The Relationship Between Geological Structures and High Temperature Geothermal Systems in the Eastern Taupo Volcanic Zone (New Zealand) as Seen from High Resolution Airborne Magnetic Data

Abstract

An airborne magnetic survey carried out in 2005 by the gold exploration company Glass Earth NZ Ltd. included several geothermal systems in the Eastern Taupo Volcanic Zone (ETVZ) in central North Island of New Zealand. The magnetic measurements were made at 60m ground clearance along EW flight lines at 150m spacing. These measurements provided a continuous set of high resolution magnetic data which allows a close interpretation of geological structures and hydrothermal demagnetisation across the survey area. Strong negative magnetic anomalies occur over the Orakeikorako, Ngatamariki and Te Kopia geothermal systems. Moderately strong negative magnetic anomalies were measured over the Wairakei (only partially covered by this study) and Waiotapu geothermal systems. The negative magnetic anomalies over the Reporoa and Ohaaki geothermal systems are subdued. No negative magnetic anomalies appear to be directly associated with the Rotokawa geothermal system. The strong negative magnetic anomalies over the three geothermal systems of Te Kopia, Orakeikorako and Ngatamariki correlate with major geological structures Paeroa and Orakeikorako Fault Zones, and edges of the Whakamaru Group Caldera. They also appear to be associated with the south-western edge of Reporoa Basin recently delineated by a gravity modelling. To further investigate this correlation, a regional scale three dimensional magnetic modelling was carried out across the ETVZ study area to obtain the likely distribution of subsurface magnetisations causing the observed magnetic anomalies. The modelling results show that the Orakeikorako, Ngatamariki and Te Kopia geothermal systems are all marked by intensive hydrothermal demagnetisation. However, the presence of reversely magnetised rocks complicates the magnetic model of Te Kopia. The Wairakei, Waiotapu, and Reporoa and Ohaaki geothermal systems have only moderate to weak intensity of hydrothermal demagnetisation, mostly at depths. The Rotokawa geothermal system is marked by a poor amount of hydrothermal demagnetisation. The hydrothermal demagnetisation at the Wairakei geothermal system may be associated with northeastern end of the Kaipo Fault Zone. The Ngapouri Fault Fault Zone runs through the northwestern part of the Waiotapu Geothermal system but the lateral extents of the hydrothermal demagnetisation do not appear to follow the fault zone direction. The Ohaaki geothermal system occurs over the southeastern edge of Reporoa Basin. No major geological structures run through the Reporoa and Rotokawa geothermal systems. The strong correlation between major geological structures and intensive hydrothermal demagnetisations at Orakeikorako, Ngatamariki and Te Kopia suggests the geological structures create vertical permeable paths that allow rigorous upflows of thermal fluids. However, the case of Wairakei, Waiotapu and Ohaaki indicates this situation is not always true. Direct relationship between intensity of hydrothermal demagnetisation and the productive part of a geothermal reservoir remains to be seen. Detailed pattern of hydrothermal demagnetisation at the smaller scale of local individual geothermal system can be modeled form the airborne magnetic data, which could then be compared with the known actual production parts of the reservoirs. Unfortunately, the Orakeikorako, Te Kopia, Waiotapu and Reporoa geothermal systems have been designated protected areas and no geothermal fluids would be produced from the reservoirs for electricity generation.