After 5 years, the Geothermal: the Next Generation programme is coming to a close.

Here, we summarise the Geology component of the research. This research is contributing to identification of prime location targets for exploration drilling to develop supercritical resources. You can also check out the summaries for Geophysics research and Modelling research.

The aim of our geology research was to refine our understanding of the basement rock and its fracture network, and to advance the understanding of the granitic and magmatic crust. For the first time, we compiled geology and geophysics models of these fields.

Key knowledge advances include:

• We refined the buried geology underneath the area by proposing a new geological model that better fits observed gravity and magnetic data.
• We characterised fracture patterns and orientation and proposed a new understanding of the distribution of basement terranes that are likely to be the reservoir rocks below the current drilled depths.

1)     New understanding of TVZ basement rocks

Our work undertaken produced a step change in the interpretation of basement terrane geometry under the TVZ. Study of basement rocks brought to the surface in volcanic eruptions (xenoliths) has produced a new interpretation of the distribution of these rocks beneath the TVZ.

Figure 1 summarises the new interpretation of terranes (blue and green units) in cross section.

The implications for deep geothermal systems and magma genesis are that:

• TVZ structures are not controlled by reactivated steep, crust-penetrating terrane boundaries (black zones in Figure 1); and
• the middle crust under much of TVZ is likely to comprise relatively quartz- and mica-rich Kaweka Terrane instead of Waipapa Terrane.

This work provides a useful, 10-100 km scale context for more detailed investigations of TVZ geothermal systems.

Accessible data & publications:

• Mortimer, N., Charlier, B.L.A., Rooyakkers, S.M., Turnbull, R.E., Wilson, C.J.N., Negrini, M., Bannister, S., Milicich, S.D., Chambefort, I., Miller, C.A., Kilgour, G., 2023. Crustal basement terranes under the Taupō Volcanic Zone, New Zealand: Context for hydrothermal and magmatic processes. J. Volcanol. Geotherm. Res. 107855. Link.

2)     Fracture permeability in the basement rocks

Fracture densities measured in borehole images in the greywacke basement at the Kawerau Geothermal Field (Figure 2) are high enough to yield a fully connected fracture network at reservoir scale, based on fracture network modelling. The fracture density at borehole scale, and the combination of varied vein orientation and long veins in outcrop suggest that, in a reservoir, some of the veins would be well-oriented for being reactivated in any stress field and could provide connectivity and/or storage for fluids. Increased veining in close vicinity of faults in outcrops suggests a very focused enhanced permeability, as suggested by other studies of greywacke in New Zealand.

A multi-scale framework of controls on fractures and fault in greywacke is being developed to aid in selecting key parameters relevant from siting the supercritical borehole to the interpretation of borehole data.

The implication from borehole and outcrop studies is that there will likely be permeable fractures in deep wells drilled into greywacke. However, modelling of fracture permeability under supercritical pressure and temperature conditions needs to be modelled. In supercritical conditions, fluids need connected fracture networks, but not necessarily fractures of elevated aperture, as fluids have low viscosity. As suggested by laboratory and numerical modelling of granite samples, small cloud-fracture networks may be sufficient to provide sufficient permeability, though the impact of the anisotropy of greywacke and schists has not been tested yet. Further studies on the effect of stress, metamorphic fabric and hydrothermal alteration are needed to confirm this, in addition to supercritical exploratory drilling.

Accessible data & publications:

• S. Milicich, C. Massiot, I. Chambefort, D. Satya (2024) Fracture Permeability in Basement Greywacke for Supercritical Geothermal Drilling Planning. GRC Transactions, Vol 48. Link.
• Kissling, W.M., Massiot, C. Modelling of flow through naturally fractured geothermal reservoirs, Taupō Volcanic Zone, New Zealand. Geotherm Energy 11, 20 (2023). Link.

3)     Guide to magmatic degassing

The composition of New Zealand’s deep magmatic fluids, that drive the TVZ geothermal systems, is relatively unknown. The current conceptual model assumes the deep heat source of hydrothermal fluids is a partial molten zone in the mantle and/or shallow intrusions (~4 km) in the crust. The targeting of deep supercritical or superhot geothermal resources requires knowing: (i) the chemical composition (i.e., the unique fingerprint) of the primary exsolved fluids, and (ii) the spatial distribution of deep-seated magma bodies.

Rock and clay mineral compositions were used to track the magmatic fluid signature in geothermal systems using its metal content. We used a 700,000 year-old buried intrusion and its hydrothermal halo at Ngatamariki Geothermal Field as a proxy for what a shallow magma in the crust could looks like. We compared its geochemistry to the rocks and clays of Rotokawa and Ohaaki geothermal fields. Based on these data, over the >20,000-year lifetime of the Ohaaki and Rotokawa geothermal systems, fluids were dominated by chloride-poor meteoric water and contained little magmatic contributions other than conducted heat, some gases (CO₂- N₂ ± H₂S), and a small fraction of the total H₂O of geothermal waters.

Therefore, the inferred magma bodies of intermediate to silicic composition that lie at shallow depth beneath these geothermal systems currently are not, and likely have not been for >20,000 years, degassing significant water and chloride despite the high water and chloride contents of the magmas. By inference, the intermediate to silicic magmas at depth have not transferred large amounts of volatiles to the geothermal systems over this period.

Accessible data & publications:

• Chambefort, I., Dilles, J.H., (2023). Chemical vectoring in continental geothermal systems: Composition of altered rocks and illite as guides to magmatic degassing. Geothermics 110, 102682. Link.  

4)     Isotopic evidence for deep fluid circulation

High-temperature (> ~300°C) hydrothermal alteration of rocks by surface-derived waters lowers the rock oxygen isotope (¹⁸O/ ¹⁶O) ratios, which can be used to track infiltration of water into the crust. Magmas that melt and assimilate these altered rocks can inherit their low ¹⁸O/ ¹⁶O ratios, providing evidence for hydrothermal circulation down to the depths of magma storage.

We analysed the ¹⁸O/ ¹⁶O ratios (expressed as δ¹⁸O values) of >700 volcanic mineral and glass samples from >90 different TVZ eruptions to look for evidence of these processes in the central TVZ. We found that most TVZ magmas have high δ¹⁸O values, consistent with melting and assimilation of high-δ¹⁸O unaltered greywacke basement. However, our geochemical models showed that the observed magma δ¹⁸O values were usually lower than expected, requiring separate assimilation of low-δ¹⁸O altered rocks as well. This mismatch between observed and expected δ¹⁸O values was widespread throughout the TVZ in both time and space, suggesting that high-temperature meteoric-hydrothermal alteration is prevalent around the upper reaches of central TVZ magmatic systems (typically at depths of >5 – 6 km).

Our results imply that the magmatic-hydrothermal interface in the TVZ is a dynamic zone where proximity between deep-circulating meteoric fluids and shallow magma bodies leads to large-scale interactions between magmas and altered materials. They also affirm the basis for modelling surface water circulation to magmatic depths in the TVZ to explore where supercritical conditions may be reached.

Accessible data & publications:

• Rooyakkers, S.M., Chambefort, I., Faure, K., Wilson, C.J.N., Barker, S.J., Mortimer, N., Elms, H.C., Troch, J., Charlier, B.L.A., Leonard, G.S., Farsky, D., (2023). Absence of low-δ18O magmas despite widespread assimilation of altered crust in a large magmatic and hydrothermal province. Geochim. Cosmochim. Acta 355, 195–209. Link.
• Rooyakkers, S.M., Faure, K., Chambefort, I., Barker, S.J., Elms, H.C., Wilson, C.J.N., Charlier, B.L.A., (2023). Tracking Magma-Crust-Fluid Interactions at High Temporal Resolution: Oxygen Isotopes in Young Silicic Magmas of the Taupō Volcanic Zone. Geochem Geophys Geosystems 24. Link.