GEOLOGICAL EVOLUTION OF THE ROTOKAWA GEOTHERMAL SYSTEM, NEW ZEALAND

Abstract

New data from the Rotokawa geothermal system (central Taup? Volcanic Zone), has constrained the magmatic, structural and hydrothermal evolution of the hottest utilised geothermal system in New Zealand. U-Pb zircon geochronology data on buried lithologies at Rotokawa provide constraints on the 3-km-thick sequence of volcanic products. The oldest volcanic rock dated is a Tahorakuri Formation ignimbrite, with an eruption age estimate of 1.84 ± 0.04 Ma. This and other old ignimbrites onlap the Rotokawa Andesite lava pile, up to 1.2 km thick, that rests on Mesozoic basement greywacke. Between ~1.8 and 0.7 Ma, there is a magmatic hiatus, with the next oldest being a rhyolite lava dated at 720 ± 90 Ma. At 350 ka, the area was buried by ignimbrites of the Whakamaru Group. Ignimbrites and sediments of the Waiora Formation were then emplaced over a 150 kyr period. Extensive rhyolitic lava bodies of the 90 ± 10 ka Oruahineawe Formation show evidence suggesting both extrusive dome and shallow intrusive emplacement. Mostly lacustrine sediments of the Huka Falls Formation and pyroclastic deposits of the 25.4 ka Oruanui eruption then cap the system. Rotokawa is typical of high gas and high enthalpy New Zealand geothermal systems with a deep chloride water reservoir and an excess steam phase. As a result, the volcanic and sedimentary succession has been variably altered, to abundant quartz, chlorite and calcite, with common but variable adularia, epidote, calcite and illite at depth.