Rare earth elements in the iron-oxide apatite (IOA) deposit: insights from apatite
Kiruna-type or IOA deposits are characterized by magnetite-apatite-actinolite associations and may contain considerable amounts of rare earth elements (REEs) that can be exploited as by-products. REEs in the Kiruna-type deposit, heterogeneously distributed, are mainly hosted in phosphates (e.g. apatite). Published data of apatite, together with other evidence, can be used to decipher the source, enrichment mechanism, and post-ore modification of REEs in the Kiruna-type deposits. Sr isotopes and melt inclusions compositions of apatite, together with other evidence, support a mantle + evaporite affinity for the ore-forming materials of many IOA deposits. Ore-forming materials of different IOA deposits may originate from primitive, depleted, or enriched mantle sources respectively. This suggests that magmatic-hydrothermal evolutions, rather than REE-enriched mantle source, are probably more important for REE enrichment in Kiruna-type deposits. Increasing evidence supports an ortho-magmatic origin of IOA deposits, yet detailed forming model is still in debate. From the perspective of models forming Fe-P-rich melts, we tentatively test the REE enrichment mechanism of IOA deposit. Sulphate-rich, Fe-P-bearing melts have been observed in the Iron Springs, Buena Vista, and El Laco IOA deposits. These melts may extract REEs from silicate magmas and thus contribute to REE enrichment. Compiled experimental data show that Fe-P melts would incorporate more REEs than the silicate-rich melts during magma Fe-Si immiscibility. As a result, magma Fe-Si immiscibility potentially occurring in IOA deposits may also assist in the enrichment of REEs. Hydrothermal events in the Kiruna-type deposits could result in formation of monazite and xenotime, which is beneficial for industrial recovery of REE. Finally, the existence of various magmatic-hydrothermal Fe-REE assemblages (e.g. IOA deposit, Bayan Obo deposit) implies that co-enrichment of iron and REEs can occur during various magmatic-hydrothermal processes.
