Formation of iron oxide–apatite deposits

Renewed economic interest in iron oxide–apatite (IOA) deposits — containing tens to hundreds of millions of tonnes of Fe and substantial amounts of rare earth elements, P, Co and V — has emerged to supply the sustainable energy transition. However, the mechanisms that efficiently concentrate dense iron-rich minerals (for example, in ores up to ~90% magnetite) at the Earth’s near-surface are widely debated. In this Review, we discuss synergistic combinations of magmatic and hydrothermal iron-enrichment processes that can explain the available geochemical, petrological and geological IOA data. IOA deposits typically evolve from subduction-related water-rich and chlorine-rich intermediate magmas under a wide temperature range, almost spanning the whole igneous–hydrothermal spectrum (from ~1,000 to 300 °C). Magmatic–hydrothermal fluids could efficiently scavenge Fe from magmas to form large IOA deposits (>100 million tonnes of Fe), whereas crystal fractionation and liquid immiscibility processes might account for more minor Fe mineralization occurrences. Igneous magnetite crystallization, volatile exsolution and highly focused transport of Fe-rich hydrothermal fluids through the crust under extensional tectonic conditions could be key factors enabling concentration of dense magnetite minerals in the less-dense upper crust. Future research should target both fertile and barren mafic–intermediate magmatic suites for distinctive signatures diagnostic of metallogenic fertility, to help unravel the genetic linkage between IOA and iron oxide–copper–gold systems.