Geology, Geochronology, Alteration and Geochemistry of the Rotokawa Geothermal System, TaupÅ Volcanic Zone, New Zealand

A more detailed understanding of the Rotokawa geothermal system, central Taupō Volcanic Zone (New Zealand), has been realised in order to constrain the magmatic, volcanic, structural and hydrothermal evolution of the hottest utilised geothermal system of New Zealand. Geochronology undertaken on buried hydrothermally altered lithologies at Rotokawa has provided constraints on the stratigraphy and volcanic evolution of the region. A 3-km-thick sequence of volcanic products is present at Rotokawa. This sequence is comprised of rhyolitic ignimbrites from large, caldera-forming events at volcanoes outside the field area, and locally sourced andesite and rhyolite lava bodies. The oldest volcanic rock from U-Pb dating of zircon is an ignimbrite, which yields an eruption age estimate of 1.84 ± 0.04 Ma. This ignimbrite, part of the Tahorakuri Formation, is among the oldest silicic volcanic deposits from the Taupō Volcanic Zone and can be linked to comparably-aged counterparts at three other nearby geothermal systems (Ngatamariki, Ohaaki and Kawerau). These ignimbrites onlap a basal andesite lava pile, up to 1.2 km thick, that rests on Mesozoic basement greywacke. Between ~1.8 and 0.7 Ma, there are no eruptives represented at Rotokawa, with the next oldest lithology being a rhyolite lava dated at 0.72 ± 0.09 Ma. At 350 ka, the Rotokawa area was buried by regionally extensive ignimbrites of the Whakamaru Group. Ignimbrites and sediments of the Waiora Formation were then emplaced over a 150 kyr period, coeval with dome-building activity forming the Maroa dome complex to the west, with this same rhyolite buried at the nearby Ngatamariki and Wairakei geothermal systems. Extensive rhyolitic lava bodies of the 90 ± 10 ka Oruahineawe Formation show stratigraphic and petrographic relationships suggesting both extrusive dome and shallow intrusive emplacement. Dominantly lacustrine sedimentary rocks of the Huka Falls Formation and pyroclastic deposits of the 25.4 ± 0.2 ka Oruanui eruption then cap and seal the system. The geothermal system is typical of the high gas and 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 by hydrothermal minerals with alteration to abundant quartz, chlorite and calcite with common but variable amount of adularia, epidote, calcite and illite at depth. Zones of acid alteration characterised by kaolinite occur at shallow depths and can occur up to 1,000 m below the ground surface. Arsenic, Sb, and S abundances increase toward the surface, likely associated with sulfur complexing and deposition upon boiling and cooling.