Our knowledge of active magmatic hydrothermal system (or magmatic vapors) is largely related to the modeling of geochemical processes including heterogeneous equilibrium calculations with variable bulk composition, temperature or pressure. With the aim to constrain the characteristics of extinct hydrothermal systems it is necessary to look for petrogenic evidence in active ones. The Kudryavy volcano in the Kurile Islands (Russia) provides a unique natural laboratory to study the formation of fumarolic rocks from high temperature fluids (measured temperature for the fumaroles up to 940 °C). By studying the minerals chemistry and assemblages and by computing a thermodynamical modeling, we investigated the mineralogical evidence for such high-temperature. We confirm the documented occurrence of Ca-Fe rich minerals as isolated patches, veins or incrustation in cavities and their formation during the alteration of primary minerals. We describe and documented secondary Na-Al rich minerals (davyne, nepheline, sodalite) and their occurrence together with Ca-Fe minerals in cavities. We also documented the presence of Fe-wollastonite, a relatively rare mineral suggesting high-T formation and we observed assemblages including wollastonite + andradite + magnetite suggesting high-temperature decomposition of andradite. This natural laboratory provides a series of concordant petrologic evidences for high-temperature fumarolic rocks and offers guidelines for the study of ancient high temperature hydrothermal environments on Earth and other planets like Mars.