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 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.
Highlights
Carbonatites are host to some of the largest REE resources (Chakhmouradian and Wall, 2012; Wall, 2014; Verplanck et al, 2016)
Particular attention was paid to the relationship between apatite and REE phases because low-tenor HREE enrichment occurs in late-stage apatite at the Tundulu, Kangankunde and Juquiá carbonatites (Ngwenya, 1994; Wall and Mariano, 1996; Broom-Fendley et al, 2016a; Walter et al, 1995)
Petrography was carried out using cold-cathodoluminescence (CL) and scanning electron microscope (SEM) before samples were analysed for trace elements using electron probe micro-analyser (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)
Summary
Carbonatites (igneous rocks containing N50% carbonate minerals; Le Maître et al, 2002) are host to some of the largest REE resources (Chakhmouradian and Wall, 2012; Wall, 2014; Verplanck et al, 2016). A few examples of carbonatite-related HREE enrichment are known (e.g. Kangankunde, Malawi; Khibiny, Russia; Deadhorse Creek, Canada; Huanglongpu and Huayangchuan, China; Lofdal, Namibia; Pivot Creek, New Zealand; and Bear Lodge, USA; Wall and Mariano, 1996; Zaitsev et al, 1998; Potter and Mitchell, 2005; Xu et al, 2007; Wall et al, 2008; Kynicky et al, 2012; Cooper et al, 2015; Andersen et al, 2016). With the exception of Lofdal, these are typically minor occurrences forming in late-stage fluid-rich environments. Particular attention was paid to the relationship between apatite and REE phases because low-tenor HREE enrichment occurs in late-stage apatite at the Tundulu, Kangankunde and Juquiá carbonatites (Ngwenya, 1994; Wall and Mariano, 1996; Broom-Fendley et al, 2016a; Walter et al, 1995)
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