Magmatic immiscibility and the origin of magnetite-(apatite) iron deposits

The origin of magnetite-(apatite) iron deposits (MtAp) is among the most contentious issue in ore geology with competing models involving purely hydrothermal to magmatic processes. We study here melt inclusions trapped in plagioclase phenocrysts in andesite hosting the emblematic MtAp mineralization at El Laco, Chile. Our high-resolution study reveals that individual melt inclusions preserve complex processes of melt immiscibility including the separation of Si- and Fe-rich melts, the latter hosting other Cu-sulfide, phosphate-rich, and C-O-HFSE-rich residual melts. This assemblage is a small-scale analogue of the ore mineralization and establishes the missing link between silicate and Fe-P-rich melts which subsequently produce extrusive magnetite. These results strongly indicate that El Laco mineralization derives from crystallization of Fe-P-rich melts providing insight into the formation of similar deposits elsewhere.