Search

Karst bauxites have recently received renewed attention for their potential as non-conventional REE sources. Karst bauxites from the Pedernales Peninsula in the Dominican Republic stand among the world’s richest in REE. Bauxite ore from two deposits from this bauxite district, Aceitillar and El Turco, have been selected for this study due to their outstanding REE contents and contrasting mineralogy. REE (La to Lu) contents in Aceitillar, range from 0.07 to 0.16 wt%, and Y from 0.01 to 0.13 wt%, whereas El Turco contains between 0.28 and 1.40 wt% REE, and 0.33 to 1.48 wt% Y. The characterisation of REE mineralisation was performed through powder and monocrystal XRD, SEM-EDS, and EMP analyses. REE phosphates and carbonates reveal textural features that suggest significant REE mobilisation and re-deposition within the bauxite profile. The identified REE minerals can be classified into: i) primary monazite(-Ce) and minor monazite(-La); ii) secondary Y- and Nd-dominant phosphates; and iii) secondary Gd- and Nd-carbonates of the (hydroxyl)bastnäsite group. While monazites are ubiquitous in the two studied deposits, secondary phosphates are predominant in El Turco while secondary carbonates are exclusive of Aceitillar. This contrasting mineralogy is explained by the total concentration of carbonate and/or phosphate in the karst bauxite groundwater solutions. REE phosphates are the most stable phases at [CO32−]total/[PO43-]total ≤ 2; whereas REE carbonates are stable at near neutral pH when the total aqueous carbonate concentration is two orders of magnitude higher than that of phosphate. Results of this investigation contribute to a better understanding of the formation REE minerals in the supergene environment and can be applied in REE separation methods. Validerad;2023;Nivå 2;2023-04-21 (sofila);Funder: MESCYT (Ministry of HigherEducation, Science and Technology of the Dominican Republic) (grant nos. 2014-1B4-133 & 2022-1A4-189) PID2019-105625RB-C21 (IP J.A. Pro-enza)

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. Refereed/Peer-reviewed

Isotopic dating studies have revealed that many carbonatite-related REE ore systems have protracted and complex temporal evolution histories. The mechanisms that cause these complex dating results remain poorly understood. Here, we present an in-situ Sm-Nd and Th-Pb isotopic investigation of monazite and bastnasite from a geologically-complicated deposit, the world-class Bayan Obo REE deposit of China, with the aim of defining the general mechanisms accounting for the protracted age spread observed in this and many other REE deposits. Monazite and bastnasite are two principle REE ore minerals in the Bayan Obo deposit. Most of the studied monazite and bastnasite grains have experienced fluid-aided modification. The modified grains have generally homogeneous Sm and Nd contents and Sm-Nd isotopic compositions, implying low mobility of Sm and Nd during the modification process. Thus, the Sm-Nd chronometer can be used to constrain the primary crystallization timing of the REE minerals at 1293 ± 48 Ma. However, altered REE minerals have apparent 208Pb-232Th ages (1248–261 Ma) that are variably younger than the primary crystallization age. In most samples, Th contents have no correlations with 208Pb-232Th ages, while Pb contents tend to decrease along with the younging trend of 208Pb-232Th ages. NanoSIMS mapping shows that 208Pb and 208Pb/232Th are heterogeneous, and irregularly distributed in the altered grains. Such results are indicative of variable Pb loss during the fluid-aided modification process. Thus, many Th-Pb ages represent partial isotopic resetting and, hence, are geologically meaningless. Completely reset ages cluster at 450–400 Ma or 280–260 Ma, representing two discrete episodes of post-ore hydrothermal activity. Small amounts of monazite and bastnasite that were newly crystallized during these two overprinting episodes have initial Nd isotopic signatures approaching the time-evolved isotopic compositions of primary ores, which suggests insignificant REE input to the primary ores during these events. This study provides critical evidence to resolve complex dating results and disparate ore genesis models for the giant Bayan Obo deposit. Moreover, our results indicate the resistance of Sm-Nd isotopes, but the susceptibility of Th-(U)-Pb isotopes, to fluid-aided modification. Therefore, younger Th-U-Pb dates in geologically-complicated deposits should be treated with caution as they may represent partial resetting results and/or may not represent events of increased metal tenor.

Ca−REE fluorocarbonates occur in various geological environments, including A-type granite systems. Here we investigate the character and paragenesis of fluorocarbonates in the A-type Rödö rapakivi complex, central Sweden, and their importance for the late stages of felsic magma evolution. Magma evolution is largely controlled by fractionation of the major phases quartz, plagioclase and K-feldspar, but these do not account for the decrease in LREE in the evolved magmas. Allanite or monazite, the major LREE-fractionating accessory minerals in common silicic igneous rocks, have not been encountered. In the Rödö suite, Ca−REE fluorocarbonates occur in rocks of various evolutionary level, typically as interstitial fibrous masses or filling small vugs. In the more evolved rocks, they are generally associated with fluorite and xenotime. In part, they are rich in Fe, Th, and Y. In composition, they range between synchysite and bastnäsite (but not pure bastnäsite) and seem to involve a number of less well-known intermediate phases. Ca−REE fluorocarbonate chemistry covaries with whole rock chemistry such that they become depleted in LREE and enriched in Y+HREE with magma evolution. This suggests that fluorocarbonates have crystallized in fluid ‘bubbles’ from fluids expelled from the evolving high-level, F- and CO2-rich magma system at various stages, which resulted in a progressive depletion in LREE coupled with enrichment in HREE in the residual magmas.

—This paper presents the results of the first geological, isotope, geochemical, and mineralogical study of carbonatite veins that were previously unknown in the Soustov pluton. The studied veins are similar in the Sm–Nd isotope composition and model age to the host rocks, which implies a common formation processs. High contents of light lanthanides, Sr, and Nb in carbonatite veins were measured. These elements are concentrated in bastnasite, strontianite, monazite, and pyrochlore. These data significantly enlarge our concepts of the geochemical and ore specialization of the massif.

The genesis of Earth’s largest rare earth element (REE) deposit, Bayan Obo (China), has been intensely debated, in particular whether the host dolomite marble is of sedimentary or igneous origin. The protracted (Mesoproterozoic to Paleozoic) and intricate (magmatic to metasomatic) geological processes complicate geochemical interpretations. In this study, we present a comprehensive petrographic and in situ, high-spatial resolution Sr-Pb isotopic and geochemical investigation of the host dolomite from the Bayan Obo marble. Based on petrographic evidence, the dolomite marble is divided into three facies including coarse-grained (CM), fine-grained (FM), and heterogeneous marble (HM). All carbonates are ferroan dolomite with high SrO and MnO contents (> 0.15 wt.%), consistent with an igneous origin. Trace element compositions of these dolomites are highly variable both among and within individual samples, with CM dolomite displaying the strongest LREE enrichment. In situ 206Pb/204Pb and 207Pb/204Pb ratios of the dolomite are generally consistent with mantle values. However, initial 208Pb/204Pb ratios define a large range from 35.45 to 39.75, which may result from the incorporation of radiogenic Pb released from decomposition of monazite and/or bastnasite during Early Paleozoic metasomatism. Moreover, in situ Sr isotope compositions of dolomite indicate a large range (87Sr/86Sr = 0.70292–0.71363). CM dolomite is characterized by a relatively consistent, unradiogenic Sr isotope composition (87Sr/86Sr = 0.70295–0.70314), which is typical for Mesoproterozoic mantle. The variation of 87Sr/86Sr ratios together with radiogenic 206Pb/204Pb signatures for dolomite within FM and HM possibly represents recrystallization during Early Paleozoic metasomatism with the contribution of radiogenic Sr and Pb from surrounding host rocks. Therefore, our in situ geochemical data support a Mesoproterozoic igneous origin for the ore-bearing dolomite marble in the Bayan Obo deposit, which subsequently underwent intensive metasomatism during the Early Paleozoic.

Bastnasite is the main ore mineral in many carbonatite-related rare earth element (REE) deposits, which account for ∼51% of rare-earth oxide reserves worldwide. However, the occurrence, geochemistry, and genetic significance of bastnasite has not been methodically investigated. The Cenozoic Mianning–Dechang (MD) REE belt in Sichuan Province, SW China, contains the Maoniuping, Dalucao, Lizhuang, and Muluozhai deposits as well as numerous smaller REE occurrences. Individual deposits within the belt contain different types of bastnasite-bearing ore, which provides a unique opportunity to explore in detail the common mechanisms controlling the formation of bastnasite-rich REE deposits. Here, we present detailed results from field observations and petrographic, geochemical, and fluid inclusion studies of bastnasite from the main MD deposits. Calcite, fluorite, and barite form stable mineral assemblages that are commonly overprinted by bastnasite. Homogenization temperatures of fluid inclusions in bastnasite of ∼150–270 °C (Dalucao and Lizhuang deposits) and 155–210 °C (Maoniuping deposit) are systematically lower than those of fluid inclusions in gangue minerals. Therefore, the petrographic studies and homogenization temperatures both show that large-scale crystallization of bastnasite took place during the later stage of the hydrothermal system. The bastnasite, relatively geochemically homogeneous within all of the MD deposits, is enriched in Ba (293–8425 ppm), Th (16.4–2527 ppm), and U (4.19–92.7 ppm), and relatively depleted in high field strength elements such as Nb (0.15–17.4 ppm), Ta (0.06–6.48 ppm), Zr (0.71–31.1 ppm), Hf (0.62–5.65 ppm), and Ti ( carbonatite , and ore veins. In comparison, the samples from the study area show an increase in average REE contents from syenites to carbonatites to ore veins (i.e., bastnasite-bearing ores) and finally to bastnasite. Lanthanum and Ce were commonly substituted by Th, U, Sc, Ba, and Sr supplied by more evolved hydrothermal fluids. Combining the present results with existing data, we present a three-stage model for the formation of carbonatite-related REE deposits. First, partial melting of metasomatized sub-continental lithospheric mantle, fluxed by REE- and CO2-rich fluids, forms the parental carbonatite–syenite magma. Second, Sr, Ba, and REEs are strongly partitioned into carbonatite melts during liquid immiscibility in the carbonatite–syenite magmatic system. Third, hydrothermal fluids exsolved from the crystalizing syenite and carbonatite magmas form ore veins with early gangue minerals and later bastnasite overgrowths. Consequently, barite, calcite, and fluorite assemblages are a valuable guide in REE exploration.

The Taipingzhen deposit is a newly discovered REE deposit in the North Qinling Orogen (NQO), Central China. The REE ores are hosted in the altered fracture zones of Paleozoic strata and a granitic intrusion. The ore minerals mainly include bastnasite-(Ce)[(Ce(CO3)F], parisite-(Ce)[CaCe2(CO3)3F2], and synchysite-(Ce)[CaCe(CO3)2F], and this deposit also contains some uncommon minerals, such as tornebohmite-(Ce)[Ce2Al(SiO4)2(OH)], gatelite-(Ce) [CaCe3AlAl2Mg(Si2O7)(SiO4)3O(OH)2], fluocerite-(La)[LaF3] and F-rich cerite-(Ce)[(Ce,Ca)9 Mg(SiO4)3[SiO3(OH)]4F3], which are identified here for the first time in China. To determine the timing of REE mineralization and explore possible genetic relationships between mineralization and granitic magmatism, the U-Pb ages of bastnasite-(Ce) from ore and zircon from ore-hosting granite are obtained. The in situ LA-MC-ICP-MS U-Pb age of the bastnasite-(Ce) provides a reliable 238U/206Pb-207Pb/206Pb lower intersection point age of 409 ± 16 Ma, and ID-TIMS analyses yield an U-Pb age of 407.8 ± 3.3 Ma; the U-Pb dating of the magmatic zircon reveals that the wall rock was emplaced at 457.1 ± 4.9 Ma, and the hydrothermal zircon age of 399.4 ± 3.5 Ma records the time of hydrothermal alteration, thus indicating that the REE mineralization in the Taipingzhen deposit occurred at approximately 409–400 Ma. This age coincides well with the U-Pb ages of the bastnasite-(Ce) and zircon, revealing a genetic association between the hydrothermal fluid and the REE mineralization. The Hf isotopic compositions imply that the partial melting of Paleoproterozoic HP and UHP eclogite from the lower crust provided ore materials for the mineralization, and the deposit was generated in a retrograde metamorphic setting during the late Paleozoic postcollisional extension event in the NQO. Furthermore, combined with the geochronological data published for other U-REE metallic deposits in the NQO, our study indicates that the late Silurian-early Devonian granite-related U-REE metallic mineralization throughout the NQO mainly occurred during 421–395 Ma.

Carbonatite-related rare-earth element (REE) deposits are the most important source of the world’s REE resources. Hydrothermal fluids have been proposed to play a significant role in the transport and precipitation of REEs, but fluid inclusion data on the hydrothermal processes in carbonatitic settings are relatively sparse. The Dalucao deposit, located in the Mianning–Dechang (MD) REE belt, Sichuan, China, is a Cenozoic carbonatite-related REE deposit (c. 12 Ma) that offers an excellent opportunity to investigate the evolution of ore-forming fluids. Brecciated and weathered ores are common in this deposit. The former are characterized by mineral assemblages comprising fluorite + barite + celestite + calcite + quartz + bastnasite (No. 1 orebody) or fluorite + celestite + pyrite + muscovite + calcite + quartz + bastnasite (No. 3 orebody), whereas the latter contain REE minerals, clay minerals, and minor gangue minerals. We present a comprehensive study of fluid inclusions from the Dalucao deposit to constrain its hydrothermal evolution. Magmatic, pegmatitic, hydrothermal, and supergene stages have been recognized. During the pegmatitic stage, the main minerals that formed were coarse-grained fluorite, barite, celestite, calcite, and quartz, which host melt inclusions, melt–fluid inclusions, and minor high-salinity fluid inclusions. The presence of melt and melt–fluid inclusions suggests a magmatic origin for the ore-forming fluids. Hydrothermal processes included at least two stages, characterized by hydrothermal veins that are developed in fractures within the carbonatite–syenite complex: (1) Fluid inclusions during the formation of the fluorite–quartz–barite veins in the pre-REE stage were trapped under immiscible conditions, as evidenced by the presence of CO2-bearing inclusions coexisting with aqueous ones. These immiscible CO2-bearing inclusions recorded a range of pressures from 1050 to 1600 bar. All of fluid inclusions in this stage exhibited homogenization temperatures varying from 278 to 442 °C, with salinities ranging from 3.2 to 45.1 wt% NaCl equivalent (equiv.). (2) The REE-stage fluids were represented by abundant aqueous inclusions, characterized by homogenization temperatures ranging from 147 to 323 °C and salinities between 1.1 and 9.5 wt% NaCl equiv. These data suggest that the ore-forming fluids forming the Dalucao deposit evolved from high-temperature, high-pressure, high-salinity, CO2-rich to low-temperature, low-pressure, low-salinity, CO2-poor. Gas- and ion-chromatographic analyses combined with mineralogical features indicate that the initial fluids were rich in REEs, (SO4)2−, Cl−, F−, Na+, K+, Ca2+, and volatile components (e.g., H2O, CO2, N2, CH4, Ar, and C2H6). H–O isotope analyses of quartz suggest that the hydrothermal fluids had a dominantly magmatic signature and were gradually diluted by meteoric waters. Hydrothermal REE transport was probably controlled by F−, (SO4)2−, and Cl− as complexing ligands. We propose that fluid cooling and mixing rather than immiscibility led to the precipitation of bastnasite during the waning stage of hydrothermal activity. Taken together, the inclusion data and observations of alteration, paragenesis and mineralization have provided insights into the development of REE mineralization and the further exploration of carbonatite-related REE resources.

Recent exploration revealed a number of rare earth element (REE) deposits that are distributed along the Qinling Orogenic Belt, Central China. These deposits have an estimated total reserve of about 2 Mt. REE2O3, thus making this belt a world-class REE metallogenic province. Understanding the metallogenesis of the belt requires direct dating of REE minerals. In this study, LA-ICP-MS U-Th-Pb dating on bastnasite and monazite from the Huangshuian, Taipingzhen, and Miaoya deposits in different units of the belt was used to precisely determine the timing of the REE mineralization. The Huangshuian deposit in the north is a carbonatite-related Mo-(REE) deposit in which the REE minerals are closely associated with molybdenite. After correction for common Pb and excessed 206Pb decayed from 230Th, a weighted average 206Pb/238U age of 207 ± 4 Ma (n = 17; MSWD = 1.9) is obtained for bastnasite grains from this deposit. Such an age is slightly younger than that of the final collision (peak at 230–220 Ma) of the Qinling Orogenic Belt, thus indicating that the Mo-REE mineralization is likely related to a post-collisional extension setting. The Taipingzhen REE deposit in the middle contains sheet-like ore bodies composed of veins where bastnasite, the dominant REE mineral, is closely associated with quartz, fluorite, and barite. In situ bastnasite U-Th-Pb dating shows that the REE mineralization in this deposit has formed at 421 ± 7 Ma (n = 17; MSWD = 1.5), synchronous with extension-related magmatism in the region. The Miaoya deposit in the south is the largest one in the belt, and it is essentially a REE-mineralized syenite-carbonatite complex. In this deposit, the monazite grains are closely associated with major minerals of the syenites or carbonatites (e.g., K-feldspar or calcite), but commonly exhibit complex internal textures. Different domains of monazite yield two groups of U-Pb ages at 414 ± 11 Ma (n = 5; MSWD = 0.91) and 231 ± 2 Ma (n = 21; MSWD = 3.1), whereas the bastnasite has an age of 206 ± 4 Ma (n = 14; MSWD = 1.5). The early age of 414 Ma is obtained from homogenous monazite grains and in good agreement with zircon U-Pb ages of the Miaoya syenite-carbonatite complex, and thus is considered to represent the timing of the major REE mineralization in the deposit. The younger ages of 231–206 Ma are obtained from monazite grains with complex internal textures and bastnasite in late veinlets, thus recording secondary, consecutive REE remobilization events likely related to the compression process during formation of the Qinling Orogenic Belt. Our new U-Th-Pb ages, in combination with previously geochronological data, demonstrate that there are two episodes of REE mineralization at 440–410 Ma and 220–200 Ma in the Qinling Orogenic Belt.

Florencite, [REEAl3(PO4)2(OH)6], is the most abundant REE-phosphate mineral in the giant Olympic Dam Cu-U-Au-Ag deposit, South Australia. Florencite typically occurs as fine-grained crystals and occasional aggregates in the matrix of the granite-dominant breccia that hosts the majority of the copper mineralization. Olympic Dam florencite, with the compositional range and extended formula (Ca0.01–0.24Sr0.03–0.40La0.14–0.49Ce0.20–0.47Pr0.00–0.03Nd0.00–0.05)Σ0.43–0.96Al2.89–3.33(P1.42–1.96S0.05–0.34As0.0–0.20)Σ1.77–2.21O4(OH)6], is LREE-enriched, typically La-dominant, while HREEs are minor. There is also compositional variability with respect to Sr, Ca, SO4, and AsO4 components. Chondrite-normalized fractionation trends are steeply downwards-sloping with a relatively low and flat HREE segment. Such a fractionation trend is markedly different from that seen for REE-fluorocarbonates, the dominant REE host at Olympic Dam, which contain relatively higher MREE and HREE components. Xenotime is relatively rare at Olympic Dam and is most commonly seen as overgrowths on zircon. Compositional data for xenotime show HREE concentrations in which Y > Yb > Er > Dy > Ho. Thorium, U, and Pb are minor components in both xenotime and florencite. Monazite is a minor phase and co-existing monazite and florencite are never observed. Texturally, florencite appears to belong to a later stage of LREE mineralization at Olympic Dam, following and less prominent than the main REE-fluorocarbonate stage. Olympic Dam florencite is more La-rich and Ca-poor than it is in most of the localities worldwide for which compositional data have been published. The REE mineralization trends of florencite are compared with published REE trends for other mineral groups from Olympic Dam. Uraninite, brannerite, coffinite, and apatite show patterns of progressive LREE depletion from generation to generation, suggesting enhanced solubility of LREE over geological time. All LREE minerals (fluorocarbonates and phosphates), on the other hand, display progressive LREE enrichment correlating with a paragenetic sequence from REE-fluorocarbonates with essential Ca, through bastnasite, to florencite. The late-stage florencite reported here is the most La-rich of all REE-bearing phases and is interpreted to record either the end of a continuous fluid evolution, or a later separate event at changed physicochemical conditions under which REE-aluminum phosphate minerals are stable.

The carbonatite dike swarm and vein stockwork at the center of the Paleogene Bear Lodge alkaline complex (BLAC), Wyoming, USA, is host to diverse REE mineral assemblages that are largely a result of subsolidus modification and REE redistribution. Pseudomorphic replacement of primary burbankite by an assemblage of ancylite, strontianite, and barite is the result of interaction with late-stage hydrothermal fluids that added Sr, Ba, S, F, and REE, analogous to the replacement processes described for some carbonatite complexes of Russia's Kola Peninsula. Carbon and oxygen stable isotope ratios indicate that the primary carbonatite mineralogy experienced degassing/pneumatolysis and alteration by fluids of variable temperature, CO2/H2O ratios, and/or meteoric water content. Isotopic differences of matrix calcite between Group 1 carbonatites (avg. δ13C = −7.3‰; δ18O = 9.1‰) and Group 2 carbonatites (avg. δ13C = −9.9‰; δ18O = 10.2‰) are consistent with loss of CO2 during degassing. The open-system alteration of burbankite caused a pronounced positive δ18O-shift in bulk ancylite pseudomorphs (δ18O: 14.3–25.7‰) relative to matrix calcite (δ18O: 8.7–11.2‰). Oxygen isotope compositions of biotite (δ18O: 4.5–5.9‰) and K-feldspar (δ18O: 7.3–7.9‰) in unoxidized carbonatite are typical of primary magmatic silicates and suggest that fluids responsible for the burbankite-to-ancylite conversion remained predominantly magmatic (carbohydrothermal). Concomitant increases toward the surface in 13C and 18O, oxidation, matrix carbonate dissolution, and the replacement of REE carbonates (ancylite, carbocernaite, and burbankite) by Ca-REE fluorocarbonates (bastnasite, parisite, synchysite) suggest interaction with late-stage, low temperature (

The Ilmeno–Vishnevogorsk (IVC), Buldym, and Chetlassky carbonatite complexes are localized in the folded regions of the Urals and Timan. These complexes differ in geochemical signatures and ore specialization: Nb-deposits of pyrochlore carbonatites are associated with the IVC, while Nb–REE-deposits with the Buldym complex and REE-deposits of bastnäsite carbonatites with the Chetlassky complex. A comparative study of these carbonatite complexes has been conducted in order to establish the reasons for their ore specialization and their sources. The IVC is characterized by low 87Sr/86Sri (0.70336–0.70399) and εNd (+2 to +6), suggesting a single moderately depleted mantle source for rocks and pyrochlore mineralization. The Buldym complex has a higher 87Sr/86Sri (0.70440–0.70513) with negative εNd (−0.2 to −3), which corresponds to enriched mantle source EMI-type. The REE carbonatites of the Chetlassky complex show low 87Sr/86Sri (0.70336–0.70369) and a high εNd (+5–+6), which is close to the DM mantle source with ~5% marine sedimentary component. Based on Sr–Nd isotope signatures, major, and trace element data, we assume that the different ore specialization of Urals and Timan carbonatites may be caused not only by crustal evolution of alkaline-carbonatite magmas, but also by the heterogeneity of their mantle sources associated with different degrees of enrichment in recycled components.

Stability relations of the REE-bearing accessory phases and alteration processes in the cancrinite-bearing nepheline syenite from the Čistá pluton (the center of the upper-crustal Tepla–Barrandian unit, Bohemian Massif, Czech Republic) were studied. Observations of rock microtextures, quantitative analyses of minerals and compositional X-ray mapping were performed using electron probe microanalysis (EPMA). The primary REE-bearing accessory minerals assemblage includes monazite-(Ce) associated with gadolinite-group minerals (i.e. gadolinite-(Ce) and gadolinite-(Y)), which were partially replaced by britholite-(Ce), bastnäsite-(Ce), aggregates of fine-grained REE-bearing phases (possibly fluorapatite and/or britholite-(Ce)) and, rarely, cerianite. K-feldspar and albite form intergrowths or symplectites with REE-phases in the investigated reaction microtextures. Furthermore, the zircon crystals demonstrate oscillatory zoning and/or extensive patchy zoning due to alteration processes. The alteration of accessory minerals are interpreted as driven by K- and Na-bearing alkali fluids with high CO2 activity during late- to post-magmatic processes.Acknowledgements: This work was supported by the National Science Centre research grant no. 2017/27/B/ST10/00813.

The Vuoriyarvi Devonian alkaline&ndash, ultramafic complex (northwest Russia) contains magnesiocarbonatites with rare earth mineralization localized in the Petyayan-Vara area. High concentrations of rare earth elements are found in two types of these rocks: (a) ancylite-dominant magnesiocarbonatites with ancylite&ndash, baryte&ndash, strontianite&ndash, calcite&ndash, quartz (&plusmn, late Ca&ndash, Fe&ndash, Mg carbonates) ore assemblage, i.e., &ldquo, ancylite ores&rdquo, (b) breccias of magnesiocarbonatites with a quartz&ndash, bastn&auml, site matrix (&plusmn, Mg carbonates), i.e., &ldquo, site ores.&rdquo, We studied fluid inclusions in quartz and late-stage Ca&ndash, Mg carbonates from these ore assemblages. Fluid inclusion data show that ore-related mineralization was formed in several stages. We propose the following TX evolution scheme for ore-related processes: (1) the formation of ancylite ores began under the influence of highly concentrated (&gt, 50 wt.%) sulphate fluids (with thenardite and anhydrite predominant in the daughter phases of inclusions) at a temperature above300&ndash, 350 &deg, C, (2) the completion of the formation of ancylite ores and their auto-metasomatic alteration occurred under the influence of concentrated (40&ndash, 45 wt.%) carbonate fluids (shortite and synchysite&ndash, Ce in fluid inclusions) at a temperature above 250&ndash, 275 &deg, (3) bastn&auml, site ores deposited from low-concentrated (20&ndash, 30 wt.%) hydrocarbonate&ndash, chloride fluids (halite, nahcolite, and/or gaylussite in fluid inclusions) at a temperature of 190&ndash, 250 &deg, C or higher. Later hydrothermal mineralization was related to the low-concentration hydrocarbonate&ndash, chloride fluids (&lt, 15 wt.% NaCl-equ.) at 150&ndash, 200 &deg, C. The presented data show the specific features of the mineral and fluid evolution of ore-related late-stage hydrothermal rare earth element (REE) mineralization of the Vuoriyarvi alkaline&ndash, ultramafic complex.

Synchysite-(Ce) at Cuasso al Monte occurs as fine needles intergrown with quartz in quartz-dikes and in association with altered K-feldspar and oxidized chalcopyrite as major constituents. Synchysite-(Ce) [Ca1.00(Ce0.43La0.26Nd0.17Y0.07Pr0.04Sm0.02Gd0.01)=1.00(CO3)2(F0.58(OH)0.42)], shows an overgrowth rim of bastn&auml, site-(Ce) [(Ce0.34La0.25Nd0.17Pb0.07C a0.06Y0.06Pr0.04S m0.02Gd0.01)=1.00C O3(F0.75(OH)0.25)]. Unit cell refinement of synchysite (C2/c) and bastn&auml, site (P&macr, 62c) led to a = 12.272(4), b = 7.100(2), c = 18.640(5) &Aring, &beta, = 102.71(5)&deg, and a = 7.085(1), c = 9.746(2) &Aring, respectively. Polysomatic faults are sporadic, but polytypic disorder is widespread. High resolution transmission electron microscopy images taken along [100] or 130 show an apparent order and the related diffraction patterns are streak-free. Conversely, along [010] or 110, a high density of stacking faults is observed and the related diffraction patterns show hhl rows with h 3n affected by streaks. No ordered domain larger than a few unit cells was detected. The stacking sequence of (Ca,REE)-fluorcarbonates can be compared with subfamily-B mica polytypes (2M2, 2O and 6H), which are characterized by n60&deg, (n = odd) rotations. Subfamily-A polytypes (1M, 2M1 and 3T), characterized by n60&deg, (n = even) rotations, should not be possible. Synchysite, characterized by 60&deg, rotations, can be likened to the 2M2 polytype.

The Vuoriyarvi Devonian carbonatite–ijolite–pyroxenite–olivinite complex comprises several carbonatite fields: Neske Vara, Tukhta-Vara, and Petyayan-Vara. The most common carbonatites in the Tukhta-Vara and Neske-Vara fields are calciocarbonatites, which host several P, Fe, Nb, and Ta deposits. This paper focuses on the Petyayan-Vara field, in which the primary magmatic carbonatites are magnesian. The least altered magnesiocarbonatites are composed of dolomite with burbankite and are rich in REE (up to 2.0 wt. %), Sr (up to 1.2 wt. %), and Ba (up to 0.8 wt. %). These carbonatites underwent several stages of metasomatism. Each metasomatic event produced a new rock type with specific mineralization. The introduction of K, Si, Al, Fe, Ti, and Nb by a F-rich fluid (or fluid-saturated melt) resulted in the formation of high-Ti magnesiocarbonatites and silicocarbonatites, composed of dolomite, microcline, Ti-rich phlogopite, and Fe–Ti oxides. Alteration by a phosphate–fluoride fluid caused the crystallization of apatite in the carbonatites. A sulfate-rich Ba–Sr–rare-earth elements (REE) fluid (probably brine-melt) promoted the massive precipitation of ancylite and baryte and, to a lesser extent, strontianite, bastnäsite, and synchysite. Varieties of carbonatite that contain the highest concentrations of REE are ancylite-dominant. The influence of sulfate-rich Ba-Sr-REE fluid on the apatite-bearing rocks resulted in the dissolution and reprecipitation of apatite in situ. The newly formed apatite generation is rich in HREE, Sr, and S. During late-stage transformations, breccias of magnesiocarbonatites with quartz-bastnäsite matrixes were formed. Simultaneously, strontianite, quartz, calcite, monazite, HREE-rich thorite, and Fe-hydroxides were deposited. Breccias with quartz-bastnäsite matrix are poorer in REE (up to 4.5 wt. % total REE) than the ancylite-dominant rocks (up to 11 wt. % total REE).

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.

The Gallinas Mountains, located at the junction of Lincoln and Torrance Counties, New Mexico, USA, are a series of alkaline volcanic rocks intruded into Permian sedimentary rocks. The Gallinas Mountains area hosts fluorite and copper as veins containing bastnäsite, whereas deposits of iron skarns and iron replacement are in the area as well. These deposits produce iron. In this study, the multispectral band-ratio method is used for surface mineral recognition, whereas 2D subsurface structure inversion modeling was applied to explore the depth extent of the magnetic ore distribution from aeromagnetic data. Bastnäsite has higher magnetic susceptibility (0.009 SI) than the host rocks and surrounding sedimentary rock. The bastnäsite and iron oxides (magnetite + hematite) can contribute to a positive aeromagnetic anomaly. Results indicate that (1) the positive magnetic anomaly observed at Gallinas Mountains area can be accounted for by a mixture of bastnäsite and iron oxides at a depth of approximately 400 m and a thickness of approximately 13–15 m. The surface of this area is dominated by the hydrothermal alteration associated with iron oxides over the trachyte intrusions as detected by Landsat 8 band-ratio imaging.

Carbonate gangue in historically important post-Variscan polymetallic Pb-Zn-Cu-Ag vein systems in the Upper Harz Mountains has elevated rare earth element contents (ΣREE+Y, 1500 ± 340 ppm; n = 16) in bulk samples. This enrichment is due to the occurrence of abundant micrometer-sized synchysite inclusions in calcite, identified via Raman spectroscopy. Calcite bulk analyses have roof-shaped PAAS-normalized REE patterns with a Eu peak but without Ce or Y anomalies and with a steeply dipping HREE pattern. Microbeam analysis (electron microprobe and LA-ICPMS) identifies these patterns as dominated by synchysite-(Ce–Nd) with strong Eu enrichment in the weight percent range and LREE/HREE fractionation (La/Lu ~ 100, PAAS-normalized). The synchysite component in calcite gangue is detected in polymetallic veins all over the Harz Mountains, which seems to be a diagnostic feature of the region and suggests a large-scale Mesozoic fluid system. However, the Upper (western) Harz systems with no fluorite have more elevated REE content, compared with the Lower (eastern) Harz fluorite-rich systems with less REE content. Carbonate gangue in the Upper Harz systems has homogeneous 87Sr/86Sri around 0.714 and eNdi of ~ − 9, while carbonate gangue in the Lower Harz systems has 87Sr/86Sri around 0.716 and eNdi of − 2 to − 5. Fluorite in the Lower Harz Mountains has strong positive Y anomalies and variable Eu anomalies at generally low REE abundances and no REE-mineral inclusions. Both Sr and Nd isotope compositions in the fluorite are very variable and indicate an open system (87Sr/86Sri, 0.710–0.718; eNdi, − 3 to − 17). The synchysite MREE enrichment in calcite gangue in the Upper Harz Mountains compares favorably with carbonatite-related LREE-dominated bastnasite from China and elsewhere and allows an interesting perspective as a by-product of Pb–Zn mining.

Rare-earth deposits associated with intrusive carbonatite complexes are the world’s most important source of these elements (REE). One of the largest deposits of this type is Maoniuping in the Mianning-Dechang metallogenic belt of eastern Tibet (Sichuan, China). In the currently mined central part of the deposit (Dagudao section), REE mineralization is hosted by a structurally and mineralogically complex Late Oligocene (26.4 ± 1.2 Ma, 40Ar/39Ar age of fluorphlogopite associated with bastnasite) hydrothermal vein system developed in a coeval syenite intrusion. Low-grade stockworks of multiple veinlets and breccias in the lower part of the orebody grade upwards into progressively thicker veins (up to 12 m in width) that are typically zoned and comprise ferromagnesian micas (biotite to fluorphlogopite), sodium clinopyroxenes (aegirine to aegirine-augite), sodium amphiboles (magnesio-arfvedsonite to fluororichterite), K-feldspar, fluorite, barite, calcite, and bastnasite. The latter four minerals are most common in the uppermost 80 m of the Dagudao section and represent the climax of hydrothermal activity. Systematic variations in the fluid inclusion data indicate a continuous hydrothermal evolution from about 230–400 °C (fluid inclusions in feldspar, clinopyroxene, and amphibole) to 140–240 °C (fluid inclusions in bastnasite, fluorite, calcite). Hydrothermal REE transport was probably controlled by F−, (SO4)2−, Cl−, and (CO3)2− as complexing ligands. We propose that at Dagudao, silicate magmas produced orthomagmatic fluids that explored and expanded a fissure system generated by strike-slip faulting. Initially, the fluids had appreciable capacity to transport REE and, consequently, no major mineralization developed. The earliest minerals to precipitate were alkali- and Fe-rich silicates containing low levels of F, which caused progressive enrichment of the fluid in Ca, Mg, F, Cl, REE, (SO4)2−, and (CO3)2−, leading to the crystallization of aegirine-augite, fluororichterite, fluorphlogopite, fluorite, barite, calcite, and bastnasite gradually. Barite, fluorite, calcite, and bastnasite are the most common minerals in typical ores, and bastnasite generally postdates these gangue minerals. Thus, it is very probable that fluid cooling and formation of large amount of fluorite, barite, and calcite triggered bastnasite precipitation in the waning stage of hydrothermal activity.

The silico-carbonatite dykes of the Huanglongpu area, Lesser Qinling, China, are unusual in that they are quartzbearing, Mo-mineralised and enriched in the heavy rare earth elements (HREE) relative to typical carbonatites. The textures of REE minerals indicate crystallisation of monazite-(Ce), bastnasite-(Ce), parisite-(Ce) and aeschynite-(Ce) as magmatic phases. Burbankite was also potentially an early crystallising phase. Monazite-(Ce) was subsequently altered to produce a second generation of apatite, which was in turn replaced and overgrown by britholite-(Ce), accompanied by the formation of allanite-(Ce). Bastnasite and parisite where replaced by synchysite-(Ce) and rontgenite-(Ce). Aeschynite-(Ce) was altered to uranopyrochlore and then pyrochlore with uraninite inclusions. The mineralogical evolution re carbonatite, to more silica-rich conditions during early hydrothermal processes, to fully hydrothermal conditions accompanied by the formation of sulphateminerals. Each alteration stage resulted in the preferential leaching of the LREE and enrichment in the HREE.Mass balance considerations indicate hydrothermal HREE to themineralisation. The evolution of the in may have been the HREE source. Leaching in the presence of strong, LREE-selective ligands (Cl for the depletion in late stage minerals in the LREE, but cannot account for subsequent preferentialHREE addition. Fluid inclusion data indicate the presence of sulphate-rich brines during alteration, and hence sulphate complexationmay have been important for preferential HREE transport. AlongsideHREE-enrichedmagmatic sources, and enrichment during magmatic processes, late stage alteration with non-LREE-selective ligands may be critical in forming HREE-enriched carbonatites.flects the evolution from magmaticfluids must have contributedfluorcarbonate mineral assemblage requires an increaseaCa2+ and aCO3 2− in the metasomatic fluid (where a is activity), and breakdown of HREE-enriched calcite−) may account

Rare metal-bearing, alkaline granites have attracted extensive interests because of their economic significance. The Early Permian (∼290 Ma) Boziguo’er alkaline granitic pluton, exposed in the middle segment of the South Tianshan Terrane, Xinjiang, NW China, contain Nb-Ta-, Zr-, Th-U- and REE-bearing accessory minerals, and host a large-scale rare metal deposit. The unalterated alkaline granites are mainly composed of alkali feldspars (∼40 vol% orthoclase/microcline and ∼30 vol% albite), quartz (∼20 vol%), arfvedsonite (∼5 vol%) and biotite (∼5 vol%), with accessory minerals at least including zircon, monazite, pyrochlore, Fe-Ti oxides, xenotime, bastnasite and astrophyllite. The existence of two types of alkaline feldspars, which respectively show nearly pure Ab and Or end member compositions, suggest that the Boziguo’er pluton can be classified as “subsolvus granites”. Rare metal elements in the pluton are mainly contained by pyrochlore (Nb-Ta), zircon (Zr-Hf, Th-U and HREE) and monazite (LREEs) grains. Mineral chemistry suggests that the pyrochlore and monazite are of primary igneous mineral. Based on morphology, internal texture and mineral chemistry, four types of zircons have been identified. Type-I zircon may have crystallized from volatile-undersaturated early-stage magmas, while type-IIA zircon crystallized from volatile (fluorine)-saturated magmas. The type-IIB and type-III zircons could have grown in an LREE-depleted silicate melt after the crystallization of the LREE-rich minerals (e.g., monazite) or an early aqueous fluid during the transition from a magmatic to hydrothermal system. The type-IV zircons can be regarded as low-temperature hydrothermal ones. A semiquantitative calculation, based on mineral compositions of biotites, yields 0.51–2.14 wt% for F contents in granitic melts. The high contents of fluorine in the hydrous minerals suggest a fluorine-enriched characteristic for melt, which would not only prolong the duration of the magmatic differentiation but can also enhance the solubility of ore-forming elements in magma. The magmatic differentiation was mainly dominated by fractional crystallization of alkali feldspar and amphibole. Extensive rare metal mineralization in the pluton likely occurred within a limited temperature interval and was related to the crystallization of a highly evolved intergranular melt at the latest magmatic stage.

Rare earth elements (REEs) are a focus of current research due to their importance in a wide range of industries and technologies. Bayan Obo in China is the largest REE deposit in the world. Although some studies have investigated the banded REE ores in the Bayan Obo deposit, the origin and formation processes of the ores are unclear. In this study, we used X-ray powder diffraction, electron microprobe, laser ablation–inductively coupled plasma–mass spectrometry, and mass spectrometry methods to investigate the mineral assemblages and geochemical characteristics of minerals in banded ores in the Bayan Obo REE deposit. Based on petrographic observations, the early minerals that precipitated were alkali- and Fe-rich silicates, such as aegirine–augite and arfvedsonite, followed by bastnasite, fluorite, barite, calcite, and monazite in the late stage. The REE minerals in the banded REE ores are mainly monazite, bastnasite, and parisite (1–10 vol%) that overprinted gangue minerals, suggesting that REE mineralization occurred during the late hydrothermal stage. The sequence of mineral formation and their evolution caused progressive enrichment in REEs, F, Sr, and Ba. For example, fluorite in the banded REE ores has high concentrations of Sr (252–1910 ppm), Ba (1040–8230 ppm), and light REEs (1001–16,079 ppm), but is depleted in Nb and Ta. The bastnasite is enriched in Sr (885–4046 ppm), Ba (1073–80,809 ppm), and ΣREEs (99,631–158,227 ppm). The fluorite, arfvedsonite, and bastnasite in the banded REE ores have ɛNd(t) values of −3.68–1.78, −5.54, and 0.05, and initial (87Sr/86Sr)i (ISr) ratios of 0.70352–0.70478, 0.70620, and 0.70346, respectively. Based on these data, and the regional geological setting and Pb isotope data, it is proposed that fluorite and bastnasite crystallized from ferrocarbonatite and fine-grained dolomite, and the REE source was a mixture of HIMU mantle and global marine sediments. We also suggest that the carbonatite- or dolomite-hosting ores in Bayan Obo formed by melting of the subcontinental lithospheric mantle, which might have been previously metasomatized by REE- and CO2-rich fluids derived from subducted marine sediments.

Rare-earth-element (REE) bearing minerals have been recognized for the first time within rare-element granitic pegmatite pockets from Lagoa Real Uranium Province (LRUP), Bahia state, NE Brazil. The Province is one of the chief U deposits in the world and the largest in South America. Notwithstanding its economic and strategic importance, there are gaps in the geological knowledge about other mineral resources which could occur in the area. One of them, is the lack of petrographic and chemical data sets on its REE content, especially in pegmatite pockets which occur in the contact between Juazeirinho (1755 ± 6 Ma) and Sao Timoteo (1741 ± 4 Ma) granites. In this paper, it is reported a petrographic description and geochemical characterization of allanite, bastnasite, monazite, xenotime, cheralite, and parisite recognized in three spongy mineral assemblages within unzoned granitic pegmatite pockets from Lagoa Real Uranium Province. The REE-bearing minerals present moderate-to-high REE content with significant amounts of Ce, La, Y, and critical REE such as Nd, Er, and Yb, totalling to up to 49 wt%. Petrographical and geochemical evidence arguably indicate that the REE-enrichment in granitic pegmatite pockets of Lagoa Real Uranium Province is a result of late-magmatic and hydrothermal events involving F−, OH−, CO32− and PO43− REE-bearing fluids. The report and characterization of these recently recognized minerals indicate that the Lagoa Real Uranium Province, might hold important REE mineralization as U by-product.

Carbonatite-related rare earth element (REE) deposits are the most significant source of REEs worldwide. The processes of REE precipitation, enrichment, and mineralization remain controversial. The Cenozoic Mianning–Dechang (MD) REE belt, located in Sichuan Province, southwestern China, comprises one giant (Maoniuping), one large (Dalucao), and two small–medium (Muluozhai and Lizhuang) deposits. These deposits provide a continuous record of fluid evolution, and thus are ideal for investigating the processes of REE mineralization in carbonatite-related REE deposits. Given that sulfate (i.e., barite and celestite) and sulfide (i.e., pyrite and galena) minerals crystallized and precipitated in the pegmatitic to hydrothermal stages, respectively, the REE minerals formed later than the sulfate minerals. However, the formation sequence of the sulfide minerals and bastnasite is unclear, although both pyrite and bastnasite formed in the late hydrothermal stage. We used S isotope data for sulfate and sulfide minerals and Fe isotope data for pyrite to investigate the composition and evolution of ore-forming fluids during the magmatic–hydrothermal stages. The sulfate minerals have positive δ34SCDT values (+3.2‰ to +8.3‰), and the sulfide minerals have negative δ34SCDT values (−13.5‰ to − 5.6‰) in the four REE deposits. In the Maoniuping deposit, δ34SCDT values for barite from the pegmatitic stage (+4.7‰ to +5.7‰) are higher than for barite from the hydrothermal stage (+4.1‰ to +4.5‰), which indicate that hydrothermal activity led to relative enrichment in isotopically light S. The δ34SCDT values for barite (+3.2‰ to +5.5‰) are lower than for celestite (+6.2‰ to +7.2‰) from the pegmatitic stage in the Dalucao deposit. The δ34SCDT values for galena (−13.5‰) are also lower than for pyrite (−13.5‰ to −7.2‰) from the hydrothermal stage in the Guangtoushan section. In general, δ34SCDT values change from positive to negative values (+8.3‰ to −16.4‰) as the fluids evolved from the pegmatitic to hydrothermal stages, which can be attributed to a decrease in oxygen fugacity (fO2) and addition of sediment containing isotopically light S. Iron isotopic compositions of pyrite from the hydrothermal stage show significant variations (δ56FeIRMM-014 = −0.03‰ to +0.65‰ for the Maoniuping deposit; −0.14‰ to 0.00‰ for the Dalucao deposit; +0.05‰ to +0.35‰ for the Lizhuang deposit), and are higher than those for the carbonatites (δ56Fe IRMM-014 = −0.47‰ to −0.17‰). These data indicate there are two sources of Fe in the MD REE belt, which are the carbonatite–nordmarkite magma and 56Fe-rich sediment. Paleozoic–Mesozoic volcanic–sedimentary and Mesozoic clastic and carbonate rocks are exposed in the MD REE belt. In general, the S–Fe isotope data, along with geological and petrographic observations, indicate that the REE minerals formed later than the sulfate minerals, and the S–Fe were derived from both carbonatite magma and sediment containing isotopically light S and heavy Fe.

Hydrothermal rare earth element (REE) precipitation in carbonatite is generally attributed to increase in pH or decrease in temperature when REE-bearing acidic fluids interact with carbonates. Here, we document microtextures comprising intergrowth of bastnasite, hydroxyl-parisite, rontgenite, synchysite, and pyrochlore, with calcite that pseudomorphically replaces patches of hematite with cellular boxwork-type structure, in calciocarbonatites from the Kamthai alkaline complex in western India. The nature of the REE mineralization grades from proximal bastnasite-dominated to distal hydroxyl-parisite dominated in the boxworks. The microtextural relations and trace element chemistry of hematite, magnetite and calcite, and C-O isotope composition of carbonate are suggestive of extensive low-temperature hydrothermal alteration of the carbonatites. The hematite boxwork structure and the REE mineral-calcite intergrowths often have squarish outlines and are interpreted to have pseudomorphed primary magnetite during fluid-rock interaction. We propose a new mechanism of REE precipitation in magnetite-rich carbonatites involving influx of acidic hydrothermal fluids, which scavenged the REE and other trace elements from magmatic carbonates and apatite. These acidic fluids were responsible for the protonation of magnetite and leaching out of Fe2+, converting them to hematite through a non-redox transformation. The reaction results in 32% volume reduction for every mole of magnetite consumed, generating significant rock porosity, which further aided and abetted fluid-rock reaction and hydrothermal alteration. More importantly, it consumed proton, which increased the fluid pH triggering precipitation of bastnӓsite-group minerals (including bastnӓsite, parasite, rontgenite, synchysite). Close to the magnetite-hematite reaction front, fluor-dominated bastnӓsite-group minerals (mainly bastnӓsite) appear, while away from such fronts, mineralization was enriched in hydroxyl-bastnӓsite group minerals (mainly parisite) as a consequence of decreasing activity of F‐ and/or increasing activity of OH‐ with the progress of magnetite-to-hematite transformation.

Growing demand for the rare earth elements, particularly the heavy REE (HREE), has fuelled a boom in mineral exploration and scientific research on carbonatite-hosted deposits, especially those with unusual HREE enrichment. The Ashram REE deposit is such a HREE-enriched carbonatite-hosted REE deposit. Magmatic processes were important in the genesis of the deposit, but hydrothermal mobilization was the main process responsible for the concentration of the REE to potentially economic levels and the fractionation of the HREE. The REE minerals in the Ashram deposit were precipitated from hydrothermal fluids. They comprise monazite-(Ce) and bastnasite-(Ce), with lesser monazite-(Nd) and trace xenotime-(Y) and aeschynite-(Nd). This mineralization occurs as disseminations in breccia matrices, in veins, and as vug fillings. Monazite-(Ce) was the earliest mineral to form, followed by xenotime-(Y) and bastnasite-(Ce). The composition of monazite-(Ce) varies with location in the deposit and is preferentially enriched in the light REE (LREE) with increasing distance from a large, irregular breccia body, underlying the zone of HREE enrichment. This body is interpreted to have been the main conduit for the mineralizing fluids which, on exiting, deposited monazite as a result of the cooling and pH buffering that accompanied their interaction with the adjacent carbonatites. Bastnasite-(Ce) replaced monazite-(Ce) through ligand exchange (F−, CO32− for PO43−), preserving the original REE distribution. Interaction of a compositionally evolving fluid with host rocks of variable bulk composition and buffering capacity resulted in a deposit-scale fractionation and zonation of the REE.

Carbonatites and associated alkaline rocks are the primary sources for REE mineralization. The Ambadongar Carbonatite Complex (ADCC) from NW Deccan Volcanic Province (DVP) constitutes the largest Carbonatite Associated REE Deposits (CARD) in India. ADCC belongs to the final stages of the Late Cretaceous alkaline-carbonatite magmatism associated with main Deccan basalt volcanic activity. The ADCC is an envisioned diatreme structure in which four carbonatitic phases are recognized, mainly calcio-carbonatites and ferro-carbonatites. Each successive carbonatite phase shows higher REE enrichment. The primary REE mineralization with bastnasite as the dominant REE phase is hosted by pervasive hydrothermally altered ferro-carbonatite plugs. The secondary mineralogy formed with barites in the main orebody during late- to post-magmatic hydrothermal fluid alteration is fluorite, quartz, ankerite, and other REE-bearing minerals like bastnasite, parisite, synchysite, strontianite, florencite, monazite and columbite. Carbonatite samples contain 18.61% to 52.42% of CaO, and the LOI varies from 5.28% to 38.79%. Most can be classified as calcio-carbonatites. Since all the samples also contain an appreciable amount of Fe2O3 (4.13% to 20.20%) and MnO (0.07% to 5.46%), some may be classified as ferro-carbonatites. Total REE content varies from 0.6 to 4%, with a high Ce concentration and LREE/HREE ratio. The highest values for La, Ce, Pr, and Nd are 1.95%, 1.56%, 0.16%, and 0.45%, respectively. Metasomatism of SCLM from asthenospheric melts followed by the low degree partial melting of the SCLM region is responsible for fertile carbonatite generation in ADCC. The multiphase liquid immiscibility of carbonatite melts from carbonate-silicate magma followed by immiscibility of REE rich carbonatite melt and REE deficient fluoride-rich aqueous fluids explain the higher level of REE enrichment in each successive phases of carbonatites in ADCC. The mineralizing fluids were probably the result of residual magmatic volatiles that brought mainly REE and later SiO2 into the overprinted rocks. Ambadongar carbonatites' stable isotopic compositions agree with a magmatic origin (δ13C = −4.1 ± 1.9‰ [PDB] and δl8O = 10.3 ± 1.7‰ [SMOW]). The C–O stable isotopic modeling indicates re-equilibration under hydrothermal conditions between 180 °C and 70 °C. Significant amounts of REE fluorocarbonate minerals, relatively Sr- and Th-rich, were deposited during re-equilibration. The REE fluorocarbonate bastnasite-(Ce) occurs as late individual crystals, overgrown on the synchysite and parisite polycrystals. Textural and chemical reactions between the REE fluorocarbonates provide insights into rare-earth elements' mobility during fluid-rock interaction. Early crystallization of synchysite/parisite indicates the high activity of Ca2+, OH−, (SO4)2−, Al and Si in the fluid. Later, the fluid was characterized by increased activity of F−, (SO4)2−, REE and Si, and decreased activity of Ca2+ as reflected in the association of barite, fluorite, quartz, and bastnasite typical of strongly overprinted ferro-carbonatites. Re-equilibration and recrystallization of the primary minerals in the presence of OH−, (SO4)2−, F−, REE, Al, and Si carried in solution by the hydrothermal fluid is the leading cause behind the refixing of REE in the form of REE fluorocarbonate in REE rich ferro-carbonatites.

Carbonatites are enriched in critical raw materials such as the rare-earth elements (REE), niobium, fluorspar and phosphate. A better understanding of their fluid regimes will improve our knowledge of how to target and exploit economic deposits. This study shows that multiple fluid phases penetrated the surrounding fenite aureole during carbonatite emplacement at Chilwa Island, Malawi. The first alkaline fluids formed the main fenite assemblage and later microscopic vein networks contain the minerals of potential economic interest such as pyrochlore in high-grade fenite and rare-earth minerals throughout the aureole. Seventeen samples of fenite rock from the metasomatic aureole around the Chilwa Island carbonatite complex were chosen for study. In addition to the main fenite assemblage of feldspar and aegirine ± arfvedsonite, riebeckite and richterite, the fenite contains micro-mineral assemblages including apatite, ilmenite, rutile, magnetite, zircon, rare-earth minerals and pyrochlore in vein networks. Petrography using a scanning electron microscope in energy-dispersive spectroscopy mode showed that the rare-earth minerals (monazite, bastnäsite and parisite) formed later than the fenite feldspar, aegirine and apatite and provide evidence of REE mobility into all grades of fenite. Fenite apatite has a distinct negative Eu anomaly (determined by laser ablation inductively coupled plasma mass spectrometry) that is rare in carbonatite-associated rocks and interpreted as related to pre-crystallization of plagioclase and co-crystallization with K-feldspar in the fenite. The fenite minerals have consistently higher mid REE/light REE ratios (La/Sm ≈ 1.3 monazite, ≈ 1.9 bastnäsite, ≈ 1.2 parisite) than their counterparts in the carbonatites (La/Sm ≈ 2.5 monazite, ≈ 4.2 bastnäsite, ≈ 3.4 parisite). Quartz in the low- and medium-grade fenite hosts fluid inclusions, typically a few micrometres in diameter, secondary and extremely heterogeneous. Single phase, 2- and 3-phase, single solid and multi solid-bearing examples are present, with 2-phase the most abundant. Calcite, nahcolite, burbankite and baryte were found in the inclusions. Decrepitation of inclusions occurred at ∼200°C before homogenization but melting-temperature data indicate that the inclusions contain relatively pure CO2. A minimum salinity of ∼24 wt.% NaCl equivalent was determined. Among the trace elements in whole-rock analyses, enrichment in Ba, Mo, Nb, Pb, Sr, Th and Y and depletion in Co, Hf and V are common to carbonatite and fenite but enrichment in carbonatitic type elements (Ba, Nb, Sr, Th, Yand REE) generally increases towards the inner parts of the aureole. A schematic model contains multiple fluid events, related to first and second boiling of the magma, accompanying intrusion of the carbonatites at Chilwa Island, each contributing to the mineralogy and chemistry of the fenite. The presence of distinct rare-earth mineral microassemblages in fenite at some distance from carbonatite could be developed as an exploration indicator of REE enrichment.

The Eocene (ca. 55–38 Ma) Bear Lodge alkaline complex in the northern Black Hills region of northeastern Wyoming (USA) is host to stockwork-style carbonatite dikes and veins with high concentrations of rare earth elements (e.g., La: 4140–21000 ppm, Ce: 9220–35800 ppm, Nd: 4800–13900 ppm). The central carbonatite dike swarm is characterized by zones of variable REE content, with peripheral zones enriched in HREE including yttrium. The principle REE-bearing phases in unoxidized carbonatite are ancylite and carbocernaite, with subordinate monazite, fluorapatite, burbankite, and Ca-REE fluorocarbonates. In oxidized carbonatite, REE are hosted primarily by Ca-REE fluorocarbonates (bastnasite, parisite, synchysite, and mixed varieties), with lesser REE phosphates (rhabdophane and monazite), fluorapatite, and cerianite. REE abundances were substantially upgraded (e.g., La: 54500–66800 ppm, Ce: 11500–92100 ppm, Nd: 4740–31200 ppm) in carbonatite that was altered by oxidizing hydrothermal and supergene processes. Vertical, near surface increases in REE concentrations correlate with replacement of REE(±Sr,Ca,Na,Ba) carbonate minerals by Ca-REE fluorocarbonate minerals, dissolution of matrix calcite, development of Fe- and Mn-rich gossan, crystallization of cerianite and accompanying negative Ce anomalies in secondary fluorocarbonates and phosphates, and increasing δ18O values. These vertical changes demonstrate the importance of oxidizing meteoric water during the most recent modifications to the carbonatite stockwork. Scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis were used to investigate variations in mineral chemistry controlling the lateral complex-wide geochemical heterogeneity. HREE-enrichment in some peripheral zones can be attributed to an increase in the abundance of secondary REE phosphates (rhabdophane group, monazite, and fluorapatite), while HREE-enrichment in other zones is a result of HREE substitution in the otherwise LREE-selective fluorocarbonate minerals. Microprobe analyses show that HREE substitution is most pronounced in Ca-rich fluorocarbonates (parisite, synchysite, and mixed syntaxial varieties). Peripheral, late-stage HREE-enrichment is attributed to: 1) fractionation during early crystallization of LREE selective minerals, such as ancylite, carbocernaite, and Ca-REE fluorocarbonates in the central Bull Hill dike swarm, 2) REE liberated during breakdown of primary calcite and apatite with higher HREE/LREE ratios, and 3) differential transport of REE in fluids with higher PO43−/CO32− and F−/CO32− ratios, leading to phosphate and pseudomorphic fluorocarbonate mineralization. Supergene weathering processes were important at the stratigraphically highest peripheral REE occurrence, which consists of fine, acicular monazite, jarosite, rutile/pseudorutile, barite, and plumbopyrochlore, an assemblage mineralogically similar to carbonatite laterites in tropical regions.

The 2060 Ma old Palabora Carbonatite Complex (PCC), South Africa, comprises diverse REE mineral assemblages formed during different stages and reflects an outstanding instance to understand the evolution of a carbonatite-related REE mineralization from orthomagmatic to late-magmatic stages and their secondary post-magmatic overprint. The 10 rare earth element minerals monazite, REE-F-carbonates (bastnasite, parisite, synchysite), ancylite, britholite, cordylite, fergusonite, REE-Ti-betafite, and anzaite are texturally described and related to the evolutionary stages of the PCC. The identification of the latter five REE minerals during this study represents their first described occurrences in the PCC as well as in a carbonatite complex in South Africa. The variable REE mineral assemblages reflect a multi-stage origin: (1) fergusonite and REE-Ti-betafite occur as inclusions in primary magnetite. Bastnasite is enclosed in primary calcite and dolomite. These three REE minerals are interpreted as orthomagmatic crystallization products. (2) The most common REE minerals are monazite replacing primary apatite, and britholite texturally related to the serpentinization of forsterite or the replacement of forsterite by chondrodite. Textural relationships suggest that these two REE-minerals precipitated from internally derived late-magmatic to hydrothermal fluids. Their presence seems to be locally controlled by favorable chemical conditions (e.g., presence of precursor minerals that contributed the necessary anions and/or cations for their formation). (3) Late-stage (post-magmatic) REE minerals include ancylite and cordylite replacing primary magmatic REE-Sr-carbonates, anzaite associated with the dissolution of ilmenite, and secondary REE-F-carbonates. The formation of these post-magmatic REE minerals depends on the local availability of a fluid, whose composition is at least partly controlled by the dissolution of primary minerals (e.g., REE-fluorocarbonates). This multi-stage REE mineralization reflects the interplay of magmatic differentiation, destabilization of early magmatic minerals during subsequent evolutionary stages of the carbonatitic system, and late-stage fluid-induced remobilization and re-/precipitation of precursor REE minerals. Based on our findings, the Palabora Carbonatite Complex experienced at least two successive stages of intense fluid–rock interaction.

Data are presented on detailed sampling of the surface and pore waters of soda Lake Doroninskoye (Eastern Transbaikalia, Russia). Particular attention has been paid to the possible REE precipitation species from both water types. Thermodynamic modeling at 25 °C and 1 bar total pressure was performed with the “HCh” code using a free-energy minimization algorithm. We modeled the heterophase 43-component system; both individual minerals and ideal solid solutions of the REE and numerous dissolved species were incorporated into the model. According to the calculations, redox conditions in the lake waters are established by the sulfur species. The main result is that the surface water is slightly supersaturated with respect to CaCO 3 -REE (65%) and F-apatite-REE (35%) solid solutions, as well as bastnasite Ce 0,5 La 0,25 Nd 0,2 Pr 0,05 CO 3 F. A trend of the HREE predominance in the solution during the process of early diagenesis has been confirmed. Nevertheless, the thermodynamic model does not entirely explain the REE fractionation in pore waters (pore solutions are enriched in the LREEs in situ but are drastically depleted according to the thermodynamic model). The results of this study suggest that further considerations about solid solutions should be taken into account to model and understand exact REE fractionation during water-rock interaction in salt lakes.

Ion-adsorbed rare earth element (REE) deposits supply the majority of world heavy REE production and substantial light REE production, but relatively little is known of their occurrence outside Southeast Asia. We examined the distribution and forms of REEs on a North American pluton located in the highly weathered and slowly eroding South Carolina Piedmont. The Hercynian Liberty Hill pluton experiences a modern climate that includes ~ 1500 mm annual rainfall and a mean annual temperature of 17 °C. The pluton is medium- to coarse-grained biotite-amphibole granite with minor biotite granite facies. REE-bearing phases are diverse and include monazite, zircon, titanite, allanite, apatite and bastnasite. Weathered profiles were sampled up to 7 m-deep across the ~ 400 km 2 pluton. In one profile, ion-adsorbed REEs plus yttrium (REE + Y) ranged up to 581 mg/kg and accounted for up to 77% of total REE + Y in saprolite. In other profiles, ion-adsorbed REE + Y ranged 12–194 mg/kg and only accounted for 3–37% of totals. The profile most enriched in ion-adsorbed REEs was located along the mapped boundary of two granite facies and contained trioctahedral smectite in the saprolite, evidence suggestive of hydrothermal alteration of biotite at that location. Post-emplacement deuteric alteration can generate easily weathered REE phases, particularly fluorocarbonates. In the case of Liberty Hill, hydrothermal alteration may have converted less soluble to more soluble REE minerals. Additionally, regolith P content was inversely correlated with the fraction ion-adsorbed REEs, and weathering related secondary REE-phosphates were found in some regolith profiles. Both patterns illustrate how low P content aids in the accumulation of ion-adsorbed REEs. The localized occurrence at Liberty Hill sheds light on conditions and processes that generate ion-adsorbed REEs.

The Weishan carbonatite-related rare earth element (REE) deposit in China contains both high- and low-grade REE mineralization and is an informative case study for the investigation of magmatic&ndash, hydrothermal REE enrichment processes in such deposits. The main REE-bearing mineral is bastn&auml, site, with lesser parisite and monazite. REE mineralization occurred at a late stage of hydrothermal evolution and was followed by a sulfide stage. Barite, calcite, and strontianite appear homogeneous in back-scattered electron images and have high REE contents of 103&ndash, 217, 146&ndash, 13,120, and 194&ndash, 16,412 ppm in their mineral lattices, respectively. Two enrichment processes were necessary for the formation of the Weishan deposit: Production of mineralized carbonatite and subsequent enrichment by magmatic&ndash, hydrothermal processes. The geological setting and petrographic characteristics of the Weishan deposit indicate that two main factors facilitated REE enrichment: (1) fractures that facilitated circulation of ore-forming fluids and provided space for REE precipitation and (2) high ore fluorite and barite contents resulting in high F&minus, and SO42&minus, concentrations in the ore-forming fluids that promoted REE transport and deposition.

Fenitization is an alkali metasomatism that widely occurs in and around the alkaline-carbonatite complex, and is closely associated with rare earth elements (REE) mineralization. However, the origin and nature of the fenitizing fluids and its role in REE mineralization are poor understood. The Lizhuang REE deposit in the Panxi region, SW China, is hosted in a syenite-carbonatite complex with low- and high-grade fenitization. In this study, we conducted detailed petrographic, compositional and isotopic studies on the fresh/unaltered syenite and low- and high-grade fenites to reveal the origin and nature of the fenitizing fluids and their effects on REE mineralization. Our results show that the whole-rock Na/K ratios gradually increase from the fresh syenite, low-grade fenites to high-grade fenites, as well as mineral replacements, such as K-feldspar is replaced by albite and Ca-rich clinopyroxene is replaced by Na-rich clinopyroxene, indicating that sodic fenitization occurred in the Lizhuang syenite system. In situ U-Pb dating of the unaltered and altered apatite shows that the hydrothermal alteration processes are nearly simultaneous with the emplacement of syenite and carbonatite during 27.4–25.3 Ma, maybe lasting for ~2 Ma. In addition, the altered regions of feldspar, clinopyroxene and apatite have in situ 87Sr/86Sr ratios ranging from 0.70580 to 0.70674, which is consistent in error with those of the unaltered regions and host rock syenite-carbonatite complex (0.70566–0.70666). It is suggested that the fenitizing fluids were exsolved from the syenite-carbonatite complex in a relatively closed system. As the increase of alteration degree, the proportion of hydrothermal minerals (e.g., barite, calcite, fluorite and bastnasite) in fenite increased gradually, indicating that the fenitizing fluids are enriched in SO42−, CO32− and F−. This is also supported by the elevated F and SO3 contents from the unaltered to altered apatite. The increasing REE concentrations, from the fresh syenite, low-grade fenite to high-grade fenite, indicate that REE minerals such as bastnasite were precipitated from the fenitizing fluids during fenitization processes. In-situ analyses on the altered clinopyroxene and apatite show variable REE losses compared to the unaltered precursors, which indicates the successive remobilization and transferring of REE by fenitization fluids. Therefore, two discrete stages of fenitization have been identified in the Lizhuang REE deposit, resulted from fluids exsolved from the syenite and carbonatite, respectively. Meanwhile, such fenitization fluids are effective extracting agents of REE in post-magmatic stage, which is crucial to REE remobilization and precipitation to form economic deposit.

Karstic bauxites in western Guangxi, China, comprise two subtypes: Permian bauxite and Quaternary bauxite. The Quaternary bauxite originated from the breaking up, rolling, and accumulating of Permian bauxite in karstic depressions in Quaternary. Various types of rare earth element (REE) minerals were discovered during the formation of the Permian and Quaternary bauxites from the Xinxu, Longhe, and Tianyang bauxite deposits in this study. Five types of REE minerals, including bastnasite, parisite, cerianite, rhabdophane, and churchite, were identified. Bastnasite and parisite are the most abundant, and they are widely developed in the Permian ore and also present in the Quaternary ore. Obvious variations in bastnasite and parisite REE compositions were observed, which is ascribed to distinctions in the source materials in the primary weathering profile from different areas. The mode of occurrence of bastnasite and parisite suggests they were mainly precipitated under alkaline and reducing conditions during the Permian bauxite-forming stage and underwent intensive corrosion in the Quaternary. Churchite was formed during the Permian weathering stage under acidic condition. Both cerianite and rhabdophane occur in fractures within the Permian bauxite ore, indicating that both formed during the Quaternary weathering stage. It is considered that the rhabdophane enriched in Ce have formed locally, in the process of that the Ce3 +, released from bastnasite rapidly, entered the rhabdophane lattice before being oxidized to Ce4 +. Cerianite was mainly found in association with Mn–Al hydroxides, suggesting that the released Ce3 + was oxidized into Ce4 + and precipitated cerianite in fractures within the Permian bauxite ore. Mass balance equations reveal a depletion in nearly all REEs during the transformation from the Permian to the Quaternary bauxite ore, mainly caused by the dissolution of bastnasite and parisite. The genesis of the REE minerals, together with the occurrence of other minerals, indicates that intensively acidic and oxidizing conditions developed before the formation of the Permian bauxite ore. Towards the end of the Permian, the conditions became reducing and alkaline, favorable for the large-scale bauxitization. The Quaternary bauxite-forming stage was characterized by variable pH and Eh conditions, with acidic (pH = 4–6) and oxidizing (Eh > 2) conditions at the surface of the exposed Permian bauxite ore.

The Bayan Obo deposit is a world-class Fe-REE-Nb deposit, and its reserves of rare earth element (REE) resources rank the first over the world. In the face of the current situation of insufficient utilization rate of rare earth resources and scarcity of middle-heavy rare earth elements (M−HREE) resources, the Bayan Obo deposit with such a huge amount of M−HREE cannot be underestimated. In this paper, the occurrence characteristics of M−HREE in different types of iron ore in the Bayan Obo main ore body are studied by using field emission scanning electron microscope (FESEM), energy dispersive spectrometer (EDS) and advanced mineral identification and characterisation system (AMICS), and the enrichment mechanism is also discussed. The results show that both Sm and Y are the most abundant M−HREE in each type of iron ore in the main ore body, and the content of M−HREE accounts for 1.41%–5.57% of total REE, among which the content of M−HREE in aegirine type Nb-REE-Fe ore (824.47 ppm) and fluorite type Nb-REE-Fe ore (794.82 ppm) are higher, and the content of M−HREE in massive type Nb-REE-Fe ore is lower (318.49 ppm). The main minerals containing M−HREE are bastnasite, parisite, Huanghoite, monazite, aeschynite and fergusonite, among which the content of M−HREE in fergusonite and aeschynite are the highest. According to the characteristics of mineral paragenetic association of REE in this ore district, it is believed that the REE migrates mainly in many different forms of complexes. Heavy rare earth elements (HREE) mainly experienced carbonatite magmatism stage, sodium-fluorine metasomatism stage and late vein mineralization stage, and finally got enrichment.

The rare earth element (REE) mineralization of Gakara (Burundi) has first been discovered in 1936 and has periodically been the subject of geological studies, at times when the exploitation of bastnasite-(Ce) and monazite-(Ce) was economically interesting. This study focuses on the establishment of a mineral paragenesis for Gakara, with special attention to the REE-bearing phases, to understand the formation history of the deposit. The paragenesis can be subdivided into 3 stages: primary ore deposition, brecciation stage and supergene alteration. Evidence for fenitization processes (i.e. pinkish-red cathodoluminescence of K-feldpar, brecciation stage) and the strong enrichment of light REEs in bastnasite and monazite substantiate the hypothesis of a structurally controlled hydrothermal mineralization with a strong carbonatitic affinity. This likely confirms the association of the Gakara REE deposit with the Neoproterozoic alignment of alkaline complexes and carbonatites along the present-day Western Rift. It suggests a direct link with a – currently unidentified – carbonatitic body at depth, possibly derived from a predominantly metasomatized lithospheric mantle.

Rare-earth elements play a crucial role in modern technologies and are necessary for a transition to a green economy. Potentially economic deposits of these elements are typically hosted in minerals such as monazite, bastnasite, and eudialyte (a complex Na-Ca-Fe-Zr silicate mineral with Cl), making these prime targets for geological research. Globally, rare-earth mineral deposits commonly show evidence of polyphase development and mineralisation processes, which need to be better understood to improve exploration strategies. The Norra Karr alkaline complex (Sweden) contains a globally significant deposit of rare-earth elements, hosted in the mineral eudialyte. In this study, we focussed on eudialyte crystals in undeformed, cross-cutting pegmatoid veins from Norra Karr. In order to determine their age, we refined an established micromilling method to enable sampling of minerals rich in rare-earth elements for precise analysis of major and trace elements, Nd isotope ratios, and Sm-Nd geochronology down to a scale of

Elevated uranium concentrations have been reported in many IOCG deposits, but detailed information about the occurrences of uranium minerals and their relationships with other minerals remains scarce, which hinders understanding of the mineralization mechanisms and the nature of the association of these metals. This paper reports a new case of uranium enrichment in an IOCG system, the Lala Cu-Fe deposit in the Kangdian region, SW China. Whole-rock analyses of ores and barren rocks yielded highly variable U concentrations ranging from 0.76 to 266 ppm. Uranium shows a strong positive correlation with Mo, broadly positive correlations with Cu and ΣREE, and a vague, negative correlation with Th. Four uranium minerals, uraninite [UO2], uranothorite [(Th,U)SiO4], polycrase [(Y,Ca,Ce,U,Th)(Ti,Nb,Ta)2O6], and coffinite [U(SiO4)1-x(OH)4x], were identified. The uraninite is divided into two phases: phase 1 (Ur1) characterized by corroded or irregular crystal shapes, and phase 2 (Ur2) showing well-developed, subhedral to euhedral crystal shapes. Ur1 is interpreted to be formed in the primary Cu – Fe – Mo – REE – U mineralization, together with the bulk of Cu and other metal sulfides, as well as other U and REE minerals, including uranothorite, polycrase, bastnasite and xenotime, and accompanied by K-alteration (K-feldspar and biotite). Ur2 is interpreted to be formed in a secondary hydrothermal event characterized by introduction of fluorite and carbonates, in which U and Mo, and to lesser extent Cu and REE, were remobilized, while Th remained relatively stable. The primary mineralization is inferred to have taken place in a period from ca. 1086 to 1053 Ma based on previous Re-Os geochronology, and the secondary mineralization (remobilization) likely took place from ca. 888 to 828 Ma, as suggested by uraninite chemical ages obtained in this study and other published geochronological data. These ages correspond to tectonomagmatic events taking place in generally extensional environments, in the western margin of the Yangtze Block during the Meso- to Neoproterozoic time. The Lala Cu-Fe deposit represents a new case of IOCG deposits with U (and REE) enrichment, the economic potential of which warrants further examination.

As a key source of strategic commodities for advanced technologies, carbonatite-related (carbonatite–nordmarkite complex) rare earth element (REE) deposits are globally important resources. Such deposits have different ore types, scales, ore-formation structures, and evolutionary processes. However, the causes of these variations have not been well studied, which prevents us from understanding the formation of such deposits. The factors that lead to the formation of large carbonatite-related REE deposits have also not been carefully documented. Fortunately, the Cenozoic Mianning–Dechang (MD) REE belt in eastern Tibet contains several medium- and large-scale carbonatite-related REE deposits, including brecciated, weathered, and disseminated ores, providing an ideal opportunity to investigate these factors. Current and this studies show that the deposits in the MD belt have similar formation stages, crystallization sequences, sources, and REE mineralization conditions. Previous studies have shown that the REE mineralization occurred alongside the formation of large-scale overprinted gangue minerals, such as barite, fluorite, and calcite during the lowest-temperature hydrothermal stage ( 4 2−, and CO2 are important for transporting and precipitating REE. Detailed geological mapping suggests that the variation in ore type was caused by local ore-controlling structures, rather than being due to different depths of formation. Bastnasite has similar initial 87Sr/86Sr (0.7059–0.7080), 143Nd/144Nd (0.5123–0.5124), 207Pb/204Pb (15.594–15.648), and 208Pb/204Pb (38.422–71.488) ratios along the whole belt, implying that all the deposits have the same source and the REE were sourced directly from a mixture of crustal and mantle material. The C–O isotopic compositions of the carbonatites (δ18OV-SMOW = 6.4‰ to 10.5‰, δ13CV-PDB = − 3.9‰ to − 8.5‰) and bastnasite (δ18OV-SMOW = 9.5‰–16.3‰, δ13CV-PDB = − 4.7‰ to − 9.3‰) suggest that low-temperature alteration occurred along with REE mineralization. Development of pegmatitic stage and aegirine–augite and arfvedsonite fenitization alteration may promote the formation of large-scale REE deposits. The giant Maoniuping and Dalucao deposits both had pegmatitic stages that involved the formation of coarse-grained barite, fluorite, and calcite with REE contents of up to 6305 ppm, suggesting the pegmatitic stage suggest another REE enrichment event after the magmatic stage, and large-scale REE mineralization. The intense alkali alteration that forms arfvedsonite and aegirine–augite in carbonatite–nordmarkite complexes was only found in the Maoniuping deposit, whereas biotitation was found in the Dalucao and Lizhuang deposits. This suggests that the Maoniuping deposit experienced more intense alteration, which may have led to more effective REE migration from the wallrock into the ore-forming fluids. The mantle source, the evolution of the magma, the presence of a pegmatitic stage, the composition of the fluids, and the high degree of alkaline alteration could have affected the scale of the deposits, in addition to the large volume of carbonatite–nordmarkite complexes and multiple phases of tectonism.

The Morro dos Seis Lagos niobium rare earth element, Ti-bearing lateritic deposit (Amazonas, Brazil) is derived from a primary siderite carbonatite. The complex is the only example of a Nb deposit in which Nb-rich rutile is the main Nb ore mineral. Apart from the laterites, at the current level of exposure the complex consists only of siderite carbonatite; silicate rocks are absent. Three types of siderite carbonatite are recognized: (1) a brecciated and oxidized core siderite carbonatite consisting of up to 95 vol% siderite together with: hematite; pyrochlore; Nb-brookite; Ti-maghemite; and thorobastnasite; (2) a REE- and P-rich variety of the core siderite carbonatite consisting of siderite (up to 95 vol%), hematite, minor pyrochlore, monazite and bastnasite; (3) a border hydrothermal siderite carbonatite with ~70 vol% siderite, barite (~15 vol%), gorceixite (~7 vol%) and minor rhabdophane and pyrochlore. The country rock gneiss in which the carbonatite was emplaced was affected by potassic fenitization, with the formation of phlogopite and orthoclase together with monazite, fluorapatite and bastnasite. The siderite carbonatites exhibit a wide variation of δ13C (−5.39‰ to −1.40‰), accompanied by a significant variation in δ18O (17.13‰ to 31.33‰), especially in the REE-rich core siderite carbonatite, and are explained as due to the presence of both H2O and CO2 in the magma. The core siderite carbonatite is the richest in Fe (48.64–70.85 wt% Fe2O3) and the poorest in Ca (up 0.82 wt% CaO) example of a siderite carbonatite yet recognized The ferrocarbonatite has significant contents of Mn, Ba, Th, Pb and LREE, and a very high Nb (up to 7667 ppm) content due to the presence of Nb-brookite. The substitution 3Ti4+ = Fe2+ + 2Nb5+ recognized in Nb-rich brookite explains enrichment of Nb in the core siderite carbonatite and indicates formation in a reducing environment. The high Nb/Ta ratio (1408–11,459) of the carbonatite is compatible with residual liquids derived by fractional crystallization. The 87Sr/86Sr (0.70411–0.70573) and 144Nd/143Nd (0.512663–0.512715) isotopic data suggest the carbonatite is mantle-derived with essentially no crustal contamination and is younger than the maximum age of 1328 ± 58 Ma (U Pb in zircon). We suggest that the Morro dos Seis Lagos carbonatite complex represents the upper-most parts of a differentiated carbonatite magmatic system, and that the siderite carbonatite is related to late-magmatic-to-carbo-hydrothermal processes.

Understanding basic material properties of rare earth element (REE) bearing minerals such as their phase stability and equations of state can assist in understanding how economically viable deposits might form. Bastn&auml, site is the most commonly mined REE bearing mineral. We synthesized the lanthanum-fluoride end member, bastn&auml, site-(La) (LaCO3F), and investigated its thermal behavior and decomposition products from 298 K to 1173 K under ambient pressure conditions through thermogravimetric analysis, differential scanning calorimetry, evolved gas analysis, and high temperature powder X-ray diffraction. We also investigated the compressibility of bastn&auml, site-(La) via single crystal X-ray diffraction in diamond anvil cells at an ambient temperature up to 11.3 GPa and from 4.9 GPa to 7.7 GPa up to 673 K. At ambient pressure, bastn&auml, site-(La) was stable up to 598 K in air, where it decomposed into CO2 and tetragonal &gamma, LaOF. Above 948 K, cubic &alpha, LaOF is stable. High temperature X-ray diffraction data were used to fit the Fei thermal equation of state and the thermal expansion coefficient &alpha, 298 for all three materials. Bastn&auml, site-(La) was fit from 298 K to 723 K with V0 = 439.82 &Aring, 3, &alpha, 298 = 4.32 &times, 10&minus, 5 K&minus, 1, a0 = &minus, 1.68 &times, 1, a1 = 8.34 &times, 8 K&minus, 1, and a2 = 3.126 K&minus, 1. Tetragonal &gamma, LaOF was fit from 723 K to 948 K with V0 = 96.51 &Aring, 298 = 2.95&times, 4 K&minus, 2.41&times, 1, a1 = 2.42&times, 7 K&minus, 1, and a2 = 41.147 K&minus, 1. Cubic &alpha, LaOF was fit from 973 K to 1123 K with V0 = 190.71 &Aring, 298 = &minus, 1.12&times, 1, a0 = 2.36&times, 1, a1 = &minus, 1.73 &times, 1, and a2 = &minus, 17.362 K&minus, 1. An ambient temperature third order Birch&ndash, Murnaghan equation of state was fit with V0 = 439.82 &Aring, 3, K0 = 105 GPa, and K&rsquo, = 5.58.