A comparison of buried andesites at Ngatamariki and Rotokawa geothermal fields, Taupo

Abstract

The Rotokawa and Ngatamariki geothermal systems are hosted by hydrothermally altered Quaternary andesitic, dacitic and rhyolitic volcanics resting on Mesozoic Torlesse greywacke basement, and covered by lacustrine sediments and surficial deposits. Andesitic lava and breccia (> 2 km thick) below 1 km depth at Rotokawa is an important production aquifer for geothermal power due to its interconnected fracture permeability. Exploration drilling at Ngatamariki has revealed andesite at several depth intervals, with an approximate age of 1.2 Ma for the shallow andesite lavas, interlayered with ~1 km of ignimbrite. Rotokawa andesite is a massive to flow-banded, fine-medium grained porphyritic lava with primary plagioclase (An51-82), clino- and orthopyroxene in an aphanitic groundmass. Accessory minerals are biotite and Fe-Ti oxides. The Ngatamariki andesite has primary plagioclase (An54-82) and clinopyroxene phenocrysts. Intercalated breccia contains andesite, minor rhyolite and greywacke lithics in a fine grained matrix. Hydrothermal alteration is propylitic, albeit with minor potassic or argillic mineralogy resulting from variable conditions within the thermally-evolving reservoirs. The andesite is commonly veined and/or has a vuggy texture. XRF analysis, particularly plots of immobile element data, e.g. Ti vs. Zr, Y vs. Zr, has been used to fingerprint the hydrothermally altered volcanics, and infer the intensity of fluid-rock interaction. The TiO2/Zr ratio in Rotokawa andesite is 27-63 whereas in the Ngatamariki andesite it is 13-48. Chemical data, combined with inferred stratigraphic relationships suggest that Rotokawa andesites derive from an older, basaltic andesite composite cone volcano, compared to a younger dacite/andesite volcanic centre at Ngatamariki.