The rare earth elements are unusual when de quoted as total rare earth oxide, but the importance of a deposit may be related to the grade for individual, or a limited group of the elements. Taking the total REE resource, only one currently known deposit (Bayan Obo) would class as giant ( classify as large ( Olympic Dam IOCG deposit, all of these deposits are related to alkaline igneous activity or agpaitic nepheline syenites. The total resource in these deposits must relate to the scale of the primary igneous source, but the grade is a complex function of igneous source, magmatic crystallisation, hydrothermal modi that the sources conducive to the formation of large REE deposits are developed in subcontinental lithospheric mantle, enriched in trace elements either by plume activity, or by previous subduction. The reactivation of such enriched mantle domains in relatively restricted geographical areas may have played a role in the formation of some of the largest deposits (e.g. Bayan Obo). Hydrothermal activity involvingfining giant-sized ore deposits, as resources are often>1.7 107 tonnes contained metal), but a range of others>1.7 106 tonnes). With the exception of unclassified resource estimates from thee either carbonatitesfication and supergene enrichment during weathering. Isotopic data suggest fl grade, depending on primary mineralogy and the availability of ligands. Weathering and supergene enrichment of carbonatite has played a role in the formation of the highest grade deposits at MountWeld (Australia) and Tomtor (Russia). For the individual REE with the current highest economic value (Nd and the HREE), the boundaries for the large and giant size classes are two orders of magnitude lower, and deposits enriched in these metals (agpaitic systems, ion absorption deposits) may have signi economic impact in the near future.uids from magmatic to meteoric sources may result in the redistribution of the REE and increases inficant
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- Bayan Obo REE-Nb-Fe deposit