Undeveloped high-enthalpy geothermal fields of the Taupo Volcanic Zone, New Zealand

Abstract

In the Taupo Volcanic Zone there are 21 high enthalpy (>225°C) geothermal fields with 7 utilised for power generation. This paper presents an overview of the undeveloped high enthalpy geothermal systems of the region; namely, Rotoma—Tikorangi, Tikitere, Taheke, Rotomahana—Waimangu, Waiotapu (including Waikite), Reporoa, Te Kopia, Orakei Korako, Mangakino, Horomatangi, Tokaanu—Waihi and Tongariro. Most of these geothermal systems are hosted in rocks of rhyolitic composition, although Tokaanu—Waihi and Tongariro are hosted in andesitic rocks. Some of the geothermal systems have extensive surface thermal manifestations (e.g. Waiotapu, 17km2), whereas others have few (e.g. Rotoma—Tikorangi), largely reflecting limited hydrological connectivity to the surface (i.e. impermeable rocks, structures). Uniquely, Horomatangi is fully submerged beneath Lake Taupo. All have liquid-dominated reservoirs of neutral pH alkali chloride waters, except for Tongariro which is inferred to have a vapour-dominated reservoir.Volcanic and tectonic activity, both past and present has impacted the near surface hydrology of geothermal systems in the Taupo Volcanic Zone. For example, the 1886 volcanic eruption of Mount Tarawera resulted in a series of phreatomagmatic and hydrothermal eruptions through the Rotomahana geothermal system, and created new fluid flow pathways and established thermal features at Waimangu Valley. Similarly, tectonic activity with movement along the Paeroa Fault has disturbed the Te Kopia geothermal field by changing the hydrology and resulted in the juxtaposition of acid on neutral alteration minerals in the upthrown fault block. But despite these volcanic and fault interruptions, thermal activity has continued. Several systems show evidence of thermal decline (Mangakino, Orakei Korako, Te Kopia), and the occurrence of altered ground and relict sinter between Te Kopia and Orakei Korako suggest they may at one time have been been hydrologically connected in the near surface.