REE minerals at the Songwe Hill carbonatite, Malawi: HREE-enrichment in late-stage apatite
Broom-Fendley, S; Brady, AE; Wall, F; et al.Gunn, G; Dawes, W
Date: 1 October 2016
Journal
Ore Geology Reviews
Publisher
Elsevier
Publisher DOI
Abstract
Compared to all published data from carbonatites and granitoids, the fluorapatite compositions in the Songwe
Hill carbonatite, determined by EPMA and LA ICP-MS, have the highest heavy (H)REE concentration of any
carbonatite apatite described so far. A combination of this fluorapatite and the REE fluorocarbonates,
synchysite-(Ce) and ...
Compared to all published data from carbonatites and granitoids, the fluorapatite compositions in the Songwe
Hill carbonatite, determined by EPMA and LA ICP-MS, have the highest heavy (H)REE concentration of any
carbonatite apatite described so far. A combination of this fluorapatite and the REE fluorocarbonates,
synchysite-(Ce) and parisite-(Ce), which are the other principal REE bearing minerals at Songwe, gives a REE deposit
with a high proportion of Nd and a higher proportion of HREE (Eu–Lu including Y) than most other
carbonatites. Since Nd and HREE are currently the most sought REE for commercial applications, the conditions
that give rise to this REE profile are particularly important to understand. Multiple apatite crystallisation stages
have been differentiated texturally and geochemically at Songwe and fluorapatite is divided into five different
types (Ap-0–4). While Ap-0 and Ap-1 are typical of apatite found in fenite and calcite-carbonatite, Ap-2, -3
and -4 are texturally atypical of apatite from carbonatite and are progressively HREE-enriched in later paragenetic
stages. Ap-3 and Ap-4 exhibit anhedral, stringer-like textures and their REE distributions display an Y anomaly.
These features attest to formation in a hydrothermal environment and fluid inclusion homogenisation temperatures
indicate crystallisation occurred between 200–350 °C. Ap-3 crystallisation is succeeded by a light (L)REE
mineral assemblage of synchysite-(Ce), strontianite and baryte. Finally, late-stage Ap-4 is associated with
minor xenotime-(Y) mineralisation and HREE-enriched fluorite. Fluid inclusions in the fluorite constrain the
minimum HREE mineralisation temperature to approximately 160 °C. A model is suggested where sub-solidus,
carbonatite-derived, (carbo)-hydrothermal fluids remobilise and fractionate the REE. Chloride or fluoride complexes
retain LREE in solution while rapid precipitation of apatite, owing to its low solubility, leads to
destabilisation of HREE complexes and substitution into the apatite structure. The LREE are retained in solution,
subsequently forming synchysite-(Ce). This model will be applicable to help guide exploration in other
carbonatite complexes.
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