Vertical and lateral changes of hydrothermal alteration in andesitic composite volcanoes – Linking flank collapse with hydrothermal activity

Andesitic composite volcanoes located at convergent plate margins can host extensive zones of hydrothermally altered rocks, which can influence their rock mechanics and potentially modulate eruptive activity. Hydrothermal minerals typically form along fractures and permeable zones above cooling and degassing magma bodies. A robust understanding of the vertical and lateral distribution of hydrothermal minerals can therefore reveal the location of (former) magma bodies/fluid source and can indicate areas susceptible for future flank collapse processes.

This study investigates the type and extent of hydrothermal alteration within the Wahianoa Formation (160-80 ky) of Ruapehu volcano in New Zealand by integrating field observations, Scanning Electron Microscopy (SEM-EDS), Shortwave Infrared (SWIR) reflectance spectroscopy, X-Ray Diffraction (XRD), sulfur isotope systematics and Inductively coupled plasma mass spectrometry (ICP-MS), thermodynamic modelling and airborne geophysics. Ruapehu shows a diverse suite of weathering and hydrothermal alteration minerals formed in relation to the present and fossil hydrothermal systems. Wahianoa Formation is one of the oldest formations, showing remarkable diversity of hydrothermal alteration that has never been studied before. The distal rock has only supergene alteration with abundant goethite, hematite and phyllosilicate mineral associations, while the hydrothermally altered rock are rich in phyllosilicates, Fe-oxides, pyrite, jarosite, alunite, gypsum anhydrite, and native sulphur. The latter is interpreted to be formed under intermediate and advanced argillic alteration conditions (>150 °C and low pH). In contrast, the some of the exposed outcrops within the upper Wahianoa valley show distinct mineralogy, that is rich in quartz, pyrite, illite(-chlorite) and tourmaline, indicating a transition from the advanced argillic conditions toward more phyllic alteration type (>220 °C and more neutral pH). Our results indicate a complex hydrothermal system developed within the Wahianoa Formation between 150-80 ky, providing a great example to study vertical and lateral mineralogical changes. A new model has been proposed to integrate hydrothermal alteration history into the Mt Ruapehu’s evolution that can better depict ongoing alteration processes and triggers for flank instability and volcanic hazards associated hydrothermal systems.