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Fluorapatite in carbonatite-related phosphate deposits: the case of the Matongo carbonatite (Burundi)

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Abstract

The Matongo carbonatite intrusive body in the Neoproterozoic Upper Ruvubu alkaline plutonic complex (URAPC) in Burundi is overlain by an economic phosphate ore deposit that is present as breccia lenses. The ore exhibits evidence of supergene enrichment but also preserves textures related to the concentration of fluorapatite in the carbonatitic system. Magmatic fluorapatite is abundant in the ore and commonly occurs as millimeter-sized aggregates. It is enriched in light rare earth elements (LREE), which is especially apparent in the final generation of magmatic fluorapatite (up to 1.32 wt% LREE2O3). After an episode of metasomatism (fenitization), which led to the formation of K-feldspar and albite, the fluorapatite-rich rocks were partly brecciated. Oxygen and carbon isotope compositions obtained on the calcite forming the breccia matrix (δ18O = 22.1 ‰ and δ13C = −1.5 ‰) are consistent with the involvement of a fluid resulting from the mixing of magmatic-derived fluids with a metamorphic fluid originating from the country rocks. In a subsequent postmagmatic event, the carbonates hosting fluorapatite were dissolved, leading to intense brecciation of the fluorapatite-rich rocks. Secondary carbonate-fluorapatite (less enriched in LREE with 0.07–0.24 wt% LREE2O3 but locally associated with monazite) and coeval siderite constitute the matrix of these breccias. Siderite has δ18O values between 25.4 and 27.7 ‰ and very low δ13C values (from −12.4 to −9.2 ‰), which are consistent with the contribution of organic-derived low δ13C carbon from groundwater. These signatures emphasize supergene alteration. Finally, the remaining voids were filled with a LREE-poor fibrous fluorapatite (0.01 wt% LREE2O3), forming hardened phosphorite, still under supergene conditions. Pyrochlore and vanadiferous magnetite are other minerals accumulated in the eluvial horizons. As a consequence of the supergene processes and fluorapatite accumulation, the phosphate ore, which contains 0.72 to 38.01 wt% P2O5, is also enriched in LREE (LaN/YbN from 47.1 to 83.5; ΣREE between 165 and 5486 ppm), Nb (up to 656 ppm), and V (up to 1232 ppm). In the case of phosphate exploitation at Matongo, REE could prove to have a subeconomic potential to be exploited as by-products of phosphates.

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Acknowledgments

The authors would like to warmly thank the MRAC for providing the study samples. Laurence Monin (MRAC) is thanked for her role in providing whole-rock analyses. Rénovat Nyandwi is thanked for his help and the preparation of the polished section. Chris Harris and an anonymous reviewer are sincerely thanked for their help to improve the quality of the manuscript. The authors are also grateful to Hartwig Frimmel and Bernd Lehmann for the editorial handling of this paper. Thomas Goovaerts is thanked for the English review of the manuscript.

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Authors and Affiliations

  1. Royal Belgian Institute of Natural Sciences, Brussels, Belgium

    Sophie Decrée

  2. Royal Museum for Central Africa, Tervuren, Belgium

    Sophie Decrée, Luc Tack & Luc André

  3. Géosciences Rennes, UMR 6118, Université de Rennes 1, Rennes, France

    Philippe Boulvais

  4. University of Mons, Mons, Belgium

    Jean-Marc Baele

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  1. Sophie Decrée
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Correspondence to Sophie Decrée.

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Editorial handling: Lawrence P. Rozas

Electronic supplementary material

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Electronic supplementary material 1

Representative cathodoluminescence spectra of minerals in the Matongo phosphatic ore (operating conditions: 15 kV–500 μA, defocussed beam of approx. 4 mm diameter, spectra not corrected for system response). Upper spectra: Mn2+-activated calcite and Fe3+-activated albite and K-feldspar. The observed luminescence color is orange, red and pink, respectively. Lower spectra: Mn2+- and REE-activated primary fluorapatite (dominant blue-CL color) and Mn2+-activated secondary fluorapatite (green CL). The spectrum for the primary fluorapatite is representative for an average of its emission (blue-green and red-CL core and blue-violet rim). (JPEG 639 kb)

Electronic supplementary material 2

Raman spectra obtained on magmatic fluorapatites (a) and hydrothermal-supergene fluorapatites (b) from the Matongo phosphate deposit. Acquired using λ = 532 nn, power: 2 mW, acquisition time: 5 × 30 s, spectral range: 0–4500 cm−1. Peaks that are not directly related to the fluorapatite structure are present on the Raman spectra at the following frequencies (presented by decreasing intensities): ~2080 cm−1, ~3290 cm−1, in the 2500–2700 cm−1 range, in the 1750–1820 cm−1, and in the 1480–1530 cm−1 range. When the Raman shift is converted to wavelength (in nm), these unresolved peaks correspond fairly well to the REE emission lines. The most important are those of Sm3+ (at ~599, 644 and 652 nm), others would correspond to Eu3+ (multiple peaks in the 614–618 nm range), Tb3+ (one or two peaks at 587–588 nm), and Dy3+ (multiple peaks in the 577–579 m range). These wavelengths, which are typical of the fluorescence induced by the REE, can be related to the presence of these elements in the studied fluorapatite, what has been confirmed by in situ analyses on magmatic fluorapatites (Table 1) (PDF 2611 kb)

Table 1

Representative microprobe analyses (oxides in wt%) of apatite and monazite from the Matongo phosphate ore (XLSX 16 kb)

Table 2

Major element contents (in wt%), REE and trace elements (in ppm) of samples from the Matongo phosphate ore (XLSX 18 kb)

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Decrée, S., Boulvais, P., Tack, L. et al. Fluorapatite in carbonatite-related phosphate deposits: the case of the Matongo carbonatite (Burundi). Miner Deposita 51, 453–466 (2016). https://doi.org/10.1007/s00126-015-0620-1

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