Nb Deposit Research Articles

Stable and radiogenic isotope constraints on the magmatic and hydrothermal evolution of the Nechalacho Layered Suite, northwest Canada

The agpaitic nepheline syenites of the Nechalacho Layered Suite represent the youngest intrusive phase of the Blatchford Lake Igneous Complex, Canada, and host hydrothermally altered eudialyte-rich layers that constitute one of the world's largest rare earth element (REE) and niobium deposits. Three distinct peripheral rare metal pegmatites, the T-, R- and S-Zones, contain high-grade Li, Be, REE and Nb deposits. Based on stable and radiogenic isotope data, we constrain the age of the Nechalacho Layered Suite, identify its magmatic source and interpret the hydrothermal alteration that modified the ore and gangue mineralogy. In situ Laser ICP-MS U-Pb dating of igneous zircon yielded a concordia age of 2176.0±2.7Ma for the Nechalacho Layered Suite. Initial εNd2176Ma between 1.1 and 3.0 and averaging 2.0 indicates a derivation from mantle magma that was isotopically similar to modern ocean island basalt. The δ18Omelt values calculated for magmatic aegirine, annite and zircon at 700°C range from 5.1 to 7.8‰; δDmelt is −60‰. For the earlier alkaline granite and quartz syenites of the Blatchford Lake Igneous Complex (Grace Lake Granite, Thor Lake Syenite, Rim Syenite), δ18Omelt calculated for fayalite, riebeckite, richterite and quartz ranges from 6.7 to 8.1‰ and δDmelt from −99 to −93‰. Together with the Nd isotope ratios, this indicates a common, deep-seated feeder system of mantle magma that evolved rapidly to generate granites, quartz syenites, and finally nepheline syenites, with little or no crustal contamination. The Nechalacho Layered Suite exsolved a late-magmatic aqueous-carbonic fluid that caused auto-metasomatism associated with ankerite-bastnäsite-(Ce) mineralization, producing hydrothermal carbonates with δ13C from −6.3 to −2.7‰ and δ18O from 11.3 to 15.0‰. Similar ratios previously reported for ankerite from the T-Zone demonstrate a genetic link to the pegmatites at the magmatic-hydrothermal interface. Hydrothermal monazite-(Ce) from the Basal Zone of the Nechalacho deposit yielded a U-Pb concordia age of 1871±14Ma and 208Pb/232Th ages of 1852±14, 1835±57 and 1854±27Ma. This indicates monazite formation during a late regional hydrothermal event associated with faulting and emplacement of the Compton Intrusive Suite along the southern margin of the Slave Craton. The temperature determined for this late overprint using oxygen isotopes of quartz and magnetite is ~300°C and the fluids had δ18OH2O of 3.1–9.3‰ and δD of −24‰, indicating the presence of a regionally pervasive amagmatic crustal fluid.

Genesis of the Bayan Obo Fe–REE–Nb deposit: Evidences from Pb–Pb age and microanalysis of the H8 Formation in Inner Mongolia, North China Craton

The Bayan Obo Fe–REE–Nb deposit is a world-largest REE deposit in Inner Mongolia, North China Craton. It also contains large Fe and Nb reserves. The REE ore body is hosted in H8 dolomite of the Bayan Obo Group. Depositional time and genesis of H8 dolomite are still controversial in addressing the genesis of the REE deposit. In this study, Pb–Pb age and geochemical features of carbonate minerals in H8 dolomite have been studied, providing constraints to genesis of H8 dolomite and the REE deposit. Results of Pb–Pb dating from unmetamorphosed and non-mineralized domains of the H8 dolomite well constrain the depositional age as 1619±150Ma, which is earlier than both REE ore and carbonatite dykes in the Bayan Obo region. This indicates that REE mineralization has characteristics of epigenetic origin. Geochemical data of carbonate minerals in H8 dolomite have low REE contents, distinctly distinguished from those carbonatitic dykes. However, the fine-grained H8 dolomite whole rocks have high REE contents, similar to those carbonatitic dykes. Mineral analysis suggests a close relationship between REE mineralization and calcite carbonatite dykes and related-derived fluids, which could transport a large amounts of REE. Integrated with these new geochronological and geochemical data, we draw conclusion that the Bayan Obo Group was a Proterozoic depositional succession, REE mineralization is as result of sedimentary carbonate rocks once being metasomatised by fluids derived from regional REE-rich calcite carbonatitic magma at depth.

New data of the Bayan Obo Fe–REE–Nb deposit, Inner Mongolia: Implications for ore genesis

The Bayan Obo deposit in the Inner Mongolia, North China Craton, is the largest rare-earth element (REE) resource in the world. The deposit is hosted in Mesoproterozoic sediments of the Bayan Obo Group, the lithological sequence of which has been divided into seventeen units (H1–H17). The REE mineralization is mainly concentrated in the H8 dolomite unit, and therefore the depositional age of this unit is important in addressing the debate over the genesis of the REE–Fe–Nb ore deposit. Here we present U–Pb and Hf model ages of zircons from the adjacent H9 slate. Our data suggest that the H9 slate formed at 1505±12Ma and the H8 dolomite was deposited during the Mesoproterozoic. We also present Ar–Ar ages of riebeckite in veins cutting the H8 dolomite and biotite in the ore bodies which show 389.5±3.0Ma and 289.1±1.8Ma, respectively. The U–Pb measurements from overgrowth zircon in H9 slate give an age of 518.8±7.5Ma. These new age data indicate that the Bayan Obo deposit was disturbed during thermal events at 520Ma to 390Ma, generating a second phase of REE–Nb mineralization. The δ7Li values of the REE mineralized dolomite are similar to those of carbonatite dykes. Combined with ɛNd features, we provide geochemical evidences that REE mineralized dolomite has undergone fluid interaction. Results reveal that the carbonatite magmatism occurred within the Bayan Obo rift at ca. 1.2 Ga, provided the primary source for REE–Nb mineralization or to fertilize the Bayan Obo crust, and the later was remobilized during the Caledonian to generate the second phase of mineralization within a convergent plate margin regime.

The Paleoproterozoic Montviel carbonatite-hosted REE–Nb deposit, Abitibi, Canada: Geology, mineralogy, geochemistry and genesis

The Montviel 250 Mt carbonatite-hosted REE–Nb deposit is hosted in a Paleoproterozoic alkaline suite located in the Sub-Province of Abitibi, in the Archean Province of the Superior. The alkaline intrusion consists of biotite clinopyroxenites, melano- to leucosyenites, a melteigite–ijolite–urtite series, riebeckite granite, a series of carbonatites and a carbonatite polygenic breccia. The carbonatite series includes silicocarbonatites, calciocarbonatites, rare magnesiocarbonatites, ferrocarbonatites and mixed carbonatites and are cut by a late, high-energy carbonatite polygenic breccia. Diamond drill hole assays and microscope observations indicate that Nb is hosted in pyrochlore from silicocarbonatite whereas the REE mineralization is mainly hosted in ferrocarbonatite, late mixed carbonatites and polygenic breccia, in REE-bearing carbonates and fluorocarbonate minerals. Diamond drill hole underground mapping and systematic assays have shed light on zones enriched in Nd and LREE with preferential Ba and Sr hydrothermal precipitation and zones enriched in Dy, Y and HREE displaying preferential F and P bearing hydrothermal precipitation. Petrographic observations, electron microprobe analyses, LA-ICPMS and X-ray diffraction were used to study the mineralization processes and to identify and quantify the REE-bearing burbankite–(Ce), carbocernaite–(Ce), ewaldite–(Y), huanghoite–(Nd), cordylite–(Ce), cordylite–(Nd), kukharenkoite–(Ce) and synchysite–(Ce). Most minerals are enriched in total LREE with values around 19.3wt.%, have total MREE values around 2.2wt.% and extremely variable total HREE values, with very high contents of Dy and Y averaging around 0.3wt.% and 1.0wt.%, respectively, and with total HREE reaching up to 10.0wt.%. A paragenetic sequence is proposed that consists of: (1) a silicocarbonatite Nb stage, and (2) a calciocarbonatite stage, dominated by magmatism but accompanied by hydrothermal fluids, (3) a main ferrocarbonatite stage, dominated by episodes of Ba- and Sr-hydrothermalism and LREE mineralization, F- and P-hydrothermalism and HREE mineralization and evolved ferrocarbonatitic magmatism, (4) a renewed, mixed carbonatite magmatic stage with minor but increasing hydrothermalism, and (5) a terminal stage of fluid pressure buildup and explosion, leading to the creation of a HREE-enriched polygenic breccia. Globular melt inclusions of Ba–Cl–F (±Si–O) may indicate the presence and contribution of barium-bearing chlorofluoride melts during hydrothermal activity and mineralization of the carbonatite.

Geology, market and supply chain of niobium and tantalum—a review

Tantalum (Ta) and niobium (Nb) are essential metals in modern society. Their use in corrosion prevention, micro-electronics, specialty alloys and high-strength low-alloy (HSLA) steel earns them a strategic designation in most industrialised countries. The Ta market is unstable due in part to historic influx of ‘conflict’ columbite-tantalite concentrate, or “Coltan,” that caused Ta mines in Australia and Canada to be placed on care and maintenance. More recently, the growing appetite of modern society for consumer goods made of ‘conflict-free’ minerals or metals has put pressure on suppliers. Pegmatites, rare-element-enriched granites, related placer deposits and weathered crusts overlying carbonatite and peralkaline complexes account for the majority of Ta production. Several carbonatite-related deposits (e.g. Upper Fir and Crevier, Canada) are being considered for potential co-production of Ta and Nb. Pyrochlore (Nb–Ta), columbite-tantalite (Nb–Ta), wodginite (Ta, Nb and Sn) and microlite (Ta and Nb) are the main ore minerals. Approximately 40 % of Ta used in 2012 came from Ta mines, 30 % from recycling, 20 % from tin slag refining and 10 % from secondary mine concentrates. Due to rapid industrialisation and increased use of Nb in steel making in countries such as China and India, demand for Nb is rising. Weathered crusts overlying carbonatite complexes in Brazil and one hard rock carbonatite deposit in Canada account for about 92 and 7 % of Nb world mine production, respectively. Since the bulk of the production is geographically and politically restricted to a single country, security of supply is considered at risk. Other prospective resources of Nb, beside carbonatites and associated weathered crusts, are peralkaline complexes (e.g. Nechalacho; where Nb is considered as a potential co-product of REE and zirconium). Economically, significant deposits of Ta and Nb contain pyrochlore, columbite-tantalite, fersmite, loparite and struverite. Assuming continued elasticity of Ta and Nb prices and that the law of the supply and demand applies, new sources of these metals can be developed. In the long term, there is no need to worry about Ta and Nb availability. Temporary disruptions in Ta and Nb supply are possible and could be difficult to cope with, so new sources of supply may be developed to diversify geographic sources of supply for strategic reasons.

Carbonatite-hosted niobium deposit at Aley, northern British Columbia (Canada): Mineralogy, geochemistry and petrogenesis

The Aley Nb deposit in northern British Columbia, Canada, is hosted by metamorphosed calcite and dolomite carbonatites of anorogenic affinity emplaced in Lower Paleozoic sedimentary carbonate rocks in the Devonian. Primary Nb mineralization consists of pyrochlore (commonly comprising a U–Ta-rich and F-poor core) and ferrocolumbite developed as discrete crystals and replacement products after the pyrochlore. These phases and associated heavy minerals (apatite±magnetite±zircon±baddeleyite) precipitated early in the magmatic history and probably formed laterally extensive cumulate layers up to at least 1.5m in thickness. Fractionation of copious amounts of pyrochlore is reflected in the chemical composition of the carbonatites and their constituent minerals, which show large variations in Nb/Ta value, but a near-chondritic Zr/Hf ratio. Alkali-rich metasomatic rocks (in particular, fenites and glimmerites) associated with the carbonatites are barren; the bulk of Nb in these rocks is contained in rutile, phlogopite and, to a much lesser extent, amphibole. When the passive margin of North America became the zone of plate convergence in the Cretaceous, the host carbonatites were strongly deformed, which is manifested in structures and textures indicative of grain comminution, ductile flow, folding and, locally, brecciation. The structure and continuity of the cumulate units enriched in Nb minerals were profoundly affected by these processes. Interaction of the carbonatites with crustal fluids of complex chemistry resulted in extensive dolomitization, replacement of the pyrochlore and ferrocolumbite by fersmite, and development of hydrothermal parageneses consistent with the lower greenschist-facies conditions. At these late evolutionary stages, Nb was mobilized only to a very limited extent and sequestered in a variety of minerals (fersmite, euxenite, Mg-rich ferrocolumbite and Nb-bearing rutile) typically occurring as scarce minute crystals associated with hydrothermal dolomite, quartz and chlorite. Progressive enrichment of the deformed dolomite carbonatites in heavy C and O isotopes relative to primary calcite, coupled with changes in the trace-element composition of Nb phases, indicate that the fluids were equilibrated with the wall-rock sedimentary rocks hosting the Aley deposit and were capable of transporting F−, (PO4)3−, U, Th and rare-earth elements, but not Nb.

Carbonatitic melts in olivine and magnetite from rare-metal carbonatite of the Belaya Zima alkaline carbonatite complex (East Sayan, Russia)

The problem of the origin of carbonatite is still a priority in Russian and foreign publications. Attention to these rocks is mostly explained by their raremetal specialization and links of Nb, Ta, Zr, Sr, Ba, and REE deposits to them. In addition to the hypothesis of the magmatic origin of carbonatites, currently some authors consider these rocks as products of hydrother� mal-metasomatic activity. This contradiction may be solved by study of the inclusions of mineralforming media in minerals, which provide direct evidence for natural melts or fluids captured in host minerals. In spite of the active investigation of melt inclu� sions in minerals from rocks of the alkaline carbonatite complexes over the last decade, there are still some data including the quantitative characteristics of the chemical composition of melts for carbonatites, as well as on other associated nonsilicate (fluorite, apa� tite, etc.) rocks (1-6). Only a few publications con� cern carbonatites properly (3, 5, 6). However, estima� tion of the role of melts and their compositions is of great importance for understanding of the origin of alkaline carbonatite complexes including the pro� cesses and conditions of accumulation of commercial minerals in them. In this paper, we report the results of the study of the melt and crystalline inclusions coexisting in min� erals of orebearing forsterite-calcite carbonatite from the Belaya Zima massif (East Sayan), which allowed us to distinguish the carbonatitic melts responsible for the formation of these rocks for the first time. A number of complex (Nb, Ta, REE, U, Pb, Zn, and P) deposits including the largest Belaya Zima nio� bium deposit related to the carbonatite massif of the same name are distinguished within the East Sayan province of ultrabasic alkaline rocks and carbonatites. Based on the data of geochronological U-Pb study of zircons, the age of this deposit is 643 Ma (7). The Belaya Zima (Nizhnesayanskii) massif is located in the northern foothill of East Sayan, in the upper reaches of the Belaya Zima River. The massif occurs in the zone of large faults of northwestern orientation, in the area of the junction of the Siberian Platform and folded complexes of East Sayan. In plan, it has a pearlike shape and spreads to the northwest for 8.5 km includ� ing a branch elongated in the same direction (8). The central part is composed of carbonatites of various types. Ultrabasic alkaline rocks form a semiring zone surrounding the stock along the periphery.

Primary and secondary niobium mineral deposits associated with carbonatites

This work reviews the character and origin of primary and supergene economic deposits of niobium associated with carbonatites. The Brazilian supergene deposits account for about 92% of the total worldwide production of Nb, with the primary St. Honoré carbonatite and other sources accounting for only for 7 and 1%, respectively. The emphasis of the review is upon the styles of Nb mineralization and the geological factors which lead to economic concentrations of Nb-bearing minerals. Primary economic deposits of Nb are associated principally with carbonatites found in diverse types of plutonic alkaline rock complexes. Primary magmas are principally those of the melilitite, nephelinite and aillikite clans. Although many primary niobium deposits are associated with carbonatites, ijolites and syenites in the same alkaline complexes can also contain significant Nb mineralization in the form of niobian titanite and diverse Nb–Zr-silicates (marianoite-wöhlerite); these potential sources of Nb have not as yet been explored or exploited. Primary Nb deposits can be regarded as large tonnage, low grade (typically <1wt.% Nb2O5) disseminated ore deposits. Niobium is hosted principally by diverse Na–Ca–U-pyrochlores, ferrocolumbite and fersmite. Every actual, and potential, primary Nb deposit is unique with respect to the varieties of pyrochlore present; extent of replacement by other minerals; and degree of alteration by deuteric/hydrothermal fluids. Within a given occurrence individual petrographically-defined units of carbonatite contain distinct suites of pyrochlore. Bulk rock analysis for Nb gives no indication of the style of mineralization and provides no information of use regarding beneficiation of the ore. Evaluation of any Nb deposit requires extensive definition drilling and detailed mineralogical studies. Primary Nb deposits result from the early crystallization of Nb-bearing minerals in magma chambers followed by crystal fractionation, magma mixing, and redistribution of Nb-minerals by density currents. Supergene Nb deposits occur in laterites formed by extensive weathering of primary carbonatites. The process results in the decomposition of apatite and magnetite, removal of soluble carbonates and physical concentration of resistant primary pyrochlore. Intense lateritization results initially in the replacement of primary pyrochlores by supergene, commonly Ba, Sr, K or Pb-bearing pyrochlores, and ultimately complete decomposition of pyrochlore and formation of Nb-bearing rutile, brookite, and anatase. The Nb contents of the laterites can be enriched up to 10 times or more above those of the primary carbonatite. Commonly, pyrochlores in laterites are fine grained and intimately intergrown with hematite, goethite and minerals of the crandallite group. The different styles of mineralization of primary and secondary Nb deposits require different methods of ore beneficiation.

A review of the genesis of the world class Bayan Obo Fe–REE–Nb deposits, Inner Mongolia, China: Multistage processes and outstanding questions

The Bayan Obo Fe–REE–Nb deposit is the world's largest rare earth element (REE) resource and with the increasing focus on critical metal resources has become a focus of global interest. The deposit is hosted in the Palaeoproterozoic Bayan Obo Group, mainly concentrated in the H8 dolomite marble. The ores consist of light REE enriched monazite and bastnäsite, with a wide array of other REE minerals. Niobium mineralisation is hosted primarily in aeschynite and pyrochlore, although there are a wide range of other Nb-minerals. The origin of the host dolomite and ore bodies has been a subject of intense debate. The host dolomite has been proposed to be both of sedimentary origin and an igneous carbonatite. Carbonatite dykes do occur widely in the area, and consideration of the textural, geochemical and isotopic composition of the dolomite suggests an origin via intrusion of magmatic carbonatite into meta-sedimentary marble, accompanied by metasomatism. The origin of the ore bodies is complex, indicated most strongly by an ~1Ga range in radiometric age determinations. Compilation of available data suggests that the ores were originally formed around 1.3Ga (Sm–Nd isochron ages; Th–Pb ages of zircon), close in time to the intrusion of the carbonatite dykes. The ores were subsequently subjected to several stages of deformation and hydrothermal overprint, culminating in deformation, metamorphism and fluid flow related to the Caledonian subduction of the Mongolian Plate under the North China Craton from ~450 to 420Ma (Th–Pb ages of monazite). This stage resulted in the formation of the strong foliation (‘banding’) of the ore. The presence of undeformed veins with alkali mineral fills, and the overprinting of the foliation by Nb minerals suggest that secondary fluid flow events may also have contributed to the metal endowment of the deposits, as well as remobilising the original Fe and REE mineralisation. The alteration mineralogy and geochemistry of the ores are comparable to those of many REE mineralised carbonatites. Initial Nd isotope ratios at 450Ma, however, suggest crustal sources for the metals. These conflicting lines of evidence can be reconciled if a (at least) two stage isotopic evolution is accepted for the deposits, with an original mantle-sourced, carbonatite-related metal accumulation forming around 1.3Ga with εNd close to 0. The ore was remobilised, with associated re-equilibration of Th–Pb isotope systematics during deformation at ~450Ma. A further stage of alkaline hydrothermal fluid was responsible for Nb mineralisation at this stage. The complex geological history, with multiple stages of alkaline, high field strength element-rich, metasomatic fluid flow, is probably the main reason for the exceptional metal endowment of the Bayan Obo area.

Applicability of handheld X-Ray fluorescence spectrometry in the exploration and development of carbonatite-related niobium deposits: a case study of the Aley Carbonatite, British Columbia, Canada

This study evaluates the suitability of portable (handheld) X-Ray fluorescence spectrometry (pXRF) in the exploration for Aley-type ‘hard-rock’ (primary) carbonatite-hosted Nb deposits. The assessment consists of comparisons between: (1) results of pXRF analyses on selected pulp samples and results of analyses of the same pulps using traditional laboratory methods; (2) results of averaged, multiple pXRF spot field analyses performed directly on 10 to 15 cm long pieces of core (before pulverization) compared with those of traditional laboratory analyses of the same pieces of core after pulverization; and (3) results of a manual core scanning method compared with the results of conventional analytical methods of the pulps of the corresponding scanned sections. A strong correlation exists between pXRF measurements on pulps and laboratory methods for most specialty metals, such as Nb (r 2 = 0.99), La (r 2 = 0.97), Ce (r 2 = 0.67), Y (r 2 = 0.93), and P (r 2 = 0.89); however, the values of r 2 for Pr and Nd are 0.19 and 0.38, respectively. As expected, textural heterogeneities within sample intervals reduced the quality of pXRF results when multiple spot readings were taken directly on the core. Nevertheless, the data can still be used to identify carbonatite-related Nb (± other specialty metal mineralization) and delimitate potentially economically significant zones within it. The core scanning reduced the degree of variation associated with spot analyses. Scanning is useful during the early exploration stages, but provides data limited by the inability of the operator to maintain constant scanning speed. The scanning results correlate with laboratory methods for Nb (r 2 = 0.88), Th (r 2 = 0.80), Fe (r 2 = 0.84), Sr (r 2 = 0.74), Ba (r 2 = 0.73), Y (r 2 = 0.59), and Zn (r 2 = 0.75). The values of r 2 for La, Ce, Pr, and Nd were only 0.31, 0.26, 0.01 and 0.03, respectively, suggesting that concentrations of these elements were too low, and/or that the light rare earth elements (LREEs) were present not only in the crystal structure of fersmite, pyrochlore and apatite, but also in minor or accessory minerals such as REE-bearing fluorocarbonates or zircon erratically distributed throughout the core. Portable XRF is a robust tool facilitating exploration-related decision-making in the field, assuming that elements of interest such as Nb are present in concentrations within the analytical range of the instrument. The pXRF core scanning reduces the need for sample preparation (no pulps) and can be done directly on the drill-site, but the precision and accuracy of the data are reduced relative to laboratory and pXRF pulp analyses. The multiple spot analyses (no pulps) approach is good for instant verification of unknown, potentially ore-bearing minerals and for analysing discrete homogeneous features, layers, veins, etc; however, under normal circumstances this method is inferior to pulp analyses in precision and accuracy, and to scanning for determining average grade of core intervals.

Electrodeposition of Niobium from 1-Butyl-1-Methylpyrrolidinium bis(trifluoromethylsulfonyl)amide Ionic Liquid

Electrodeposition of Niobium from the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]TFSA) was investigated at various temperatures ranging from RT to 170°C. The effect of addition of salts such as LiF and LiTFSA on the quality of the Nb deposits was also investigated. Cyclic voltammetry was employed to examine the electrochemical behaviour of NbF5 alone in [Py1,4]TFSA and of mixtures of NbF5 with LiF (or NbF5 with LiTFSA) in [Py1,4]TFSA. Potentiostatic electrolysis was performed to deposit Nb. The obtained Nb deposits were characterized using SEM and XRD. The results show that Nb can be electrodeposited along with nonstoichiometric compounds of niobium subfluorides from NbF5/[Py1,4]TFSA. Cubic facets were seen in the SEM analysis of the Nb deposit from (NbF5+LiTFSA)/[Py1,4]TFSA. Glancing Angle XRD analysis of the deposits obtained from (NbF5+LiTFSA)/[Py1,4]TFSA and (NbF5+LiF)/[Py1,4]TFSA at 170°C proved the existence of elemental Nb in addition to oxides of Nb. The addition of either LiF or LiTFSA to NbF5/[Py1,4]TFSA significantly enhanced the quality of the Nb deposits.

Age boundaries of formation of the Tomtor alkaline-ultramafic pluton: U-Pb and 40Ar/39Ar geochronological studies

This paper presents the results of geochronological studies of the Tomtor alkaline-ultramafic pluton, one of the largest Nb, Y, Sc, and TR deposits. A new scheme of its magmatism is given. The current K-Ar and Rb-Sr ages of different igneous rocks of the Tomtor pluton range from 800 to 250 Ma [Zaitsev et al., 1992; Frolov et al., 2003]. Such dispersion is probably related to the intense carbonatization of the rocks. The U-Pb zircon and 40Ar/39Ar mica ages indicate two stages of the formation of the pluton (700 and 400 Ma), which agrees well with the age of cycles of rift-related tectonogenesis of the Siberian platform.

Geochemistry of magmatic and hydrothermal zircon from the highly evolved Baerzhe alkaline granite: implications for Zr–REE–Nb mineralization

The Baerzhe alkaline granite pluton hosts one of the largest rare metal (Zr, rare earth elements, and Nb) deposits in Asia. It contains a geological resource of about 100 Mt at 1.84 % ZrO2, 0.30 % Ce2O3, and 0.26 % Nb2O5. Zirconium, rare earth elements (REE), and Nb are primarily hosted by zircon, yttroceberysite, fergusonite, ferrocolumbite, and pyrochlore. Three types of zircon can be identified in the deposit: magmatic, metamict, and hydrothermal. Primary magmatic zircon grains occur in the barren hypersolvus granite and are commonly prismatic, with oscillatory zones and abundant melt and mineral inclusions. The occurrence of aegirine and fluorite in the recrystallized melt inclusions hosted in the magmatic zircon indicates that the parental magma of the Baerzhe pluton is alkali- and F-rich. Metamict zircon grains occur in the mineralized subsolvus granite and are commonly prismatic and murky with cracks, pores, and mineral inclusions. They commonly show dissolution textures, indicating a magmatic origin with later metamictization due to deuteric hydrothermal alteration. Hydrothermal zircon grains occur in mineralized subsolvus granite and are dipyramidal with quartz inclusions, with murky CL images. They have 608 to 2,502 ppm light REE and 787 to 2,521 ppm Nb, much higher than magmatic zircon. The texture and composition of the three types of zircon indicate that they experienced remobilization and recrystallization during the transition from a magmatic to a hydrothermal system. Large amounts of Zr, REE, and Nb were enriched and precipitated during the transitional period to form the giant low-grade Baerzhe Zr–REE–Nb deposit.

Geochemistry of pyrochlore minerals from the Motzfeldt Center, South Greenland: The mineralogy of a syenite-hosted Ta, Nb deposit

Pyrochlore minerals are common accessory phases in the syenite and nepheline syenite rocks of the Motzfeldt alkaline center, Gardar Province, South Greenland, and are of important economic interest due to their exceptional ability to host various high field strength elements. In this study pyrochlore from two distinct intrusive units within the Motzfeldt Center have been studied. The largely homogeneous syenite and nepheline syenite rocks of the Flinks Dal formation host oscillatory and sector-zoned pyrochlore, which have experienced minor alteration in the presence of high-temperature, dominantly juvenile magmatic fluids. Alteration is characterized by variable decrease in the Na, Ca, and F contents and addition of Sr and K, accompanied by introduction of A and Y site vacancies. Pyrochlore from the more heterogeneous and intensely altered syenite rocks of the Motzfeldt So Formation are pervasively altered through interaction with lower temperature evolved hydrothermal fluids. During this phase of alteration cations are preferentially leached from the A and Y sites, accompanied by progressively increasing hydration. In both formations the proportion of B -site cations remains unaltered regardless of the degree of alteration. The alteration encoded in the pyrochlores of the Motzfeldt Center records the different hydrothermal conditions the two formations experienced during the subsolidus, showing how evolution of fluids within one intrusive unit can vary both spatially and temporally.

Late Paleozoic and Early Mesozoic rare-metal magmatism of Central Asia: Stages, provinces, and formation settings

Late Paleozoic and Early Mesozoic epochs in the formation history of the Central Asian Foldbelt are distinguished by high rare-metal productivity. A number of large REE, Ta, Nb, Zr, Be, Sn, Li, Mo, Re, and other deposits were formed at that time. As a rule, they are of the magmatic origin and related to the intrusions of highly evolved igneous rocks varying in composition from alkaline ultramafic with carbonatites to alkali and Li-F granites. In general, the occurrences of rare-metal magmatism are related to the rift zones of the Central Asian Rift System formed 310–190 Ma ago and conjugated with a consecutive series of the Barguzin, Hangay, and Hentiy zonal igneous provinces characterized by the large batholiths in their centers and rift zones in the framework. Such a structure indicates that these provinces were formed above isometric mantle sources or plumes with participation of large-scale crustal anatexis. The evident links of rare-metal deposits to occurrences of mantle magmatism within the zonal igneous provinces show that plume sources played an important role in their formation.

Experimental study of concentration dependence of Ta2O5 and Nb2O5 solubility in the alkaline and carbonate solutions at T=550oC, P=500 bar and low oxygen fugacity (Co-CoO buffer)

This study is a continuation of long-term work to build a quantitative model of genesis of deposits of Ta, Nb and Li, connected with granites. Importance of the problem is the need to use the available experimental data to estimate quantitatively a possible role of hydrothermal transport and redeposition of tantalum and niobium under physical-chemical conditions typical for Ta and Nb deposits, generally connected with alkaline rocks. Earlier we investigated concentration and temperature dependence of the tantalum and niobium oxides solubility in the sodium alkaline and carbonate solutions in a wide range of temperature, compositions and concentrations of solutions at P=1000 bar [Kotova and Zaraisky, 2009; Zaraisky et al., 2009]. The choice of sodium specificity of the solutions is related to the fact, that Ta-Nb mineralization in lithium-fluoric granites (“apogranites”), alkaline granites, alkaline metasomatites, sienites and carbonates is closely associated with sodium metasomatism (albitization, ribektization, egirinization). In order to practically cover the entire range of conditions for the formation of Ta-Nb deposits the experimental studies of Та2О5 and Nb2O5 solubility were carried out in aqueous alkaline solutions at P=500 bar. The tantalum and niobium oxides- chemical reagents Та2О5 and Nb2O5, being the analogues of tantite and nioboxide, seldom occurring in nature, were used as initial material in the runs. The concentration dependence of the Та2О5 and Nb2O5 solubility at T=550 o C, P=500 bar in the solutions of Na2CO3 and NaOH with the concentration (0.01; 0.1; 0.5, 1.0 и 2.0 m) in the presence of oxygen buffer Co–CoO was studied. Run duration was 21 days. Experiments were performed on a hydrothermal line. The capsules and the container with buffer were sealed into cold-seal pressure vessel of Tuttle type with big working volume that gives possibility to isolate the capsules from the container with buffer. The same technique was used to study Та2О5 solubility in fluoride and chloride solutions. After the run, the quenched aqueous solutions and the solid products were separated using a centrifuge. The solutions were then analyzed using ICP/MS (Inductively Coupled Plasma Mass Spectrometry) and ICP/AES (Atomic Emission Spectroscopy) for Nb, Ta, Mn, and Fe and admixture elements Ti, W, and Sn,. The composition of the solid product was characterized using optical microscopy, X-ray diffraction, and electron microprobe analysis (Cam Scan MV 2300 (VEGA TS5130MM). Experimental data are shown in fig. 1, 2, 3 and 4. The Ta2O5 solubility isotherms was found to have a negative trend in all studied alkaline and carbonate solutions at P=1000 bar. The concentration dependence of the Та2О5 was found to be more complicated at P=500 bar. In NaOH solutions a pressure decrease from 1000 bar to 500 leads to a decrease of tantalum content by a factor of 1 between 0.01 and 0.05 m NaOH. However, the Ta2O5 solubility increases by a factor of 1 in aqueous solutions with NaOH concentrations of 1 m and greater (fig. 1). In the Na2CO3 solutions with decreasing pressure from 1000 bar to 500 the tantalum content decreases by a factor of 1 (fig. 2). Experimentally established fact of the weak solubility of tantalum oxide in aqueous alkaline and carbonate solutions is fundamentally important for understanding of genesis of Ta deposits. The experimental data show that, for the range of concentrations, temperatures and pressures considered in this study, the Ta2O5 solubility is very low with concentrations on the order of 10 -5 to 10 -7 m in aqueous alkaline and carbonate solutions. The investigations have shown that at such low concentrations it is difficult to speak about possibility of Ta hydrothermal transport by alkaline and carbonate solutions. In NaOH and Na2CO3 solutions oxide tantalum has incongruent solubility. As solid phases wonderfully cut crystals of Na-tantite Na2Ta4O11 with the hexaoctahedric form typical for pyrochloride are formed in 0.01 and 0.1m NaOH and

Source and Evolution of Molybdenum in the Porphyry Mo(-Nb) Deposit at Cave Peak, Texas

The 37 � 1 Ma breccia-hosted porphyry Mo(^Nb) deposit at Cave Peak is genetically related to a mafic, alkaline intrusion nearby (the Marble Canyon Stock), thus providing an excellent opportunity to study the influence of mafic magmatism on porphyry Mo formation. Laser ablation inductively coupled plasma mass spectrometry analysis of crystallized melt inclusions allowed reconstruction of pristine concentrations of Mo and other fluid-mobile elements in melts ranging from phonotephrite (� 50 wt % SiO2 )t o rhyolite (� 75 wt % SiO2) in composition. Incompatible trace element abundances in these melt inclusions define smooth, linear trends reaching up to 100 times average upper continental crust, implying that the felsic magmas evolved from a mafic parent by fractional crystallization.The mafic parent has a geochemical signature typical of primitive melts generated in within-plate tectonic settings. Molybdenum increases from 4 ppm in the mafic endmember to 12 ppm in felsic melts immediately before Mo mineralization. After breccia formation, Mo concentrations in the residual melt decreased gradually to 5 ppm, suggesting that Mo was removed from the residual melts by exsolving fluids. This interpretation is supported by fluid^melt partition coefficients (DMo, fluid/melt ¼ 17^20) measured on assemblages of coexisting fluid and melt inclusions. A high sulfur fugacity in the mafic input magma is indicated by early pyrrhotite saturation. To account for the 0·4^1·5 � 10 6 t Mo present in the deposit a volume of � 30^100 km 3 alkali feldspar granite magma is needed, or up to 10 times more of the mafic melt. Significantly less magma is required to provide all the sulfur present in the deposit.The results of this study provide direct evidence for derivation of metals (and sulfur) in porphyry Mo deposits from associated mafic, alkaline magmas through fractional crystallization.

Metasomatic silicate chemistry at the Bayan Obo Fe–REE–Nb deposit, Inner Mongolia, China: Contrasting chemistry and evolution of fenitising and mineralising fluids

Fenite aureoles around carbonatite dykes, and alteration associated with Fe–REE–Nb ore bodies at Bayan Obo, Inner Mongolia, China, show alkali silicate assemblages containing aegirine–augite, (magnesio-)riebeckite, (magnesio-)arfvedsonite, and phlogopite, accompanied by varying amounts of apatite, albite and quartz. In both fenites and orebodies simple thermodynamic constraints indicate mineral parageneses are consistent with rock buffered cooling accompanied by the infiltration of a range of externally buffered hydrothermal fluids. Statistical analysis of amphibole chemistry indicates that even in apparently texturally well constrained paragenetic stages wide variations in chemistry occur in both the ore bodies and fenites. Much of this variation is attributable to the Mg and F content of amphibole, and is therefore interpreted as a result of variation in externally controlled variables ( P, T, initial fluid composition) rather than internally controlled variables such as protolith composition. Similarities in chemistry exist between fenite and some ore body amphiboles. Thermodynamic analysis of the composition of biotite and apatite allows constraints to be placed on the F-content of hydrothermal fluids, and indicates relatively consistent compositions in fenites and orebodies (log a HF/ a H 2O = − 3.8 to − 3.6 at 300 °C and 1 kbar). Amphibole and biotite associated with niobate mineralization are both enriched in fluorine relative to the rest of the paragenesis, and biotite compositions indicate significantly higher HF activities in the hydrothermal fluid (log a HF/ a H 2O = − 2.6 at 300 °C and 1 kbar). The data presented here reinforce previous interpretations of the complex, multistage nature of mineralisation at Bayan Obo, but are still consistent with a direct involvement of carbonatite derived fluids during ore genesis.

EIS study of niobium films sputtered at different target–substrate angles

Nb films have been magnetron sputtered onto quartz sheets oriented with respect to the target at angles varying between 0° and 90°, with 15° steps. Impedance plots have been obtained by contacting these films with aqueous Na 2SO 4, either at the open circuit potential or at a potential where Nb is covered by an anodic passive Nb 2O 5 film. As the target–substrate angle θ increases, the shape of the impedance plots changes from that of a smooth electrode to that of a porous one, characterised in the high frequency range by a straight line forming a 45° angle with the real axis. The surface roughness of the Nb deposits, calculated from their double layer capacity, is low and constant at low θ, significantly increases at θ = 45°, goes through a maximum in the range 60–75° and drops at θ = 90°. AFM surface profiling confirms this trend, but estimates a lower surface roughness. Attempts to obtain Nb deposits with a surface roughness less strongly dependent on θ have been made by performing the depositions under pulsed conditions or by heating the substrates at 400–600 °C. Heating at the higher temperature was a fairly effective method for decreasing the roughness of deposits formed at large θ.

Production and characterization of electrolytic nickel–niobium composite coatings

Ni–Nb composite coatings were obtained on carbon steel by galvanostatic electrolysis of Watts nickel bath, containing suspended niobium powders. The effect of cathodic current density, stirring rate of the electrolyte, and size of the Nb particles on the volume fraction of co-deposited Nb particles and deposit morphology was investigated. The microhardness and electrochemical behavior of the composites were also evaluated and compared with pure nickel coatings. The bath containing the largest Nb powders (50 μm mean size) led to low Nb content coatings and inhomogeneous distribution of particles. On the contrary, using 20-μm mean size particles, coatings with 8.5–19% Nb content and good particles distribution were obtained. In this case, the 400 rpm stirring rate led to the higher Nb content for all current densities investigated and the smoother coatings were obtained for both 400 and 550 rpm stirring rates. The microhardness of the composite coatings was higher than that of electrolytic Ni obtained under the same electrolysis conditions due to grain refining. Incorporation of Nb particles in Ni coatings improved the corrosion resistance of the deposits in NaCl and H 2SO 4 solutions.