Nb Ore Research Articles

Hydrothermal niobium (Nb) mineralization and mobilization in the world-class Madeira Sn-Nb-Ta granitic deposit (Amazonas, Brazil)

The Madeira deposit is a world-class tin (Sn) deposit characterized by a unique mineralogical assemblage composed of a massive cryolite (NaAlF3) deposit associated with economically important metals like Nb (0.20 wt% Nb2O5). Although hydrothermal alteration has long been recognized in the cryolite formation, its effects on the mineralization and mobility of Nb remain obscure. This study presents new data on the Nb mineralization of the Pitinga core and border albite-enriched granites provided by nanoscale and site-selective approaches, using transmission electron microscopy and synchrotron-radiation analyses. Pyrochlore is the main Nb ore mineral with three distinct compositional types (U-Pb, Pb-U and Y-bearing varieties). Hydrothermal processes lead to the extensive alteration of pyrochlore into columbite (later designated as columbitization) by a coupled dissolution-reprecipitation mechanism, which evidences the alterability of pyrochlore in a hydrothermal context. Nanoscale analyses of veins and reaction interfaces reveal the presence of additional Nb hosts including fergusonite-(Y), Nb-bearing uraninite and Nb-bearing coffinite which formed from the alteration of pyrochlore. The nature of the altered phases mainly depends on the composition of the parent pyrochlore. Their formation, occurring in absence of direct proximity with remaining pyrochlore, shows that Nb mobilization at macroscopic scale is possible in F-rich reducing fluids. Niobium L3-edge XANES spectroscopy on bulk samples representative of the different facies show that hydrothermal processes change the Nb mineralization by converting primary U-Pb-bearing pyrochlore into columbite and Pb-U/Y-bearing pyrochlores. The columbitization process of pyrochlore led to the increase of the Nb ore grade. Nonetheless, hydrothermal alteration modified Nb mineral liberation, thereby limiting the recovery of the full range of Nb host phases.

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.

Multiscale processes controlling niobium mobility during supergene weathering

Niobium (Nb) is one of the most immobile elements during supergene weathering, widely used for mass-balance calculations, despite elusive information on the mechanisms controlling its dynamics in the critical zone. Here, a multiscale approach, from weathering profile to atomic-scale, is developed to monitor Nb speciation along a thick (about 50 m) lateritic regolith formed over the Pitinga pluton (Amazonas, Brazil). In the A-type parent granite, Nb is mainly hosted in ilmenite (1.5–2 wt% Nb2O5) and, to a lesser extent, in rutile (2–3 wt% Nb2O5). A quantitative assessment of the average Nb speciation from the parent rock to the upper horizons has been carried out by combining spectroscopic and spatially-resolved chemical techniques. The contribution of Nb-bearing Ti oxides (2–6 wt% Nb2O5) and Fe oxides (0.1–0.3 wt% Nb2O5) in the average Nb speciation increases with the degree of weathering and reaches 80% in the most altered horizon. This unusual Nb speciation results from the nature of the primary Nb carriers, less resistant to weathering than common Nb ore minerals such as pyrochlore. X-ray absorption spectroscopy demonstrates that Nb released from the weathering of ilmenite substitutes for Fe and Ti in goethite and Ti oxides, respectively, providing atomic-scale evidence of the high affinity of Nb for these phases. Elemental mobility of Nb, Ta and Sn followed by geochemical mass-transfer calculations evidences lateral transport from Nb-Sn-enriched laterites developed over the surrounding Nb-enriched facies of the Pitinga granite to the studied profile. Elements such as Ti, Zr and Hf, often considered immobile, are leached or redistributed in the studied profile. Our results demonstrate Nb remobilization during intense weathering. Although Nb scavenging by secondary Fe and Ti oxides limits Nb mobility at the mineral scale, this work questions the unrestricted use of Nb as a chemical invariant during surficial alteration processes.

Discovery of high-grade REE-Nb-Fe mineralization associated with calciocarbonatite in south Morocco

The recently discovered REE and Nb mineralization in the Twihinat area in the western part of the Oulad Dlim Massif (Adrar Souttouf) in South Morocco is linked to a Cretaceous calciocarbonatite intrusion which was likely formed in an intracontinental rift setting and crops out locally within a ring structure that mainly consists of massive Fe-oxide mineralization and silica breccia. The carbonatite shows intensively metasomatized zones, which contain bastnaesite and pyrochlore-group minerals as the main REE and Nb ore minerals. They are usually associated with apatite, quartz and Fe-oxides, or trapped in calcite voids, suggesting a secondary ore formation. Within the associated Fe-oxide mineralization, pyrochlore and monazite-(Ce) are the main ore minerals occurring closely associated with quartz and magnetite or hematite. The silica breccia also shows significant subsequent infill of barite, bastnaesite-(Ce) and hydrated ceriopyrochlore, which was identified by EPMA and Raman spectroscopy. Bastnaesite commonly forms prismatic aggregates whereas pyrochlore and ceriopyrochlore usually display subhedral grains along tiny fractures. Structural and textural relationships clearly indicate epigenetic ore formation induced by multiple stages of hydrothermal fluid flow and fracturing. Ore precipitation likely resulted from interaction between low-pH mineralizing hydrothermal fluids and the wall-rock. The latter efficiently buffered the acidity of the fluids and allowed significant amounts of REE and Nb ore minerals to precipitate. Trace element ICP-MS analyses show very high REE and Nb concentrations of up to 0.76 wt% ΣREE and 0.21 wt% Nb in carbonatite and up to 3 wt% ΣREE and 1.3 wt% Nb in the associated silica and Fe-oxide mineralization. The results clearly demonstrate that the Twihinat REE-Nb deposits are significant and represent a potential new high-grade resource for these critical metals.

Mineralogy and geochemistry of the Morro dos Seis Lagos siderite carbonatite, Amazonas, Brazil

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

Characterization of REE-Nb ores by a combination of spectrochemical techniques

Various spectroscopic techniques and different sample pretreatment schemes were used for the characterization of the uniquely rich Tomtor REE-Nb ores and for the technological solutions used for hydro-metallurgy processing of these ores. The ranges of the target analyte concentrations available for the techniques used, XRF, MP-AES and ICP-MS, were experimentally found. The determination of REE by XRF was possible for concentrations in solid samples above 10−3%. A unique procedure for solid sample borate fusion which resulted in homogeneous, transparent glass disks which were stable during long-term storage was established. Actual concentrations of REE in solid samples were determined using an improved data processing algorithm of XRF spectra evaluation. Metrological parameters for XRF analysis of ore samples were evaluated by the analysis of certified reference materials CRM OSO №250-91 NFS-23 and Nb ore concentrate CRM DH SX18-03.Solid sample digestion was used for the determination of low concentrations of REEs by MP-AES and ICP-MS. The metrological parameters of MP-AES are identical to XRF. ICP-MS was used for determination of 40 elements in technological solutions. In the case of ICP-MS, La, Ce and Nd cause severe spectral interferences for Ga, As and Gd. The quantitative contribution of these interferences was estimated. Correction equations were developed, improving the accuracy of target analyte determination.

Sources of Ore Substance of Carbonatite Complexes of the Ural Fold Belt: Rb–Sr and Sm–Nd Isotope Data

The results of study of the Nd and Sr isotope compositions of the Nb ore minerals (pyrochlore and aeschynite groups) and rocks from the Ilmenogorsk–Vishnevogorsk and Buldym carbonatite complexes of the Ural Fold Belt are presented. It has been established that pyrochlores of the early stages of ore formation and the IVC miaskite-carbonatite rocks have a single substance source corresponding to a mantle moderately depleted source according to isotope parameters. The crustal components, along with mantle, participate in the processes of ore formation within the Buldym complex.

Mineralogy and geochemistry of laterites from the Morro dos Seis Lagos Nb (Ti, REE) deposit (Amazonas, Brazil)

The Morro dos Seis Lagos niobium deposit (2897.9Mt at 2.81wt% Nb2O5) is associated with laterites formed by the weathering of siderite carbonatite. This iron-rich lateritic profile (>100m in thickness) is divided into six textural and compositional types, which from the top to the base of the sequence is: (1) pisolitic laterite, (2) fragmented laterite, (3) mottled laterite, (4) purple laterite, (5) manganiferous laterite, and (6) brown laterite. All the laterites are composed mainly of goethite (predominant in the lower and upper varieties) and hematite (predominant in the intermediate types, formed from goethite dehydroxylation). The upper laterites were reworked, resulting in goethite formation. In the manganiferous laterite (10m thick), the manganese oxides (mainly hollandite, with associated cerianite) occur as veins or irregular masses, formed in a late event during the development of the lateritic profile, precipitated from a solution with higher oxidation potential than that for Fe oxides, closer to the water table. Siderite is the source for the Mn. The main Nb ore mineral is Nb-rich rutile (with 11.26–22.23wt% Nb2O5), which occurs in all of the laterites and formed at expense of a former secondary pyrochlore, together with Ce-pyrochlore (last pyrochore before final breakdown), Nb-rich goethite and minor cerianite. The paragenesis results of lateritization have been extremely intense. Minor Nb-rich brookite formed from Nb-rich rutile occurs as broken spherules with an “oolitic” (or Liesegang ring structure). Nb-rich rutile and Nb-rich brookite incorporate Nb following the [Fe3++(Nb, Ta) for 2Ti] substitution and both contain up to 2wt% WO3. The laterites have an average Nb2O5 content of 2.91wt% and average TiO2 5.00wt% in the upper parts of the sequence. Average CeO2 concentration increases with increasing depth, from 0.12wt% in the pisolitic type to 3.50wt% in the brown laterite. HREE concentration is very low.

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.

An assessment of a handheld X-ray fluorescence instrument for use in exploration and development with an emphasis on REEs and related specialty metals

Handheld (portable) X-ray fluorescence (pXRF) instruments are designed for use in the exploration for base metals, precious metals, and specialty metals (e.g. rare earth elements (REE), Ta, and Nb) and allow rapid decision-making directly in the field. This paper evaluates the technical merits and limitations of pXRF technology in the exploration for specialty metals using data generated from the analysis of three geochemical standards and a silica blank: Standard Reference Material NIST 2780 from the National Institute of Standards and Technology, Gaithersburg; the Certified Reference Material “TRLK” Rare Earth Ore “CGL 124” from the Mongolia Central Geological Laboratory; the Reference Niobium Ore OKA-1 (CANMET); and a silica blank described as Si (IV) oxide (99.8 % on metal basis) from Alpha Aesar (Ward Hill, MA, USA). The data was acquired over a period of nearly 2 years as a by-product of several distinct specialty metal-related projects using the same pXRF instrument and the same settings. Instrumental analytical accuracy was determined using the percent difference (%diff) between the average concentrations of the pXRF instrument readings and the reported certified values of the standard. Percent relative standard deviation (%RSD) was used as a measure of precision. Smaller %diff and %RSD indicate more accurate and precise data, and the accuracy and precision of the pXRF depended strongly on the elemental concentrations in the standards used. Box and whisker diagrams were used to illustrate characteristics of pXRF data sets (mean, lower and upper quartiles, and range) corresponding to individual standards. The bias of the pXRF determinations (under/overestimation) relative to certified values of individual standards are also depicted on these diagrams. This study indicates that the pXRF was capable of producing readings for Si, K, Al, Fe, Ca, Ti, Pb, Zn, Sr, Ag, Cd, Th, Sb, P, S, Mo, Mn, Mg, As, Nb, Rb, La, Ce, Pr, Nd, and Y within 10 %RSD of the reported certified value in at least one of the three standards measured, depending on the concentrations within that standard. The instrument was able to provide relatively accurate and precise data for Nd, Pr, Ce, La, and Y (%diff, ≤17; %RSD, ≤3.7) when the concentrations were greater than 1,000 ppm (e.g., rare earth ore CGL 124). It generated lower quality data (in terms of %diff and %RSD) when concentrations of these elements were in the hundreds parts per million range or lower (NIST 2780). As for Nb, the best %RSD (2.2) was obtained on the Nb Ore OKA-1 with an accompanying %diff of 34. This indicates that the instrument is suitable for use in the exploration and development of carbonatite-related REE deposits such as: Mountain Pass (California), Bayan Obo (Inner Mongolia), Wicheeda Lake (British Columbia), St Honore, Oka, and Eldor (Quebec). By extension, the instrument is also suitable for the exploration of apatite–monazite veins (Steenkampskraal) and peralkaline intrusion-related deposits such as Kipawa, Nechalacho (Northwest Territories), or Strange Lake (Quebec; Labrador, Newfoundland). A pXRF without a radioactive source cannot be used to analyze for most of the heavy lanthanides; however, Y can be used as a pathfinder for these elements. Limited but effective use of the pXRF in exploration for ion adsorption clay deposits with grades as low as 500 ppm total REE appears possible. The pXRF could also be used in the exploration for carbonatite-related Nb-bearing deposits such as St Honore (Quebec) and Aley (British Columbia), or in the exploration for lower-grade Nb–Ta deposits such as Upper Fir (British Columbia), justifying the need for future case studies.

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.

Fe SPECIATION IN WEATHERED PYROCHLORE-GROUP MINERALS FROM THE LUESHE AND ARAXA (BARREIRO) CARBONATITES BY 57Fe MOSSBAUER SPECTROSCOPY

The minerals of the pyrochlore group exhibit a large range of chemical composition. The general structural formula can be written as A 2− m B 2 X 6− w Y 1− n · p H 2 O. The occupancy of the eight-fold-coordinated A site can reflect the degree of alteration of the pyrochlore. In unaltered pyrochlore-group minerals, this site is completely filled by Na and Ca, whereas in the altered equivalents, the A site is only partly filled by cations as K, Ba, Sr and Ce. In the literature, the structural formula is systematically calculated assuming an ideal stoichiometry, i.e. , a total of two six-fold-coordinated B -site cations per formula unit, dominantly Nb, with subordinate Ti and Ta in certain rock-types, e.g. , carbonatites. Fe is systematically considered as Fe 3+ and assigned to the B site. The present 57 Fe Mossbauer study focuses on altered pyrochlore-group minerals from two Nb ore deposits in laterite: Lueshe in the Democratic Republic of Congo, and Araxa in Brazil. In contrast to the common assumption that all Fe is present as Fe 3+ , 47 ± 2% of the total Fe is in the Fe 2+ oxidation state in the kalipyrochlore sample from Lueshe. Spectra of both samples are consistent with the presence of all the Fe 3+ at the B site, but with two different coordination spheres. Fe 3+ coordinated by 6 O 2− is observed in both cases. In the Lueshe sample, it is also coordinated by 5 O 2− and 1 OH − , whereas in bariopyrochlore from Araxa, the unusually large value of the quadrupole splitting, 3 mm/s, affecting ~19 ± 2% of the Fe 3+ , suggests that this Fe 3+ is coordinated by only 5 O 2− owing to the presence of anion vacancies at the X site.

Rare metal mineralization at the Salpeterkop carbonatite complex, Western Cape Province, South Africa

The Salpeterkop complex is an eroded central volcano of Late Cretaceous (66 Ma) age with a sediment-filled crater remnant 1 km in diameter associated with radial and concentric carbonatite/K-trachyte dykes as well as K-trachyte, breccia and olivine melilitite plugs. The sedimentary country rocks have been domed. Geochemical anomalies for Ti, Y, Zr, Nb, Th, V, Zn and Mo, based on a core-drilling programme, were found to outline a mineralized zone coinciding with the crater rim, especially on the western and southern sides. The rim rocks are volcanic breccias intimately mixed and veined by decomposed, silicified and ferruginized carbonatite and carbonatitic tuff. A large variety of very fine-grained minerals have been identified in a groundmass consisting mainly of goethite and chalcedony. The principal Nb ore mineral is niobian brookite occurring as microscopic disseminated grains of several morphological types. High-Nb (12% Nb 2O 5) and low-Nb (8% Nb 2O 5) brookites have been distinguished. Some of them contain inclusions of ilmenorutile. Delicate brookite crystal aggregates crystallized in situ. No discrete V and Zn minerals could be identified but Th occurs as thorianite and thorite, Zr occurs in eudialyte and an unidentified KNb silicate and Y occurs as xenotime. Nepheline and eudialyte are probably xenocrysts from an unexposed body of nepheline syenite. It is concluded that mineralization took place in three overlapping stages: 1. i)primary magmatic, with concentration of Nb, Zr, Th, V in both carbonatite and associated silicate rocks; 2. ii)mesothermal, with oxidation and introduction of Fe, Ti, REE, Mo and F; 3. iii)epithermal, with hydration and dissolution of carbonates, removal of Ca and Mg, redistribution of Y, Ti, Nb, Zn and deposition of Si, Ba, Mn and other mobile elements. The suggestion is made that the preservation and extent of the third stage, which may be linked to fumarolic activity, distinguishes the multi-element mineralization at Salpeterkop from other carbonatite complexes. Secondary alteration in the weathered zone merged with the third stage, but is less important.

Host-rock controlled epigenetic, hydrothermal metasomatic origin of the Bayan Obo REE [sbnd]Fe [sbnd]-Nb ore deposit, Inner Mongolia, P.R.C.

Bayan Obo, a complex rare earth element (REE) Fe Nb ore deposit, located in Inner Mongolia, P.R.C. is the world's largest known REE deposit. The deposit is chiefly in a marble unit (H8), but extends into an overlying unit of black shale, slate and schist unit (H9), both of which are in the upper part of the Middle Proterozoic Bayan Obo Group. Based on sedimentary structures, the presence of detrital quartz and algal fossil remains, and the 16-km long geographic extent, the H8 marble is a sedimentary deposit, and not a carbonatite of magmatic origin, as proposed by some previous investigators. The unit was weakly regionally metamorphosed (most probably the lower part of the green schist facies) into marble and quartzite prior to mineralization. Tectonically, the deposit is located on the northern flank of the Sino-Korean craton. Many hypotheses have been proposed for the origin of the Bayan Obo deposit; the studies reported here support an epigenetic, hydrothermal, metasomatic origin. Such an origin is supported by field and laboratory textural evidence; 232Th/ 208Pb internal isochron mineral ages of selected monazite and bastnaesite samples; 40Ar/ 39Ar incremental heating minimum mineral ages of selected alkali amphiboles; chemical compositions of different generations of both REE ore minerals and alkali amphiboles; and evidence of host-rock influence on the various types of Bayan Obo ores. The internal isochron ages of the REE minerals indicate Caledonian ages for various episodes of REE and Fe mineralization. No evidence was found to indicate a genetic relation between the extensive biotite granitic rocks of Hercynian age in the mine region and the Bayan Obo are deposit, as suggested by previous workers.

Geochemical characteristics of some rare element-bearing granites in southeast China and physico-chemical conditions of their formation

The formation of ore-bearing granites in the Yenshanian movement in southeast China and the geochemical characteristics of some RE-bearing granites have been studied through multivariate statistical analysis and physicochemical approach. The main conclusions have Been drawn as follows: (1) The granites are believed to be products of anatexis of the crustal materials. The formation temperature of granitic magma is estimated at about 600°C and the crystallization temperatures range from 600° to 500°C. The temperature of Li-, Fand H2O-rich residual magma in the latest stage of magmatism is probably below 500°C. (2) A rock series from early lepidomelane-granite through protolithionite-and zinnwalditegranite to lithioniteor lepidolite-granite is considered as a result of actual crystallization. (3) The mineral paragenesis and fades zonation of granite plutons are mainly controlled by Ph2o, μNa2O and μK2O in the magma. (4) During the magmatic evolution the ore-forming elements (REE, Nb, Ta, etc.) are variable in geochemistry. REEs, similar to mafic components, were highly concentrated at the early stage of the magmatic evolution and deposited under favourable conditions in the zinnwaldite-and protolithionite-granites; Nb and Ta have a preference for felsic and volatile components, thus are mainly concentrated at later stages of the magmatic evolution. Nb ore deposits are formed in the lithionite and lepidolite granites, for Ta is intimately associated with Na2O, Li2O, F and H2O.