Angola): Implications of Nb Geochemistry in Metallogenesis

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Versão online: https://www.lneg.pt/wp-content/uploads/2020/05/Volume_107_CIG.pdf Comunicações Geológicas (2020) 107, Especial II, 75-80 ISSN: 0873-948X; e-ISSN: 1647-581X

Nb-bearing mineral phases in the Bailundo Carbonatite Complex (Angola): implications of Nb geochemistry in metallogenesis

Fases minerais portadoras de Nb no Complexo Carbonatítico do Bailundo (Angola): implicações da geoquímica do Nb em metalogenia

J. Roseiro 1,2*, I. Ribeiro da Costa 1, J. Figueiras 1,3, P. Rodrigues 1,3, A. Mateus 1,3

3 Abstract: Pyrochlore group minerals are common accessory phases in Instituto Dom Luiz (IDL), Faculdade de Ciências, Universidade de Lisboa, Ed. C1, Campo Grande, 1749-016 Lisboa, Portugal. many rock types of the Bailundo Carbonatite Complex. These minerals *Corresponding author/Autor correspondente: [email protected] record compositional and textural features that provide useful information regarding their genesis and accumulation, monitoring magmatic, metasomatic and weathering events. In drill core samples, primary compositions (significant Ta and U contents, and relatively low Nb and F 1. Introduction values) are found in relict cores of strongly metasomatized pyrochlore Pyrochlore group minerals and their weathered products are the grains; irregular patches in pyrochlore rims, typically enriched in F, Na and Nb, reflect fluid alteration fronts. At shallower levels, preserved main economic Nb-bearing mineral phases, most of them pyrochlores show well-defined concentric zoning and substantially higher occurring in alkaline-carbonatite complexes. Current Nb-mining values of F and Nb. In the weathering profile, alteration processes include activity is restricted to just three active mines: Araxá and replacement of F, Na and Ca by Ba, Sr, Pb and H2O. These data suggest Catalão-I, in Brazil (> 90% of global production; e.g. Gunn, the possibility of Nb concentration in late-magmatic fluids as fluoride 2014) followed by St. Honoré, in Canadá (e.g. Mitchell, 2015). complexes, and its subsequent mobilization and crystallization in the Moreover, the considerable risk of supply disruption has led the form of pyrochlore at shallower levels of the Bailundo Carbonatite European Union to consider Nb a critical raw material for its Complex. economy. Keywords: Pyrochlore Group Minerals, Bailundo Carbonatite Complex, Niobium-bearing alkaline-carbonatite melts are usually Nb-mineralization. thought to derive from multi-stage processes of low-degree partial melting of carbonated metasomatized mantle rocks Resumo: Os minerais do grupo do pirocloro são fases acessórias comuns (previously enriched in volatiles, Nb, REE and other nos diferentes litótipos do Complexo Carbonatítico do Bailundo. Estes incompatible elements), followed by liquid immiscibility or minerais registam particularidades composicionais e texturais que fractional crystallization mechanisms, segregating carbonatite proporcionam informações úteis sobre a sua génese e acumulação, melts from the residual silicate fraction (e.g. Woolley, 1982; monitorizando eventos magmáticos, hidrotermais e meteóricos. Em Panina and Motorina, 2008; Mitchell, 2015). The generation of amostras de sondagem, as composições primárias (com teores consideráveis em Ta e U, e teores relativamente baixos de Nb e F) these melts and subsequent emplacement of their rock derivatives ocorrem como relíquias em núcleos de grãos de pirocloro fortemente are mostly associated with intra-cratonic rifting environments. metassomatizados; domínios periféricos irregulares, enriquecidos em F, Late hydrothermal processes (such as fenitization), as well as Na e Nb, refletem frentes de alteração. Nos níveis mais superficiais, os weathering events, are known to enhance Nb concentration in pirocloros bem preservados apresentam zonamento concêntrico evidente Nb-bearing minerals (e.g. Lapido Loureiro, 1995; Melgarejo et e teores bastante elevados de Nb e F. No perfil de meteorização, os al., 2012; Mitchell, 2015; Elliot et al., 2018). Two main Nb processos de alteração consistem na substituição de F, Na e Ca por Ba, deposit types can thus be considered: (i) primary carbonatite Sr, Pb e H2O. Estes dados sugerem a possibilidade de fluidos tardi- intrusions, such as the St. Honoré deposit; and (ii) residual magmáticos concentrarem Nb sob a forma de complexos fluoretados, e de posterior mobilização e cristalização na forma de pirocloro nos níveis deposits resulting from extensive chemical weathering of primary superiores do Complexo Carbonatítico do Bailundo. Nb-bearing carbonatite rocks, as is the case of the large-scale Palavras-chave: Minerais do grupo do pirocloro, Complexo Araxá, Catalão-I and Seis Lagos deposits (e.g. Guilbert and Park, Carbonatítico do Bailundo, Mineralização de Nb. 1989; Mitchell, 2015). Notwithstanding the irregular distribution of carbonatite complexes in the world (most of them hosting

1Dept. Geologia, Faculdade de Ciências, Universidade de Lisboa, Ed. C6, pyrochlore mineralization), the largest known Nb-bearing Campo Grande, 1749-016 Lisboa, Portugal. alkaline-carbonatite deposits are located in the Brazil-Angola 2Instituto de Ciências da Terra (ICT), Polo de Évora; CLAV, Rua Romão Carbonatite Mega-Province (e.g. Lapido Loureiro, 1995). Ramalho 59, 7000-671 Évora, Portugal (present address).

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2. Geological Setting Th), apatite (F, P), jadeite (Na), galena (Pb), almandine (Al, Fe), diopside (Ca, Si), pyrope (Mg), benitoite (Ti, Ba), sanidine (K), 2.1. Age and Geotectonic Setting bustamite (Mn), celestite (Sr), tugtupite (Cl), stibnite (S), LiNbO3 (Nb), LiTaO3 (Ta), zirconia (Y, Zr) vanadium (V), UO2 The Brasil-Angola Carbonatite Mega-Province (included in the (U), GaAs (As), hafnium (Hf); for other REE, ASTIMEX Paraná-Angola-Namibia magmatic system; e.g. Comin- REEP30-15+FC (REE phosphates), ASTIMEX REEM25-15 Chiaramoti et al., 2007) is a large geological unit defined by (REE metals) and several REE-bearing synthetic glasses were Lapido Loureiro (1995), which encompasses numerous alkaline- used. The data was fully corrected for ZAF matrix effects, and carbonatite complexes intruding the São Francisco and Congo the analytical error was < 1% for the elements analysed. Cratons. This unit is related to the large Cretaceous alkaline The ionic distribution per formula unit of pyrochlore group magmatic event that took place in both peri-Atlantic continental minerals was obtained after normalization on a basis of 2 cations margins (Peri-Atlantic Alkaline Pulse; Matton and Jébrak, 2009), in the 6-coordinated B site; OH contents were estimated by and generated some of the most important known carbonatite- charge balance over the available anionic positions, assuming full related Nb deposits (Araxá and Catalão I; Lapido Loureiro, 1995; occupancy. In this work, the Atencio et al. (2010) and Christy Gunn, 2014; Mitchell, 2015). and Atencio (2013) systematics are followed. In the Angolan segment of the Brazil-Angola Carbonatite Mega-Province, many Cretaceous carbonatite complexes occur 4. Pyrochlore in the BCC along regional NE-SW tectonic lineaments (Fig. 1A, B; Issa Filho et al., 1991; Lapido Loureiro, 1995; Alberti et al., 1999; Comin-Chiaramoti et al., 2007), and some of them are known to 4.1. Primary pyrochlore host significant amounts of pyrochlore, as is the case of the Fluor-calciopyrochlore and hydroxy-calciopyrochlore are Bailundo, Tchivira, Bonga, Longonjo, Virulundo and Catanda dominant in the fresh carbonatite and glimmerite rocks (deep carbonatites (Lapido Loureiro, 1995; Santos, 2010; Melgarejo et drill-core samples; Fig. 2E), showing similar compositional al., 2012; Bambi et al., 2012; Torró et al., 2012; Zheng et al., features in both lithotypes. Though some grains display euhedral 2014; Bambi, 2015; Roseiro, 2017; Amores-Casals et al., 2019). contours with concentric zonation (mainly in the upper levels of the intrusion), most are fractured and display effects of corrosion 2.2. The Bailundo Carbonatite Complex and irregular alteration patches, outlining the progress of metasomatic processes affecting primary grains (e.g. Figs. 2A, The Bailundo Carbonatite Complex (BCC; Fig. 1C), at ~11 km to B). In Ca-carbonatite rocks, pyrochlore grains with irregular the NE of the Bailundo village (Huambo province), consists of an textures display quite variable Nb contents (22.4 < Nb2O5 wt% < intrusive ring-shaped structure emplaced during intra-continental 62.8, median = 45.3 wt%) and F (median = 0.6 wt%, up to 3.6 rifting of the Proterozoic granitoids of the Congo Craton (Lapido wt%), and relatively high contents of Ta (median =7.3 wt% Loureiro, 1995). Ta2O5) and U (median 6.2 wt% UO2), as well as noticeable Th The BCC comprises two main outcropping carbonatite facies: and Na contents (Santos, 2010; Roseiro, 2017). The hydrothermal (i) a central Fe-carbonatite core, mainly composed of ankerite alteration patches (e.g. Figs. 2A, B) show higher F and OH and siderite, with accessory iron oxides(-hydroxides), apatite, contents, whereas relict primary domains (that could be resorbed pyrochlore, barite and REE-phosphates, and (ii) a peripheral Ca- cores) tend to display higher Ta and U contents. Intergrowths carbonatite aureole, mostly formed by calcite and, to a lesser between Ta-U-rich pyrochlore and other oxides (e.g. baddeleyite, extent, ankerite and dolomite, with accessory iron oxides (mainly Fig. 2C) also occur in deep-seated lithotypes. magnetite), apatite, pyrochlore, barite and REE-phosphates. At Pyrochlore grains from samples of shallower levels of the depth (Fig. 1D), the Ca-carbonatite facies may include other intrusion are concentrically zoned (Fig. 2D) and show accessory oxide mineral phases (baddeleyite and other titanates significantly higher Nb (median = 57.8 wt%, up to 66.9 wt% and zirconates), as well as REE-fluorcarbonates (Beleque, 2010; Nb2O5; Nb increasing from core to rim) and F (median 2.37 wt%, Santos, 2010; Pereira, 2011). Occasional deep-seated glimmerites up to 3.8 wt%), and lower Ta (median = 1.1 wt% Ta2O5) and U occur as vein-like structures, composed of tetraferriphlogopite (median 0.23 wt% UO2). Vacancies in the 8-coordinated cationic with accessory apatite, pyroxene, amphibole and pyrochlore. The A site (Avac) do not show significant increase from fresh to BCC is surrounded by an asymmetrical fenite aureole, developed metasomatized pyrochlore grains in carbonatite. by intrusion-related pervasive metasomatism of the country Pyrochlore in the glimmerite tends to show comparable rocks. These fenites contain K-feldspar, albite, aegirine-augite contents of Nb (39.5 < Nb2O5 wt% < 66.8, median = 46.7 wt%), and richterite, with lesser magnetite and pyrochlore (Santos, Ta (median 6.5 wt% Ta2O5) and U (< 3.4 wt% UO2). 2010; Roseiro, 2017). Colluvial-elluvial deposits derived from intense chemical weathering of the BCC partially cover the outcropping rocks. 4.2. Surface pyrochlore In the BBC weathering profile (outcrop samples), altered 3. Materials and Methods hydroxy-calciopyrochlore is the main Nb-bearing phase, followed by hydroxy-bariopyrochlore (Fig. 2C). The mineral Both outcrop (n = 81) and drill-core (n = 37) samples from the grains are usually very fractured and display some particular main BCC lithotypes were provided by Genius Mineira Ldt. A features, such as bleached or corroded patches along large set of pyrochlore grains was identified and selected for microfractures or around their rims (Fig. 3A); in some cases, detailed textural study and for chemical analysis with a JEOL- relicts of previous pyrochlore phases are observed (Figs. 3A, B), JXA8200 electron microprobe at the University of Lisbon. displaying chemical features similar to concentrically zoned Quantitative analyses were obtained with an accelerating voltage pyrochlores from shallower levels of the drill-core. of 15 kV, counting times of 20s and a beam current of 10 nA Pyrochlore grains of the Fe-carbonatite core often show Fe- (with 5 µm diameter). Standards used were: monazite (Ce, La, Pr, oxides sealing fractures and cracks (e.g. Fig. 3A).

Pyrochlore from the Bailundo Carbonatite Complex 77

Figure 1. (A) Location and main tectonic lineament of the Angolan sector of the Brazil-Angola Carbonatite Mega-Province (adapted from Issa Filho et al., 1991); (B) geographic distribution of the main carbonatite-alkaline complexes in Angola, adapted from Issa Filho et al. (1991); (C) simplified geological map of the Bailundo Carbonatite Complex, adapted from Lapido Loureiro (1995); (D) schematic representation of the borehole drilled by Genius Mineira Ltd; (E) schematic cross-section of the Bailundo Carbonatite Complex, adapted from Lapido Loureiro, 1995). Figura 1. (A) Localização e principais lineamentos tectónicos do sector angolano da Mega-Província Carbonatítica Brazil-Angola (adaptada de Issa Filho et al., 1991); (B) distribuição Geográfica dos principais complexos alcalino-carbonatíticos em Angola, adaptado de Issa Filho et al. (1991); (C) mapa geológico simplificado do Complexo Carbonatítico do Bailundo, adaptado de Lapido Loureiro (1995); (D) representação esquemática do testemunho de sondagem disponibilizado pela Genius Mineira Ltd; (E) Perfil esquemático Complexo Carbonatítico do Bailundo, adaptado de Lapido Loureiro (1995). 78 Roseiro et al. / Comunicações Geológicas (2020) 107, Especial II, 75-80

The most significant compositional features shown by only minor corrosion marks are visible (Fig. 2D). Moreover, Ta pyrochlore from the variably altered outcropping carbonatites are and U depletion and increased Nb and F contents suggest a late- due to extreme depletion of most major components (Ta, Ti, Zr, magmatic origin for these pyrochlore grains, because early Ta-U U, Th, F, Ca and Na), and relative concentration of Nb (median = joint fractionation from melt to rock, and Nb-Ta decoupling 66.3 wt%, up to 72.9 wt% Nb2O5), Pb and, to a lesser extent, Sr during carbonatite-silicate melt immiscibility processes and/or and REE. carbonatite metasomatism (reflecting high Nb/Ta ratios; Fig. 4A) are common in these environments (e.g. Green et al., 1992; Brod et al., 2013; Kynicky et al., 2018; Walter et al., 2018; Amores- Casals et al., 2019).

Figure 3. Back-scattered electron images of pyrochlore grains from surface (outcrop) samples. (A) strongly fractured and weathered grain with intense Ba- and Pb-enriched bleached/corroded patches along micro-fractures; (B) extremely weathered pyrochlore grain with preserved patches of the previous pyrochlore phase; (C) key ions in the 6- coordinated B-site (Nb-Ta-Ti), in the anionic site (O-F-OH), and in the 8-coordinated A-site (Ca-Na-cationic vacancies), in pyrochlore from surface samples, following the nomenclature system proposed by Atencio et al. (2010) and Christy and Atencio (2013). Figure 2. Back-scattered electron images of pyrochlore grains from drill-core samples. Figura 3. Imagens de electrões retrodifundidos de pirocloros de amostras superficiais (A) partly metasomatized pyrochlore with primary relict core; (B) strong metasomatic (aflorantes). (A) grão fortemente fracturado e meteorizado com marcas de overprint in primary pyrochlore; (C) intergrowth of primary pyrochlore and lixiviação/corrosão, enriquecidas em Ba e Pb, ao longo de microfracturas; (B) grão de baddeleyite; (D) less fractured and corroded concentrically zoned pyrochlore grain pirocloro extremamente meteorizado com preservação da fase anterior; (C) iões from shallower levels of the intrusion; (E) key ions in the 6-coordinated B-site (Nb-Ta- principais na posição B, de coordenação 6 (Nb-Ta-Ti), na posição aniónica (O-F-OH) e Ti), in the anionic site (O-F-OH), and in the 8-coordinated A-site (Ca-Na-vacancies), in na posição A, de coordenação 8 (Ca-Na-lacunas) dos pirocloros superficiais, de acordo pyrochlore from drill-core samples, following the nomenclature system proposed by com o sistema de nomenclatura proposto por Atencio et al. (2010) e Christy e Atencio Atencio et al. (2010) and Christy and Atencio (2013). (2013). Figura 2. Imagens de electrões retrodifundidos de pirocloros do testemunho de sondagem. (A) pirocloro parcialmente metassomatizado com núcleo com composição In surface rocks, weathering effects are represented by primária; (B) forte carácter metassomático sobreposto num pirocloro primário; (C) extreme F depletion, along with hydration, and Ca and Na intercrescimento de um cristal de pirocloro primário com um cristal de badeleiíte; (D) replacement by components presumably leached from weathered grão de pirocloro zonado concentricamente, ligeiramente fracturado e corroído, dos níveis mais superficiais da intrusão; (E) iões principais na posição de coordenação B, 6 carbonatite phases and host-rocks, namely Ba, Pb and Sr (Santos, (Nb-Ta-Ti), na posição aniónica (O-F-OH) e na posição A, de coordenação 8 (Ca-Na- 2010; Roseiro, 2017). No significant differences regarding Nb, lacunas) dos pirocloros do testemunho de sondagem, de acordo com o sistema de Ta and U were observed between weathered pyrochlore and Ta- nomenclatura proposto por Atencio et al. (2010) e Christy e Atencio (2013). U-depleted late-magmatic pyrochlore (Figs. 4A, B). Though some authors report Nb mobility during weathering (e.g. 5. Discussion Melgarejo et al., 2012), no evidence for such process was observed in this study. 5.1. Monitoring magmatic, hydrothermal and weathering events 5.2. Nb metallogenesis in the BCC Although most BCC deep-seated pyrochlore show evidence of The late-magmatic and weathered pyrochlore of BCC shows strong metasomatic overprint, and few primary crystals were significant Nb enrichment when compared with pyrochlore from observed, some primary features can be inferred from the deep-seated carbonatite and glimmerite samples, whereas other available data set for early pyrochlore in both carbonatite and incompatible metals (e.g. Ta, U) are nearly absent. Several glimmerite rocks: high Na, Ca, Ta and U and relatively low Nb studies (e.g. Bambi et al., 2012; Mitchell, 2015; Timofeev et al., and F contents (e.g. Fig. 4B). At shallower levels of the drill- 2015, Elliot et al., 2018; Sokól, 2018) propose that late-magmatic core, concentrically zoned euhedral pyrochlore grains may have solubilization and mobility of Nb towards the upper levels of been formed in a later stage, as grains are well preserved and carbonatite intrusions might be enhanced by Nb-bonding with F- Pyrochlore from the Bailundo Carbonatite Complex 79 and OH- ligands present in late-magmatic and hydrothermal mechanism for Nb-bearing minerals, promoting the development (fenitizing) fluids, precipitating when they interact with of secondary Nb deposits, as previously observed by Lapido carbonate rocks or are mixed with meteoric-derived fluids, Loureiro (1995). achieving neutral pH (e.g. Timofeev et al., 2015). In the BCC, both magmatic and hydrothermal fluids may have been enriched 6. Conclusion and final remarks in F-, as suggested by the presence of F-rich minerals such as fluorcarbonates and F-rich pyrochlores (e.g. Santos, 2010; The compositional variability displayed by pyrochlore in the Roseiro, 2017). Such high HF activity in the melt might have Bailundo Carbonatite Complex strongly suggests that late- favoured Nb-solubilization and mobilization to shallower levels magmatic, hydrothermal and weathering events were responsible of the intrusion during late-magmatic stages. for the accumulation of Nb-rich pyrochlore in shallower levels of this intrusion and for late relative Nb-enrichment in the pyrochlore lattice. Few primary pyrochlore grains were found in the sampled rock types. Pyrochlore with typical primary composition show strong hydrothermal overprint: Ta- and U-rich relict cores were detected in some deeply metasomatized grains. Residual Nb- and F-enriched fluids from the latest stages of the intrusion might have been responsible for the alteration of the deep-seated rocks (possible carbonatite autometasomatism), as well as for Nb mobilization to shallower levels, later incorporated in the Nb-rich pyrochlore grains. In intensely weathered rocks of the BCC, pyrochlore was affected by leaching and hydration, and some of its constituents (mostly Ca and Na) were replaced by Ba, Pb and Sr. The evolving path of BCC pyrochlore corroborates chemical trends reported in previous studies worldwide, its compositional features being consistent with common alkaline-carbonatite systems and with the factors involved in related Nb metallogenetic processes.

Acknowledgments The authors are grateful for the large set of samples provided by Genius Mineira Ltd., which were the basis for the present work. The authors also thank Alberto Verde (LABLAPOL, Dept. Geologia, Faculdade de Ciências, Universidade de Lisboa), for preparing all thin-sections used in this study. FCT funding, through project FCT/UID/GEO/50019/2019 – IDL, supported part of this work.

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