Pristine vs. contaminated trends in Nb,Ta-oxide minerals of the Jihlava Pegmatite District,Czech Republic

Summary Columbite-tantalite is widespread in the lepidolite-subtype rare-element pegmatites of the Jihlava pegmatite district, western Moravia, Czechoslovakia. In most pegmatites, fractionation of columbite-tantalite shows initial enrichment in Mn followed by increasing Ta, in accordance with the usual trend from ferrocolumbite through mangano-columbite to manganotantalite, typical of the lepidolite pegmatites. Many dikes, however, show local deviations toward Fe- and Ti-rich compositions. In extreme cases, all columbite-tantalite is strongly enriched in Fe and Ti, and is associated with ixiolite and tantalian rutile. The degree of enrichment of the Nb,Ta oxide-mineral assemblage in Fe and Ti is proportional to tectonic introduction of wallrock xenoliths of mafic pyroxene-biotite syenite into the pegmatites during late stages of their consolidation. Extensive reaction of the residual pegmatite melt with the xenoliths contaminated the near-by melt, and generated Nb,Ta oxide minerals and tourmaline of non-typical chemistries. Late fersmite probably formed after thermal equilibration of the pegmatites with their syenitic country rocks, from Ca-bearing regional interstitial fluids pervading through the solidified pegmatites.

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Zusammenfassung In Selten-Element Pegmatiten (Lepidolith-Subtyp) des Jihlava Pegmatit Distriktes, West-Mähren, Tschechien, ist Columbit-Tantalit weitverbreitet. In den meisten Pegmatiten führte die Fraktionierung von Columbit-Tantalit zur einer anflinglichen Anreicherung von Mn gefolgt von einer Zunahme in Ta, vergleichbar mit dem für Lepidolith-Pegmatit bekannten Trend Ferrocolumbit-Manganocolumbit-Manganotantalit. Viele Gänge zeigen jedoch lokale Abweichungen zu Fe- und Ti-reichen Zusammensetzungen. In extremen Fällen ist Columbit-Tantalit stark an Fe und Ti angereichert und mit Ixiolith und Tantalo-Rutil vergesellschaftet. Das Ausmaß der Fe und Ti Anreicherung in (Nb,Ta)-Oxid-Assoziationen ist proportional dem tektonisch bedingten Eindringen von maischen Pyroxen-Biotit-reichen Syenit-Nebengesteinsxenolithen in die Pegmatite gegen Ende ihrer Verfestigung. Tiefgreifende Reaktion der Pegmatit-Restschmelze mit den Xenolithen kontaminierte die Schmelze und führte zur Bildung von (Nb,Ta)-Oxid-Mineralen und Turmalin ungewöhnlicher Zusammensetzung. Möglicherweise nach der thermischen Gleichgewichtseinstellung der Pegmatite mit ihren syenitischen Nebengesteinen bildete sich Fersmit unter Beteiligung von Ca-führenden regionalen Porenlösungen, welche die verfestigten Pegmatite durchdrangen.

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Nb-Ta-minerals from the cap de creus pegmatite field,eastern Pyrenees: distribution and geochemical trends

Summary Internal structure and mineralogy facilitate distinction of four main pegmatite types at the eastern end of the Pyrenees. Three main trends in compositional variations in Nb-Ta-Sn-REE-Ti minerals have been established: a regional trend, with Ta/(Ta + Nb) ratio increasing towards the more evolved pegmatites, Mn/(Mn + Fe) being relatively low and increasing only slightly; a single-body trend, with similar enrichment toward the late pegmatite units; a single-crystal trend, with zoning related to both Ta/(Ta + Nb) and Mn/(Mn + Fe) ratios and a tendency toward Ta-enrichment in the late growth stages. The regional geochemical enrichment trends in the Mn/(Mn + Fe) ratios and Ta/(Ta + Nb) are those expected for a beryl-columbite pegmatite type. In a single pegmatite, the evolution depends on the simultaneous growth of other mineral species. Three factors seem to control the development of zoning in columbite-tantalite crystals: availability of Mn, Ta, Fe, Nb, significant differences in solubility between mineral group end members and re-equilibria with late pegmatite fluids.

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Nb-Ta-Minerale aus dem Pegmatit-Feld vom Cap de Creus, östliche Pyrenäen: Verteilung und geochemische Trends

Zusammenfassung Am Ostrand der Pyrenäen können anhand des inneren Aufbaus und der Mineralogie vier Haupttypen von Pegmatiten unterschieden werden. Die Zusammensetzungen von Nb-Ta-Sn-SEE-Ti-Mineralen folgen drei Haupttrends: einem regionalen Trend, bei dem das Verhältnis Ta/(Ta + Nb) zu den höher entwickelten Pegmatiten hin zunimmt, während Mn/(Mn + Fe) relativ niedrig ist und nur leicht zunimmt; einem lokalen (auf das jeweilige Vorkommen beschränkten) Trend mit einer ähnlichen Anreicherung zu den spätpegmatitischen Einheiten hin; einem auf Einzelkristalle bezogenen Trend mit Zonierung in bezug auf die Verhältnisse Ta/(Ta + Nb) und Mn/(Mn + Fe) und einer Tendenz zur T a-Anreicherung in den späten Wachstumsphasen. Die regionalen geochemischen Anreicherungstrends in den Mn/(Mn + Fe)- und Ta/(Ta + Nb)-Verhältnissen entsprechen jenen, wie sie für den Beryll-Columbit-Pegmatit-Typ erwartet werden. In einem einzelnen Pegmatit hängt die Entwicklung vom gleichzeitigen Wachstum anderer Mineral-Spezies ab. Drei Faktoren scheinen die Ausbildung einer Zonierung in Columbit-Tantalit-Kristallen zu kontrollieren: das Angebot an Mn, Ta, Fe und Nb, deutliche Unterschiede in der Löslichkeit der Endglieder von Mineralgruppen und die Iteequilibrierung mit spätpegmatitischen Lösungen.

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新疆阿尔泰可可托海3号伟晶岩脉重钽铁矿研究

重钽铁矿是主要见于高度分馏花岗伟晶岩中的一种罕见的钽矿物。新疆阿尔泰可可托海3号伟晶岩脉中存在的重钽铁矿呈他形，与钽猛矿(因Ta含量的明显差异可分为钽锰矿I和钽锰矿Ⅱ)和铀细晶石共生形成铌钽矿物集合体。钽锰矿I构成集合体的主体，而重钽铁矿与相对富Mn和Ta的钽锰矿Ⅱ紧密共生并分布在集合体的边部。重组铁矿的RTa[RTa＝Ta／(Nb Ta)]在0．96—0．99间变化，RMn[RMn＝Mn／(Fe Mn)]在0．10—0．2l间变化。铌钽铁矿的共生组合和化学成分特征表明，重钽铁矿是钽锰矿I在非平衡条件下快速结晶后残余亚稳定相出溶的产物。     

Primary compositional range and alteration trends of microlite from the yellowknife pegmatite field,Northwest territories,Canada

Summary Microlite occurs as a rare accessory mineral in beryl-columbite, beryl-columbitephosphate, complex spodumene, albite-spodumene and amblygonite type rare-element granitic pegmatites in the Archean Yellowknife pegmatite field of the Canadian Shield. The chemistry of microlite is variable but consistent with the accepted structural formula A_2–mB₂X₆Y_1–n pH₂O, where generally A = Ca,Na; B = Ta,Nb; X = O; Y = O,OH, F; m = 0 - 2; n = 0 - 1 and p = 0 - l. The chemistry of the Yellowknife microlite is dominated by Ca, Na, Ta, and Nb with minor amounts of U, Pb, Fe, Mn, and Ti. The compositions of microlite are interpreted to reflect primary variability and effects of late-stage alteration.Two principal types of microlite can be distinguished by their primary composition and alteration trends. U-poor microlite originated by the metasomatic replacement of pre-existing manganocolumbite, manganotantalite, and ferrotapiolite; with progressive alteration, its composition evolves from early Ca-rich, Fe,Mn-poor members to late Ca,Na-poor, Fe,Mn-enriched members. In contrast, U-bearing microlite formed from U-enriched, moderately fractionated pegmatitec fluids acting upon ferrocolumbite, manganocolumbite, and manganotantalite; with progressive alteration, its composition evolves from U,Ca,Na-enriched members to U,Ca,Na-poor, Fe,Mn-enriched members.

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Primärer zusammensetzungsbereich und umwandlungstrends der mikrolithe aus dem yellowknife pegmatitfeld, Northwest Territories, Kanada

Zusammenfassung Primärer Zusammensetzungsbereich und Umwandlungstrends der Mikrolithe aus dem Yellowknife Pegmatitfeld, Northwest Territories, Kanada Mikrolith kommt in den Beryll-Columbit-, Beryll-Columbit-Phosphat-, komplexen Spodumeri-, Albit-Spodumen- und Amblygonit-Typen der Seltene-Element-Granitpegmatite im archäischen Yellowknife Pegmatitfeld des Kanadischen Schildes als seltenes akzessorisches Mineral vor. Der Chemismus des Mikroliths variiert, ist aber mit der gebräuchlichen Strukturformel A_2–mB₂X_1–mY_1–n·pH₂O verträglich, mit im allgemeinen A = Ca,Na, B = Ta,Nb, X = O, m=0–2, n =O–1 und p=0–1. Der Chemismus des Yellowknife Mikroliths wird durch Ca, Na, Ta und Nb dominiert, U, Pb, Fe, Mn und Ti treten in kleineren Mengen auf. Die Zusammensetzungen des Mikroliths spiegeln die primäre Variabilität sowie die Auswirkungen späterer Umwand lungen wieder.Zwei Haupttypen des Mikroliths können nach ihrer primären Zusammensetzung und den Umwandlungstrends unterschieden werden. U-armer Mikrolith entstand durch metasomatischen Ersatz von früherem Manganocolumbit-Manganotantalit und Ferrotapiolith, mit fortschreitender Umwandlung entwickelt sich seine Zusammensetzung von frühen Ca-reichen, Fe,Mn-armen Gliedern zu späten Ca,Na-armen, Fe,Mn-angereicherten Gliedern. Im Gegensatz dazu bildete sich U-haltiger Mikrolith aus an U angerereicherten, mäßig fraktionierten pegmatitischen Fluiden, die auf Manganocolumbit-Manganotantalit einwirkten, mit fortschreitender Umwandlung entwickelt sich seine Zusammensetzung von U,Ca,Na-angereicherten Gliedern zu U,Ca,Na-armen, Mn,Feangereicherten Gliedern.

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Phyllosilicate and rutile compositions as indicators of Sn specialization in some southeastern Australian granites

Biotites from unaltered Sn granites in southeastern Australia are highly ferroan, Fe/(Fe+Mg+Mn) >0.75, whereas biotites from barren granites are less Ferich, Fe/(Fe+Mg+Mn)<0.65. Similar distinctions between Sn-specialized and barren granites can be observed in the other phyllosilicates, especially chlorite. Biotites and muscovites from Sn granites have greater Be, Cs, (F), Li, Mo, Rb, Sc, Sn, Tl, (Y) and Zn and lesser Ba abundances than corresponding micas from barren granites in the same district. Alteration of barren granites also results in similar enrichments in micas. Of these elements, Sn and Zn, because of their abundance and retention during degradation of biotite to chlorite, are the best trace element discriminants between barren granites and Sn granites/altered granites, with the Sn content of phyllosilicates being a better indicator than Zn. Rutile inclusions within phyllosilicates from unaltered Sn granites have Nb₂O₅ contents up to 26%. The Ta content tends to increase with Nb content but especially high Ta contents occur in the rutile inclusions of granites that give rise to pegmatitic deposits. The rutile inclusions in Sn granites may also have substantial Sn and W contents. The rutiles of barren granites have low Nb, Ta, Sn and W contents but Sn and W increase with alteration. Together, the ratio Fe/(Fe+Mg+Mn) and Sn contents in phyllosilicates and rutile compositions can be used to identify the Sn mineralization potential of a granite.     

Nb-Ta-Ti-W-Sn-oxide minerals as indicators of a peraluminous P- and F-rich granitic system evolution: Podlesí, Czech Republic

Summary The strongly peraluminous, P- and F-rich granitic system at Podlesí in the Krušné Hory Mountains, Czech Republic, resembles the zonation of rare element pegmatites in its magmatic evolution (biotite → protolithionite → zinnwaldite granites). All granite types contain disseminated Nb-Ta-Ti-W-Sn minerals that crystallized in the following succession: rutile + cassiterite (in biotite granite), rutile + cassiterite → ferrocolumbite (in protolithionite granite) and ferrocolumbite → ixiolite → ferberite (in zinnwaldite granite). Textural features of Nb-Ta-Ti-W minerals indicate a pre-dominantly magmatic origin with only minor post-magmatic replacement phenomena. HFSE remained in the residual melt during the fractionation of the biotite granite. An effective separation of Nb + Ta into the melt and Sn into fluid took place during subsequent fractionation of the protolithionite granite, and the tin-bearing fluid escaped into the exocontact. To the contrast, W contents are similar in both protolithionite and zinnwaldite granites. Although the system was F-rich, only limited Mn-Fe and Ta-Nb fractionation appeared. Enrichment of Mn and Ta was suppressed due to foregoing crystallization of Mn-rich apatite and relatively low Li content, respectively. The content of W in columbite increases during fractionation and enrichment in P and F in the melt. Ixiolite (up to 1 apfu W) instead of columbite crystallized from the most fluxes-enriched portions of the melt (unidirectional solidification textures, late breccia).     

Nb-Ta-Ti oxides fractionation in rare-metal granites: Kr��sno-Horn�� Slavkov ore district, Czech Republic

Nb-Ta-Ti-bearing oxide minerals (Nb-Ta-bearing rutile, columbite-group minerals) represent the most common Nb-Ta host in topaz-albite granites and related rocks from the Krásno-Horní Slavkov ore district. Tungsten-bearing columbite-(Fe), W-bearing ixiolite, wodginite and tapiolite-(Fe) are extremely rare in these rocks. Rutile contains significant levels of Ta (up to 37?wt.% Ta₂O₅) and Nb (up to 24?wt.% Nb₂O₅), with Ta/(Ta?+?Nb) ratio ranging from 0.04 to 0.61. Columbite-group minerals are represented mostly by columbite-(Fe) and rarely by columbite-(Mn), with Mn/(Mn?+?Fe) ratio ranging from 0.23 to 0.94. The exceptionally rare Fe-rich, W-bearing ixiolite occurs only as inclusions in Nb-Ta-bearing rutile from quartz-free alkali-feldspar syenites (Vysoky Kámen stock). Wodginite was found only in the topaz-albite microgranite of gneissic breccia matrix that occurs in the upper most part of the Hub topaz-albite granite stock. In wodginite, the Mn/(Mn?+?Fe) ratio is 0.42?C0.51, whereas the coexisting tapiolite-(Fe) has a distinctly lower Mn/(Mn?+?Fe) ratio close to 0.06.     

A ferrotantalite-ferrotapiolite intergrowth from Spittal a.d. Drau,Carinthia, Austria

Summary Intimate intergrowths of ferrotantalite and ferrotapiolite occur in a pegmatite in Spittal a.d. Drau, Carinthia. They are associated with muscovite, albite, smoky quartz, cassiterite, and microscopic uranmicrolite, zircon and uraninite. An assemblage of secondary uranium minerals is also present, generated by extensive alteration and leaching of the uranmicrolite and zircon. Textures of the ferrotantalite-ferrotapiolite intergrowths suggest considerable recrystallization that obliterated most of their primary features; neither coprecipitation nor exsolution can be recognized with certainty. Despite intersecting tielines indicating disequilibrium, the ferrotantalite and ferrotapiolite compositions show very restricted ranges (Mn/(Mn + Fe) 0.08–0.11, Ta/(Ta + Nb) 0.53–0.57 for ferrotantalite, and 0.01–0.04, 0.84–0.89 for ferrotapiolite, respectively), particularly in comparison with compositions from other localities featuring primary textures. A degree of compositional equilibration could have been attained during recrystallization. This process may also explain the high level of structural order characterizing both minerals; they are considerably disordered in other localities. Extensive deformation typical of pegmatites in the southern Ostalpen in general, and specifically of the Spittal pegmatite, is probably responsible for the recrystallization phenomena in the Ta, Nb, Sn-bearing mineral assemblage.

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Eine Ferrotantalit-Ferrotapiolit Verwachsung von Spittal a.d. Drau, Kärnten, österreich

Zusammenfassung In einem Pegmatit von Spittal a.d. Drau, Kärnten, treten enge Verwachsungen von Ferrotantalit und Ferrotapiolit auf. Diese werden von Muskovit, Albit, Rauchquarz, Zinnstein sowie-in mikroskopischem Masstab-von Uranmikrolith und Zirkon begleitet. Die Textur der Verwachsungen lässt Rekristallisation erheblichen Ausmasses erkennen, die die primären Merkmale weitgehend auslöscht. Weder eine gemeinsame Auskristallisation der beiden Mineralphasen noch eine Bildung durch Entmischung kann mit Sicherheit erkannt werden. Trotz einander kreuzender Verbindungslinien, die einen Hinweis auf Ungleichgewicht darstellen, zeigen die Zusammensetzungen des Ferrotantalits und des Ferrotapiolits lediglich geringe Schwankungsbreiten: Mn/(Mn + Fe) 0,08–0,11, Ta/(Ta + Nb) 0,53–0,57 für den Ferrotantalit beziehungsweise 0,01–0,04 und 0,84–0,89 für den Ferrotapiolit. Dies gilt insbesondere für den Vergleich mit Zusammensetzungen solcher Mineralphasen mit jenen von Fundarten, die primäre Verwachsungstrukturen aufweisen. Bis zu einem gewissen Ausmass ist diese homogene Zusammensetzung möglicherweise auf die Rekristallisation zurück zuführen. Diese Rekristallisation könnte auch den hohen strukturellen Ordnungsgrad der beiden Mineralphasen erklären. An anderen Fundorten zeigen diese Minerale strukturell merklich geringeren Ordnungsgrad. Intensive metamorphe überprägung, wie sie für die Pegmatite in den südlichen Ostalpen und insbesondere für jenen von Spittal typisch sind, kann wahrscheinlich als Ursache der Rekristallisationsphänomene der Ta-Nb-Sn Mineralparagenese angenommen werden.

     

The origin of rutile-ilmenite aggregates (“nigrine”) in alluvial-fluvial placers of the Hagendorf pegmatite province, NE Bavaria, Germany

Summary Titanium placer deposits occur in alluvial-fluvial drainage systems which dissect Moldanubian gneisses intruded by Late Variscan pegmatites (Hagendorf province) in southern Germany. Based upon their texture (zonation, exsolution lamellae, intergrowth), microchemical data (Nb, Cr, Ta, V, Fe, W, Sn) and mineral inclusions, two major grain types of intergrown rutile and ilmenite have been established. Grains of type A are always zoned and consist of rutile cores enveloped by ilmenite containing small inclusions of wolframite. A core-rim transition zone is characterized by complex relations of rutile and ilmenite, with rutile lamellae being rich in Nb, V and Fe. Types B1 and B2 aggregates consist of ilmenite with lamellae of niobian rutile and/or ilmenorutile, and additionally have inclusions of ferrocolumbite, pyrochlore, betafite, sphalerite, pyrrhotite and Fe oxides. Such grain types featuring an intimate intergrowth of rutile and ilmenite were called nigrine. Type-C grains are quite similar in their morphological appearance but consist of W-enriched rutile devoid of mineral inclusions and reaction products. Pseudorutile and leucoxene replacing minerals of the nigrine aggregates are presumably caused by supergene alteration under fluctuating redox conditions. Phosphate and aluminum remobilized by supergene processes led to the formation of hydrous Ti-rich phases containing Al, P and Fe. High Nb and W concentrations in nigrine aggregates and in rutile type C may be taken as a marker for highly differentiated granites or pegmatites. This has implications for both, heavy-mineral-based provenance analysis and stream sediment exploration.     

Intercrystalline cation partitioning between minerals of the triplite-zwieselite-magniotriplite and the triphylite-lithiophilite series in granitic pegmatites

Minerals of the triphylite-lithiophilite, Li(Fe, Mn)PO₄, and the triplite-zwieselite-magniotriplite series, (Mn, Fe, Mg)₂PO₄F, occur in the late stage period of pegmatite evolution. Unfortunately, neither are the genetic relationships between these phosphates fully understood nor are thermodynamic data known. Consequently, phosphate associations and assemblages from 8 granitic pegmatites — Clementine II, Rubicon II and III, and Tsaobismund (Namibia); Hagendorf-Süd and Rabenstein (Germany); Valmy (France); Viitaniemi (Finland) — have been tested for compositional zoning and intercrystalline partitioning of main elements by electron microprobe techniques. Although the selected pegmatites display varying degrees of fractionation, and the intergrowth textures indicate different genetic relationships between the phosphates, the plots of mole fractions X _Fe=Fe/(Fe+Mn+Mg+Ca), X _Mn=Mn/(Fe+Mn+Mg+Ca), and X _Mg=Mg/(Fe+Mn+Mg+Ca) can be fitted relatively well with smooth curves in Roozeboom diagrams. Their deviations from symmetrical distribution curves are mainly dependent upon X _Mg or X _Ca, and upon non-ideal solutions. Surprisingly small differences between the partition coefficients were detected for intergrowths of different origin. However, the partitioning of shared components among coexisting phases is clearly dependent upon the conditions of formation. Compositional zoning is observed only when both Fe–Mn phosphates are intergrown mutually or with other Fe–Mn–Mg mineral solid-solutios. Thus, the zoning does not seem to be due to continuous crystallization, but to later diffusion processes. The triplite structure has preference for Mn, Mg, and Ca, while Fe prefers minerals of the triphylite series. A quantification of main element fractionation between minerals of the triphylite and the triplite series is possible in the cases where diffusion can be excluded. For the Fe/(Fe+Mn) ratios of core compositions an equation with a high correlation coefficient (R=0.988) was determined: Fe/(Fe+Mn)_Tr=[Fe/(Fe+Mn)_Li]/{2.737-(1.737)[Fe/(Fe+Mn)_Li]} (Tr=triplite series, Li=triphylite series). Consequently, the Fe/(Fe+Mn) ratio of the triplite series can now also be used in the interpretation of pegmatite evolution, just like that of the triphylite series which has been successfully applied in the past.     

Accessory minerals of the Cínovec (Zinnwald) granite cupola,Czech Republic Part 1: Nb-, Ta- and Ti-bearing oxides

Summary In 1961–63 the Czechoslovakian Geological Survey drilled a 1596 m deep borehole in the Sn-W-mineralized Cinovec (Zinnwald) granite cupola. The hole traversed zinnwaldite granite (ZG) to 730 m, then protolithionite granite (PG). The boundary between the two granites is a transition zone (TZ) about 10 m thick. The oxides of Nb, Ta and Ti, present in accessory amounts, are columbite, ilmenorutile, rutile and pyrochlore. The columbite occurs in both granites, but in the PG only below 1558 m depth. Its crystals are strongly zoned, the zoning representing variations in Nb/(Nb + Ta) on the one hand, and non-uniform distribution of W on the other. The columbite in the TZ is strongly enriched in W, up to 32.6 wt% WO₃. The columbites with W < M⁴⁺ show the substitutions W⁶⁺ + M⁴⁺ 2(Nb, Ta)⁵⁺, where (M⁴⁺ = Ti, Sn, Th, U, Zr) and 6M⁴⁺ + 3M³⁺ 4Fe²⁺ 5(Nb, Ta)⁵⁺, where (M³⁺ = Sc, Y). In columbites with W > M⁴⁺, tungsten is introduced by the substitution W⁶⁺ + M⁴⁺ 2(Nb, Ta)⁵⁺, but also through the appearance of Fe³⁺ in the B site according to the replacement 2W⁶⁺ + Fe³⁺ 3(Nb, Ta)⁵⁺. The ratio Fe/(Fe + Mn + Ca) increases with depth, and Nb/(Nb + Ta) is higher in the PG.The ZG is characterized by the presence of ilmenorutile, which does not occur in the PG, where rutile contains at most only 4 wt% Nb₂O₅. Two types of substitution have been found in the ilmenorutile: Fe³⁺ + (Nb, Ta)⁵⁺ 2Ti⁴⁺; (Fe, Mn)²⁺ + 2(Nb, Ta)⁵⁺ 3Ti^4-. For the ilmenorutiles studied, the ratio [Fe³⁺/(Fe, Mn)²⁺]at is near 1.AU- and Nb-rich phase, containing up to 36.2 wt% UO₂, included in protolithionite, and missing from the ZG, has the composition of a defect pyrochlore, A²⁺ ₂ ⁵⁺(O₆), and forms a solid solution with U⁴⁺B₂ ⁴⁺(O₆]), where B^4-= Ti, Si, Zr, Sn. Electron microprobe analyses indicate that this phase is strongly hydrated.The crystal chemistry of Nb-, Ta- and Ti-oxides in the Cinovec cupola reflects the complex geochemistry of its component granites and the interaction of the minerals with an F- and CO₂-rich fluid phase. Among the thermodynamic parameters, fO₂ plays a predominant role in the early evolutionary stages.

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Résumé Un sondage profond (jusqu'à -1596m), a été réalisé en 1961–63 par le Service géologique tchécoslovaque, dans la coupole granitique, minéralisée en Sn-W, de Cinovec (Zinnwald), République tchèque. Ce sondage a recoupé un granite à zinnwaldite (ZG), relayé en profondeur (–730 m) par un granite à protolithionite (PG). Le contact entre ces deux granites est matérialisé par une zone de transition (TZ) puissante de 10 m environ. Les oxydes de Nb, Ta et Ti, présents en quantité accessoire, sont représentés par: columbite, ilménorutile, rutile et pyrochlore.La columbite apparaît tant dans ZG que dans PG, mais dans ce dernier uniquement dans la zone profonde (-1558.0 m). Ses cristaux sont fortement zonés. Le zonage reflète des variations du rapport Nb/(Nb + Ta) d'une part et une distribution hétérogène de W, d'autre part. La columbite de la zone de transition ZG-PG est très enrichie en W (jusqu'à 32.6 wt.~/ 0 W03). Les coiumbites à W < SM4+ présentent des substitutions W⁶⁺ + M⁴⁺ 2(Nb, Ta)⁵⁺, où (M4⁴⁺ = Ti, Sn, Th, U, Zr) et 6M⁴⁺ + 3M³⁺ 4Fe²⁺ + 5(Nb, Ta)^5-, où (M³⁺ = Se, Y). Dans celles à W > EM⁴⁺, outre la substitution W⁶⁺ + M⁴⁺ 2(Nb, T)⁵⁺, le tungstène est introduit grâce à l'apparition de Fe 3+ sur le site B suivant le schéma: 2W⁶⁺ + Fe³⁺ 3(Nb, Ta)⁵⁺. Le rapport Fe/(Fe + Mn + Ca) croit avec profondeur; celui Nb/(Nb + Ta) augmente dans PG.Le ZG est caractérisé par la présence de l'ilménorutile; par contre, celui-ci est absent dans PG, oú le rutile ne contient que 4 wt.% Nb₂0₅ au maximum. Deux types de substitution sont mis en évidence dans l'ilménorutile: Fe³⁺ + (Nb, Ta)⁵⁺ 2Ti4⁺; (Fe, Mn)²⁺ + 2(Nb, Ta)⁵⁺ 3Ti⁴⁺. Pour les ilménorutiles étudiés, le rapport [Fe3+/(Fe, Mn)2+]à, est proche de 1.Une phase riche en U (jusqu'à 36.2 wt.% UO₂) et Nb, incluse dans la protolithionite et absente dans ZG, a composition d'un pyrochlore lacunaire A²⁺[B₂ ⁵⁺(06), formant une solution solide avec U4+E:B₂4+(O₆), où B⁴⁺ = Ti, Si, Zr, Sn. Les analyses à la microsonde électronique indiquent que cette phase est fortement hydratée.La cristallochimie des oxydes de Nb, Ta et Ti dans la coupole de Cinovec reflète tant la complexité géochimique des granites qui la composent que l'interaction des minéraux avec une phase fluide riche en F et CO,. Parmi les paramètres thermodynamiques, fO₂ joue un râle prépondérant lors des stades d'évolution précoces.

     

Tantalite in Suzhou Granite：mineralogy and Geological Implications

Tantalite,occurring as intergranular tabular crystals,was reported for the first time in the Suzhou granite.Electron microprobe analyses show that it is rich in W and Ti,with a Ta/(Ta Nb) ratio ranging from 0.5 to 0.73 and a Mn(Mn Fe) ratio between 0.20 and 0.40.It is structurally distinct from isomorphic tapiolite by a remarked Ag Raman peak at 880cm^-1.The associated zircon is striking by significant enrichment of Hf,with the HfO2 content amounting up to 35%-40%,The discovery of tantalite suggests that the Suzhou granite should be classified as a S-type granite instead of A-type as considered previously.     

The Kenticha rare-element pegmatite,Ethiopia: internal differentiation,U–Pb age and Ta mineralization

The Kenticha rare-element pegmatite, a globally important tantalite source in the Neoproterozoic Adola Belt of southern Ethiopia, is a highly fractionated, huge (2,000 m long and up to 100 m thick), subhorizontal, sheet-like body, discordantly emplaced in ultramafic host rock. It corresponds to the spodumene subtype of the rare-element pegmatite class and belongs to the lithium–cesium–tantalum petrogenetic family. The Kenticha pegmatite is asymmetrically zoned from bottom to top into granitic lower zone, spodumene-free intermediate zone, and spodumene-bearing upper zone. A monomineralic quartz unit is discontinuously developed within the upper zone. Whole-rock data indicate an internal geochemical differentiation of the pegmatite sheet proceeding from the lower zone (K/Rb ~36, K/Cs ~440, Al/Ga ~2,060, Nb/Ta ~2.6) to the upper zone (K/Rb ~19, K/Cs ~96, Al/Ga ~1,600, Nb/Ta ~0.7). The latter one is strongly enriched in Li₂O (up to 3.21%), Rb (up to 4,570 ppm), Cs (up to 730 ppm), Ga (up to 71 ppm), and Ta (up to 554 ppm). Similar trends of increasing fractionation from lower zone to upper zone were obtained in muscovite (K/Rb 23–14, K/Cs 580–290, K/Tl 6,790–3,730, Fe/Mn 19–10, Nb/Ta 6.5–3.8) and columbite–tantalite (Mn/Mn + Fe 0.4–1, Ta/Ta + Nb 0.1–0.9). The bottom-to-top differentiation of the Kenticha pegmatite and the Ta mineralization in its upper part are principally attributed to upward in situ fractionation of a residual leucogranitic to pegmatitic melt, largely under closed system conditions. High MgO contents (up to 5.05%) in parts of the upper zone are the result of postmagmatic hydrothermal alteration and contamination by hanging wall serpentinite. U–Pb dating of Mn-tantalite from two zones of the Kenticha pegmatite gave ages of 530.2 ± 1.3 and 530.0 ± 2.3 Ma. Mn-tantalite from the Bupo pegmatite, situated 9 km north of Kenticha, gave an age of 529.2 ± 4.1 Ma, indicating coeval emplacement of the two pegmatites. The emplacement of the pegmatites is temporally related to postorogenic granite magmatism, producing slightly peraluminous, I-type plutons in the area surrounding the Kenticha pegmatite field. Fractionated members of this suite might be envisaged as potential parental magmas.     

Ti-poor hoegbomite in kornerupine-cordierite-sillimanite rocks from Ellammankovilpatti,Tamil Nadu,India

Hoegbomite occurs sparingly in minute (mostly 0.1 mm) grains with fine-grained hercynite, magnetite, and rutile in two coarse-grained kornerupine-cordierite-sillimanite rocks from Ellammankovilpatti, Tamil Nadu, India. The hoegbomite is Ti-poor (2.5 wt% TiO₂), Fe-rich (25–26% Fe as FeO), and contains 6.2–6.8% MgO, 59.8–60.1% Al₂O₃, 1.0–1.3% ZnO, 0.3–0.7% Cr₂O₃ and 0.02% Li₂O. Minor amounts (estimated not to exceed 0.2 wt% oxide) of V, Co, Ni, Ga, and Sn were detected on the electron microprobe, but Be, Nb, and Zr were not detected with the ion microprobe mass analyser. Assuming the crystal structure refined by Gatehouse and Grey (1982) to be applicable to the Ellammankovilpatti hoegbomite, the analyses were recalculated on a basis of 22 cations, 30 oxygens, and two hydroxyls, resulting in 49 to 53% of the iron being ferric. Identification of hoegbomite was confirmed by X-ray powder diffraction. Associated cordierite (Fe/(Fe+Mg)=0.14) and kornerupine (Fe/(Fe+Mg)= 0.27) contain 0.02 weight % Li₂O and 0.05–0.07% BeO, while only the kornerupine contains B₂O₃ — 1.57% (ion microprobe analyses). Hoegbomite and the other oxides may have crystallized at temperatures between 680 and 720° C (P6.5 kbar) following attainment of peak conditions by the reaction: kornerupine+sillimanite±rutile+ZnO+H₂O+O₂ =cordierite+chlorite+hercynite+hoegbomite +magnetite+B₂O₃.The conditions for hoegbomite formation at Ellammankovilpatti appear to be characteristic of many hoegbomite parageneses. Critical for hoegbomite are silica undersaturation and relatively high oxygen and water activities at fairly high temperatures, conditions which are most commonly attained in later phases of a metamorphic cycle in upper amphibolite- and granulite-facies terrains.     

Nb/Ta fractionation observed in eclogites from the Chinese Continental Scientific Drilling Project

This paper reports detailed analyses of Nb and Ta concentrations of 19 eclogite samples and their principal mineral constituents from the main drill hole of the Chinese Continental Scientific Drilling Project (CCSD) and nearby outcrops. We observe highly fractionated and overall suprachondritic Nb/Ta values in minerals, e.g., rutile (4.8–87), titanite (12–62) and amphibole (2.0–67). Amphiboles in amphibolites (retrograded from eclogite) can be classified into two groups: a low Nb/Ta group that bears higher Al contents and is thus of higher pressure origin, and a high Nb/Ta, lower pressure group. The former group was likely formed during subduction; the latter may have formed during exhumation in the presence of rutile and titanite. The significant Nb/Ta fractionation in rutile and other minerals may reflect early dehydration of the subducted slab at shallow depths before the formation of rutile, which occurs at depths ≥50 km. The dehydration, with amphiboles existing as the main Nb–Ta-bearing phase, would lead to Nb/Ta fractionation, i.e., forming subchondritic Nb/Ta ratios in the released fluids and, complementarily, suprachondritic Nb/Ta ratios in the residual phases. While a large proportion of the fluids may escape from the slab to the mantle wedge, considerable amounts of the fluids can be retained in hydrous minerals within the descending slab, thus forming hydrated cold eclogites with subchondritic Nb/Ta characteristics. As subduction continues to depths over 50 km, rutile appears and consequently controls the Nb–Ta budget. In the presence of rutile, melting of the hydrated cold eclogites with very low Nb/Ta ratios would form magmas with negative Nb, Ta anomalies and subchondritic Nb/Ta. Further dehydration of the continuously descending slab results in even more fractionated Nb/Ta ratios in subsequently released fluids and residues, providing a feasible explanation for the large Nb/Ta variation observed in the modern arc magmas and residual eclogites.     

Chemical variations and significance of phosphates from the Fregeneda-Almendra pegmatite field, Central Iberian Zone (Spain and Portugal)

Granitic pegmatites are widespread within a schist-metagreywacke complex in the Fregeneda-Almendra area (Central Iberian Zone). They intrude pre-Ordovician metasedimentary rocks and show a zonal distribution relative to the Meda-Penedono-Lumbrales granitic complex, from barren bodies to those enriched in Li, F, Sn, Nb>Ta, P and Be. Based on mineralogical criteria, these pegmatites are classified into three main categories: barren, intermediate and rare-element pegmatites, with each type including various subtypes. Phosphates are present in many pegmatites that usually occur as fine-grained accessory minerals. The most complex association of such minerals includes numerous Fe–Mn phosphates that occur in intermediate pegmatites. Al-phosphates are characteristic of Li-rich pegmatites. Electron microprobe analyses of representative phosphates reflect compositional differences depending on the pegmatite type. The Fe/(Fe+Mn) ratio of phosphates tends to decrease as the evolution degree of the pegmatites increases.     

THE GEOCHEMISTRY OF TANTALUM AND NIOBIUM

Ta and Nb are associated in nature. Both are oxyphile and are related geochemically to Fe, Mn, Ti, rare earths U, Th, Zr, W, Sn, Bi, and Sb. Both accompany the alkali metals,especially Na and Li. Their close relationship explains their isomorphism in mineral-forming processes. Zr, W, and Sn entrain Ta and Nb in the crystal lattices of their minerals in limited amounts. The concentration of Ta and Nb increases in the course of magma evolution from ultrabasic to alkalic. Nb predominates over Ta in the main kinds of rocks by from 5:1 to 17:1. Only in granite pegmatites is Ta dominant. In granitic rocks Ta and Nb are associated with Fe, Mn, Bi, Sb, W, and Sn. In granosyenitic complexes they form complex minerals with Ti, rare earths of the Y subgroup, U, and Th. Concentrations of Ta and Nb in granitic and granosyenitic complexes increase toward the end of the magmatic and pegmatitic processes, and afterward diminish toward the end of the pneumatolytic-hydrothermal processes. In alkalic complexes Ta and Nb are associated with Ti, rare earths of the Ce group, and Th. Concentrations of Ta and Ni in alkalic massifs are caused by magmatic differentiation. In alkalic ultrabasic complexes, in magmatic and pegmatitic processes, Ta and Nb do not form independent minerals but enter into minerals of Ti and Fe, i. e. perovskite, titanomagnitite, and pyroxenes. --M. Russell.     

Mobilization of Ti-Nb-Ta during subduction: Evidence from rutile-bearing dehydration segregations and veins hosted in eclogite, Tianshan, NW China

Field evidence from the western Tianshan subduction complex in northwestern China indicates that the high field strength elements Ti, Nb, and Ta were mobilized and thereby fractionated from Zr and Hf during the dehydration process that transformed blueschist into eclogite. Both a segregation with a depletion halo, thought to represent initial mobilization during dehydration, and a transport vein, indicative of the long distance transport were investigated. In each case, centimeter-sized rutile grains grew as needle-like crystals in the segregation and as prismatic crystals in the vein. Within the host rock of the segregation, the Ti contents of garnet and omphacite, the modal abundances of rutile and titanite and the bulk rock Ti, Nb, and Ta contents decrease towards the segregation. These observations are consistent with transport of Ti, Nb, and Ta from the host rock into the segregation. Textural and geochemical data for the eclogite-facies vein minerals indicate that Ti-Nb-Ta-rich fluids were transported over long-distances (at minimum meter-scale) during fracture-controlled fluid flow. Complex forming ligands (e.g., Na-Si-Al polymers and F⁻) may have enhanced the solubility of Ti, Nb, and Ta in the fluid. Changes in fluid composition (e.g., X_CO2) may both precipitate rutile and fractionate Ti, Nb, and Ta from LILE and REE.     

喜马拉雅东段库曲岩体锂、铍和铌钽稀有金属矿物研究及指示意义

稀有金属矿物记录了花岗伟晶岩成岩成矿的重要信息。喜马拉雅是全球著名的淡色花岗岩带,库曲岩体位于喜马拉雅东段的特提斯喜马拉雅岩系中。本文调查了库曲岩体的二云母花岗岩、白云母花岗岩、电气石花岗岩和花岗伟晶岩,其中,花岗伟晶岩涉及花岗岩的伟晶岩相和独立伟晶岩脉。库曲岩体产出的稀有金属矿物包括锂辉石、锂绿泥石、绿柱石、铌铁矿-钽铁矿、钇铀钽烧绿石和细晶石,它们主要赋存于似文象伟晶岩、石英-钠长石-白云母伟晶岩、块体长石-钠质细晶岩、块体长石-电气石钠质细晶岩、锂辉石-块体长石-细晶岩、白云母花岗岩的伟晶岩相以及电气石花岗岩内。显微镜观察、电子探针和LA-ICP-MS测试结果显示锂辉石具有四种产状,包括粗粒锂辉石自形-半自形晶、细粒锂辉石-石英镶嵌晶、中细粒锂辉石-钾长石-钠长石-云母镶嵌晶以及发育锂绿泥石的粗粒锂辉石,揭示了其形成时复杂的熔流体动荡结晶环境。绿柱石背散射电子图像（BSE）下呈均一结构和不均一结构（蚀变边、不规则分带和补丁分带）,元素替代机制包括通道-八面体替代、通道-四面体替代以及通道中碱金属阳离子间的置换。铌铁矿族矿物包括原生、蚀变边和不规则分带结构,部分被钇铀钽烧绿石和细晶石交代。与原生铌铁矿相比,蚀变边和不规则分带铌铁矿族矿物总体上富钽贫锰,显示了结晶分异、过冷却引起的过饱和以及流体作用。根据稀有金属矿物揭示的成因信息,独立伟晶岩脉（似文象伟晶岩）、白云母花岗岩的伟晶岩相和电气石花岗岩在岩浆分异程度、经历的演化过程、以及流体活动方面存在差异,很可能是不同期次岩浆活动的产物。库曲岩体绿柱石的Rb和Zn含量、以及铌铁矿族矿物的Sc₂O₃、SiO₂和PbO含量,与已有指示标志存在相关性,作为潜在指示标志仍需开展更多的研究工作。综合含锂辉石伟晶岩的产出、岩浆分异演化程度、多期花岗质岩浆活动、复杂的流体作用以及所属锂丰度高值区等因素,库曲岩体是喜马拉雅东段找锂的有利地段。     

Mineralogy,geochemistry, and genesis of co-genetic granite-pegmatite-hosted rare metal and rare earth deposits of the Kawadgaon area,Bastar craton,Central India

Co-genetic pegmatites associated with the granite of the Kawadgaon area in the Bastar craton, Central India, contain a wide range of ore minerals of Nb, Ta, Be, Sn, Zr, Ti, and REE, including columbite-tantalite, ixiolite, pseudo-ixiolite, wodginite, tapiolite, microlite, fersmite, euxenite, aeschynite, beryl, cassiterite, monazite, xenotime, zircon, ilmenite, triplite, and magnetite. There is a distinct vertical zonation between the rare metal and tin pegmatites in apical parts of the host granite. Geochemically, these are LCT-S type, beryl-columbite-phosphate pegmatites that have notably high contents of SiO₂ (av. 73.80%), Rb (av. 381 ppm), and Nb (av. 132 ppm). The investigated granites probably were derived from the melting of older crustal rocks, as indicated by a high initial ⁸⁷Sr/⁸⁶Sr isotopic ratio, and the major-element geochemistry of the granites and pegmatites. Plots of mol. CaO/(MgO+FeO^t) vs. mol. Al₂O₃/(MgO+FeO^t) suggest that the source rock was pelitic metasediments. Based on the available data, it is postulated that the derivation of pegmatites from the parent granite occurred shortly after granite emplacement in the late Archaean-early Proterozoic (~2500 Ma). The K/Rb, Ba/Rb, and Rb/Sr ratios of the felsic bodies reveal that a substantial part of the granite formed from evolved melts, and further fractionation produced the co-genetic pegmatites and associated rare metal and rare earth deposits.