| Title | Relict Magmatic-Hydrothermal Alteration at Ngatamariki: Implications for the Deep Roots of TVZ Geothermal Systems |
|---|---|
| Authors | B. Lewis, I. Chambefort, A. Rae, G. Bignall, C. Coutts |
| Year | 2013 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Ngatamariki geothermal field, Taupo Volcanic Zone, potassic alteration, phyllic alteration, advanced argillic alteration, SWIR spectrometry |
| Abstract | Zones of hypogene potassic and advanced argillic alteration have been encountered for the first time in the central Taupo Volcanic Zone (TVZ). Geothermal drilling at Ngatamariki in 2012 for Mighty River Power intercepted the diorite pluton and phyllic alteration halo first encountered by NM4 in 1985. Wells NM8 and NM9 encountered not only the diorite and phyllic alteration halo, but also a tonalite body, mafic dykes, and high-temperature magmatic-hydrothermal alteration minerals not previously recognised at Ngatamariki or in any TVZ geothermal fields. The phyllic alteration halo is a vertically extensive (~1.5 km thick) zone characterised by texturally destructive quartz-muscovite-pyrite alteration. Detailed study of alteration mineralogy and textures in wells from the northern part of the field using short-wave infrared spectroscopy, XRD, and thin section microscopy indicate that high-temperature potassic and advanced argillic alteration minerals are also present. Biotite-magnetite ± Kfeldspar alteration is present within and immediately above the intrusive complex, constituting a potassic alteration halo formed by high-temperature magmatic-hydrothermal fluids exsolved during magma degassing. Above this potassic core and cross-cutting the phyllic zone is a zone of pyrophylliteandalusite- topaz (± anhydrite and vuggy silica) advanced argillic alteration formed by upward migration of acidic magmatic-hydrothermal fluids as far as 1.4 km above the pluton. These zones of hypogene potassic and advanced argillic alteration are the first to be recognised in a TVZ geothermal field, and are analogous to those found in porphyry copper systems, high-sulfidation epithermal gold systems, and andesite-arc setting geothermal systems (e.g. Indonesia, Philippines). Furthermore, the record of magmatic-hydrothermal fluid-rock interactions preserved at Ngatamariki may be analogous to processes occurring at depth currently beneath other TVZ geothermal systems. |