Structure of Concealed Rhyolites and Dacites in the Broadlands-Ohaaki Geothermal Field

Abstract

The structure of concealed igneous bodies in the Broadlands- Ohaaki geothermalfield have been studied by analysis of seismic, gravity, and magnetic data. By using a combination of constraints imposed by borehole stratigraphy, density information, and interpretation of seismic data, 3-dimensional gravity modelling outlines concealed rhyoliteand dacite flows. Broadlands Dacite has been intersected by nearly all wells drilled in the east of the geothermal field and extends over an area of about 16 The flow is thickest m) near the eastern boundary of the field. A magnetic high associated with this body is centred 2 km east of the productive borefield. The anomaly i s attributed to the unaltered, normally magnetised (magnetisation contrast 2.5-3.0 dacite east of the resistivity boundary. This is consistent with the Broadlands Dacite inside the geothermal field having partially or wholly lost its magnetisation through a process of thermal alteration. A smaller rhyolite flow (Broadlands Rhyolite) located in the southern part of the field (magnetisation contrast -1 may be associated with a poorly defined magnetic low Isopachs of the Ohaaki Rhyolite suggest that flows thicken to the northwest. On the basis of this evidence it is conceivablethat the source of the rhyolite is to the west of the Broadlands-Ohaakigeothermal field and part of more extensive rhyolites outcropping to the north and west.