Nb Mineralization Research Articles

Aeschynite Group Minerals Are a Potential Recovery Target for Niobium Resources at the Giant Bayan Obo Nb–REE–Fe Deposit in China

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

LEAD-LEAD DATING REVEALS PERMIAN REMOBILIZATION OF NIOBIUM MINERALIZATION AT BAYAN OBO

Abstract The Bayan Obo deposit hosted by the H8 unit is a world-class rare earth element (REE) deposit with considerable niobium (Nb) and iron (Fe). Permian granites are widely exposed in the mining area and have a close spatial association with the Nb mineralization. Whether the granites contributed Nb or only remobilized existing mineralization is important for understanding the controls of ore formation. Previous studies have mostly focused on the REEs, whereas research on Nb has been limited. This is due mainly to the difficulty of accurately determining the age of the Nb mineralization because of the fine-grained and texturally complex nature of the Nb-bearing minerals and their exceptionally low U content. Although microbeam techniques show promise in tackling the aforementioned challenges, their application is hampered by matrix effects caused by the diverse composition of Nb-bearing minerals. Here we report the application of a high-precision secondary ion mass spectrometry (SIMS) Pb-Pb isochron approach that enables young samples (i.e., <500 Ma) to be dated without matrix-matched reference materials. A variety of Nb-bearing minerals from eastern Bayan Obo were analyzed, yielding Pb-Pb isochron ages of 276 ± 10 Ma (pyrochlore, 394–6,864 ppm U in the rim and 6,563–19,858 ppm in the core), 277 ± 36 Ma (fersmite, 18–61 ppm U; fergusonite-Ce, 45–95 ppm U), and 257 ± 46 Ma (aeschynite, 342–1,006 ppm U). In combination with the deposit geology and petrographic observations, these ages link the Nb mineralization to ~270 Ma granites. As these granites are not particularly rich in Nb, skarn formation during granite emplacement is interpreted to have remobilized the existing Nb mineralization, which increased the grain size of the Nb-bearing minerals—a key factor facilitating their extraction. Our study shows that high-precision SIMS Pb-Pb analysis holds promise for directly dating mineralization without matrix-matched reference materials. It also emphasizes the need to consider the role of the Nb remobilization at Bayan Obo and elsewhere.

Dissolution of Ta–Nb and Nb Minerals in Granitoid Melts

The effective solubilities (maximum contents) of Ta and Nb in model felsic lithium-fluoride melts of variable alkalinity and alumina content were experimentally determined at the dissolution of Ta–Nb and Nb minerals: pyrochlore, microlite, ilmenorutile, and ferrotapiolite at T = 650–850°C and P = 100 and 400 MPa. The Ta and Nb partitioning in the mineral-melt systems was also studied. When pyrochlore is dissolved in granitoid melts at P = 100 MPa and T = 650–850°C, the highest effective solubilities of Nb (0.7–1.8 wt %) are obtained in alkaline melt, and they significantly decrease (to 0.03–0.5 wt %) in subaluminous and peraluminous melts. A temperature increase increases the solubility (content) of Nb in the melt. Similar dependences were obtained for Ta solubility by dissolving microlite. In peraluminous granitoid melt, microlite remains stable, while pyrochlore becomes unstable. A pressure decrease from 400 to 100 MPa in alkaline and subaluminous melts was found out to not significantly affect on the dissolution of microlite and pyrochlore, whereas Ta and Nb contents in the peraluminous melt noticeably decrease. The dependences of Nb solubility and its partitioning between granitoid melts and ilmenorutile on the alkalinity–alumina content of the melt are similar to those for the dissolution of columbite and tantalite. The dependences obtained by dissolving ferrotapiolite, pyrochlore, and microlite differ from them.

Magmatic-hydrothermal evolution of the Dangba rare-metal granitic pegmatites in the Songpan–Ganzê orogenic belt, Eastern Tibet: Insights from muscovite and columbite-group minerals

Rare-metal granitic pegmatites are commonly considered to be the results of a combination of magmatic and hydrothermal processes. Although magmatic crystallization and fractionation, and magmatic-hydrothermal transition have been extensively investigated, the processes related economically valuable mineralization of rare-metals and strong fractionation of geochemical twins remain controversial. This study presents a comprehensive analysis of the geochemical evolution of rare-metal mineralized pegmatites, focusing on the mineral chemistry of muscovite and columbite-group minerals (CGM) from the Dangba rare-metal granitic pegmatites in the Songpan–Ganzê orogenic belt. Our findings suggest that the random orientation growth of spodumene crystals and widespread distribution of disordered columbite-group minerals crystals are indicative of lithium saturation in the initial melt at the Dangba No. VII dike. Magmatic fractionation is indicated by gradually increasing ratios of alkali metals (K/Rb ratios) and Ta# (Ta/(Ta + Nb)) in muscovite and CGM, and is proposed to be an important factor for enrichment of rare-metals. During the magmatic-hydrothermal transition, muscovite from metasomatism of spodumene exhibits similar or slightly higher levels of Li, Rb, and Cs, compared to primary muscovite, but shows strong depletion of Ta, Nb, Sn, and W. Alteration of primary minerals (e.g., spodumene, alkali feldspar, and columbite-group minerals) results in the release of these elements into the reactive media (melt/fluid) during the metasomatism. This process ultimately leads to the depletion of rare metals in the early crystallized minerals. However, this process is advantageous for the subsequent mineralization of Ta and Nb, as well as their fractionation. The chemical compositions of exsolved aqueous fluid are influenced by the mineralogy of pegmatites and the partitioning behavior between melt and fluid. Furthermore, pervasive albitization serves as an exploration criterion for Li–Ta–Nb in the Ke'eryin Orefield.

Enrichment mechanisms of Nb, Zr, and REY in the Late Permian coal-bearing strata in western Guizhou, SW China

Niobium, Zr, REE, and Y are widely enriched in the Upper Permian coal-bearing strata in southwestern China and are regarded as a substantial resource. To better understand the occurrence modes and enrichment mechanisms of these rare metals, comprehensive heavy-mineral separation, Micro-Raman spectroscopy, EPMA, and LA-ICP-MS analyses of primary and secondary metal minerals were conducted in the polymetallic beds in western Guizhou. In addition, the XRD, XRF, and ICP-MS analyses of bulk rocks were carried out. Our results suggest that Nb is primarily hosted in secondary TiO2 polymorphs which converted from primary Nb-rich rutile, and occurs as minor ilmenorutile and columbite-(Fe). The Zr is sequestered in both detrital zircon and secondary microcrystalline zircon. REE and Y (REY) are mainly incorporated into secondary florencite-(Ce), rhabdophane-(Ce), churchite-(Y), and zircon, also partially adsorbed by clay minerals. The assemblage of detrital zircon, Nb-rich rutile, and discrete Nb minerals reveals that rare metals are preconcentrated during the waning-stage Emeishan alkaline magmatisms. Complex zonings in rutile crystals may result from the diffusion of Nb5+ and Fe2+, and these grains are further overprinted by magmatic-hydrothermal and enveloped by ilmenorutile and columbite-(Fe). Furthermore, magmatic rutile is commonly converted into anatase, brookite, and leucoxene under supergene conditions, and then persisted as stable phases. Authigenic REY-rich (Al)-phosphates and zircon are sourced from the leaching, even decomposition of primary REY-phosphates and zircons. Their colloidal aggregates are precipitated from the acidic and oxidizing low-temperature fluids, which are superposed onto clay matrix. The mineralogical and geochemical features suggest that the polymetallic beds are enriched by both alkaline magmatic-hydrothermal processes in source rocks and supergene fluid alterations in sediments. This study gives new insights into the enrichment processes and conditions, and further extraction of critical metals in the coeval coal-bearing strata in southwestern China.

Magmatic–hydrothermal stage recorded by mica zonation and Nb–Ta–W–Sn mineralization: New insight into the genesis of highly evolved Songshugang Nb–Ta deposit (Jiangxi, SE China)

The role of magmatic and hydrothermal processes in niobium-tantalum mineralization is still controversial and has been limited by a lack of mineralogical records in the evolving magmatic–hydrothermal system. Micas as essential rock-forming minerals and comprise the amount of elements (especially rare-metal elements), become one of the ideal indicator. The Songshugang rare-metal granite (RMG) located in the southern part of Jiangxi Province, China, is renowned as the largest tantalum reservoir in South China. The albite–topaz–K-feldspar granite is the major granite, with layers of pegmatite, topaz-K-feldspar granite, and greisen overlay on it. It is the typical highly evolved peraluminous with low phosphorous RMGs (PLP-type) according to geochemistry. The Songshugang pluton preserves analogical characters in topaz-albite-K feldspar granites and topaz-K feldspar granite, and partly reserves in greisen. Detailed petrographic, mineralogy, and chemical composition study of zoned mica and accessory Nb–Ta minerals suggest that zinnwaldite and Nb-Ta oxides have experienced magmatic fractionation with gradually decrease of K/Rb and Nb/Ta ratio in prismatic zinnwaldite and columbite-(Fe). The leap of Rb (Rb2O up to 4.08 wt%) and Ta (Ta# up to 0.29) at the rim of zinnwaldite and columbite is likely attributed to supersaturation. Afterwards the overgrowth of Rb-deficit zinnwaldite (0.56–0.72 wt% Rb2O) is coeval with the Ta-highest wodginite, taking place during the magmatic-hydrothermal transition associated with a fluid-melt interaction in hydrosaline melt. The ascent of fluxing elements contributes to the formation of hydrothermal zinnwaldite and cassiterite through greisenization. The systemic coupling mineralization of zinnwaldite and Nb–Ta oxides altogether record the both the melt-driven and fluid-driven processes in highly evolved Songshugang deposit that is critical for rare-metal mineralization. The Songshugang displays a great similarity to the typical PLP RMGs Cínovec cupola in geochemistry, which are metaluminous to peraluminous, containing low phosphorous, intermediate REE, Zr, Hf, and Th content. But Songshugang has experienced strong supersaturation; the latter is more akin to highly evolved melts alike Yichun or Beauvoir. The strong supersaturation and crystallization demonstrate its discrepant mineralization mechanism in Songshugang RMGs.

Principal component analysis of mineral and element composition of ores from the Bayan Obo Nb-Fe-REE deposit: Implication for mineralization process and ore classification

The Bayan Obo Nb-Fe-REE polymetallic deposit in northern China is the world’s largest light REE deposit. The deposit was formed through multiperiodic alteration and mineralization, and thus mineral species and assemblages are extremely complex. To date, there is still no consensus on the classification of ore types, and research on the paragenetic association of minerals is still inadequate. Eighty ore samples are divided into seven types according to altered minerals, including dolomite, iron oxide, aegirine, riebeckite, biotite, fluorite, and barite types. These samples were analyzed by a TESCAN Integrated Mineral Analyzer (TIMA) for their mineral and element composition. The data set was investigated by principal component analysis (PCA) to reveal the mineralization and alteration information and classify the ores.PCM1 to PCM3 and PCE1 to PCE3 represent the first three principal components for PCA results of minerals and elements, respectively. The results indicate that PCM1 may distinguish rare earth mineralization from barite and silicification alteration that is associated with the latest fluids. PCM2 can distinguish mineralization and alteration associated with Nb from those associated with REE. The combination of PCM2 and PCE1 indicates that the precipitation of REE may be closely related to fluorination and phosphate, while the combination of PCM2 and PCE3 indicates that the Nb mineralization may be more closely related to alteration of riebeckite and biotite, as well as to Ti-rich fluids. In addition, PCM3 distinguishes the Th mineralization and associated fluorination from other alteration and mineralization. Based on PCA results, the above seven ore types can be classified into three types: dolomite REE ore, Fe-REE-F ore, and Nb-rich ore, which provides a simplified and comprehensive classification for Bayan Obo ores. PCA-based approach combining geochemical and mineralogical data can quickly identify ore types, which is helpful in mineral exploration.

Three-stage niobium mineralization at Bayan Obo, China.

The Chinese Bayan Obo deposit is a world-class rare earth element (REE) deposit with considerable niobium (Nb) and iron (Fe) resources. A complete genetic understanding on all metals is fundamental for establishing genetic models at Bayan Obo. With extensive research being focused on REE enrichment, the timing and controls of Nb enrichment remain unresolved at Bayan Obo, which is mainly due to the challenges in dating, i.e. multistage thermal events, fine-grained minerals with complex textures and the rare occurrence of uranium-enriched minerals with mature dating methods. Based on robust geological and petrographic frameworks, here we conducted ion probe uranium-lead (U-Pb) dating of ferrocolumbite to unravel the timing, hence the genesis of Nb mineralization. Three types of hydrothermal ferrocolumbites-key Nb-bearing minerals-are identified based on their textures and mineral assemblages. They yield U-Pb ages of 1312±47 Ma (n=99), 438±7 Ma (n=93), and 268±5 Ma (n=19), respectively. In line with deposit geology, we tentatively link the first, second and third stage Nb mineralization to Mesoproterozoic carbonatite magmatism, ubiquitous early Paleozoic hydrothermal activity, and Permian granitic magmatism, respectively. While quantifying the contribution of metal endowment from each stage requires further investigation, our new dates highlight that multi-stage mineralization is critical for Nb enrichment at Bayan Obo, which may also have implications for the enrichment mechanism of Nb in REE deposits in general.

Aptian Li-F Granites of the Northern Verkhoyansk–Kolyma Orogenic Belt, Eastern Russia: Composition, Genesis, and Ore Potential

This paper reports the results from an investigation on the geochemistry and petrogenesis of the Aptian Li-F granites from the Omchikandya, Burgali, and Arga Ynnakh Khaya ore fields in the northern Verkhoyansk–Kolyma orogenic belt in eastern Russia. Li-F microcline–albite granites intrude the Late Jurassic to Early Cretaceous syn-collisional granitoids. According to their geochemical composition, they are close to A-type granites and can be subdivided into low-P and high-P varieties, differing in their geochemistry and genesis. The low-P microcline–albite granites (Omchikandya massif) intrude syn-collisional biotite granites. It is assumed that the formation of their parent melt occurred at deep levels in the same magma chamber that produced biotite granites. The high-P granites (Verkhne–Burgali ethmolith and Kester harpolith) are supposed to have been derived from melts originated from a high-grade metamorphosed lower crustal protolith under the influence of deep-seated fluid flows related to diapirs of alkaline-ultrabasic or alkaline-basic composition. It is supposed that their formation was related to post-collisional extension during the early stages of the evolution of the Aptian–Late Cretaceous Indigirka belt of crust extension. All studied Li-F granites are enriched with rare metals and have associated Li deposits with accompanying Sn, W, Ta, and Nb mineralization. In the low-P Li-F Omchikandya massif, mineralization tends to occur within greisenized granites and greisens in their apical parts. In the high-P granite massifs, mineralization is found throughout their volume, and, therefore, the Verkhne–Burgali ethmolith and Kester harpolith can be considered as large ore bodies. There is a direct dependence of the content and reserves of Li2O on the content of P2O5. Minimum Li2O reserves are established in low-P Li-F microcline–albite granites of the Polyarnoe deposit of the Omchikandya ore field, whereas in the high-P granites of the Verkhne–Burgali and Kester deposits, the Li2O reserves are significantly higher.

Atomic-scale environment of niobium in ore minerals as revealed by XANES and EXAFS at the Nb K-edge

Abstract. The mineralogy of niobium (Nb) is characterized by multicomponent oxides such as AB2O6, A2B2O7, ABO4, and ABO3 in which Nb is incorporated in the B site. Such complex crystal-chemistry prevents their unambiguous identification in ore deposits such as hydrothermal rocks and laterites which exhibit complex and fine-grained textures. The understanding of the processes controlling Nb ore deposit formation in various geological settings is therefore limited, although Nb is a critical element. In this study, we use X-ray absorption spectroscopy (XAS) at the Nb K-edge to investigate the local atomic-scale structure around Nb in a large set of natural and synthetic minerals of geological and technological importance. Our X-ray absorption near-edge structure (XANES) data at the Nb K-edge show three major features of variable position and intensity and then can be related to the local distortion and coordination number of the Nb site. Shell-by-shell fits of the extended X-ray absorption fine structure (EXAFS) data reveal that the NbO6 octahedra are distorted in a variety of pyrochlore species. At least two distinct first shells of O atoms are present while reported crystallographic data yield regular octahedra in the same minerals. Next-nearest Nb–Nb distances in pyrochlore and Nb-bearing perovskite mirror a corner-sharing NbO6 network, whereas the two Nb–Nb distances in columbite are typical of edge- and corner-sharing NbO6 octahedra. Such a resolution on the Nb site geometry and the intersite relationships between the next-nearest NbO6 octahedra is made possible by collecting EXAFS data under optimal conditions at 20 K and up to 16 Å−1. The local structure around substituted Nb5+ in Fe3+, Ti4+, and Ce4+ oxides suffers major changes relative to the unsubstituted structures. The substitution of Nb5+ for Ti4+ in anatase leads to the increase in the interatomic distances between Nb and its first and second Ti4+ neighbors. The substitution of Nb5+ for Ce4+ in cerianite reduces the coordination number of the cation from eight to four, and the Nb–O bonds are shortened compared to Ce–O ones. In hematite, Nb5+ occupies a regular site, whereas the Fe3+ site is strongly distorted suggesting major site relaxation due to charge mismatch. The sensitivity of XANES and EXAFS spectroscopies at the Nb K-edge to the local site geometry and next-nearest neighbors demonstrated in this study would help decipher Nb speciation and investigate mineralogical reactions of Nb minerals in deposit-related contexts such as hydrothermal and lateritic deposits.

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

The Bayan Obo deposit is world-famous for its giant rare earth reserves, and also accompanied with 2.2 Mt Nb2O5 resources at an average grade of 0.13 %, renowned as the second largest Nb deposit globally. Previous studies have focused on the origin of carbonatite (igneous/sedimentary), and variation composition during evolution, as well as its super-enrichment of light rare earth elements (LREE), while the metallogenic mechanism of niobium is being overlooked despite the identification of plenty Nb-bearing minerals. In this study, we identify four types of columbite as major Nb-bearing minerals, instead of pyrochlore in the west pit of Bayan Obo. Detailed mineralogical study has revealed large primitive columbite (Clb1-I, 100–200 μm) is hosted in the coarse-grained Fe-dolomite carbonatite (FeO = 5.46–10.47 wt%), being an ideal target for future mineral beneficiation. Those columbites (Clb1-I) are featured by lowest Sr, Y, and ∑REE but enriched in iron in composition, revealing its magmatic origin. Sieve-textured columbite (Clb1-II, ∼100 μm) dispersed in the fluorination and chloritization dolomite carbonatite, with a U–Pb age of 1359 ± 31 Ma, contemporary with ages of the regional carbonatite magma intrusion and rift-related magmatism, indicating niobium being sourced from mantle-derived carbonatite. The occurrence of fluorite as inclusions in Clb1-I indicates the solubility of Nb was enhanced by fluxes, and the columbite saturation was easily achieved during the carbonatite magma evolution into ferroan carbonatite. Strong fenitization metasomatism further locally concentrated columbite either within the mineral assemblages of pyrite + phlogopite + allanite (Clb2) or in the mica-dominated fenite (Clb3), and crystallized at 346 ± 18 Ma and 284 ± 16 Ma, respectively. The latter metalogenetic epoch is accordant with late Permian granitic magmatism event (ca. 280 Ma) while the former age may attribute to its occurrences related to the early Paleozoic arc-continent collision. Differ from pristine columbite (Clb1-I and Clb1-II), the metamorphic columbite (Clb2 and Cl3, especially Clb3) are enriched in Ta (1803–4092 ppm), Y (560–2029 ppm) and ∑REE (720–6317 ppm) content, probably suggesting the highest degree of evolution. The similar metallogenesis framework between Nb and REE, combined with the coexistence of columbite and REE minerals assemblages, implies the spatial and temporal coupling of Nb and REE mineralization. This first innovative and systematic mineralogical and geochronological work directly constrains the nature of Mesoproterozoic mantle-derived carbonatite with fertile Fe and fluxes, and the Paleozoic and Permian fenitization metasomatism, are the major factors contribute to the metallogenic specialization of niobium in Bayan Obo.

Mineral Chemistry of Pyrochlore Supergroup Minerals as Records of Nb Mineralization Processes in NYF-Type Pegmatites: A Case Study of the Emeishan Large Igneous Province, SW China

Alkaline igneous rocks have become a potentially important source of Nb, except for the carbonatites. The metallogenetic mechanism of Nb during the magmatic-hydrothermal evolution of alkaline rocks remains ambiguous. From the perspective of Nb minerals, the mineral chemistry of pyrochlore supergroup minerals provides the mineralogical evidence for indicating the respective contributions of magma and hydrothermal fluids to Nb mineralization. In the Miyi County of the Panzhihua-Xichang (Pan-Xi) area, the central zone of the Permian Emeishan large igneous province (ELIP) in SW China, hundreds of Nb-Y-F mineralized pegmatites (NYF-type) are exposed. This study conducted in situ mineral chemistry analyses on four types of pyrochlores to elucidate the Nb mineralization process. Both Pcl-I and Pcl-II exhibit well-developed oscillatory zoning (OZ), representing magmatic pyrochlore formed through disequilibrium crystallization in an oscillatory environment. Pcl-III, with a homogeneous and less variable composition, is also considered of magmatic origin due to its coherent chemical evolution with Pcl-II. Pcl-IV is considered of hydrothermal origin based on its irregular zoning texture, extremely high Na2O contents, and compositional gap compared with magmatic types. The decrease in TiO2 contents, coupled with the increase in Na2O, F, and Nb2O5 contents from Pcl-I to Pcl-III and from the core to the rim of zoned Pcl-II, indicates that fractional crystallization facilitates the crystallization of albite and the enrichment of volatiles, as well as the precipitation of Nb from the early to late stages. During the magmatic-hydrothermal transition stage, the reductive, Na- and F-enriched fluid transports Nb more effectively, leading to further Nb enrichment in pyrochlore. Consequently, there are two-stage Nb mineralization processes associated with the magmatic-hydrothermal evolution in the Miyi pegmatite, with the primary magmatic ore assemblages being the dominant Nb mineralization, which may be a general model for the mineralization of NYF-type pegmatites.

Distribution and mineralogical features of thorite in the Bayan Obo deposit: Implications for hydrothermal metasomatic Th re-enrichment

Thorite, as a principally Th-bearing independent mineral, is an important indicator for Th mineralization. However, its genesis and significance for Th enrichment processes are still unclear. Herein, we address this issue based on systematic mineralogical observation and major element analyses of thorites with related monazite and bastnäsite from different type ores in the Bayan Obo deposit. The thorites are mainly hosted in biotite-type and riebeckite-type ores with minor distribution in aegirine-type and banded-type ores, but are scarce in dolomite-type ores. The thorites in each type ores principally contain Th, O and Si with variable LREE and Fe concentrations. They commonly occur as vein (or veinlet) or aggregation crosscutting, cavity-infilling or around REE, Fe and Nb minerals, and some of these minerals exist as relict or inclusion within the thorites. In addition, late-stage huanghoite and pyrite are intergrown with the thorites or appear as inclusions within the thorites. These petrographic characteristics suggest that thorite formation was posterior to REE, Fe and Nb mineralization and was related to Paleozoic hydrothermal mineralization events. Barite and alkali alteration minerals (e.g., biotite and riebeckite) show a closely spatial relationship with the thorites, which indicates that hydrothermal fluids for thorite formation were enriched in alkalis (K+ and Na+) and sulfate (SO42-). Major element analyses of monazite and bastnäsite exhibit variable ThO2 concentration (unaltered region: 7.13 ∼ 14.34 wt% for monazite, 0.64 ∼ 1.33 wt% for bastnäsite; altered region: 2.50 ∼ 5.41 wt% for monazite, 0.17 ∼ 0.53 wt% for bastnäsite), which indicates that later-formed monazite and bastnäsite underwent Th leaching by chemical-coupled substitutions including: (1) 2REE3+ = Th4+ + Ca2+; and (2) P5+ + REE3+ = Si4+ + Th4+. In combination with relevant published data, our study suggests that later-stage Th-rich monazite and bastnäsite in the Bayan Obo deposit experienced mineral dissolution and Th leaching by metasomatism of hydrothermal fluids, which induced Th re-enrichment in form of thorite precipitation. On the other hand, as intrinsically high Th content of the thorites, this study also evaluates grain size and Th abundance of the thorites within different type ores, and proposes that biotite-type and riebeckite-type are targeted type ores for thorite extraction and Th collection.

Multistage ore formation in the world’s largest REE-Nb-Fe deposit of Bayan Obo, North China Craton: New insights and implications

The Bayan Obo deposit in the northern margin of the North China Craton is a world-class REE-Nb-Fe deposit, the complex mineralization history of which remains unresolved. In this study, we employ SEM and Image J software combined with mineralogical data obtained by in-situ EDS and EPMA analyses to analyze petrographic images rapidly and to investigate the mineralogical assemblages of each type of rocks/ores from the Main Orebody in an attempt to understand the mineralization process and ore-formation mechanism of this deposit. We identify three metallogenic periods and six ore-forming stages based on the field occurrence of ores combined with published geochronological data. The Mesoproterozoic magmatic event comprises two stages, which were distinguished as the coarse-grained dolomite stage (stage 1) and the fine-grained dolomite stage (stage 2). Three stages were recognized in the Mesoproterozoic shear deformation-hydrothermal mineralization period, including the disseminated mineralization stage (stage 3), the banded mineralization stage (stage 4) and the massive mineralization stage (stage 5). The Paleozoic hydrothermal period involved the vein mineralization stage (stage 6). Based on detailed studies of the mineral assemblages and paragenesis, as well as the complex textures of the ores, we propose a model of the multi-stage metallogenic process as follows: (1) The REE minerals and rutile occurring as inclusions wrapped within magmatic idiomorphic dolomite grains indicate that the REE-Nb mineralization might have started during the magmatic period. (2) The Nb minerals, such as fergusonite and ilmenorutile, intergrown with REE minerals, such as monazite and bastnäsite, indicate that the Nb and REE were transported by the same ore-forming magma or fluid and precipitated during the same metallogenic process. (3) From the fine-grained dolomite stage to the massive mineralization stage (from stage 2 to stage 5), the mineral assemblages become more complex, with a gradual increase in the degree of hydrothermal alteration suggesting that more hydrothermal fluid was involved in the ore mineralization. (4) The complex mineralogical assemblages and textural relationships indicate multiple formation mechanisms for the REE-Nb-Fe minerals. We propose that the unusually large volume of REE-Nb-Fe resources in the Bayan Obo deposit was the result of combined magma and hydrothermal fluid activities in the Mesoproterozoic (from stage 1 to stage 5), whereas the fluids only caused reactivation of the ore materials previously deposited without the addition of exogenous materials in the Paleozoic stage (stage 6).

Study on the complex magmatic and hydrothermal metallogenic processes in the giant Huayangchuan uranium (U)-polymetallic ore district: Constraints from geochemical and geochronological evidences

The supergiant Huayangchuan uranium (U)-polymetallic deposit is situated in the west part of Lesser Qinling Orogen, Central China, which is renowned for its unique metallogenic type and complex mineralization processes. This newly-discovered deposit contains economic endowments of U, Nb, Pb, Se, Sr, Ba and REEs. The genesis of this deposit remains hugely controversial and the related magmatic and metallogenic framework have not been well established. In our study, we present newly obtained zircon U-Pb age data, whole-rock trace elements, detailed SEM observation, carbon and oxygen isotope determination and titanite U-Pb age result to provide geochemical and geochronological constraints on the magmatic and hydrothermal metallogenic processes in Huayangchuan ore district and give insight into the REE, Nb and U mineralization and remobilization mechanism. Carbonatite in Huayangchuan deposit is rich in Ba, Sr, Nb, U and REE and the δ18OSMOW and δ13CPDB (‰) values of carbonatite vary between + 7.30 ‰ to + 9.80 ‰ and − 6.00 ‰ to − 7.20 ‰, respectively, which is the unique characteristic of igneous carbonatite. The total REE contents of carbonatite range from 1319.5 to 2535.8 ppm, significantly higher than that of the granite pegmatite dykes and Taihua Group granitic gneiss. Two zircon grains from the Huayangchuan carbonatite yield the 206 Pb / 238 U age of 223 ± 5.6 Ma and 217 ± 8.6 Ma. Uranium and niobium are mainly hosted in the primary mineral of betafite, minorly in uraninite and fergusonite. REE are remained in various REE carbonates, phosphates and silicates, e.g. allanite, parite, monazite, apatite, yttrialite and xenotime. Betafite contains numerous mineral inclusions of calcite, allanite and apatite, which displays as vermicular relicts with metasomatic corrosion texture. The intimate relationship between betafite, calcite, apatite, allanite and REE carbonates implies that U and Nb mineralization occurred during carbonatite-related magmatic stage. Late Triassic carbonatite magmatism (∼220 Ma) was responsible for the world-class U-Nb-Mo-REE mineralization in the Qinling Orogen. Whereas, LA-ICP-MS U-Pb data of titanite collected from Huayangchuan carbonatite reveal two sets of ages in the Huayangchuan deposit: during the Late Triassic and the Early Cretaceous. LA-ICP-MS data of the Triassic titanite and Cretaceous titanite in Huayangchuan deposit yield the lower intercept age of 223.1 ± 9.4 Ma and 141.9 ± 4.7 Ma respectively in the Tera-Wasserburg (238U/206Pb - 207Pb/206Pb) diagram. Cretaceous hydrothermal titanites show enrichment in Y, HREEs and depletion in TiO2 and LREEs relative to the Triassic magmatic titanites. Primary betafite and titanite were subjected to later hydrothermal alteration. The close genetic relation between altered titanite and Ti-Fe-Nb-Pb-U mineral inclusions demonstrates that Early Cretaceous hydrothermal fluids contributed to the Nb, U and Ti remobilization. Early Cretaceous regional granitic magmatic activity brought hydrothermal fluids to alter the early carbonatite magmatic pyrochlore and titanite. U, Nb, Ti, Y, Pb and HREEs were transferred and subsequently re-precipitated into uraninite, and hydrothermal titanite.

Geology and mineralization of Nb, P, Fe and light rare earth elements of the Araxá alkaline-carbonatite complex, Minas Gerais state, Brazil

New 40Sr/39Ar dating revealed that the magnetite phlogopitite at the center of the Araxá complex reached the surface at 98 Ma, and generated volcanoes with mineralized volcanic tuffs and crater lakes. At the bottom of the crater lakes, micro-graywacke deposited. Based on the mineral composition, hydrothermal structures, and δ18O and δD values, a hydrothermal episode occurred between ∼98 and 86 Ma Ma. The δ18O of phlogopites of phlogopities vary between 4.8‰ and 5.7‰, and those of magnetitites are 4.3‰ and 6.8‰. The δ18O of the water in equilibrium with phlogopites of the phlogopitites vary between 2.3‰ and 3.2‰, and those with magnetitite are 4.3‰ and 1.8‰. The δD of phlogopites of phlogopitites vary between −86.5‰ and −95.2‰, and those of magnetitite are −84.9‰, and −85.8‰. The δD of the water in equilibrium with phlogopites of the phlogopitites vary between −26.8‰ and −35.5‰, and with magnetitite are −25.6‰, and −26.26‰. During the hydrothermal event, magnetite phlogopitites were hydrothermally altered into phlogopite magnetitite, magnetite apatitite (epifoskorites), and mineralized with pyrochlore. At 84 Ma, the phlogopitic core was permeated by veins and micro-veins of Sr-rich norsethite carbonatites. At 83.5 Ma, calcitic carbonatites and micro-phlogopitites originating from the same mantle source as the magnetite phlogopitite intruded, which resulted in the formation of a carbonatite ring around the core and the assimilation of part of the phlogopitic core. This magmatism generated a new hydrothermal event of limited coverage that mineralized phlogopitites and carbonatites with monazite and massive pyrite ± chalcopyrite. The magmatism ended at 77 Ma, when phosphorous-saturated calcitic carbonatite surrounded the core. The saturation of phosphorous favored the crystallization of apatite, which generated phosphate deposits.

Petrogenesis of Late Cretaceous A1-type alkali monogenetic volcanoes, Wadi Natash, South Eastern Desert, Egypt, and their implications for Nb mineralization and tectonic evolution

Petrogenesis of Late Cretaceous A1-type alkali monogenetic volcanoes, Wadi Natash, South Eastern Desert, Egypt, and their implications for Nb mineralization and tectonic evolution

Mineralogical study on the distribution regularity of niobium in various types of ores in the giant Bayan Obo Fe-REE-Nb deposit

The Bayan Obo Fe-REE-Nb deposit is the largest rare earth deposit in the world, which also contains huge amounts of niobium resources, hosting 83% of industrial reserves in China. The previous researches focused on the REE mineralogy, mineralization age and REE source, whereas only a few studies paid attention to the genesis of Nb mineralization. The REE and Nb might present systematic mineralization difference as the degree of REE and Nb mineralization is distinct in different types of ores in host dolomite. Many complicated factors such as low grade, fine-grained dissemination and complex mineral composition are the main obstacles to hinder the effective utilization of Nb. In this study, a variety of niobium-bearing minerals have been identified, including aeschynite, fergusonite, columbite, pyrochlore, fersmite, baotite, and ilmenorutile. The distribution regularity of niobium in various types of ores in the giant Bayan Obo Fe-REE-Nb deposit is asserted. In massive Nb-REE-Fe ore and dolomite-type Nb-REE-(Fe) ore, niobium-bearing minerals are represented by columbite, while niobium mineralization in aegirine-type Nb-REE-Fe ore is characterized by pyrochlore and aeschynite. The major niobium-bearing mineral in the riebeckite -type Nb-REE-(Fe) ore and fluorite-type (banded) Nb-REE-Fe ore is aeschynite, and major niobium-bearing mineral in biotite-type Nb-REE-Fe ore and diopside-type niobium ore is fergusonite. The μ-XRF results and SEM observations display that columbite contains dolomite, magnetite and monazite inclusions and show obvious genetic links with apatite. Columbite, apatite and magnetite in massive Fe-REE-Nb ore and dolomite-type Nb-REE-(Fe) ore occur at same mineral generation, both having magmatic origins. Pyrochlore from aegirine-type Fe-REE-Nb ore also showsclose genetic relationship with apatite, which is formed by fractional crystallization during the magmatic stage of carbonatite. Aeschynite in fluorite-type (banded) Fe-REE-Nb ore, riebeckite-type Nb-REE-(Fe) ore, and fergusonite in biotite-type Fe-REE-Nb ore and diopside-type niobium ore are large in grain size and have hydrothermal origins. Moreover, intensive Nb mineralization in a series of ores related to fluoritization and alkali-metasomatism demonstrates the critical role of hydrothermal process in niobium enrichment in the Bayan Obo deposit. Fluid-rock interaction leads to the generation of alteration minerals, such as fluorite, biotite and aegirine, and contributes to a slash of K, Na and/or F in hydrothermal fluids, facilitating precipitation of niobium-bearing minerals. By using scanning electron microscope (SEM), X-ray energy dispersive analysis (EDS), Micro-XRF spectral analysis (μ-XRF) and TESCAN Intergrated Mineral Analysis (TIMA), detailed mineralogical study on niobium occurrence and typical niobium mineral compositions in various types of Fe-REE-Nb ores was conducted to provide an important reference for mineral processing and niobium prospecting and give a deeper understanding of the niobium anomalous enrichment mechanism of the Bayan Obo Fe-REE-Nb deposit.

The magmatic-hydrothermal transition in P-rich pegmatitic melts: Crystal-melt-fluid interactions recorded by phosphate minerals

Rare-element pegmatites are the crystallization product of melts strongly enriched in incompatible elements. Some of these elements, e.g., Li, B, P, F, and H2O, function as melt structure modifiers, allowing such melts to reach unusually low crystallization temperatures and regulating the saturation of aqueous-carbonic fluids from the melt. However, the timing of fluid saturation is highly debated, and the processes that mark the magmatic-hydrothermal transition in rare-element pegmatites are still elusive. The crystallization sequence of phosphate minerals can be used to assess the timing and nature of fluid-related processes in pegmatitic systems. In the Buranga pegmatite, western Rwanda, magmatic phosphates are replaced by (hydrous) alkali-rich phosphates and later by strongly hydrated phosphates as the dike cools. This reaction series indicates a disequilibrium in the crystal-melt-fluid system and records complex fluid-rock interactions during dike consolidation. This contribution compares these interactions with the fluid record preserved in fluid inclusions in the main phosphates of the crystallization sequence. Three major fluid inclusion types are observed, aqueous-carbonic (mostly-two-phase, L-V), carbonic-aqueous (two-phase, V-L, or three-phase, L-L-V), and crystal-rich inclusions (multiphase, L-V-Solids). Crystal-rich fluid inclusions in trolleite [Al4(PO4)3(OH)3], the last primary phosphate to precipitate from the melt, show a paragenesis of solid phases similar to the secondary phosphates observed in the dike. LA-ICPMS analyses of individual fluid inclusions show high concentrations of Na and K, with some content of Li, Rb, Cs, B, Fe, and Mn in the fluid. A similar composition is present in all inclusion types within the same mineral. Crystal-rich fluid inclusions demonstrate that during the magmatic-hydrothermal transition, primary phosphates react with the fluid in the presence of melt to generate secondary phosphates. No evidence of external fluid sources has been observed during the magmatic-hydrothermal transition. The fluid inclusion record shows that the fluid salinity is kept constant due to fluid-mineral interactions and corroborates the hypothesis that fluid-soluble elements are internally remobilized in the melt-crystal-fluid system during the magmatic-hydrothermal stage. These interpretations have implications for the general evolution of P-rich pegmatitic melts and the mineralization of Nb and Ta in these systems.

Magmatic and hydrothermal controls on diverse Nb mineralization associated with carbonatite-alkaline complexes in the southern Qinling orogenic belt, Central China

Abstract Although carbonatite-alkaline complexes are the primary source of the world’s niobium (Nb) supply, the mineralization style is largely variable in these complexes and the processes behind their formation are still poorly understood. Exemplifying with our new observations on the ~430 Ma Miaoya and Shaxiongdong carbonatite-syenite complexes in the southern Qinling orogenic belt, central China, show that disseminated Nb mineralization in these two deposits is pervasive throughout the entire complexes in both syenite and carbonatite. Both magmatic and hydrothermal processes have contributed to Nb mineralization in both deposits, despite differences in the mineralization style. The Nb-bearing minerals in the mineralized Miaoya syenites include magmatic U-poor pyrochlore, rutile, and ilmenite with minor amounts of columbite, and hydrothermal columbite and rutile, whereas those in the mineralized carbonatites are mainly magmatic U-poor pyrochlore, uranpyrochlore, U-rich betafite, and rutile with minor amounts of columbite, and hydrothermal columbite and rutile. On the other hand, the Nb-bearing minerals in the mineralized Shaxiongdong syenites include magmatic U-poor pyrochlore, titanite, rutile, and ilmenite, and hydrothermal fersmite, rutile, and ilmenite, whereas those in the mineralized carbonatites are mainly magmatic U-poor pyrochlore without any hydrothermal Nb-bearing minerals. Field observations, whole-rock chemical and Sr-Nd isotopic compositions strongly constrained that assimilation of U-rich rocks (e.g., the hosting Yaolinghe and Meiziya Groups) and magma differentiation are responsible for diverse magmatic Nb mineralization in the two deposits. On the other hand, the diverse assemblages of hydrothermal Nb minerals in Miaoya and Shaxiongdong are mainly controlled by variations in the nature of the fluids, which is constrained to be genetically related to ~220 and ~420 Ma hydrothermal events, respectively. In summary, both magma evolution (e.g., differentiation, assimilation) and late hydrothermal overprinting are responsible for the diverse Nb mineralogy in carbonatite-alkaline complexes, a situation that is commonly observed worldwide.