types: R1 is characterized by hydrous minerals (hornblende ± biotite), low FeO*/MgO (calc-alkaline series), low MREE, Y, and Zr, and high Sr; and R2 is characterized by anhydrous minerals (orthopyroxene ± clinopyroxene), high FeO*/MgO (tholeiitic series), high MREE, Y, and Zr, and low Sr. Slab-derived aqueous fluid components (Ba, Cl) correlate well with oxygen fugacity, and other well defined characteristics of silicic magmas in the Taupo Volcanic Zone (TVZ) between a cold-wet-oxidizing magma type (R1: amphibole ± biotite; high Sr, low Zr and FeO*/MgO, depleted MREE) and a hot-dry-reducing magma type (R2: orthopyroxene ± clinopyroxene; low Sr, high Zr, and FeO*/MgO, less depleted MREE). Oxygen fugacity was obtained from analysis of Fe-Ti oxides and ranges between -0.039 to +2.054 log units (ΔQFM; where QFM = quartz + fayalite + magnetite buffer) and is positively correlated with the bulk-rock Ba/La ratio, indicating that slab-derived fluid is the oxidizing agent in the rhyolites. Chlorine contents in hornblende also correlate with the bulk-rock Ba/La ratio. Hence, high fluid-flux typically correlates with the R1 and low fluid-flux with R2 rhyolite magma types. A geochemical evolution and distribution can be tracked in time and space throughout the central region of the TVZ from 550 ka to present and has revealed two distinct magmatic cycles that vary in length. The first cycle included widespread R1 type magmatism across the central TVZ beginning ca. 550 ka and was directly associated with previously unreported dome-building and ignimbrite-forming volcanism, and led to a voluminous (>3000 km³) ignimbrite 'flare-up' between ca. 340 and 240 ka. These magmas also display the highest K₂O and Pb isotopic compositions compared to those erupted more recently, and is consistent with a peak in slab-derived sediment input. The second cycle began roughly 180 ka, erupting ca. 800 km³ of magma, and continues to the present. The duration, rate, and composition of melt production within these cycles appears to be governed by the flux of fluid/sediment released from the subducting slab, while the distribution of melts may be governed more by extension along the central rift axis. The Matahina Ignimbrite (~160 km³ rhyolite magma; 330 ka) was deposited during a caldera-forming eruption from the Okataina Volcanic Centre, TVZ. The outflow sheet is distributed primarily from the northeast to southeast and consists of a basal plinian fall member and three ash-flow members. Pumice clasts are separated into three groups defined by differences in bulk geochemistry and mineral contents: high CaO, MgO, Fe₂O₃T, TiO₂, and low Al₂O₃, +hornblende (A2), low CaO, MgO, Fe2O3T, TiO2, ±hornblende (A1), and a subset to A1, which has high-K, +biotite (B). Two types of crystal-rich mafic clasts were also deposited during the final stages of the eruption. The distinct A and B rhyolite magma types are petrogenetically related to corresponding type A and B andesitic magma by up to 50% crystal fractionation under varying ƒO₂-ƒH₂O conditions. Further variations in the low- to high-silica rhyolites can be accounted for by up to 25% crystal fractionation, again under distinct ƒO₂-ƒH₂O conditions. Reconstruction of the P-T-ƒO₂-ƒ’H₂O conditions of the andesite to rhyolite magmas are consistent with the existence of a compositional and thermal gradient prior to the eruption. Magma mingling/mixing between the basalt to andesite and main compositionally zoned rhyolitic magma occurred during caldera-collapse, modifying the least-evolved rhyolite at the bottom of the reservoir and effectively destroying the pre-eruptive gradients. A detailed examination of the diverse range of calcic-amphibole compositions from the ca. 330 ka Matahina eruption (ca. 160 km³ rhyolitic magma) of the Okataina Volcanic Complex, Taupo Volcanic Zone, including crystal-rich basalt to dacite pumice from post-collapse deposits, reveals several pre- and syn-eruption magmatic processes. (1) Amphibole phenocrysts in the basaltic-andesite and andesite crystallized at the highest pressures and temperatures (P: up to 0.6±0.06 GPa and T: up to 950°C), equivalent to mid-crustal depths (13-22 km). Inter- and intra-crystalline compositions range from Ti-magnesiohornblende → Ti-tschermakite → tschermakite → magnesiohornblende and some display gradual decreases in T from core to rim, both consistent with magma differentiation by cooling at depth. (2) The largest amphibole crystals from the basaltic-andesite to andesite display several core to rim increases in T (up to 70°C), indicating new hotter magma periodically fluxed the crystal mush. (3) The dominant population of amphibole (magnesiohornblende) from the rhyolite is small and bladed and crystallized at low P-T conditions (P: 0.3 GPa, T: 765°C), equivalent to the eruptive P-T conditions. Amphibole (tschermakite-magnesiohornblende) from the dacitic and low-silica rhyolitic pumice form two distinct populations, which nucleated at two different T (High: 820°C and Low: 750°C). These compositional variations, governed primarily by differences in T conditions during crystal growth, record the mixing of two distinct amphibole populations that approached a thermal equilibrium at the eruptive T. Therefore, the diversity in amphibole compositions can be reconciled as an exchange of crystals+liquid between the basaltic-andesite to dacite from the mid-crust and rhyolite from the upper-crust, which quenched against one another, modifying the dacite to low-silica rhyolite compositions as the eruption progressed.

The Taupe Volcanic Zone (TVZ), New Zealand, is an area of active rifting, intense volcanism and geothermal activity. In this thesis, the use of C-band InSAR to measure and separate the geothermal, tectonic and magmatic deformation signals has been explored, in order to better understand the partitioning of the strain between these different sources.

The goal of this thesis is to evaluate the effect of fractures on the bulk permeability of rocks. Several methods are used to address this problem: 1) surface radon gas measurements, 2) stress induced fracture permeability 3) fracture generation conditions. Each method was variably effective in providing answer to the initial question. The radioactive radon isotopes (220Rn and 222Rn) were measured in soil gas extracted from 1 m depth in two areas and the concentrations for both isotopes tended to be higher near mapped faults. Soil samples recovered from 1 m depth indicate that the isotopic anomalies are coincident with changes in soil colour and the emanation of 220Rn, but are unrelated to the 222Rn emanation. The lack of a relationship for the latter can be explained by small-scale (~1m) diffusion for >90% of the soil gas measurements. However, diffusion cannot explain all of the observed patterns in the data, and in some specific locations along the fault, 222Rn concentrations are more likely sensitive to advective flow of sub-surface gases, suggesting channelizing of flow along faults. Stress is estimated using Leak-off Tests, estimating overburden weigth, and using drilling induced features observable in boreholes to model stress conditions. The results of the stress interpretation in the Rotokawa Geothermal Field show a relationship between the differential stress and alteration zones containing smectite, where the presence of smectite lowers the differential stress in the crust. This confirms a well-recorded relationship between the friction of rocks, and the strength of the crust. The magnitude of the principal stress axes, which are determined in this thesis, are used to predict the fracture orientations prone to slip in the Rotokawa Reservoir. The precise range of fracture orientations prone to slip is critically dependent on the poorly constrained intermediate stress. However, analysis of stresses on fracture orientations observed in the Rotokawa Andesite, coupled with independent permeability estimates reveal a complex relationship between fracture slip, and permeability, suggesting that slip on fractures can have both a positive or negative effect on slip. This is will depend on the degree of alteration of the Rotokawa Andesite. Failure in the Rotokawa Andesite is a result of: 1) the constant tectonic strain and 2) the increase in fluid pressure. Mathematical models used in this thesis show that if failure occurs through increase in fluid pressure, it is unlikely that the overpressures required to induce rock mass failure are solely generated by porosity/permeability reduction in the Rotokawa geothermal reservoir, requiring a constant external flux of fluids to induce the overpressures. Large-scale failure of the Rotokawa Andesite is modelled as a rock mass using the Hoek-Brown failure criterion, and indicates that the current dominant mode of failure is for the Rotokawa Andesite is shear failure at depth. However, small scale changes in stress, or an increase in rock mass strength will favour tensile failure. High fracture densities observed in three wells of the Rotokawa Andesite are oriented consistent with fractures formed in shear mode, consistent with ‘Healy’ faulting being the main mode of fracture formation in the Rotokawa Andesite.

The Tahorakuri Formation was introduced as a stratigraphic term to simplify the sometimes complex and inconsistent naming conventions in subsurface deposits within the geothermal fields of the central Taupo Volcanic Zone (TVZ). It consists of all volcaniclastic and sedimentary deposits between the ~350 ka Whakamaru-group ignimbrites and the greywacke basement that cannot be correlated with known ignimbrites. As such, it represents a long period in which relatively little is known about the volcano-tectonic history of the TVZ. The thesis focuses on the Tahorakuri Formation at Ngatamariki and Rotokawa geothermal fields and the implications for the volcano-tectonic evolution of the TVZ. Drill cuttings from wells NM5 and NM6 are re-examined, and new U-Pb zircon dates from the Tahorakuri Formation are presented and implications discussed. Potassium feldspars identified in the drill cuttings from NM5 were examined by Raman spectroscopy and electron microprobe (EMP) analysis. Although petrographically many of the feldspars appear similar to sanidine, a primary volcanic mineral phase, this showed them to be adularia which formed during hydrothermal alteration. Raman spectroscopy was found to be ideal for analysing a large number of grains quickly, with the spectral peak at ~140 cm⁻¹ being particularly useful for identifying adularia as it is absent in sanidine. EMP analysis was found to be somewhat slower, but definitively identified the feldspars as adularia, with typical potassium-rich compositions of Or₉₄-Or₉₉. U-Pb dating shows that the Tahorakuri Formation formed over a very long time, with pyroclastic deposits ranging from 1.89 - 0.70 Ma. This was followed by a period with little or no explosive volcanism until ~0.35 Ma during which sediments were deposited at Ngatamariki. The periods at ~1.9 Ma and ~0.9 Ma were particularly active phases of pyroclastic deposition, with the second phase likely correlating with the Akatarewa ignimbrite. The oldest deposits overlie a large andesitic composite cone volcano. Significant subsidence of the andesite must have preceded emplacement of the silicic deposits, indicating that rifting within the central TVZ may have started earlier than previously thought. While the origin of the deposits is uncertain, the distribution of the oldest deposits outcropping at the surface, as well as the likely early initiation of rifting, would suggest a source within the TVZ is likely.

Phreatomagmatic eruptions result from the explosive interaction between magma and some external source of water, and produce deposits which are usually distinctive in nature from those of magmatic eruptions. The widespread deposits of large-scale phreatomagmatic eruptions (usually termed Phreatoplinian) are poorly studied relative to their magmatic counterparts and, consequently, current models for large-scale phreatomagmatic volcanism remain speculative. The Hatepe ash and Rotongaio ash (units 3 and 4 of the 1.85 ka Taupo eruption) are two classical widespread phreatomagmatic fall deposits. These have been examined in fine detail and sampled, for the first time, at a mm-scale, with the intention of quantifying vertical and lateral variations within these deposits and improving our understanding of the eruptive mechanisms and depositional processes during large-scale 'wet' eruptions. The Hatepe ash (1.75 km3) is a widespread (>15 000 km2, individual subunit bt values = 4.4 to 5.5 km), multiple-bedded, poorly-sorted pumiceous fall deposit. The fines-rich character and widespread occurrence of ash aggregates in the proximal to medial dispersal areas are indicators of a phreatomagmatic origin. Subunits contain multiple layers with a wide range of dispersal and grain size characteristics, and a number of distinctive primary lithofacies have been defined which characterise the changes in eruptive conditions and main depositional modes during Hatepe volcanism. The predominantly fine grained clasts (Mdø= 3.3-4.5), along with perhaps 20-25 wt.% liquid, were transported and deposited in the form of damp to wet 'mud lumps' and accretionary lapilli. Dispersal was from dense, 'wet' plumes which promoted the cohesion and aggregation of liquid-coated fine particles. This mode of transport and deposition was dominant during relatively long-lived episodes of relatively low discharge rate, with higher water/magma ratios at the vent and liquid/particle ratios in plumes. When magma discharge rate was relatively high and water/magma ratios low, fines-poor, plinian-style deposits (Mdø = -2.2 to 0.63) were produced by discrete particle fall from high (~25-30 km), relatively 'dry' plumes. Minor, short-lived fluctuations in discharge rate produced episodes of mixed discrete and ash aggregate fall which produced poly- and bimodal deposits (Mdø = 2.5-3) in proximal and inner-medial areas. Lateral emplacement by dilute, turbulent pyroclastic density currents was important in the proximal environment. The range and indices of Hatepe ash juvenile clast vesicularities (50-90%, and 75% vesicles, respectively) indicate that fragmentation was driven by magmatic volatiles but that water played some part in quenching. The minimal variation in juvenile clast vesicularity through the deposit and between the facies types indicates that the state of the Hatepe magma remained a uniform foam, and that the mechanism of fragmentation (but not the water/magma ratio) was consistent throughout Hatepe volcanism. Facies analysis and mapping of internal variations in ash dispersal confirm that the Hatepe ash is not the product of simple sustained magma discharge, but was actually the result of a continuous but highly irregular flux, with fluctuations in magma supply, sometimes over very short time intervals, resulting in a range of eruptive styles and depositional modes. The Rotongaio ash (0.8 km3) is a widespread (>10 000 km2, subunit bt values = 2.9 to 5km), poorly-sorted fall deposit with abundant evidence for the important involvement of liquid water at the vent and in the plume. Modes of deposition were similar to the Hatepe ash; dominantly damp to wet mud lump fallout (Mdø= 3.9 to 5.5), but with minor episodes of discrete particle fall (Mdø = -1.1 to 1.9) and mixed discrete and aggregate fall (Mdø= 1.2 to 2.9) caused by fluctuations in discharge rate. An additional depositional mode in medial areas during Rotongaio volcanism was by dilute, turbulent density currents, derived from particle-laden downbursts from the umbrella region of dense, wet, convectively-unstable plumes. Such a process may account for occurrences of cross-stratification in the medial-distal parts of other widespread ash falls. Secondary processes such as fluvial erosion and reworking, and soft-sediment deformation and slurry-flow were important depositional modes that operated syneruptively during Rotongaio (and Hatepe ash) volcanism. The very close association in time and space between primary and secondary lithofacies implies that there was a strong genetic link between the style of primary eruptive processes and the nature and extent of the secondary modification. In many cases the 'secondary' processes formed a continuum with primary depositional processes, influenced by the liquid/particle ratio of ash fallout and inherent to the mode of eruption. Throughout deposition of the Rotongaio ash a delicate balance always existed between primary accumulation, erosion and redeposition. The Rotongaio ash differs from the Hatepe ash, and most other widespread ash fall deposits, in a number of important ways which indicate the Rotongaio ash is not a typical phreatoplinian deposit; 1) it is extremely finely laminated in proximal exposures and many of these beds cannot be traced into the medial environment indicating it is the product of multiple, discrete and non-sustained explosions which dispersed material along a number of axes and with a wide range of thinning rates, 2) juvenile clasts are mostly poorly- to non-vesicular and clast populations span a very wide range of densities (0-65% vesicles) indicating that the Rotongaio magma was partially degassed and heterogeneous (unlike the Hatepe ash and other pumiceous phreatoplinian deposits), and fragmentation was driven not by vesiculation, but largely by external volatiles, 3) the lack of any significant coarse component compared to the Hatepe ash at anyone site supports a fundamentally different mode of fragmentation for Rotongaio volcanism and vent processes which probably involved significant recycling of clasts through the vent. Detailed analysis of the Hatepe ash and Rotongaio ash has provided some interesting insights into the nature of large-scale phreatomagmatic eruptions. Ash dispersal patterns for subunits of the two deposits indicate that 'wet' and 'dry' plumes, even of comparatively small magnitudes (0.02 to 0.8 km3 subunit volumes) behave in distinctive ways which hint at fundamentally different dynamics of dispersal. Assessment of lateral variations in clast size populations suggest the differences between proximal strongly fines-segregated 'dry' facies and the fines-rich 'wet' facies is an artefact controlled mostly by the initial liquid/solid ratio in the plume rather than the mechanism of fragmentation.

The Whakamaru group ignimbrites are a Widespread and voluminous group of welded crystal-rich ignimbrites which outcrop along the eastern and western margins of the Taupo Volcanic Zone (TVZ), New Zealand. They have previously been mapped as Whakamaru (s.s), Manunui, Rangitaiki, Te Whaiti, and Paeroa ignimbrites, and have a combined volume of more than 1000km³ (DRE). The ignimbrites were erupted from a large vent area within central TVZ at 340ka, following a c.350ka hiatus in caldera forming activity in TVZ. This study investigates field and volcanological aspects of the ignimbrites, the geochemistry of pumice clasts and plutonic lithics, and the geochemistry of rhyolite lavas of the Western Dome Belt (WDB). The postulated vent area for the ignimbrites lies to the north of Lake Taupo and overlaps with the younger Taupo and Maroa volcanoes. Maximum lithic data indicate that the western margin of the vent area was located at or within a few kilometres east of the WDB, and probably overlapped with the northern part of Lake Taupo, providing clear support for a North Taupo/ Maroa caldera source. Isopleths close around an area previously modelled as a deep basement collapse structure, suggesting this area may have been an important focus of eruption and collapse within a broad 'Whakamaru Centre' comprising several nested collapse structures. On the basis of field evidence, mineral chemistry, and new Ar-Ar dates, Whakamaru, Manunui, Rangitaiki, Te Whaiti, Wairakei, and Paeroa Range Group (PRG) ignimbrites are considered to be correlatives. Manunui ignimbrite represents the stratigraphically lowest unit(s) of Whakamaru that is locally more highly welded and is less crystal rich in distal areas. Manunui ignimbrite therefore correlates with unit A of Briggs (1976) at Maraetai. East of TVZ, Te Whaiti ignimbrite also corresponds to the lowermost part of Lower Rangitaiki ignimbrite, with a gradational boundary between the two. There is no clear evidence for a significant time break between either Manunui and Whakamaru, or Te Whaiti and Rangitaiki ignimbrites. High precision Ar-Ar dating indicates eruptions occurred over a period of less than c.5ka, and lack of field evidence for a significant time break suggests a duration of no more than hundreds of years. Electron microprobe analysis of whole-rock samples throughout the ignimbrite sequence identify multiple populations of hornblende and biotite, whereas orthopyroxene has a relatively narrow compositional range. There is apparently no systematic variation in the chemistry of ferromagnesian silicate minerals with stratigraphic height. In contrast, Fe-Ti oxide minerals show considerable variability with stratigraphic height, becoming more Mg-rich toward the base of the ignimbrite. There is a corresponding trend in calculated Fe-Ti oxide temperatures, with generally high equilibrium temperatures (800-820°C) at the base, and generally lower, but widely variable (730-900+°C) temperatures in middle and upper parts. Study of juvenile pumices has identified five distinct magma types (rhyolites A-D, and high alumina basalt) and significant gradients in temperature, water content, and Sr isotopic composition in the preeruptive magma system. Rhyolite pumice clasts range from 70 to 77 wt% SiO₂, and mixed basalt/rhyolite clasts range from 51.7 to 68.0% SiO₂. There is a marked variation in mineral assemblage with composition. The low silica type A rhyolite pumices contain plagioclase, quartz, orthopyroxene, hornblende, biotite, and magnetite with distinctive large rounded quartz phenocrysts. High silica type B and C pumices contain quartz (smaller, subhedral phenocrysts), plagioclase, sanidine, biotite, and magnetite/ilmenite. Biotite therefore becomes the dominant mafic phase at high silica compositions as orthopyroxene and hornblende disappear in response to increasing P(H20) and decreasing temperature conditions. Calculated Fe-Ti oxide equilibrium temperatures range from 730°C in high silica pumices to 820°C in low silica type A pumices. Rare earth elements show a general enrichment in the more evolved pumices, and progressively increasing Eu* from type A to C. More evolved rhyolite types B and C are related to type A magma by a two-stage crystal fractionation process, probably by side wall crystallisation and convective fractionation within a large, zoned magma chamber. The first step involved 30-40% fractionation of a plagioclase-dominated (but sanidine-free) assemblage to produce a type B magma, which in turn underwent fractionation of a plagioclase/quartzlsanidine assemblage to produce the highly evolved, but relatively Ba-depleted type C magmas. Petrographic and temperature trends in ignimbrite wholerock suggest that eruptions commenced with the hottest, least evolved magmas, and more evolved magmas became important at a later stage in the eruption. This sequence precludes simple sequential tapping of a large zoned magma chamber, and indicates a complex magma chamber configuration and/or withdrawal dynamics during eruption. Two types of plutonic lithics have been recovered from Whakamaru group ignimbrite; leucocratic biotite monzogranite, and medium- to fine-grained dolerites. Whakamaru granites are chemicallymore evolved, and are strongly depleted in HREE compared to granitoid lithics from Atiamuri and Tarawera. They are chemically unlike pumices from Whakamaru group ignimbrite, and are not comagmatic. Rhyolite lavas of the WDB were extruded along a N-S trending curvilinear structure that marks the western boundary of the TVZ, and also coincides with the western margin of the Whakamaru caldera. Analyses fall into two compositional groups; the Western Dome Complex, south of the Waikato River are chemically variable (73.4-76.4% SiO₂), whereas the Northwestern Dome Complex are predominantly high-silica rhyolites (>77% SiO₂). The lavas have similar trace element and REE characteristics to Whakamaru pumices, but have lower ⁸⁷Sr/⁸⁶Sr ratios, indicating they are not simply degassed remnants of the Whakamaru magma system, but represent new crustal melts. The Whakamaru magma system provides clear evidence that (less evolved) low silica rhyolites undergo significant fractionation at shallow crustal levels in TVZ, to produce the generally more evolved rhyolites most commonly erupted at the surface. Type A magma with its relatively high Sr, low Rb and SiO₂, and lack of a significant Eu anomaly may be close to a 'primary' crustal melt composition. Trace element and REE characteristics for selected rhyolite domes and ignimbrites suggest the crustal source for TVZ rhyolites is not homogenous, but may be variable, at least with respect to mineral composition and melting behaviour in space and time.

The Ngatamariki Geothermal Field is located 20 km north of Taupo in the Taupo Volcanic Zone and has a boundary of 12 km² as delineated by magneto-telluric surveys (Urzua 2008). Rhyolitic deposits, derived from the Maroa Volcanic Centre, dominate the geology of the area with the 186 AD (Wilson et al. 2009) Taupo pumice mantling stream valleys in the area. The majority of thermal features at Ngatamariki are located along the Orakonui Stream on the western boundary of the field; the stream area is dominated by a 50x30 m geothermal pool filling a hydrothermal eruption crater. This crater was formed during a hydrothermal eruption in 1948, with a subsequent eruption in April 2005. Orakei Korako is located 7 km north of Ngatamariki and has one of the largest collections of thermal features in New Zealand. The geology at Orakei Korako is similar to Ngatamariki, but the area is dominated by a series of south-west trending normal faults which create sinter terraces on the eastern bank of Lake Ohakuri. Water samples from springs and wells at Ngatamariki and Orakei Korako were taken to assess the nature of both fields. Spring waters at Ngatamariki have chloride contents of 56 to 647 mg/l with deep waters from wells ranging from 1183 to 1574 mg/l. This variation is caused by mixing of deep waters with a steam heated groundwater, above clay caps within the reservoir. Stable isotopic results (δ¹⁸O and δD) suggest that reservoir waters are meteoric waters mixed with magmatic (andesitic) water at Ngatamariki. Reservoir water chemistry at Orakei Korako exhibits low chloride contents, which is anomalous in the Taupo Volcanic Zone. Chloride content in well and spring waters is similar ranging from 546 to 147 mg/l, due to mixing of reservoir fluids with a ‘hot water’ diluent at depth. Isotopic compositions of spring waters suggest that they are meteoric waters which mix with magmatic (rhyolitic) water, more enriched in δ¹⁸O and δD than ‘andesitic’ water. Relationships between major ion concentrations and known subsurface geology suggest there is no hydraulic connection between the two fields.

Soil CO2 flux (φCO₂) has increasingly become important as a global exploration and monitoring tool in geothermal and volcanic fields. As CO₂ is the second most abundant gas in magma-hydrothermal systems, its study is vital for the location or management of those systems. Often one of the only surface expressions is the diffuse gas flux streaming through the soil zone. This thesis reports the investigations into heat and mass at the Rotokawa geothermal field’s thermal area, and White Island volcano’s crater floor hydrothermal system. Surface measurements were taken at high spatial resolution across the fields in a large sampling campaign during the summers of 2010/2011 and 2011/2012. A large dataset was built up which allowed for greater accuracy during geospatial modelling. The models are 2d pixel plots of the soil gas flux and temperature and are used to estimate values of heat and mass flow for the respective magma-hydrothermal systems. Both field areas have a large anomalous diffuse gas flux through the soil zone and related conductive heat flow anomaly, which indicates relative permeability from the source to the surface in these areas. That the rising fluids from the deep source can be sampled at the surface simply is a powerful tool for the exploration and management of these systems. Rotokawa has a diffuse gas release of over 600 t d⁻¹ and an associated heat flow through soil of 37 MWt while White Island has a diffuse gas release of 116 t d⁻¹ and 19.5 MWt of heat flow through the soil. Translating these values to total heat and mass flow values: Rotokawa has a mass flow 125 kg s⁻¹ and a heat flow of 314 MWt and White Island’s crater floor has a mass flow of 100 kg s⁻¹ and a heat flow of 22 MWt. Fluid flow pathways are mapped from the surface and show arcuate and hot spot spatiality, controlled by fault related permeability and structure. soil gas and temperature surveying elucidates Shallow structures that otherwise may have been hidden from status quo surface mapping. The method used in this study is applicable to both known thermal areas and blind thermal areas by addressing not only the flux but also the nature of the soil gases. Further study of White Island has found more evidence for the existence of seawater infiltration of the crater magma-hydrothermal system.

The active precipitation of precious and heavy metals at the surface of hot springs in the Taupo Volcanic Zone (TVZ), North Island, New Zealand, provides a modern analogue for fossilised epithermal metal deposits found throughout the globe. This project explores the relationship, if any, between the microbial populations in these springs and the elevated concentration of metal(loid)s in their precipitates by attempting to replicate the geochemical environment present at the surface of the springs in the laboratory through a series of microcosm experiments. Molecular analysis of microbial mat samples from Waimangu found diverse microbial communities present in the microbial mats, each with a unique community structure. A wide variation in the number and type of microbial phylotypes was found in the spring waters with both bacterial and archaeal DNA amplified. A rod shaped, Gram-positive bacterium was isolated from sediment samples from Echo Crater. Sequence analysis revealed this bacterium has 99% identity with the 16S rRNA gene sequence of Alicyclobacillus herbarius, an acidophilic Gram-positive bacterium, which grows aerobically at an optimum temperature of 55-60°C and an optimum pH of 4.5-5.00. The colonies of the A. herbarius-like isolate appear to be surrounded by a mucous-like material, which may be an exopolysacchride (EPS). Some EPS are known to precipitate metals from dilute solutions and the ability of an EPS layer from the isolate to precipitate metal(loid)s would be of particular interest. A microcosm experiment was constructed to attempt to replicate the geochemistry of the surface hydrothermal environment. Initially, abiotic experiments were conducted to determine the effect of pH on the solubility of metal(loid)s, commonly found in the precipitates of these systems, in the presence of iron and sulphur in the form of sulphate. Though many surface hydrothermal fluids contain sulphur as sulphide, sulphide was not included due to experimental difficulties in preventing loss to oxidation prior to the fluid entering the microcosms. Samples of the precipitates, suspended material and the solution were gathered and analysed using a variety of techniques to determine their chemical and mineralogical composition. It was found that while the precipitates in the AMEs partially resembled precipitates from the natural systems, the lack of sulphide in the fluid prevented the formation of iron pyrite and other metallic sulphides, which are abundant in the natural systems. Interestingly, particles of gold between one to ten microns in width were precipitated in one of the AMEs. The precipitation of hexagonal gold microparticles within the microcosms could represent a new environmentally friendly method for producing these platelets for use in micro- and nanoparticle technology along with other elements of interest.

This thesis is a study of the large scale flows of water and heat which give rise to the geothermal fields in the Taupo Volcanic Zone (TVZ), New Zealand. To carry out this study, a super-critical equation of state module has been developed for the geothermal simulator TOUGH2, which can describe the flow of water at the conditions expected deep in the TVZ. The code is used to simulate the behaviour of a range of idealised TVZ models in 2D and 3D settings. Hydrothermal plumes which remain stable for periods comparable to the lifetime of the TVZ can occur when there is a contrast between the high permeability of the inner TVZ 'infill' region and the lower permeability exterior region. In this case, downflows of cool surface fluid in the inner TVZ 'sweep' the geothermal heat across the TVZ at depth to the permeability barrier, where the heated fluid ascends to the surface in discrete plumes. This behaviour occurs in 2D models, where separate plumes form at each side of the high permeability infill region, and also in 3D models of caldera-like structures, where perhaps four hot plumes can form around the perimeter of the caldera. This notion is then applied to the complete TVZ hydrological system, where a permeable ‘envelope’ is defined by the location of the Taupo Fault Belt and the currently known volcanic centres in the TVZ. The permeability within this envelope varies spatially according to the geothermal heat flux, and the region outside has relatively low permeability. The spatial variation of the geothermal heat flux is obtained by summing the measured heat flows from the geothermal fields for a number of areas across the TVZ. In this model, the geothermal fields form about the boundary of the envelope, as in the TVZ, and bear a striking resemblance to the actual TVZ geothermal fields. Finally, a new simulation code, NaCl-TOUGH2, is developed to provide a tool for future modelling involving the commonest chemical species in the TVZ - salt. The code incorporates the complete phase diagram for salt-water mixtures and involves liquid, vapour and solid phases over a wide range of temperatures, pressures and salt concentrations. The code is used to solve a number of simple geothermal and mathematical problems.

The active Orakeikorako-Te Kopia geothermal system was drilled in the mid-1960’s, down to 1405m, as part of a programme to investigate its electrical generation capability. Four wells were completed at Orakeikorako (23km NNE of Taupo) and two at Te Kopia, 9.5km further northeast. The exploration drilling provided information on the present day hydrological and thermal regime which is as hot as 265°C (1137m drilled depth (-801m RL) in OK-2). Major flows into the wells occurred at depths down to 850m, although poor permeability and decline in mass output discouraged development. The waters discharged were of near neutral pH and had low salinities (highest Cl content from OK-2 ≈546mg/kg), low discharge enthalpies and indicated water temperatures (TSiO2 and TNaKCa) of 2l0°C to 240°C. A hydrologic model proposed here envisages a hot water reservoir in the OK-2 area (northeastern part of the Orakeikorako thermal area) with a lateral flow supplying water to the Red Hill (OK-4 area) in the southern part of the system and a concealed northeast flow which reaches the surface at Te Kopia. The Orakeikorako thermal area occupies a surface area of about 1.8km2, mainly on the east bank of the Waikato River, where dilute chloride-bicarbonate water discharges along faults and fractures in association with an extensive silica sinter sheet, boiling springs and geysers. The occurrence of a mordenite-smectite assemblage at shallow depths, plus the oxygen and hydrogen isotopic composition of surface discharge waters, indicate that the ascending chloride fluids are diluted by near surface (heated?) groundwaters. The δD shift from local groundwater composition may be evidence for a magmatic component to the convecting hydrothermal system. Incursion of fluids from the relatively cool (300°C to ~250°C as it ascended, resulting in the deposition of adularia, quartz and bladed calcite. The system has cooled, resulting in lower subsurface temperatures (as recorded by fluid inclusion geothermometry) suppressing boiling, and migrated northwards as a consequence of self sealing. The thermal decline and retention of CO2 in the deep alkali-chloride fluid shifted the alteration assemblage from one of albite-adularia stability to illite stability. The homogenisation (Th) temperatures of primary and secondary liquid-rich inclusions in 27 cores from different depths mostly match measured temperature profiles (e.g. OK-1 (shallow levels) and OK-2). Never-the-less, fluid inclusion data support mineral-inferred stability temperatures which indicate that parts of the Orakeikorako-Te Kopia system have cooled appreciably (e.g. OK-1, deep levels) and OK-4 (maximum Tbore=238°C, maximum Th=312°C; epidote abundant). In contrast, the northwestern margin (OK-6 area) has heated (OK-6:1113.4m; Tbore=261°C, Th=210-221°C). Some inclusions in the Te Kopia drillholes have Th values that exceed Tbore by as much as 50°C, and are deduced to have been uplifted by movement on the Paeroa Fault. Freezing data indicate that the trapped fluid was dilute (~0.2 to 1.7 wt% NaCl equivalent) since most Tm values range from -0.1 to -0.5°C. The outflow portion of the Orakeikorako-Te Kopia system has evolved recently, both chemically and physically. Movement on the Paeroa Fault, that uplifted pyroclastic rocks hosting a quartz-adularia-illite assemblage, combined with a lowering of the watertable has resulted in an overprinting of the neutral pH hydrothermal mineral assemblage by a kaolinite-alunite type assemblage which derives from an acid sulphate fluid. Quartz crystals found 150m above the base of the Paeroa Fault scarp host dilute (~1.5wt% NaCl equivalent) fluid inclusions with Th values that range from 180-206°C (average 196°C). Bladed quartz (after calcite) did not contain usable inclusions. It is deduced that the inclusions formed about 120-160m below the ground, which indicates uplift in the order of ~300m. Assuming a constant rate of uplift of 4m/ka (based on the offset of 330ka Paeroa Ignimbrite), the minimum duration of activity at Te Kopia is 75,000 years.