Targeting Heavy Rare Earth Elements in Carbonatite Complexes

The HREE are generally considered to be the most critical of the REE, indispensable for many high-tech applications such as smart-phones and electric vehicles. Currently, carbonatites are the main source of REE due to their high REE grade; most carbonatites, however, are HREE-poor. This thesis presents the findings on HREE mineralisation at the Songwe Hill carbonatite, in the CAP of south-eastern Malawi. Across all carbonatite types at Songwe, whole-rock Y and P2O5 concentrations correlate positively, indicating that phosphate minerals have a strong control over the HREE contents. This is confirmed through textural and geochemical analyses (LA ICP-MS and EPMA) of apatite, which show that it can be subdivided into 5 different types (Ap-0–4), found at different stages of the paragenetic sequence. The chemistry of each of these apatite types becomes progressively more HREE-enriched, up to 3 wt. % Y2O3, and ultimately culminating in xenotime crystallisation. Cross-cutting relationships indicate that HREE-enriched apatite formed as an early crystallisation product from a late-stage, carbonatite-derived hydrothermal fluid. It is evident that LREE-fluorcarbonate mineralisation occurred after apatite crystallisation and it is assumed that crystallisation of all hydrothermal phases was though the evolution of a single fluid, rather than several different fluids. The apatite composition is compared to a compilation of analyses of apatite from other carbonatites and granitoids, as well as new analyses of late-stage apatite from the Kangankunde and Tundulu carbonatites, Malawi. Based on these analyses, it is concluded that apatite from Songwe has the highest HREE concentration compared to apatite from any previously analysed carbonatite. However, apatite from the Tundulu carbonatite has a similar geochemistry and paragenesis to the HREE-rich apatite from Songwe, suggesting that late-stage HREE enrichment may be a common process in carbonatites. In order to elucidate the fluid conditions which led to HREE mineralisation, new fluid inclusion and stable isotope data are presented to complement the mineralogical data. The fluid inclusions constrain the minimum temperature of apatite crystallisation of 160 ◦C, and most homogenisation temperatures in apatite are between 160–360 ◦C. Inclusions from apatite are CO2-rich, and it is suggested that transport of the REE occurred in carbonate complexes. Stable isotope data were obtained from both conventional C and O analyses of carbonates and from a novel method developed for acquiring δ18OPO4 from apatite. A conceptual model involving the simultaneous cooling and mixing of magmatically-derived and meteoric fluids is suggested. Two possible causes of REE fractionation are suggested: (1) a crystal-chemical control and (2) control through preferential stability of LREE and HREE complexes. However, neither mechanism is equivocal and further work on the stability of carbonate complexes is suggested in order to better understand REE mineralisation at carbonatites In addition to results on the HREE mineralisation in carbonatites, new data on the mineralogy, geochemistr y and age of the Songwe Hill carbonatite and the closely-associated Mauze nepheline syenite intr usion are presented. Songwe compr ises three stages of intr usion (C1–3): (C1) sovitic calcite carbonatite, (C2) alvikitic calcite-carbonatite and (C3) Fe-rich carbonatite. The LREE grade increases with the increasing Fe-content of the intrusion, as is common at many REE-rich carbonatites. Later-stages of the intrusion include apatite-fluorite veins (C4) and Mn-Fe-veins. The former is a volumetrically minor stage, but can contain up to 1 wt. % Y2O3, and the latter is formed through oxidation of carbonatite by supergene fluids. Samples analysed from Mauze show that it is REE- and P2O5-poor, with MREE-depleted REE distributions. U-Pb dating of zircons from Songwe and Mauze show that they are 131.5 ± 1.3 and 133.1 ± 2.0 Ma, respectively. The close temporal association of each intrusion suggests that Mauze could be a ‘heat-engine’ for hydrothermal mineralisation at Songwe.NER

Tracing the fluid source of heavy REE mineralisation in carbonatites using a novel method of oxygen-isotope analysis in apatite: the example of Songwe Hill, Malawi

Stable (C and O) isotope data from carbonates are one of the most important methods used to infer genetic processes in carbonatites. However despite their ubiquitous use in geological studies, it is suspected that carbonates are susceptible to dissolution-reprecipitation and isotopic resetting, especially in shallow intrusions, and may not be the best records of either igneous or hydrothermal processes. Apatite, however, should be much less susceptible to these resetting problems but has not been used for O isotope analysis. In this contribution, a novel bulk-carbonatite method for the analysis of O isotopes in the apatite PO4 site demonstrates a more robust record of stable isotope values. Analyses of apatite from five carbonatites with magmatic textures establishes a preliminary Primary Igneous Apatite (PIA) field of δ18O = + 2.5 to + 6.0‰ (VSMOW), comparable to Primary Igneous Carbonatite (PIC) compositions from carbonates. Carbonate and apatite stable isotope data are compared in 10 carbonatite samples from Songwe Hill, Malawi. Apatite is heavy rare earth element (HREE) enriched at Songwe and, therefore, oxygen isotope analyses of this mineral are ideal for understanding HREE-related mineralisation in carbonatites. Carbonate C and O isotope ratios show a general trend, from early to late in the evolution, towards higher δ18O values (+ 7.8 to + 26.7‰, VSMOW), with a slight increase in δ13C (− 4.6 to − 0.1‰, VPDB). Oxygen isotope ratios from apatite show a contrary trend, decreasing from a PIA field towards more negative values (+ 2.5 to − 0.7‰, VSMOW). The contrasting results are interpreted as the product of the different minerals recording fluid interaction at different temperatures and compositions. Modelling indicates the possibility of both a CO2 rich fluid and mixing between meteoric and deuteric waters. A model is proposed where brecciation leads to depressurisation and rapid apatite precipitation. Subsequently, a convection cell develops from a carbonatite, interacting with surrounding meteoric water. REE are likely to be transported in this convection cell and precipitate owing to decreasing salinity and/or temperature

Critical metal mineralogy and ore genesis revisited: thematic set arising from the Third International Critical Metals Meeting, Edinburgh

This is the author accepted manuscript. The final version is available from Cambridge University Press via the DOI in this recordNatural Environment Research Council (NERC

Light rare earth element redistribution during hydrothermal alteration at the Okorusu carbonatite complex, Namibia

The Cretaceous Okorusu carbonatite, Namibia, includes diopside-bearing and pegmatitic calcite carbonatites, both exhibiting hydrothermally altered mineral assemblages. In unaltered carbonatite, Sr, Ba and rare earth elements (REE) are hosted principally by calcite and fluorapatite. However, in hydrothermally altered carbonatites, small (<50 µm) parisite-(Ce) grains are the dominant REE host, while Ba and Sr are hosted in baryte, celestine, strontianite and witherite. Hydrothermal calcite has a much lower trace-element content than the original, magmatic calcite. Regardless of the low REE contents of the hydrothermal calcite, the REE patterns are similar to those of parisite-(Ce), magmatic minerals and mafic rocks associated with the carbonatites. These similarities suggest that hydrothermal alteration remobilised REE from magmatic minerals, predominantly calcite, without significant fractionation or addition from an external source. Barium and Sr released during alteration were mainly reprecipitated as sulfates. The breakdown of magmatic pyrite into iron hydroxide is inferred to be the main source of sulfate. The behaviour of sulfur suggests that the hydrothermal fluid was somewhat oxidising and it may have been part of a geothermal circulation system. Late hydrothermal massive fluorite replaced the calcite carbonatites at Okorusu and resulted in extensive chemical change, suggesting continued magmatic contributions to the fluid system

Formation of rare earth deposits in carbonatites

This is the final version. Available on open access from the Mineralogical Society of America via the DOI in this recordCarbonatites and carbonatite-related rocks are the premier source for light rare earth element (LREE) deposits. Here we outline a simplified ore formation model for LREE mineralised carbonatites, reconciling field and petrological observations with recent experimental and isotopic advances. REE can strongly partition to carbonatite melts, which are either directly mantle-derived or immiscible from silicate melts. As carbonatite melts evolve, alkalis and REE concentrate in the residual melt due to their incompatibility in early crystallising minerals. In most carbonatites, additional fractionation of calcite or ferroan dolomite leads to evolution into a mobile alkaline “brine-melt” from which primary alkali REE carbonates can form. These carbonates are rarely preserved owing to dissolution by later fluids and are replaced in-situ by monazite and alkali-free REE-(fluor)carbonates.Natural Environment Research Council (NERC)Australian Research Council (ARC)National Natural Science Foundation of Chin

REE minerals at the Songwe Hill carbonatite, Malawi: HREE-enrichment in late-stage apatite

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

Apatite texture and composition in the Tonglushan porphyry-related skarn system, eastern China: implications for mineral exploration

This is the final version. Available on open access from Elsevier via the DOI in this recordData availability: Data will be made available on request.The composition of ‘indicator’ minerals is thought to provide a guide to the potential of magmatic arc systems to form porphyry Cu deposits, but whether this is also the case for endoskarn- and exoskarn-dominated systems remains unclear, despite their importance as a source of Cu, Fe and Au. In a first step to address this, we compare the texture, cathodoluminescence (CL) colour and composition of apatite between relatively fresh quartz monzodiorite (QMD) and porphyry-type-, endoskarn- and Fe-(Cu)-mineralised exoskarn components of the Tonglushan porphyry-skarn system of the Daye ore district, China. In the relatively fresh QMD, apatite luminesces yellow-green due to elevated Mn contents. However, where affected by potassic-sodic alteration, it shows green–blue CL thought to reflect partial removal of Mn and an associated increase in REE. Apatite in the endoskarn is more pervasively replaced and veined, and shows mid-blue luminescence due to relatively low Mn, Mg and Cl. The exoskarns contain apatite with variable grain shapes and navy blue-violet or bright to dark blue CL colours, caused by low Mn and elevated Ce, and with only small patches of pale yellow-green CL. Apatite is near absent in the limestone wall rocks and xenoliths and, therefore, where present in the exoskarns is interpreted to have precipitated from the same fluids as the Fe-(Cu) mineralization. Apatite CL colour and chemistry is indicative of the different styles of alteration and mineralisation in the Tonglushan system and provides insights into the composition of skarn-forming fluids. Our results offer a potentially effective method for utilising apatite as a porphyry and skarn deposit indicator mineral in a range of exploration materials including regolith and stream sediments.China Scholarship CouncilUniversity of ExeterNatural Environment Research Council (NERC

Evidence for dissolution-reprecipitation of apatite and preferential LREE mobility in carbonatite-derived late-stage hydrothermal processes

The Tundulu and Kangankunde carbonatite complexes in the Chilwa Alkaline Province, Malawi, contain late-stage, apatite-rich lithologies termed quartz-apatite rocks. Apatite in these rocks can reach up to 90 modal% and displays a distinctive texture of turbid cores and euhedral rims. Previous studies of the paragenesis and rare earth element (REE) content of the apatite suggest that heavy REE (HREE)-enrichment occurred during the late-stages of crystallization. This is a highly unusual occurrence in intrusions that are otherwise light REE (LREE) enriched. In this contribution, the paragenesis and formation of the quartz-apatite rocks from each intrusion is investigated and re-evaluated, supported by new electron microprobe (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data to better understand the mechanism of HREE enrichment. In contrast to the previous work at Tundulu, we recognize three separate stages of apatite formation, comprising an “original” euhedral apatite, “turbid” apatite, and “overgrowths” of euhedral late apatite. The crystallization of synchysite-(Ce) is interpreted to have occurred subsequent to all phases of apatite crystallization. The REE concentrations and distributions in the different minerals vary, but generally higher REE contents are found in later-stage apatite generations. These generations are also more LREE-enriched, relative to apatite that formed earlier. A similar pattern of increasing LREE-enrichment and increased REE concentrations toward later stages of the paragenetic sequence is observed at Kangankunde, where two generations of apatite are observed, the second showing higher REE concentrations, and relatively higher LREE contents. The changing REE distribution in the apatite, from early to late in the paragenetic sequence, is interpreted to be caused by a combination of dissolution-reprecipitation of the original apatite and the preferential transport of the LREE complexes by F- and Cl-bearing hydrothermal fluids. Successive pulses of these fluids transport the LREE out of the original apatite, preferentially re-precipitating it on the rim. Some LREE remained in solution, precipitating later in the paragenetic sequence, as synchysite-(Ce). The presence of F is supported by the F content of the apatites, and presence of REE-fluorcarbonates. Cl is not detected in the apatite structure, but the role of Cl is suggested from comparison with apatite dissolution experiments, where CaCl2 or NaCl cause the reprecipitation of apatite without associated monazite. This study implies that, despite the typically LREE enriched nature of carbonatites, significant degrees of hydrothermal alteration can lead to certain phases becoming residually enriched in the HREE. Although at Tundulu the LREE-bearing products are re-precipitated relatively close to the REE source, it is possible that extensive hydrothermal activity in other carbonatite complexes could lead to significant, late-stage fractionation of the REE and the formation of HREE minerals. Keywords: Apatite, carbonatite, rare earth elements, Chilwa Alkaline Province, Tundulu, Kangankunde, REE mobility, dissolution-reprecipitatio

Geology, geochemistry and geochronology of the Songwe Hill carbonatite, Malawi

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.Songwe Hill, Malawi, is one of the least studied carbonatites but has now become particularly important as it hosts a relatively large rare earth deposit. The results of new mapping, petrography, geochemistry and geochronology indicate that the 0.8 km diameter Songwe Hill is distinct from the other Chilwa Alkaline Province carbonatites in that it intruded the side of the much larger (4 x 6 km) and slightly older (134.6 ± 4.4 Ma) Mauze nepheline syenite and then evolved through three different carbonatite compositions (C1–C3). Early C1 carbonatite is scarce and is composed of medium–coarse-grained calcite carbonatite containing zircons with a U–Pb age of 132.9 ± 6.7 Ma. It is similar to magmatic carbonatite in other carbonatite complexes at Chilwa Island and Tundulu in the Chilwa Alkaline Province and others worldwide. The fine-grained calcite carbonatite (C2) is the most abundant stage at Songwe Hill, followed by a more REE- and Sr-rich ferroan calcite carbonatite (C3). Both stages C2 and C3 display evidence of extensive (carbo)-hydrothermal overprinting that has produced apatite enriched in HREE (<2000 ppm Y) and, in C3, synchysite-(Ce). The final stages comprise HREE-rich apatite fluorite veins and Mn-Fe-rich veins. Widespread brecciation and incorporation of fenite into carbonatite, brittle fracturing, rounded clasts and a fenite carapace at the top of the hill indicate a shallow level of emplacement into the crust. This shallow intrusion level acted as a reservoir for multiple stages of carbonatite-derived fluid and HREE-enriched apatite mineralisation as well as LREE-enriched synchysite-(Ce). The close proximity and similar age of the large Mauze nepheline syenite suggests it may have acted as a heat source driving a hydrothermal system that has differentiated Songwe Hill from other Chilwa carbonatites.Thanks are due to A. Lemon, A. Zabula, C. Mcheka, I. Nkukumila (Mkango Resources Ltd.), É. Deady (BGS) and P. Armitage (Paul Armitage Consulting Ltd.) for logistical support and enthusiastic discussions in the field. This contribution benefitted from reviews by Jindřich Kynický and Ray Macdonald, as well as anonymous reviewers, who we thank for their time and insightful comments. This work was funded by a NERC BGS studentship to SBF (NEE/J50318/1; S208), the NERC SoS RARE consortium (NE/M011429/1) and by Mkango Resources Ltd. AGG publishes with the permission of the Executive Director of the British Geological Survey (NERC)

Geology, geochemistry and geochronology of the Songwe Hill carbonatite, Malawi

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.Songwe Hill, Malawi, is one of the least studied carbonatites but has now become particularly important as it hosts a relatively large rare earth deposit. The results of new mapping, petrography, geochemistry and geochronology indicate that the 0.8 km diameter Songwe Hill is distinct from the other Chilwa Alkaline Province carbonatites in that it intruded the side of the much larger (4 x 6 km) and slightly older (134.6 ± 4.4 Ma) Mauze nepheline syenite and then evolved through three different carbonatite compositions (C1–C3). Early C1 carbonatite is scarce and is composed of medium–coarse-grained calcite carbonatite containing zircons with a U–Pb age of 132.9 ± 6.7 Ma. It is similar to magmatic carbonatite in other carbonatite complexes at Chilwa Island and Tundulu in the Chilwa Alkaline Province and others worldwide. The fine-grained calcite carbonatite (C2) is the most abundant stage at Songwe Hill, followed by a more REE- and Sr-rich ferroan calcite carbonatite (C3). Both stages C2 and C3 display evidence of extensive (carbo)-hydrothermal overprinting that has produced apatite enriched in HREE (<2000 ppm Y) and, in C3, synchysite-(Ce). The final stages comprise HREE-rich apatite fluorite veins and Mn-Fe-rich veins. Widespread brecciation and incorporation of fenite into carbonatite, brittle fracturing, rounded clasts and a fenite carapace at the top of the hill indicate a shallow level of emplacement into the crust. This shallow intrusion level acted as a reservoir for multiple stages of carbonatite-derived fluid and HREE-enriched apatite mineralisation as well as LREE-enriched synchysite-(Ce). The close proximity and similar age of the large Mauze nepheline syenite suggests it may have acted as a heat source driving a hydrothermal system that has differentiated Songwe Hill from other Chilwa carbonatites.Thanks are due to A. Lemon, A. Zabula, C. Mcheka, I. Nkukumila (Mkango Resources Ltd.), É. Deady (BGS) and P. Armitage (Paul Armitage Consulting Ltd.) for logistical support and enthusiastic discussions in the field. This contribution benefitted from reviews by Jindřich Kynický and Ray Macdonald, as well as anonymous reviewers, who we thank for their time and insightful comments. This work was funded by a NERC BGS studentship to SBF (NEE/J50318/1; S208), the NERC SoS RARE consortium (NE/M011429/1) and by Mkango Resources Ltd. AGG publishes with the permission of the Executive Director of the British Geological Survey (NERC)