Geological Results from Drilling in the Poihipi (Western) Sector of the Wairakei Geothermal Field, NZ

Abstract

Four wells drilled into the Poihipi Sector on the margin of the Wairakei geothermal field have found similar lithostratigraphy to that encountered in wells previously drilled in the general area. Young pumice breccias overly Falls Formation, with the latter containing of the Rautehuia Breccia. This in turn overlies ignimbrites and tuffaceous sediments of the Waiora Formation, which contains flows of Haparangi Rhyolite. This sequence is by steeply dipping normal faults which strike to the northeast and for the most part dip towards the northwest. Hydrothermal alteration is virtually limited to the Waiora and Haparangi units where a sequence of interlayered illitesmectite and illite clays are found along with chlorite, quartz, pyrite and calcite. There is a minor occurrence of zeolites. Despite large changes in the area's hydrology in response to exploitation, changes in alteration are limited to a comparatively deep occurrence of kaolinite and minor overprintingof epidote by illitic clay.