Pyrochlore Supergroup Minerals Research Articles

The role of magmatic versus metasomatic processes in Ta-Nb mineralization at the Jiabusi Deposit, Inner Mongolia: Evidence from textural and chemical features of rare metal minerals and mica

The exact role magmatic versus metasomatic processes play in Ta-Nb mineralization is a question that has been debatable for several decades. The Jiabusi deposit is chosen to address this question, for its ample endowments of a variety of Ta-Nb oxides and mica in the various rock types that could provide insights into the whole evolutional history of ore-forming processes during magmatic-hydrothermal evolution. Textural and chemical evidence suggests that Fe-mica in the Fe-mica granite and pegmatitic portion is magmatic, indicating that the co-precipitated columbite-group minerals (CGM) and pyrochlore-supergroup minerals (PSM) are also magmatic. In the lepidolite granite and aplite, on the other hand, two stages of mineralization occured, represented by unzoned CGM that are in equilibrium with lepidolite, snowball quartz, and fluorite that all have a metasomatic origin and subsequent CGM/PSM overgrowths that co-precipitated with metasomatic Cs-lepidolite and saccharoidal albite. These CGM overgrowths have 1–2 time(s) higher Ta/(Nb + Ta) values than the preceded CGM, generating a clear compositional gap between them. Both stages of CGM and CGM/PSM precipitated from a hydrosilicate liquid (HSL), the partial migration of which results in the disequilibrium textures observed. The lepidolite granite is largely a metasomatic rock, the protolith of which has been obscured by intense metasomatism governed by the HSL. This HSL, however, is mainly constrained to the lepidolite granite but a few migrated into the Fe-mica granite. This is the main reason for the different mineralizing styles and intensities between the two rock types.

WinPclclas, a Windows Program for Pyrochlore Supergroup Minerals

Abstract A Microsoft® Visual Basic program, WinPclclas, has been developed to calculate the chemical formulae of pyrochlore supergroup minerals (PSM) based on data obtained from wet-chemical and electron-microprobe analyses. These are also classified following the nomenclature scheme adopted by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association for PSM (IMA-10) and including both subsequent group and species updates. The program considers classification among the 33 IMA-approved PSM, as well as ten species that have not been approved, adhering to the general formula A2B2X6Y and considering the dominant ions at the A, B, and Y sites. It considers PSM represented by the pyrochlore, microlite, roméite, elsmoreite, and betafite groups, along with unassigned members of the ralstonite and coulsellite groups. Mineral formulae of the PSM are calculated based on the ideal number of B cations being two apfu, owing to the potential for vacancies at the A, X, and Y sites. WinPclclas operates in four stages: (1) it estimates cation and anion contents provided by input chemical data; (2) it determines the dominant cation and anion at the A, B, and Y sites; (3) it assigns the PSM to one of the seven groups, based on the dominant cation at the B site (i.e., W6+, Nb5+, Ta5+, Sb5+, Ti4+, Al3+, and Mg2+); and (4) it classifies the pyrochlore species into an appropriate group, based on the dominant cation at the A site and the dominant anion at the Y site. The H2O (wt.%) content for the PSM is estimated by stoichiometric constraints. WinPclclas allows users to: (1) enter up to 51 input variables for mineral-chemical analyses; (2) type and load multiple PSM compositions in the data entry section; (3) edit and load the Microsoft® Excel files used in calculating, classifying, and naming the PSM, together with the total monovalent to hexavalent cation, and (4) store all the calculated parameters in the output of a Microsoft® Excel file for further data evaluations. The program is distributed as a self-extracting setup file, including the necessary support files used by the program, a help file, and representative sample data files.

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

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

Mineral Chemistry of Pyrochlore Supergroup Minerals from the Boziguoer Nb-Ta-Zr-Rb-REE Deposit, NW China: Implications for Nb Enrichment by Alkaline Magma Differentiation

Alkaline rocks are generally enriched in rare metals (e.g., Nb, Ta, and Zr) and rare earth elements (REE), but the key factors controlling Nb-Ta-REE enrichment remain unclear. The Boziguoer Nb (Ta-Zr-Rb-REE) deposit in Southwest Tianshan (northern margin of Tarim Basin) is China’s largest, with reserves of 0.32 Mt Nb2O5 and 0.02 Mt Ta2O5. It is an alkaline felsic complex 4.45 km in length and 0.5–1.3 km in width, composed of alkalic granite and syenite, which can be subdivided into syenite I and syenite II. The main minerals in each lithofacies are the same (albite, K-feldspar, quartz, arfvedsonite and aegirine). The Nb in the deposit is mainly hosted in pyrochlore supergroup minerals, ubiquitous in alkalic granite and syenite of the Boziguoer deposit. The wide variation in cations (Ca, Na, REE, U, Th) in the A-site further classifies the Boziguoer pyrochlore supergroup minerals as fluornatropyrochlore, fluorcalciopyrochlore and fluorkenopyrochlore. All Boziguoer pyrochlore supergroup minerals are Nb-rich and Ta-poor at the B-site and dominated by F at the Y-site. These cation occurrence illustrate a new mechanism of substitution in the Boziguoer pyrochlore supergroup minerals (2Ca2+ +Ti4+ +4Ta5+ = REE3+ +A-V + 5Nb5+, where A-V is the A-site vacancy). This substitution mechanism is different from that in the pyrochlore supergroup minerals from other rocks such as carbonatite and nepheline syenite, which are dominated by the replacement of Ba (Rb, Sr) with Ca+ Na + A-V. In addition, the substitution of REE (mainly La, Ce) for Ca in the Boziguoer pyrochlore supergroup minerals is likely a result of either REE enrichment or a change in the REE partition coefficient during the evolution of the alkaline magma. Both the pyrochlore supergroup minerals and their host rocks display negative large ion lithophile element (LILE; K, Rb, Sr, and Ba) anomalies, positive high-field-strength element (HFSE) anomalies and light rare earth element (LREE) enrichment with negative Eu anomalies. This is consistent with the crystallization of the pyrochlore supergroup minerals from the magma rather than from hydrothermal fluids, suggesting a magmatic origin. These findings indicate that the mechanisms of pyrochlore supergroup minerals crystallization in alkaline magma may be significantly different from those in carbonatite and nepheline syenite, and that magmatic differentiation processes may have played a role in the enrichment of the Boziguoer deposit by Nb.

Evolution of Nb–Ta Oxide Minerals and Their Relationship to the Magmatic-Hydrothermal Processes of the Nb–Ta Mineralized Syenitic Dikes in the Panxi Region, SW China

Previous geochemical and petrological studies have concluded that initially magmatic Nb–Ta mineralization is often modified by post-magmatic hydrothermal fluids; however, there is still a lack of mineralogical evidence for the syenite-related Nb–Ta deposit. From the perspective of Nb–Ta minerals, the pyrochlore supergroup minerals have significance for indicating the fluid evolution of alkaline rock or related carbonatite type Nb–Ta deposits. The Panzhihua–Xichang (Panxi) region is a famous polymetallic metallogenic belt in southwestern China, abound with a huge amount of Nb–Ta mineralized syenitic dikes. This study focuses on the mineral textures and chemical compositions of the main Nb–Ta oxide minerals (including columbite-(Fe), fersmite, fergusonite-(Y), and especially pyrochlore group minerals) in samples from the Baicao and Xiaoheiqing deposits, in the Huili area, Panxi region, to reveal the magma evolution process of syenitic-dike-related Nb–Ta deposits. The Nb–Ta oxides in the Huili syenites are commonly characterized by a specific two-stage texture on the crystal scale, exhibiting a complex metasomatic structure and compositional zoning. Four types of pyrochlore group minerals (pyrochlores I, II, III, and IV) formed in different stages were identified. The euhedral columbite-(Fe), fersmite, and pyrochlores I and II minerals formed in the magmatic fractional crystallization stage. Anhedral pyrochlore III minerals are linked to the activity of magma-derived hydrothermal fluids at the late stages of magma evolution. The pyrochlore IV minerals and fergusonite-(Y) tend to be more concentrated in areas that have undergone strong albitization, which is a typical phenomenon of hydrothermal alteration. These mineralogical phenomena provide strong evidences that the magmatic-hydrothermal transitional stage is the favored model for explaining the Nb–Ta mineralization process. It is also concluded that the changes in chemical composition and texture characteristics for pyrochlore group minerals could serve as a proxy for syenite-related Nb–Ta mineralization processes.

Pyrochlore-Supergroup Minerals Nomenclature: An Update.

The general formula of the pyrochlore-supergroup minerals is A 2 B 2 X 6 Y. The mineral names are composed of two prefixes and one root name (identical to the name of the group). The first prefix refers to the dominant anion (or cation or H2O or vacancy) of the dominant valence at the Y-site. The second prefix refers to the dominant cation of the dominant valence [or H2O or vacancy] at the A-site. Thirty-one pyrochlore-supergroup mineral species are currently distributed into four groups [pyrochlore (B = Nb, X = O), microlite (B = Ta, X = O), roméite (B = Sb5+, X = O), and elsmoreite (B = W, X = O)] and two unassigned members [hydrokenoralstonite (B = Al, X = F) and fluornatrocoulsellite (B = Mg, X = F)]. However, when the new nomenclature system of this supergroup was introduced (2010) only seven mineral species, namely, oxycalciopyrochlore, hydropyrochlore, hydroxykenomicrolite, oxystannomicrolite, oxystibiomicrolite, hydroxycalcioroméite, and hydrokenoelsmoreite, were valid. The seven species belong to the cubic crystal system and space group Fd m and O is predominant in the X structural site. The 24 new mineral species described between 2010 and 2021 are cesiokenopyrochlore, fluorcalciopyrochlore, fluornatropyrochlore, hydrokenopyrochlore, hydroxycalciopyrochlore, hydroxynatropyrochlore, hydroxykenopyrochlore, hydroxymanganopyrochlore, hydroxyplumbopyrochlore, fluorcalciomicrolite, fluornatromicrolite, hydrokenomicrolite, hydroxycalciomicrolite, kenoplumbomicrolite, oxynatromicrolite, oxycalciomicrolite, oxybismutomicrolite, fluorcalcioroméite, hydroxyferroroméite, oxycalcioroméite, oxyplumboroméite, fluornatrocoulsellite, hydrokenoralstonite, and hydroxykenoelsmoreite. Among the new species, hydroxycalciomicrolite belongs to a different space group of the cubic system, i.e., P4232. There are also some mineral species that crystallize in the trigonal system. Hydrokenoelsmoreite occurs as 3C (Fd m) and 6R (R ) polytypes. Hydrokenomicrolite occurs as 3C (Fd m) and 3R (R m) polytypes, of which the latter corresponds to the discredited “parabariomicrolite.” Fluornatrocoulsellite crystallizes as 3R (R m) polytype. Surely there are several new pyrochlore-supergroup minerals to be described.

Composition and probable ore igneous rocks source of columbite from alluvial deposits of Mayoko district (Republic of the Congo)

The article presents the results of optical, electron microscopic and electron microprobe studies of columbite group minerals, collected during heavy mineral concentrate sampling of alluvial deposits in the Mayoko region (Republic of the Congo). The aim of the study is revealing tantalum niobates ore body in this region. We found that these minerals in loose deposits are represented by two grain-size groups: less than 1.6 mm (fine fraction) and 1.6-15 mm (coarse fraction). The grains of both fractions belong mainly to columbite-(Fe), less often to columbite-(Mn), tantalite-(Mn) and tantalite-(Fe), contain impurities of Sc, Ti, and W. The crystals have micro-scaled zoning (zones varies slightly in the Ta/Nb ratio values) and contains a lot of mineral inclusions and veins represented by zircon, pyrochlore supergroup minerals and others. Columbite-(Fe) and columbite-(Mn) are characterized by an increased content of Ta2O5 up to the transition to tantalite-(Fe) and tantalite-(Mn). This allows us to exclude the formation of subalkaline rare-metal granites, their metasomatites (albitites and greisenes) and carbonatites, from the list of possible columbite ore rocks source in the Mayoko district. Thus, beryl type and complex spodumene subtype rare-element pegmatites of the mixed petrogenetic family LCT-NYF (according to P.Černý) should be considered as a probable root source. The results of the research should be taken into account when developing the methodology for prospecting in this area.

Oxides of the Pyrochlore Supergroup from a Nonsulfide Endogenic Assemblage of Pb–Zn–Sb–As Minerals in the Pelagonian Massif, Macedonia

The specific features of the chemical composition, isomorphism, and zoning have been studied for pyrochlore supergroup minerals (PSM) from metasomatic rocks of ore occurrence no. 9 (Nezilovo, Pelagonian Massif, Republic of Macedonia). Ore occurrence no. 9 has an unusual nonsulfide association of chalcophile elements: Zn, As, Sb and Pb. Pyrochlore supergroup minerals crystallized at all stages of metasomatism. Zoning of pyrochlore supergroup minerals and their relationships with associated minerals (Zn-bearing silicates, tilasite, barite, carbonates, various accessory Pb-, Sb- and Zn-oxides, etc.) suggest a complex geochemical evolution of ore occurrence no. 9. The possible mechanism for the formation of nonsulfide parageneses of chalcophile elements is discussed.

New Mineral Names*,†

In this issue This New Mineral Names has entries for 23 new minerals, including alpeite, ammoniozippeite, batagayite, centennialite, dagenaisite, davidsmithite, epifanovite, ferro-tschermakite, greenlizardite, grootfonteinite, janchevite, javorieite, kampelite, roymillerite, tredouxite, vanadio-pargasite, and wilancookite. Included also is a series of pyrochlore supergroup minerals: fluorcalciopyrochlore, hydroxycalciomicrolite, hydroxyferroroméite, hydroxykenoelsmoreite, hydroxynatropyrochlore, oxynatromicrolite along with a new data on hydrokenoelsmoreite-3C and hydrokenoelsmoreite-6R.

Radionuclide disposal using the pyrochlore supergroup of minerals as a host matrix-A review.

Since the first large scale commercial nuclear power plant became operational in 1958, the nuclear power industry has been faced with the growing problem of disposal of radionuclides produced from nuclear fission. The current global production of high level nuclear waste is approximately 10,000 metric tons p.a., consisting predominantly of uranium, plutonium, actinides and other minor radionuclides. Developing a safe and cost-effective method for long term storage and disposal of nuclear waste is a key issue of concern to the nuclear power industry. A promising approach to radionuclide disposal is incorporation of the nuclear waste into refractory oxide host minerals or mineral matrices. This technique offers lower leaching rates when compared to the commonly used glass-based vitrification approaches. The refractory pyrochlore supergroup of minerals are particularly attractive for this purpose as they can incorporate considerable amounts of the radionuclides: 93Zr, 133Ba, 135Cs, Th, U, 238Pu, and 244Cm, while demonstrating very low leachability. This review examines the structural, compositional and chemical properties of radionuclide-containing pyrochlore supergroup minerals. Compiled leaching data for radionuclides hosted in pyrochlores demonstrates that these materials offer a high degree of aqueous durability making them strong candidates for radionuclide disposal, offering a viable storage alternative to traditional vitrification methods.

Three-D Mineralogical Mapping of the Kovdor Phoscorite-Carbonatite Complex, NW Russia: III. Pyrochlore Supergroup Minerals

The pyrochlore supergroup minerals (PSM) are typical secondary phases that replace (with zirconolite–laachite) earlier Sc-Nb-rich baddeleyite under the influence of F-bearing hydrothermal solutions, and form individual well-shaped crystals in surrounding carbonatites. Like primary Sc-Nb-rich baddeleyite, the PSM are concentrated in the axial carbonate-rich zone of the phoscorite-carbonatite complex, so their content, grain size and chemical diversity increase from the pipe margins to axis. There are 12 members of the PSM in the phoscorite-carbonatite complex. Fluorine- and oxygen-dominant phases are spread in host silicate rocks and marginal carbonate-poor phoscorite, while hydroxide-dominant PSM occur mainly in the axial carbonate-rich zone of the ore-pipe. Ti-rich PSM (up to oxycalciobetafite) occur in host silicate rocks and calcite carbonatite veins, and Ta-rich phases (up to microlites) are spread in intermediate and axial magnetite-rich phoscorite. In marginal (apatite)-forsterite phoscorite, there are only Ca-dominant PSM, and the rest of the rocks include Ca-, Na- and vacancy-dominant phases. The crystal structures of oxycalciopyrochlore and hydroxynatropyrochlore were refined in the Fd3¯m space group with R1 values of 0.032 and 0.054 respectively. The total difference in scattering parameters of B sites are in agreement with substitution scheme BTi4+ + YOH‒ = BNb5+ + YO2‒. The perspective process flow diagram for rare-metal “anomalous ore” processing includes sulfur-acidic cleaning of baddeleyite concentrate from PSM and zirconolite–laachite impurities followed by deep metal recovery from baddeleyite concentrate and Nb-Ta-Zr-U-Th-rich sulfatic product from its cleaning.

Evidence for Nb-Ta Occurrences in the Syn-Tectonic Pan-African Mayo Salah Leucogranite (Northern Cameroon): Constraints from Nb-Ta Oxide Mineralogy, Geochemistry and U-Pb LA-ICP-MS Geochronology on Columbite and Monazite

The Mayo Salah pluton, which is located in the North-Cameroon domain of Central African Bold Belt (CAFB), is emplaced as a laccolith in volcano-sedimentary schists of Poli series, and displays features of Rare-metal Granite (RMG). It is made of two main rock groups: (1) the metaluminous barren muscovite granite (MsG) and (2) the Nb-Ta bearing peraluminous leucogranite (MsL) which expresses four subtypes. The evolved Rare-element MsL is subalkaline, slightly peraluminous (ASI = 1.01–1.21), and it displays flat REE chondrite-normalized patterns with a strong negative Eu anomaly (Eu/Eu* = 0.02–0.20). It belongs to the peraluminous low phosphorus Rare-element Granites and L-type igneous rocks, as shown by the relatively low Zr/Hf (4.8–14) and Nb/Ta (1.4–9.0) ratios and the positive slope of the Zr-Hf-Nb-Ta profile in spider diagrams. The rare-element-bearing mineral is represented by columbite-group minerals (CGM) and other Nb-Ta-oxides (Nb-rutile and pyrochlore supergroup minerals). The CGM is classified as Mn-columbite, with Ta# and Mn# ratios increasing from core to rim. Two stages of mineralization are identified; the earliest stage (CGM-I) consists in scattered tabular or prismatic euhedral grains that were related to magmatic fractionation. The latest stage (CGM-II) is expressed as a Ta-rich Mn hydrothermal CGM episode represented as rims and/overgrowths around and/or as veinlet crosscutting CGM-I or in cleavage planes of muscovite. The U-Pb dating of columbite and monazite of the Mayo Salah leucogranite indicates a late-Neoproterozoic magmatic-hydrothermal mineralization event from 603.2 ± 5.3 to 581.6 ± 7.2 Ma, as consistent with both late D2 to D3 events that were recorded in the CAFB in Cameroon, and the associated continental collision environment. The Nb-Ta mineralization of the Mayo Salah pluton provides evidence for the presence of RMG in Northern Cameroon of CAFB, and its temporal association with the youngest period of metallogenic epoch of Nb-Ta-ore formation in Africa associated to Pan-African times.

Diversity of Ti–Sn–W–Nb–Ta oxide minerals in the classic granite-related magmatic–hydrothermal Cínovec/Zinnwald Sn–W–Li deposit (Czech Republic)

The diversity of Ti–Sn–W–Nb–Ta oxide minerals within a complex granite-related magmatic–hydrothermal system was studied by electron microprobe on the example of the Cinovec (Zinnwald) granite cupola containing a well-known Sn–W–Li deposit. The Cinovec granite cupola is a late Variscan ( ca . 322 Ma), strongly fractionated A-type pluton composed of upper zinnwaldite granite and lower biotite granite. The zinnwaldite granite contains numerous subhorizontal quartz veins and greisen bodies, both composed of quartz, zinnwaldite and topaz and minor amounts of fluorite and Ti–Sn–W–Nb–Ta oxide minerals. Common Nb–Ta-rich rutile and rare cassiterite, columbite-(Fe) and -(Mn), and minerals of the pyrochlore-supergroup were found in biotite granite in deeper parts of the pluton, whereas common columbite occurs with rare W-rich ixiolite, qitianlingite, pyrochlores, and scheelite in some varieties of zinnwaldite granite. In the uppermost part of the cupola, Ta-rich cassiterite, microlite and pyrochlore are frequently observed. The greisens host predominantly wolframite, scheelite and Ta-poor cassiterite, which are accompanied by columbite-group minerals, rare W-rich ixiolite and qitianlingite. Throughout the pluton, the molar Mn/(Fe + Mn) ratio in columbite increases from 0.15–0.40 in the deeper part of the zinnwaldite granite (depths of 500–635 m) to 0.5–0.9 in granite and greisen in the uppermost part of the pluton (depths of 0–180 m), but no systematic changes in the Ta/(Nb + Ta) ratio were found. On the scale of individual zoned crystals, the rims are always Ta-enriched, whereas the Mn/(Fe + Mn) values in the rims are equal or lower. The Nb,Ta-rich rutile, interiors of the columbite crystals and Ta-rich cassiterite disseminated in the granites are interpreted as products of magmatic crystallization. The Ta-rich rims of columbite-(Fe) to -(Mn), Ta-poor cassiterite and all wolframite found in the greisen should be attributed to the early hydrothermal (greisenization) stage, whereas W-rich ixiolite, qitianlingite, and pyrochlore-supergroup minerals appear to belong to the later, hydrothermal event at comparatively lower temperature.

Tantalum-(niobium-tin) mineralisation in pegmatites and rare-metal granites of Africa

Tantalum is an important metal for high-technology applications of the modern world. It is recovered from oxide minerals that are present as minor constituents in rare-metal granites (RMG) and granitic rare-element pegmatites (REP). Significant potential exists as a by-product of spodumene mining from pegmatites, and from Nb-Zr-rare earth element (REE) mining from carbonatites and peralkaline igneous rocks. Columbite-group minerals (CGM) account for the majority of the current Ta production; other Ta-Nb oxides such as tapiolite, wodginite, ixiolite, rutile and pyrochlore-supergroup minerals are of minor importance. The estimated Ta resources of Africa are >50,000 tons of contained Ta 2 O 5 , representing 16% of the world resources. Currently, the economically most important mining districts in Africa are located within the Kibara Belt (mainly Rwanda and the DR Congo), around the Kenticha mine (Ethiopia) and in the Alto Ligonha Province (Mozambique). Production from pegmatites and related placer deposits is mostly from hundreds of artisanal and small-scale workings with only a handful of semi-industrial Ta mines. Collectively, African operations are currently responsible for more than half of the world’s primary Ta. Rare-metal granite and REP districts in Africa are part of Archaean, Palaeoproterozoic, Neoproterozoic, Palaeozoic and Mesozoic provinces. Each period of Ta ore formation is characterised by specific mineralogical and geochemical features. Compositions of CGM are variable: Fe-rich types predominate in the Man Shield (Sierra Leone), the Congo Craton (DR Congo), the Kamativi Belt (Zimbabwe) and the Jos Plateau (Nigeria); Mn-rich columbite-tantalite is typical of the Alto Ligonha Province (Mozambique), the Arabian-Nubian Shield (Egypt, Ethiopia) and the Tantalite Valley pegmatites (southern Namibia) and Large compositional variations through Fe-Mn fractionation, followed by Nb-Ta fractionation are typical for pegmatites of the Kibara Belt of Central Africa, pegmatites associated with the Older Granites of Nigeria and many pegmatites in the Damara Belt of Namibia. Trace element concentrations in CGM and other Ta-Nb oxides are highly variable, with Ti, W, Sn, Zr, Hf, Y, Sc and REE displaying significant regional differences that may be used to discriminate Ta provinces.

The Euxenite-Group Minerals and Products of Their Alteration In The Hybrid Julianna Granitic Pegmatite, Piława GÓrna, Sudetes, Southwestern Poland

Abstract Polycrase-(Y) and euxenite-(Y) represent the euxenite-group minerals (EGM) that formed as magmatic minerals in the sequence of (Y,REE,U,Th)–(Nb,Ta,Ti) oxides: columbite-(Fe) → Ti-bearing ixiolite ± ferrowodginite → samarskite-group minerals [ishikawaite → samarskite-(Y)] → EGM in the moderately fractionated granitic pegmatites of the hybrid NYF + LCT Julianna pegmatitic system at Pilawa Gorna, Sudetes, SW Poland. The EGM crystallized with increasing Ti activity and decreasing Ta/(Ta + Nb) and Mn/(Mn + Fe) from F-poor and LREE-depleted melt. The variation in their composition results mainly from substitution along the A (U,Th) B Ti A (Y,REE) –1 B (Nb,Ta) –1 vector. Metasomatic alteration of the EGM resulted in the formation of a variety of secondary pyrochlore supergroup minerals (PSM). The chemical composition of the PSM depends on the access gained by the metasomatic fluid to the replacement front. Alteration at sites of restricted access of the fluid led first to the formation of atypical Ca-enriched EGM evolving to zero-valence-dominant betafite with Y + REE as the second-rank A -site component, and finally to (Y,REE)-rich zero-valence-dominant pyrochlore. These PSM closely reflect the compositions of the parental EGM as indicated by similar Ti/(Ti + Ta + Nb), Ta/(Ta + Nb), and Mn/(Mn + Fe) values. If the transport of ions to and from the replacement front was relatively easy, the compositions of the alteration products were less limited by the composition of the primary oxides and zero-valence-dominant pyrochlore with Ca or U + Th as the second-rank A -site components formed. This type of alteration took place with decreasing Ti/(Ti + Nb + Ta), Y/(Y + REE), and Mn/(Mn + Fe) but highly variable Ta/(Ta + Nb). The addition of Ca, Fe, Mg, Sn, and Pb to the pegmatitic system during metasomatic alteration of the EGM could have been achieved through: (1) reaction of the orthomagmatic fluid with the amphibolite wall rock, circulating in a convective cell around the pegmatites; (2) assimilation of the wall rock by the pegmatite-forming melt; or (3) autometasomatic processes. However, given the current state of knowledge about the geochemical evolution of the Julianna pegmatitic system, the first scenario seems most likely.

Mineralogical and chemical evolution of tantalum–(niobium–tin) mineralisation in pegmatites and granites. Part 2: Worldwide examples (excluding Africa) and an overview of global metallogenetic patterns

Mineralogical and chemical evolution of tantalum–(niobium–tin) mineralisation in pegmatites and granites. Part 2: Worldwide examples (excluding Africa) and an overview of global metallogenetic patterns

Pyrochlore and columbite-tantalite as indicator minerals for specialty metal deposits

Pyrochlore supergroup and columbite-tantalite series minerals can be used as indicators for specialty metal deposits. Their mineral chemistry can be used to target specific specialty metal deposit-types and provide a vector towards mineralisation. A ternary Nb-Ti-Ta discrimination diagram for the pyrochlore supergroup shows that minerals from carbonatites and related deposits occupy the pyrochlore field and extend slightly into the betafite and microlite fields (>35% Nb, <65% Ti, and <45% Ta). Those from peralkaline complexes define a narrow zone paralleling the Nb-Ti join (>40% Nb, <60% Ti, and <10% Ta). Pyrochlore supergroup minerals from Li-Cs-Ta enriched (LCT) pegmatites occupy the microlite field (<90% Nb, ≤40% Ti, and > 15% Ta) whereas those from peraluminous granites plot in the microlite field along the Nb-Ta join, extending slightly into the pyrochlore field (<65% Nb, <10% Ti, and >35% Ta) on the Nb-Ti-Ta ternary diagram. The discrimination diagram for pyrochlore supergroup minerals is based on 349 new electron microprobe analyses from British Columbia and Québec, Canada, combined with 879 published analyses from 50 deposits worldwide. A columbite-tantalite series discrimination quadrilateral indicates that minerals from carbonatites and related deposits are Nb-rich, with Mn/(Mn+Fe) and Ta/(Ta+Nb) atomic ratios ≤ 0.25 and ≤ 0.20, respectively. Columbite from peralkaline complexes plot along the FeNb 2 O 6 –MnNb 2 O 6 join of the quadrilateral and have Mn/(Mn+Fe) ratios of ≤ 0.65 and Ta/(Ta+Nb) ratios of ≤ 0.15. Fe-Nb-rich varieties within this zone slightly overlap with those from carbonatites. Columbite-tantalite series minerals from Li-Cs-Ta (LCT) pegmatites and peraluminous granites are not well constrained but do slightly overlap with columbite-(Fe) from carbonatites. A minor overlap exists between the fields from carbonatites, peralkaline complexes, LCT pegmatites, and peraluminous granites near the columbite-(Fe) corner of the quadrilateral. The discrimination diagram for columbite-tantalite series minerals is based on 153 new electron microprobe analyses from British Columbia combined with over 2500 analyses complied from the literature. Combining the two discrimination diagrams presented here enhances their effectiveness to explore for specialty metal deposits.

"Silicified" pyrochlore from nepheline syenite (mariupolite) of the Mariupol Massif, SE Ukraine: A new insight into the role of silicon in the pyrochlore structure

Pyrochlore-supergroup minerals containing relatively high Si concentration are quite common in various geochemical parageneses, e.g., carbonatites, alkaline syenites, pegmatites. However, the role of Si and the mechanism of its incorporation into the structure of these minerals, although widely discussed, have not been explained definitively. Our paper reports the results of comprehensive SEM, EPMA, XRD, TEM, and MAS-NMR studies performed for the first time on a natural pyrochlore, which is the late-magmatic to early hydrothermal accessory component of the nepheline syenite in the alkaline Mariupol massif in Ukraine. It represents partly metamict, patchy-zoned, A -cation depleted, REE-, U-, and Th-bearing fluornatropyrochlore, locally exceptionally rich in SiO 2 (up to 13.01 wt%) that underwent both primary and secondary alterations, leading to kenopyrochlore or hydropyrochlore species. The primary alteration was induced by high-temperature, Ca 2+ - and Si 4+ -rich, and F − moderate fluids, which affected only some domains of the pyrochlore crystals and resulted in filling the A site vacancies mainly by Ca 2+ , but also Mn 2+ , Sr 2+ , and K + . The secondary alteration, induced by the exposure of the host rock to ground water driving fluid-mediated coupled dissolution-reprecipitation process, affected the whole pyrochlore crystals (both Si-enriched and Si-free domains) and caused, among others, the leaching of some A- and Y- site components. TEM investigations indicate that the selected-area electron diffraction patterns taken from Si-poor areas show strong and sharp diffraction spots related to well-crystalline pyrochlore, whereas the Si-rich areas show weaker spots with a diffuse diffraction halo that are typical of metamict material. Due to the fact that no intergrowth with other Si-bearing phases was observed in the TEM images even at very high magnification, it might be concluded that Si 4+ can occupy severely α-decay damaged and chemically altered portions of this structure. The absence of Si in the sixfold-coordinated B site has been corroborated both by compositional relationships, and by the lack of any [6] Si 4+ signal around −200 ppm in the MAS-NMR spectrum. A broad signal in the spectrum appearing at around −84 ppm, points to an amorphous species with tetrahedrally coordinated Si, close to Q (2) species defined as Si atom with two bridging O atoms, i.e., [Si(OSi) 2 (−) 2 ], in the form of finite-length chain-like structures, located in the damaged A and B sites of the primary structure.

Tantalum–(niobium–tin) mineralisation in African pegmatites and rare metal granites: Constraints from Ta–Nb oxide mineralogy, geochemistry and U–Pb geochronology

Tantalum, an important metal for high-technology applications, is recovered from oxide minerals that are present as minor constituents in rare-metal granites and granitic rare-element pegmatites. Columbite-group minerals (CGM) account for the majority of the current tantalum production; other Ta–Nb oxides (TNO) such as tapiolite, wodginite, ixiolite, rutile and pyrochlore-supergroup minerals may also be used.In this paper mineralogical and geochemical data with a focus on opaque minerals as well as age determinations on CGM using the U–Pb method are presented for 13 rare-element granite and pegmatite districts in Africa, covering Archean, Paleoproterozoic, Neoproterozoic, Paleozoic and Mesozoic provinces. Geological, economic and geochronological data are reviewed.Each period of Ta-ore formation is characterised by peculiar mineralogical and geochemical features that assist in discriminating these provinces. Compositions of CGM are extremely variable: Fe-rich types predominate in the Man Shield (Sierra Leone), the Congo Craton (Democratic Republic of the Congo), the Kamativi Belt (Zimbabwe) and the Jos Plateau (Nigeria). Mn-rich columbite–tantalite is typical of the Alto Ligonha Province (Mozambique), the Arabian–Nubian Shield and the Tantalite Valley pegmatites (southern Namibia). Large compositional variations through Fe–Mn fractionation, followed by Nb–Ta fractionation are typical for pegmatites of the Kibara Belt of Central Africa, pegmatites associated with the Older Granites of Nigeria and some pegmatites in the Damara Belt of Namibia. CGM, tapiolite, wodginite and ixiolite accommodate minor and trace elements at the sub-ppm to weight-percent level. Trace elements are incorporated in TNO in a systematic fashion, e.g. wodginite and ixiolite carry higher Ti, Zr, Hf, Sn and Li concentrations than CGM and tapiolite. Compared to tapiolite, CGM have higher concentrations of all trace elements except Hf and occasionally Zr, Ti, Sn and Mg. The composition of TNO related to rare-element pegmatites is rather different from rare-metal granites: the latter have high REE and Th concentrations, and low Li and Mg. Pegmatite-hosted TNO are highly variable in composition, with types poor in REE, typical of LCT-family pegmatites, and types rich in REE — showing affinity for NYF-family or mixed LCT–NYF pegmatites. Major and trace elements show regional characteristics that are conspicuous in normalised trace element and REE diagrams. In general, CGM from Ta-ore provinces are characterised by the predominance of one type of REE distribution pattern characterised by ratios between individual groups of REE (light, middle, heavy REE) and the presence and intensity of anomalies (e.g. Eu/Eu*).Despite textural complexities such as complex zoning patterns and multiple mineralisation stages, the chemical compositions of CGM, tapiolite and wodginite–ixiolite from rare-metal granite and rare-element pegmatite provinces indicate that they are cogenetic and reflect specific source characteristics that may be used to discriminate among rocks of different origin.Geochronological data produced for CGM from ore districts are discussed together with the respective ore mineralogy and minor and trace element geochemistry of TNO to reconsider the geodynamics of pegmatite formation. In Africa, formation of rare element-bearing pegmatites and granites is related to syn- to late-orogenic (e.g., West African Craton, Zimbabwe Craton), post-orogenic (Kibara Belt, Damara Belt, Older Granites of Nigeria, Adola Belt of Ethiopia) and anorogenic (Younger Granites of Nigeria) tectonic and magmatic episodes. The late-orogenic TNO mineralisation associated with A-type granites in the Eastern Desert of Egypt shares geochemical features with the anorogenic Younger Granites of Nigeria.

THE CRYSTAL STRUCTURE OF A MICROLITE-GROUP MINERAL WITH A FORMULA NEAR NaCaTa2O6F FROM THE MORRO REDONDO MINE, CORONEL MURTA, MINAS GERAIS, BRAZIL

We present electron-microprobe and single-crystal X-ray-diffraction data for a microlite-group mineral with a formula near NaCaTa 2 O 6 F from the Morro Redondo mine, Coronel Murta, Minas Gerais, Brazil. On the basis of these data, the formula is A (Na 0.88 Ca 0.88 Pb 0.02 □ 0.22 ) Σ2.00 B (Ta 1.70 Nb 0.14 Si 0.12 As 0.04 ) Σ2.00 X [(O 5.75 (OH) 0.25 ] Σ6.00 Y (F 0.73 □ 0.27 ) Σ1.00 . According to the new nomenclature for the pyrochlore-supergroup minerals, it is intermediate between fluornatromicrolite and “fluorcalciomicrolite”. The crystal structure, Fd 3 m , a = 10.4396(12) A, has been refined to an R 1 value of 0.0258 ( wR 2 = 0.0715) for 107 reflections (Mo K α radiation). There is a scarcity of crystal-chemical data for pyrochlore-supergroup minerals in the literature. A compilation of these data is presented here.