REE-rich Phases Research Articles

Recent advances in electrochemical recovery of rare earth elements from NdFeB magnets

Recent advances in electrochemical methods promise a more sustainable recycling of rare earth elements (REEs) from discarded NdFeB permanent magnets. The demand for NdFeB magnets for clean energy applications is rapidly increasing, motivating recycling efforts to diversify REE supply. The main electrochemical steps include the selective dissolution of REE-rich phases at the anode and the reduction of REE ions at the cathode. Pretreatment with demagnetization, mechanical size reduction, and leaching contributes to the release and concentration of REEs. Thermal demagnetization and mechanical crushing make the magnets brittle and improve the penetration of leaching agents. Acid leaching dissolves the REEs, but also dissolves the iron. To facilitate the extraction of REEs at high temperatures, molten salt electrolytes such as chlorides are used, while ionic liquids allow extraction under milder conditions, but with the caveat of possible decomposition during the process. Aqueous solutions have been most thoroughly investigated due to their versatility and affordability. Fluoride-based molten salt electrolytes effectively dissolve RREs and provide a stable environment for hightemperature electrodeposition, improving the efficiency and sustainability of rare earth element recovery. To isolate highpurity REE oxides and metals, additional processing is required using techniques such as solvent extraction, selective precipitation, and electroseparation. Key factors for optimal electrochemical recycling are maximizing selectivity for REEs, minimizing energy consumption and waste generation, and simplifying integration. Although technical challenges remain, recent advances show that electrochemical technologies can improve the sustainability of recycling critical REEs from permanent magnets.

Occurrence, geochemistry and provenance of REE-bearing minerals in marine placers on the West Coast of the South Island, New Zealand

ABSTRACT The occurrence, concentration and geochemistry of REE-bearing minerals in beach sands along the West Coast of the South Island of New Zealand is reported as a proxy for the composition of adjacent uplifted Holocene marine terraces. The strandlines of 13 beaches along this coast contain REE-rich allanite (average: 15% REO), monazite (82% REO) and xenotime (83% REO), with allanite being most common and xenotime having a restricted range. Other REE-rich phases are apatite (average: 2259 ppm), zircon (1756ppm), epidote (761 ppm), titanite (699 ppm) and scheelite. Geographical trends to the distribution of these minerals are not entirely clear, but there is an influx of titanite and allanite north of the Grey River and an increase in apatite, allanite, monazite and epidote north of Little Wanganui River. In-situ trace elements and/or 87Sr/86Sr isotopes indicate allanite, monazite and xenotime to be likely mainly derived from exhumed Paleozoic and Cretaceous granites, with minor amounts from the Alpine Schist metasedimentary rocks. The main source of the analysed apatite is likely also the granitoids, with contributions from the Greenland Group and Alpine Schist. When the REE abundances in the beach sands are compared to the size of the uplifted adjacent Holocene terraces, Waimangaroa terrace may contain the highest concentration of REE, followed by the Little Wanganui and Barrytown terraces.

Hydrothermal alteration of magmatic zircon related to NaCl-rich brines: Diffusion-reaction and dissolution-reprecipitation processes

Magmatic zircon from altered gabbros adjacent to the ∼1.07 Ga Lala Fe-Cu deposit, SW China was modified by NaCl-rich brines related to the Fe-Cu mineralization. The modified zircon grains are composed mostly of both inclusion-free and porous domains, and some grains also have overgrowth/rims. The inclusion-free domains, commonly overgrown by the porous domains, are roughly homogeneous under BSE imaging but display oscillatory or sector zoning under CL imaging. In contrast, the porous domains are distinctly mosaic-like under CL imaging, and contain abundant pores and mineral inclusions such as thorite, xenotime, REE-rich phases, actinolite, albite, biotite and calcite. The inclusion-free domains have concentrations of “non-formula” elements (for example Al, Ca, and Fe) much higher than the magmatic zircon, and are thus interpreted to be products of interaction between magmatic zircon (possibly metamictized) and the fluids via a diffusion-reaction process. However, these resultant domains have retained the Th and U contents, REE patterns, U-Pb ages and Hf isotopes of the precursor, magmatic zircon. On the other hand, the porous domains have stoichiometric end-member compositions (that is lowered Th, U, REE, Y, and P), and given that they are porous and inclusion-rich, we proposed that they have formed from the precursor, element-rich inclusion-free domains or magmatic zircon via a fluid-induced dissolution-reprecipitation process. It is notable that the porous domains retained the 177Hf/176Hf ratios of the precursors, but possess modified and meaningless U-Pb ages. Instead, a meaningful U-Pb age (1006 ± 62 Ma), similar to the timing of the Lala deposit, is well recorded by the overgrowth/rims with distinctly high LREEs (La = 50–700 ppm) and elevated Hf isotopic ratios. Our new results reveal that in the presence of the NaCl-rich brines, the nature of the precursor, magmatic zircon (that is high element budgets and/or metamictization) plays a key role on hydrothermal alteration of zircon.

Diamonds in ophiolites: Contamination or a new diamond growth environment?

For more than 20 years, the reported occurrence of diamonds in the chromites and peridotites of the Luobusa massif in Tibet (a complex described as an ophiolite) has been widely ignored by the diamond research community. This skepticism has persisted because the diamonds are similar in many respects to high-pressure high-temperature (HPHT) synthetic/industrial diamonds (grown from metal solvents), and the finding previously has not been independently replicated. We present a detailed examination of the Luobusa diamonds (recovered from both peridotites and chromitites), including morphology, size, color, impurity characteristics (by infrared spectroscopy), internal growth structures, trace-element patterns, and C and N isotopes. A detailed comparison with synthetic industrial diamonds shows many similarities. Cubo-octahedral morphology, yellow color due to unaggregated nitrogen (C centres only, Type Ib), metal–alloy inclusions and highly negative δC13 values are present in both sets of diamonds. The Tibetan diamonds (n=3) show an exceptionally large range in δN15 (−5.6 to +28.7‰) within individual crystals, and inconsistent fractionation between {111} and {100} growth sectors. This in contrast to large synthetic HPHT diamonds grown by the temperature gradient method, which have with δN15=0‰ in {111} sectors and +30‰ in {100} sectors, as reported in the literature. This comparison is limited by the small sample set combined with the fact the diamonds probably grew by different processes. However, the Tibetan diamonds do have generally higher concentrations and different ratios of trace elements; most inclusions are a NiMnCo alloy, but there are also some small REE-rich phases never seen in HPHT synthetics. These characteristics indicate that the Tibetan diamonds grew in contact with a C-saturated Ni–Mn–Co-rich melt in a highly reduced environment. The stable isotopes indicate a major subduction-related contribution to the chemical environment. The unaggregated nitrogen, combined with the lack of evidence for resorption or plastic deformation, suggests a short (geologically speaking) residence in the mantle. Previously published models to explain the occurrence of the diamonds, and other phases indicative of highly reduced conditions and very high pressures, have failed to take into account the characteristics of the diamonds and the implications for their formation. For these diamonds to be seriously considered as the result of a natural growth environment requires a new understanding of mantle conditions that could produce them.

Geochemistry of the Neoarchaean Volcanic Rocks of the Kilimafedha Greenstone Belt, Northeastern Tanzania

The Neoarchaean volcanic rocks of the Kilimafedha greenstone belt consist of three petrological types that are closely associated in space and time: the predominant intermediate volcanic rocks with intermediate calc-alkaline to tholeiitic affinities, the volumetrically minor tholeiitic basalts, and rhyolites. The tholeiitic basalts are characterized by slightly depleted LREE to nearly flat REE patterns with no Eu anomalies but have negative anomalies of Nb. The intermediate volcanic rocks exhibit very coherent, fractionated REE patterns, slightly negative to absent Eu anomalies, depletion in Nb, Ta, and Ti in multielement spidergrams, and enrichment of HFSE relative to MORB. Compared to the other two suites, the rhyolites are characterized by low concentrations of TiO2 and overall low abundances of total REE, as well as large negative Ti, Sr, and Eu anomalies. The three suites have a εNd (2.7 Ga) values in the range of −0.51 to +5.17. The geochemical features of the tholeiitic basalts are interpreted in terms of derivation from higher degrees of partial melting of a peridotite mantle wedge that has been variably metasomatized by aqueous fluids derived from dehydration of the subducting slab. The rocks showing intermediate affinities are interpreted to have been formed as differentiates of a primary magma formed later by lower degrees of partial melting of a garnet free mantle wedge that was strongly metasomatized by both fluid and melt derived from the subducting oceanic slab. The rhyolites are best interpreted as having been formed by shallow level fractional crystallization of the intermediate volcanic rocks involving plagioclase and Ti-rich phases like ilmenite and magnetite as well as REE-rich phases like apatite, zircon, monazite, and allanite. The close spatial association of the three petrological types in the Kilimafedha greenstone belt is interpreted as reflecting their formation in an evolving late Archaean island arc.

Zircon-Rich Ta-Nb-REE Mineralization in the McKeel Lake Pegmatite-Aplite System, Welsford Intrusion, Southwestern New Brunswick

The Ta-Nb-Zr-REE-rich McKeel Lake alkali granite pegmatite-aplite dike swarm is hosted within the late Silurian multiphase alkali granite intrusion (Welsford) near Browns Flats in southwestern New Brunswick. A steep easterly dipping series of pegmatite-aplite dikes trending 170° can be traced 150 m along strike; they are up to 40 cm wide, closely spaced (10–20 cm apart), and interconnected by narrow branching dikelets that generally trend perpendicular to the overall strike direction. The Welsford intrusion has mantle A-type affinities characterized by enrichment of Zr, Ta, Nb, Th, and rare earth elements (REE). Two separate sets of dikes have been identified: (1) an unmineralized fine-grained porphyry, which occurs at depth and seems to be the youngest intrusive phase; and (2) a mineralized pegmatite-aplite dike swarm (419 ± 20 Ma) that crops out at surface. The mineralized pegmatite-aplite dikes likely represent a late magmatic, highly fractionated phase of the Welsford intrusion. XRF and INAA data indicate that the Welsford intrusions and related pegmatite-aplite dikes show elevated levels of Ta (74–220 ppm), Nb (750–3,040 ppm), Zr (1,450–36,000 ppm), Y (540–3,070 ppm), Th (170–870 ppm), U (60–250 ppm), Ce (430–1,950 ppm) and Yb (110–290 ppm). Electron microprobe analysis of two samples identified Ta-Nb- and REE-rich phases: euxenite, fergusonite, aeschynite, (Y)-aeschynite, and a REE carbonate.

Sm-Nd and U-Pb zircon isotopic constraints on the provenance of sediments from the Amadeus Basin, central Australia: Evidence for REE fractionation

The Amadeus Basin of central Australia is a Late Proterozoic to Late Palaeozoic ensialic depositional basin located between two Proterozoic basement blocks of different Nd crustal formation ages, the older Arunta Block ( T DM Nd = 2.0 to 2.2 Ga, U-Pb zircon ages = 1.5–1.9 Ga) to the north and the younger Musgrave Block ( T DM Nd = 1.7 to 1.9 Ga, U-Pb zircon ages = 1.0–1.7 Ga) to the south. Initial Nd isotopic compositions of the Amadeus Basin sediments generally fall into the region defined by the evolutionary trajectories of these two basement blocks, indicating that the sediments are dominated by essentially two source components: the older Arunta Block [ 147Sm/ 144Nd ≈ 0.114 and ϵ Nd(0) ≈ −20.4 ] and younger Musgrave Block [ 147Sm/ 144Nd ≈ 0.118 and ϵ Nd(0) ≈ −14.7 ] and/or their equivalents. Stratigraphically higher sediments plot more closely to the evolutionary trajectory of the Musgrave Block. This result, together with the average provenance ages, indicates the proportion of material from the Musgrave Block increases as the sediments become younger. The Sm-Nd isotopic data preclude a substantial involvement of Archaean sources. U-Pb zircon ion probe analyses of the Late Proterozoic Heavitree Quartzite and the late Cambrian Goyder Formation are consistent with the Sm-Nd isotopic constraints. Most of the zircons from the basal Heavitree Quartzite give concordant U-Pb zircon ages of 1500 to 1900 Ma, consistent with derivation from an Arunta-type source, whilst zircons from the younger Goyder sandstone give six discrete U-Pb zircon age groups of 511 ±20, 615 ± 15, 960 ± 109, 1190 ± 54, 1633 ±24 and 1878 ±48 Ma, indicative of a dominantly Musgrave-type source with a minor contribution from the Arunta Block or reworked preexisting Amadeus Basin sediments. The two youngest U-Pb zircon ages probably indicate some contribution from Late Proterozoic-Cambrian volcanics in central Australia or a previously unrecognised younger local source in the basement blocks. The 511 ±20 Ma constrains the maximum depositional age of the sediment. The Amadeus Basin sediments have a surprisingly wide range of 147Sm/ 144Nd ratios (0.077–0.136), irrespective of grain size and rock types. In addition, the calculated T DM Nd values for samples from the same stratigraphic unit are well correlated with 147Sm/ 144Nd ratios, with samples of different stratigraphic units forming sub-parallel arrays. These phenomena are interpreted in terms of REE fractionation and preferential sorting of preexisting REE-rich phases during sedimentary recycling and deposition. This process can lead to either an increase or a decrease in 147Sm/ 144Nd ratio of a sediment relative to its source and therefore an erroneous estimate of its T DM Nd provenance age. A theoretical model is developed which accounts for the observed correlation between T DM Nd and 147Sm/ 144Nd ratios for suites of Stratigraphically related sediments and allows a reliable estimate to be made of the provenance age of individual stratigraphic units, assuming the average 147Sm/ 144Nd ratio of the source is known or can be estimated. The provenance age for each stratigraphic unit of the Amadeus Basin has been calculated using this method. The Sm-Nd isotopic systematics in the Amadeus Basin sediments suggest that 1. (1) REE fractionation during sedimentary recycling can in some cases be an important factor and needs to be considered; 2. (2) the provenance of the Amadeus Basin sediments was controlled by local source regions and cannot be used to infer large-scale continental averages. Hence, some caution must be evoked in using Sm-Nd isotopic constraints from restricted sampling for more generalised models of crustal evolution.

Protoliths and petrogenesis of Archean gneisses from the Kenora area, English River Subprovince, northwest Ontario

Archean gneissic rocks in the Kenora area can be divided into three groups: (1) remnants of a dominantly mafic supracrustal sequence; (2) biotite tonalite gneisses; (3) granitic pegmatoid gneisses. From field, petrographic and compositional data, the biotite tonalite gneisses are interpreted as plutonic rocks derived by partial melting of the supracrustal sequence. The granitic pegmatoid gneisses are considered to be the product of fractional crystallization of the tonalitic magma. Attempts were made to reconstruct this scheme by computer-assisted trace element modelling using REEs. With the source rock constrained to one having a bulk composition similar to that of the fine-grained amphibolites within the supracrustal sequence (comparable to Archean tholeiite elsewhere), the most satisfactory source rock was found to be a two-pyroxene garnet granulite (quartz 5%, plagioclase 25–35%, clinopyroxene 30–40%, orthopyroxene 15–25%, garnet 5–15%). This, when 10% melted under equilibrium, non-modal conditions, will yield a model melt with a REE pattern similar to that observed in the tonalites. Quartz eclogite, granulite without garnet, and amphibolite are not acceptable source rocks under the modelling conditions used. The mineralogy of the optimum model source rock is similar to that expected in a quartz tholeiite melting between 45 and 50 km depth. The granitic pegmatoid gneisses have very low REE content, especially LREEs, and their patterns defied conventional modelling. They may result from REE loss in residual fluids or the end result, after precipitation of REE-rich phases, such as allanite.