Nb-Ta Deposits Research Articles

Petrogenesis and tectonic setting of granitic pegmatites in the Beishan orogenic Belt, Northwest China: Evidence from geology, geochronology and geochemistry

The Weiboshan (Li-, Cs- and Ta-enriched) LCT-type Nb–Ta deposit located in Ejin Banner, Inner Mongolia, represents a newly discovered pegmatite-type rare metal deposit of significant interest. In this paper, we describe the petrography and present new mineralogical and whole-rock geochemical data and zircon U–Pb ages of ore-related granitic pegmatites in this deposit. Pegmatite dikes can be divided into three distinct zones on the basis of their contents: the K-feldspar graphic granite pegmatite zone (Kf), microcline graphic granite pegmatite zone (Mic), and albite–muscovite granite pegmatite zone (Ab). The pegmatite dikes and associated mineralization formed during the Mesozoic. Granite pegmatites in various zones display distinctive geochemical features characterized by high silicon contents, substantial alkali enrichment, and relatively low iron, calcium, and magnesium levels. These pegmatites are classified as peraluminous or quasialuminous and exhibit low total rare earth element concentrations. Notably, these pegmatites are enriched in large ion lithophile elements (LILEs), such as Rb, K, and U, but depleted in high field strength elements (HFSEs), such as P and Ti. The progression from the Kf to Mic to Ab zones signifies the thorough evolution of the granitic magma, where the late-stage near-hydrothermal system assumed a crucial role in the mineralization of rare metals during magma differentiation. The Weiboshan pegmatites, which exhibit syncollisional attributes, are A-type granites formed through crustal partial melting induced by decompression. This process occurred against the background of postorogenic extension driven by asthenospheric mantle upwelling resulting from lithospheric delamination due to crustal collision and thickening.

Magmatic Evolution and Nb-Ta Enrichment of Early Jurassic Granitic Porphyry from the Shangxiahu Nb-Ta Deposit of the Nanling Range

Magmatic Evolution and Nb-Ta Enrichment of Early Jurassic Granitic Porphyry from the Shangxiahu Nb-Ta Deposit of the Nanling Range

Age and petrogenesis of the Madi intrusion in the Huashi area, northern margin of the North China Craton: Implications for magma evolution and Nb–Ta mineralization

Although the mineralization of rare earth elements (REEs) and rare metals is intimately associated with the extreme fractionation of granitic magmas, the metallogenic intrusions of many granite-hosted Nb–Ta deposits have undergone fluid–melt interaction. Nevertheless, the precise mechanisms by which fluid–melt interaction influences mineralization remain poorly understood. The present investigation examines the issues of the fluid–melt interaction in highly fractionated granites with Nb–Ta mineralization, utilizing data from the newfound Huashi deposit in the northern margin of the North China Craton (NNCC). The Huashi Nb–Ta–Rb–Li deposit hosted in the Madi intrusion consists of two lithologies that have evolved continuously, namely medium–fine grained granite (MGG) in the lower section and alkali-feldspar granite (AG) at the top. The ages of the MGG and AG were determined using LA–ICP–MS columbite U–Pb dating, yielding values of 182.9 ± 1.7 Ma and 184.7 ± 1.3 Ma, respectively. The Madi intrusion has high SiO2, Al2O3, and total alkali contents, along with low CaO, MgO, MnO, and TFe2O3 contents and high Al2O3 / (CaO + Na2O + K2O) (A/CNK) values, classifying it as highly peraluminous granite with a high-K calc-alkaline affinity. Additionally, the intrusion also exhibits enrichment in Rb, U, Th, and Nb alongside significant depletion in Sr, Ba, Ti, Eu, and P, with a noticeable tetrad effect of REEs. The investigation of mica and feldspar minerals in the Madi intrusion using electron probe microanalysis (EPMA) indicates that the mica is mainly zinnwaldite, while the plagioclase belongs to albite. In summary, the Madi intrusion exhibits a highly I-type fractionated granite affinity. The extreme fractionation, intense fluid–melt interaction, and hydrothermal alteration of the intrusion contribute to the formation of the Huashi deposit.

Geochronology, mineralization and prospecting implication of the Paleoproterozoic Shaliuquan Nb-Ta pegmatite deposit in the northern Tibetan Plateau

A large number of Archean and Paleoproterozoic pegmatite rare-metal deposits exist worldwide, but few Precambrian pegmatite deposits have been reported in China, especially in the Tibetan Plateau. The Precambrian pegmatite deposits are still poorly studied. The Shaliuquan pegmatite, located in Quanji Massif, is a rare pegmatite-type Nb-Ta deposit in the northern Tibetan Plateau. Columbite-tantalite group minerals (CGMs) from three pegmatite samples give the U-Pb ages of 1844.2 ± 8.1 Ma, 1831.2 ± 12.2 Ma and 1848.0 ± 18.7 Ma, respectively. The U-Pb ages of CGMs show that the Shaliuquan pegmatite is a rare Paleoproterozoic pegmatite rare-metal deposit in the Tibetan Plateau. This is supported by the Precambrian 40Ar/39Ar plateau age of muscovites from two pegmatite samples (1094–1518 Ma), representing the metamorphic events after formation. As for the formation of Shaliuquan pegmatites, we argue that the hydrothermal processes in the late stage of the pegmatite evolution play a vital role for the enrichment and mineralization of Ta, which is evidenced by texture and compositions of CGMs and zircons. The formation of the Shaliuquan pegmatites is a response to the assembly of the Columbia supercontinent. The discovery of the Precambrian Shaliuquan pegmatite deposit implies that the Quanji Massif in the northern margin of the Qaidam Basin, which has an ancient mature metasediments basement and thick continental crust, has great potential for prospecting the Paleoproterozoic pegmatite rare-metal deposits.

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.

Mn–Fe-rich genthelvite from pegmatites associated with the Madeira Sn–Nb–Ta deposit, Pitinga, Brazil: new constraints on the magmatic-hydrothermal transition in the albite-enriched granite system

AbstractGenthelvite from pegmatite veins hosted by the albite-enriched granite (ca.1.8 Ga) corresponding to the Sn–Nb–Ta (F, REE, Li, Zr, U, Th) Madeira deposit, Amazonas, Brazil was studied. Genthelvite, the exclusive Be-bearing mineral within the deposit, occurs as massive crystals of up to 4.7 cm in size. Compositions are homogeneous within individual crystals, although there is moderate variation in the overall composition reflecting relatively limited substitutions within the helvine–genthelvite–danalite solid-solution series, with relatively high Zn contents (36.96 to 49.45 wt.% ZnO), lower Mn contents (0.61 to 3.03 wt.% MnO), and variable Fe contents (2.10 to 10.94 wt.% FeO), completing an existing compositional gap in this system. Genthelvite formed in an alkaline and subaluminous environment, under stable conditions within the late-evolved fluids, at relatively high temperature (>400°C), in a reducing environment. The extremely high concentration of fluorine in the magma and the crystallisation of magmatic galena resulted in an effective reduction of H2S fugacity. This resulted in the stabilisation of genthelvite during the transition from the late magmatic to early hydrothermal stages of the albite-enriched granite evolution. The variability in Fe content within genthelvite is associated primarily with localised variations in the mineral assemblage (e.g. the presence of riebeckite and polylithionite). Genthelvite was altered by low-temperature aqueous fluids rich in F which resulted in the incorporation of Fe, Mn, Mg, Pb, Ba, Na, K, U and REE into the Zn2+ structural site and the allocation of excess Si, Al, Ti and P in the IVSi and IVBe structural sites. The substantial content of U and REE substituting for Zn, together with Si substituting for Be, is charge balanced by the presence of vacancies at the A site.

Application of high-resolution laser multi collector ICP-MS U–Pb dating to columbite-group minerals with compositional zonation: reassessment of matrix effects among columbite-group minerals

Columbite-group minerals (CGMs) have been widely used in U–Pb geochronological analysis of granite and pegmatite Nb–Ta deposits due to their high U and low common Pb characteristics.

Rare-metal Be–Nb–Ta mineralisation by pegmatite remelting: Insights from Dakalasu deposit in the Chinese Altai orogenic belt

The rare metals Li, Be, Cs, Nb and Ta are mainly extracted from pegmatite deposits that may have formed through extensive fractional crystallisation or local partial melting of crustal rocks. There are a large number of rare-metal pegmatites that formed through crustal melting in the Chinese Altai, but what controls rare-metal enrichment remains enigmatic. Here, a new columbite UPb age of pegmatite mineralisation (247 ± 1 Ma) that is distinct from the ages of surrounding biotite granites (290–270 Ma) in the Dakalasu Be–Nb–Ta deposit in the Altai is reported. Zircon from the border zone with a medium to coarse-grained graphic texture of pegmatite yields two age groups: 290–270 Ma and 249 ± 7 Ma, corresponding to the age of biotite granite and the mineralisation age of pegmatite, respectively. The consistent age of the older group and biotite granite (290–270 Ma) and the elevated εHf(t) ratios and lower Nb/Ta ratios of pegmatite suggest that pegmatite is the end member of biotite granite after fractional crystallisation. The similar age of the younger group and mineralisation, the myrmekitic texture and albite trace elements of pegmatite imply pegmatite remelting and mineralisation occurred at ∼250 Ma. Our modelling agrees with geochronology, petrology and geochemistry that Be–Nb–Ta pegmatite mineralisation requires two melting events. The first step with anatexis of metasedimentary rocks generated biotite granitic melts whose extensive fractional crystallisation generated pegmatite with preliminary Be–Nb–Ta enrichment. The second step with pegmatite remelting (<5% degree) generated the significant enrichments of Be, Nb and Ta, which are sufficient for beryl and columbite crystallisation. The Be–Nb–Ta mineralisation mechanism through pegmatite remelting is likely common in orogenic belts with high-grade metamorphism and, therefore, should be considered in future explorations.

Contrasting Nb-Ta mineralization between the Mufushan and Lianyunshan granites, South China: Evidence from whole-rock and zircon geochemistry and geochronology

The Mufushan (MFS) and Lianyunshan (LYS) granitic complexes are located in northeastern Hunan (South China) and host important Nb-Ta mineralization. Fine-grained muscovite granites (FGMGs) occur as intrusions within the Lengjiaxi Group and show a spatial relationship with the ore-bearing pegmatites. Here, geochemical, geochronological, and isotopic investigations were carried out on the FGMGs of the MFS and LYS complexes to compare their magma sources and metallogenic potential. The MFS and LYS FGMGs are strongly peraluminous (A/CNK > 1.5) and highly fractionated S-type granites. Comparatively, the MFS FGMGs possess more negative Eu (Eu/Eu* = 0.19–0.60), Ba (3.3–360 ppm), Sr (6.2–156 ppm), and Zr (4–26 ppm) anomalies, and more positive Ta (1.1–15.8 ppm) anomalies than the LYS FGMGs, suggesting a stronger magma differentiation. In addition, the zircons from the MFS FGMGs exhibit higher contents of Hf (MFS = 34336 to 60010 ppm, LYS = 6795 to 28937 ppm), Nb (MFS = 16.49 to 401.56 ppm, LYS = 1.12 to 20.22 ppm), Ta (MFS = 33.45 to 138.65 ppm, LYS = 1.11 to 7.19 ppm), Th (MFS = 458 to 3630 ppm, LYS = 11 to 710 ppm), and U (MFS = 8400 to 30100 ppm, LYS = 1285 to 5220 ppm) than those from the LYS FGMGs, suggesting that the MFS FGMGs are more fractionated and their zircons have undergone hydrothermal alteration. Zircon U-Pb dating yielded concordant ages of 140.6 ± 0.5 Ma and 139.9 ± 0.7 Ma for the MFS and LYS FGMGs, respectively, implying an Early Cretaceous magmatic-hydrothermal event. Moreover, the geochemical and isotopic (whole-rock Sm-Nd and zircon Lu-Hf) compositions show younger sources for the MFS FGMGs compared to the LYS FGMGs, which were derived from the partial melting of Paleoproterozoic basement rocks with clay-rich material in a post-collision extensional setting. Our results, combined with previous studies, indicate that the studied FGMGs are the parental rocks of extensive Nb-Ta mineralization, and the highly fractionated MFS FGMGs are more conducive to hosting Nb-Ta mineralization than the LYS FGMGs. The strongly peraluminous and highly-fractionated S-type FGMGs can be used as good targets for further exploration of Nb-Ta deposits in South China and elsewhere.

Geochronology, geochemistry, and mineral chemistry of the Lingshan-Huangshan complex, South China: Insights into Nb and Ta enrichment

The Huangshan Nb-Ta deposit, located at the northern segment of the Qin-Hang metallogenic belt of South China, is closely associated with the Lingshan-Huangshan complex. As a large Nb-Ta deposit, with the Nb-Ta mineralization hosted by the late two phases of the Lingshan-Huangshan complex, the factors that led to Nb and Ta enrichment in the Huangshan deposit remain unclear. Therefore, we conducted a comprehensive investigation of petrography, geochronology, whole-rock geochemistry, and mineral compositions (zircon and mica) of the Lingshan-Huangshan complex. The Lingshan-Huangshan complex comprises the Lingshan coarse-grained porphyritic amphibole-biotite granite, the Huangshan coarse-grained biotite granite, the fine-grained albite-biotite granite, and the pegmatite from the center outwards. Zircon U-Pb dating shows that the different phases within this complex formed at 133.4–132.7 Ma. They are characterized by high SiO2 (66.46–79.11 wt%), total alkali (7.38–10.39 wt%) contents, moderate A/CNK values (average of 1.01), and high Ga/Al and Zr + Nb + Ce + Y values, presenting geochemical affinities to A-type granites. Combined with the pre-existing Nd-Hf isotopic data, they originated from the Proterozoic crust of South China with some input of juvenile crustal or mantle-derived materials. The whole-rock Al2O3, TiO2, and Fe2O3 contents and Nb/Ta and Zr/Hf ratios decrease with increasing SiO2 contents from the Huangshan coarse-grained biotite granite, the fine-grained albite-biotite granite to the pegmatite. These variations, together with the increasing Hf and Th + U + Y in zircon and increasing Rb, Nb, and Ta in mica, indicate that these phases (especially the late two) underwent highly fractional crystallization. The Lingshan coarse-grained porphyritic amphibole-biotite granite shares part of the whole-rock geochemical variations with the other phases, but its Nb/Ta ratios varied discordantly, which may be attributed to the solidified residual crystal-rich mush. The increasing F and Li in micas from the early to late phases of the Lingshan-Huangshan complex reflect enrichment of F and Li content during the magmatic evolution process, which is conducive to enhancing the solubilities of Nb and Ta in the melt. Accordingly, we suggest that the Nb-Ta enrichment in the Huangshan deposit is closely associated with highly fractionated magma and elevated volatile elements F and Li in melts.

The petrogenesis and mineralization of Zhaojinggou Nb–Ta deposit, Inner Mongolia: Evidence from geochronology, rock, and mineral geochemistry

The Zhaojinggou Nb–Ta deposit is one of large rare metal deposits newly discovered in the northern margin of the North China Craton in recent years. This paper reports petrography, petrochemistry, columbite‐group minerals U–Pb chronology study of the amazonite granitic pegmatite (AGP) exposed in this deposit, and composition of columbite‐group minerals and biotite are obtained by electron probe microanalyzer and LA‐ICP‐MS. Eighteen analyses of columbite‐group minerals yielded weighted mean 206Pb/238U age of 116.9 ± 1.4 Ma. The crystallization temperature of biotite is 630–650°C, and the oxygen fugacity is 10−17–10−18 bars. The biotite has low MgO contents and high Rb, Rb/Sr, and FeOT/(FeOT + MgO) ratios. The AGP has extremely low MgO, Cr, Co and Ni contents, with Nb/Ta ratios range from 1.63 to 9.05 and Rb/Sr ratios range from 303.30 to 648.90, and obvious ‘M’ type tetrad effect of rare earth element indicating that the formation of the AGP is related to crust‐derived magma. The contents of Nb2O5 and FeO decrease, while the Ta2O5 and WO3 contents, Mn# and Ta# values increase gradually from the core to the rim of columbite‐group minerals. Some columbite‐group minerals have unobvious oscillatory zoning, and some have a clear bright zoing with high Ta contents on the rim, indicating that the genesis of Zhaojinggou Nb–Ta deposit is mainly magmatic crystallization differentiation, accompanied by hydrothermal autometasomatism in the late stage.

Zircon geochronology, Hf isotope, and rare earth element geochemistry of the Apdong Nb-Ta deposit in North Korea

The Apdong Nb-Ta deposit is hosted within the syenite and in the Hoamsan area in the northern part of the Imjingang Belt in North Korea. Our new sensitive high-resolution microprobe zircon U-Pb results reveal two stages of Neoproterozoic to Ordovician inherited cores (887 Ma to 465 Ma) and Silurian emplacement and mineralization (425.5 ± 7.4 Ma) events. The zircon grains from the Apdong syenite show typical magmatic or textureless and sponge-like hydrothermal domains. These textural analyses supported the rare earth element characteristics obtained from the laser ablation-induced coupled plasma mass-spectrometry analyses (LA-ICP-MS). Neoproterozoic magmatic activities reportedly occur in the Pyeongnam Basin, Imjingang Belt [present study], Gyeonggi Massif, and Okcheon metamorphic belt in the central part of the Korean Peninsula and China (North China Craton, Qinling Orogenic Belt, and South China Craton). Notably, the Caledonian mineralization events in the southeastern part of the South China Craton coincided with emplacement and mineralization ages from the Apdong syenite (425.5 ± 7.4 Ma). The LA multi-collector ICP-MS Hf isotope of zircons from the Apdong syenite yielded positive initial εHf values plotted along the Neoproterozoic crustal evolution path, indicating that the host rock of Nb-Ta ore was formed from the pre-existing Neoproterozoic source. The source materials of Neoproterozoic intrusives in the central part of the Korean Peninsula were generally derived from Mesoproterozoic depleted mantle sources. Considering the previously reported Hf isotopic results, the involvement of pre-existing crustal materials is more prominent in the Neoproterozoic intrusives in the southern part of the central Korean Peninsula, i.e., the Gyeonggi Massif, than in the northern part of the central Korean Peninsula, such as the Imjingang Belt and Pyeongnam Basin in the northern part of the central Korean Peninsula.

Polyphase deformation-controlled giant Renli Nb–Ta deposit, South China: Constraints from systematic structural analysis

The Early Cretaceous Renli Nb–Ta deposit was recently discovered near the southern margin of the Late Jurassic–Early Cretaceous Mufushan composite granitic batholith, central Jiangnan Orogenic Belt, and represents the largest known rare metal deposit in South China. Substantial breakthroughs have been made in prospecting the pegmatite-dike-type of Nb–Ta mineralization in the deposit, especially from the perspective of geochemical exploration. However, there is still a lack of systematic structural studies in this area which has seriously hindered the in-depth understanding of the emplacement of pegmatite dikes and the distribution patterns of Nb–Ta ore bodies. In order to shed new light on this key issue, this study presents new and detailed structural data from the Mufushan region. The results indicate that the Renli and adjacent areas have experienced several deformational events, including D1 Neoproterozoic NE–SW contraction and upright-folding, D2 latest Late Jurassic to Early Cretaceous regional extension accompanied by granitic magma intrusion, and D3 Early Cretaceous brittle normal faulting and fracturing with granitic magma emplacement and Nb–Ta mineralization. The D1 event, corresponding to the Jiangnan Orogeny, formed regional-scale NW–SE-striking upright folds with ubiquitous subvertical S1 cleavages and subhorizontal L1 crenulation lineations. The D2 event represented orogenic exhumation and produced a domal structure cored primarily by gneissic/mylonitic rocks and granites and mantled by schists, which dominated the bulk structural architecture of the Mufushan area. This was followed by the D3 event which controlled the formation of numerous extension fractures in the study area. The Nb–Ta-bearing magmatic-hydrothermal fluids migrated along the pre-existing D2 shear zone and subsequently filled in the widespread D3 extension fractures to form pegmatite dikes. The D2 to D3 phases can be considered as a progressive exhumation event from plastic to brittle regimes. The D2 and D3 structures dictate the spatial distribution of Nb–Ta ore bodies.

Chemical and boron isotopic compositions of tourmalines from the Lianyunshan Nb-Ta pegmatite in northeastern Hunan, China: Insights into fluid and metallogenic sources

Although tourmaline is common in pegmatite-related Nb-Ta deposits and metasedimentary cover rocks in the Lianyunshan Complex of northeastern Hunan Province, South China, its fluid sources and genetic relationship to the host pegmatite remain unresolved. In this contribution, we chose the less-studied tourmaline minerals and analyzed their chemical (major, trace, and rare earth elements) and boron isotopic compositions to shed light on the process of pegmatite-related Nb-Ta mineralization. Lianyunshan tourmalines are both metasedimentary-hosted (Tourmalines I and II) and pegmatite-hosted (Tourmaline III), belong to the alkali group, have mainly schorlitic compositions, and contain dominantly hydroxyl anionic species. Element substitution is controlled by the activities MgFe-1, alkali-deficiency vector R1Al[Na(FeMg)]-1, AlO[R(OH)]-1, and □Al(NaR)-1. Lianyunshan tourmalines define a trend consistent with foitite to magnesio-foitite compositions. Lianyunshan tourmalines are poor in Fe3+, with a Fe3+/Fe2+ ratio of zero in all samples. This feature, as well as their non-alignment with the FeAl-1 vector, suggests precipitation from a reduced fluid. Fluid salinity may have been moderate to low given the low Na contents of all samples (0.60–0.66 apfu). Tourmaline I is rich in Mg and Ti, but the contents of these elements decrease in Tourmalines II and III. Fluid fractionation may have played a role in the decreasing Mg and Ti contents with corresponding enrichment in the contents of Fe, Na and X□ as Tourmalines II and III precipitated. Unusual enrichment of Li is found in the pegmatite-hosted Tourmaline III (mean 97 ppm) relative to Tourmalines I and II (means ∼ 31–33 ppm), and of Sn in Tourmaline II (mean 291 ppm) relative to Tourmalines I and III (means ∼ 3–10 ppm). δ11B values are similar for Tourmalines I (–14.21 to –12.09 ‰), II (–15.07 to –13.71 ‰), and III (–14.48 to –13.55 ‰), being consistent with the δ11B of typical S-type granites (∼−16 to −8 ‰) such as the S-type muscovite monzogranite host of the Lianyunshan tourmalines. We conclude that the Lianyunshan tourmalines precipitated from an evolved melt that formed muscovite monzogranite.

Carbonation and serpentinization of diopsidite in the Altun Mountains, NW China.

Mineral carbonation of mafic-ultramafic rocks has been highlighted as a promising way for permanent carbon capture and storage. Carbonatization involves the release of Ca, Mg and Fe from silicate minerals by dissolution and reaction in the aqueous phase to form stable carbonate minerals. Diopside is one of the most abundant mafic minerals in the lithosphere and contributes a portion of Mg and Ca to surface weathering. Here, we present detailed processes of the carbonation-coupled serpentinization of diopsidite from the Yushishan Nb-Ta deposit in the Altun Mountain, northwest China. Diopsidite is the prograde metamorphic product of siliceous dolomitic marble by full decarbonation process. Retrograde serpentinization and carbonation of diopsidite lead to the addition of CO2, H2O, light rare earth elements and fluid-mobile elements but the loss of SiO2. The diopsides are replaced by calcite and chrysotile by mineral alteration to form pseudomorphic textures. Dissolution-precipitation processes significantly affect diopside serpentinization and carbonation. The carbonation of diopside-rich rocks may be suitable for permanent CO2 storage.

Ultrastructure of a Columbite-Tantalite Mineral from the Zhaojinggou Ta-Nb Deposit in the North China Craton: Direct Evidence of the Formation Mechanism of the Columbite-Group Minerals

The formation of columbite-group mineral phases in peraluminous granite has not been demonstrated to date. Here, a nanoscale study of the columbite-tantalite mineral in the Zhaojinggou Nb-Ta deposit in North China Craton elucidated its formation mechanism and the role of fluids in Nb-Ta mineralization. Transmission electron microscopy (TEM) analysis of a focused ion beam cut of the columbite-tantalite mineral revealed a comparatively well-ordered mineral structure. Energy-dispersive X-ray spectroscopy (EDS) revealed the presence of Nb, Ta, Mn, W, Fe, Sn, and Pb in the columbite-tantalite mineral. Furthermore, detailed TEM images depicted the nanoscale hydrothermal fluid occurring within the columbite-group mineral grain as well as between columbite-tantalite mineral and quartz grains. K, Al, Si, and O were found to be enriched in the hydrothermal fluid that was present between the quartz grains and the columbite-tantalite mineral. It did not react with the mineral grains of the columbite group. The ultrastructure of the columbite-tantalite mineral suggested the columbite-group mineral in the Zhaojinggou Nb-Ta deposit formed during magmatic crystallization rather than from hydrothermal fluids. Furthermore, HR-TEM images provided the first nanoscale observations of the fluid-mediated mineral dissolution and amorphous phase formation. This study also revealed that the mineral dissolution, element transport, and reprecipitation were significantly influenced by the fluid amorphous phase in the Nb-Ta deposits.

Prospectivity Mapping of Heavy Mineral Ore Deposits Based upon Machine-Learning Algorithms: Columbite-Tantalite Deposits in West- Central Côte d’Ivoire

This study aimed to model the prospectivity for placer deposits using geomorphic and landscape parameters. Within a geographic information system (GIS), spatial autocorrelation analysis of 3709 geochemical samples was used to identify prospective and non-prospective targets for columbite-tantalite (Nb-Ta) placer deposits of Hana-Lobo (H-L) Geological Complex (West- Central Côte d’Ivoire, West Africa). Based on mineralization system analysis, hydrologic, geomorphologic and landscape parameters were extracted at the locations of the identified targets. Supervised automatic classification approaches were applied, including Random Forest (RF), K-Nearest Neighbors (KNN) and Support Vector Machines (SVM) to find a prospectivity model complex enough to capture the nature of the data. Metrics such as cross-validation accuracy (CVA), Receiver Operating Characteristic (ROC) curves, Area Under Curve (AUC) values and F-score values were used to evaluate the performance and robustness of output models. Results of applying machine-learning algorithms demonstrated that predictions provided by the final RF and KNN models were very close (κ = 0.56 and CVA = 0.69; κ = 0.54 and CVA = 0.68, respectively) and those provided by the SVM models were slightly lower with κ = 0.46 and CVA = 0.63. Independent validation results confirmed the slightly higher performance of both KNN and RF prospectivity models, compared to final SVM. Sensitivity analyses of both KNN and RF prospectivity models for medium and high-grade Nb-Ta deposits show a prediction rate of up to 90%.

Continental-scale distribution of niobium and tantalum in catchment sediments throughout China: Prospecting implications from the China geochemical Baselines project

Niobium (Nb) and tantalum (Ta), which are chemical twins with very similar geochemical properties, are critical mineral resources for emerging global industries. Owing to increasing demands, understanding their concentrations and distributions in the pedosphere is essential for assessing potential Nb and Ta resources in China. To address this, we used geochemical data obtained from the China Geochemical Baselines (CGB) project, which began in 2008 and has since then yielded continental-scale datasets. With a density of approximately-one sampling site per 3,000 km2, the CGB project involved catchment sediments from two horizons at each site: a top sample collected at a depth of 0–25 cm and a deep sample collected at a depth greater than 100 cm. In total, 3,394 samples each of top and deep catchment sediments were collected, and Nb and Ta concentrations were measured using inductively coupled plasma mass spectrometry under strict quality control conditions. The Nb concentrations in the top and deep sediment samples varied in the ranges 0.04–89.86 ppm with a median value of 12.71 ppm and 1.20–104.12 ppm with a median value of 12.41 ppm, respectively. Further, the Ta concentrations in the top samples ranged from 0.01 to 26.41 ppm with a median of 0.97 ppm, while those in the deep samples ranged from 0.14 to 26.41 ppm with a median of 0.96 ppm. Furthermore, both the top and deep samples had similar 2.5th, 25th, 50th, 75th, and 97.5th percentile Nb and Ta contents, indicating that Nb and Ta in the catchment sediments underwent similar and consistent formation processes. Our results also indicated that the concentrations of both elements varied widely across regions, and their distributions in the catchment sediments throughout China were predominantly governed by the parent rock, particularly granites and alkaline rocks. Mineralization and sedimentation properties also played roles at a regional scale. Overall, the spatial distributions of Nb and Ta in the top and deep catchment sediments were relatively consistent, with concentrations being higher in southeastern China and lower in northwestern China. Twelve large-scale geochemical Nb-Ta belts were delineated based on continuous anomalous point distributions wherein many well known Nb-Ta deposits are already located. These nationwide Nb and Ta concentrations and distributions provide foundational large-scale data for further exploration and geological studies.

SOURCES AND MECHANISMS OF FORMATION OF ALKALINE RARE-METAL GRANITES AT THE ZASHIKHINSKY MASSIF BASED ON GEOCHEMICAL AND Nd ISOTOPE DATA

The intraplate alkaline-granite magmatism essentially contributes to formation of rare-metal strategic raw materials. In the Major Sayan Fault of the East Sayan Mountains, the rocks of the Zashikhinsky (Pz3) massif were studied through the isotope-geochemical analysis to identify probable sources of alkaline-granite magma and mechanisms of their evolution resulting in ore accumulations, up to the formation of Nb-Ta deposits. The Nd isotopic characteristics of its alkaline granites were obtained for the first time. Together with the results of mineralogical and geochemical studies, they were applied for modeling its formation, in which crystallization differentiation of alkaline granite melts proceeds simultaneously with their assimilation of the enclosing granite-metamorphic formations.

Apatite U-Pb Dating and Composition Constraints for Magmatic–Hydrothermal Evolution in the Giant Renli Nb-Ta Deposit, South China

Apatite is a nearly ubiquitous accessory phase in igneous rocks that crystallizes during the entire magma evolution process and has great implications for geochronology and petrogenesis. Previous studies suggested that Nb-Ta mineralization in the giant Renli deposit was genetically related to Late Jurassic two-mica monzogranite or Early Cretaceous muscovite monzogranite. Moreover, the magmatic–hydrothermal evolution of these two stages is poorly understood. In our study, we confirm that the muscovite monzogranite, biotite monzogranite, and two-mica monzogranite are all spatially associated with Nb-Ta pegmatites. We present new apatite U-Pb ages to constrain the timing of Nb-Ta mineralization and related magmatism. The results show that apatite from the two-mica pegmatite yield a lower intercept age of 130 ± 2 Ma (2σ), and apatite grains from two two-mica pegmatite samples yield a lower intercept age of 135 ± 8 Ma (2σ) and 134 ± 3 Ma (2σ), respectively. Apatite and whole-rock geochemistry suggest the oxidation degree of the Nb-Ta mineralization increases from north (RL-6) to south (RL-16) in the giant Renli deposit. This study demonstrates that a combination of apatite composition and U-Pb ages can be used to constrain the magmatic–hydrothermal evolution of granite and pegmatite-type Nb-Ta deposits.