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Paragenesis and precipitation stages of Nb-Ta-oxide minerals in phosphorus-rich rare-element pegmatites (Buranga dike, Rwanda)

  • Fernando Prado Araujo ORCID logo , Niels Hulsbosch and Philippe Muchez
Published/Copyright: January 29, 2023
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American Mineralogist
From the journal American Mineralogist Volume 108 Issue 2

Abstract

Nb-Ta-oxide minerals (NTO) are commonly associated with rare-element pegmatites where they are interpreted to precipitate at magmatic to magmatic-hydrothermal conditions. Although high-temperature experiments show that phosphorus and other fluxing elements (e.g., Li, B, F) can affect the saturation of NTO in pegmatitic systems, it is still uncertain how NTO saturation occurs in natural, flux-rich pegmatitic melts and whether crystallization occurs at multiple stages during magmatic or subsolidus conditions. The lithium-cesium-tantalum (LCT) family P-rich Buranga granitic pegmatite (western Rwanda) is used as a type locality to address this question. NTO mineralization in the Buranga dike occurs in two mineralogical units: in mineralogically complex phosphatic nodules, and in albitized parts. In the phosphatic nodules, Fe-Nb-Ta-rich rutile and columbite-group minerals (CGM) are observed, while in the albitized parts, only CGM is found. Fe-Nb-Ta-rich rutile precipitates at the magmatic stage along with early primary phosphates (i.e., F-rich montebrasite, wyllieite, and fluorapatite). Conversely, CGM mineralization occurs at the magmatic-hydrothermal stage in association with replacement phosphates like bertossaite, after primary minerals (i.e., rutile, wyllieite, rosemaryite, and trolleite) are destabilized due to crystal-melt-fluid interactions. NTO textures and chemical zoning show uneven evolution from core to rim and are related to localized alteration phenomena of the surrounding minerals. This indicates that local processes and element transfers are more important than dike-scale fractionation processes for NTO mineralization in P-rich granitic pegmatites. The restricted availability of Fe and Mn in the system, which is related to the competition between phosphate and oxide minerals, is identified as the main control on the CGM mineralization in the Buranga dike. CGM precipitation is only possible during the magmatic-hydrothermal stage when Fe and Mn are leached from the primary phosphates and remobilized to the Nb-Ta-bearing residual melt by the exsolved fluid.

Keywords: Nb-Ta mineralization; granitic pegmatite; pegmatitic phosphates; mineral paragenesis; Raman mapping; Buranga pegmatite

Funding statement: Fernando Prado Araujo is funded by a Ph.D. scholarship from KU Leuven. Research of Niels Hulsbosch was supported by an FWO Junior Postdoctoral Fellowship. The Raman equipment was acquired via the medium-scale research infrastructure FWO grant Raman-SIM2 (number I000718N), and the research is financially supported by Research Grant C14/17/056 of the KU Leuven Research Fund.

Acknowledgments

The authors are grateful to Stijn Dewaele (UGent), Florias Mees (Royal Museum for Central Africa), and Frederic Hatert (ULiège) for providing the samples. Herman Nijs (KU Leuven) is thanked for preparing the high-quality thin sections. Valeria Fonseca and Marco Dalla Vecchia are thanked for helping with the python scripts. The help of Luiza Freitas in the edition of the manuscript is much appreciated. Careful reviews and constructive comments of the associate editor David Dolejš, and reviewers Pavel Uher, and Marieke Van Lichtervelde helped to improve the quality of this manuscript. The first author thanks Robert Linnen, Alysha McNeil, Olivier Namur, Anouk Borst, and Laura van der Does for the discussions about the magmatic-hydrothermal textures.

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Received: 2021-06-29
Accepted: 2022-01-13
Published Online: 2023-01-29
Published in Print: 2023-02-23

© 2023 Mineralogical Society of America

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  7. 3D zoning of barium in alkali feldspar
  8. In situ Raman vibrational spectra of siderite (FeCO3) and rhodochrosite (MnCO3) up to 47 GPa and 1100 K
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  11. A neutron diffraction study of boussingaultite, (NH4)2[Mg(H2O)6](SO4)2
  12. Zn-clays in the Kihabe and Nxuu prospects (Aha Hills, Botswana): A XRD and TEM study
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  15. American Mineralogist thanks the Reviewers for 2022
https://doi.org/10.2138/am-2022-8201
Keywords for this article
Nb-Ta mineralization; granitic pegmatite; pegmatitic phosphates; mineral paragenesis; Raman mapping; Buranga pegmatite

Articles in the same Issue

  1. Highlights and Breakthroughs
  2. Analyses under the curve, identifying how invisible gold is held in pyrite
  3. Titanite geochemistry and textures: Implications for magmatic and post-magmatic processes in the Notch Peak and Little Cottonwood granitic intrusions, Utah
  4. Gismondine-Sr, Sr4(Al8Si8O32)·9H2O, a new strontium dominant, orthorhombic zeolite of the gismondine series from the Hatrurim Complex, Israel
  5. Lifting the cloak of invisibility: Gold in pyrite from the Olympic Dam Cu-U-Au-Ag deposit, South Australia
  6. Paragenesis and precipitation stages of Nb-Ta-oxide minerals in phosphorus-rich rare-element pegmatites (Buranga dike, Rwanda)
  7. 3D zoning of barium in alkali feldspar
  8. In situ Raman vibrational spectra of siderite (FeCO3) and rhodochrosite (MnCO3) up to 47 GPa and 1100 K
  9. Isotopic responses of magnesium to two types of dissolution-reprecipitation processes for the growth of the double-carbonate mineral norsethite
  10. Fluid-rock interaction and fluid mixing in the large Furong tin deposit, South China: New insights from tourmaline and apatite chemistry and in situ B-Nd-Sr isotope composition
  11. A neutron diffraction study of boussingaultite, (NH4)2[Mg(H2O)6](SO4)2
  12. Zn-clays in the Kihabe and Nxuu prospects (Aha Hills, Botswana): A XRD and TEM study
  13. Finchite, Sr(UO2)2(V2O8)·5H2O, a new uranyl sorovanadate with the francevillite anion topology
  14. Multi-stage metasomatic Zr mineralization in the world-class Baerzhe rare earth element Nb-Zr-Be deposit, China
  15. American Mineralogist thanks the Reviewers for 2022
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