- © 2017 Society of Economic Geologists.
The Ngatamariki geothermal system is one of more than 20 high enthalpy (>225°C) geothermal systems in the Taupo Volcanic Zone, North Island, New Zealand. At shallow levels (<2 km), they are analogous to low-intermediate sulfidation state epithermal ore-forming environments. Unique to Ngatamariki is the occurrence of an intrusive complex with an associated magmatic-hydrothermal alteration halo that was intersected by deep geothermal drilling (~3 km depth) and that resembles hydrothermal alteration associated with high-intermediate sulfidation state epithermal mineralization. This paper describes the results of a study involving alteration petrography, X-ray diffraction, shortwave infrared (SWIR) reflectance spectroscopy, backscattered electron scanning electron microscopy/energy dispersive spectroscopy (BSE-SEM/EDS), plus whole-rock and trace element geochemistry to document and characterize distinct hydrothermal alteration found nowhere else in the Taupo Volcanic Zone. Two separate phases of hydrothermal activity are distinguished, old and modern, as defined by a paleosurface that is dated at 0.68 Ma, which occurs ~500 m below sea level.
A composite plutonic body comprising intrusions of diorite to tonalite was encountered in three adjacent drill holes (NM4, NM8, and NM9) between 2,000 and 3,200 m below sea level (~2,300–3,500 m depth below the surface) in the northern part of the Ngatamariki system. The associated hydrothermal alteration is zoned and made up of potassic, advanced argillic, phyllic, and propylitic mineral assemblages that occur between 500 and 2,500 m below sea level. Subtle potassic alteration consisting of biotite + magnetite ± K-feldspar mantles the intrusive complex. It is crosscut by a hypogene advanced argillic alteration containing pyrophyllite ± minor andalusite ± topaz ± anhydrite ± rare aluminophosphates (AP) and fluorine-bearing minerals, but lacks alunite. The deep-formed advanced argillic alteration appears in some samples to be overprinted by phyllic alteration, made up of quartz + muscovite + pyrite. Between 500 and 1,000 m below sea level, the intense phyllic alteration is less pervasive, and the hydrothermal alteration is dominated by kaolinite, rare dickite, and localized occurrences of highly silicified rocks that resemble vuggy quartz, which is bounded at the top by the paleosurface (defined as the unconformity at the base of the overlying Whakamaru group ignimbrite). In the central and southern part of the system below the paleosurface, propylitic hydrothermal alteration consisting of chlorite + calcite + epidote ± wairakite ± actinolite (along with ± albite and ± illite) is widespread, and could equally have formed during old hydrothermal activity associated with emplacement of the intrusive complex or in the deep hot parts of the modern hydrothermal system.
The mineralogy and geochemistry of advanced argillic altered rock indicates that acidic magmatic-hydrothermal fluids leached base cations, resulting in the loss of elements typically considered immobile (Al, Ti, Y, Zr, Nb, as well as rare earth elements) to form cation-depleted minerals such as pyrophyllite, andalusite, and topaz. The strongest enrichments in Au (0.6 g/t), Ag (4.6 g/t), Te, As, Sb, and Bi coincide with intense acid alteration at <250-m depth beneath the paleosurface. The results of this study reveal a complex history of intrusion and hydrothermal activity that provides a modern example of successive development (~600,000 years apart) of acid and neutral pH hydrothermal alteration assemblages, which are associated with the two end member types of epithermal mineralization.