Global resources of heavy Rare Earth Elements (REE) are dominantly sourced from Chinese regolith-hosted ion-adsorption deposits in which the REE are inferred to be weakly adsorbed onto clay minerals. Similar deposits elsewhere might provide alternative supply for these high-tech metals, but the adsorption mechanisms remain unclear and the adsorbed state of REE to clays has never been demonstrated in situ. This study compares the mineralogy and speciation of REE in economic weathering profiles from China to prospective regoliths developed on peralkaline rocks from Madagascar. We use synchrotron X-ray absorption spectroscopy to study the distribution and local bonding environment of Y and Nd, as proxies for heavy and light REE, in the deposits. Our results show that REE are truly adsorbed as easily leachable 8- to 9-coordinated outer-sphere hydrated complexes, dominantly onto kaolinite. Hence, at the atomic level, the Malagasy clays are genuine mineralogical analogues to those currently exploited in China.

Garnets from skarns in the Beinn an Dubhaich granite aureole, Isle of Skye, Scotland, have a large range of concentrations of uranium (0·2–358 ppm) and the rare earth elements (REE) (23–4724 ppm). Variations in these concentrations correlate with major element zonation within the garnets, and with changes in the shape of REE patterns. Typical patterns in most garnets display light REE (LREE) enrichment, flat heavy REE (HREE) distribution and a negative Eu anomaly. These patterns are interpreted to represent equilibrium trace element exchange between pre-existing pyroxene, hydrothermal fluid and calcic garnets. Iron-rich zones are characterized by positive Eu anomalies and an increase in the abundance of the LREE relative to the HREE. These patterns are interpreted as resulting from changes in REE speciation related to the introduction of externally buffered fluid to the skarn system. Relatively Fe-poor zones show strongly HREE-enriched patterns with negative Eu anomalies and in some instances depletions in Y relative to Ho and Dy, which are interpreted as resulting from surface sorption of the REE during rapid, disequilibrium garnet growth. Strong correlations between U abundance and the REE patterns indicate that the same processes have affected U distribution. Both types of pattern can be modified by the effects of closed-system crystallization on REE abundance in the fluid, and changes in fluid major element chemistry.

MS, AF and FW acknowledge the support of the NERC SoS:RARE consortium grant (NE/M011267/1). D. Kavecsanszki acknowledges the support of a postgraduate fellowship from the College of Engineering, Mathematics and Physical Sciences at the University of Exeter.The rare earth elements are unusual when defining giant-sized ore deposits, as resources are often 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 (>1.7×107 tonnes contained metal), but a range of others classify as large (>1.7×106 tonnes). With the exception of unclassified resource estimates from the Olympic Dam IOCG deposit, all of these deposits are related to alkaline igneous activity – either carbonatites 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 modification and supergene enrichment during weathering. Isotopic data suggest 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 involving fluids from magmatic to meteoric sources may result in the redistribution of the REE and increases in 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 Mount Weld (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 2 orders of magnitude lower, and deposits enriched in these metals (agpaitic systems, ion absorption deposits) may have significant economic impact in the near future.Publisher PDFPeer reviewe