We propose an integrated model that explains the magmatic and hydrothermal features of the unique El Laco iron deposit that is located in a Pliocene–Pleistocene volcano of the Chilean Andes. (Sub)volcanic crystallization of an iron-rich melt as massive magnetite promoted the exsolution of a small volume of a hydrosaline melt and of large amounts of vapor that led to the formation of an alkali-calcic hydrothermal assemblage replacing the host andesite; this assemblage is capped and overprinted by a large zone of acid-sulfate steam-heated alteration forming as a whole a protracted shallow-level magmatic-hydrothermal system. Oxygen isotopic data for the massive magnetite (δ18O: 4.3‰–5.0‰) and the alkali-calcic altered rock (diopside δ18O: 7.2‰–8.7‰; magnetite δ18O: 4.4‰–6.7‰) suggest that these rocks are genetically related to the host andesite (δ18O: 7.4‰–9.6‰). The estimated temperature of the mineral assemblage (>∼900 °C) may be the highest recorded in hydrothermal systems, is attributed to exsolution of fluids from the crystallizing iron-rich melts, and is considered unlikely to reflect the convection of surficial fluids.

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