Reactions among magmas, evaporites, and iron oxide−phosphate-rich strata promote iron oxide-apatite (IOA) deposit formation

Competing hypotheses for the origin of iron oxide-apatite (IOA) deposits include magnetite flotation, immiscible iron-rich melts, salt melts, hydrothermal fluids, or metasomatism. However, no individual hypothesis explains all observations in natural IOA deposits. IOA deposits are spatially associated with evaporites and iron oxide−phosphate-rich (FeP-rich) sediments, but no clear genetic link exists. We ran piston-cylinder experiments at 0.4 GPa and 1000−1200 °C to react silicate magmas with sulfate evaporites and FeP-rich analogues of red beds and phosphorites. Quenched products show chlorine-rich silicate glass, dendritic magnetite, iron oxide-phosphate−silicate (FeP−Si) liquid immiscibility, Fe-bearing salt melts, and fluid bubble adherence to magnetite and apatite. These reproduce key features of all proposed competing hypotheses and are similarly observed in natural IOA deposits. Our experiments reveal that these distinct genetic models are not mutually exclusive but could instead work together to form IOA deposits. When silicate magmas intrude a ferrous sedimentary sequence with evaporites and phosphates, immiscibility, magnetite-fluid interaction, and salt melts can all form.