| Abstract |
At least eight large hydrothermal eruptions have occurred within the Rotokawa Geothermal Field in the last 20,000 years; the latest took place about 3700 years ago. However, the r e are likely to have been many smaller eruptions also - some possibly within the last 1800 years of which no record remains. . The products of the large hydrothermal eruptions cover an area of approximately 15km2; they consist of poorly sorted, matrix supported, polylithologic materials. Clasts comprise several rocks types, including Wairakei Breccia - exposed at the surface - and deeper formations penetrated only by drillholes (lacustrine sedimentary rocks, pyroclastic rocks and rhyolite lava). Many of the clasts are hydrothermally altered; a common assemblage consists of abundant alunite and kaolinite. Other clasts contain hydrothermal feldspars and zeolites. Very few clasts show hydraulic fracture textures. Some of the eruptions may have had deep focal depths. There is mineralogial evidence that alkali chloride waters discharged at the surface at one time but ceased to do so as the water table lowered, possibly causing some of the hydrothermal eruptions as pressures in the shallow part of the field declined. The deduced locations of the eruption vents, aligned in a northeast direction, extend over a distance of 1.7km, parallel to the regional and local fault trends. |