A multidisciplinary approach to constrain hydrothermal alteration history on Mt Ruapehu (New Zealand)

Composite volcanoes can progressively weaken through hydrothermal alteration, which may lead to volcano collapse, forming far-reaching debris avalanches. Hydrothermal minerals can also contribute to flank instability as they play a critical role in moderating volcanic degassing by changing the porosity and permeability of the rock and thereby changing the local pore-pressure distribution. Therefore, a robust model and understanding of hydrothermal alteration within a volcanic edifice is important to improve hazard assessment efforts. This study investigates the type and extent of hydrothermal alteration on Mt Ruapehu, New Zealand, using a combination of mineralogical, hyperspectral imaging, and aero-magnetic studies.

Mt Ruapehu shows a diverse suite of surface weathering and hydrothermal alteration minerals, which are distributed heterogeneously on the surface. The surface weathering has abundant goethite, hematite and phyllosilicate mineral associations, while the hydrothermal alteration is characterised by phyllosilicates, Fe-oxides, pyrite, jarosite, alunite, gypsum anhydrite, and native sulphur minerals. Although surficial evidence of alteration on Mt Ruapehu is limited, aero-magnetic data and inversion modelling indicate deep-seated (≤500 m) alteration of demagnetized rocks. The decrease of magnetic susceptibility can be linked to the dissolution of (Ti-) magnetite phases, as well as the deposition of brecciated horizons between lava flows and intercalated glacial till and volcaniclastics. Surface outcrops mapped by airborne hyperspectral imaging combined with Scanning Electron Microscopy (SEM-EDS), Short wavelength Infrared (SWIR) Spectroscopy, X-Ray Diffraction and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) data of ground samples reveal a complex alteration history developed around older vent/crater systems of Mt. Ruapehu in last 250 ky. This study provides a simplified geological model to capture the hydrothermal processes on Mt Ruapehu, aiding future studies on delineating areas prone to mass movements.