The Se-Chahun magnetite-apatite deposit (Bafq district, central Iran) comprises several orebodies with large-scale replacement and brecciation textures, and a sodic-calcic alteration envelope. Such members of the iron-oxide copper-gold (IOCG) deposit family commonly exhibit REE enrichment, but the source and evolution of the REE component is arguable. In the Anomaly X orebody, semi-massive ilmenite-rich magnetite and coarse REE-rich fluorapatite formed at 510 +/- 8 Ma (U-Pb LA-ICPMS age), at the end of the main regional sodic magmatic event (525 +/- 7 Ma). Prior to a major brecciation, a metasomatic event removed Ti and Al from magnetite grain boundaries, and leached primary fluorapatite (BSE-bright) of LREE, V, Na, Cl, Mg, Mn and Fe, leaving BSE-dark apatite that contained LREE-rich monazite and co-existing vapor- and liquid-dominated fluid inclusions. Subsequent brecciation and hydrothermal infill produced a matrix dominated by actinolite. K-feldspar, biotite, chlorite, calcite, hematite, rutile, and titanite, but apatite was not further metasomatized. It is concluded that P. REE, Ti, Al and Fe were readily transported in the early sodic-calcic fluids, but P and the REE had very restricted mobility (mm's to cm's) in the CO2-K-Cl-rich fluids responsible for apatite metasomatism. A role for fluid over-pressuring as a means of increasing the efficacy of apatite REE leaching is circumstantially suggested by the observation that the metasomatic event was terminated by tensile failure. Overall, a case is emerging for fluid-induced nucleation of monazite, and accompanying apatite REE depletion, to be included as a typical but subtle paragenetic feature within the iron oxide-apatite deposit model.