Magnetite-apatite deposits at El Laco volcano, Chile, formed by extrusion of an iron-rich melt

The origin of magnetite-apatite (MtAp) deposits has recently come under renewed scrutiny. Geological and geochemical studies describe volcanic-hosted magnetite deposits at El Laco, an andesitic stratovolcano in the Central Volcanic Zone, northern Chile, which include vesiculated lava flows and pyroclastic deposits made of magnetite. Observations indicate a formation by eruptive emplacement of a highly iron-rich melt. Recent experiments confirm that such an ore-forming liquid may form by spontaneous separation of an andesitic parent magma into Fe-rich and Si-rich immiscible liquids in a shallow magma reservoir. Their effusive to explosive extrusion, however, poses a puzzle: how would such high density Fe-rich melt have ascended and erupted to generate the ore deposits? Observations of ubiquitous vesiculation, outgassing pipes, brecciation, and steam-heated alteration provide important clues as to the pivotal role of volatiles as drivers of extrusion.

In the present study, we combine high-resolution geochemical analyses, thermodynamic calculations of liquid immiscibility, mechanical modelling of volcano deformation, and scaling analysis of bubble- driven extrusion along collapse faults to test a formation hypothesis where an Fe-rich liquid forms in a shallow magma reservoir and subsequently ascends as a bubbly suspension along volcano collapse faults. Our results indicate that the liquid immiscibility is a robust feature for compositions in the range of El Laco andesites. Our results show that magma reservoir deflation combined with the topographic load of the edifice and minor tectonic extension may give rise to collapse faults connecting the base of a shallow crustal magma reservoir, where the liquid is gravitationally concentrated, with locations on the volcano flanks, where ore deposits are found today. Finally, our analysis suggests that volatile exsolution and bubble expansion upon ascent along fractures may provide sufficient driving force for an eruptive emplacement despite the high density of the ore-forming liquid.

Ore formation by volcanic emplacement of exotic melts likely applies to MtAp deposits globally. Our work furthermore has important ramifications for understanding resource exploration, magma differentiation, crust formation, and planetary research.