WHANGAIROROHEA THERMAL AREA AND THE DEMISE OF A HIGH TEMPERATURE TVZ GEOTHERMAL SYSTEM

Abstract

Whangairorohea, Taupo Volcanic Zone (TVZ), New Zealand, occurs on the Waikato River approximately 7 km directly west of
Ohaaki and 5.5 km east-northeast of Ngatamariki geothermal fields. Several warm seeps occur along a 1.2 km stretch of the northern
bank of the river. There are also extinct silica sinters and silicified surface deposits, as well as a few small circular warm and cold pools (20-50 m diameter). These surface features are evidence that the area once hosted a high temperature (>210°C) geothermal reservoir, with vigorous surface activity, that has since waned. The area is situated on the southern Paeroa block, an asymmetric horst feature that has been displaced by the Paeroa Fault, the most active fault in the TVZ and one that defines the eastern edge of the Taupo Fault Belt. Recently published geological mapping in the area north of Whangairorohea shows additional sinter and hydrothermal eruption deposits that demonstrate geothermal activity was once more widespread (an area approximately 10 x 5 km). Aeromagnetic data suggests a subsurface, shallow rhyolite body has been locally demagnetised by hydrothermal alteration. This work proposes that Whangairorohea geothermal area was once part of a high temperature geothermal reservoir with vigorous and widespread surface manifestations. Its demise resulted from vertical displacement of the Paeroa block, which affected reservoir fluid pathways and deepened aquifers, and demonstrates how high temperature TVZ geothermal systems are prone to irreversible change from active faulting.