Rare-earth mobility as a result of multiple phases of fluid activity in fenite around the Chilwa Island Carbonatite, Malawi

Abstract

Carbonatites are enriched in critical raw materials such as the rare-earth elements (REE), niobium, fluorspar and phosphate. A better understanding of their fluid regimes will improve our knowledge of how to target and exploit economic deposits. This study shows that multiple fluid phases penetrated the surrounding fenite aureole during carbonatite emplacement at Chilwa Island, Malawi. The first alkaline fluids formed the main fenite assemblage and later microscopic vein networks contain the minerals of potential economic interest such as pyrochlore in high-grade fenite and rare-earth minerals throughout the aureole. Seventeen samples of fenite rock from the metasomatic aureole around the Chilwa Island carbonatite complex were chosen for study. In addition to the main fenite assemblage of feldspar and aegirine ± arfvedsonite, riebeckite and richterite, the fenite contains micro-mineral assemblages including apatite, ilmenite, rutile, magnetite, zircon, rare-earth minerals and pyrochlore in vein networks. Petrography using a scanning electron microscope in energy-dispersive spectroscopy mode showed that the rare-earth minerals (monazite, bastnäsite and parisite) formed later than the fenite feldspar, aegirine and apatite and provide evidence ofREEmobility into all grades of fenite. Fenite apatite has a distinct negative Eu anomaly (determined by laser ablation inductively coupled plasma mass spectrometry) that is rare in carbonatite-associated rocks and interpreted as related to pre-crystallization of plagioclase and co-crystallization with K-feldspar in the fenite. The fenite minerals have consistently higher midREE/lightREEratios (La/Sm ≈ 1.3 monazite, ≈ 1.9 bastnäsite, ≈ 1.2 parisite) than their counterparts in the carbonatites (La/Sm ≈ 2.5 monazite, ≈ 4.2 bastnäsite, ≈ 3.4 parisite). Quartz in the low- and medium-grade fenite hosts fluid inclusions, typically a few micrometres in diameter, secondary and extremely heterogeneous. Single phase, 2- and 3-phase, single solid and multi solid-bearing examples are present, with 2-phase the most abundant. Calcite, nahcolite, burbankite and baryte were found in the inclusions. Decrepitation of inclusions occurred at ∼200°C before homogenization but melting-temperature data indicate that the inclusions contain relatively pure CO2. A minimum salinity of ∼24 wt.% NaCl equivalent was determined. Among the trace elements in whole-rock analyses, enrichment in Ba, Mo, Nb, Pb, Sr, Th and Y and depletion in Co, Hf and V are common to carbonatite and fenite but enrichment in carbonatitic type elements (Ba, Nb, Sr, Th, YandREE) generally increases towards the inner parts of the aureole. A schematic model contains multiple fluid events, related to first and second boiling of the magma, accompanying intrusion of the carbonatites at Chilwa Island, each contributing to the mineralogy and chemistry of the fenite. The presence of distinct rare-earth mineral microassemblages in fenite at some distance from carbonatite could be developed as an exploration indicator ofREEenrichment.

Fenites associated with carbonatite complexes: A review

“…Calcite carbonatite ring dykes on Chilwa Island were depleted in HREE relative to high grade fenites and breccias which have higher HREE:LREE ratios. This fractionation most likely resulted from F − in late-stage fenitizing fluids preferentially mobilizing HREE, as evidenced by fluorite, quartz and barite veining (Dowman et al, 2017). Similar trends have been observed at Fen and Alnö, both of which display increasing REE fractionation with decreasing intensity of fenitization (Morogan, 1989;Verschure and Maijer, 2005).…”

“…There is a distinct lack of emphasis in the literature regarding fenites, with only a few recent publications appreciating the importance of fenitizing fluids with regard to element mobility within the system (e.g. Arzamastev et al, 2011;Al Ani and Sarapää, 2013;Trofanenko et al, 2014;Dowman et al, 2017). As such, our comprehension of the relationship between intrusions, fluids and country rock is relatively restricted.…”

“…Here they precipitate to form mineral assemblages of REE, Nb and fenite minerals (see Fig. 7E) (Hogarth, 2016;Bodeving et al, 2017;Dowman et al, 2017).…”

“…These were originally constituents of the magma that are not retained in the carbonatite rock, therefore descriptions of intrusions are incomplete without also characterizing their associated fenite. Fenitizing fluids emanating from evolved and subsequently enriched magmas, form REE and Nb micro-mineral assemblages in the country rock (Elliott et al, 2016;Dowman et al, 2017). These can provide information regarding their source magma and any associated mineral deposits, effectively acting as exploration indicators.…”

Ti-Nb Mineralization of Late Carbonatites and Role of Fluids in Its Formation: Petyayan-Vara Rare-Earth Carbonatites (Vuoriyarvi Massif, Russia)

“…The pyrochlores reflect the general pattern of imposed metasomatic processes in such areas, including the Petyayan-Vara ore field rocks. Direct transitions to Pb-pyrochlore are only present in fenite around the Chilwa Island Carbonatite, Malawi [64]. An association of Pb-pyroclore and later monazite is typical in both cases, as well as the mentioned fenites.…”