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The rare-earth elements (REEs, La–Lu, Y) are essential for the development of renewable technologies. Bastnäsite (REECO3F) is a common REE ore mineral that is often subject to hydrothermal alteration at all crustal levels. Mechanisms of hydrothermal bastnäsite alteration therefore govern the evolution of REE deposits, though these mechanisms remain poorly understood. This experimental work investigates the hydrothermal replacement of bastnäsite by rhabdophane (REEPO4∙xH2O, x = 0–1) and monazite (REEPO4) in phosphatic fluids. Two temperature-dependent alteration pathways were identified; both follow the coupled dissolution-reprecipitation (CDR) mechanism. At 90 °C, bastnäsite was replaced by highly-porous metastable rhabdophane which was then replaced by monazite, forming an inner layer of rhabdophane and an outer layer of monazite. At 220 °C, bastnäsite was replaced directly by monazite. Although replacement initiated more quickly at 220 °C, greater overall replacement occurred at 90 °C (~ 61 wt.% after 500 h, compared to ~ 13 wt.% at 220 °C) due to surface passivation by monazite at 220 °C. Geochemical analyses showed REE fractionation during bastnäsite alteration. At 90 °C, rhabdophane was enriched in heavy REEs (Eu–Lu, Y), likely due to the evolving fluid chemistry, while at 220 °C secondary monazite was enriched in Sm and Ho compared to bastnäsite. These results indicate that: 1) the hydrothermal alteration of bastnäsite by rhabdophane and monazite in ore deposits leads to REE immobilisation, with little net loss of REEs to solution; 2) rhabdophane is metastable relative to monazite at 90 °C, and; 3) variable temperatures can cause different mineral textures and REE fractionation trends during hydrothermal alteration and mineral replacement. [ABSTRACT FROM AUTHOR]

Objective The Chishan rare earth deposit in Weishan County, Shandong Province is located on the southeastern edge of the North China Plate. It is a magmatic hydrothermal rare earth deposit mainly composed of light rare earths, and is currently the third largest light rare earth deposit in China. There is still controversy over its mineralization age. Methods Therefore, taking the latest exploration borehole ore samples from Chishan rare earth deposit as the research object, the mineralization age of Chishan rare earth deposit was accurately determined through field investigation, mineralogy, and chronology research, using Th-Pb isotope dating of fluorocarbon cerium ore. Results The Th-Pb isotope dating results of 5 fluorocarbon cerium deposits indicate that the mineralization age of the Chishan rare earth deposit can be divided into two periods: the early period is (125.5±0.5)-(122.2±0.5)Ma, and the late period is (112.3±0.6) Ma. Conclusion The early age is basically consistent with the mineralization age of the deposit determined by previous studies, while the late mineralization age is the first report, indicating that there may be two periods of mineralization activity in the Chishan rare earth deposit, which is worthy of attention and further work.

In this paper, we evaluated the potentialities of Raman spectroscopy and electron backscattered diffraction (EBSD) in the microscopic characterization of Ca-REE fluorcarbonates (CRFC) belonging to the bastnäsite-synchysite series to provide a "road map" for further investigations with transmission electron microscopy (TEM). EBSD was efective in establishing the sample orientation, setting up the oriented cuts, and ascertaining the effective syntactic relationship among all the detected CRFC phases; however, it failed to distinguish between diferent polysomes. On samples with diferent orientations that were preventively ascertained by EBSD and characterized by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), micro-Raman spectroscopy allows for distinguishing between polysomes based on the diferences in intensity and position of the symmetric stretching vibration (v1) of the carbonate group C O 3 2 − in the region around 1080–1099 cm–1. However, as evidenced by TEM-EDS, what appears as a homogeneous polysome in backscattered electrons (BSE) images may be a disordered intergrowth of compositional faults with a bulk composition being matched with that of a real polysome only by accident. Therefore, we conclude that the Raman signal is sensitive to diferent Ca/(Ca+REE) ratios but not to any ordered distribution of Ca-poor and Ca-rich lamellae within the analyzed volume, making the unambiguous identification of a polysome tricky. Finally, several ordered polysomes were detected at the TEM scale, including a B2S and a long-range polytype with a 32 nm repeat distance along c. The possible implications of the detected microstructure for ore mineral formation are discussed. [ABSTRACT FROM AUTHOR]

Fluocerite has been shown experimentally and by thermodynamic modeling to be an important phase for the evolution of magmatic-hydrothermal REE mineralizing systems, but one that is rarely observed in nature. The reason for the latter is poorly understood. The newly discovered Taipingzhen REE deposit in Henan province, Central China, is unique for containing abundant fluocerite and thus provides a great opportunity to explore the role of fluocerite in REE mineralization. Fluocerite was the earliest REE mineral to crystallize in this deposit and was replaced extensively by bastnäsite. It has a higher La/Nd ratio than the later-depositing REE minerals, e.g., bastnäsite and monazite. This decrease in the La/Nd ratio with time is interpreted to have resulted from the fluocerite-mediated fractionation of REE (fluocerite prefers the lightest REE during its deposition from a high temperature, sulfate-rich hydrothermal fluid, in which the REE were transported as sulfate complexes). This study provides compelling evidence that fluocerite is, indeed, an important, early crystallizing mineral in magmatic-hydrothermal REE systems, which elsewhere was replaced entirely by bastnäsite during subsequent fluid evolution. Moreover, the study shows that the bastnäsite, which replaced fluocerite, inherited its high La/Nd ratio, suggesting that this ratio could be used as a tool with which to identify the former presence of fluocerite. The preservation of fluocerite in the Taipingzhen deposit is interpreted to have been due to the lack of significant cooling until a late stage of mineralization, at which time most of the fluid was lost from the system. [ABSTRACT FROM AUTHOR]

Bastnäsite contains considerable amounts of U and Th and has been widely used for U-Th-Pb dating. Hydrothermal alteration of bastnäsite is common in nature but its effects on U-Th-Pb dating are not currently well constrained. Hence the significance of U-Th-Pb ages obtained from altered bastnäsite cannot be evaluated. Here, we present a detailed geochronologic as well as micro- and nano-scale mineralogical study of altered bastnäsite in a Mo-REE deposit, Central China. The original bastnäsite grains were confirmed to have crystalized at 208 Ma but were variably overprinted by a hydrothermal event at 150 Ma. They commonly exhibit typical replacement textures that appear to have formed from a coupled dissolution-reprecipitation process, i.e., a primary unaltered domain surrounded by a porous altered domain. Micro- and nano-scale mineralogical observations strongly suggest that during the coupled dissolution-reprecipitation process, non-radiogenic (common) Pb was incorporated into the altered domains in the form of nanoscale galena inclusions. Such incorporation (even minor) has significantly affected the 206Pb/238U and 207Pb/206Pb ratios due to the low contents of U and its daughter isotopes in bastnäsite, resulting in highly variable, discordant U-Pb dates for the altered domains. In contrast, incorporation of the non-radiogenic Pb has very limited effects (<5%) on the Th-Pb system due to the remarkably high contents of Th and radiogenic 208Pb in bastnäsite. Instead, the scattered 208Pb/232Th ages (208 to 150 Ma) of the altered domains were essentially affected by incomplete replacement, and thus can be used to approximate the lower age limit of the primary hydrothermal activity or the upper age limit of the secondary hydrothermal activity. The results from this study highlight that because of the different orders of magnitude between the U and Th contents in bastnäsite, the mobilization of radiogenic and non-radiogenic Pb during alteration may have significantly different impacts on the U-Pb and Th-Pb systems. Therefore, the two systems should be treated separately during the dating of bastnäsite resulting from secondary hydrothermal events. [ABSTRACT FROM AUTHOR]

The interaction between rare earth element (REE)‐rich (La, Pr, Nd, Dy) aqueous solutions, dolomite (CaMg(CO3)2), and aragonite (CaCO3) at low temperature hydrothermal conditions (25–220 °C) is studied. The experiments result in the solvent‐mediated surface precipitation and subsequent pseudomorphic mineral replacement of the dolomite and aragonite seeds by newly formed REE‐carbonates. The host grains are replaced from periphery inward. The newly formed REE‐bearing carbonates in La‐, Pr‐, and Nd‐doped systems follow the crystallization sequence: lanthanite [REE2(CO3)3·8H2O] → kozoite [orthorhombic REECO3(OH)] → hydroxylbastnasite [hexagonal REECO3(OH)]. The interaction of Dy‐bearing solutions with dolomite results only in the crystallization of kozoite [orthorhombic DyCO3(OH)]. However, experiments with aragonite reveal a two‐step crystallization pathway: tengerite [Dy2(CO3)3·2‐3(H2O)] → kozoite [orthorhombic DyCO3(OH)]. The temperature, the dissolution rate of the host mineral, and the ionic radii of the REE3+ in question are found to control the kinetics of the replacement reaction, the polymorph selection, and the crystallization pathways toward bastnasite. The findings allow to gain a more in‐depth understanding of the formation REE‐bearing carbonates, particularly the mineral bastnasite, which is the main source of REEs for industry. This knowledge can be used to improve REE separation, exploration, exploitation methods, as well to produce carbonate minerals with tailored structures. [ABSTRACT FROM AUTHOR]

The rapid depletion of high-grade rare earth elements (REE) resources implies that future supplies may be augmented with low-grade ores, tailings, and other unconventional resources to meet cut-off grades and, subsequently, supply demands. This paper presents an amalgamation of studies conducted on selected complex low-grade iron-oxide-silicate-rich tailings, with the overall aim of developing efficacious methods and associated process mineralogy characterization for enhanced REE minerals recovery and upgrade. To this end, a summary of the overarching key results from froth flotation, magnetic separation, and gravity separation studies of the tailings and their implications are presented in this review. Reconciliation of all the findings reveals lucid links between feed ore properties, mainly mineralogy and particle size distribution, as the key influential factors that affect the beneficiation of real complex low-grade tailings, although distinct differences in physicochemical properties of the valuable and gangue minerals may exist. It is clearly established that the unliberated association between REE and gangue minerals within the ore can lead to either synergistic or antagonistic effects on the quality of concentrates produced. Furthermore, the limitations presented by the poorly liberated minerals are exacerbated by their “fine” nature. With appreciable recoveries obtained using such readily available conventional separation methods, the tailings provide additional REE value to the primary commodities; hence, such material could be considered a potential resource for REE beneficiation. The learnings from the respective beneficiation studies are significantly important as they provide the knowledge base and greater understanding of the mineralogical characteristics and beneficiation response of REE minerals in typical complex, low-grade tailings.

Abstract The interaction between rare earth element (REE)‐rich (La, Pr, Nd, Dy) aqueous solutions, dolomite (CaMg(CO3)2), and aragonite (CaCO3) at low temperature hydrothermal conditions (25–220 °C) is studied. The experiments result in the solvent‐mediated surface precipitation and subsequent pseudomorphic mineral replacement of the dolomite and aragonite seeds by newly formed REE‐carbonates. The host grains are replaced from periphery inward. The newly formed REE‐bearing carbonates in La‐, Pr‐, and Nd‐doped systems follow the crystallization sequence: lanthanite [REE2(CO3)3·8H2O] → kozoite [orthorhombic REECO3(OH)] → hydroxylbastnasite [hexagonal REECO3(OH)]. The interaction of Dy‐bearing solutions with dolomite results only in the crystallization of kozoite [orthorhombic DyCO3(OH)]. However, experiments with aragonite reveal a two‐step crystallization pathway: tengerite [Dy2(CO3)3·2‐3(H2O)] → kozoite [orthorhombic DyCO3(OH)]. The temperature, the dissolution rate of the host mineral, and the ionic radii of the REE3+ in question are found to control the kinetics of the replacement reaction, the polymorph selection, and the crystallization pathways toward bastnasite. The findings allow to gain a more in‐depth understanding of the formation REE‐bearing carbonates, particularly the mineral bastnasite, which is the main source of REEs for industry. This knowledge can be used to improve REE separation, exploration, exploitation methods, as well to produce carbonate minerals with tailored structures.

Rare earth element (REE) fluorcarbonates are found for the first time in the ore-hosting sequence of the Saf’yanovka Cu–Zn massive sulfide deposit, Middle Urals: bastnäsite, synchysite-(Ce), and synchysite-(Y) in an assemblage with fluorite, calcite, barite, pyrite, and sphalerite. Samples with fluorcarbonates were found at a depth of 262 m in quartz veinlets of an altered zone of pyrite–carbonate–kaolinite–sericite–quartz rocks at the contact with massive pyrite–chalcopyrite and pyrite–sphalerite ores. It is suggested that the REE fluorcarbonates formed from a hydrothermal fluid synchronously with fluorite and calcite. This event can be related to hydrothermal activity at the postcollision stage of the evolution of the Urals fold system. [ABSTRACT FROM AUTHOR]

This study presented the first in a series of investigations currently underway to develop efficacious, cost-effective, and benign processing opportunities to produce rare earth elements (REE)–rich concentrate from an Australian complex low–grade saprolite ore [1.14% total rare earth oxides (TREO) grade], which is primarily exploited for its gold and copper values. This work specifically presented a preliminary flotation investigation carried out on the ore using sodium oleate as a collector. The relative effects of pulp pH, desliming, and depressants were investigated to ascertain any chance of recovering and upgrading REE minerals in saprolite ore using three different processing configurations. Based on the experimental results, flotation processes carried out on raw feed allowed the recovery of the majority of REE minerals (>50%), but the process was unselective, where clay and silicate gangue minerals reported into the flotation concentrate along with the REE minerals. However, desliming before flotation in the presence of depressants (starch and sodium silicate) improved REE minerals flotation selectivity, which produced concentrates assaying 5.87% and 4.22% TREO grades, with corresponding recoveries of 45% and 50% at pulp pH 9 and 10.5, respectively. Mineralogical analysis conducted on selected flotation concentrate indicated that silicate and clay gauge minerals were recovered via the synergistic act of surface activation and entrainment due to their fine to ultrafine nature. A comparison of all the test results revealed a haphazard grade–recovery relationship suggesting that there is an opportunity to further maximize both REE recovery and grade through further flotation studies where other process parameters may be investigated and optimized. The prospect of using magnetic separation has also been suggested. [ABSTRACT FROM AUTHOR]

The rapid depletion of high-grade rare earth elements (REE) resources implies that future supplies may be augmented with low-grade ores, mining tailings and other unconventional resources to meet cutoff grades and subsequently supply demands. In this work, detailed characterization of tailings from a copper processing plant was carried out to expand the knowledge of their chemical and mineralogical characteristics for REE recovery. The results showed that the REE are mainly mineralized as bastnäsite, monazite, and stetindite, along with uncommon minerals, such as brannerite, stillwellite, and florencite, for which there is limited processing data. In a preliminary effort to recover the REE, the tailings were subjected to gravity, magnetic, and froth flotation separation. Over 90% REE recovery was achieved via froth flotation, whereas magnetic and gravity separation tests recovered up to 86% and 54% REE, respectively. The results from this study suggest that, with systematic optimization studies, REE recovery and upgrade could be enhanced, with significant efforts of minimizing the recovery of key gangue minerals, such as goethite, hematite, and quartz. Overall, the study confirms that the tailings used in this work could be regarded as unconventional REE resources subject to tactical development and optimization of beneficiation methods. [ABSTRACT FROM AUTHOR]

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Bauxite deposits are an important source of Al and also host a variety of critical metals, including rare earth elements (REEs). However, the parent rocks of bauxite deposits generally have low REE contents, leading to uncertainty over the REE sources and enrichment mechanisms. In this study, we report high REE (2095 ppm; Ce = 1340 ppm) contents in the Lijiatian bauxite deposit in Luxi County, western Hunan Province, South China. The Lijiatian bauxite deposit is the largest deposit in Hunan Province, with a total Al resources exceeding 7 × 106 tons. The ore-bearing strata in the lower Permian Liangshan Formation (ca. 275 Ma) are comprised a 3–5-m-thick bauxite layer (bauxite ore/clay) and an underlying 4–8-m-thick Fe layer (Fe ore/Fe-rich clay). The high REE contents in the bauxites can be attributed to the widespread presence of bastnäsite. The bastnäsite commonly coexists with authigenic chlorite, which formed at temperatures of 221–285 °C, indicative of a hydrothermal origin. Uranium–Pb dating of the bastnäsite yielded an age of 141.4 ± 7.0 Ma, indicating a hydrothermal overprint occurred during the Early Cretaceous, which resulted in the enrichment of REEs in the bauxites. The high contents of REEs, Ba, and P in the Cambrian Niutitang Formation black shales indicate it was a potential source of these elements for bastnäsite and coexisting barite and apatite. The deep-seated hydrothermal fluids (enriched in HF, CO 2 , and H 2 S) associated with the late Yanshanian extensional tectonic event in South China (155–123 Ma) ascended along fractures. The ascending fluids interacted with the black shales, extracting REEs, Ba, and P. When the fluids reached a certain depth, they mixed with the infiltrating basinal waters to form a mineralising fluid. The fluid then migrated towards the bauxite layers, underwent cooling and depressurisation, and precipitated bastnäsite, barite, apatite, and chlorite. We propose that a hydrothermal event led to the enrichment of REEs in the bauxites in Hunan Province. Considering the scarcity of REEs in the parent rocks of bauxites worldwide, hydrothermal activity likely has a key role in REE enrichment in bauxites. [Display omitted] • The main mineral hosting REE (Ce) in Permian bauxite from southern China is Bastnäsite. • The bastnäsite formed during the Cretaceous period due to hydrothermal fluid activity. • The REEs are source from the basement black shale. [ABSTRACT FROM AUTHOR]

[Display omitted] • Bastnäsite formed in the Vuoriyarvi carbonatites during Triassic at ca. 235 Ma. • Bastnäsite is not related to the magmatic event that formed the Vuoriyarvi complex. • Bastnäsite of the Vuoriyarvi is not supergene. The Vuoriyarvi massif is a Devonian multistage alkaline-ultrabasic carbonatite complex within the Kola alkaline province. Dolomite carbonatites of the Vuoriyarvi massif contain abundant rare-earth mineralization mainly represented by ancylite-(Ce) and bastnäsite-(Ce). Ancylite was previously shown to have probably formed in the Devonian (ca. 365 Ma) during an early postmagmatic overprint. Previous geological observations have revealed a much later crystallization of bastnäsite but have not been able to specify the exact age of the mineralization. The in situ U-Pb dating of bastnäsite allowed us to constrain its genesis. Bastnäsite for this study was extracted from two varieties of dolomite carbonatite breccias cemented by (1) quartz-bastnäsite and (2) strontianite aggregates (hereafter bastnäsite-rich and strontianite-rich carbonatites – BRC and SRC, respectively). The obtained age estimations (237.7 ± 9.8 Ma and 239.9 ± 4.1 Ma, respectively) indicate that both studied rocks were formed during a single event. The revealed age difference (∼125 Ma) excludes the genetic link between the bastnäsite origin and regional alkaline magmatism, pointing out an additional source for the Vuoriyarvi bastnäsite-bearing rocks. Moreover, the obtained U-Pb ages provide strong evidence that a Triassic event is responsible for the occurrence of bastnäsite mineralization due to hydrothermal REE redistribution from the Devonian ancylite-rich carbonatites. Most of the REEs released during this process via dissolution of ancylite were precipitated in situ as bastnäsite, while strontium was transported and incorporated into strontianite. The Pb isotopic characteristics of bastnäsite (206Pb/204Pb = 18.1 ± 0.1, 207Pb/204Pb = 15.3 ± 0.1, and 207Pb/206Pb = 0.84 ± 0.01) are most probably inherited from the Devonian host rocks of the Vuoriyarvi massif involved in the Triassic overprint. Isotopic signatures of Pb, Sr, and Nd show that the depleted mantle and lower crust played the leading role in formation of the Vuoriyarvi alkaline complex. Taken together, the results of the present study negate the supergene origin of the Vuoriyarvi bastnäsite, implying that the bastnäsite mineralization is not confined to near-surface layers and, therefore, may be dispersed more broadly throughout the complex. These findings raise the question on underestimation of the probable REE reserves and lay the groundwork for a reassessment of the economic potential of the Vuoriyarvi complex. [ABSTRACT FROM AUTHOR]

The rapid depletion of high-grade rare earth elements (REE) resources implies that future supplies may be augmented with low-grade ores, tailings, and other unconventional resources to meet cut-off grades and, subsequently, supply demands. This paper presents an amalgamation of studies conducted on selected complex low-grade iron-oxide-silicate-rich tailings, with the overall aim of developing efficacious methods and associated process mineralogy characterization for enhanced REE minerals recovery and upgrade. To this end, a summary of the overarching key results from froth flotation, magnetic separation, and gravity separation studies of the tailings and their implications are presented in this review. Reconciliation of all the findings reveals lucid links between feed ore properties, mainly mineralogy and particle size distribution, as the key influential factors that affect the beneficiation of real complex low-grade tailings, although distinct differences in physicochemical properties of the valuable and gangue minerals may exist. It is clearly established that the unliberated association between REE and gangue minerals within the ore can lead to either synergistic or antagonistic effects on the quality of concentrates produced. Furthermore, the limitations presented by the poorly liberated minerals are exacerbated by their "fine" nature. With appreciable recoveries obtained using such readily available conventional separation methods, the tailings provide additional REE value to the primary commodities; hence, such material could be considered a potential resource for REE beneficiation. The learnings from the respective beneficiation studies are significantly important as they provide the knowledge base and greater understanding of the mineralogical characteristics and beneficiation response of REE minerals in typical complex, low-grade tailings. [ABSTRACT FROM AUTHOR]

Rare earth elements (REEs) are considered critical elements in modern society due to their irreplaceable role in new innovative and energy technologies. The giant carbonatite-related Bayan Obo deposit contributes most REE resources in the world's market, while its origin is still unclear because of the complicated and diverse REE ore mineralogy and texture. Thescanning electron mircroscopy SEM)-based automated mineralogy allows for the numeric assessment of rocks and ores' compositional and textural properties. Here, we use TIMA (TESCAN Integrated Mineral Analyzer) to quantitatively characterize REE ore mineralogy from the deep drill core within the H8 unit ("dolomite marble") to better understand the deposit. The mineral composition, occurrence, and Ce elemental deportment of the borehole ores at different depths (i.e., 1107 m, 1246 m, 1406 m, 1546 m, and 1682 m) were obtained. The results show that the main types of ores in the investigated samples can be divided into banded REE-Fe ores, banded REE ores, disseminated REE-Fe ores, and veined REE ores. REE and gangue minerals vary significantly in abundance and occurrence. Monazite-(Ce) and bastnäsite-(Ce) are the primary REE host minerals, and both contribute the most to the REE budget. Other REE minerals, such as parisite-(Ce)/synchysite-(Ce), cerite-(Ce), huanghoite-(Ce)/cebaite-(Ce), and aeschynite-(Ce), are significant contributors. The gangue minerals generally include fluorite, barite, magnetite, pyrite, quartz, magnesio-arfvedsonite, and minerals of the biotite and apatite groups, among others. Combined with the newly published mineral-scale chronological and isotopic geochemical analyses, it is reasonable to conclude that the later hydrothermal fluids remobilized and redistributed the original Mesoproterozoic carbonatitic REE minerals and formed a high variable ore mineral assemblage. Furthermore, it is demonstrated that the mineralogical study using TIMA can provide accurate and reliable mineralogy data for the comprehensive interpretation of the complex REE ores, and extend our understanding of the deposit. [ABSTRACT FROM AUTHOR]

The stability relations of cerium and its species in the hydrometallurgical processing of bastnasite were studied using Pourbaix diagrams. Different systems of Ce-(F)-(CO3)-(SO4)-(C2O4)-H2O were constructed using HSC Chemistry software with the necessary thermodynamic data. The Ce-F-H2O system exhibits a large stability region for CeF3 (pH∼ −1.5 – 12). The stability domain of was determined by Ce-CO3-H2 system in the pH of 4.5 – 12. The decomposition behaviours of CeFCO3 were studied by Ce-F-CO3-H2O system which shows the stability domain of CeFCO3 from neutral to basic media. The treatment of CeFCO3 with H2SO4 was investigated by depicting diagrams of Ce-F-CO3-SO4-H2O system. According to this system, CeFCO3 can be decomposed to produce soluble Ce sulphates at around pH 6.2. In contrast, alkaline treatment of CeFCO3 converts it to Ce(OH)3 at pH 11. Finally, the recovery of Ce by oxalate precipitation was studied by Ce-C2O4-H2O system which showed the domain of Ce oxalate decahydrate. [ABSTRACT FROM AUTHOR]

The time and processes of hydrothermal mineralization are long-standing problems in geology. This work addresses these questions with reference to the Maoniuping giant rare earth elements (REE) deposit (southwest China), which has rare earth oxides (REO) reserves of 3.17 million tons with an average grade of 2.95 wt%. Bastnäsite is the dominant economic mineral, occurring as four distinct paragenetic types in the Maoniuping syenite–carbonatite complex: (1) primary euhedral bastnäsite (type-A) in syenite, with isolated melt inclusions; (2) macro-crystalline tabular euhedral bastnäsite (type-B) in pegmatitic dikes, with a diverse variety of fluid inclusions; (3) fine-grained, anhedral veinlet-disseminated bastnäsite (type-C) in syenite; and (4) coarse-grained anhedral bastnäsite (type-D) in carbonatite dikes, occurring as veinlets or interstitial to calcite, fluorite, and barite. From the paragenetic and compositional variations, it is inferred that type-A bastnäsite is of primary magmatic origin, whereas the other three types have characteristics of hydrothermal origins. In situ LA-ICP-MS U-Pb geochronology of the four types of bastnäsite results in lower intercept ages of 28.2 ± 0.5 Ma (n = 95, MSWD = 5.10), 27.8 ± 0.4 Ma (n = 43, MSWD = 0.73), 26.8 ± 0.7 Ma (n = 50, MSWD = 0.83), and 25.8 ± 0.7 Ma (n = 55, MSWD = 1.70), respectively, which are consistent with the weighted average 206Pb/238U and 208Pb/232Th ages by 207Pb-correction method. Compositional variations of clinopyroxene and apatite from the associated syenite, pegmatitic and carbonatitic dikes indicate a genetic relationship of the Maoniuping alkaline complex. The compositions of clinopyroxene range from Ae44–67Di14–18Hd17–41 in pegmatitic dikes, Ae43–66Di6–20Hd21–38 in carbonatitic dikes to Ae68–90Di0–3Hd10–30 in syenite. Apatites in the pegmatitic and carbonatitic dikes have similar compositions with higher F, total REE, and Sr, and lower CaO contents than those in the syenite, which suggests a cogenetic origin for the associated pegmatite and carbonatite. Clinopyroxene and apatite compositions suggest that the pegmatitic melt might differentiate directly from the initial carbonatitic melt rather than the syenitic magma. The bastnäsite U-Pb geochronology and minerals data indicate continuous magmatic-hydrothermal evolution for the REE mineralization in the Maoniuping alkaline complex. [ABSTRACT FROM AUTHOR]

Thermodynamic calculations on the impact of carbonate–bicarbonate fluids cooling from 500 to 100°C on a monazite + calcite association with a simultaneous decrease in pressure from 2000 to 125 bar are performed. It is shown that the REE carbonate–bicarbonate complexes gain importance only at low temperatures. In this case, the total concentration of lanthanides in the fluid turns out to be rather low. This indicates that, in the presence of even insignificant amounts of fluorine and calcium in the hydrothermal system, carbonate–bicarbonate fluids do not lead to the removal of REEs but contribute to their accumulation in the form of precipitating fluorocarbonates. Thus, the role of carbonate-bicarbonate hydrothermal fluids in the transport and deposition of rare earth elements has been assessed for the first time for a wide range of TP-parameters and for the entire series of lanthanides. [ABSTRACT FROM AUTHOR]

[Display omitted] • Monazite and bastnäsite U-Th-Pb dating demonstrate that the Boziguoer deposit formed at ∼ 290–280 Ma. • Post-magmatic hydrothermal processes play a significant role in the mineralization process at Boziguoer deposit. • The alkaline rocks and associated Nb-Ta-Rb-Zr-REE mineralization of the South Tianshan formed in a Tarim mantle plume setting. The Boziguoer alkaline intrusion is located in the South Tianshan Orogenic Belt on the northern margin of the Tarim Craton. The intrusion is entirely mineralized and forms a super-large rare metal (RM) and rare-earth element (REE) deposit dominated by Nb-Ta-Rb, accompanied by Zr-REE. The primary RM minerals include pyrochlore, astrophyllite, and zircon, while the main REE minerals consist of fluocerite, monazite, xenotime, and bastnäsite, which are commonly present as granular aggregates or singularly filling the interstices between gangue minerals. Through a combination of geochronological and geochemical analyses of different types of monazite and bastnäsite in the mineralized alkaline rocks, this study elucidates the role of magmatic-hydrothermal evolution on the Boziguoer RM-REE mineralization and reconstructs the geochronological framework of alkalic magmatic-hydrothermal evolution and mineralization processes, as well as establishing mechanisms responsible for RM-REE enrichment. Petrographic observations and back-scattered electron (BSE) imaging revealed several types of monazite and bastnäsite with different characteristics as follows: (1) type Ia monazite (Mnz-Ia) experienced intense hydrothermal alteration, forming residual cores of monazite mantled by apatite and allanite coronas; (2) type Ib monazite (Mnz-Ib) was partially eroded, forming concentric zoning patterns with a core of monazite, a mantle of apatite, and an outer rim of allanite coronas; (3) type II monazite (Mnz-II) is slightly modified, and commonly associated with fluorite; (4) bastnäsite and fluocerite exhibit a core-rim structure, with the core of fluocerite being brighter than the rim of bastnäsite in BSE images. According to the paragenetic relationships and compositional variations, all two types of monazite are of primary magmatic origin, while bastnäsite is of hydrothermal origin. In addition, principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) results indicate that Mnz-Ia and Mnz-Ib belong to the same group, different from Mnz-II. The latter has higher (Ce/Gd) N and (Th/U) N ratios, indicating relatively low-temperature crystallization of Mnz-II at late stages. In-situ LA-ICP-MS U-Th-Pb dating of monazite and bastnäsite show that Mnz-Ia and Mnz-Ib have ages of ca. 290 Ma, consistent with the zircon age of the intrusion, whereas the Mnz-II and bastnäsite have younger ages of ca. 280 Ma, representing a post-magmatic hydrothermal mineralization event at Boziguoer. These new ages indicate that both magmatic and subsequent hydrothermal processes played critical roles in the RM-REE mineralization in this deposit. The latest findings also support the model that the formation of the alkaline belt where the Boziguoer ore-bearing intrusion was temporally and spatially linked to the Tarim Large Igneous Province that is genetically related to a mantle plume. [ABSTRACT FROM AUTHOR]

[Display omitted] • CL colors and peaks of minerals in ores containing bastnäsite were investigated. • Bastnäsite can be identified from color and luminescence intensity in CL images. • Rough content of the bastnäsite in the ores can be estimated from the CL images. • CL imaging can aid the effective exploration of mines with REE reserves. A rapid determination of the rough content of the major rare earth element (REE)-bearing minerals (bastnäsite, monazite, and xenotime) can lead to a drastic reduction of the time required to analyze mineral ores for the extraction of REE metals, thus enabling the efficient exploration of mines with REE reserves. This study presents a method of using cathodoluminescence (CL) imaging to identify bastnäsite in mineral ores and determine the rough bastnäsite content rapidly. By investigating the luminescence colors in CL images and the peaks in CL spectra emitted by minerals in ores that include bastnäsite, the author found that bastnäsite can be identified by detecting dark red or red–orange luminescence in CL images of the ores in the wavelength range 420–680 nm or by detecting areas that emitted no luminescence in the 420–680 nm CL images but that do emit pale red luminescence in CL images in the broader wavelength range 350–1000 nm. The bastnäsite content in the ores can be estimated roughly from the fractional areas identified as bastnäsite in the CL images. The CL images can be obtained within 30 s. Therefore, CL imaging can prescreen mineral ores for subsequent more precise quantitative analyses (e.g., using inductively coupled plasma mass spectrometry or electron-probe microanalysis) by identifying mineral ores having a probable high bastnäsite content. This approach can drastically reduce the time required to explore mines with potential REE reserves. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Huanglongpu deposit developed two stages of REE mineralization (219.4 Ma and 145.9–142.6 Ma). • The early REE mineralization is intimately related to the carbonatite Mo mineralization, which was created in the late Triassic post-collision extensional environment. • The late REE mineralization may be generated by REE remobilization and recrystallization resulting from the Yanshanian magmtism. • The REE mineralization that occurred at 145.9–142.6 Ma was first reported in the Qinling Region. The Huanglongpu deposit is one of the most representative igneous carbonatite-type molybdenum-rare earth element (Mo-REE) deposits in the eastern Qinling metallogenic region. The molybdenum resources exceed 180 kt, whilst the total rare earth resources are about 37.8 kt. However, the age of REE mineralization, sources of ore-forming minerals, and tectonic setting are not well characterized. Therefore, this study carried out detailed mineralogical, geochronological, and geochemical analyses to provide new insights. Monazite, bastnäsite, parisite, and xenotime were found to be the dominant REE-bearing minerals in the Huanglongpu deposit. LA-ICP-MS U-Pb dating of the REE minerals revealed a two-stage REE mineralization: monazite formed at 219.4 Ma, whereas bastnäsite and xenotime formed at 145.9 Ma and 142.6 Ma, respectively. The carbonatites, as the main ore host, were characterized by a strong enrichment of light rare earth element (LREE) and moderately high heavy rare earth element (HREE) contents. Monazite showed different Nd isotopic compositions, with lower ε Nd (t) values ranging between −7.12 and −4.18, while the ε Nd (t) values of bastnäsite and xenotime ranged from −9.57 to −6.49 and from −11.13 to −6.28, respectively. These contrasting geochemical and isotopic signatures suggest that the Huanglongpu REE mineralization occurred over multiple magmatic-hydrothermal events and that the ore-forming material was unlikely to have stemmed from a single enriched mantle (EMI) source. The early REE mineralization stage (219.4 Ma) was closely associated with carbonatite-related Mo mineralization, both of which were products of magmatic mineralization during the crystallization of the host carbonatite emplaced in an extensional environment after the Late Triassic collision. The ore-related magmas may have originated from a hybrid source composed of EMI and partial melting of the lower crust. In contrast, due to the extensive Yanshanian magmatism which could provide massive heat, the later REE mineralization stage (145.9–142.6 Ma) in the Huanglongpu deposit was likely driven by the reactivation and recrystallization of early-formed REE minerals within the host carbonatites, hence representing a hydrothermal redistribution of the primary magmatic ore. [ABSTRACT FROM AUTHOR]

The interaction between rare earth element (REE)-rich (La, Pr, Nd, Dy) aqueous solutions, dolomite (CaMg(CO 3 ) 2 ), and aragonite (CaCO 3 ) at low temperature hydrothermal conditions (25-220 °C) is studied. The experiments result in the solvent-mediated surface precipitation and subsequent pseudomorphic mineral replacement of the dolomite and aragonite seeds by newly formed REE-carbonates. The host grains are replaced from periphery inward. The newly formed REE-bearing carbonates in La-, Pr-, and Nd-doped systems follow the crystallization sequence: lanthanite [REE 2 (CO 3 ) 3 ·8H 2 O] → kozoite [orthorhombic REECO 3 (OH)] → hydroxylbastnasite [hexagonal REECO 3 (OH)]. The interaction of Dy-bearing solutions with dolomite results only in the crystallization of kozoite [orthorhombic DyCO 3 (OH)]. However, experiments with aragonite reveal a two-step crystallization pathway: tengerite [Dy 2 (CO 3 ) 3 ·2-3(H 2 O)] → kozoite [orthorhombic DyCO 3 (OH)]. The temperature, the dissolution rate of the host mineral, and the ionic radii of the REE 3+ in question are found to control the kinetics of the replacement reaction, the polymorph selection, and the crystallization pathways toward bastnasite. The findings allow to gain a more in-depth understanding of the formation REE-bearing carbonates, particularly the mineral bastnasite, which is the main source of REEs for industry. This knowledge can be used to improve REE separation, exploration, exploitation methods, as well to produce carbonate minerals with tailored structures., Competing Interests: The authors declare no conflict of interest., (© 2022 The Authors. Global Challenges published by Wiley‐VCH GmbH.)