Fenites associated with carbonatite complexes : a review

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.Carbonatites and alkaline-silicate rocks are the most important sources of rare earth elements (REE) and niobium (Nb), both of which are metals imperative to technological advancement and associated with high risks of supply interruption. Cooling and crystallizing carbonatitic and alkaline melts expel multiple pulses of alkali-rich aqueous fluids which metasomatize the surrounding country rocks, forming fenites during a process called fenitization. These alkalis and volatiles are original constituents of the magma that are not recorded in the carbonatite rock, and therefore fenites should not be dismissed during the description of a carbonatite system. This paper reviews the existing literature, focusing on 17 worldwide carbonatite complexes whose attributes are used to discuss the main features and processes of fenitization. Although many attempts have been made in the literature to categorize and name fenites, it is recommended that the IUGS metamorphic nomenclature be used to describe predominant mineralogy and textures. Complexing anions greatly enhance the solubility of REE and Nb in these fenitizing fluids, mobilizing them into the surrounding country rock, and precipitating REE- and Nb-enriched micro-mineral assemblages. As such, fenites have significant potential to be used as an exploration tool to find mineralized intrusions in a similar way alteration patterns are used in other ore systems, such as porphyry copper deposits. Strong trends have been identified between the presence of more complex veining textures, mineralogy and brecciation in fenites with intermediate stage Nb-enriched and later stage REE enriched magmas. However, compiling this evidence has also highlighted large gaps in the literature relating to fenitization. These need to be addressed before fenite can be used as a comprehensive and effective exploration tool.This research has received funding from the European Union’s Horizon 2020 research and innovation programme under grant No 689909

Petrogenesis of the Lofdal intrusive suite: Implications for rare earth elements mineralisation

The Lofdal Intrusive Suite is currently the target of mineral exploration for its heavy rare earth element (HREE) mineralisation. As a result, there is a need to develop a model that can guide this exploration. This study examines the magmatic evolution of the igneous system, which consists of silica-undersaturated alkaline rocks (phono-tephrite, phonolite and its plutonic equivalent, nepheline syenite) and calcio-carbonatite, and develops a genetic model that includes a source for the HREE.Emplacement of the Lofdal Intrusive Suite occurred during a period of rifting prior to the Damara Orogeny, into Paleoproterozoic basement rocks of the Huab Metamorphic Complex. Trace element ratios (Y/Ho, Zr/Hf and Nb/Ta), as well as oxygen and carbon isotope ratios for calcite from the calcio-carbonatite close to mantle values suggest that the corresponding magmas had a mantle origin. The phono-tephrite is the most basic of the silicate magmas and has the most restricted range of compositions. This and the observation that Harker diagrams display linear trends from phono-tephrite to phonolite and nepheline syenite suggest that the phono-tephrite evolved to phonolite and nepheline syenite by fractional crystallisation. Modelling of this process is consistent with fractionation of plagioclase and clinopyroxene, as well as minor proportions of apatite and Ti-bearing magnetite. The fact that the nepheline syenite has a lower content of REE compared to the phonolite is likely due to fractionation of the REE into apatite, an early crystallising phase that settled under the influence of gravity to the lower, currently unexposed, part of the magma chamber. The calcio-carbonatite has chondrite-normalised REE profiles that are parallel to those of the phonolite and nepheline syenite. However, the absolute REE content of the calcio-carbonatite is higher and there is a systematic increase in the La/Lu ratio from phono-tephrite to phonolite and carbonatite. The formation of the calcio-carbonatite is therefore attributed to continued fractionation of the phono-tephrite magma after crystallisation of the most evolved phonolites. This would have driven the melt to a more Ca- and volatile-rich composition, culminating in a small residue of calcio-carbonatite liquid that produced calcio-carbonatites, which unlike most carbonatites elsewhere are unusually enriched in HREE and represent a possible source for the albitite-hosted HREE mineralisation at Lofdal.La suite intrusive de Lofdal fait présentement l'objet d' exploration minière pour sa minéralisation de terres rares lourdes. Cela crée un besoin pour le développement d'un model permettant de guider cette exploration. L'étude présentée examine l'évolution magmatique du système ignée de Lofdal, qui consiste de roches alcalines a basse saturation de silice (phono-téphrite, phonolite et son équivalent plutonique, la néphéline syénite) et de la calcio-carbonatite, et développe un modèle génétique qui suggère une source pour les terres rares lourdes.La formation de la suite intrusive de Lofdal date d'une période de rift précédant la période d'orogenèse de Damara dans les roches primitives paléoprotérozoiques du complexe métamorphique de Huab. Les ratios d'éléments traces (Y/Ho, Zr/Hf et Nb/Ta), ainsi que les ratios d'isotopes d'oxygène et de carbone dans les calcites provenant de la calcio-carbonatite, approchent des valeurs correspondents à celles du manteau. Ainsi, cela démontre que les magmas d'origine de ce système proviennent du manteau. La phono-téphrite est le plus basique des magmas siliciques et possède l'intervalle de compositions le plus restreint. De plus, l'observation des diagrammes de Harker révèle une tendance linéaire depuis la phono-téphrite jusqu'à la phonolite et la néphéline syénite et suggère que la néphéline syénite et la phonolite proviennent de la crystallisation fractionnée de la phono-téphrite. Ce processus a été modèlisé avec les fractions de plagioclase et de clinopyroxène, ainsi que des proportions mineures d'apatite et de magnétite contenant du titane. Le fait que la néphéline syénite ait une plus basse teneur en terres rares comparée à la phonolite est probablement due à la fraction des terres rares en apatite, une phase de cristallisation précoce qui se dépose, par la force de gravité, dans le bas de la chambre magmatique qui n'est pas exposée à la surface. La calcio-carbonatite présente des profils de terres rares normalisés aux chondrites parallèles à ceux de la phonolite et de la néphéline syénite. Néanmoins, la concentration absolue en terres rares de la calcio-carbonatite est plus élevée et il y a une augmentation systématique du ratio de La/Lu de la phono-téphrite vers la phonolite et la carbonatite. La formation de cette calcio-carbonatite serait donc attribuable à la fraction continue du magma phono-téphrique après la cristallisation des phonolites les plus évolués. Ce processus amenerait le magma à une composition à haute teneur en Ca- et en espèces volatiles, produisant un résidu de carbonatite qui produit des calcio-carbonatite exceptionnellement enrichies en terres rares lourdes. Ces calcio-carbonatites représentent une source potentielle pour l'albitite contenant des terres rares lourdes mineralisées à Lofdal

Carbonate–silicate melt immiscibility, REE mineralising fluids, and the evolution of the Lofdal Intrusive Suite, Namibia

The Lofdal Intrusive Suite, Namibia, consists of calcio-carbonatite and silica-undersaturated alkaline intrusive rocks ranging in composition from phono-tephrite to phonolite (and nepheline syenite). The most primitive of these rocks is the phono-tephrite, which, on the basis of its Y/Ho and Nb/Ta ratios, is interpreted to have formed by partial melting of the mantle. Roughly linear trends in major and trace element contents from phono-tephrite to phonolite and nepheline syenite indicate that the latter two rock types evolved from the phono-tephrite by fractional crystallisation. [...] Fluids released from the corresponding magma are interpreted to have been the source of the REE mineralisation that is currently the target of exploration