Lichen have been used as indicators of atmospheric pollutants since Grindon (1859) observed lichen populations declining in a polluted Southern Lancashire in the mid-1800s. Since then lichen have been used in a number of atmospheric studies. A study by Tozer et al. (2005) attempted to use nitrogen isotopes of lichen and free-living algae as indicators of geothermal ‘pollution’ near Rotorua and the Te Kopia Geothermal Area, but was unable to show a correlation with distance to geothermal features. This thesis aims to build from Tozer et al. (2005) and use both carbon and nitrogen isotopes in lichen as an exploration tool in geothermal areas. Three transects were completed: one across the South Island from Christchurch to Greymouth (non-geothermally influenced area), and two along (north-south) and across (east-west) the Taupo Volcanic Zone (TVZ) in the North Island (geothermally influenced area). In addition to these three transects, sampling at higher spatial resolution was conducted in the immediate vicinity of the Orakonui Stream geothermal springs at the Ngatamariki Geothermal Area. The three transects showed large variation, largely due to the type of land use from which the sample was collected. The highest nitrogen contents (1.62 ± 0.39%) and less negative nitrogen isotopic compositions (-9.44 ± 0.39‰) were found over farmland, while both exotic and native forests had low nitrogen (1.08 ± 0.35% and 1.03 ± 0.44‰, respectively) and highly negative isotopic compositions (-12.94 ± 0.26‰ and -12.09 ± 0.45‰, respectively). The statistical difference between land use classes is hypothetically explained by variations in nitrogen sources, with intensive farmland volatilizing NH3 with δ15N values of -6 to -10‰ (Tozer et al., 2005), while forest areas are expected to produce biogenic nitrogen from decomposition with more negative δ15N. At Ngatamariki, δ13C and δ15N isoscapes were produced, with both showing a large isotopic anomaly (>-23.5 and >-8‰, respectively) to the north and north-west of the study area, correlating with areas of farmland, although in some places the δ15N values exceed 0‰, which is unexplained. A study by Hanson (2014) identified diffuse soil flux using δ13C in the vicinity of the Orakonui South Main Crater to have a geothermal signature, the same location in which a small relatively less-negative δ13C anomaly (>-23.5‰) is seen in lichen isotopes. While this could be attributed to a geothermal influence, it could also be due to the effect of substrate the lichen lives on and a reduction in carbon sourced from biogenic respiration. Ultimately, there is the potential for isotopes in lichen to be used as a geothermal exploration tool, although this method needs to be investigated in a higher flux geothermal area, such as Rotokawa, 7km to the south of Ngatamariki.

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.

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.