Iron-rich melts, magmatic magnetite, and superheated hydrothermal systems: The El Laco deposit, Chile

Tornos, F., Velasco, F., Hanchar, J. M. 2016. Iron-rich melts, magmatic magnetite, and superheated hydrothermal systems: The El Laco deposit, Chile. Geology 44, 6, 427-430, DOI: 10.1130/G37705.1

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 (delta O-18: 4.3 parts per thousand-5.0 parts per thousand) and the alkali-calcic altered rock (diopside delta O-18: 7.2 parts per thousand-8.7 parts per thousand; magnetite delta O-18: 4.4 parts per thousand-6.7 parts per thousand) suggest that these rocks are genetically related to the host andesite (delta O-18: 7.4 parts per thousand-9.6 parts per thousand). The estimated temperature of the mineral assemblage (>similar to 900 degrees 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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