Evaluated Utilization of Middle–Heavy REE Resources in Bayan Obo Deposit: Insight from Geochemical Composition and Process Mineralogy

Comparison of the Daluxiang and Maoniuping carbonatitic REE deposits with Bayan Obo REE deposit, China

With the development of the steel industry, China’s demand for niobium is increasing. However, domestic niobium resources are not yet stably supplied and are heavily dependent on imports from abroad (nearly 100%). It is urgent to develop domestic niobium resources. The Bayan Obo deposit is the largest rare earth element deposit in the world and contains a huge amount of niobium resources. However, the niobium resource has not been exploited due to the fine-grained size and heterogeneous and scattered occurrences of Nb minerals. To promote the utilization of niobium resources in the Bayan Obo deposit, we focused on the mineralogical and geochemical characterization of six types of ores and mineral processing samples from the Bayan Obo deposit, using optical microscopes, EPMA, TIMA, and LA–ICP–MS. Our results show that: (1) the niobium mineral compositions are complex, with the main Nb minerals including aeschynite group minerals, columbite–(Fe), fluorcalciopyrochlore, Nb–bearing rutile, baotite, fergusonite–(Y), fersmite, and a small amount of samarskite–(Y). Aeschynite group minerals, columbite–(Fe), and fluorcalciopyrochlore are the main niobium-carrying minerals and should be the primary focus of industrial recycling and utilization. Based on mineralogical and geochemical investigation, the size of the aeschynite group minerals is large enough for mineral processing. Aeschynite group minerals are thus a significant potential recovery target for niobium, as well as for medium–heavy REE resources. The Nb–rich aegirine-type ores with aeschynite group mineral megacrysts are suggested to be the most significant niobium resource for mineral processing and prospecting. Combined with geological features, mining, and mineral processing, niobium beneficiation efforts of aeschynite group minerals are crucial for making breakthroughs in the utilization of niobium resources at the Bayan Obo.

The giant Bayan Obo deposit in China represents the largest rare earth element (REE) resource in the world, but the mechanisms for its highly anomalous REE enrichment have long been controversial. The central debate concerns the nature and origin of the ore-hosting dolomite. In this study, a texturally constrained microanalytical study of carbonate minerals from the Bayan Obo ore-hosting dolomite is presented to resolve this controversy. The dolomite rocks contain two major components: the early-stage coarse-grained dolomite (CD) and the late-stage fine-grained dolomite (FD). The CD dolomite grains have C-O isotope ratios (δ13CVienna-PeeDee Belemnite (V-PDB): –4.8 to –3.3‰; δ18OVienna-standard mean ocean water (V-SMOW): 7.1 to 11.9‰) plotting in or adjacent to the primary igneous carbonatite field, with a narrow range of low 87Sr/86Sr ratios (0.70262–0.70327). The mantle-like C-O and Sr isotopes indicate that the coarse-grained dolomite rocks are magmatic in origin. Dolomite grains from the FD have experienced extensive hydrothermal alteration related to both REE mineralization and post-ore metamorphism of the Bayan Obo deposit. The domains of primary unaltered dolomite have high SrO and MnO contents that clearly distinguish them from sedimentary carbonates. The 87Sr/86Sr ratios of unaltered dolomite domains range from 0.70271 to 0.70473, with the majority lower than 0.7035, contrasting with higher 87Sr/86Sr ratios of Mesoproterozoic sedimentary carbonates globally. Thus, the primary unaltered fine-grained dolomite, the precursor to the FD, is also proposed to be a carbonatite. Compared with dolomite grains from the CD, most of the unaltered dolomite domains within the FD are more enriched in FeO and MnO and have higher δ13CV-PDB (-4.9 to 0.3 ‰) and δ18OV-SMOW (9.4 to 17.1 ‰) values. In addition, the FD contains abundant REE- and volatile-rich hydrothermal minerals and Fe-Mg carbonates, which are rare in the CD. The geochemical and mineralogical data in conjunction indicate that the melts forming the late-stage FD were much more evolved than those forming the early-stage CD. It is noteworthy that the unaltered dolomite domains within the FD have a wider range of 87Sr/86Sr ratios than those within the CD, which implies that the CD and FD, at least a proportion of them, are unlikely to have crystallized from the same progenitor magmas. Some FD was possibly the product of fractionation of less-evolved carbonatitic magma that generated the CD, whereas other FD crystallized from new pulses of magmas that were highly fractionated at depth. When compared with other carbonatite complexes, the Bayan Obo carbonatite suite is notable for having a large surface area (~48 km2 in outcrop) and containing an anomalously large proportion of highly evolved components. The voluminous evolved carbonatite clearly provided a basis for the accumulation of significant ore metals. Therefore, the giant-sized REE deposit is proposed to be associated with large-volume, highly evolved carbonatite at Bayan Obo.

Temporal variations in Rare Earth Element distributions in the Cenozoic succession and modern sediments of the Cayman Islands

Detail mineralogical study and geochronological framework of Bayan Obo (China) Nb mineralization recorded by in situ U-Pb dating of columbite

(1) Background: The spontaneous combustion of coal (SCC) not only consumes huge amounts of coal resources but also causes environmental degradation. Rare earth elements (REE) can be taken as an effective indicator to evaluate the environmental effects of SCC. Coal in the Rujigou Mine has been spontaneously combusting for hundreds of years. (2) Methods: The geochemical properties of REE and major elements in the soil of the Rujigou coal mine are methodically examined to reveal the environmental effects of SCC. (3) Results: Soil REE concentration in the Rujigou mine is 216.09 mg/kg, and there is an enrichment of light rare earth elements (LREE) and a depletion of heavy rare earth elements (HREE), LREE/HREE in Rujigou mine was 5.52. The spontaneous combustion of coal could change the vertical distribution of REE, which is conducive to the enrichment of LREE. According to the Eu anomaly and δCe/δEu, the source of material in this mine may be derived from the terrigenous clastic rock controlled by weak reduction. Aluminum and titanium have similar geochemical behavior to REE, especially LREE. The concentration of sulfur is negatively correlated with REE, especially HREE. Calcium, sodium, and magnesium all had a negative correlation with LREE. (4) Conclusions: The spontaneous combustion of coal can lead to the fractionation of light and heavy rare earth elements, resulting in the enrichment of LREE and depletion of HREE.

Pedochemical behaviors of rare earth elements in soil profiles along a lithosequence in eastern Taiwan

Carbonatite and alkaline intrusive complexes, as well as their weathering products, are the primary sources of rare earth elements. A wide variety of other commodities have been exploited from carbonatites and alkaline igneous rocks including niobium, phosphate, titanium, vermiculite, barite, fluorite, copper, calcite, and zirconium. Other elements enriched in these deposits include manganese, strontium, tantalum, thorium, vanadium, and uranium. Carbonatite and peralkaline intrusion-related rare earth element deposits are presented together in this report because of the spatial, and potentially genetic, association between carbonatite and alkaline rocks. Although these rock types occur together at many locations, carbonatite and peralkaline intrusion-related rare earth element deposits are not generally found together. Carbonatite hosted rare earth element deposits are found throughout the world, but currently only five are being mined for rare earth elements: Bayan Obo, Daluxiang, Maoniuping, and Weishan deposits in China and the Mountain Pass deposit in California, United States. These deposits are enriched in light rare earth elements, including lanthanum, cerium, praseodynium, and neodynium. The principal rare earth element-minerals associated with carbonatites are fluocarbonates (bastnäsite, parisite, and synchysite), hydrated carbonates (ancylite), and phosphates (monazite) with bastnäsite being the primary ore mineral. Calcite and dolomite are the primary gangue minerals. At present, the only rare earth element production from a peralkaline intrusion-related deposit is as a byproduct commodity at the Lovozero deposit in Russia. Important rare earth element minerals found in various deposits include apatite, eudialyte, loparite, gittinsite, xenotime, gadolinite, monazite, bastnäsite, kainosite, mosandrite, britholite, allanite, fergusonite, and zircon, and these minerals tend to be enriched in heavy rare earth elements. Carbonatite and alkaline intrusive complexes are derived from partial melts of mantle material, and neodymium isotopic data are consistent with the rare earth elements being derived from the parental magma. Deposits and these associated rock types tend to occur within stable continental tectonic units, in areas defined as shields, cratons, and crystalline blocks; they are generally associated with intracontinental rift and fault systems. Protracted fractional crystallization of the magma leads to enrichment in rare earth elements and other incompatible elements. Rare earth element mineralization associated with carbonatites can occur as either primary mineral phases or as mineralization associated with late stage orthomagmatic fluids. Rare earth element mineralization associated with alkaline intrusive complexes may occur as primary phases in magmatic layered complexes or as late-stage dikes and veins. The greatest environmental challenges associated with carbonatite and peralkaline intrusion-related rare earth element deposits center on the associated uranium and thorium. Considerable uncertainty exists around the toxicity of rare earth elements and warrants further investigation. The acid-generating potential of carbonatites and peralkaline intrusion-related deposits is low due to the dominance of carbonate minerals in carbonatite deposits, the presence of feldspars and minor calcite within the alkaline intrusion deposits, and only minor quantities of potentially acid-generating sulfides. Therefore, acid-drainage issues are not likely to be a major concern associated with these deposits. Uranium has the potential to be recovered as a byproduct, which would mitigate some of its environmental effects. However, thorium will likely remain a waste-stream product that will require management since progress is not being made towards the development of thorium-based nuclear reactors in the United States or other large scale commercial uses. Because some deposits are rich in fluorine and beryllium, these elements may be of environmental concern in certain locations.

Rare earth elements (REE) concentrations have been analyzed in 13 sediment samples from Tolo Harbour. The highest concentrations of ∑REE, ∑LREE (light rare earth elements) and ∑HREE (heavy rare earth elements) are all at site G as the enrichment of Mn–Fe oxyhydroxides and the highest pH of site G relative to its nearby areas. The relatively higher ∑LREE/∑HREE ratio at site G is probably due to the stronger REE fractionation. During the fractionation process, LREE are preferentially scavenged on particle surfaces such as Mn–Fe oxyhydroxides and clay minerals. Concentrations of ∑REE, ∑LREE and ∑HREE at sites Y and Z are much lower than those at other sites as calcareous biological debris dilute the REE abundances of the sediments in Tolo Harbour. REE are released from sediments into waters due to the high salinity in Tolo Harbour and REE fractionation occurs in this release process. The provenance of sediments in Tolo Harbour is terrigenous and the North American shale composite (NASC)-normalized REE patterns of the 13 samples in Tolo Harbour all display strongly negative Eu anomalies. ∑REE is poorly correlated with Zr and Hf, which suggests that Tolo Harbour is heavily polluted by human activities.

Three lateritic bauxites representing felsic (Rompin area), intermediate (Johor area), and mafic (Kuantan area) parent rocks from Malaysia were subjected to detailed mineralogical and geochemical investigations to examine the factors controlling their geochemical variations. The bauxitic mineral gibbsite occurs as in all areas. Non-bauxitic minerals are represented by kaolinite, quartz, and goethite in bauxites from granites and intermediate tuffs and by goethite only in bauxite from basalts. Bauxites from basalts and intermediate tuffs have higher TiO2, Fe2O3, Ba, V, Cu, Pb, and Zn but lower SiO2, Th, Rb, and rare earth elements (REE) when compared with bauxite from granites. Bauxite from basalts has higher concentrations of Cr, Nb, Cu, and Zn and lower Ba, As, and REE when compared with bauxite from intermediate tuffs. Chondrite-normalized REE patterns of bauxite from granites showed heavy rare earth elements (HREE) enrichments over light rare earth elements (LREE), pronounced negative EuN/EuN* anomalies and strong positive CeN/CeN* anomalies. In contrast, bauxites from intermediate tuffs and basalts exhibited LREE enrichment relative to HREE, slight to strong negative CeN/CeN* anomalies, and absence or slight negative EuN/EuN* anomalies. Compositional variations among the studied bauxites are controlled not only by the parent rock compositions but also by the degree of lateritization and mineralogy of its products. The effect of each of these factors can be traced by a number of compositional parameters in these bauxites. The influence of parent rock compositions is indicated by absence of kaolinite and quartz, high TiO2 and Fe2O3 and low SiO2 in bauxites from basalts and intermediate tuffs as well as negative EuN/EuN* anomalies in bauxite from granites and absence of EuN/EuN* anomalies in bauxites from basalts and intermediate tuffs. The impact of lateritization degree was evident by abundance of quartz and kaolinite in bauxite from granites as well as relative LREE enrichments over HREE in bauxites from basalts and intermediate tuffs. The effect of lateritic products mineralogy was observed in the variations of REE patterns between bauxite from granites that are characterized by HREE enrichment and Ce anomaly due to occurrence of REE as residual zircon and bauxites from intermediate tuffs and basalts that are characterized by low HREE and high LREE due to occurrence of REE as authigenic florencite and/or cerianite. The distinct geochemical composition of Fe concretions such as lower TiO2 and most of the trace elements, enrichment of HREE relative to LREE and negative EuN/EuN* anomaly compared to the host bauxite at Johor area also indicates the effect of mineralogy of lateritic products on their geochemistry.

Composition of rare earth elements in settling particles collected in the highly productive North Pacific Ocean and Bering Sea: Implications for siliceous-matter dissolution kinetics and formation of two REE-enriched phases

Submarine groundwater discharge is an important net source of light and middle REEs to coastal waters of the Indian River Lagoon, Florida, USA

ABSTRACTLate Triassic–Early Jurassic intrusions of the Erguna Block, Northeast China, are located along the southern margin of the Mongol–Okhotsk orogenic belt. They comprise granodiorite, monzogranite, syenogranite, and lesser gabbro–diorite, of adakitic and calc­alkaline affinity. The adakite-like and calc­alkaline granites share similar light rare earth elements (LREE) characteristics; however, their heavy rare earth elements (HREE) trends differ from one another. The relative abundances of HREE in the calc­alkaline granites are relatively consistent and are similar to those of intrusive rocks formed from dehydration melting of garnet-free amphibolitic source rocks at relatively low pressures. In contrast, the adakite-like granites show more prominent HREE fractionation trends, indicating that they crystallized at higher pressures, where garnet in the source rocks was stable. At least two isotopically distinct sources were involved in the petrogenesis of the granites, but the extent to which they contributed varies between plutons. Most intrusions have incorporated an isotopically primitive component, possibly juvenile mafic crust. The other sources include a small proportion of old continental crustal material and isotopically evolved wall rocks. The gabbro–diorites have high MgO contents (>7 wt.%), a high Mg# (>0.6), and show moderate LREE and HREE fractionation, indicating they formed from the melting of subducted metasomatized lithospheric mantle. All of the intrusions in the study area are characterized by a relative enrichment in large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), indicating they were emplaced in an Andean-type active continental margin setting related to southward subduction of the Mongol–Okhotsk oceanic plate.

To interpret the mechanism of cocrystallization and sorptive properties of rare earth elements (REE) with calcium sulfate dihydrate (CSD) crystallization, synthetic crystals were prepared by the crystallization of CSD from solutions containing Ce, Nd, Er, and Tb as heavy and light REE. The obtained crystals were characterized using an array of characterization techniques, including X-ray diffraction, thermogravimetric analysis, differential thermal analysis, time-of-flight secondary ion mass spectrometry, high resolution scanning electron microscopy, and transmission electron microscopy. The results showed that the heavy REE are less prone to enter the CSD structure than the light REE. Further, the main mechanism of incorporation of REE in the CSD structure is via a surface adsorption process rather than an isomorphous substitution process into the CSD crystal lattice.

Rare earth element cycling in a sandy subterranean estuary in Florida, USA