Mineralogical and chemical evolution of tantalum ...

OREGEO-01763; No of Pages 42 Ore Geology Reviews xxx (2016) xxx–xxx

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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 Frank Melcher a, Torsten Graupner b, Hans-Eike Gäbler b, Maria Sitnikova b, Thomas Oberthür b, Axel Gerdes c, Elena Badanina d, Thomas Chudy e a

Montanuniversität Leoben, Peter-Tunnerstrasse 5, 8700 Leoben, Austria Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655, Hannover, Germany c Institut für Geowissenschaften, Petrologie und Geochemie, Universität Frankfurt, Altenhöferallee 1, D-60438 Frankfurt am Main, Germany d St. Petersburg State University, Universitetskaya Emb. 7/9, 199034 St. Petersburg, Russia e Department for Earth, Ocean, and Atmospheric Sciences, University of British Columbia, Vancouver, BC V6T 1Z4, Canada b

a r t i c l e

i n f o

Article history: Received 3 September 2015 Received in revised form 15 March 2016 Accepted 18 March 2016 Available online xxxx

a b s t r a c t Columbite-group minerals (CGM) account for the majority of the production of tantalum, an important metal for high-technology applications. Along with other Ta–Nb oxides such as tapiolite, wodginite, ixiolite and pyrochlore supergroup minerals, CGM are recovered from rare-metal granites and granitic rare-element pegmatites. In this paper mineralogical and geochemical data with a focus on CGM, tapiolite, wodginite and ixiolite are presented for rare-element granites and pegmatites from worldwide occurrences except Africa that has been covered in a previous contribution (Melcher et al., 2015). Major and trace element data of the Ta–Nb oxides are presented and compared for a total of 25 granite/pegmatite provinces, and one carbonatite for comparison. Based on CGM compositions, the data allow to distinguish between various subgroups of Li–Cs–Ta (LCT)-family pegmatites, Nb–Y–F (NYF)-family pegmatites, mixed LCT–NYF pegmatites, and rare-element granites. Each period of Ta-ore formation in Earth history is characterised by peculiar mineralogical and geochemical features. Some of the largest and economically most important rare-element pegmatite bodies are located within Archean terrains and intruded ultramafic and mafic host rocks (e.g., Tanco/Canada, Wodgina and Greenbushes/Western Australia, Kolmozero/Kola). They are highly fractionated, of LCT affinity throughout and yield complex mineralogical compositions. The variety of minor and trace elements incorporated attests to a rather insignificant role of the immediate host rocks to their geochemical signature and rather points to the significance of the composition of the underlying crustal protoliths, internal fractionation and the processes of melt generation. Many of the Archean pegmatites carry significant Li mineralization as spodumene, petalite, and amblygonite, and all of them are also characterised by elevated Li in CGM. In addition, Sb and Bi are important trace elements, also reflected by the occasional presence of stibiotantalite and bismutotantalite. REEN patterns of CGM are dominated by the MREE or HREE, and range from very low to high total REE concentrations. Negative Eu anomalies are omnipresent. Scandium contents are also highly variable, from very high (Tanco) to very low concentrations (Wodgina, Kolmozero). A second period of worldwide pegmatite formation was in the Paleoproterozoic. All CGM analysed derive from LCT-family pegmatites except samples from the Amazonas region where Ta is mined from rare-metal granites at Pitinga. Pegmatites intruded highly variable lithologies including metasediments, metabasites, gneiss, granite and quartzite within a variety of structural and paleogeographic settings; however, most of them are syn- to postorogenic with respect to major Paleoproterozoic orogenic events. Minor and trace element signatures are similar to CGM from Archean pegmatites. Some are characterised by considerable REE enrichment (São João del Rei/ Brazil; Amapá/Brazil; Finnish Lapland/Finland), whereas others have normal to low total REE concentrations (Black Hills/USA, Bastar/India). Examples with high REE commonly are enriched in Sc and Y as well, and are often transitional to NYF-family pegmatites. The Mesoproterozoic period is comparatively poor in rare-element pegmatites and rare-metal granites. Mineralogical and chemical attributes of ixiolite–wodginite, tapiolite, CGM and rutile from placer material in Colombia point to an unusual pegmatite source of NYF affinity, yielding high total REE, Sc and Th at low Li and Bi. REE patterns have typical negative Eu and Y anomalies. A third major period of pegmatite formation was the Early Neoproterozoic at around 1 Ga, documented in the Grenvillian (North America), the Sveconorwegian (northern Europe) and the Kibaran in central Africa. CGM

E-mail address: [email protected] (F. Melcher).

http://dx.doi.org/10.1016/j.oregeorev.2016.03.014 0169-1368/© 2016 Elsevier B.V. All rights reserved.

Please cite this article as: Melcher, F., et al., Mineralogical and chemical evolution of tantalum–(niobium–tin) mineralisation in pegmatites and granites. Part 2: Worldwide ex..., Ore Geol. Rev. (2016), http://dx.doi.org/10.1016/j.oregeorev.2016.03.014

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F. Melcher et al. / Ore Geology Reviews xxx (2016) xxx–xxx

are present in numerous, mostly small pegmatites, although larger examples also occur (e.g., Manono in the D.R. Congo; Melcher et al., 2015). Pegmatite fields often display a zonal arrangement of mineralised pegmatites with respect to assumed “fertile” parent granites. They intrude metasediments, metabasites, gneiss and granite of middle to upper crustal levels and display a variety of mineralogical and chemical characteristics. Pegmatites of the Sveconorwegian and Grenville domains are usually of the NYF type and CGM are characterised by elevated Y, REE, Th and Sc. In contrast, the pegmatites of central (Kibara Belt) and southwestern Africa (Orange River Belt) are commonly of LCT affinity carrying spodumene, beryl and cassiterite (Melcher et al., 2015). These CGM have elevated concentrations of Li, Mg, Sn and Hf. Total REE concentrations are low except for the Sveconorwegian, and exhibit a variety of shapes in normalised diagrams. The fourth major pegmatite-forming event coincides with amalgamation of Gondwana at the Neoproterozoic/ Paleozoic boundary around 550 Ma ago. This event is omnipresent in Africa (“Panafrican”) and South America (“Brasiliano event” documented in the Eastern Brazilian pegmatite and Borborema provinces). Pegmatites often intruded high-grade metamorphic terrains composed of metasediments including schist, marble, quartzite, as well as gneiss, amphibolite, ultramafic rocks, and granite. Within the Neoproterozoic, rare-metal granites of NYF affinity are locally abundant. Pegmatites show both LCT and NYF affinities, and mixed types occur in Mozambique. The Alto Ligonha and Madagascar provinces are characterised by abundant REE and Sc both within Ta–Nb-oxides and as separate mineral phases. Notably, some pegmatite provinces are almost devoid of cassiterite, whereas others carry cassiterite in economic amounts. In the Phanerozoic (younger than 542 Ma), pegmatites formed at all times in response to orogenetic processes involving various continents and terranes during the long-time amalgamation of Pangea and the Alpine orogenies. Whereas some activity is related to the Pampean, Acadian and Caledonian orogenies, the Variscan/Hercynian and Alleghanian orogenies are of utmost importance as manifested in pegmatite formation associated with Sn–W mineralised granites in central and western Europe as well as in the Appalachians. Most of the Variscan and Alleghanian pegmatites are of LCT affinity, although NYF and some mixed types have been described as well. Variscan pegmatite formation culminated at ca. 330 to 300 Ma, whereas Alleghanian pegmatites range in age from about 390 Ma to about 240 Ma. Most are syn- to post-orogenic and were emplaced at different crustal levels and into a variety of host rocks. Degree of fractionation as well as minor and trace element geochemistry of Ta– Nb oxides are rather variable and cover the complete field of CGM compositions. REE patterns are characterised by prominent negative Eu anomalies. Some Mesozoic and Cenozoic pegmatites and rare-metal granites from Southeast Asia and the Russian Far East are included in the compilation. Rare-metal granites of the Jos Plateau (Nigeria) were previously investigated (Melcher et al., 2015). The proportion of NYF pegmatites and rare-metal granites in the Mesozoic is striking, i.e. illustrated by Jos, Orlovka, Ulug Tanzek as well as the southeast Asian deposits related to tin granites. CGM from these areas are invariably rich in REE, Sc, Y and Th. In all rare-metal granites, Ta–Nb oxides are characterised by high total REE concentrations and both, negative Eu and Y anomalies in chondrite-normalised REE diagrams. Although constituting a vastly different magmatic system compared to rare metal pegmatites and granites, we included the Upper Fir carbonatite from the Canadian Cordillera, for comparison, because it is characterised by unusal high Ta contents. As expected, the CGM differ from the pegmatitic CGM by having high Mg and Th, and low U concentrations in columbite-(Fe) and lack an Eu anomaly. However, they also show similarities to primitive CGM from rare metal pegmatites of the NYF family in terms of the REE pattern and the increase in #Ta and #Mn towards the margins of the CGM. Our findings support recent results presented in Chudy (2014) indicating that the Ta enrichment in some carbonatites might be attributed to magmatic processes and conditions that are similar to the pegmatitic systems. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Tantalum is an important metal for high-technology applications and is recovered from oxide minerals that are present as minor constituents in rare-metal granites and granitic rare-element pegmatites. Significant potential exists as a by-product of Nb–Zr–REE mining from carbonatites and peralkaline igneous rocks. The most important Ta carriers are represented by columbite-group minerals (CGM), followed by tapiolite and minerals of the wodginite, pyrochlore and aeschynite groups. A large variety of Ta–Nb-bearing phases are recovered from rare-metal granites, granitic rare-element pegmatites and their derivatives to be further processed as Ta sources (e.g., Černý et al., 2005). The paper presented here provides insights into the compositional variation of CGM and other Ta carriers, with special emphasis on the trace elements. The dataset derives from scanning electron microscopy (SEM with EDX and automated mineralogy, MLA — Mineral Liberation Analysis), electron microprobe, laser ablation ICP-MS (LA-ICP-MS), solution ICP-MS, ICP-OES and thermal ion mass spectrometry (TIMS) analysis on ore concentrates and individual Ta–Nb oxide mineral grains from various Ta districts worldwide (Fig. 1). In a previous paper (Part 1), Melcher et al. (2015) reported on trace element and isotopic data of the U–Pb system (mineral formation ages) for Ta–Nb-oxides from African Ta deposits and occurrences. The work

mainly focused on the establishment of regional and local differences (“signatures”) for African coltan (i.e. columbite–tantalite ore concentrates) to be applied for a fingerprint of individual deposits. This fingerprint forms a significant contribution to an unequivocal identification of conflict minerals (e.g., from the Kivu provinces/DRC) along the trading chains of coltan concentrates (e.g., Melcher et al., 2008a, 2008b, 2009; Savu-Krohn et al., 2011). The current paper presents a compilation and discussion of new geochemical and mineralogical data from important Ta-bearing deposits outside of Africa. The presentation of major, minor and trace element data as well as age information (U–Pb system) is structured according to the geological eras in Earth's history that produced significant Ta mineralisation. 2. Rare-element pegmatites and rare-metal granites: global distribution, mineralogy Tantalum(–Nb–Sn)-bearing granites and pegmatites hold an estimated 80% share in Ta production and occur on all continents in synto post-orogenic structures spanning an age range from the early Archean (ca. 3 Ga) to the Tertiary (Tkachev, 2011). Comprehensive descriptions on the global distribution of pegmatites are given, among others, in Landes (1935); Schneiderhöhn (1961); Černý (1989) and Dill



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Fig. 1. Map of rare-element pegmatite and rare-metal granite provinces (different symbols) covered in the paper. Colours indicate age provinces. Provinces in Africa have been treated by Melcher et al. (2015).

(2015). Collision environments generating granite–pegmatite systems include continent–continent (e.g., the Himalayas), continent–island arc (e.g., central Svecofennian basin), possibly island arc–island arc and closures of ensialic rifts (e.g., Damara Province, Namibia) (Černý, 1991b). Emplacement of granitic pegmatites is commonly related to fold structures, shears and fault systems forming bodies of variable shape and size (metres to kilometres) outside of outcropping or postulated “source” plutons; many pegmatites lack outcropping parental granitic intrusions (e.g., pegmatites of the Bernic Lake Group, of which the Tanco pegmatite is a member, cf. Stilling et al., 2006). Pegmatites hosted by granitic rocks are less abundant. Various detailed investigations provided conclusive evidence of systematic variations in degree of fractionation, concentration of rare elements, and distance from the assumed host pluton (e.g., Varlamoff, 1972; Trueman and Černý, 1982). In granitic pegmatites, two families are distinguished based on their diagnostic trace element signatures (Černý, 1991a): the LCT (Li–Cs–Ta) and NYF (Nb–Y–F) families. Based on the parameters (1) metamorphic environment, (2) mineralogy, (3) elemental composition and (4) texture, Černý and Ercit (2005) and Černý et al. (2012a) defined five classes of pegmatites from which four may host Ta–Nb–Sn mineralisation (namely the abyssal, muscovite-rare-element, rare-element and miarolitic classes). These are further subdivided into 10 subclasses. Within the rare-element class as the most important for Nb–Ta mineralization, the two subclasses (REE and Li) are split into 7 types, and two types are further split into 7 subtypes based on their mineralogical composition. The types and subtypes in this classification are defined by major and minor minerals carrying Li, Be, REE and Nb–Ta. Černý et al. (2012a) furthermore illustrated how the subclasses fit into the overarching pegmatite family classification. In the LCT family of the rare-element class, which is economically most important for Ta, four pegmatite types (beryl, complex, albite–spodumene, spodumene) and further on, six subtypes are distinguished (Černý, 1991a; Černý and Ercit, 2005; Černý et al., 2012a; Simmons and Webber, 2008). 3. Methods Tantalum minerals from pegmatites and rare-metal granites covering all continents except Antarctica have been investigated (Fig. 1). The results of the comprehensive work on African Ta ores have already

been published (Melcher et al., 2015); more details on the analytical methods were provided by Gäbler et al. (2011). The present paper presents results from ore concentrates, drill core, hard rock samples and single grains collected from pegmatite, granite and placer deposits in Australia, Asia, Europe, North and South America, which were sampled during the study or obtained from mining companies and museum collections. For details of the sample locations and the sample characteristics, the reader is referred to Fig. 1 and the Supplementary material S1. Polished sections were prepared from concentrate, single crystal and hard rock samples. The mineralogical composition of concentrates was quantitatively determined using SEM/MLA (scanning electron microscope/Mineral Liberation Analysis) techniques. In Table 1, the results of these measurements are presented in a condensed form and some examples of classified concentrates are displayed in Fig. 2. Selected Ta mineral grains were subsequently analysed using electron microprobe (EPMA) and LA-ICP-MS techniques. A CAMECA SX 100 electron microprobe equipped with five wavelength-dispersive spectrometers and an energy-dispersive PGT system was used to determine major and minor element concentrations in CGM, tapiolite, wodginite–ixiolite and rutile, applying 30 kV acceleration voltage, 40 nA sample current and appropriate counting times to reach detection limits of 200 ppm for minor and trace elements. The following lines, spectrometer crystals and standards were used: SiKα, PET, rhodonite; CaKα, PET, apatite; ScKα, LPET, metal; TiKα, PET, rutile; MnKα, LLIF, rhodonite; FeKα, LLIF, magnetite; ZrLα, PET, metal; NbLα, PET, columbite; SnLα, PET, metal; HfLα, LLIF, metal; TaLα, LLIF, tapiolite; WLα, LLIF, metal; UMα, LPET, metal. Ta–Nb-oxides of the pyrochlore supergroup, the aeschynite, euxenite, samarskite and fergusonite groups, as well as stibiotantalite and bismutotantalite were analysed at 20 kV and 20 nA using a modified program comprising up to 37 elements. The following lines, spectrometer crystals and standards were used in addition to the above mentioned settings: FKα, PC0, fluorite; NaKα, TAP, albite; MgKα, TAP, chromite; AlKα, TAP, chromite; PKα, LPET, apatite; SKα, PET, pentlandite; KKα, LPET, sanidine; ZnKα, LLIF, willemite; AsLα, TAP, synthetic GaAs; YLα, LPET, synthetic YAG; SbLα, LPET, metal; CsLα, LLIF, pollucite; BaLα, PET, barite; LaLα, LPET, monazite; CeLα, LPET, monazite; SmLα, LLIF, REE glass; GdLα, LLIF, REE glass; DyLα, LLIF, REE glass; ErLα, LLIF, REE glass; YbLα, LLIF, REE glass; PbMα, PET, PbS; BiMα, LPET, metal; ThMα, PET, metal. Detection limits are below


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Table 1 Mineralogical composition (heavy minerals) of rare-element pegmatites and granites from typical ore mineral concentrates (in area percent), MLA data. Locality

Oxenbushes Greenbushes Greenbushes Greenbushes Wodgina Wodgina New South Wales Sao Joao del Rei Sao Joao del Rei Sao Joao del Rei Volta Grande Lourenco Pitinga Aracuai Aracuai Equador Picui unknown unknown Tanco Tanco Tanco Bernic Lake Vampès Vampès Vampès Bastar Bastar Bastar Kim Bee Taikeu Ust-Mramornoe Longot-Yugan Taikeu Ulug-Tanzek Ulug-Tanzek Ulug-Tanzek unknown South Dakota Pennington Pegmatite

Province

District

Western Australia Western Australia Western Australia Western Australia Western Australia Western Australia New South Wales Minas Gerais Minas Gerais Minas Gerais Minas Gerais Amapá Amazonas Minas Gerais, EBPP Minas Gerais, EBPP Rio Grande do Norte Rio Grande do Norte

Balingup belt Balingup belt Balingup belt Balingup belt Pilbata craton Pilbata craton

Aracuai Aracuai Paraiba, Alto Do Serido Paraiba, Alto Do Serido

Manitoba Manitoba Manitoba Manitoba

Bernic Lake Bernic Lake Bernic Lake Bernic Lake

Polar Ural Polar Ural Polar Ural Polar Ural West Siberia West Siberia West Siberia

East Sayan East Sayan East Sayan

South Dakota South Dakota

Black Hills Black Hills

Sao Joao del Rei Sao Joao del Rei Sao Joao del Rei Sao Joao del Rei

Country

Australia Australia Australia Australia Australia Australia Australia Brazil Brazil Brazil Brazil Brazil Brazil Brazil Brazil Brazil Brazil Brazil Brazil Canada Canada Canada Canada Colombia Colombia Colombia India India India Malaysia Russia Russia Russia Russia Russia Russia Russia Thailand USA USA

Sample

011 413 413 577 412 412 717 012 062 537 395 051 920 017 017 657 658 032 033 490 490 490 716 1043 1042 1041 751 752 753 879 364 365 366 367 368 369 370 576 715 719

Age (Ga)

2.5 2.5 2.5 2.5 2.9 2.9 2.0 2.0 2.0 2.0 2.0 1.8 0.5 0.5 0.5 0.5 0.5 0.5 2.6 2.6 2.6 2.6 1.3 1.3 1.3 2.0 2.0 2.0 0.2 0.6 0.6 0.6 0.6 0.2 0.2 0.2 0.2 1.6 1.6

total Ta–Nb minerals

CGM total

90 13 11 75 17 18 7 79 95 25 86 92 6 40 39 81 88 97 87 26 28 32 1 1 82 89 0 78 58 66 92 83 88 69 95 43 97 99 6 5

70 9 8 9 8 9 3 77 93 2 83 90 6 24 26 64 35 59 87 22 21 26 1 1 31 59 0 46 42 55 83 71 85 65 95 43 97 97 4 4

Relative proportion of end member

Ratio to total Ta–Nb minerals

FeC

FeT

MnC

MnT

Tapiolite

Wodginite, ixiolite

38.1 18.8 25.4 19.5 0 0 68.1 0 0 0 0.6 90.9 37.6 46.3 42.7 45.2 22.6 25.3 94.3 0.6 0.4 0.8 26.2 86.5 76.5 10.0 0 47.2 48.6 21.3 90.7 95.3 95.1 96.4 94.9 20.2 98.2 83.5 80.5 70.9

43.2 32.6 32.6 42.3 6.9 6.5 30.1 2.2 0 12.7 9.6 4.9 0.0 1.5 0.4 4.4 10.4 64.4 0.2 10.2 0.3 7.7 13.1 13.5 0 50.8 0 5.5 10.3 0 0 0.1 0 0.1 0 0 0 2.7 15.5 26.5

16.0 35.9 30.2 30.1 4.9 6.1 1.8 17.3 0.4 23.9 29.5 4.2 62.4 49.3 54.4 48.9 15.8 7.0 5.6 8.8 0.1 13.3 50.8 0 0 17.2 0 7.9 14.6 78.4 9.3 4.6 4.9 3.5 5.1 79.8 1.8 13.4 4.1 1.8

2.8 12.7 11.8 8.1 88.2 87.4 0 80.5 99.6 63.4 60.3 0 0 2.9 2.5 1.6 51.2 3.4 0 80.5 99.2 78.2 9.8 0 23.5 22.0 100 39.4 26.5 0.3 0 0 0 0 0 0 0 0.4 0.0 0.8

7.9 1.4 3.5 0 1.2 1.0 58 0 0 0 0 1.7 0 0 0 21 32 24 0 2.5 0.9 1.0 0 0 0 0 0 26 5.7 0 0 0 0 0 0 0 0 0 25 26

8.2 14 11 1.6 27 30 0 0.5 1.0 4.4 2.2 0 0 37 29 0 16 3.5 0 4.6 11 4.9 5.6 0 7.0 20 0 10 18 0 0 0 0 0 0 0 0 1.3 0 0.9

FeC, columbite-(Fe); FeT, tantalite-(Fe); MnC, columbite-(Mn); MnT, tantalite-(Mn); BiT, bismutotantalite; SbT, stibiotantalite.

400 ppm for Na, Mg, Al, Si, P, K, Ca, Sc, Ti, As, between 400 and 800 ppm for F, S, Mn, Fe, Zn, Zr, Sn, La, Ce, between 800 and 1200 ppm for Y, Sm, Gd, Dy, Er, Yb, Sb, Nb, Hf, Bi, and higher than 1200 ppm for Cs, Ba, Ta, W, Pb, Th and U. Laser ablation systems used for in-situ determination of trace elements included a 266 nm Nd:YAG laser (New Wave) coupled to an Agilent 7500i quadrupole mass spectrometer instrument at the University of Erlangen and a 193 nm excimer laser (New Wave UP193FX) coupled to a Thermo Scientific ELEMENT XR sector field mass spectrometer, with an additional Faraday cup, at the BGR. Details of the methodology and standardisation are provided by Gäbler et al. (2011). More than 150 single Ta–Nb-oxide grains were pre-selected by SEM work and subsequently analysed following complete dissolution. Finely ground sample material (5 mg) was dissolved in a mixture of 48% m/m HF(100 μl) and 65% m/m HNO3 (200 μl). After dissolution (reaction time 2–7 days), de-ionised water was added to bring the volume to 20 ml. The solution was filled up to 50 ml with 0.15 M HNO3 and analysed by ICP-AES (Nb, Ta, Mn, Fe, Sn) and sector field ICP-MS (Al, Ba, Bi, Li, Pb, Sb, Sc, Sr, Th, Ti, U, W and REE). Minor and trace elements were analysed using conventional solution ICP-MS, LA-ICP-MS and electron microprobe techniques. Solution ICP-MS analyses have superior detection limits, but suffer from the

possible presence of inclusions that may remain undetected even after recalculation. Compared to conventional solution ICP-MS, the advantages of the laser method are a better spatial resolution and the possibility to avoid impurities such as inclusions if careful optical screening is done before analysis; nevertheless, the comparatively large ablated volumes (spot sizes are usually 50 μm) often include small hidden inclusions, and also several zones within heterogeneous grains. Electron beam methods have a superior spatial resolution enabling the detection of very thin (down to a few μm thickness) zones; however, detection limits for trace elements better than 200 ppm are difficult to achieve using appropriate analysis times. Nevertheless, analyses carried out by all methods described here yielded comparable results for the major and most minor elements. However, a few trace elements had unrealistically high concentrations in solution analyses, e.g., Rb, Sr, Li, Bi (Badanina et al., 2015). This is interpreted as indicating the contribution of inclusions such as micas or feldspar; monitoring Si, Al, K and Ca in the prepared solutions may help to detect such inclusions. Uranium–lead dating was carried out in-situ using a ThermoScientific Element II sector field ICP-MS, coupled to a New Wave UP213 ultraviolet laser system with low-volume ablation cell (University of Frankfurt). Spot size varied from 30 to 60 μm. Raw data were corrected for background signal, common Pb, laser-induced elemental fractionation, instrumental mass discrimination and time-dependant



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Table 1 Mineralogical composition (heavy minerals) of rare-element pegmatites and granites from typical ore mineral concentrates (in area percent), MLA data. Ratio to total Ta–Nb minerals Microlite-group

BiT, SbT

Samarskite, euxenite, fergusonite

6.8 8.0 8.9 0.9 23 17 2.0 2.4 0.7 87 1.3 0 0 4.1 4.9 0 11 11 0 10 14 11 27 0 0 0 0 3.7 2.8 13 1.7 3.0 0 1.5 0 0 0 0 6.7 0.6

0 6.7 10 85 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 3.9 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.7 0 0 8.2 12 2.8 5.4 0 0 0 0 0 0

Cassiterite

Wolframite scheelite

Rutile

Ilmenite

Gahnite

Zircon

Monazite

Xenotime

Apatite

Sulfides

Garnet

1.7 11 17 20 6.0 7.7 66 0 0 60 1.8 0 69 11 13 0 0 0 12 5.0 5.5 6.2 93 99 18 11 97 21 41 18 0 0.5 3.1 1.2 0 0 0 0 71 44

0 0 0 0 0 0 4.3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3.0 4.5

1.0 0 0 0 0 0 0 0 0 0 0 0 0 6.3 8.4 0 0 0 0 0 0 0 0 0 45 11 0 0 0 2.6 0 0 0 0 0 0 0 0 0 0

0 4.9 5.0 0 0 0 1.6 1.0 2.0 0 0.9 4.4 0.7 0 1.7 0.6 5.3 0 0 0.9 1.2 0.8 1.4 0 0 0 1.8 0 0 2.8 0 0 0 0 0 0 0 0 0.8 4.7

1.8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.8 0 0 0 0 0 0 0 0 0 0

0 0.5 0 4 0.8 0.6 0 0 0 1.4 0.3 0 18 0 0 2.3 0 0 0 0 0 0 0 0 0 0 0 0 0 1.2 0 1.5 0 0.8 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0.6 1.2 0 0 0 0 0 0.6 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.0 0 0 0 0 0 0 0 0 0 0

0 1.7 1.6 0 1.1 1.7 0 0 0 2.1 0 0 0 0 0 0 0 0 0 1.3 0.7 0.6 1.3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 4.2 4.7 0 9.1 6.0 0 0 0 1.9 0 0 1.1 0 0 0 0 0 0 0 0 0.6 1.7 0 0 0 0 0 0 5.4 0 0 0 0 0 0.6 0 0 0 0

0 0 1.3 0 1.0 1.2 3.1 0.5 0 0 0 0 0 1.1 1.9 0 1.0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2.6 2.5

elemental fractionation (Gerdes and Zeh, 2006). The analytical reproducibility (e.g., GJ-1 reference zircon) of the 206Pb/238U and 207 Pb/206Pb ratios were commonly about 0.8 and 0.5%, respectively. No matrix dependent U/Pb fractionation was observed. Results are summarised in Table 2. The reliability of in-situ U–Pb dating was evaluated by Che et al. (2015a, 2015b), taking into account the use of different standards (zircon vs. columbite), including the BGR-internal standard Coltan 139 from Madagascar (Gäbler et al., 2011; Melcher et al., 2015). U–Pb ages were also estimated from about 15 to 40 single spot analyses of CGM per sample in polished sections using the sector field LAICP-MS data (Gäbler et al., 2011). Spots with low radiogenic Pb portions (e.g. 207Pbrad b 50% of total 207Pb) were excluded from the age estimation. 206Pbrad/238U and 207Pbrad/235U ratios were calculated for each single spot and plotted on a concordia diagram. A robust linear regression algorithm based on the minimisation of the sum of the absolute values of the distances in y-direction was used to calculate the intersections with the Concordia curve. The upper intersection was used as the U– Pb age estimate for discordant samples. For samples which did not show discordance, the age of the intersection with the Concordia curve close to the sample points was used as U–Pb age estimate. Following, data obtained by this method is termed “age estimate” (Supplementary material S2).

4. Granitic/pegmatitic Ta–(Nb–Sn) provinces worldwide (excluding Africa) Pegmatite and rare-element provinces are described chronologically from the Archean to the Cenozoic. Altogether, 25 granite/pegmatite provinces hosting Ta–Nb-oxides are discussed (Supplementary material S1 for locations, coordinates, samples, and collection). Each chapter contains brief geological information, followed by a discussion of radiometric data and the mineralogy, mainly based on Ta ore concentrates and thus focussing on opaque heavy minerals (SEM/MLA data; Table 1 and Fig. 2). Radiometric data giving U–Pb ages of CGM and other Tabearing phases are presented in Table 2 and Supplementary material S2, and in Fig. 3. Selected data of associated cassiterite are included for comparison. Back-scatter electron images showing characteristic textures of Ta– Nb oxides are shown in Fig. 4. The major element variation of CGM, tapiolite and wodginite–ixiolite in each province is illustrated in the “columbite quadrilateral” diagrams (#Mn = 100 ∗ Mn / (Mn + Fe) versus #Ta = 100 ∗ Ta / (Ta + Nb)) (Figs. 5, 8, 11). Chemical analyses commonly are insufficient to adequately distinguish monoclinic wodginite (ordered structure) from orthorhombic ixiolite (disordered structure) and, in the absence of structural (X-ray diffraction) data, these minerals are simply referred to as wodginite–ixiolite in the present paper. A


6


Fig. 2. Representative SEM/MLA maps from ore concentrates of Ta–Nb–Sn pegmatites. (a) Tanco, Canada; sample 490; (b) Wodgina, Australia; sample 412; (c) Greenbushes, Australia; sample 413; (d) Araçuai, Brazil; sample 017.

miscibility gap to Fe–Ta-rich phases collectively attributed to tapiolite is evident. In the following, however, minerals have been classified using their chemical composition only. The trace element geochemistry, including the REE, of the major ore minerals is treated separately in Section 5.

Wodgina: Sweetapple and Collins, 2002), and Russia (e.g., Kolmozero and Voronja Tundra, Kola). Studied concentrate material from Tanco, Wodgina and Greenbushes was accompanied by thin section work using drill core and mine samples. These pegmatites of the LCT family are mineralogically complex, yielding a large variety of Nb–Ta–Sn phases.

4.1. Archean

4.1.1. Southeast Manitoba, Canada (Bernic Lake) The Tanco pegmatite (Bernic Lake Pegmatite Group; Fig. 1; for numbers on Fig. 1 see Supplementary material S1) was emplaced into a metagabbro in the Bird River Greenstone Belt (Černý, 1982, 2005). According to Stilling et al. (2006) it represents a highly fractionated petalite-subtype of the LCT family of granitic pegmatites. The mineralogy of its nine zones (zones 10 to 90) is well documented (e.g., Černý, 2005).

Important Archean rare-metal pegmatite provinces are found in Canada (e.g., Tanco, Manitoba: Černý, 1982, 2005; Tindle and Breaks, 1998; Yellowknife District, Northwest Territories: Wise and Černý, 1990; Red Cross Lake, Manitoba: Černý et al., 2012b), Western Australia (e.g., Greenbushes: Hatcher and Bolitho, 1982; Partington et al., 1995;



7

Table 2 Radiometric ages (U–Pb) of Nb–Ta mineralization, LA-ICP-MS data, University of Frankfurt. Location

Country

Province

Sample no.

Minerals dated

Type of material dated

Geologically significant age(s) (Ma)

±2s (Ma)

MSWD

Degree of concordance (%)a

LA-ICP-MS: n (spots)

Tanco São João del Rei

Canada Brazil

Manitoba São João del Rei

490 012

Brazil

EBPP

017

Passeiertal

Italy

Austroalpine

532

Concentrate Concentrate Concentrate Concentrate Concentrate Crystal Crystal Crystal Crystal Concentrate Crystal Crystal Crystal

2624 2025b 2023 502c 504 301c 319 206 303 608 301e 302c 300e

22 7 7.2/−7.6 3 7.5/−7.1 4 48 5 6 70 4 3 2

24 1.2 1.19 1.6 1.1 0.98 1.03 4 0.82 1.4 1.5 1.5 1.15

27–105 (85)

Aracuai

Columbite–tantalite, cassiterite Columbite, tantalite Columbite, tantalite Columbite, tantalite Columbite, tantalite Ixiolite/columbite aggregate Ixiolite/columbite aggregate Ixiolite/columbite aggregate Columbite Columbite, fergusonite, cassiterite Columbite Columbite Columbite

45 14 36 27 50 9 37 12 30 39 15 15 15

Feli Longotyugan Trutzhofmühle

Spain Russia Germany

Ural Bavaria

Hagendorf

Germany

Bavaria

266 366 d d

a b c d e

d

90–116 (100) 95–104 (100) 32–319 (98) 90–115 (103) 50–214 (95)

Weighted mean 206Pb/238U age. 14 spots with 206Pb/204Pb N 5000. Concordia age. From Dill et al. (2008). Degree of concordance = (206Pb/238U age × 100 / 207Pb/206Pb age), range and average (in brackets).

In addition to common CGM, cassiterite, microlite, uranmicrolite and a large variety of wodginite-group minerals, the pegmatite contains rare Ta–Nb-oxides such as tantite, simpsonite, stibiotantalite, cesstibtantite, calciotantite, and rankamaite–sosedkoite. Significant Ta mineralisation occurs restricted to the aplitic albite zone (zone 30; ~ 0.5 vol.% of Ta oxides), the central intermediate zone (zone 60; ≤ 5 vol.% of Ta oxides) and the lepidolite zone (zone 90) (e.g., Van Lichtervelde et al., 2008). LA-ICP-MS analyses of tantalite and cassiterite from an ore concentrate collected at the Tanco processing plant yielded a highly discordant age of 2624 ± 24 Ma (Fig. 3a; Table 2), largely in accord with a 2640 ± 7 Ma date published for tantalite (Baadsgaard and Černý, 1993). Camacho et al. (2012) give U–Pb (TIMS) ages between ca. 2640 and 2620 Ma for tantalite, and the six oldest ages at 2641 ± 3 Ma were interpreted as a major period of crystallisation. According to SEM-MLA data, Ta–Nb-oxides make up about 29% of the concentrate investigated, with CGM prevailing (67% of the Ta–Nboxides, mainly tantalite-(Mn)), followed by cassiterite (16%), microlite (10%), wodginite–ixiolite (6%), and tapiolite (1.2%) (Fig. 2a). Drill core material from the Lower Tanco orebody revealed abundant columbite(Fe,Mn) in addition to tantalite-(Mn), wodginite–ixiolite, microlite and cassiterite (Nkengafac, 2012). Typical hand specimens from the main orebody consist of complex intergrowths of zoned wodginite– ixiolite, columbite-(Mn), tantalite-(Mn), tapiolite, microlite, cassiterite and occasionally Ta-rich rutile (25–34% Ta2O5) (Fig. 4a). The lepidolite zone (zone 90) contains fine-grained tapiolite intergrown with wodginite–ixiolite and microlite. Zircon is an abundant accessory mineral at Tanco and is commonly intergrown with Ta mineralisation (Van Lichtervelde et al., 2009). Literature (e.g., Van Lichtervelde et al., 2006, 2007) and own data reveal that CGM in the Tanco pegmatite follow an evolutionary trend from columbite-(Fe) to columbite-(Mn) and tantalite-(Fe) with increasing fractionation of the pegmatitic melt (Fig. 5a). 4.1.2. Western Australia, Australia 4.1.2.1. Wodgina Greenstone Belt. The Wodgina deposit (Wodgina Pegmatite District) is hosted by metakomatiites and metasedimentary rocks of the Wodgina Greenstone Belt in the Archean North Pilbara Craton (Sweetapple et al., 2001). The pegmatites are assigned to the albite– spodumene (Mt. Cassiterite Group) and the albite types (Wodgina Group) of the LCT family according to their bulk geochemistry and mineral assemblages, including spodumene, lepidolite, lithiophyllite, beryl, and spessartine; at other places gadolinite, monazite, tanteuxenite,

calciotantite, yttrotantalite, fergusonite, samarskite and rynersonite are reported (Sweetapple and Collins, 2002). Wodgina hosts reserves of ~23,200 t of Ta2O5 at 370 g/t Ta2O5 (Fetherston, 2004). The oldest columbites ever dated are from Wodgina and Woodstock (Pilbara Craton) which have 207Pb/206Pb SHRIMP ages of 2829 and 2879 Ma (Sweetapple and Collins, 2002). Our U–Pb age estimate for CGM from Wodgina is close to 2900 Ma (Supplementary material S2). An ore concentrate from the Wodgina plant carries 18 vol.% Ta–Nboxides and 7% cassiterite (Fig. 2b). Sulfides, apatite, zircon and garnet are further important heavy minerals. CGM, all Mn- and Ta-rich (Fig. 5b), make up 36% of the Ta–Nb-oxides and Sn phases, followed by cassiterite (28%), wodginite–ixiolite (20%) and microlite (15%). Tapiolite (0.8%) is subordinate. CGM are dominated by tantalite-(Mn) (88%); columbite-(Mn) and tantalite-(Fe) are minor constituents (Fig. 4b). 4.1.2.2. Balingup Metamorphic Belt. The Greenbushes Pegmatite was intruded into rocks of the Balingup Metamorphic Belt as a series of linear dikes (Archean Western Gneiss Terrain, Gee et al., 1981; Partington, 1990; Partington et al., 1995; Fig. 1). It has four major and four subsidiary compositional zones (Paterson, 1983; Černý, 1989) and represents a spodumene and albite-rich pegmatite of the LCT family, containing beryl, apatite, tourmaline, K-feldspar, albite, and uraninite. Greenbushes hosts reserves of ~20,000 t of Ta2O5 at 220 g/t Ta2O5 (Fetherston, 2004). Zircons from the Greenbushes Pegmatite Group have been dated at 2527 ± 2 Ma (Partington et al., 1995), chiefly in accordance with our age estimates for CGM that range from 2423 to 2587 Ma (Supplementary material S2). A Greenbushes concentrate provided by Talison Minerals Pty Ltd. in 2008 carries 12.5 vol.% Ta–Nb-oxides and 15% cassiterite (Figs. 2c and 4c; Table 1). Ilmenite, sulfides, apatite, garnet, zircon and rutile follow in abundance. Among the Ta–Nb-oxide + Sn minerals, cassiterite accounts for 53%, whereas CGM make up 33%, followed by wodginite– ixiolite (6%), microlite (4%), stibiotantalite (4%) and tapiolite (1%). The different CGM compositions are present in similar proportions; Mnrich columbite and tantalite (#Mn N 60, #Ta = 24–93) with intermediate to low trace element concentration levels were observed. Another concentrate collected in the 1980s is rich in Fe-rich CGM, and contains tapiolite, wodginite–ixiolite and microlite in appreciable quantities (Table 1). Among the remaining minerals, cassiterite and gahnite are most abundant. At Greenbushes, stibiotantalite was separated from CGM during processing. In a stibiotantalite concentrate (77% of the Ta–Nb–Sn oxides), cassiterite (21%) and zircon are also present (Table 1). Thin sections of ore specimen from the different zones of


8


Fig. 3. U–Pb concordia diagrams of columbite–tantalite from (a) Tanco (Canada), columbite–tantalite and cassiterite, sample 490; (b) Sao Joao del Rei (Brazil), sample 12; (c) Araçuai, Brazil, sample 17; (d) Longot–Yugan, Polar Urals (Russia), sample 366; (e) Feli (Spain), sample 266; (f) Passeier Valley (Italy), sample 532.

the Greenbushes Pegmatite yielded similar results, being rich in stibiotantalite, wodginite and CGM; however, cassiterite is more abundant in samples bare of stibiotantalite that are composed of tantalite-

(Mn) as the main CGM phase. CGM from Greenbushes are intermediate in their major element composition and do not form prominent trends in the #Mn–#Ta diagram (Fig. 5c).



9

Fig. 4. Back-scatter electron images of Ta–Nb oxide assemblages from various Ta provinces. (a) complex Ta–Sn mineralisation in aplite at the Tanco mine, sample 489; (b) intergrowth of tantalite-(Mn) and wodginite, replaced by microlite, ore concentrate from Wodgina, sample 412; (c) ore concentrate from Greenbushes, sample 413; (d); CGM from Sao Joao del Rei, sample 012; (e) zoned columbite-(Fe), ore concentrate from Bastar, sample 752; (f) CGM intergrown with rutile, concentrate from Araçuai, sample 017; (g) intergrowth of CGM, fersmite and microlite from the Pusterwald area, sample 725; (h) CGM from Orlovka, sample 628. FeC, columbite-(Fe); FeT, tantalite-(Fe); MnC, columbite-(Mn); MnT, tantalite-(Mn); Tap, tapiolite; Wdg, wodginite; Mic, microlite; PbMic, plumbomicrolite; SbT, stibiotantalite; Fsm, fersmite; Cas, cassiterite; Rt, rutile; Zir, zircon.

4.1.2.3. Norseman–Wiluna Belt. The Norseman–Wiluna Belt of the Eastern Goldfields Province (Yilgarn Craton) was intruded by numerous Ta-bearing pegmatites. About 60 km westward of the Binneringie site (Fetherston, 2004), several vein-like pegmatite bodies occur southeast of Spargoville (e.g., Witt, 1990) (Fig. 1). These zoned or simple pegmatites are rich in microcline and contain fine-grained albite-rich portions

(± beryl, garnet). Simpson (1952) describes the occurrence of columbite-(Mn) with minor tantalite-(Mn) at this location. The total recorded tantalite–columbite production from this site is 5.3 t (Witt, 1990). Six large (up to some centimetres in size) and homogeneous columbite-(Fe) grains (Fig. 5d) from one of the pegmatites close to


10


Fig. 5. Diagrams of 100 ∗ Mn / (Mn + Fe) versus 100 ∗ Ta / (Ta + Nb) for columbite–tantalite group minerals, tapiolite (Tap, open quadrangles) and wodginite/ixiolite (Wdg) in Ta ore provinces outside of Africa. Diagrams are arranged according to their formation ages, starting with the Archean. (a) Tanco, Canada. Additional data points from van Lichtervelde (unpubl.); (b) Wodgina, Western Australia; additional data points from Broken Hill, New South Wales; (c) Greenbushes, Western Australia; (d) Northern Kola peninsula, Russia (Kolmozero, VT = Voronja Tundra); (e) Paleoproterozoic provinces of Brazil: São João del Rei pegmatite province (SJR) including Dattas/Diamantina; northern Brazilian pegmatite province including Lourenco, Marowgne and Pitinga; (f) alluvial grains derived from the Miessi and Pusku rivers, Finnish Lapland. Additional data from Erajärvi/Finland (Lahti, 1987) and Sweden (Romer and Smeds, 1996; Černý et al., 2004).



Spargoville were studied. They show slight replacement by Pb- and Ubearing pyrochlore-group minerals, and occasional inclusions of uraninite and zircon. Fetherston (2004) also mentions the presence of tantalite-(Mn) (carrying 64% Ta2O5 and 20% Nb2O5) in these pegmatites. 4.1.3. Kola Peninsula, Russia The Archean Kolmozero–Voronya Zone is situated in the NE part of the Kola Peninsula (Fig. 1). The rocks were metamorphosed under amphibolite-facies conditions and were intruded by granodiorites, granites and pegmatite veins (e.g., Kudryashov et al., 2003). The extensive Kolmozero–Voronya zone is characterised by zoning of pegmatite fields, changing from cesium mineralization (pollucite) in the northwest (Vasin Mylk) to lithium mineralization (spodumene) in the south-east (Kolmozero), with a variety of pegmatites of intermediate composition with columbite–tantalite and beryl in the middle of this zone (Polmos) (Gavrilenko, 2001). Tantalite from the Vasin Mylk Pegmatite revealed U–Pb ages of 2518 ± 9 Ma (Kudryashov et al., 2004). Spodumene-type pegmatites of the Kolmozero Pegmatite Field are intrusive into biotite–garnet–staurolite-bearing gneisses and carry beryl, tourmaline, pyrochlore, CGM, garnet, molybdenite, apatite, and phosphates (Fersman, 1962). The Kolmozero deposit contains Ta–Nb-oxides in four zones (Gordienko, 1970; Badanina et al., 2010a,b, 2015) with estimated total rare metal reserves of 74 million tons at 0.0075% Ta, 0.011% Nb, 0.53% Li and 0.019% Be (Weihed et al., 2013). Kolmozero hosts CGM ranging from columbite-(Fe) to tantalite-(Mn) following a fractionation trend (Fig. 5d) superimposed by a later metasomatic overprint (Badanina et al., 2015). Ta–Nb oxide phases separated from the Kolmozero Pegmatite are mainly columbite-(Mn, Fe) and some tantalite-(Mn, Fe), tapiolite and rutile. Tantalite-(Mn), columbite-(Mn), stibiotantalite and microlite were also studied from the Voronya Tundra Pegmatite Field, intruding amphibolite (Fersman, 1962). Most of the pegmatites in the area are barren, but some belong to the complex (rare-element) type of the LCTfamily and carry spodumene, pollucite, lepidolite, beryl, montebrasite, tourmaline, tantalite-(Mn), simpsonite and microlite. 4.2. Paleoproterozoic 4.2.1. São João del Rei Pegmatite Province, Brazil The São João del Rei Pegmatite Province in the southern Minas Gerais State, Brazil (Fig. 1), which includes the economically important Sn–Ta-rich Volta Grande Pegmatite Field at Nazareno (e.g., MIBRA mine), is associated with Paleoproterozoic Transamazonian granites that intruded an Archean greenstone belt at the southern border of the São Francisco Craton (Heinrich, 1964; Lagache and Quéméneur, 1997). The “layered pegmatites” are of the albite–spodumene type and in addition carry K-feldspar, muscovite, lepidolite, holmquistite and zinnwaldite. The main deposit of the MIBRA operations, the Orebody A Pegmatite, hosts 2359 t of Ta2O5 at 375 g/t Ta2O5 (Mining Journal Spec. Publ. — Tantalum, 2010). A Paleoproterozoic age (U–Pb of columbite; 2024 ± 7 Ma; Fig. 3b, Table 2) was obtained for a Ta concentrate from São João del Rei. Age estimates based on U–Pb isotopes of CGM in several samples range from 2050 to 2090 Ma, and are 2115 Ma for cassiterite. Ávila (2000) gives a zircon age of 2121 ± 7 Ma from a granite associated with pegmatite mineralisation. The Transamazonian granites in the area were dated at 1930 ± 30 Ma (Rb–Sr; Quéméneur and Vidal, 1989). Several concentrates from the São João del Rei Province yield similar mineralogical compositions (Table 1). As an average of three Ta concentrates, Ta–Nb-oxides make up 86% of the total. Ta–Nb–Sn minerals are dominated by CGM (96%), composed of tantalite-(Mn) (69%) and columbite-(Mn) (25%) (Fig. 4d). Cassiterite (0.9%), wodginite (1.1%) and microlite (1.5%) are present in minor amounts. A Sn-rich concentrate from the same region consists of cassiterite (71%) and microlite (26%), followed by Mn-rich CGM (2.3%) and wodginite–ixiolite (1.3%).

11

CGM from São João del Rei are generally Mn-rich (columbite and tantalite with #Mn N 60; Fig. 5e). Diamond-bearing conglomerates from the Diamantina area in the central Minas Gerais State (e.g., at Dattas) are Lower to Middle Proterozoic in age (Machado et al., 1989)). They commonly contain columbite and brookite as well as minor cassiterite, and the rare mineral senaite [Pb(Ti,Fe,Mn)21O38] (Hussak, 1899; Hussak and Prior, 1898). 4.2.2. Further examples from northern Brazil and adjoining areas Paleoproterozoic Ta–Nb and Sn-bearing pegmatites intruding granites and gneisses occur along the eastern border of the Guyana Shield, e.g., at Lourenço in Amapá, Brazil (Putzer, 1976) (Fig. 1, district 2.2, label “L”). The pegmatites carry CGM (mainly columbite-(Fe); Fig. 5e), cassiterite, tourmaline, beryl, nigerite (Kloosterman, 1974) and gahnite. Ta–Nb-oxides recovered from a concentrate consisting of columbite(Fe), tapiolite and ilmenite yielded an age estimate of 2000 Ma (Table 1, Supplementary material S2). Columbite-bearing placers are associated with pegmatites in French Guiana, Suriname and Guyana (Putzer, 1976). Small production is recorded from placers in Guyana (Morabisi area; Fetherston, 2004; Kantharaja, 2011) and French Guiana, where 80–90 t of alluvial columbite–tantalite have been produced from 1969 to 1991 (Fetherston, 2004). Coarse alluvial grains consisting of homogeneous tapiolite and columbite–tantalite, occasionally intergrown with tapiolite, from the Marowgne river, Suriname (Fig. 1, district 2.2, label “M”), give an age estimate of 500 Ma (Supplementary material S2); however, this is in contradiction to a 2083 ± 8 Ma age of zircon from granite of the North Guiana pegmatite belt (Marowijne–Paramaca greenstone belt; Tkachev, 2011). The tin, rare metal (Zr, Nb, Ta, Y, REE) and cryolite mineralisation at the economically important Pitinga Sn–Ta Mine (12,000 t per year cassiterite, 227 tons Ta2O5 in 2008) in Amazonas State, Brazil (Fig. 1, district 2.2, label “P”), is hosted by an oxidised, highly evolved peralkaline albite granite of the Madeira Igneous Complex and by biotite and topaz granites of the adjacent Agua Boa Igneous Complex. Both are about 1820 Ma old (Lenharo et al., 2003; Fetherston, 2004). Mineralisation at Pitinga comprises cassiterite, CGM, microlite, “plumbocolumbite”, zircon, thorite, ilmenite, rutile, magnetite, apatite, xenotime and others (Costi et al., 2000; Lenharo et al., 2003). A fine-grained tin concentrate from the Pitinga Mine yielded, besides cassiterite (~ 69%) and zircon (18%), moderate quantities of columbite-(Mn) and columbite-(Fe) (about 3% each) as well as minor plumbopyrochlore, ilmenite, pyrite, thorium silicate and rare fergusonite (Table 1). 4.2.3. Black Hills Pegmatite Province, South Dakota, USA The Black Hills Pegmatite Province comprises a large number (ca. 24,000) of pegmatite bodies that accompany the Harney Peak Granite, South Dakota (Fig. 1; e.g., Jolliff et al., 1986; Shearer et al., 1987, 1992; Norton and Redden, 1990). Zoned pegmatites in this province range from rather simple ones to, mostly small, rare-element pegmatites with minerals of Li, Be, Nb, Ta, Sn and Cs (e.g., Hugo and Peerless Pegmatites at Keystone; Černý, 1989; Norton and Redden, 1990). Some of the zoned pegmatites yielded economic amounts of spodumene, amblygonite–montebrasite, Ta–Nb minerals, cassiterite and pollucite (Norton and Redden, 1990). The lepidolite-bearing Bob Ingersoll No. 1 Pegmatite and the spodumene-bearing Tin Mountain Pegmatite carry a sequence of CGM from columbite-(Fe) → columbite-(Mn) → tantalite-(Mn) → microlite + tapiolite + wodginite (Fig. 8b; Spilde and Shearer, 1992). Radiometric ages of the pegmatites are ca. 1702 to 1718 Ma (Černý, 1991c; Tkachev, 2011). Age estimates from this study are around 1610 Ma (Supplementary material S2). Two concentrates from Pennington County (probably from the Cowboy Pegmatite) consist chiefly of cassiterite (90%); Ta–Nb-oxides make up 6% and are dominated by columbite-(Fe), with minor tantalite-(Fe) and tapiolite. The concentrates also yielded appreciable wolframite and garnet (Table 1).


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4.2.4. Svecofennian and Svecokarelian Provinces, Sweden, Finland and Russia Paleoproterozoic pegmatites related to the Svecofennian events and yielding columbite ages from 1821 to 1750 Ma are widespread in central and northern Sweden (Romer and Wright, 1992; Romer and Smeds, 1994, 1996, 1997). Various types of the rare-element class of granitic pegmatites (including LCT family pegmatites) occur in the Ostrobothnia region, belonging to the Svecofennian accretionary arc complex of central and western Finland. The mineralogy of these pegmatites has been extensively studied (Lahti, 1987; Romer and Smeds, 1996; Alviola et al., 2001; Mäkitie et al., 2001) and ages between 1789 and 1831 Ma have been obtained for zircon, tapiolite and columbite– tantalite (Lindroos et al., 1996; Alviola et al., 2001). Placer columbite–tantalite and tapiolite grains were studied from tributaries of the Lemmenjoki river in the Paleoproterozoic (ca. 2 Ga) granulite belt in Finnish Lapland, northern Finland (Fig. 1, district 2.4, label “L”). The grains were recovered from glaciofluvial river gravels, sands and terraces of the Miessijoki and Puskuoja rivers. The glacial transport has been in the direction of SW–NE trending river valleys that cut the general strike of the strongly strained metasedimentary granulites and magmatic mafic–ultramafic rocks. The CGM and tapiolite grains (Fig. 5f) occur with wolframite, scheelite, cassiterite, native gold, lead and bismuth, platinum group minerals, and thorianite–uraninite. Occasionally, fractures in tapiolite grains are filled with microlite and plumbomicrolite. Tapiolite and columbite–tantalite sometimes contain inclusions of strüverite. Other heavy minerals in the placers include magnetite, ilmenite, rutile, hematite, chromite, limonite and vanadinite (Kojonen et al., 2008, 2012). In accordance with recent age determinations of rocks within the granulite belt, U–Pb age estimates of detrital tapiolite and CGM grains found in rivers of Finish Lapland yield Paleoproterozoic ages (1856–2051 Ma; Supplementary material S2). The rare-element (Li, Be, Nb, Ta, Sn, Cs, Y) pegmatites of SomeroTammela in southwestern Finland (Fig. 1, district 2.4, label “T”), historically explored for Li and Sn, are hosted by Svecokarelian schists and intrusive rocks (Mäkinen, 1913; Aurola, 1963; Alviola, 1989a,b). More than 100 pegmatites are known and about 50% of them host rareelement minerals (Alviola, 1989b). The pegmatites carry CGM, tapiolite, microlite, stannomicrolite (type locality), wodginite, strüverite– ilmenorutile, samarskite and cassiterite (Alviola, 1989b). Northern Karelia, Russia (Fig. 1, district 2.4, label “K”), contains numerous zoned pegmatites including rare-element pegmatites (Li–Cs), some of which have been mined for feldspar and muscovite in the past. Pb isotope measurements point to emplacement about 1800– 1900 Ma ago (Schneiderhöhn, 1961), respectively1860–1645 Ma (Ovchinnikov et al., 1975). The Urosozero (Uros Lake) pegmatite field contains approximately 60 narrow veins (up to 20 m wide), some of which contain abundant CGM (up to 40% of the heavy mineral fraction), monazite, thorite, fergusonite, pyrochlore and topaz. The pegmatites have NYF affinity and were intruded into a magmatic assemblage of andesite, dacite and basalt. At Pertima, a 340 m long and up to 30 m wide pegmatite vein hosts CGM and monazite in metasomatic portions composed of amazonite, muscovite and albite (Fig. 5f). 4.2.5. Bastar–Malkangiri Pegmatite Belt, India The Bastar–Malkangiri Pegmatite Belt in Central India (Fig. 1) hosts Sn and rare metal mineralisation associated with LCT-family pegmatites that are probably of Paleoproterozoic age (Pal et al., 2007); the latter carry K-feldspar, albite, muscovite, quartz, lepidolite, amblygonite, beryl, fluorite, tourmaline, cassiterite, Ta–Nb-oxides, and zircon. A general mineralogy of the Sn, Nb and Ta mineralisation in the Bastar Belt has been presented by Babu (1983) and Pal et al. (2007). The mineralogy of a Sn ore concentrate and two Ta concentrates from the Bastar Belt includes cassiterite (20–96%), CGM (up to 46%), tapiolite, wodginite–ixiolite and microlite replacing tantalite-(Fe) (Table 1). CGM comprise both columbite (Fe, Mn) and tantalite-(Fe, Mn) (Figs. 4e, 8a). Another secondary Ca-bearing Ta–Nb-oxides

replacing tapiolite possibly represents fersmite or rynersonite. The major element chemistry of CGM follows a fractionation trend from columbite-(Fe) to tantalite-(Mn), but at overall low Ta concentrations (Fig. 8a). Age estimates based on U–Pb isotopes in CGM range from 2098 to 2149 Ma (Supplementary material S2). 4.2.6. New South Wales, Australia Mineralogically zoned pegmatites occur in the Precambrian Carpentarian Willyama Complex of the Paleo- to Mesoproterozoic Broken Hill district (Lishmund, 1976). These so-called Type I pegmatites have been mined for feldspar, beryl and quartz since the 1940s (Lishmund, 1982). Tantalum-bearing pegmatites, which vary strongly in size, are described for the Thackaringa area. In the Lady Beryl pegmatite (County Yancowinna), tantalite occurs in lens-shaped bodies together with quartz and beryl. The Pearces Prospect pegmatite (northeast of Lady Beryl) hosts garnet and radioactive tantalite in its central zone (Tonkin, 1969). A concentrate from an unknown locality in New South Wales yielded 66% cassiterite, 7% Ta–Nb-oxides and 4% wolframite (Table 1); tapiolite is the most abundant Ta–Nb-oxide, followed by columbite-(Fe) and tantalite-(Fe). Furthermore, wodginite from a pegmatite at Broken Hill was investigated (Supplementary material S1). 4.3. Mesoproterozoic Ca. 1700 Ma old granites with associated pegmatite mineralisation at 1440 Ma are known from the Colorado Province (Black Wonder Granite). Moderately mineralised pegmatites of the Wausau Complex in Wisconsin postdate the Paleoproterozoic Penokean Orogeny by ca. 150 Ma (1485 ± 15 Ma; Černý, 1991c; Falster et al., 1999; Simmons et al., 2012). Miarolitic pegmatites within the Nine Mile Pluton, an alkali granite to monzonite within the Wausau complex, carry a complex mineralogy including Nb–Ta oxides and abundant REE minerals (Falster et al., 2012). Pegmatites associated with late stages of the Rapakivi granite intrusions in Southern Finland record ages of 1570 to 1500 Ma (Černý, 1991c). In South America, rare-element mineralisation is associated with both, the Rio Negro (ca. 1700–1600 Ma) and Rondonia (ca. 1400– 1250 Ma) cycles. The latter presumably includes recently discovered alluvial Ta–Nb–Ti–Sn–Zr mineralisation in Eastern Colombia (Cramer et al., 2010; Fig. 1). Age estimates based on U–Pb isotopes in wodginite–ixiolite, CGM, rutile and cassiterite point to a period of mineralisation ranging from 1277 to 1453 Ma (Supplementary material S2). Columbite-(Mn) recovered from placers originating from pegmatites of the Mesoproterozoic Sunsás belt, Bolivia, is replaced by fersmite, polycrase and microlite (Alfonso et al., 2015). 4.4. Early Neoproterozoic Early Neoproterozoic pegmatite provinces are abundant worldwide and include the Kibara and Orange River Belts in Africa, the Grenvillian Central Metasedimentary Belt (Ontario, Quebec; mainly NYF family), the Sveconorwegian in southern Norway and southern Sweden (both, NYF and LCT families; Smeds, 1990), the Pikes Peak batholith in Colorado (NYF) and the Llano uplift in Texas (NYF; Černý, 1991c). Pegmatites emplaced into amphibolite- to granulite-facies metamorphic rocks in the Adirondack Mountains, New York State, belong to the abyssal class, NYF and mixed NYF–LCT families (Lupulescu et al., 2012). They carry accessory columbite-(Fe) and rare element-bearing Nb–Ta minerals such as uranopolycrase, euxenite-(Y) and fergusonite-(Y). These pegmatites are related to two different tectonic events, (1) calcalkaline arc magmatism during the Shawingian orogeny (ca. 1180 Ma), and (2) extensional A-type granitic magmatism following collapse of the Ottawan orogeny (ca. 1000–1090 Ma; Lupulescu et al., 2012).



4.4.1. Sveconorwegian Province Radiometric ages of CGM, xenotime, titanite, K-feldspar, gadolinite and molybdenite from granite and pegmatite in southern Norway and Sweden (Sveconorwegian) range from 910 to 1094 Ma (Romer and Smeds, 1996; Tkachev, 2011). Högsbo is a famous pegmatite in southern Sweden (near Gothenburg). One of the pegmatite bodies at this location is zoned and contains albite veins with garnet, columbite, monazite, beryl, yttrotantalite and other rare element-bearing minerals (Brotzen, 1959, 1961). Several columbite-(Mn) crystals from Högsbo were investigated in this study (Fig. 8d). The homogeneous columbite contains scarce inclusions of pyrochlore and is partly replaced by patchy microlite aggregates. The U–Pb age estimate is ca. 1000 Ma (Supplementary material S2), in accordance with U–Pb (TIMS) ages of CGM from Orust and Högsbo that range from 1029.7 ± 1.4 to 1041.3 ± 1.6 Ma (Romer and Smeds, 1996). In southern Norway, peraluminous NYF-type pegmatites occur in several pegmatite fields ranging in age from 1060 to 910 Ma (Brøgger, 1906; Schneiderhöhn, 1961; Ihlen and Müller, 2009; Müller et al., 2012, 2015). The Evje-Iveland Pegmatite Field consists of more than 350 major pegmatite bodies associated with granite dated at 917 + 63/− 34 Ma (Andersen et al., 2002), intruding amphibolite, metadiorite and gneiss within the Telemark lithotectonic domain. Gadolinite from an unspecified pegmatite from the Evje-Iveland Pegmatite Field yielded a U–Pb age of 910 ± 14 Ma (Scherer et al., 2001). The zoned pegmatites are members of the rare-element REE and muscovite-rare-element REE classes (according to Černý and Ercit, 2005), and consist of K-feldspar, plagioclase, quartz, biotite, muscovite, magnetite and garnet, with accessory euxenite, aeschynite, samarskite, fergusonite, yttrotantalite, microlite, and (Sc-rich) columbite. Single grains of columbite, tapiolite and fergusonite from several pegmatites in southern Norway (Moss, Kragerø, Risør) were studied. The pegmatites near Moss and Råde in southeastern Norway, spatially associated with the Bohus–Fredrikshalde Granite (ca. 920 Ma), crystallised more than 50 Ma before the granite (Brøgger, 1906; Eliasson and Schöberg, 1989; Smeds, 1990; Romer and Smeds, 1996; Ihlen and Müller, 2009).

4.4.2. South Platte District, Colorado, USA The South Platte granite–pegmatite system in Jefferson County, Colorado (Fig. 1), is a prime example of well-developed internally zoned pegmatites enriched in the REE, that are hosted by a zoned granite batholith (Simmons et al., 1987; Simmons et al., 2012). The South Platte pegmatite district covers more than 75 pegmatites that occur as segregations within the parental Pikes Peak batholith. Samarskite-(Y) is the most abundant REE mineral, with additional samarskite-(Yb) and CGM. One CGM-bearing sample studied from the Rayleigh Peak area yielded an age estimate corresponding to the intrusion age of the Pikes Peak batholith (1010 Ma) (Supplementary material S2).

4.5. Late Neoproterozoic to Early Paleozoic Important pegmatite provinces are associated with “Pan-African” magmatism in mobile belts surrounding the African Craton. The PanAfrican orogeny is equivalent to the Brasiliano orogeny in Brazil that produced numerous rare-element pegmatite provinces. Pegmatites of similar age are also known from the Highland Complex Belt (Sri Lanka) and the Kerala Kondalite Belt in India (Tkachev, 2011). In the Eastern Brazilian Shield, several belts of rocks were affected by late Proterozoic deformation and magmatic emplacement, including the Borborema (Paraiba) and the Eastern Brazilian Pegmatite Provinces (Minas Gerais), both being important rare-metal pegmatite provinces with associated gemstones (e.g., Putzer, 1976; Viana et al., 2003; Baumgartner et al., 2006; Beurlen et al., 2008; Pedrosa-Soares et al., 2011).

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4.5.1. Eastern Brazilian Pegmatite Province (EBPP) Six pegmatite districts belong to the 800 km long and 150 km wide EBPP — Itambé, Araçuai, Safira, Nova Era, Aimorés and Espera Feliz (Fig. 1; e.g., Morteani et al., 2000). The districts differ in size, number of individual pegmatites and geology. Geochronological data for the pegmatites range between 1100 and 408 Ma (K–Ar, Rb–Sr, U–Pb); however, most data scatter between 600 and 450 Ma, with major periods of granite formation at 582–573 Ma (first generation of gem quality tourmaline-rich and zoned spodumene pegmatites) and granite and pegmatite intrusion (second generation; beryl-bearing ceramic simple pegmatites) from 520 to 498 Ma (Bilal et al., 2000; Viana et al., 2003; Tkachev, 2011). U–Pb age estimates of minerals from Araçuai range from 486 to 500 Ma (Supplementary material S2). CGM in a concentrate from Araçuai yielded a “Pan-African”, respectively Braziliano age (502 ± 3 Ma; Fig. 3c, Table 2). The material is dominated by columbite-(Fe,Mn) (43% of the Ta–Nb-oxides) and wodginite–ixiolite (23%), with abundant cassiterite (20%) and Nb–Ta-rich rutile (12%; Table 1, Figs. 2d, 4f, 8e). Tourmaline, muscovite, K-feldspar, garnet, CGM, beryl (including morganite), amblygonite, spodumene, various phosphates and Li-rich mica, have been reported. 4.5.2. Borborema Pegmatite Province (BPP), northeastern Brazil The BPP, comprising ~1500 pegmatite bodies, is located in the Seridó Fold Belt and extends over an area of 75 by 150 km (Fig. 1). “Heterogeneous”, often ore-bearing LCT-family pegmatites of the beryl–columbite–phosphate and complex spodumene subtypes exhibit up to four zones and contain beryl, tantalite, spodumene, amblygonite, lepidolite, K-feldspar, tourmaline, CGM, cassiterite and others (Beurlen et al., 2008; Da Silva et al., 1995). The historic minimum production is around 3000 t of tantalite and 1000 t of cassiterite (Beurlen et al., 2008). Columbite ages of mineralised pegmatites range from 509 to 515 Ma (Baumgartner et al., 2006). Two concentrates analysed in this study yielded different mineralogical compositions (Table 1). One concentrate from a pegmatite near Ecuador was rich in columbite-(Fe, Mn) and tapiolite (U–Pb age estimate for CGM: 560 Ma), another one from a pegmatite near Picui was rich in tantalite-(Mn), tapiolite, wodginite– ixiolite (Fig. 8e), microlite and ilmenite (U–Pb age estimate for CGM: 521 Ma, Supplementary material S2). 4.5.3. Polar Urals, Russia The Taikeu Ore Cluster forms part of the Kharbei–Longot–Yugan Ore District in the Polar Urals (Fig. 1; Vasil'ev et al., 2009). Four Nb–(Ta) deposits hosted by altered granitic rocks are outlined: Longot–Yugan (resources of 30,100 t Nb2O5 at an average grade of 0.126% Nb2O5), Taikeu (26,200 t Nb2O5 at an average grade of 0.046% Nb2O5), Ust–Mramornoe (7920 t Nb2O5 at grades of 0.032–0.28% Nb2O5) and Nemur–Jugan (0.08 to 0.25% Nb2O5; Urazova and Buchholz, 2012). Ta2O5 concentrations range from 0.007 to 0.3%. Mineralisation is found along the margins of metasomatised granite and Riphean greenschist facies rocks. Major ore minerals are fergusonite and plumbopyrochlore, with minor samarskite, columbite, Ta–Nb-bearing cassiterite and ilmenorutile (Vasil'ev et al., 2009), besides aegirine, fluorite, thorite, zircon, gadolinite, allanite, cassiterite, sphalerite, genthelvite and Pb-bearing chlorite. A three stage formation history is suggested for the magmatic rocks hosting the Taikeu deposits with (i) 605–570 Ma (collisional magmatism; U–Pb, zircon), (ii) 447–420 Ma (magmatic event, metasomatism; Sm–Nd and U–Pb, ore minerals) and (iii) 350–300 Ma (collisional magmatism; Rb–Sr, rocks and minerals) for the individual stages (O. Udoratina, pers. comm.). Four fine grained concentrates from the Longot–Yugan and Ust– Mramorne Nb–Ta deposits of the Taikeu Ore Cluster were studied. All concentrates are dominated (60–80%) by columbite-(Fe) and contain moderate amounts of fergusonite (3–10%) and columbite-(Mn) (2.5– 8%) (Table 1). Pyrochlore, plumbopyrochlore, Th-microlite and cassiterite occur in minor amounts (b3%). U–Pb isotopes of columbite from the Longot–Yugan deposit yield a poorly defined U–Pb age of 608 ± 70 Ma (Fig. 3d, Table 2). Considerable scatter along the concordia towards ages


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as young as 250 Ma points to a complex polyphase metamorphic history. 4.6. Paleozoic Pegmatites yielding formation ages between 250 and 550 Ma are common in South America (e.g., Argentina), North America (e.g., the Appalachians), Russia (e.g., the Urals), Asia and Europe. Variscan pegmatites occur in southwestern Europe (Portugal, Spain), central Europe, e.g., the Massif Central (France), the Austroalpine Basement Units (Austria, Italy), the Bavarian Forest, Oberpfalz, Bohemian Massif (Germany, Czech Republic), Western Carpathians (Slovakia) and others. Literature data of CGM and tapiolite compositions from several pegmatites in southern and central Europe indicate a wide compositional range; both pegmatites with abundant columbite-(Fe) (Hagendorf Province) and occurrences with almost iron-free columbite (Iberian province) to Ta-rich CGM in Slovakia and the Czech Republic are reported (Černý et al., 1992, 1998; Chudík et al., 2011; Novák and Černý, 1998, 2001; Novák et al., 2000, 2003; Uher et al., 1994, 1998, 2007; Leal Gomes et al., 2009; Dias et al., 2009; Martins et al., 2009; Fuertes-Fuente and Martin-Izard, 1998; Ramos, 2007; Mücke and Keck, 2008; Fig. 11a). LCT and NYF family pegmatites that in some cases carry complex Ta–Nb-oxide assemblages are known to occur in the Bohemian Massif, especially in the Moldanubian domain (Škoda and Novák, 2007; Novák et al., 2011; Melleton et al., 2012). A number of unusual chemical compositions have been reported from Nb–Ta oxides in Moldanubian and other Variscan pegmatites, including W-rich Nb–Ta oxides (Novák et al., 2008), Sc-rich Nb–Ta oxides (Wise et al., 1998; Novák et al., 2008; Dill et al., 2008) and U–Y–REE-rich Nb–Ta–Ti minerals (e.g., Škoda et al., 2011; Uher et al., 1998). High concentrations of Sn, Zr, Sb, Pb and Bi are reported from CGM (Černý et al., 1998; Novák and Černý, 1998; Novák et al., 2000). Radiometric ages of monazite and CGM from Moldanubian pegmatites constrain the emplacement to the period 333 to 325 Ma, postdating the 350–340 Ma high-pressure–high temperature metamorphism (Novák et al., 1998; Melleton et al., 2012), whereas those in the Hagendorf province of Germany reveal U–Pb ages of 300–310 Ma (Dill et al., 2008). In southern China Ta–Nb-oxides are mined from granitic pegmatites and from rare-metal (Li–F) granites of Paleozoic age. The Nanping rareelement granitic pegmatite field with more than 500 pegmatite veins lies at the southeastern part of the Cathaysian block in the Fujian Province (Yang et al., 2003; Rao et al., 2009). The No. 31 pegmatite group has economic significance (4230 tons of reserves at 0.030% Ta2O5) and represents highly evolved spodumene-type pegmatites (LCT pegmatites; Fetherston, 2004; Rao et al., 2009). Rare-metal mineralisation includes CGM of a wide range of compositions (although columbite-(Mn) is scarce), tapiolite(Fe), wodginite, microlite, niobian rutile and fergusonite (Yang et al., 2003). It is suggested that the Nanping pegmatites are the product of differentiation of spatially related migmatitic granites (295 ± 30 Ma, different authors in Yang et al., 2003); however, whole-rock Rb/Sr isotopic ages of the metamorphic host rocks range from 328 to 235 Ma (Yang et al., 2003). 4.6.1. Pampean Pegmatite Province, Argentina The Pampean Pegmatite Province in the Sierra de San Luis, Argentina (Fig. 1), hosts Sn- and Ta–Nb-mineralised LCT pegmatites that intruded a polyphase metamorphic basement during the Ordovician to Silurian (Sosa et al., 2002; Galliski and Černý, 2006). The El Totoral Field is composed of LCT pegmatites of the albite (albite, K-feldspar, spodumene relics, muscovite: Los Chilenitos, Independencia Argentina) and spodumene types (e.g., Victor Hugo; Galliski and Černý, 2006). Pegmatites in the area comprise beryl-type (beryl-columbite and beryl-columbite– phosphate subtypes with CGM, uranmicrolite and bismuth minerals), complex, albite, and albite–spodumene types. Petalite and lepidolite subtypes are rare. Post-orogenic pegmatites within the Acahala batholith carry beryl, phosphates, CGM, and minor amblygonite.

Samples from the El Totoral Field analysed in our study confirm the mineralogical data of Sosa et al. (2002) with the presence of columbite(Mn) and tantalite-(Mn) (Fig. 8f); cassiterite is mostly present in separate pegmatite bodies (Sosa et al., 2002). Age estimates based on U–Pb isotopes range from 437 Ma (Los Chilenitos pegmatite) and 442 Ma (Victor Hugo pegmatite) to 456–481 Ma (Independencia pegmatite) (Supplementary material S2) which is within the range of columbite ages published for the San Luis II (450 ± 12 Ma) and La Totora pegmatites (476 ± 12 Ma; Von Quadt and Galliski, 2011). K–Ar ages of muscovite bracket mineralisation between 404 Ma (Los Chilenitos) and 444 Ma (La Esmeralda) (Sosa et al., 2002). 4.6.2. Central Iberian Pegmatite Province The Bajoca, Gonçalo (Portugal) and Feli pegmatites (Spain) host minor amounts of columbite-(Mn) and tantalite-(Mn), microlite and rutile besides abundant cassiterite, lepidolite, spodumene, petalite, Kfeldspar, albite, muscovite and quartz (e.g., Mangas and Arribas, 1988; Vieira and Lima, 2007; Neiva and Ramos, 2010). According to the latter authors, the aplite–pegmatite sills at Gonçalo represent LCT pegmatites of the amblygonite and the lepidolite subtypes of Černý and Ercit (2005). The CGM at Feli were dated to 303 Ma (U–Pb age, Fig. 3e, Table 2), those at Gonçalo gave an unrealistically young date of 239 Ma (age estimate for CGM, Supplementary material S2); radiometric K–Ar ages of 270–277 Ma are reported for lepidolite from lithian veins at Gonçalo (Ramos, 2007). Zircon and monazite from granite associated with pegmatite yielded a 311 ± 1 Ma age (Almeida et al., 1998; Lima and Roda-Robles, 2007; Tkachev, 2011). 4.6.3. Hagendorf Province, Germany The pegmatites in south-eastern Germany (e.g., Hagendorf, Pleystein, Püllersreuth, Hühnerkobel/Zwiesel) are phosphate-rich, but carry rather “primitive” columbite-(Fe) (Fig. 11a) associated with uraninite and petscheckite–liandratite (Dill et al., 2007, 2008, 2012; Dill, 2015; Mücke and Keck, 2008). U–Pb ages for CGM range from 300 to 310 Ma (Dill et al., 2008). Columbite is associated with albite, quartz, beryl, garnet, and schoerl in an abandoned pegmatite mine near Püllersreuth (Linhardt, 2000). CGM from other pegmatites in the Moldanubian domain in Czech Republic crystallized at 333 Ma and 325 Ma, respectively (Melleton et al., 2012). 4.6.4. Austroalpine Province, Austria and Italy The mineralisations investigated from the Eastern Alps are mostly spodumene-bearing pegmatites (e.g., Postl and Golob, 1979; Göd, 1989; Černý et al., 1989, 1992; Melcher et al., 2010) although CGM and cassiterite-bearing pegmatite veins carrying beryl and phosphates have also been reported (Schneider et al., 2012). Major element compositions of Ta–Nb-oxides vary from a dominance of Fe to Mn whereas tapiolite, microlite, fersmite, wodginite–ixiolite and U–REE-rich Ta– Nb-oxides are occasionally present; minor amounts of cassiterite occur (Fig. 11c). In the Hohe Kreuzspitze area, Passeier Valley, South Tyrol (Italy), a vein-like pegmatite hosted by metapelites of the polymetamorphic Ötztal–Stubai crystalline basement forms boudins of meter thickness, and is composed of quartz, albite, muscovite, with accessory beryl, garnet, zircon, staurolite, various phosphates, and Sn- and Ta-oxide phases (wodginite–ixiolite, tantalite-(Fe), columbite-(Fe), ferrotapiolite, Tarich rutile, cassiterite) (Konzett, 1990; Konzett et al., 2013; Schneider, 2013). U–Pb analysis of one large and complexly zoned wodginite– ixiolite crystal using LA-ICP-MS revealed two age populations at 300 and 200 Ma (Fig. 3f), whereas zircon yielded an age of 240 Ma (U–Pb; Konzett et al., 2013). In the Pusterwald–Bretstein–Lachtal area, Styria, zoned spodumenebearing pegmatites are hosted by mica schist, marble, amphibolite and quartzite of the Rappold Complex which is part of the Austroalpine Koralpe–Wölz nappe system (Mali, 2004). The pegmatites of likely Permian age (U–Pb age estimates of 206–228 Ma for CGM are affected



by a strong Alpine, Late Cretaceous metamorphic overprint) carry accessory graphite, garnet, tourmaline, apatite, beryl, pollucite, cassiterite, Ta–Nb-oxides (columbite-(Mn), tantalite-(Mn), pyrochlore, microlite, fersmite, aeschynite, wodginite–ixiolite, tapiolite; Fig. 4g, 11b), zircon, and uraninite. At the Weinebene–Brandrücken, Koralpe, Carinthia, unzoned, dike-like spodumene-bearing pegmatites are hosted by eclogitic amphibolites and kyanite-bearing micaschists (Göd, 1989). The pegmatites carry accessory apatite, beryl, cassiterite, columbite(Fe) (Fig. 11b) and zircon; titaniferous columbite with exsolved niobian rutile was investigated by Černý et al. (1989). Rb–Sr whole rock dating yielded an “approximate age” of 280 Ma (Göd, 1989). However, zircons from the Weinebene pegmatite define an upper intercept age of 240 ± 1.5 Ma, which may be considered as the minimum age of formation (Heede, 1997). 4.6.5. Southern Urals, Russia In the Ural Mountains (Fig. 1), both rare-element pegmatites and metasomatically altered granitic rocks host Ta–Nb-oxides. Granites related to rare-element pegmatites in the Ural Pegmatite Province (eastern Ural fold belt) reveal Variscan ages (e.g., Montero et al., 2000). The Lipovy Log Ta–Nb–Mo pegmatite deposit with 300 tons of Ta2O5 reserves at 0.007% yielded an age of 262 ± 7.3 Ma (Re–Os; Mao et al., 2003). Tantalite and columbite are the major Ta–Nb-oxides. In the Svetlinskoje Pegmatite Field, located in the western exocontact of the Borisov Massif in the southern Urals, miarolitic pegmatites are associated with leucogranite dykes intruding metasedimentary rocks of Lower Paleozoic age (Talantseva, 1988). Some of the pegmatites yielded gem-quality beryl, topaz and tourmaline and were intermittently explored for piezo-optic quartz, kaoline and gemstones. The pegmatite carries columbite, tantalite, tapiolite and stibiotantalite. The Ilmen Mountains in the southern Urals, located along the Main Uralian Fault zone, host a large district of miarolitic pegmatites of NYF affinity (Schneiderhöhn, 1961; Popov and Popova, 2006). Related to the Urals Orogen are bodies of feldspathoidal syenites, of which the Ilmen miaskite (nepheline syenite) massif is the largest, and granites emplaced in metamorphic rocks of Riphean (Late Proterozoic) age. The associated pegmatites, grouped into six principal types, span an age range of 290–240 Ma (Belogub and Bazhenov, 1997; Popov and Popova, 2006). The types comprise: (1) pre-miaskite granite pegmatites; (2a) feldspar (syenite) pegmatites; (2b) miaskite pegmatites; (2c) corundum-feldspar pegmatites; (3) alkaline ultramafic pegmatites, (4) “carbonatite pegmatites”; (5) post-miaskite granite pegmatites; and (6) granite amazonite pegmatites. Types 2a, 2c and 5 carry abundant columbite-(Fe), whereas columbite-(Mn) is restricted to types 5 and 6, and tantalite-(Fe, Mn) to type 6. Type 5 also carries abundant ilmenorutile. REE-rich Ta–Nb-oxides are associated with types 2, 4, 5 and 6. Niobian rutile breaking down to wolframoixiolite has been described by Černý and Chapman (2001) from a mineral specimen of unknown origin in the Ilmen Mountains. 4.6.6. Appalachian Fold Belt, USA and Canada In the Appalachian Fold Belt (Fig. 1), pegmatites formed from Devonian to Permian times (i.e., from 395 to 272 Ma; Tkachev, 2011). They are particularly abundant in the southwestern (Virginia, North Carolina, South Carolina, Georgia, Alabama) and northeastern Appalachians (Maine, New Hampshire, Massachusetts, Connecticut, eastern Canada) (Schneiderhöhn, 1961, for a summary; Wise, 2006, Wise et al., 2012). As an example, the Kings Mountain Pegmatites, North Carolina, are part of an extensive tin–spodumene belt, hosted predominantly by amphibolites. The deformed, moderately zoned pegmatites of the albite– spodumene type sensu Černý (1989) are related to the 345 Ma old Cherryville quartz monzonite (347 ± 2 Ma columbite–tantalite U/Pb age from the Foote pegmatite, Kings Mountain; see McCauley and Bradley, 2014). Although reserves of spodumene are large, no data are available about the contents and reserves of cassiterite and columbite–tantalite, the latter having Nb/Ta ratios (wt.%) between 8 and 0.6

15

(Černý, 1989). The LCT-type pegmatite at Brazil Lake, southwestern Nova Scotia, carrying abundant spodumene, Rb-rich K-feldspar, mica and Nb–Ta oxides was emplaced into metasedimentary and metavolcanic rocks of the Silurian White Rock Formation that overlies CambroOrodovician sediments of the Meguma Group; U–Pb TIMS dates of five fractions from one tantalite grain range from 395 to 366 Ma (Kontak et al., 2005). Columbite crystals from North Carolina (Spruce Pine District, Mitchell County) in the Southern Appalachian Pegmatite Subprovince, and from Chesterfield (Clarke Ledge or Chesterfield Hollow) in Massachusetts (New England Subprovince) were investigated in this study (Fig. 11d). The New England Subprovince mainly hosts shallow emplaced LCT-family pegmatites with rare-element enrichment (Li, Rb, Cs, Be, Ga, Sn, Ta N Nb, B, P and F) related to S-type parental granites, although there are a few NYF type pegmatites scattered throughout. Geochemically more primitive pegmatites, dominated by muscovite — rare-element types (which by definition cannot be given a LCT or NYF designation; Černý and Ercit, 2005), in the Mid-Atlantic and Southern Subprovinces (e.g., the Spruce Pine mica pegmatites; Maurice, 1940; Swanson and Veil, 2010) were emplaced at deeper levels and are related to poorly differentiated S- to I-type granitoids. NYF-family pegmatites (Nb N Ta, Ti, Y, REE, Zr, Be, U, Th, F) related to A-type granitoids are more prominent in the New England Subprovince (Wise, 2006, Wise et al., 2012); however, except at Topsham, Maine, these pegmatites do not carry significant CGM (M. Wise, pers. communication, 2013; Hanson et al., 1998). 4.6.7. Kalba–Narym Pegmatite Belt, Kazakhstan The large pegmatite province in the Altai Fold Belt (Pri-Irtysh orogeny; Fig. 1) comprises Permian (283–295 Ma) rare-metal pegmatites (Vladimirov et al., 2001; Dyachkov and Maiorova, 1996; Kotler et al., 2014). Associated granitoids have been dated at 289 ± 3 to 275 ± 3 Ma by the Ar–Ar method; zircons from granitoids of the Monastery Complex yielded 284 ± 4 Ma (Kuibida et al., 2009), whereas recent U– Pb geochronological data of biotite granites related to the main Kalba complex I phase yielded 300 ± 3 Ma (Kotler et al., 2014). Numerous large (150 to 1000 m in length, 2–6 m in thickness), concentrically zoned Ta, Nb, Sn, Be and Cs-bearing pegmatites carrying beryl, schorl, garnet, tantalite, Ta-bearing cassiterite, microlite and pollucite are hosted by biotite granites. Lithium minerals such as spodumene, amblygonite, petalite, lepidolite and lithiophyllite–sicklerite are common. 4.7. Mesozoic and Cenozoic Pegmatite provinces younger than Paleozoic are known from central and eastern Asia, western North America and southern and central Europe (Tkachev, 2011). The Little Nahanni Pegmatite Group, Northwest Territories of Canada, intruded Upper Proterozoic schists ca. 82 Ma ago. Columbite–tantalite-(Mn), wodginite, cassiterite, Ta-rutile and uraninite have been reported (Groat et al., 2003). The LCT pegmatites are assigned to the albite-spodumene type and show simple to chaotic internal zoning (Černý et al., 2007). The pegmatite group of Mount Begbie, British Columbia, comprises more than 50 pegmatite bodies of diverse mineralogical composition. Some of them carry columbite–tantalite, bismutotantalite, Nb-rutile, cassiterite and qitianlingite in addition to beryl, tourmaline, cordierite, garnet, Li minerals, pollucite and phosphates (Dixon et al., 2014). Pegmatites intruded a sequence of metapelites and calc-silicate gneisses of the Monashee complex during decompression and exhumation from an unexposed S-type granitic magma ca. 50 Ma ago. The major, low-grade Yichun Ta–Nb–Li deposit (6800 tons of reserves at 0.017–0.020% Ta2O5) in the southwestern Jiangxi Province, China, is hosted by a topaz-lepidolite granite forming part of the Yashan Batholith (Yin et al., 1995; Fetherston, 2004; Wang et al., 2004). According to Yin et al. (1995), crystallisation of the ore minerals columbite– tantalite, microlite and Ta-rich cassiterite represents an integral part of


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the crystallisation of magma. The age of the ore hosting topaz-lepidolite granite was determined using the Rb–Sr isotopic method with 178 ± 3.7 Ma (Yin et al., 1995). Extremely fractionated granitic pegmatites of Triassic age (ca. 218 Ma; Che et al., 2015a,b) are found in the Chinese Altai Mountains. The extensive (up to 1400 m long) Koktokay No. 1 and No. 3 LCTfamily pegmatites intruded into Paleozoic metagabbro and reveal strong asymmetric zonation. Especially in zone III (cleavelandite– quartz–spodumene zone), the Koktokay No. 1 pegmatite carries abundant lepidolite, apatite, zircon, CGM, microlite, beryl, garnet, pollucite and lithiophilite (Yin et al., 2015). Within the Southern Steep Belt of the Alpine nappe system in the Central Alps (Italy), pegmatite dikes of Oligocene age (29–20 Ma) occasionally carry Ta–Nb-oxides including columbite-(Fe), euxenite-(Y) and wodginite (Guastoni et al., 2014). The mineral assemblages are consistent with LCT and mixed LCT–NYF pegmatites. The rare-element mineralized pegmatites from the Island of Elba, Italy, are among the youngest rare-element pegmatites (6.87 Ma; Dini et al., 2002). Miarolithic pockets carry complex Ta–Nb-oxides assemblages (Pezzotta, 2000; Aurisicchio et al., 2002). 4.7.1. Thailand, Malaysia and Burma Tin-mineralised and often Ta-bearing pegmatites are concentrated in the Western Granitoid Province of Thailand and Burma (22–149 Ma; Schwartz et al., 1995). Praditwan (1988) describes the association of cassiterite with columbite–tantalite, Nb–Ta rutile and some samarskite and tapiolite for samples from northern, central and southern Thailand. LCT pegmatites on Phuket Island are related to Cretaceous granites (Suwimonprecha et al., 1995) and belong to the beryl-columbite and lepidolite subtypes of Černý and Ercit (2005); ore minerals include cassiterite, columbite-(Fe,Mn), Nb–Ta rutile, “wolframoixiolite” and minor tantalite-(Fe,Mn) and microlite (Suwimonprecha et al., 1995). A concentrate from Thailand studied in this project is composed of columbite-(Fe), making up 84% of the CGM, followed by columbite(Mn), some wodginite–ixiolite, tapiolite, samarskite and cassiterite (Table 1). At the Kim Bee mine near Bakri, Malaysia, columbite–tantalite, cassiterite as well as wodginite, fersmite, tapiolite, euxenite and fergusonite are hosted by garnet-rich pegmatite (Praditwan, 1988; Schwartz et al., 1995). A concentrate composed of 55% columbite-(Fe,Mn), 13% microlite and 18% cassiterite gave an age estimate of 200 Ma (Table 1, Supplementary material S2). 4.7.2. Japan In the Fukushima Prefecture (Fig. 1), several granitic pegmatites hosting Nb–Ta mineralisation occur (Ishikawa district; Ryu et al., 2005); an example is the Uzumine pegmatite of probable Cretaceous age (Nakajima and Kurosawa, 2006). The Ishikawa granite has been dated to 111 ± 42 Ma (Sm–Nd mineral-whole rock isochron) and 106 ± 16 Ma (Rb–Sr whole-rock isochron; Shibata and Tanaka, 1987). Columbite-(Fe) is common, however, aggregates of Nb rutile with ilmenite and columbite-(Fe) were observed at Uzumine (Ryu et al., 2005; Nakajima and Kurosawa, 2006). A few single crystals of columbite-(Fe) from the Ishikawa-yama pegmatite field were investigated in our study. 4.7.3. Western Siberia Three major types of Ta deposits are recognised within the Altai– Sayan Foldbelt in western Siberia (Fig. 1): (i) rare-element pegmatites, (ii) alkaline metasomatically altered rocks and (iii) Ta-bearing spodumene granite and granite porphyry. Their formation is related to raremetal orthomagmatic alkaline–granitoid systems evolved within intraplate rift zones and shear zones in the south of the Siberian Craton above inferred mantle plumes (Dobretsov, 1997). The Ulug-Tanzek deposit (Ta, Nb, Zr, Th, Li, U and REE; Fig. 1, district 7.3, label “U”) occurs in the northern Sangilen Ridge of southeastern

Tuva, as a fragment of the Tuva–Mongolian massif. It is related to alkali granite intrusions of the Mesozoic Ulug-Tanzek intrusive complex (Grechishchev et al., 1997, 2010). The Ulug-Tanzek deposit is related to Li-mica-bearing riebeckite microcline–albite granites, which contain disseminated columbite and pyrochlore as the main ore minerals. Minor contents of zircon (malakon), thorite, fergusonite, monazite, xenotime, cryolite, gagarinite, bastnesite, yttrofluorite and galenite are reported. Some researchers attribute the deposit to alkaline rare metal granites (according to the “ongonitic” concept of Kovalenko et al., 1971; Kovalenko, 1977), whereas others favour a metasomatic origin related to albitized greisenized apogranites (Beus et al., 1962). All researchers recognized the intensive post-magmatic metasomatic processes such as microclinization, albitization and formation of a wide range of mica compositions (Li–Fe mica, including Li–Fe muscovite, protolitionite, zinnwaldite, polylithionite). Abundant riebeckitebearing quartz–microcline pegmatite bodies are typical for the inner contact of the massif, where metasomatic processes are best developed. These so called “pegmatoid schlieren” or “stockscheider” are a typical feature of rare metal deposits related to fluid-volatile-saturated granitic melts. Beryllium deposits hosting bertrandite, phenacite, feldspar and fluorite are located in the host marbles along the exocontact of the granite massif. According to K–Ar data, the ages of the alkaline granites, quartz–albite–microcline metasomatites and ore structures are 228– 231 Ma, 217–229 Ma and 209–219 Ma, respectively (Höll et al., 2000). In contrast, riebeckite granite from Ulug-Tanzek has been dated at ca. 300 Ma using Ar-Ar (amphibole) and U–Pb (zircon) dating (Yarmolyuk et al., 2010). Four concentrates were investigated in the present study (Table 1). Main Ta–Nb minerals are represented by columbite-(Fe), pyrochlore-(U) and plumbopyrochlore (Fig. 11f); minor minerals are fergusonite and euxenite, confirming the results of Vasil'ev and Borodulin (2010). The Khangilay Complex is situated in the central part of the Paleozoic Aginsk Block of Eastern Transbaikalia (Zanvilevich et al., 1985; Fig. 1, district 7.3, label “K”). The shallow-level intrusion of Cretaceous age (142.9 ± 0.9 Ma: Negrey et al., 1995, Kovalenko et al., 1999) consists of the main central Khangilay intrusion of biotite or two-mica granites and two mineralised granitic satellite intrusions, the (i) Orlovka intrusion with associated Ta deposits, a layered lithionite–amazonite–albite Li–F granite with topaz, Fe-lepidolite, rare tourmaline (Fe-elbaite), fluorite, monazite, xenotime, CGM, microlite, zircon, apatite, rutile, strüverite, cassiterite (Beskin et al., 1994), and (ii) the Spokojnoje intrusion with W mineralisation (Syritso, 2002). Based on zircon data, the parental Khangilay (139.9 ± 1.7 Ma) and Ta-bearing Orlovka massifs (141 ± 3 Ma) formed synchronously, whereas the W-bearing Spokojnoje massif formed earlier (144.5 ± 2.5 Ma; Rb–Sr dating, Abushkevich and Syritso, 2007). U–Pb dating of columbite–tantalite from ore-bearing granites of the Orlovka massif indicates a crystallization age of to 145 ± 1 Ma (MSWD =0.001) which coincides with the time of formation of the Li–F granites. This confirms that CGM crystallization occurred in a late-magmatic stage. The formation of wolframite from the Spokojnoje massif was dated by Rb–Sr and Sm–Nd methods to 139.8 ± 1.3 Ma (Abushkevich et al., 2010), confirming a metasomatic nature of tungsten mineralization. Both the main body and the satellites represent strongly differentiated granitic rocks from parental biotite granites to pegmatite bodies and greisens (Badanina et al., 2004, 2006, 2010b; Dolgopolova et al., 2005). The Orlovka Ta deposit has been in intermittent production since 1964. Six mineral separates representing the major rock types from the differentiated Orlovka massif were studied, from the parent biotite granites to the orebearing lepidolite–albite–amazonite granites and topaz greisen. Orlovka dominantly carries Mn-dominant CGM (#Mn: 0.40–0.95; #Ta: generally b 0.60; Figs. 4h, 11f) with minor microlite. Euxenite, cassiterite, rutile, Ta–Nb rutile and wolframite are trace components. In contrast, 80–85% of the Ta is associated with microlite in lithionite–amazonite–albite granites from the Etyka Ta-deposit, 300 km to the southeast from Orlovka.



The Malkhany Pegmatite Field of central Transbaikalia is located in the southwestern edge of the Caledonian Malkhany–Yablonevaya structural element (Fig. 1, district 7.3, label “M”), comprising metasedimentary rocks of Upper Proterozoic age, Lower Paleozoic intrusives, as well as Mesozoic granites. The latter are associated with numerous differentiated pegmatite bodies with rare metal and gem mineralisation, including the Bolsherechensky and Oreshny massifs composed of biotite, muscovite, and two-mica granites (Zagorski and Peretyazhko, 1992; Badanina and Gordienko, 1997; Badanina et al., 2008). Based on 40Ar/39Ar-data for biotite and muscovite, the age of the granite–pegmatite system ranges between 127.6 and 123.8 Ma (Zagorski and Peretyazhko, 2010). Some of the numerous miarolitic pegmatites are mined for gemstone-quality elbaite and polychrome tourmaline. The zoned Sosedka pegmatite vein is the largest pegmatite body of the Malkhany field (length ca. 100 m, thickness to 60 m). The main rock-forming minerals are K-feldspar, quartz, albite (cleavelandite), lepidolite, pollucite (rare), petalite and garnet (spessartine). The miarolitic pegmatite carries CGM, bismutomicrolite, stibiomicrolite, pyrochlore, betafite as well as a complex association of Sc–REE-bearing ixiolite, polycrase– euxenite, strüverite–ilmenorutile, monazite, zircon and bismuthinite (Badanina et al., 2008). Mineralised cavities up to 5 m in size contain euhedral elbaite, rare polychrome tourmaline (up to 15 × 30 cm), cleavelandite, lepidolite beryl (morganite), hambergite and occasionally danburite and zeolites. According to the study of melt and fluid inclusions in quartz this assemblage may have crystallized from a hydrosline melt (Thomas et al., 2012).

4.8. Upper Fir Carbonatite, Canada The Fir carbonatite system (Fig. 1) is located in the Canadian Cordillera within the northeastern margin of the Shuswap Metamorphic Complex (12 km north of Blue River) that is comprised of metamorphosed Late Proterozoic (ca. 700–550 Ma) supracrustal rocks and minor (ultra)mafic rocks (amphibolites, pyroxenites and meta-peridotites; Sevigny, 1988). The Fir carbonatite system is the largest of at least 14 carbonatite occurrences (measuring approximately 1200 m by 800 m) in the Blue River area and is composed of sills that reach up to 90 m in thickness (average 30 m). The carbonatite sills intruded the metasedimentary host rocks at 332.5 ± 5.7 Ma (based on the U–Pb age of zircon; Gorham et al., 2009) and were subsequently repeatedly folded and metamorphosed (140 Ma and 110–100 Ma) at amphibolite facies conditions (620–640 °C and 6–7 kbar; Digel et al., 1998; Chudy and Groat, 2013). The Fir carbonatite system consists of dolomite carbonatite and minor calcite carbonatite (b 5%) which occurs only within the dolomite carbonatite. Significant amounts of alkaline silica-undersaturated or (ultra-)mafic rocks that are typically associated with many other carbonatite complexes were not identified and an assignment to one of the proposed carbonatite clans (Mitchell, 2005) is not possible. The mineralogical composition of the Fir carbonatite includes ferroan dolomite (N80 vol.%), fluorapatite (10–15 vol.%), sodium–calcium amphibole (5–10 vol.%), and minor phlogopite, zircon, ilmenite, magnetite, monazite and thorite (Chudy and Groat, 2011; Chudy et al., 2014). The main Nb–Ta phases are pyrochlore and columbite(–Fe), and minor microlite and fersmite. Based on the Nb–Ta mineralisation and the amphibole composition, two main mineralogical facies are distinguished for the dolomite carbonatite: a columbite-bearing winchite facies along the intrusion margins and a pyrochlore-bearing katophorite (transitional to richterite) facies towards the center of the intrusion. The columbite forms fine- to coarse-grained, strongly poikilitic to sievetextured grains that are generally strongly metasomatised and contain abundant secondary high-Ta pyrochlore (transitional to microlite) and thorite. The pyrochlore in the katophorite facies forms fine- to coarsegrained subhedral to euhedral crystals that are evenly disseminated in the carbonatite or concentrated into chain-clusters, often together with zircon, fluorapatite or katophorite.

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Unlike the vast majority of carbonatite-hosted deposits, which are primarily known for their economic resources of REE, Nb, Fe, Ti, Zr, and P (Linnen et al., 2014), the Fir carbonatite system has been developed by Commerce Resources Corp. (labeled as “Blue River Tantalum–Niobium Project — Upper Fir property”) as a potential ethical source of Ta and contains an indicated resource of approximately 48.4 Mt of ore grading 197 ppm and 1610 ppm of Ta2O5 and Nb 2 O 5, respectively, and an inferred resource of approximately 5.4 Mt of ore grading 191 ppm and 1760 ppm Ta 2 O 5 and Nb2 O 5 , respectively (Kulla et al., 2013).

5. Mineral chemistry of Ta–Nb oxides The chemical composition of Ta–Nb-oxides in pegmatites and rareelement granites is characterised by strong variability. However, as already shown for Ta ore concentrates from African deposits (Melcher et al., 2015), major as well as trace elements show regional characteristics that, in many cases, allow the distinction of ore provinces based on the composition of a representative number of Ta–Nb-oxide grains. The #Mn–#Ta diagrams (Figs. 5, 8, 11) illustrate the major element variation in CGM, tapiolite and wodginite–ixiolite that basically reflects differentiation of, and fractional crystallisation from melts. Summary tables (Appendices 3 to 5) showing such characteristics, and normalised diagrams for Ta-deposits worldwide except Africa are presented to account for the heterogeneity (Figs. 6, 9, 12). Identical diagrams for African Ta deposits were used by Melcher et al. (2015). The degree and type of the variation of trace element concentration in Ta–Nb-oxides is complex and depends on multiple factors, including crystal–chemical parameters (e.g., ion radius, charge), melt chemistry (e.g., presence of fluxing elements), internal differentiation of the melt, competition of cocrystallising phases such as zircon, apatite, garnet, tourmaline adjacent to Ta–Nb-oxides, reaction with host rocks, and melt source characteristics. All these factors are superimposed on each other and result in characteristic trace element signatures for Ta–Nb-oxides from different ore provinces. For illustration of regional differences, trace element patterns were constructed normalised to a global CGM average. For normalisation, a “global CGM median” was calculated from the median values for each element in a suite of CGM from 29 ore provinces (Melcher et al., 2015). The normalising values obtained by this approach are, in ppm: Li 7.2; Mg 29.8; Sc 17; Y 20.8; Yb 1.6; Ti 4073; Zr 1051; Hf 169; Sn 858; W 1982; Sb 0.15; Bi 0.57; Th 6.1; Pb 58; and U 384. As trace element concentrations in the dataset do not show a normal distribution, robust statistical parameters were chosen to illustrate variations within each province: the median (Md), P25 (25th percentile) and P75 (75th percentile) to illustrate the variation around the median, and P90 (90th percentile) to approximate the maximum values but eliminate outliers. The REE + Y concentrations were normalised to average chondrites (C1 chondrite; McDonough and Sun, 1995), and the median, P25, P75 and P90 values are presented for CGM, wodginite–ixiolite and tapiolite in Figs. 7, 10 and 13. REE + Y patterns are classified using the nomenclature and parameters developed by Graupner et al. (2010) that are based on the relative abundances of the LREE (La to Sm), MREE (Gd to Y) and HREE (Er to Lu) and the presence or absence of anomalies quantified as Eu/Eu* = (EuN / (0.5 ∗ (SmN + GdN))) and Ce/Ce* = (CeN / (0.5 ∗ (LaN + NdN))). In the following sections, the main characteristics of Ta–Nb-oxide chemical compositions from each province are discussed and some extraordinary median or maximum values are quoted. For illustration the reader is referred to Figs. 5–14. Pyrochlore-group minerals that are abundant in many of the provinces studied, are mostly excluded for two reasons, (i) incomplete data, and (ii) often secondary nature of their occurrences (i.e., alteration of primary CGM by secondary microlite; Lumpkin and Ewing, 1992, 1995). Therefore, remarks are only made if important for the particular deposit.


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Fig. 6. Trace element diagrams of Ta–Nb oxides normalised to an average CGM composition (see text for explanation). Diagrams are arranged according to the emplacement age of the pegmatites, from the Archean to the Mesozoic. For each province, the median CGM composition (Md, blue line with diamond symbols), P25 (lower line with cross symbols), P75 (upper line with cross symbols) and P90 (upper stippled line) are presented. Patterns in red refer to wodginite/ixiolite (median), those in brown with yellow diamond symbols are tapiolites. (a) Tanco, Canada, CGM (N = 367 measurements of Ti, Zr, Hf, Sn, W, U by EPMA and LA-ICP-MS; 129 measurements of additional trace elements by LA-ICP-MS); wodginite (298/5). (b) Wodgina, Australia, CGM (127/17), wodginite (47/0; red line); (c) Greenbushes, Australia, CGM (215/119), wodginite (46/7; red line), tapiolite (8/5; orange line); (d) Spargoville, Australia, CGM (347/61); wodginite from Broken Hill (18/18); (e) Kola, Russia, CGM (195/107), tapiolite (9/9); (f) São João del Rei, Brazil, CGM (407/157), wodginite (23/5); light blue line with diamond symbols: columbite-(Fe) from Dattas (Md, 4/4); (g) Lourenco, Amapa, CGM (50/50); Marowgne, Suriname, tapiolite (Md, 198/15), columbite-Fe (Md, 23/1), tantalite-(Mn) (Md, 15/1); (h) Finnish Lapland, Svecokarelian, CGM (205/30), tapiolite (332/122); brown broken line is P90 of tapiolite. Tapiolite from Tammela, southern Finland (solution ICP-MS data); (i) Karelia, Russia; columbite-(Fe) from Urosozero, columbite-(Mn) from Pertima, solution-ICP-MS data.

5.1. Archean 5.1.1. Tanco In the #Mn vs #Ta diagram, CGM compositions display a continuous trend from columbite-(Fe) with #Mn = 35, #Ta = 20 to tantalite-(Mn) (#Ta = 70); some grains are Ta-rich (#Ta N 90), confirming compositional data published by Černý (2005) and van Lichtervelde et al. (2006) (Fig. 5a). The trace elements in CGM display a unique signature due to their high concentrations of Li (maximum 465 ppm, Md 70 ppm, n = 129), Sc (Md, 3976 ppm), Ti, Sn, Sb, and low Mg, Y (Md, 0.11 ppm) and REE (Fig. 6a). For Be (Md, 0.72 ppm), As (Md, 128 ppm), Sr (Md, 36 ppm), Sb (P90, 210 ppm; Md, 18.6 ppm) and Ba (Md, 63 ppm), the median and P90 values are higher than in any other pegmatite province investigated. The REE concentrations, however, are among the lowest observed in CGM (b0.5 ppm for each element) and reveal subtype [4b] according to Graupner et al. (2010), trough-shaped normalised patterns slightly enriched in LREEN and HREEN over the MREEN (Fig. 7a). Wodginite is common at Tanco, and is frequently intergrown with CGM, cassiterite, tapiolite and microlite (Fig. 4a). It is Ta-rich in composition (#Ta = 80–95) and #Mn values (40–100, Fig. 5a) are similar to those in CGM. Wodginite is rich in SnO2 (Md,

12.8 wt.%) and concentrations of TiO2 (Md, 3.5 wt.%), Sc, Zr and Hf exceed those in CGM; W and U are low. The few trace element measurements available indicate low Y and REE concentrations, as well as elevated Li and Sb (Fig. 6a). A small number of ferrotapiolite grains investigated by EPMA has low concentrations of SnO2 (Md, 0.39 wt.%) and TiO2 (0.1 wt.%). Van Lichtervelde et al. (2007) report the following numbers as an average of 77 ferrotapiolite analyses from the Lower Pegmatite at Tanco: 0.51 wt.% TiO2, 0.73 wt.% SnO2, 0.14 wt.% Sc2O3, 0.05 wt.% WO3 and 0.05 wt.% UO2. Ta-bearing rutile (55–66 wt.% TiO2, 25–34 wt.% Ta2O5, 6–8 wt.% FeO, 3.3–4.0 wt.% Nb2O5, 0.8–1.2 wt.% SnO2) is uncommon at Tanco, and occurs as a late-stage mineral associated with CGM, wodginite, cassiterite and zircon. 5.1.2. Wodgina CGM are Mn-rich columbite and tantalite (#Mn N 60, #Ta = 24–93; Fig. 5b) with intermediate to low trace element concentration levels. Lithium (Md, 25 ppm) is elevated compared to average CGM. Some elements, namely Mg, Sc (Md, b 1 ppm), Ti and W are rather depleted; Sb and Bi concentrations are highly variable (Fig. 6b). The REE patterns are of subtypes [1b] and [2a], with MREEN ≈ HREEN, negative Eu



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Fig. 7. Chondrite-normalised REEY diagrams for CGM, wodginite/ixiolite and tapiolite from Ta provinces. If not otherwise stated, the median (blue line with diamond symbols) and P25 and P75 (black lines) are given; the upper broken line is P90. (a) Tanco, Canada; (b) Wodgina, Australia; (c) Greenbushes, Australia; (d) Spargoville, Australia; wodginite from Broken Hill; (e) Kola, Russia; (f) São João del Rei, Brazil; (g) Lourenco, Amapa; Marowgne, Suriname; (h) Finnish Lapland and Tammela, Svecokarelian province; (i) Karelia, Russia.

anomalies and ca. 10–50 times MREEN and HREEN enrichment relative to chondrites (Fig. 7b). Wodginites are enriched in Ti, Sn, Zr and Hf compared to CGM (Fig. 6b); data of other trace elements are not available. 5.1.3. Greenbushes and Spargoville CGM are more Fe–Nb-rich in composition than at Wodgina, displaying a trend from almost pure columbite-(Fe) to intermediate columbite–tantalite (#Mn = 60, #Ta = 60); some compositions are attributed to tantalite-(Mn) ranging up to #Mn 95, #Ta 80 (Fig. 5c). They have elevated As (Md, 3.4 ppm), Sb (14 ppm), Bi (2 ppm), Sc (Md, 400 ppm), Mg (458 ppm), W, Sn and Ti concentrations, but are low in Zr, Hf, LREE, Y, U (Md, 60 ppm) and Th (Fig. 6c). Columbite–tantalite-(Fe) from “Oxenbushes” is similar, i.e. high in Mg, Sc, Sb, and low in Li, LREE, Zr and Hf. REE concentrations of Ta–Nb-oxides from Greenbushes are low to intermediate (Md Yb, 1 ppm) and normalised patterns are of subtypes [1a] and [1d], respectively, with HREEN N MREEN and presumably small negative Eu anomalies (Fig. 7c). The trace element pattern of wodginite from Greenbushes is similar to the CGM pattern, but displays the elevated Ti, Zr, Hf and Sn concentrations usually observed in cogenetic CGM–wodginite assemblages (Fig. 6c). Tapiolite, on the other hand, has lower concentrations of most minor and trace elements, except for Sb, which is higher than in CGM and wodginite. Lithium, Hf and Sn concentrations in tapiolite are higher than in CGM (Fig. 6c). Electron microprobe analyses of stibiotantalite grains from Greenbushes, which are often zoned and intergrown with wodginite, microlite and columbite-(Fe), reveal a wide variation of #Ta from 44

to 89, and low Bi concentrations (b0.3 wt.%); Sc, Ti, Sn, Mn, Fe, W and Y occur as minor elements. Stibiotantalite is sometimes associated with an Sb–Ta oxide having appreciable concentrations of Sn, Ti, Sc, Mn, Fe, Zr, Hf and #Ta = 65; this compound probably represents stibioixiolite. Bismutotantalite is present as alteration rims of stibiotantalite, uranmicrolite and tantalite only; its apparently nonstoichiometric measured composition (high Si, Ca, Al) is caused by intergrowth with secondary silicates. Bi, As and Sb are present in wt.% levels (100 ∗ Bi / (Bi + Sb + As) = 50–66). A homogeneous, slightly altered grain of bariomicrolite contains small concentrations of Ca, Na, Ti, Fe, Zr, Ce, Pb (all are b1 wt.%), and high Nb, Sn, BaO (7.3 wt.%) and Ta2O5 (66.7 wt.%). Columbite-(Fe) from Spargoville has high REE (subtype [2a] pattern), Li (Md, 37 ppm), elevated Sc (113 ppm) and Mg (340 ppm), and very low Sn and Bi concentrations (Figs. 5b, 6d, 7d). 5.1.4. Kola peninsula Chemical compositions of CGM from the Kolmozero LCT pegmatite span a continuous range from columbite-(Fe) to tantalite-(Mn) (Badanina et al., 2015); those from Voronja Tundra resemble pure tantalite-(Mn) with #Mn N 90 (Fig. 5d). Columbite-(Fe) to tantalite(Mn) show near average trace element concentrations. Magnesium (Md, 107 ppm), Hf, Sb and Pb are enriched in the CGM compared to other trace elements (Fig. 6e). Yttrium and REE (Md Yb, 0.3 ppm) are low and reveal subtype [2a] patterns with negative Eu anomalies (Fig. 7e). Tapiolites from pegmatite veins in the pegmatite field are even more depleted in trace elements except for Li, Ti and Sb and contain


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Fig. 8. Diagrams of 100 ∗ Mn / (Mn + Fe) versus 100 ∗ Ta / (Ta + Nb) for columbite–tantalite group minerals, tapiolite (Tap, open quadrangles) and wodginite/ixiolite (Wdg) in Ta ore provinces outside of Africa. (a) Bastar-Malkangiri province, India. Additional data from Karnataka (Sarbajna et al., 2000) and Bastar (Pal et al., 2007); (b) Black Hills, South Dakota, USA; field delineates analyses from the Bob Ingersoll and Tin Mountain pegmatites published by Spilde and Shearer (1992). Literature data from Černý et al. (1992) and Anthony et al. (1997); (c) placer grains from eastern Colombia; (d) single crystals from Högsbo, Sweden, and southern Norway. Literature data from Anthony et al. (1997); Černý et al. (1992) and Romer and Smeds (1996); (e) pegmatites of Brasiliano age in eastern Brazil, including the Borborema and Eastern Brazilian Pegmatite Provinces (EBPP); (f) Sierra de San Luis pegmatites, Pampean province, Argentina; literature data (open circles) are from Galliski and Černý (2006). Field delineates data from the Los Chilenitos (LC), Victor Hugo (VH) and Independencia Argentina (IA) pegmatites published by Sosa et al. (2002).

very low W, Y and Th concentrations (Figs. 6e, 7e). Nb–Ta-rich rutile from another pegmatite at Kolmozero has the following composition: 30–65 wt.% TiO2, 13–36 wt.% Nb2O5, 9–18 wt.% Ta2O5, 10–20 wt.% FeO,

1.2–3.2 wt.% SnO2, b2.3 wt.% Sc2O3, b 0.4 wt.% WO3. Stibiotantalite from Voronja Tundra is characterised by #Ta = 88–95 and low concentrations of Sc2O3 (b 0.14 wt.%), SnO2 (0.6–1.1 wt.%) and WO3 (1.0–2.5 wt.%).



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Fig. 9. Trace element diagrams of Ta–Nb oxides normalised to an average CGM composition (see Fig. 6 for explanations). (a) Bastar-Malkangiri province, India; CGM (144/61), wodginite (Md; 28/9), tapiolite (Md; 13/13); (b) Black Hills, South Dakota, USA; CGM (64/64), tapiolite (Md; 13/13), wodginite (2/2); (c) placer grains from eastern Colombia; CGM (148/18), ixiolite (Md, 254/23); (d) single grains of CGM from Högsbo, Sweden (25/15), and southern Norway (Risör, 12/12; remaining are solution-ICP-MS data, N = 13); (e) CGM from pegmatites of Brasiliano age in eastern Brazil: Borborema province (black lines and symbols; 156/156) and Eastern Brazilian Pegmatite Province (EBPP; red lines and symbols; 106/57); (f) Sierra de San Luis pegmatites, Pampean province, Argentina (CGM, 99/99).

5.2. Paleoproterozoic 5.2.1. São João del Rei, Rio Grande del Norte, Brazil CGM are Mn-rich columbite and tantalite (#Mn N 60; Fig. 5e) characterised by higher than average Li (Md, 13 ppm), Y (617 ppm), Th (46 ppm), Sb (0.7 ppm) and REE (Md Yb, 78 ppm), as well as relative depletion in Mg (Md, 3 ppm), Sc (Md, 3 ppm), WO3 (Md, 0.12 wt.%) and Bi (Md, 0.6 ppm) (Fig. 6f). The REE produce subtype [2a] patterns with prominent negative Eu anomalies (Fig. 7f). Wodginite is Mn- and Tarich (#Mn N 70, #Ta N 80) and has lower Y, REE, Bi, Th, Pb and U concentrations than CGM (Figs. 5e, 6f, 7f). Bariomicrolite is a fairly common alteration mineral in concentrates from São João del Rei, replacing tantalite-(Mn) and wodginite. Microprobe analysis of bariomicrolite reveals 52–72 wt.% Ta2O5, 4.2–22.9 wt.% Nb2O5, 3.3–11.3 wt.% BaO, as well as 0.1–2.9 wt.% TiO2, b0.6 wt.% MnO, b0.3 wt.% FeO, 1.1–4.3 wt.% SnO2, b3.5 wt.% UO2, b1.6 wt.% Ce2O3, b 0.25 wt.% ThO2 and b8.1 wt.% PbO.

Columbite crystals from Dattas, Diamantina (Rio Grande del Norte, Brazil), are of intermediate composition (#Mn 50, #Ta 40, Fig. 5e); their trace element composition is significantly different from those of the Volta Grande pegmatites, with lower Li, Y, Sn, REE, and higher Mg, Sb, Bi, Pb, U (Fig. 6f). The subtype [2a] REE patterns show small Eu anomalies only (Fig. 7f). 5.2.2. Lourenço, Amapá (Brazil) Columbite-(Fe) shows a small range in #Mn (5–25), whereas #Ta ranges from 5 to 45 (Fig. 5e). The CGM are highly enriched in Mg (Md, 716 ppm), Sc (1060 ppm), Y (1100 ppm), REE and less in WO3 (0.58 wt.%), but are depleted in SnO2 (0.02 wt.%), Sb, Bi and U (140 ppm) (Fig. 6g). The MREEN and HREEN concentrations are 1000 times chondritic, whereas those of the LREEN are low; the REE patterns are of subtype [1a], displaying prominent negative Eu anomalies (Fig. 7g).


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Fig. 10. Chondrite-normalised REEY diagrams for CGM from Ta provinces. (a) Bastar-Malkangiri province, India; CGM; (b) Black Hills, South Dakota, USA; CGM and tapiolite (Md and P90); (c) placer grains from eastern Colombia; CGM, ixiolite (Md) and Ta–Nb-rutile (Md); (d) single grains of CGM from Högsbo, Sweden, and southern Norway; (e) pegmatites of Brasiliano age in eastern Brazil: Borborema province red lines and symbols) and Eastern Brazilian Pegmatite Province (EBPP; blue lines and symbols; CGM; brown line and open symbols: wodginite); (f) Sierra de San Luis pegmatites, Pampean province, Argentina.

5.2.3. Marowgne river (Marowijne, Suriname) Alluvial grains of tapiolite (Fig. 5e) have low concentrations of Li, Mg, Sc, and very low Y, REE, W, Bi, Th; only Hf and Sb are slightly enriched compared to the average CGM composition (Fig. 6g). REE concentrations are mostly below the detection limit of the LA-ICP-MS technique. The less common CGM studied have tantalite-(Mn) or columbite-(Fe) composition; the columbite is rich in Sc (4000 ppm), Y (1820 ppm), Zr (5000 ppm) and U (2500 ppm) (Fig. 6g). The subtype [1a] REE pattern of the columbite-(Fe) is indistinguishable from that of the CGM from Lourenco, Amapa (Fig. 7g). The tantalite is depleted in minor elements, especially in SnO2 (0.02 wt.%), Sc, Y, REE, Zr, Hf, Th, U and Pb; its REE pattern is of subtype [2b] (Figs. 6g, 7g).

5.2.4. Pitinga, Brazil #Mn of columbite-(Fe) from a cassiterite–columbite concentrate varies from 15 to 90, whereas #Ta is below 20 (Fig. 5e). Compared to the global average (0.2 wt%), the columbites are enriched in WO3

(0.6 wt.%), but low in Sc, Sn, Zr, Hf, and U. Rutile is Nb- and Fe-bearing (6–16 wt.% Nb2O5, 4–9 wt.% FeO, b3.2 wt.% Ta2O5, 0.8–2.2 wt.% SnO2, b2.3 wt.% WO3). The prevailing pyrochlore-group mineral is Pb-rich pyrochlore (#Ta = 5–20; 9–44 wt.% PbO, up to 17 wt.% UO2, up to 5 wt.% ThO2, occasionally up to 11 wt.% Bi2O3, up to 5 wt.% Ce2O3, up to 5 wt.% SnO2, and some Ti, Mn, Fe).

5.2.5. Miessijoki and Puskuoja rivers, Svecofennian Province (Finnish Lapland, Finland) CGM compositions follow a Ta-rich fractionation trend from columbite-(Fe) to tantalite-(Mn) (Fig. 5f). They have high concentrations of Mg, Sc (115 ppm), Y (530 ppm), REE (Md Yb, 60 ppm), Hf (1230 ppm) and Th (60 ppm), but very low Li (b 1 ppm), SnO2 (0.07%) and WO3 (0.2 wt.%) (Fig. 6h). REE patterns are of subtype [2b] (Fig. 7h). The trace element pattern of abundant tapiolite is similar to that of the CGM, but with lower Y, REE, Bi and Th (Fig. 6h).



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Fig. 11. Diagrams of 100 ∗ Mn / (Mn + Fe) versus 100 ∗ Ta / (Ta + Nb) for columbite–tantalite group minerals, tapiolite (Tap, open quadrangles) and wodginite/ixiolite (Wdg) in Ta ore provinces outside of Africa. (a) Variscan pegmatites of Europe: Hagendorf province, Germany; Central Iberian province (Goncalo, Feli). Literature data from Černý et al. (1992, 1998), Novák and Černý (1998, 2001), Novák et al. (2000, 2003), Uher et al. (1998), Leal Gomes et al. (2009), Dias et al. (2009), Martins et al. (2009), Fuertes-Fuente and Martin-Izard (1998), Ramos (2007), and Mücke and Keck (2008); (b) Weinebene and Pusterwald pegmatites, Austria. Literature data are from Postl and Golob (1979) and Černý et al. (1989, 1992, 1998); (c) Passeier Valley pegmatite, Italy; (d) Appalachians and Urals. Literature data from Anthony et al. (1997), Wise et al. (1998), Hanson et al. (1998), and Popov and Popova (2006). The shaded field represents compositions of CGM (N = 471) from the Brunswick and Oxford fields in the Appalachians of Maine (Wise et al., 2012); (e) various pegmatite provinces from Asia, including Kalba (Kazakhstan), Phuket (Thailand), Kim Bee (Malaysia) and Ishikawa (Japan); (f) Ulug Tanzek (Sayan), Orlovka and Malkhany (Transbaikalia).


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Fig. 12. Trace element diagrams of Ta–Nb oxides from Paleozoic and Mesozoic provinces normalised to an average CGM composition (see Fig. 8 for explanations). (a) Hagendorf province, Germany, CGM (85/45); (b) Central Iberian province, CGM (291/109); (c) Austroalpine province: Weinebene CGM (106/12), Pusterwald CGM (217/17), Passeiertal CGM (271/9), ixiolite (Md, 50/38); (d) various CGM from USA, including the Appalachians (solution data, N = 9) and Raleigh, Colorado (Md, 23/23); (e) Urals, Russia: CGM Polar Urals (39/39), Svetlinskoje (Md, 22/6), Ilmen (solution data, N = 2); (f) various pegmatite provinces from SE Asia, including Phuket (Thailand, CGM (50/50)), Kim Bee (Malaysia, CGM (50/50)) and Ishikawa (Japan, CGM, solution data, N = 10); (g) Kalba (Kazakhstan), CGM (44/9), wodginite (Md, solution data, N = 7); (h) Ulug Tanzek, Sayan, Russia, CGM (109/96); (i) Orlovka (CGM, 321/52) and Malkhany (Md, solution-ICPMS data, N = 3), Transbaikalia, Russia.

Tapiolite from Tammela (Finland) is enriched in Sn, Sb, Bi, and depleted in Sc, Y, REE, W and Th (Fig. 6h) and has a subtype [2b] REE pattern (Fig. 7h). 5.2.6. Northern Karelia, Russia Columbite-(Fe) from Urosozero and columbite-(Mn) from Pertima (two crystals from each location) were investigated by solution ICPMS (Fig. 5f). Both locations have high REE, as well as elevated Sb, Bi, and Th (ca. 30 ppm) (Fig. 6i). Urosozero is characterised by a very flat subtype [1b] REE pattern at 1000 times chondritic values with a prominent negative Eu anomaly, whereas Pertima has an inclined subtype [1a] pattern with both, negative Eu and Y anomalies (Fig. 7i). 5.2.7. Bastar Belt, India CGM follow a fractionation trend from columbite-(Fe) to tantalite(Mn) (Fig. 8a) and have intermediate trace element levels, with enrichment in Sb and depletion in Bi (Fig. 9a). The REE are of subtype [1b] with negative Eu anomalies and intermediate REE concentrations (Md Yb, 3 ppm) (Fig. 10a). Wodginite has high Li, Zr, Hf and Sn (Fig. 9a). Compared to average CGM, the analyses of tapiolite reveal elevated Hf, Sn, and considerably lower Sc, Y, REE, W and Th (Fig. 9a). 5.2.8. Black Hills, South Dakota CGM investigated in this study are mainly columbite-(Fe) that shows elevated concentrations of Li (Md, 20 ppm), Mg, Sc and SnO2 (0.33 wt.%), as well as low Y (1.3 ppm), Bi, Th and U (175 ppm)

(Figs. 8b, 9b). REE concentrations are low (HREEN 10–50 times chondritic) and of subtype [1a] (Fig. 10b). Tapiolite has lower Y, REE, W, Bi and U, and higher Mg, Hf, Sn and Sb than CGM (Fig. 9b). The P75 and P90 values for the REE point to subtype [2a] REE patterns, with additional LREE enrichment (Fig. 10b). Wodginite–ixiolite from the Etta mine revealed high concentrations of Ti, Sn and Sb, and low Y, W, Bi, Th and U (Fig. 9b); the REE concentrations are low (LuN b 4 times chondritic). 5.2.9. Broken Hill, New South Wales, Australia A wodginite crystal from the Broken Hill district yields high concentrations of all trace elements except Mg and Sc. The MREEN are enriched (N 1000 × chondrite), the REE pattern is of subtype [2a] (Figs. 5b, 6d, 7d). 5.3. Mesoproterozoic 5.3.1. Eastern Colombia Mineralisation recovered from placer material comprises rutile, commonly Ta- and Nb-rich, wodginite–ixiolite, rare CGM, tapiolite, cassiterite and zircon (Fig. 8c). Wodginite–ixiolite and CGM reveal extreme concentrations of some trace elements, such as Sc (Md in CGM, 2500 ppm; Md in wodginite–ixiolite, 2 wt.%), REE (Md of Yb in CGM, 3883 ppm; Md of Yb in wodginite–ixiolite, 285 ppm), Zr (Md in CGM, 3500 ppm; Md in wodginite–ixiolite, 1 wt.%), Hf (Md in CGM, 734 ppm; Md in wodginite–ixiolite, 5880 ppm), Mo (Md in CGM, 14 ppm), WO3 (Md in CGM, 2.8 wt.%, Md in wodginite–ixiolite,



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Fig. 13. Chondrite-normalised REEY diagrams for CGM from Paleozoic and Mesozoic Ta provinces. (a) Hagendorf province, Germany, CGM; (b) Central Iberian province, CGM; (c) Austroalpine province: Weinebene CGM, Pusterwald CGM, Passeiertal CGM, ixiolite (Md); (d) various CGM from USA, including the Appalachians and Raleigh, Colorado; (e) Urals, Russia, CGM Polar Urals, Svetlinskoje, Ilmen; (f) various pegmatite provinces from SE Asia, including Phuket (Thailand), Kim Bee (Malaysia) and Ishikawa (Japan); (g) Kalba (Kazakhstan), CGM, wodginite (Md); (h) Ulug Tanzek, Sayan, Russia, CGM; (i) Orlovka and Malkhany, Transbaikalia, Russia.

1.8 wt.%) and Th (Md in CGM, 84 ppm; Md in wodginite–ixioliteix, 250 ppm) (Fig. 9c). The REE patterns of CGM display negative Eu and Y anomalies and are of subtype [1d] (Fig. 10c). Wodginite–ixiolite is even more enriched in minor elements, but depleted in REE compared to CGM, however maintaining the subtype [1d] REE pattern (Figs. 9c, 10c). Nb–Ta-bearing rutile has low REE concentrations (Fig. 10c). Mineralogy and chemical composition of columbite-(Mn) with #Mn = 31–89 and #Ta = 8–55, recovered from alluvial placers derived from Neo-Mesoproterozoic pegmatites in Bolivia, differ from placer

material in Eastern Colombia. Maximum concentrations of 2.09 wt.% TiO2, 1.88 wt.% WO3 and 0.32 wt.% SnO2 are reported for columbite(Mn) (Alfonso et al., 2015). 5.4. Early Neoproterozoic 5.4.1. Sveconorwegian The Högsbo columbite-(Mn) is rich in the REE, Y (Md, 4200 ppm) and Th (40 ppm), moderately enriched in Sb, Bi, and depleted in Li

Fig. 14. Columbite composition from the Upper Fir carbonatite, Canada. (a) Trace elements normalised to an average CGM composition; (b) chondrite-normalised REEY diagram for columbite. The median (line with diamond symbols) and P25 and P75 (cross symbols) are given; the upper broken line is P90.


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and Sn (0.02 wt.%) (Figs. 8d, 9d). The REE pattern is of subtype [1a], with negative Eu and small negative Y anomalies (Fig. 10d). Columbite-(Fe) and columbite-(Mn) from southern Norway (Moss, Råde, Kragerø, Risør) have REE patterns of subtypes [1a], [1b], and [1c] (Fig. 10d), and are enriched in Y, HREE, Sb and Th, but depleted in Sn, Bi and occasionally in W (Fig. 9d). REE patterns resemble those of garnet grains in pegmatites (Müller et al., 2012). The Sc–Y–REE–Ti–Zr–Hf–Urich trace element pattern of columbite-(Mn) from Kragerø (“Sanökedal”) closely resembles that from Högsbo in Sweden. However, Sn, Sb and Th are considerably depleted, whereas Y, HREE and W are enriched, compared to CGM from Högsbo. Columbite-(Fe) from the South Platte Pegmatite District (Raleigh, Colorado) displays low #Mn (20) and #Ta (8) and has high REE concentrations (Md, 8080 ppm of total REE) and a subtype [1d] REE pattern. In addition, it is rich in Sc, W and Th, and low in Li, Sn, Hf and U (Figs. 12d, 13d). Compositional data of CGM from the district indicate a trend from almost pure columbite-(Fe) to columbite-(Mn) with about #Mn = 60 and #Ta = 40 (Simmons et al., 2012). 5.5. Late Neoproterozoic to early Paleozoic 5.5.1. Eastern Brazilian Pegmatite Province CGM from various pegmatites in the province are commonly columbites that reveal a large within-province variation of #Mn (30–90) and #Ta (10–70; Fig. 8e). Magnesium (Md, 300 ppm), HREE and WO3 (0.48 wt.%) are enriched compared to average CGM, whereas Sc, Hf and Bi display minima (Fig. 9e). The subtype [1a] REE patterns have prominent negative Eu anomalies and are moderately enriched in MREEN and HREEN (Fig. 10e). Magnesium, W and Bi concentrations in wodginite from Araçuai are considerably lower than in CGM, and the REE have a subtype [2a] pattern (Fig. 10e). Barium-rich microlite (#Ta = 78–93) carrying 6–13 wt.% BaO, 2–5 wt.% TiO2, 1.1–2.9 wt.% SnO2 and 0.5–13 wt.% PbO is intergrown with wodginite. Rutile is Fe(3–11 wt.% total FeO, #Mn b 5) and Ta-dominated (8–45 wt.% Ta2O5, #Ta = 47–79), and carries significant SnO2 (2.4–4.4 wt.%). 5.5.2. Borborema Pegmatite Province CGM compositions scatter widely in terms of major elements (Fig. 8e). Trace element concentrations are intermediate, with high values for Mg (Md, 170 ppm), Sb (0.7 ppm) and Bi, and low Li, HREE and SnO2 (0.01 wt.%) (Fig. 9e). Concentrations of the REE are low (especially the LREE and HREE) to intermediate; the REE patterns are of subtype [2b/2c] and show prominent negative Eu anomalies (Fig. 10e). Tapiolite, which is abundant in many of the pegmatites, has low concentrations of trace elements, especially of Sc, Y, REE, Sb, Bi and Th. Magnesium, Hf and Sn are higher in tapiolite than in CGM. 5.5.3. Polar Urals, Russia Ta-poor columbite-(Fe) from rare-metal granites in the Polar Urals is characterised by extreme concentrations of Y (Md, 537 ppm) and HREE (Yb, 750 ppm), elevated Mg, Sc, Bi (2.8 ppm), and low Li, Zr (100 ppm), Hf, Sn, Th and U (4.5 ppm) (Figs. 11d, 12e). The REE patterns are of subtype [1a] with negative Eu anomalies (Fig. 13e). 5.6. Paleozoic 5.6.1. Pampean Pegmatite Province, Argentina CGM from the San Luis Province pegmatites follow fractionation trends from columbite-(Fe N Mn) to tantalite-(Mn) (Sosa et al., 2002; Galliski and Černý, 2006; Fig. 8f). A few columbite-(Mn,Fe) and tantalite-(Mn) crystals analysed from the Victor Hugo, Los Chilenitos and Independencia pegmatites reveal consistent trace element signatures characterised by low Li, Sc, W and Th, and by elevated Sb and Bi (P90, 4.5 ppm) (Fig. 9f). The REE concentrations are low and their patterns of subtype [2b] with depletion in LREEN and negative Eu anomalies (Fig. 10f).

5.6.2. Central Iberian Pegmatite Province (Portugal and Spain) Mn-rich columbite and tantalite from the Goncalo and Feli pegmatites (Fig. 11a) have low concentrations of Mg, REE, Ti, Sb, Bi and Pb, high Li (187 ppm), and elevated Zr (2690 ppm), SnO 2 (0.25 wt.%) and WO3 (0.42 wt.%) (Fig. 12b). The REE patterns are of subtype [1b] (TbN/YbN = 0.65) (Fig. 13b). 5.6.3. Hagendorf Province, Germany A few columbite-(Fe) crystals from various pegmatites in the province reveal extremely high concentrations of Zr (Md, 7000 ppm), Hf (555 ppm) and U (1420 ppm), and low Mg, Sn, Sb, Bi contents (Figs. 11a, 12a). The LREE and Eu are frequently below the detection limits, and the MREEN–HREEN part of the normalised diagrams indicates subtype [2b] patterns (Fig. 13a). Most columbites are Fe-rich (#Mn b30); the most Mn-rich samples from Püchersreuth have #Mn ranging from 39 to 47 (Fig. 11a). The most Ta-rich composition was found in columbite from Zwiesel with #Ta of 23. 5.6.4. Austroalpine province CGM from the Weinebene, Pusterwald and Passeiertal pegmatite districts range from columbite-(Fe) and tantalite-(Fe) to columbite(Mn) and tantalite-(Mn) (Fig. 11b, c), and are characterised by low concentrations of Sc, Y and the REE, and elevated Mg, Zr, Sn and Sb (Fig. 12c). Their REE patterns are often similar to the subtype [2b] patterns, observed, e.g., for the Hagendorf province, Kolmozero, and the Pampean and Borborema provinces (Fig. 13c). In the Passeiertal pegmatite, columbite–tantalite has very low REE concentrations and subtype [1a] patterns, similar to wodginite–ixiolite that is significantly enriched in Zr, Hf and Sn (Fig. 12c). In tapiolite, Ti and Sn are the only trace elements detectable by EPMA. Late-stage rutile coating wodginite–ixiolite blasts from the Passeiertal pegmatite is rich in Ta2O5 (35–40 wt.%) and FeO (13 wt.% FeO total), with significant Sn and W. Fersmite, associated with microlite and columbite-(Mn), is common in some of the pegmatites from the Pusterwald district (Fig. 4g). It is chemically rather homogeneous (CaO 15.7–16.8 wt.%, 56–68 wt.% Nb2O5, 15–26 wt.% Ta2O5; #Ta = 13–21) and carries low concentrations of MnO (0.5–0.9 wt.%), TiO2 (0.1–0.6 wt.%) and SnO2 (b 0.85 wt.%). 5.6.5. Southern Urals Compared to the Polar Urals, columbite-(Fe) from Svetlinskoje is less enriched in Y and HREE, but high in Bi (Md, 83 ppm) and U (4100 ppm), and low in SnO2 (0.04 wt.%) (Figs. 11d, 12e); REE patterns are of subtype [2a] (Fig. 13e). CGM from the pegmatites in the Ilmen Mountains cover a large range in #Mn and #Ta (Popov and Popova, 2006; Fig. 11d). Two columbite-(Fe) crystals analysed from the Ilmen Mountains carry high W (1.7 wt.%), Sb and Bi (6–61 ppm) (Fig. 12e), and reveal weakly fractionated subtype [2a] REE pattern at 1 to 10 times chondritic concentration for the MREEN and HREEN (Fig. 13e). 5.6.6. Appalachian Fold Belt Nine single crystals of columbite-(Fe) and columbite-(Mn) from North Carolina and Massachusetts were investigated by solution-ICPMS (Fig. 11d). They have REE patterns of subtype [1b] (columbite-Fe), [1c] and [2b] (columbite-Mn) (Fig. 13d), and are enriched in Ti, Y, HREE, Sb, Bi and occasionally Th, but depleted in Sn and W (Fig. 12d). The analyses plot within the compositional area constructed from measurements of 471 CGM grains from the Brunswick and Oxford pegmatite fields, Maine (Fig. 11d; Wise et al., 2012). CGM from the Brunswick field are from poorly fractionated pegmatites and show elevated levels of Ti and Mg, but low Sn. In the more evolved and fractionated Oxford field, CGM cover a larger range in #Mn–#Ta space and attain lower levels of minor elements Sc, Ti and Sn. One large grain of samarskite from North Carolina (sample #688, Supplementary material S2) contains b 2 wt.% CaO, 1.5–2.3 wt.% TiO2, ca. 1 wt.% MnO, 10–12 wt.% FeO total, 0.2–0.3 wt.% ZrO2, 37–43 wt.% Nb2O5, 4.5–6.6 wt.% Ta2O5, 0.3– 0.5 wt.% WO3, 14–24 wt.% UO2, and 7–10 wt.% Y2O3.



5.6.7. Kalba–Narym Pegmatite Belt, Kazakhstan Columbite-(Mn) and tantalite-(Mn) analysed by solution ICP-MS have low trace element concentrations except for Sn and Sb (Figs. 11e, 12g); the REE concentrations are low (MREEN 1–2 times chondritic) and form subtype [1b] patterns (Fig. 13g). Wodginite has even lower trace and REE concentrations, and is only enriched in Hf and Sn.

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and subtype [2b] REE patterns (Fig. 13i). The concentrations of As (up to 10 ppm), Sb (up to 9 ppm) and Bi (0.1–1 wt.%) are also distinctly higher than in “average” CGM. The data are best explained by intergrowth of different Ta–Nb-oxides, including a Bi-rich phase; Badanina et al. (2008) have described a complex association of Sc–REE-bearing ixiolite, polycrase–euxenite, strüverite–ilmenorutile, monazite and zircon from Sosedka.

5.7. Mesozoic and Cenozoic 5.8. Upper Fir carbonatite, British Columbia 5.7.1. Phuket Island, Thailand Columbite-(Fe) grains from one concentrate investigated have consistent trace element characteristics including elevated Sc (Md, 500 ppm), Y, HREE and WO3 (1.3%), and low Li, Ce, Sb, Bi and Pb (Figs. 11e, 12f). REE concentrations are intermediate (Md Yb, 33 ppm) and patterns are of subtype [1b], with negative Eu anomalies (Fig. 13f). 5.7.2. Kim Bee mine, Malaysia The trace element pattern of columbite-(Fe, Mn) from the Kim Bee mine is indistinguishable from the material from Thailand (Figs. 11e, 12f); the REE patterns are of subtype [1b] and [2a] with MREEN N HREEN (Fig. 13f). 5.7.3. Ishikawa, Japan Four crystal fragments each of columbite-(Fe) and tantalite-(Fe) were analysed by solution-ICP-MS, and yielded REE patterns of subtypes [2b] and [2c], respectively, with prominent negative Eu anomalies (Figs. 11e, 13f). Tantalite-(Fe) is more depleted in the HREE compared to columbite-(Fe). The latter is also enriched in Sb, Th and U, whereas tantalite-(Fe) contains significantly more Hf; both minerals have low Sn and Pb concentrations (Fig. 12f). 5.7.4. Ulug-Tanzek deposit, Altai–Sayan Foldbelt, western Siberia The CGM are represented by Ta-poor columbite (#Mn = 19–69, #Ta N9; Fig. 11f) that is extremely enriched in Y (1250 ppm), the HREE (Yb = 2300 ppm) and Th (70 ppm) (Figs. 12h, 13h). The concentrations of Li, Mg, Sc, LREE, W, Sb, Bi, Pb and U (28 ppm) are low. 5.7.5. Khangilay intrusion, Orlovka massif, Eastern Transbaikalia Mineral phases containing Ta and Nb evolve and form a sequence from biotite granite of the parent Khangilay massif to amazonite Li–F granites of the Orlovka satellite: early euxenite and fergusonite → struverite → columbite → tantalite → microlite. The chemical composition of CGM from various parts of the Orlovka rare-metal granite ranges from columbite-(Fe) to tantalite-(Mn), with maximum #Ta values of 66 (Md = 27) (Fig. 11f). The trace element patterns for all CGM are characterised by high Sc (115 ppm), HREE (Yb = 566 ppm) and Th (55 ppm), as well as low Mg, Sb and Pb (Fig. 12i). REE patterns are of subtype [1d] (MREEN b HREEN, negative Eu and Y anomalies) (Fig. 13i). Electron microprobe analyses of primary microlite from Orlovka reveal Tarich compositions (#Ta, 64–92) with significant SnO2 (Md, 1.5 wt.%), UO2 (Md, 3.4 wt.%), and some REE (Md, 0.22 wt.% CeO2), WO3 (Md, 0.3 wt.%), and Th (Md, 0.12 wt.% ThO2). The concentrations of F, Na2O and CaO average to 3.3, 5.6 and 10.6 wt.%, respectively. Euxenite from Orlovka is rich in TiO2 (21–24 wt.%), Nb2O5 (21–27 wt.%), UO2 (13– 21 wt.%), Y and HREE (10–13 wt.% Y2O3), besides 6–8 wt.% Ta2O5, 1– 2 wt.% CaO, 1–3 wt.% ThO2 and below 1 wt.% of MnO, FeO, PbO and WO3. Rutile from Orlovka is moderately enriched in Nb2O5 (up to 17 wt.%), Ta2O5 (up to 18 wt.%) and FeO (up to 10 wt.%) and carries minor SnO2 (up to 3 wt.%) and WO3 (up to 0.5 wt.%); #Ta ranges from 22 to 62. 5.7.6. Sosedka, Malkhany Pegmatite Field, central Transbaikalia Three crystals of CGM analysed by solution-ICP-MS have homogeneous major element compositions (#Mn 85, #Ta 21; Fig. 11f), high TiO2 (4.5–5.9 wt.%), Sc (0.7–0.9 wt.%), Y (1.1–2.6 wt.%), MREE (up to 0.53 wt.% Gd), Zr, Hf, Th (0.2–0.8 wt.%) and U (1.7–4 wt.%) (Fig. 12i),

Columbite-(Fe) associated with pyrochlore in the only carbonatitehosted Ta–Nb mineralization covered in this contribution is poor in Ta2O5 (Md, 1.8 wt.%), Sn, W, REE (Md, 40 ppm) and has extremely low U/Th (Md, 0.14). REE patterns are of subtype [3a] (HREEN N MREEN), lacking Eu anomalies, but show distinct negative Y anomalies (Fig. 14b). Columbite is, however, rich in Ti and Mg (Fig. 14a). 6. Discussion Rare-metal granite and rare-element pegmatites are significant hosts of HFSE and LILE mineralisation, most importantly of Ta and Nb, Li, Cs and Rb. However, the mechanisms of HFSE and LILE enrichment in these systems are neither well known nor fully understood (Linnen and Cuney, 2005). Ta–Nb oxides are capable of incorporating significant amounts of Zr, Hf, Y, REE, Sc, Ti, Sn, W and U with concentration levels varying over orders of magnitude, e.g., from a few ppm up to several weight percent. The reasons for these variations seem manyfold. They include crystallisation from fractionating volatile-rich melts in differentiating plutons and smaller-scale pegmatite bodies as well as contamination from wall rocks and other external sources. In this and in the companion paper (Melcher et al., 2015), we present new data on oxide mineralogy, major and trace element mineral chemistry of Ta–Nb oxides, and radiometric U–Pb data to characterise and discriminate between Ta–Nb–Sn provinces worldwide. Melcher et al. (2015) discussed the mineral chemical constraints and the origin of trace element patterns in Ta–Nb oxides, including rare element enrichment processes in the light of petrogenetic processes forming granite–pegmatite systems (cf. also Graupner et al., 2010). The following discussion now focuses on (1) a summary and critical evaluation of the data, (2) the link of major oxide and trace element chemistry to established pegmatite families and types, (3) inter-element dependencies (fractionation), and (4) the application of Ta–Nb-oxide compositions to shed light on regional geological processes. 6.1. Major and trace element composition of Ta–Nb-oxides Columbite–tantalite is the most abundant Ta–Nb-oxide in the studied pegmatites and granites. About 15,000 electron microprobe and ca. 10,000 LA-ICP-MS datasets are evaluated here and demonstrate variations over two to four orders of magnitude for most trace elements. The crystal structure of the AB2O6 CGM is a derivative of the α-PbO2 structure with two distinct octahedral sites (4c and 8d); according to Shannon (1976), the valence and ionic radii of the major elements in the structure are Fe2 + (0.78 Å), Mn2 + (0.83 Å), Nb5 + (0.64 Å), and Ta5+ (0.64 Å). Although there is evidence for different valences for Nb and Ta in the crust (Martin and Wülser, 2014), it is likely that most of the Nb and Ta in CGM is in the pentavalent state. The following trace elements were investigated in this study, grouped according to their preferred, possible or assumed valence, and with their ionic radii in octahedral coordination (in Å): M1+ (Li 0.76; Rb 1.52); M2+ (Be 0.45; Mg 0.72; Ca 1.0; Sr 1.18; Pb 1.19; Ba 1.35); M3+ (Al 0.54; As 0.58; Sc 0.745; Sb 0.76; Y 0.9; REE 0.9–1.0; Bi 1.03); M4+ (Ti 0.605; Sn 0.69; Hf 0.71; Zr 0.72; U 0.89; Th 0.94); M5+ (As 0.46, Sb 0.6, Bi 0.76) and M6+ (Mo 0.59; W 0.6). Following the geochemical rules of Goldschmidt (1954), most of the elements listed are likely incorporated into the columbite structure by simple or heterovalent substitution. Lead is


28


continuously produced by radioactive decay of U and Th. Unusually large ions such as Rb1 +, Sr2 +, Ba2 +, Bi3 +, U4 +, Th4 + and small ions such as Be2+, Al3+ and As5+ should not be accommodated in the columbite structure (Fig. 15). The detection of such “unlikely” elements by EPMA and LA-ICP-MS might be explained by the accidental analysis of finely dispersed silicates in the analysed sample volume, e.g., introducing Al, Rb, Sr, Li and probably Be. Nevertheless, the absence of visible contaminants in μm-scale in most of the analysed grains, and correlations with major and minor elements, indicate that most of the elements plotting outside of the limits calculated from Goldschmidt's rules, may be incorporated in the structure of columbite–tantalite in trace amounts. In some cases, metamictisation may have played a role because U and Th concentrations correlate with some of the “forbidden” elements, most notably with Bi. Martin and Wülser (2014) explained the unusual behaviour of CGM during heating – they reach an ordered state – by magmas containing mixed valences of Mn, Fe, Nb and Ta which makes a disordered distribution of octahedrally coordinated cations energetically more favourable. Due to their insolubility in aqueous solutions, CGM remain metastably “frozen” in a partly disordered state. This provides an additional mechanism explaining high concentrations of minor and trace elements in CGM and phases with related structures. Elements of different ionic radius and charge may more easily be accommodated in the highly disordered orthorhombic structure during crystallization; this structure tolerates divalent and trivalent Fe and Mn, as well as tetravalent and pentavalent Nb and Ta, and thus will offer more options for heterovalent substitution with minor and trace elements. Supplementary material S3 provides an overview on the distribution of major and trace elements in CGM based on a dataset of 22,786 single measurements by EPMA and ICP-MS from 45 Ta provinces (Fig. 1). In Fig. 16a, median, mean and range – bracketed by the P10, P25, P75 and P90 values – are presented for the complete dataset. The similar mean and median values of the major elements (Fe, Mn, Nb, Ta) indicate their normal distribution; however, trace elements are log-normally distributed, as indicated by a considerable deviation of median and mean values (e.g., Md ≪ x). The compositionally most important minor elements are Ti, W, Zr and Sn (Md N 1000 ppm), followed by U, Hf (100–500 ppm), Al, Mg, Pb (10–100 ppm), Sc, Li, Mo, Y, Th and Tl (1–10 ppm). The median concentrations of the REE, As, Sb, Bi and Sr are below 2 ppm; concentrations of Be, Rb and Ba in CGM are usually below the detection limit of the LA-ICP-MS method (Fig. 16a). In terms of maximum concentrations, the following values were encountered in the whole dataset (location in brackets): UO2 10 wt.%

Fig. 15. Ionic radius versus cation charge for ions in octahedral coordination that were detected by in-situ analysis of columbite–tantalite grains.

(Shabunda, Kibara Belt), WO3 6.3 wt.% (Damara, Namibia), ZrO2 2.9 wt.% (Jos, Nigeria), Hf 2.7 wt.% (Abu Dabbab, Egypt), Y 2.65 wt.% (Orlovka, Russia), Mg 2.6 wt.% (Upper Fir carbonatite), Sb 2.2 wt.% (Tanco), Pb 2 wt.% (Orlovka, Russia), Sc 1.9 wt.% (Marropino, Mozambique), Al 1.4 wt.% (Egypt), Ba 1.3 wt.% (Tanco), Sr 1.2 wt.% (Tanco), Bi 1 wt.% (Malkany, Russia), Th 0.76 wt.% (Malkany, Russia), As 1350 ppm (Greenbushes, Australia), Li 1320 ppm (Rutsiro, Rwanda), Rb 950 ppm (Taikeu, Russia), Be 600 ppm (Ruhanga, Rwanda), Mo 540 ppm (Jos, Nigeria), and Tl 235 ppm (Sao Joao del Rei, Brazil). The highest REE concentrations in CGM were measured at Jos, Nigeria (2734 ppm Ce), Sierra Leone (750 ppm La, 219 ppm Pr), Malkhany, Russia (1127 ppm Nd, 2728 ppm Sm, 5300 ppm Gd, 1044 ppm Tb, 4500 ppm Dy), Nkegete, Rwanda (131 ppm Eu), Orlovka, Russia (742 ppm Ho, 2378 ppm Er), and Abu Dabbab, Egypt (843 ppm Tm, 11,277 ppm Yb, 1486 ppm Lu). Some of the maximum concentrations listed above may have been affected by contamination with matrix or inclusion material, or be due to intergrowth with additional phases. The latter is difficult to exclude using LA-ICP-MS spot measurements of 50 μm diameter. TiO2 and SnO2 concentrations may reach several wt.%, but due to the problems of possible intergrowth with Ti- and Snrich Ta–Nb oxides (e.g., wodginite/ixiolite, Nb–Ta-bearing rutile), no maximum values are quoted. Considering these difficulties, the P90 values (Fig. 16a) should be used as approximations of the upper concentration levels in naturally occurring CGM. To put the minor element concentrations in a context, other data considered as “high” or “elevated” found in the literature are quoted. Černý et al. (2007), e.g., determined the highest concentrations of ZrO2 and HfO2 in a suite of samples from different pegmatites to 1.26 wt.% and 0.12 wt.%, respectively. The highest concentration of Sc encountered in a study by Wise et al. (1998) is 2.27 wt.% Sc2O3 (1.48 wt.% Sc) in a columbite-(Fe) from Luster, Colorado. The CGM in zone III of the Koktokay No. 1 pegmatite, China, are Mn-rich (#Mn = 98–99) and carry up to 1.68 wt.% ZrO2, 1.28 wt.% UO2, 0.73 wt.% HfO2, intermediate TiO2 (0.5–1 wt.%) and WO3 (0.3–0.8 wt.%), and low Sn and Sc (Yin et al., 2015). The high #Ta (N 0.6) of these CGM suggests that the minor elements are controlled by the composition of the pegmatite melt. Tungstenian columbite with up to 26.5 wt.% WO3 was described from Jos, Nigeria (Mücke and Neumann, 2006). The maximum TiO2 value found in the relevant literature is 8.78 wt.% in columbite(Fe), McGuire pegmatite, South Platte District, Colorado (Černý et al., 1999); for SnO2, 6.8 wt.% are reported from columbite–tantalite-(Mn) (Central Eastern Desert, Egypt; Jahn, 1996). Columbite-(Mn), often highly enriched in TiO2 (up to 5 wt.%), WO3 (up to 10 wt.%), Y2O3 (up to 2.45 wt.%) and REE (e.g., up to 1.92 wt.% Dy2O3 and 1.97 wt.% Yb2O3) at the 6.87 Ma old Fonte del Prete pegmatite, island of Elba (Italy), carries up to 2.71 wt.% UO2 (Aurisicchio et al., 2002). Concentrations of additional minor elements have only rarely been reported in the literature; from N 1000 analyses compiled, b20% have reported values for MgO (maximum 1.72 wt.% from Vezna, Czech Republic; Černý et al., 1998), ZrO2 (1.21 wt.% from the Muro Alto pegmatite, Portugal; Dias et al., 2009), HfO2 (0.12 wt.% from Quadeville, Canada; Černý et al., 2007), As2O3 (0.06 wt.% from Scheibengraben, Czech Republic; Novák et al., 2003), Sb2O3 (0.8 wt.% from the Animikie Red Ace pegmatite, Wisconsin; Falster et al., 2001), Bi2O3 (1.0 wt.% from the Animikie Red Ace pegmatite, Wisconsin; Falster et al., 2001), ThO2 (0.47 wt.% from Separation Rapids, Canada; Tindle and Breaks, 1998), PbO (2.98 wt.% from Chvalovice, Czech Republic; Novák and Černý, 1998), SrO (0.11 wt.% from Chvalovice, Czech Republic; Novák and Černý, 1998), and ZnO (0.51 wt.% from Abu Rusheid, Egypt; Abdalla et al., 1998). About 1150 datasets of wodginite–ixiolite were also collected from 49 pegmatites in 18 provinces; 12 of them are located outside of Africa. The chemical composition of wodginite–ixiolite is highly variable (Supplementary material S4). Mn–Fe substitution is complete (Md #Mn = 66), whereas #Ta commonly ranges from 65 to 95 only. The SnO2 concentrations in phases classified as wodginite–ixiolite range from 0.1 to



29

Fig. 16. Concentration levels of major and trace elements in (a) columbite–tantalite (CGM), (b) ixiolite/wodginite and (c) tapiolite, arranged in decreasing order of abundance (median CGM). Median and mean are given to indicate normal and log-normal distributions, respectively. Percentile values bracket ranges for 50% (P75, P25) and 80% of the data (P90, P10). Note that element order is the same for the three mineral groups. The data represent the complete datasets for CGM (n = 22.786), ixiolite–wodginite (n = 1150) and tapiolite (n = 1317).

33 wt.%, with an average and median of 12 wt.%. The concentrations of TiO2 (up to 21 wt.%; Md, 1.60 wt.%), WO3 (up to 10 wt.% in “ordinary” wodginite/ixiolite with a Md at 0.10 wt.%; up to 50 wt% in wolframixiolite from Nigeria, Melcher et al., 2015), ZrO2 (up to

10 wt.%; Md, 0.59 wt.%) and Hf (up to 8500 ppm; Md, 2300 ppm) are often significant, with average concentration levels commonly higher than in CGM and tapiolite (Fig. 16b). In all cases, Zr and Hf concentrations are higher in wodginite/ixiolite than in associated CGM, and Zr/


30


Hf ratios are lower. Černý et al. (2007) found up to 9.59 wt.% ZrO2 and 1.15 wt.% HfO2 in wodginite from pegmatites and also observed that Zr/Hf ratios in wodginite are lower (1 to 4) than in coexisting columbite–tantalite (6 to 13). Trace elements that may be present in significant concentrations in wodginite–ixiolite include Li (up to 0.9 wt.% in lithiowodginite, up to 1100 ppm in ordinary wodginite–ixiolite; Md, 27 ppm), Mg (up to 2200 ppm; Md, 24 ppm), Sc (up to 1.7 wt.%; Md, 22 ppm), Th (up to 825 ppm; Md, 12 ppm), U (up to 6743 ppm; Md, 633 ppm), As (up to 1200 ppm), Sb (up to 300 ppm) and Bi (up to 3200 ppm) (Supplementary material S4, Fig. 16b). Scandian ixiolite containing between 4 and 19 wt.% Sc2O3 has been reported from Madagascar, Mozambique, Norway, Maine and the Czech Republic (Wise et al., 1998). The absolute concentrations and chondrite-normalised patterns of the REE and Y are similar in size and shape to those in coexisting CGM, and REE concentrations are higher than in tapiolite (Fig. 16b,c). The rutile structure of tapiolite [FeTa2O6] allows accommodation of tetravalent elements, such as Sn, Ti, Zr and Hf (Ercit, 2010). More than 1300 analyses of tapiolite from 19 provinces, including 500 datasets from LA-ICP-MS, were evaluated. Compared to CGM, tapiolite has a rather invariable major element composition close to [FeTa2O6]. The major elements range from 9.3–16.5 wt.% for FeO, b3.8 wt.% for MnO, b14 wt.% for Nb2O5 and 61–88 wt.% for Ta2O5 in our data set. The maximum #Mn value is close to 25, and the lowest #Ta is close to 74. Maximum and median concentrations of the minor elements are: TiO2 (12.7 wt.%, 0.80 wt.%); SnO2 (5.4 wt.%, 0.18 wt.%); WO3 (1.1 wt.%, 0.01 wt.%), respectively (Supplementary material S5). Trace elements that are frequently detected by LA-ICP-MS include (in ppm; minimum − maximum; median): Li (b1–140; 4.6); Mg (b 1–4500; 189); Sc (b 1–1100; 2.2); Y (b1–180; 0.05); Zr (23–16,500; 902); Hf (12– 4000; 416); U (1–4500; 187); Pb (1–900; 29), respectively (Fig. 16c). Concentration levels of Zr are similar to those in coexisting CGM, but Hf concentrations are higher, and Zr/Hf values are lower than in CGM; they range from 1.6 to 2.5 in a number of provinces, whereas coexisting CGM have more variable, and higher Zr/Hf. Our data corroborate the deductions of Novák et al. (2004) that tapiolite does not accommodate significant amounts of trivalent cations (Al, As, Sb, Bi, and REE). The REE concentrations are below the detection limits in N 75% of the measured values of a given element. If REE are detected, chondrite-normalised patterns of tapiolite resemble those of coexisting CGM in shape, but are of lower magnitude (Figs. 7, 10, 13). Incorporation of trace elements in Ta–Nb-oxides occurs in a systematic fashion. Following Fig. 16, some “rules” may be established if cogenetic phases are considered: (i) wodginite/ixiolite carries higher concentrations of Li, Zr, Hf, Sn, and in some cases also Ti, Sb, Bi than CGM and tapiolite; (ii) CGM host higher concentrations of all trace elements except Hf (in some cases also of Zr, Ti, Sn, Sb) compared to tapiolite. However, it has to be taken into account that detailed analytical work on cogenetic phases is needed to fully explore this issue. The trace element patterns of Ta–Nb-oxides reflect regional geochemical peculiarities including source characteristics. This is demonstrated in spider diagrams of trace elements normalised to the median composition of CGM (Figs. 6, 9, 12). The normalised median, P25 and P75 values of CGM, tapiolite and wodginite–ixiolite from rare metal pegmatite provinces indicate common features to all of these mineral groups, suggesting most of them are cogenetic and they reflect certain source characteristics. 6.2. CGM in LCT vs NYF family pegmatites and rare-element granites The common association of Ta–Nb-oxides, especially of CGM, with granitic pegmatites has long been known. Attempts to classify pegmatites into classes, types, subtypes and families also take the HFSE into account. Most importantly, the pegmatite classification after Černý and Ercit (2005) distinguishes LCT (Li–Cs–Ta) from NYF (Nb–Y–F) families. LCT-family pegmatites are part of the rare-element pegmatite class,

subdivided into the beryl, complex (rare-element), albite–spodumene and albite types, whereas NYF-family pegmatites may occur in the rare-earth element type of the rare-element class and in the miarolitic class. In an early attempt to link Ta–Nb-oxides composition to pegmatite types, Černý (1989) demonstrated that assemblages of cogenetic CGM, wodginite and tapiolite form distinct trends in #Mn–#Ta diagrams and postulated that CGM in beryl-subtype pegmatites develop from Fe-rich columbite to Fe-rich tantalite with insignificant Fe–Mn fractionation. Fe–Mn fractionation in CGM will be largely governed by Fe–Mg-bearing silicates co-crystallizing with CGM, e.g. garnet, tourmaline, mica and others (Van Lichtervelde et al., 2006). Spodumenesubtype pegmatites are also dominated by Nb–Ta fractionation but are slightly more enriched in Mn. In the complex class, including the complex spodumene, petalite, lepidolite and amblygonite subclasses, significant enrichment of Mn takes place before fractionation of Ta from Nb. The #Mn–#Ta diagram, therefore, is of potential use to pin-point pegmatite types using CGM compositions. This is particularly important in those cases where associated minerals (e.g., the major silicates and phosphates including K-feldspar, albite, spodumene, beryl, garnet, tourmaline, apatite and other primary phosphates) were decomposed (i.e., in a lateritic environment), or where CGM grains are found in eluvial or alluvial placer deposits. Therefore, CGM compositions may serve as proxies and could be used for exploration purposes as well (Selway et al., 2006). The datasets presented here and in Melcher et al. (2015) provide further insights into the behaviour of major and trace elements during the evolution of granite–pegmatite systems enriched in rare elements. The major characteristics of CGM in provinces discussed in this paper are summarised in Table 3 which demonstrates that two major groupings related to the LCT and NYF families, exist. A third grouping may represent a mixed LCT–NYF type (cf. Černý and Ercit, 2005; Novák et al., 2012). Subgroups may relate to types and subtypes as proposed by Černý and Ercit (2005). However, attribution of individual heavy mineral ore concentrates to pegmatite types is often hampered by lack of information on associated silicates that are instrumental to distinguish them. In the present approach, therefore, mineralogical information obtained from literature had to be used. However, the information extracted is often equivocal or questionable. Our careful evaluation of the data shows that the chemistry of Ta–Nb-oxides allows to classify mineralisation into the LCT, NYF, mixed LCT/NYF, and rare-element granite categories. Some of the trace elements fingerprint to ore provinces and some may allow to estimate the economic potential of certain localities for rare-element mining. In Fig. 17, median values calculated from the present dataset and from the one published by Melcher et al. (2015) on African tantalum provinces are presented. The data used for these diagrams are included in the supplementary data as Supplementary material S6. 6.2.1. LCT family In individual LCT-family pegmatites and in pegmatite provinces, CGM are characterised by Mn-rich compositions (50% of the province averages N#Mn 60), low to intermediate REE (Md, 5–500 ppm), intermediate to high Zr/Hf (Md, 6–11) and high U/Th ratios (Md, N 20). REE patterns are frequently MREEN-dominated and of types [1] and [2], occasionally of types [3], [4] and [5] according to Graupner et al. (2010). Typically enriched minor and trace elements in CGM are Li, Zr, Hf, Sn and Sb, less frequently also Mg, Bi and U. Other elements such as Sc, Y, the REE, W and Th are often depleted compared to the global average CGM composition. In some of the LCT pegmatites, significant concentrations of As, Be and Ba were detected. Within those pegmatites assigned to the LCT-family, the #Mn and #Ta values cluster in four distinct subgroups: (i) #Mn = 20–35, #Ta b 40 (Hagendorf subgroup): representative of the beryl-type and complex spodumene subtype; (ii) #Mn = 40–60, #Ta b 50 (Kibara subgroup): mainly spodumene– albite and complex spodumene types


No.

Province

Countries present

Ocurrences sampled (examples)

Age (Ga)

REG/REP+ LCT/NYFa Mineralogy TNO

Cassiterite

4.1.1. 4.1.2.1. 4.1.2.2.

Southeast Manitoba Wodgina Greenstone Belt Balingup Metamorphic Belt

Canada Australia Australia

Bernic Lake (Tanco) Wodgina, Mount Tinstone Greenbushes

2.65 2.9 2.53

REP REP REP

LCT LCT LCT

MnC–MnT, Wdg, Tap, Mc MnC–MnT, Wdg, Tap, Mc FeC–FeT, Wdg, SbT, Mc, Tap

x x x

4.1.2.3. 4.1.3.

Norseman–Wiluna Belt Kolmozero–Voronja Zone

Australia Kola, Russia

2.52

REP REP

LCT

FeC, MnC, Mc FeC–MnT, Wdg, Tap, Rt, SbT

4.2.1.

São João del Rei Pegmatite Province Amapa Amazonas Guyana Shield

Brazil

Spargoville Kolmozero, Voronja Tundra, Vasin Mýlk Volta Grande, Mibra, Dattas

2.1

REP

LCT

MnC, MnT, Wdg, Mc

x

Brazil Brazil Suriname

Lourenco Pitinga Marowgne

2.0 1.8 2.1

Placer? REG Placer?

NYF?

x

LCT?

FeC, Tap FeC, Mc, Pbpcl, Fg Tap, MnT, FeC

4.2.3.

Black Hills Pegmatite Province

REP

LCT

FeC, FeT, MnC, MnT, Wdg, Tap

Svecofennian/Svecokarelian Province

Etta, Hugo, Peerless, Bob Ingersoll, Cowboy Miessi, Pusku rivers

1.7

4.2.4.

South Dakota, USA Sweden, Finland Finland Karelia, Russia India

1.9

Placer

LCT?

Tammela Uros-ozero, Pertima

1.8

REP REP

LCT NYF

2.0

REP

LCT

FeC, FeT, MnC, MnT, Wdg, Tap, Mc, Fs

Australia Colombia Norway, Sweden Colorado, USA Brazil

Broken Hill Högsbo, Råde, Kragerø, Risør

1.7 1.3–1.4 0.9–1.0

Placer REP

NYF? NYF

Raleigh

1.0

REP

NYF

Wdg Ix, FeC, Rt FeC, MnC, Tap, Eux, Aesch, Fg, Sam, Yta, Mc FeC

Aracuai

0.45–0.6

REP

LCT

FeC, MnC, Wdg, Rt

4.2.2.

4.2.5.

Geochemistry REE typeb

Trace element specialization

4b 1b, 2a 1a, 1d 2a 2a

Li, Sc, Ti, Sn, Sb; (–Mg, Y, REE) Li, Sb, Bi; (–Mg, Sc, Ti, W) Mg, Sc, Ti, Sn, W, As, Sb, Bi; (–Zr, Hf, Y, LREE, U, Th) Li, Sc, Mg, REE; (–Sn,Bi) Mg, Hf, Sb, Pb; (–Y, REE)

2a

Li, Y, REE, Sn, Sb, Th; (–Mg, Sc, W, Bi)

x

1a n.a. 1a (FeC), 2b (MnT) 1a, 2a

Mg, Sc, Y, REE; (–Sn, Sb, Bi, U) W Col: Sc, Ti, Y, Zr, Hf, W, U; Tap: Hf, Sb; (–Li, Mg, Sc, Y, REE, W, Bi, Th) Li, Mg, Sc, Sn; (–Y, Bi, Th, U)

Tap, FeC, FeT, MnT

x

2b

Mg,Sc, Y, REE, Hf, Th; (–Li, Sn, W)

Tap, CGM, Mc, Sbmc FeC, MnC

x 1a, 1b

Y, REE, Sb, Bi, Th

x

1b

Sb; (–Bi)

x

2a 1d 1a

(–Mg,Sc) Sc, REE, Mo, W, Sb, Th; (–Li, Bi) Y, REE, Th, Sb, Bi; (–Li, Sn)

1d

REE, Sc, W, Th; (–Li, Sn, Hf)

1a

Mg, HREE, W; (–Sc, Hf, Bi)

2a

Mg, Sb, Bi; (–Li, HREE, Sn)

4.2.6. 4.3. 4.4.1.

Bastar–Malkangiri Pegmatite Belt New South Wales Eastern Colombia Sveconorwegian

4.4.2.

South Platte District

4.5.1.

Eastern Brazilian Pegmatite Province (EBPP) Borborema Pegmatite Province Polar Urals Pampean Pegmatite Province

Brazil

Equador, Picui

0.5

REP

LCT

FeC, FeT, MnC, MnT, Tap, Wdg, Mc

Russia Argentina

0.3–0.6 0.4–0.5

REP/REG REP

NYF LCT

FeC, Fg, Pcl, Sam, Rt FeC, MnC, MnT

x (x)

1a 2b

Y, HREE, Mg, Bi; (–Li, Zr, Hf, W, U) Sb, Bi; (–Li, Sc, W, Th)

Spain, Portugal Germany Austria, Italy

0.3

REP

LCT

MnC, MnT

x

1b

Li, Zr, Sn, W; (–Mg, REE, Ti, Sb, Bi, Pb)

4.6.3. 4.6.4.

Central Iberian Pegmatite Province Hagendorf Province Austroalpine Province

Taikeu, Longot-Yugan Victor Hugo, Los Chilenitos, Independencia Argentina Bajoca, Feli, Goncalo Hagendorf, Pleystein Weinebene, Pusterwald, Passeier

0.3 0.2–0.3

REP REP

LCT LCT

(x) x

2b 1a, 2b

Zr, Hf, U; (–Mg, Sn, Sb, Bi) Mg, Zr, Sn, Sb; (–Sc, Y, REE)

4.6.5.

Southern Urals

Russia

Ilmen, Svetlinskoje

0.2–0.3

REP

NYF

x

2a

W, Sb, Bi; (–Sc, Y, REE, Th, U)

4.6.6. 4.6.7. 4.8. 4.7.1.

Appalachian Province Kalba–Narym Pegmatite Belt Blue River

0.39–0.27 0.25 0.33 0.2 0.2 0.1 0.3 0.14 0.12

REP REP CARB REP REP REP REP/REG REG REP

x

Ishikawa Altai–Sayan Khangilay Malkhany

Mitchell, Chesterfield Kalba Upper Fir Phuket Kim Bee Uzumine Ulug Tanzek Orlovka Sosedka

LCT LCT

4.7.2. 4.7.3.

USA Kazakhstan Canada Thailand Malaysia Japan Russia Russia Russia

FeC FeC, FeT, MnC, MnT, Tap, Wdg, Ix, Mc, Fs, Pcl, Aesch CGM, Tap, Ix, Wdg, SbT, Fg, Eux, Fs, Aesch, Sam, Pcl, Mc FeC, MnC MnC, MnT, Wdg Pcl, FeC, Fs FeC, Wdg FeC, Mc FeC, FeT FeC, MnC, Pcl, Fg, Eux Mc, MnC, MnT, Eux, Rt CGM, Bimc, Sbmc, Pcl, Beta, Ix, Eux

1b, 1c, 2b 1b 3a 1b 2a 2b, 2c 1d 1d 2b

Y, HREE, Sb, Bi, Th; (–Sn, W) Sn, Sb, U; (–Y, REE, Ti, W) Mg, HREE, Th; (–Li, Y, Sn, W, Sb, Bi, U) Sc, Y, HREE, W; (–Li, Sb, Bi, Pb) Sc, Y, HREE, W; (–Li, Sb, Bi, Pb) Sc, Y, HREE, Sb; (–Sn, Pb) Y, HREE, Th; (–Li, Mg, Hf, W, Sb, Pb, U) Sc, HREE, Th; (–Mg, Sb, Pb) Sc, Y, HREE, Sb, Bi, Th; (–W, Pb)

4.5.2. 4.5.3. 4.6.1. 4.6.2.

NYF LCT

x

x

x


REG/REP+ Rare-element granite; rare-element pegmatite; CARB, carbonatite; n.a., not analysed; CGM, columbite–tantalite (unspecified); FeC, columbite-(Fe); MnC, columbite-(Mn); FeT, tantalite-(Fe); MnT, tantalite-(Mn); Wdg, wodginite; Tap, tapiolite; Mc, microlite; SbT, stibiotantalite; Sbmc, stibiomicrolite; Rt, rutile; Pcl, pyrochlore; Pbpcl, plumbopyrochlore; Fg, fergusonite; Fs, fersmite; Eux, euxenite; Sam, samarskite; Ix, ixiolite; Aesch, aeschynite; Yta, yttrotantalite; BiT, bismutotantalite; Bimc, bismutomicrolite; Beta, betafite. a LCT: Li, Cs, Ta-family; NYF: Nb, Y, F-family. b Types according to Graupner et al. (2010). 31


Table 3 Tantalum provinces outside of Africa.

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Fig. 17. Fractionation indices for Ta provinces. Each point represents the median composition of CGM from one Ta province. LCT, NYF, mixed LCT–NYF pegmatite and rare element granites (RE granite) are distinguished. A median value for columbite from the Upper Fir carbonatite has been added for comparison. a) Variation of #Mn and #Ta; inset shows typical fractionation paths of CGM in individual pegmatites and in pegmatite provinces; b) #Ta vs. Zr/Hf; c) total REE vs. U/Th; d) Y vs. Sc; e) total REE vs. Bi; f) U/Th vs. Li.

(iii) #Mn = 70–100, #Ta b50 (Tanco subgroup): complex types, with amblygonite, lepidolite, petalite, elbaite and spodumene subtypes; (iv) #Mn = 70–100, #Ta N 50 (Wodgina subgroup): complex types, with amblygonite, lepidolite, petalite, elbaite and spodumene subtypes.

6.2.2. NYF family CGM in NYF-family pegmatites are Fe-rich (92% of the province averages b#Mn 40) or intermediate Fe–Mn columbite and always have

Nb N Ta. REE concentrations are intermediate to very high (Md, 100– 8000 ppm), and U/Th ratios are low (Md, b 60). Zr/Hf ratios scatter widely from 4 to 18, but do not show preferred values. REE patterns are dominated by both, the MREEN and HREEN (type [1] and [2] REE patterns). Scandium, Y, HREE, W and Th are abundant trace elements, whereas Li, Sn, Sb and U are usually lower than in LCT-family pegmatites. 6.2.3. Mixed LCT–NYF family No special preference for Fe–Mn or Nb–Ta is observed, but all pegmatites have intermediate to high REE (all are MREEN dominated, type [2] patterns), and low to intermediate U/Th. Zr/Hf behaves indifferent.



Minor and trace element patterns in CGM resemble NYF-types regarding their high Sc, Y, REE and W concentrations, and LCT-types in their Sb, Zr, Hf and Mg values. Mixed LCT–NYF pegmatites are either Fe-rich (Guyana, Appalachians, Svecofennian) or Mn-rich (Alto Ligonha); their #Ta values are within the range typical of LCT pegmatite, but their REE concentrations are higher and U/Th is lower (ca. 10). Miarolitic NYFfamily pegmatites in the Wausau syenite complex, Wisconsin, carry a complex Nb–Ta–REE assemblage including euxenite-(Y), microlite, pyrochlore, polycrase-(Y), samarskite, tapiolite and CGM (Falster et al., 2012; Simmons et al., 2012). CGM compositions range from near-endmember columbite-(Fe) to near-end-member tantalite-(Mn) with moderate to low concentrations of minor elements (e.g., 1–1.7 wt.% TiO2, 0.2– 0.3 wt.% UO2, 0.11–0.23 wt.% Y2O3), and low Sn and Sc. Such compositions are unusual for NYF, but typical for LCT family pegmatites; Falster et al. (2012) state that the most evolved CGM are restricted to small miarolitic cavities only. 6.2.4. Rare-element granites In rare-element granites CGM are Fe- or Mn-rich or both, but always have Nb N Ta. REE concentrations are high to very high (Md, 300– 6000 ppm), and U/Th is very low to low (Md, b13). Zr/Hf is either very low (b4 = highly fractionated; Orlovka, Egypt) or very high (N13 = less fractionated; Polar Urals, Jos, Altai), thus defining two extreme subgroups. Normalised REE patterns are always dominated by the HREEN, resulting in subtype [1d] patterns. Granite-hosted CGM appear to have similar trace element compositions to CGM in NYF-family pegmatites, although the concentration levels of Y and REE may be extremely high, and Sn is not as low as in NYF-family pegmatites. Molybdenum is commonly present in elevated concentrations compared to both, LCT and NYF-family pegmatites. In some rare-element granites, petrological and mineralogical arguments indicate affinity to the LCT family, e.g. at Orlovka where Ta–Nb oxides occur with F-rich mica, lepidolite, and topaz. CGM from rare element granites indicate a process of fractionation. They may carry very high concentrations of Sn, W, Li, Rb and REE. Furthermore, in rare metal granites, pure Ta CGM end members do not occur, probably due to the competition with other Ta oxide phases such as microlite with up to 75 wt.% Ta2O5. In small pegmatite conduits, the fractional crystallization process may lead to the formation of a number of spatially separated parageneses with distinct CGM generations. In contrast, a number of processes including fractional crystallization, liquid immiscibility and metasomatism act in rare element granites. These processes overlap, and as melt evolves slowly, processes do not reach completion. This diversity of processes is reflected by the complex internal zoning of CGM (Badanina et al., 2010a, 2010b). 6.2.5. Carbonatite CGM compositions have rarely been reported from carbonatites, because CGM are commonly interpreted as a secondary phase replacing primary HFSE minerals. Most of the focus of petrogenetic studies was therefore on the typical HFSE-bearing minerals such as pyrochloregroup minerals, perovskite, ilmenite, zirconolite, baddeleyite, and zircon (Chakhmouradian, 2006; Linnen et al., 2014). The Upper Fir carbonatite, British Columbia, offers a unique possibility to study the composition of primary CGM, specifically from a carbonatite occurrence that represents a Ta(–Nb) resource. CGM are Fe- and Nb-dominated, with low to intermediate REE (Md, 40 ppm, range 17–570 ppm) and low U/Th (Md, 0.14, range 0.01 to 18), but high, suprachondritic (the C1 ratio being 37) Zr/Hf (Md, 42). Ratios of light to heavy HFSE range from 7 to 237 for Zr/Hf and from 4 to 1100 for Nb/Ta, respectively, thus indicating significant evolution within the system. This compositional evolution is also observed at the grain scale with both ratios decreasing towards the margins and in highly altered zones (see also Chudy, 2014, for major element data). In many aspects, the CGM at Upper Fir are similar to a REE-poor NYF pegmatite system. A Nb–Ta assemblage including primary and secondary columbite-(Fe) with pyrochlore and secondary

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euxenite and fersmite was recently described from the carbonatitehosted Nb deposit at Aley, British Columbia (Chakhmouradian et al., 2015). Columbite is close to the Fe–Nb endmember, poor in Mg, Mn, and variable in Ti (0.9–4.6 wt.% TiO2) and Ta (0.1–5.1 wt.% Ta2O5); trace element compositions measured by LA-ICP-MS indicate high levels for Zr (Md, 5723 ppm) and Th (Md, 4153 ppm), but low U (224 ppm), Pb (129 ppm) and Sc (35 ppm). Nb/Ta and U/Th ratios range from 90 to 1700 and 10–47, respectively, whereas Zr/Hf is uniform at 33 ± 9 (Chakhmouradian et al., 2015). 6.3. Dependency between fractionation indices and trace elements Due to the heterogeneous composition of CGM at all scales (grain, deposit, district and global scales) caused by regional (precursor- and wallrock-related) as well as process-related factors related to evolution of the melt (differentiation, fractionation, melt–crystal interaction, cooling history, co-crystallizing phases competing with Ta–Nb oxides for major and trace elements, hydrothermal overprint, metasomatism), no significant inter-element correlations are obvious when the global dataset is jointly evaluated. Exceptions are #Mn and #Ta as well as ratios or factors involving chemically coherent high field-strength elements (HFSE) like Zr/Hf or U/Th, total REE or Y (Fig. 17). Several approaches to study the fractionation behavior of pegmatitic melts and/or to characterise hybridisation reactions between pegmatite melt and wallrock, based on the evaluation of ratios between HFSE, were applied by Beurlen et al. (2008); Llorens and Moro (2010); London (2008); McKeough et al. (2013); Möller (1989); Pal et al. (2007) and others; however, these studies commonly focused on a specific geological situation valid for individual pegmatite bodies or pegmatite fields only. In contrast, we calculated average HFSE ratios for pegmatite districts using the new global data set to generalise the behavior of the differentiated families of rare-element pegmatites and granites. Median values of fractionation indices were calculated for five categories of CGM chemistry, representing the LCT, mixed LCT/NYF, NYF pegmatite, and rare-element granite associations. For comparison, median values of a columbite dataset from the Upper Fir carbonatite in British Columbia are also included in Fig. 17. LCT-family pegmatite data were further subdivided using the four #Mn/#Ta subgroups as described before. In Fig. 17, the results obtained are illustrated as trends and groupings whereby the individual data points represent individual Ta provinces as provided in Fig. 1, Table 3, and Supplementary material S6. Many CGM grains from worldwide pegmatite locations of various ages are internally zoned, as a rule displaying increasing Ta concentrations and thus increasing #Ta values from core to rim. Therefore, an increased #Ta ratio is commonly interpreted as an indicator of evolved fractionation of the rare-element-rich melt (e.g., Černý, 1989), corroborated by experimental data of Linnen and Keppler (1997) which indicate that the solubility of tantalite is higher than that of columbite in granitic melts. LCT pegmatites show a large range of median #Mn values, grossly correlating with #Ta (Fig. 17a). Subtypes i (Hagendorf) and ii (Kibara) of this family both form clusters with a trend mainly determined by the #Ta variability. In many individual pegmatites/granites and pegmatite/granite provinces, #Ta correlates negatively with Zr/Hf with the most evolved pegmatites and granites having Zr/HfCGM below 5 (Fig. 17b; P25, 4.6; P10, 2.9). In the “global” CGM dataset, #Ta and Zr/Hf are linked by a logarithmic regression function of #Ta = −26 ∗ ln(Zr/Hf) + 86.5, with R2 = 0.565. The median values of #Ta and Zr/Hf plot on an exponential trend (see above), starting from Zr/Hf ratios in carbonatite N40 (higher than average crustal values) and developing towards low Zr/Hf and high #Ta in LCT pegmatites. Average data points for LCT pegmatites, when evaluated alone, form a trend indicating a negative correlation between Zr/Hf and #Ta (Fig. 17b; blue arrow). The NYF pegmatite and rare-element granite data points follow a similar but slightly steeper trend to low Zr/Hf with increasing #Ta (black arrow). Likewise, the #Mn ratio increases during fractionation, but also depends on factors intrinsic to the melt (e.g., the concentration of fluxing elements) and on the availability of coexisting


34


Fe–Mn phases (e.g., garnet, tourmaline). Therefore, #Mn does not correlate significantly with Zr/Hf in the complete dataset (not shown here). Linnen and Keppler (2002) determined that the solubilities of zircon and hafnon in metaluminous or peraluminous granitic melts are orders of magnitude lower than in peralkaline melts. Furthermore they stated that zircon on a molar basis is up to five times more soluble than hafnon in peraluminous melts, whereas the solubilities are nearly identical in peralkaline melts. On the other hand high F contents in melts were interpreted to result in elevated zircon–hafnon solubility by Keppler (1993). Aseri et al. (2015) confirmed such a positive dependency of zircon and hafnon solubilities in a haplogranitic melt on the fluorine content in the minimum composition; the solubilities increased from 2.03 ± 0.03 × 10−4 (mol/kg) ZrO2 and 4.04 ± 0.2 × 10−4 (mol/kg) HfO2 for Ffree melts to 3.81 ± 0.3 × 10−4 (mol/kg) ZrO2 and 6.18 ± 0.04 × 10−4 (mol/kg) HfO2 for melts with 8 wt.% F. However, the presence of Li in a melt is also assumed to influence (reduce) the solubilities of zircon and hafnon (Linnen, 1998; Aseri et al., 2015). Further detailed evaluation of possible effects of F and Li on the Zr/Hf ratios of the pegmatite/granite provinces plotted in Fig. 17b needs additional data. The rather similar general trends in the Zr/Hf vs. #Ta plot for both, LCT pegmatites, and NYF pegmatites and rare-element granites, as indicated by the two arrows in Fig. 17b, allow the inference that all pegmatites/granites studied here formed from metaluminous to peraluminous granitic melts and none formed from peralkaline melts (cf. Linnen and Keppler, 2002; Černý, 2005). U/Th may be considered a fractionation index, as the ratio commonly increases during evolution of rare-element-rich melts (Fig. 17c). In individual pegmatites, this is occasionally the case (e.g., Tanco). However, in the complete dataset there is no correlation of U/Th with #Mn, #Ta or Zr/Hf (diagrams not shown). A negative correlation of total REE and U/ Th is well expressed (arrow), defining a trend from rare-element granites (high REE, low U/Th) via NYF pegmatites and mixed LCT–NYF pegmatites to LCT pegmatites characterised by low REE and high U/Th (Fig. 17c). The U/Th ratio and total REE concentrations clearly discriminate LCT from NYF family pegmatites. Similar REE relationships are apparent from wholerock lithogeochemistry (Linnen and Cuney, 2005). Discrimination of NYF pegmatites and rare-element granites from LCT pegmatites is also possible in binary variation diagrams like Y vs. Sc (Fig. 17d), total REE vs. Bi (Fig. 17e) and U/Th vs. Li in CGM (Fig. 17f). Yttrium behaves similar to the HREE and thus discriminates CGM in LCT pegmatites (b 100 ppm Y) from NYF pegmatites and rareelement granites. However, variations in Sc concentrations over 4 orders of magnitude do not correlate with Y, and are difficult to interpret. Most LCT pegmatites follow a negative fractionation trend from about 100 ppm Y and 20 ppm Sc in less fractionated to b1 ppm Y and b5 ppm Sc in more fractionated types (arrow; Fig. 17d). Significant deviations from this trend to high Sc concentrations at low Y are obvious, and are best demonstrated by the exceptionally high median values for Tanco and Greenbushes. In both pegmatites, Sc concentrations are similar to those in many NYF pegmatites and rare-element granites. Likewise, high Sc concentrations are not a prerequisite for the NYF family. Three pegmatites classified as “mixed LCT/NYF type” based on their high REE and Y have intermediate to low Sc. NYF pegmatites most enriched in Sc (N5000 ppm) are those from Somipe (Alto Ligonha Province, Mozambique; Melcher et al., 2015) and South Platte, Colorado, followed by placer CGM associated with ixiolite from Eastern Colombia. In summary, scandium behavior in pegmatites is complex and not well understood at present. This is also reflected by the finding of Sc-rich alteration products and secondary Sc phases in Moldanubian pegmatites (Dill et al., 2006, 2008; Novák et al., 2008) and the extremely variable Sc concentrations in beryl from both, LCT (12–2823 ppm Sc, n = 3) and NYF pegmatites (24–758 ppm Sc; n = 2; Přikryl et al., 2014). The total REE concentrations in CGM also correlate with Bi (Fig. 17e). Whereas Bi increases with total REE (and decreases with fractionation) in LCT pegmatites, Bi concentrations are low but variable in the NYF family; however, those classified as “mixed LCT/NYF” have high Bi

concentrations following the LCT-family trend. Some trace elements show complex behaviour in some diagrams using fractionation indicators; this is illustrated in a plot of U/Th versus Li (Fig. 17f). CGM of the NYF and mixed LCT/NYF families and from the Upper Fir carbonatite have low U/Th (b 50) and low Li. LCT pegmatites, however, show two positive correlation trends. Based on the #Mn–#Ta systematics outlined above, one trend is outlined by Kibara- and Hagendorf-group pegmatites (low Li concentrations and high U/Th, up to 350; highlighted by a red arrow), whereas the second trend is outlined by Tanco-type and Wodgina-type pegmatites, with median Li concentrations exceeding 100 ppm at low U/Th (b100; black arrow). The data indicate progressive Li and U fractionation during evolution, but point to the presence of both, Li-poor systems rich in U and Li-rich systems less enriched in U. 6.4. Implications of Ta–Nb-oxide compositions for regional processes Recently, compilations on the distribution of granitic pegmatites during Earth's history have been published (e.g., Tkachev, 2011; McCauley and Bradley, 2014). Accordingly, LCT pegmatites display age peaks centered at 2638, 1800, 962, 529, 485, 371, 309 and 274 Ma, whereas NYF pegmatites range from 2652 to 33 Ma with a prominent peak at 1000 Ma representing the Grenvillian and Sveconorwegian pegmatites (McCauley and Bradley, 2014). The total number of mineralised pegmatites and granites appears to increase with time progressing, and the distribution of pegmatite ages suggests a correlation with supercontinent cycles (McCauley and Bradley, 2014). In Fig. 18, data from the present study and the earlier work on African Ta provinces (Melcher et al., 2015) are combined and summarised at a resolution of 100 Ma. Although each data point on this diagram may represent either a Ta province (such as the Kibara Belt, central Africa) or a single pegmatite (such as the Tanco pegmatite), the distribution of Ta pegmatites shows distinct maxima in Earth's history reflecting major orogenic events, namely: the late Archean, the Paleoproterozoic, the Meso-/Neoproterozoic boundary, the Neoproterozoic/Paleozoic boundary, the Carboniferous/ Permian and the Mesozoic/Cenozoic. Some of the largest and economically most important rare-element pegmatite bodies are located within Archean terrains and intruded ultramafic and mafic host rocks (e.g., Tanco/Canada, Wodgina and Greenbushes/Western Australia, Kolmozero/Kola, Bikita/Zimbabwe). They are highly fractionated and yield complex mineralogical compositions (Table 3); the most fractionated pegmatite known to date has been found at Red Cross Lake, Manitoba (Černý et al., 2012a). With the exception of a small number of ambiguous examples from western and central Africa (e.g., the Man Shield and Northern Congo Craton;

Fig. 18. Age distribution of Ta-bearing pegmatites and granites. Blue bars represent Tabearing LCT and NYF pegmatites extracted from the database of McCauley and Bradley (2014). Grey bars represent Ta ore provinces described in this paper and by Melcher et al. (2015). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)



Melcher et al., 2015), the Archean pegmatites are of LCT affinity throughout. The variety of minor and trace elements incorporated attests to a rather insignificant role of the immediate host rocks to their geochemical signature and rather points to the significance of the composition of the underlying crustal protoliths and the processes of melt generation (Martin and de Vito, 2005). The degree of contamination in most of these large pegmatites is low (e.g. Van Lichtervelde et al., 2006). Many of the Archean pegmatites carry significant Li mineralization as spodumene, petalite, and amblygonite, and all of them are also characterised by elevated Li in CGM (Fig. 6) underlining the importance of mica and K-feldspar in crustal protoliths. In addition, Sb and Bi are important trace elements in CGM, also reflected by the occasional presence of stibio- and bismutotantalite. REEN patterns of CGM are variable, from very low (i.e. Tanco, Kolmozero) to very high concentrations (i.e., Man Shield; Spargoville/WA) (Fig. 7). Negative Eu anomalies are omnipresent. Scandium plays a rather interesting role as its contents are highly variable, from very high (Tanco) to very low concentrations (Wodgina, Kolmozero). A second period of worldwide pegmatite formation was in the Paleoproterozoic, in the time range from 2.1 to 1.7 Ga. All CGM analysed derive from LCT pegmatites except samples from the Amazonas region where trace element data point to an NYF affinity for material from Amapá, and the rare element granites of Pitinga which are a major economic source of alluvial and eluvial cassiterite and columbite-(Fe) (Table 3). Pegmatites intruded highly variable lithologies including metasediments, metabasites, gneiss, granite and quartzite within a variety of structural settings; however, most of them are syn- to postorogenic with respect to major Paleoproterozoic orogenic events (i.e., Eburnean event in West Africa; Svecofennian event in northeastern Europe). Minor and trace element signatures are similar to CGM from Archean pegmatites (Figs. 6, 9). Some are characterised by considerable REE enrichment (Sao Joao del Rei/Brazil; Amapá/Brazil; Finnish Lapland/Finland), whereas others have normal to low total REE concentrations (Black Hills/USA, Bastar/India) (Figs. 7, 10). Kokobin/Ghana represents the only documented case of LREE-enriched REE in CGM, yielding a type [5b] REEN pattern (cf. Graupner et al., 2010). Examples with high REE commonly are enriched in Sc and Y as well, and are often transitional to NYF-family pegmatites. The Mesoproterozoic period is comparatively poor in rare-element pegmatites and rare-metal granites. The only example documented here is placer material from Colombia (Table 3). Mineralogical and chemical attributes of the Ta–Nb-oxides (ixiolite–wodginite, tapiolite, CGM, rutile) point to an unusual pegmatite source of NYF affinity, yielding high total REE, Sc and Th at low Li and Bi. REE patterns have typical negative Eu and Y anomalies (Fig. 10c). Meso- to Neoproterozoic REErich pegmatites are abundant in a trend from Wisconsin (Wausau complex, 1.5 Ga) through Colorado (South Platte district, 1.0 Ga) to Arizona (Mojave district, 1.5 Ga; Simmons et al., 2012). They have an anorogenic character and are all related to post-orogenic tectonic events. Regional geochemical and mineralogical differences are attributed to variable source-rock composition (e.g., the abundance of F-rich minerals in the source), different degrees of partial melting, and the extent of rifting, yielding pegmatites richer in incompatible elements from smaller degrees of partial melting (Simmons et al., 2012). A third major period of pegmatite formation was the Early Neoproterozoic at around 1 Ga, documented in the Kibara, Kamativi and Orange River belts in Africa (Melcher et al., 2015), the Grenvillian (North America) and the Sveconorwegian (northern Europe). CGM are present in numerous, mostly small pegmatites, although larger examples such as Manono (DR Congo) also occur. Pegmatite fields often display a zonal arrangement of mineralised pegmatites with respect to “fertile” parent granites (e.g., Hulsbosch et al., 2013). They intrude metasediments, metabasites, gneiss and granite of middle to upper crustal levels and display a variety of mineralogical and chemical characteristics. Pegmatites of the Sveconorwegian and Grenville domains (e.g., Ercit, 1994; Lupulescu et al., 2012) are usually of the NYF type

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and CGM are characterised by elevated Y, REE, Th and Sc (Figs. 9d, 10d). The pegmatites of central and southwestern Africa are commonly of LCT affinity carrying spodumene, beryl and cassiterite (Melcher et al., 2015). CGM have elevated concentrations of Li, Mg, Sn and Hf. Total REE concentrations are low except for the Sveconorwegian, and exhibit a variety of shapes. Most noteworthy are the overall low REE concentrations in the Kibara and Kamativi belts, accompanied by only weak Eu anomalies. The fourth major pegmatite-forming event coincides with amalgamation of Gondwana at the Neoproterozoic/Paleozoic boundary around 550 Ma ago. This event is omnipresent in Africa (“Panafrican”) and South America (EBPP and Borborema provinces, Table 3). Pegmatites often intruded high-grade metamorphic terrains composed of metasediments including various schists, marble, quartzite, as well as gneiss, amphibolite, ultramafic rocks, and granite. Within the Panafrican, raremetal granites of NYF affinity are locally abundant (i.e. Eastern Desert/ Egypt; Melcher et al., 2015). Pegmatites show both LCT and NYF affinities, and mixed types occur in Mozambique. The Alto Ligonha and Madagascar provinces are characterised by abundant REE and Sc both within Ta–Nb-oxides and as separate mineral phases. Notably, some pegmatite provinces are almost devoid of cassiterite (e.g., Alto Ligonha belt/ Mozambique, Adola belt/Ethiopia), whereas others carry cassiterite in economic amounts (e.g. Damara belt/Namibia; Eastern Desert/Egypt). The presence of Sn and W mineralisation in magmatic provinces has been associated with source enrichment processes through prolonged paleoweathering of sediments for the Variscan belt of Europe (Romer and Kroner, 2015). In this scenario, chemical weathering on a stable craton and redeposition of erosional products into extensional sedimentary basins will lead to partial melting of such pre-enriched sediments during later tectonic events. This concept has yet to be applied to the Panafrican granite–pegmatite systems but it appears that certain geochemical signatures such as the presence and absence of Sn, W, REE and Sc may be explained by geodynamic and probably paleoclimatic reasons. In the Phanerozoic (younger than 542 Ma), pegmatites formed at all times in response to orogenetic processes involving various continents and terranes during the long-time amalgamation of Pangea and the Alpine orogenies. Whereas some activity is related to the Pampean, Acadian and Caledonian orogenies (Fig. 18), the Variscan/Hercynian/ Alleghanian orogeny is of utmost importance and is manifested in pegmatite formation associated with Sn–W mineralised granites in central and western Europe as well as in the Appalachians (Romer and Kroner, 2015). Most of the Appalachian and European Variscan pegmatites are of LCT affinity, although NYF types (Škoda and Novák, 2007; Novák et al., 2011) and mixed types have been described as well (Novák et al., 2012). Unusual pegmatites of NYF affinity occur in the Ilmen massif, Urals (Table 3). Variscan pegmatite formation culminated at ca. 330 to 300 Ma (Melleton et al., 2012), and ranged in age from about 390 Ma (Brazil Lake, Nova Scotia; Kontak et al., 2005) to about 240 Ma in the Austroalpine basement units. Most are syn- to post-orogenic and were emplaced in different crustal levels and into a variety of host rocks. Degree of fractionation as well as minor and trace element geochemistry of Ta–Nb-oxides are rather variable and cover the complete field of CGM compositions (Figs. 11a–d, 12a–d). REE patterns are characterised by prominent negative Eu anomalies (Fig. 13a–d). Mesozoic and Cenozoic pegmatites and rare-metal granites are briefly covered here. Most of them are from Southeast Asia and the Russian Far East (Table 3). In Africa, rare-metal granites of the Jos Plateau (Nigeria) were previously investigated (Melcher et al., 2015). The proportion of NYF-family pegmatites and rare-metal granites having NYF affinities in the Mesozoic is striking, i.e. illustrated by Jos, Ulug Tanzek as well as the southeast Asian deposits related to tin granites. CGM from these areas are invariably rich in REE, Sc, Y and Th (Fig. 12h–i). In all rare-metal granites, Ta–Nb-oxides are characterised by high total REE concentrations and both, negative Eu and Y anomalies in chondrite-normalised REE diagrams (Fig. 13h–i).


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6.5. Comparison of CGM from pegmatites and the Upper Fir carbonatite

7. Conclusions

Compared to rare metal pegmatites and granites, carbonatite complexes are entirely different geochemical systems characterised by magmatic carbonates and silica-undersaturated alkaline or mafic rocks. The two rock systems occur in very different tectonic settings with carbonatites restricted to extensional regimes, mostly rift systems, and rare metal pegmatites typically forming during the waning stages of major orogenies (syn- to post-tectonic). The ultimate source for the parental melts for the two rock suites is also vastly different as carbonatites are sourced directly from the mantle (previously metasomatized) and fertile melts for pegmatitic systems are generated mostly in the crust (S-type, I-type and A-type granites). Nevertheless, some carbonatites show similar HFSE mineralogy and enrichment patterns, in particular for heavy HFSE which is reflected in the economic potential of carbonatites as resources for Ta. The ‘pegmatitic carbonatites’ associated with silica-undersaturated rocks from the Ilmen Mountains in southern Urals (Popov and Popova, 2006) serve not only as an example for potential Ta deposits associated with these complexes, but they also imply certain petrogenetic processes which are commonly associated with the term “pegmatitic” and are well established in granitic systems. A cursory comparison is therefore justified. It is important to note that the available trace element data from CGM from the Upper Fir carbonatite (this study) and the Aley carbonatite (Chakhmouradian et al., 2015) is not representative of the bulk of the carbonatite–alkaline rock occurrences, but rather exemplifies a specific clan (or group) which is best described as linear (dykes or sills) carbonatites without voluminous amounts of associated silicate rocks. As noted earlier, CGM from the Upper Fir carbonatite show similarities to the NYF pegmatite family by having REE patterns comparable to subtype [3a] and lack an Eu anomaly. CGM from both carbonatite occurrences have low REE contents as well as very low #Ta and #Mn, indicating a rather primitive and unevolved composition. Similarly, the low U/Th ratios and relatively high Zr/Hf ratios are indicative of a low a degree of fractionation, at least when compared to the pegmatitic systems. These findings are in agreement with petrographic observations that CGM are early phases in the magmatic evolution of the examined carbonatite systems. However, the CGM from the Upper Fir carbonatite record a significant evolution in the composition as evidenced by the large spread in Zr/Hf and Nb/Ta ratios. Compared to CGM from pegmatites, they are poor in Sn and W, but enriched in Ti and Mg, which can be attributed to the different source characteristics of the parental melts. The results from this study are twofold: (1) CGM in the examined carbonatites are early crystallizing phases as opposed to late-stage secondary minerals as reported for the majority of other carbonatite occurrences, and (2) the conditions (e.g., temperature, alkalinity, bulk chemistry of magma, etc.) during the enrichment of Ta were different than in pegmatites, although the processes (e.g., transport of HFSE, solubility of CGM, fractionation of Nb and Ta) must have been similar, at least in the Upper Fir carbonatite. This interpretation is based on the fact that CGM from this study show a systematic increase of #Ta and #Mn towards the margins of the grains and a decrease towards the center of the deposit (Chudy, 2014; Chudy et al., 2014). This indicates that fractionation processes similar to those in rare metal pegmatites were involved in the concentration of Ta in this carbonatite system. In contrast to pegmatites, the bulk of the Ta–Nb mineralization is located in pyrochlore supergroup minerals, which are typically stable under more alkaline conditions than CGM (Lumpkin and Ewing, 1995). Hence, it is unlikely that CGM from alkaline rocks such as carbonatites will reach such highly fractionated compositions as observed in many rare metal pegmatites. It is important to note that the inferred magmatic evolution of the Upper Fir carbonatite differs significantly from the Aley carbonatite, which is primarily considered a Nb deposit and is not particularly enriched in Ta, and from the Ilmen Mountains, where silicaundersaturated rocks prevail in the complex.

Nb–Ta oxides are important accessory minerals in many granitic and some alkaline and carbonatite systems. Their chemical composition is highly variable; due to their relative stability during (late-stage) alteration, compositions often record multiple stages of their genesis. CGM are especially abundant, and therefore useful to distinguish (“fingerprint”) ore provinces, single deposits, but also processes of their formation. The vast dataset assembled in the course of a study to develop an analytical fingerprint for “coltan” from central Africa (Melcher et al., 2008a,b, 2009, 2015; Gäbler et al., 2011; Graupner et al., 2010; Savu-Krohn et al., 2011) was further extended to include important Ta–Nb ores from worldwide locations. It was demonstrated that compositions of CGM and related minerals such as wodginite–ixiolite, tapiolite and others, can be correlated within deposits and provinces. Thus, mechanism of their formation may be deduced from compositional and textural data. In granitic rare-element pegmatites and rare-metal granites, the processes leading to enrichment and fractionation of the HFSE, especially of Nb and Ta, are complex. Their behavior in melts is controlled by columbite–tantalite solubilities, leading to progressively decreasing Nb/Ta in crystallizing Ta–Nb oxides, mostly CGM. Additionally, hydrothermal processes are believed to play a significant role at the hydrothermal– magmatic transition (Ballouard et al., 2016). The concentrations and ratios of HFSE in melts depend on the degrees of partial melting and on melting temperature, which are ultimately related to the tectonic setting. Both processes will affect the melting of Nb–Ta-bearing minerals in crustal sources that are most important for the formation of peraluminous granitic melts from which S-type granite batholiths crystallise in zones of crustal thickening during subduction or collision (Černý et al., 2012a). Under circumstances not yet fully explored, LCTfamily pegmatites may evolve from such systems. Source materials include metamorphic schists and gneisses of sedimentary origin. The melting behavior of mica, rutile, titanite, ilmenite and amphibole in crustal rocks is paramount to HFSE behavior: for mica and amphibole, DNb N DTa, whereas for Ti minerals and magnetite, DTa N DNb, with Di = partitioning coefficient of element i between mineral and melt. Thus, biotite melting at lower temperature will produce high Ta/Nb and low Ti in melts, while Nb is partitioned into residual biotite. For amphibole, Ti will leave the melt and partition into the residual phase, while Ta/Nb will become slightly fractionated in the melt; Ti minerals will produce high Nb/Ta in melts because Ta will be retained (Stepanov et al., 2014). High-temperature anatexis leading to complete removal of biotite produces lower Ta/Nb melts that will hardly evolve to Ta-rich, but likely to high-Ti melts. In a similar manner, low degrees of anatexis also will not produce Ta-rich melts because Ti minerals staying in the residuum will become enriched in Ta (Stepanov et al., 2014). In summary, the trace element composition of LCT-family rareelement pegmatites generated from peraluminous crustal melts will be strongly controlled by source composition and conditions of partial melting. Later on during crystallization and fractionation, factors intrinsic to the melt as previously discussed will control incorporation of trace elements into solidus phases such as CGM and other Ta–Nb oxides. Finally, metasomatic and hydrothermal processes may affect previously formed minerals, producing complex replacement textures and zoning patterns. CGM and other Ta–Nb oxides commonly preserve their trace element compositions and primary or secondary zoning patterns. Because the relative contribution of the factors governing CGM crystallization in a given pegmatite will never be the same or even similar, most pegmatite provinces may be distinguished from each other by CGM chemistry, and even pegmatites within regionally confined pegmatite fields are often different. NYF-family pegmatites evolve from A-type granites in anorogenic settings (Martin and de Vito, 2005). Their sources are mostly lower crustal metasedimenary rocks with some mantle material. Melting



temperatures are higher than for LCT-family pegmatites resulting in less biotite fractionation and saturation of monazite and zircon (Stepanov et al., 2014). Although A-type magmas are poor in H2O, fluorine released from breakdown of amphibole and mica will help to transport and fractionate HFSE. There is an ongoing debate as to whether trace elements in NYF-family pegmatites and related granites reflect source rocks, or if fractionation processes play an important role (Černý et al., 2012a). The present study illustrates that CGM compositions sometimes develop in a similar manner to those in LCT pegmatites, although to much lower Ta/Nb, thus suggesting a fractionation process to have taken place. The typical trace elements, namely the REE, Y, Th, Sc and W derive from low degree partial melting of lower crustal protoliths. Rare-metal granites form by similar processes in anorogenic settings, therefore producing CGM compositions similar to those found in many NYF-family pegmatites. However, differences are obvious (e.g., negative Y anomalies in chondrite-normalised diagrams, probably due to xenotime fractionation) and some require further studies, e.g. the Mo anomalies consistently observed in rare-metal granites. As outlined above, fractionation will overlap with other processes in larger evolving magma chambers prohibiting completion and thus producing complex internal zoning patterns (Badanina et al., 2010a,b). It is far beyond the scope of this contribution to comment on the complex processes affecting HFSE enrichment in carbonatites. But in general terms, carbonatites and alkaline rock systems show opposite fractionation trends to rare metal pegmatites. The magmatic evolution of a typical carbonatite commences with the precipitation of early liquidus phases such as uranpyrochlore which is characterised by low Nb/Ta ratios and therefore effectively fractionates Nb from Ta (uranpyrochlore trend after Chakhmouradian and Zaitsev, 1999; Linnen et al., 2014). The bulk of the HFSE mineralization is hosted by later generations of pyrochlore which become increasingly enriched in Nb and depleted in Ta and U during the magmatic evolution. Eventually, the conditions change from alkaline to acidic and CGM are stabilized, in particular the Fe–Nb endmember, replacing earlier phases such as pyrochlore supergroup minerals or perovskite. This generalized model becomes more complicated when fractionation processes such as liquid immiscibility associated with carbonated silica-undersaturated melts are taken into consideration. In contrast, CGM from the Upper Fir carbonatite show a different petrogenetic signature, which resembles early, primitive CGM from rare metal pegmatites of the NYF family. They differ, however, significantly from rare metal pegmatites and granites by having high Mg and Th, and low U concentrations in columbite-(Fe) (Fig. 14), and lack an Eu anomaly. Despite the differences, which can be chiefly attributed to the source composition of both parental melts, the enrichment processes in particular of Ta over Nb are strikingly similar. Preliminary results presented in Chudy (2014) constrain the factors that contributed to the Ta enrichment in the Upper Fir carbonatite. One important finding is that the carbonatite sill shows a systematic compositional zoning from the margins to the rims indicating an in-situ crystallization and differentiation, as opposed to a prolonged fractional crystallization at different crustal levels. The inferred magmatic evolution commences with the precipitation of primitive CGM at the carbonatite margins followed by pyrochlore towards the center that is characterised by increasingly lower Nb/Ta ratios resulting in late-stage microlite precipitation (Chudy et al., 2014). This observation is supported by the distribution of Na–Ca amphiboles and their extremely high fluorine contents. The latter play an important role for the solubility of Ta in carbonatites whereby small additions of F into the haplocarbonatitic melt drastically increase the solubility of Ta (microlite) and lower the solidus of the system (Kjarsgaard and Mitchell, 2008). Regardless of the effects of fluorine on the solubility of HFSE in pegmatitic and granitic systems (Aseri et al., 2015), the similarities between the Nb–Ta mineralization in the Upper Fir carbonatite and peraluminous systems are intriguing and require further detailed studies.

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Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.oregeorev.2016.03.014. Acknowledgements A large number of persons are gratefully acknowledged; for submitting samples: Kari Kojonen (GSF), Jürgen Konzett (University of Innsbruck), Heinz Mali (Montanuniversität Leoben), Richard Göd (University of Vienna), Graciela Sosa (University of Göttingen), Bernhardt Saini-Eidukat (Fargo State University); Uwe Kolitsch, Naturhistorisches Museum Wien (samples F9491, L3277, L8303); O.V. Udoratina (Sykyvtar), and N.V. Vasiljev (Moscow). Far-reaching analytical help came from Helene Brätz (University of Erlangen) and Friedhelm Henjes-Kunst (BGR). Technical support was provided by Jerzy Lodziak, Peter Rendschmidt and Don Henry (BGR). Finally, thanks for helpful advice to Michael Wise (Smithsonian) and Kari Kojonen (GTK). We thank Milan Novák and an anonymous reviewer for constructive comments, and A.V. Tkachev for helpful comments. 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