Pyrochlore Chemistry from the Bonga Carbonatite‐type Nb Deposit, Huila Province, Angola: Implications for Magmatic‐Hydrothermal Processes of Carbonatite.

Luo, Zheng; Gu, Xiaohong; Zhang, Yongmei

doi:10.1111/1755-6724.12373_34

Cited by 5 publications

(7 citation statements)

References 5 publications

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“…Up to five generations have been identified in the ICML domains of carbonatite groundmass (Figures 7 and 8). Pyrochlore types present compositions which may differ from the generations previously defined in other studies of the Bonga carbonatites carried out on the plug or the concentric dykes [67,68,88]. Nb also accumulates inside perovskite, baddeleyite, zircon, ilmenorutile and appear rarely concentrated in the phlogopite crystals from the phlogopite-rich xenoliths.…”

Section: Mineral Chemistry Of Nb Minerals Of the Imcl Bodiesmentioning

confidence: 55%

“…Apatite, pyrochlore, magnetite, phlogopite and amphibole are characteristic minerals of carbonatites [7]. Indeed, they appear in most of the studied Angolan carbonatites such as Tchivira [66,68], Bailundo [101], Virulundo [71], Bonga [67,68,70,76,88] and Monte Verde [69,70] but pyrochlore is irregularly distributed and has different compositions. In the Angolan carbonatite complexes, the highest pyrochlore concentrations usually occur in the calciocarbonatite concentric dykes (pyrochlore 0 with high F and Na) and magnetite-apatite phoscoritic rocks (pyrochlore I with low F and high Na), as well as in the calciocarbonatite groundmass of the external IMCL bodies of Bonga (pyrochlore I with low F and high Na), as observed in this work, instead of being enriched in the central calciocarbonatite plugs.…”

Section: Pyrochlore As Indicator Of the Dynamics Of The Carbonatite Cmentioning

confidence: 99%

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Lamprophyre-Carbonatite Magma Mingling and Subsolidus Processes as Key Controls on Critical Element Concentration in Carbonatites—The Bonga Complex (Angola)

et al. 2019

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The Bonga complex is composed of a central carbonatite plug (with a ferrocarbonatite core) surrounded by carbonatite cone sheets and igneous breccias of carbonatitic, fenitic, phoscoritic and lamprophyric xenoliths set in a carbonatitic, lamprophyric or mingled mesostase. To reconstruct the dynamics of the complex, the pyrochlore composition and distribution have been used as a proxy of magmatic-hydrothermal evolution of the complex. An early Na-, F-rich pyrochlore is disseminated throughout the carbonatite plug and in some concentric dykes. Crystal accumulation led to enrichment of pyrochlore crystals in the plug margins, phoscoritic units producing high-grade concentric dykes. Degassing of the carbonatite magma and fenitization reduced F and Na activity, leading to the crystallization of magmatic Na-, F- poor pyrochlore but progressively enriched in LILE and HFSE. Mingling of lamprophyric and carbonatite magmas produced explosive processes and the formation of carbonatite breccia. Pyrochlore is the main Nb carrier in mingled carbonatites and phoscorites, whereas Nb is concentrated in perovskite within mingled lamprophyres. During subsolidus processes, hydrothermal fluids produced dolomitization, ankeritization and silicification. At least three pyrochlore generations are associated with late processes, progressively enriched in HFSE, LILE and REE. In the lamprophyric units, perovskite is replaced by secondary Nb-rich perovskite and Nb-rich rutile. REE-bearing carbonates and phosphates formed only in subsolidus stages, along with late quartz; they may have been deposited due to the release of the REE from magmatic carbonates during the hydrothermal processes.

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Section: Mineral Chemistry Of Nb Minerals Of the Imcl Bodiesmentioning

confidence: 55%

Section: Pyrochlore As Indicator Of the Dynamics Of The Carbonatite Cmentioning

confidence: 99%

Lamprophyre-Carbonatite Magma Mingling and Subsolidus Processes as Key Controls on Critical Element Concentration in Carbonatites—The Bonga Complex (Angola)

et al. 2019

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“…Na and Ca commonly decrease in these pyrochlores, and thus vacancies, Sr, Ba, U, Th, and REE increase to balance charges. This evolution trend observed in all Angolan carbonatites [25][26][27][28][29][30][31]33]. Na decrease can be related to the decrease of Na activity in the magma, which can be associated with the loss of alkalis during fenitization processes [29].…”

Section: Pyrochlore Evolution In Monte Verdementioning

confidence: 95%

“…No intrusive carbonatite bodies occur, and only a few small outcrops of carbonatite breccia have been distinguished. However, carbonatitic rocks are very common in most of the other sub-volcanic Angolan complexes, such as Tchivira, Bonga, Virulundo, or Bailundo [26,27,31,32].…”

Section: Monte Verde Carbonatite Structurementioning

confidence: 99%

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Nb and REE Distribution in the Monte Verde Carbonatite–Alkaline–Agpaitic Complex (Angola)

et al. 2019

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The Angolan alkaline–carbonatite complex of Monte Verde has a semi-circular shape and is comprised of a central intrusion of foidolite rocks surrounded by concentrically arranged minor bodies of other alkaline rocks and carbonatite magmatic breccias. This rock association is hosted by fenitized Eburnean granites. Concentric swarms of alkaline dykes of late formation, mostly of nepheline trachyte composition, crosscut the previous units. Most high-field strength elements (HFSE) and rare earth elements (REE) are concentrated in pyrochlore crystals in the carbonatite and alkaline breccias. Magmatic fluornatropyrochlore is replaced and overgrown by five secondary generations of pyrochlore formed during subsolidus stages and have higher Th, REE, Si, U, Sr, Ba, Zr, and Ti contents. The second, third, and fourth pyrochlore generations are associated with late fluids also producing quartz and REE rich minerals; whereas fifth and sixth pyrochlore generations are linked to the fenitization process. On the other hand, minerals of the rinkite, rosenbuschite, wöhlerite, eudialyte groups, as well as loparite-(Ce), occur in accessory amounts in nepheline trachyte, recording low to moderate agpaicity. In addition, minor REE-bearing carbonates, silicates, and phosphates crystallize as late minor secondary minerals into carbonatite breccia and alkaline dykes. In conclusion, the scarcity of HFSE and REE minerals at the Monte Verde carbonatite-alkaline-agpaitic complex suggests low metallogenetic interest and economic potential for the outcrops analysed in this study. However, the potential for buried resources should not be neglected.

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Silicate-Carbonate Liquid Immiscibility: Insights from the Crevier Alkaline Intrusion (Quebec)

Groulier

André-Mayer

Ohnenstetter

et al. 2020

Journal of Petrology

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This contribution explores the petrogenetic relationships between silicate and carbonatitic rocks in the Crevier Alkaline Intrusion (CAI, Québec, Canada). The CAI is located in the Proterozoic Grenville Province and is composed of a suite of undersaturated peralkaline rocks from ijolite to nepheline syenite and carbonatites. Petrogenetic relationships between different undersaturated alkaline igneous rocks, carbonate-bearing and carbonate-free nepheline syenite and carbonatites observed in the CAI suggest that (i) carbonate-bearing and carbonate-free silicate rocks are comagmatic with carbonatite, and that (ii) both silicate and carbonatitic liquids are fractionated from an ijolitic parental magma that has undergone liquid immiscibility. One of the observed facies is characterized by spectacular ocelli of carbonate-bearing nepheline syenite in a matrix of carbonatite. The younger nepheline syenite facies can be divided into two groups based on the presence or absence of magmatic carbonates. Both groups are characterized by the presence of pyrochlore-group minerals that carry the Nb-Ta mineralization. We specifically use accessory minerals such as zircon, pyrochlore and apatite to constrain the temporal and physicochemical parameters of the immiscibility process. By coupling (i) mineral textures, (ii) trace elements, (iii) Ti-in-zircon thermometry, and (iv) oxygen isotope compositions, we have traced the crystallization of zircon before, during and after the immiscibility process. The results allowed us to constrain the minimum temperature of this process at ∼815-865 °C. In addition, two magmatic populations of pyrochlore are identified through their petrographic and geochemical characteristics within the younger nepheline syenite facies. Pyrochlore from the earlier ocelli facies of carbonate-bearing nepheline syenite follow a Nb-Ta differentiation trend, whereas pyrochlore from the younger carbonate-free nepheline syenite follow a more classical Nb-Ti trend. Following the complete immiscibility between the silicate and carbonatitic liquids, the fractionation between Nb and Ta stopped while a new generation of Nb-rich pyrochlore grew, displaying a more classical Nb-Ti fractionation trend and a more constant Nb/Ta ratio in the nepheline syenite.

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Pyrochlore Chemistry from the Bonga Carbonatite‐type Nb Deposit, Huila Province, Angola: Implications for Magmatic‐Hydrothermal Processes of Carbonatite.

Cited by 5 publications

References 5 publications

Lamprophyre-Carbonatite Magma Mingling and Subsolidus Processes as Key Controls on Critical Element Concentration in Carbonatites—The Bonga Complex (Angola)

Lamprophyre-Carbonatite Magma Mingling and Subsolidus Processes as Key Controls on Critical Element Concentration in Carbonatites—The Bonga Complex (Angola)

Nb and REE Distribution in the Monte Verde Carbonatite–Alkaline–Agpaitic Complex (Angola)

Silicate-Carbonate Liquid Immiscibility: Insights from the Crevier Alkaline Intrusion (Quebec)

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