The role of fossil hydrothermal systems on volcanic flank collapses—Deciphering the volcanic history of Ruapehu, New Zealand

Hydrothermal systems are common to many andesitic composite volcanoes, yet their evolution, mineralogy, and significance often remain underexplored compared to eruptive and magmatic histories in such settings. This study highlights new approaches to understand the evolution of composite volcanoes by leveraging insights gained from geological mapping of hydrothermally altered deposits. We applied extensive fieldwork and sampling, combined with scanning electron microscopy–energy dispersive X-ray spectroscopy, shortwave infrared reflectance spectroscopy, X-ray diffraction, stable isotope geochemistry, X-ray fluorescence, and aeromagnetic surveys. Using Ruapehu (New Zealand) as a case study, we demonstrate how such analyses can provide critical information for improving hazard assessment for flank instability and collapses. We examine fossil and active hydrothermal systems exposed in the southern sector of Ruapehu that show a diverse suite of weathering and hydrothermal mineralogy formed since 160 ka. Most of these are located with the Wahianoa Formation (160–80 ka), ranging from deeper phyllic (>220 °C and more neutral pH) to shallower argillic alteration zones (80–200 °C and low pH). This transition reflects variations in fluid temperature and pH within the exposed fossil hydrothermal system. Additionally, the outcrops reveal a complete lateral progression, from core alteration zones, dominated by alunite, pyrite, silica polymorphs, kaolinite, pyrophyllite, and smectites, to the more distal regions characterized by intermediate argillic and supergene alterations, featuring abundant goethite/hematite, pyrite, Fe-sulphates, and phyllosilicate mineral assemblages. We found a complex interplay between the volcano growth and hydrothermal alteration histories. A new model has been proposed to integrate hydrothermal alteration history into Ruapehu’s evolution that can better depict ongoing alteration processes and triggers for flank instability and volcanic hazards associated with hydrothermal systems.