Cook and 4 co-authors reported in 2022 that nanoscale mineral inclusions identified include REE-minerals common in the deposit such as florencite, xenotime, and monazite, but also less abundant minerals such as cerianite, fluocerite, and rhabdophane. Other significant NP hosts for REEs include uraninite (with characteristic HREE enrichment), scheelite and ferberite.
Variation in inclusion speciation contributes to the diverse REE fractionation patterns. Inclusion-hosted REEs were either formed from discharge of high-volatile fluids or were initially present within primary hydrothermal Fe-oxides and subsequently released and reprecipitated following superimposed fluid-assisted overprinting. Incorporation and release of trace elements during overprinting events show a preferential partitioning of REEs into magnetite and the association U-W-Sn into coexisting hematite.
Hematite from mineralized breccias hosts abundant monazite nano-inclusions. Transient porosity created during CDR reactions promote formation of REE-minerals in Feoxides. The discharge of volatile-rich fluids, possibly at multiple stages during ore formation, results in co-crystallization of a wide variety of REE-minerals together with hematite.
Conversely, REE-minerals such as florencite can host nano-inclusions of hematite. Magnetite is the better host for lattice-bound REEs than hematite. However, the two-fold superstructure of hematite with ordered O vacancies (Verdugo-Ihl et al., 2020b) is a common crystal structural modification and supports (re) mobilization of REEs and other trace elements in this Fe-oxide.
Although the data presented here is specific to the Olympic Dam deposit, there are broad implications for IOCGs elsewhere, and for other deposit types. The authors show that Fe-oxides carry REEs, both structurally-bound, and within nm- to μm-sized inclusions of discrete REE- and REE-bearing minerals and that, in magnetite, REEs are incorporated within the structure.
A wide variation in inclusion speciation contributes to diverse REE fractionation patterns. These observations emphasize the contribution of fluid-assisted
overprinting events at Olympic Dam to observed features, here causing release of REEs from Fe-oxides and reprecipitation as discrete REE-minerals as inclusions within the source host or at grain boundaries.
Although oxide-bound REEs are unlikely to account for a major proportion of the overall REE budget in the ores, they deserve consideration when designing strategies for recovery of REE from, for example, flotation tailings, and given the persistent association of REEs with U (and Th), for generation of clean concentrates.
More broadly, the findings described here carry implications for REE recovery from a wide range of ores and wastes in which Fe-oxides dominate, including lateritic REE resources, and equally importantly, from future targets for REE recovery such as red muds from alumina production.
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