Alteration minerals and their formation environments in intrusion-related hydrothermal systems at Mt. Ruapehu (New Zealand) – New insights from mass balance, thermodynamic modelling and stable isotopes

In this study, we combined field observations and microtextural analyses of the altered rocks with mass balance calculations, thermodynamic modelling, and stable isotopes to establish the formation and equilibria environments of alteration minerals and hydrothermal fluids at Mt. Ruapehu (New Zealand). Results indicate that the secondary minerals follow different precipitation sequences as a function of temperature and water type (i.e., either acid at the proximal distant areas from the summit/crater, or distal areas affected with rain-glacial composition), interacting with the magmas of intermediate composition. Variations in composition and abundances of the hydrothermal paragenesis in Mt. Ruapehu determine the evolution of its acid–sulfate alteration that also envisage a crystallization-driven degassing. We conclude that the combination of the proposed petrologic-geochemical-isotopic approach, along with geological, geophysical and the regional and local tectonic features, can effectively be used for the assessment of volcanic-hydrothermal eruptions, interpretation of monitoring data (degassing) and/or flanks instabilities. • Integration of petrology, thermodynamics and stable isotopes of each equilibria environment of alteration minerals and hydrothermal fluids at Ruapehu (New Zealand). • Newly-grown alteration minerals result from water-rock reactions within different hydrothermal environments of the volcanic edifice. • Precise physico-chemical conditions of the connection between the plumbing system and the volcano's surface.