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

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

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

Sulfur-bearing monazite-(Ce) from the Eureka carbonatite, Namibia: oxidation state, substitution mechanism, and formation conditions

Sulfur-bearing monazite-(Ce) occurs in silicified carbonatite at Eureka, Namibia, forming rims up to ~0.5 mm thick on earlier-formed monazite-(Ce) megacrysts. We present X-ray photoelectron spectroscopy data demonstrating that sulfur is accommodated predominantly in monazite-(Ce) as sulfate, via a clino-anhydrite-type coupled substitution mechanism. Minor sulfide and sulfite peaks in the X-ray photoelectron spectra, however, also indicate that more complex substitution mechanisms incorporating S2– and S4+ are possible. Incorporation of S6+ through clino-anhydrite-type substitution results in an excess of M2+ cations, which previous workers have suggested is accommodated by auxiliary substitution of OH– for O2–. However, Raman data show no indication of OH–, and instead we suggest charge imbalance is accommodated through F– substituting for O2–. The accommodation of S in the monazite-(Ce) results in considerable structural distortion that may account for relatively high contents of ions with radii beyond those normally found in monazite-(Ce), such as the heavy rare earth elements, Mo, Zr and V. In contrast to S-bearing monazite-(Ce) in other carbonatites, S-bearing monazite-(Ce) at Eureka formed via a dissolution–precipitation mechanism during prolonged weathering, with S derived from an aeolian source. While large S-bearing monazite-(Ce) grains are likely to be rare in the geological record, formation of secondary S-bearing monazite-(Ce) in these conditions may be a feasible mineral for dating palaeo-weathering horizons

On the feasibility of imaging carbonatite-hosted rare earth element deposits using remote sensing

© 2016 Gold Open Access: this paper is published under the terms of the CC-BY license. Rare earth elements (REEs) generate characteristic absorption features in visible to shortwave infrared (VNIRSWIR) reflectance spectra. Neodymium (Nd) has among the most prominent absorption features of the REEs and thus represents a key pathfinder element for the REEs as a whole. Given that the world's largest REE deposits are associated with carbonatites, we present spectral, petrographic, and geochemical data from a predominantly carbonatitic suite of rocks that we use to assess the feasibility of imaging REE deposits using remote sensing. Samples were selected to cover a wide range of extents and styles of REE mineralization, and encompass calcio-, ferro-and magnesio-carbonatites. REE ores from the Bayan Obo (China) and Mountain Pass (United States) mines, as well as REE-rich alkaline rocks from the Motzfeldt and Ilímaussaq intrusions in Greenland, were also included in the sample suite. The depth and area of Nd absorption features in spectra collected under laboratory conditions correlate positively with the Nd content of whole-rock samples. The wavelength of Nd absorption features is predominantly independent of sample lithology and mineralogy. Correlations are most reliable for the two absorption features centered at ∼744 and ∼802 nm that can be observed in samples containing as little as ∼1,000 ppm Nd. By convolving laboratory spectra to the spectral response functions of a variety of remote sensing instruments we demonstrate that hyperspectral instruments with capabilities equivalent to the operational Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) and planned Environmental Mapping and Analysis Program (EnMAP) systems have the spectral resolutions necessary to detect Nd absorption features, especially in high-grade samples with economically relevant REE accumulations (Nd> 30,000 ppm). Adding synthetic noise to convolved spectra indicates that correlations between Nd absorption area and whole-rock Nd content only remain robust when spectra have signal-to-noise ratios in excess of ∼250:1. Although atmospheric interferences are modest across the wavelength intervals relevant for Nd detection, most REE-rich outcrops are too small to be detectable using satellite-based platforms with>30-m spatial resolutions. However, our results indicate that Nd absorption features should be identifiable in high-quality, airborne, hyperspectral datasets collected at meter-scale spatial resolutions. Future deployment of hyperspec-tral instruments on unmanned aerial vehicles could enable REE grade to be mapped at the centimeter scale across whole deposits

Enrichment of heavy REE and Th in carbonatite-derived fenite breccia

This is the final version. Available on open access from Cambridge University Press via the DOI in this recordEnrichment of the heavy rare earth elements (HREE) in carbonatites is rare as carbonatite petrogenesis favours the light (L)REE. We describe HREE enrichment in fenitised phonolite breccia, focussing on small satellite occurrences 1–2 km from the Songwe Hill carbonatite from the Chilwa Alkaline Province, Malawi. Within the breccia groundmass, a HREE-bearing mineral assemblage comprises xenotime, zircon, anatase/rutile, and minor huttonite/thorite, as well as fluorite and apatite. A genetic link between HREE mineralisation and carbonatite emplacement is indicated by the presence of Sr-bearing carbonate veins, carbonatite xenoliths and extensive fenitisation. We propose that the HREE are retained in hydrothermal fluids which are residually derived from a carbonatite after precipitation of LREE minerals. Brecciation provides a focussing conduit for such fluids, enabling HREE transport and xenotime precipitation in the fenite. Continued fluid-rock interaction leads to dissolution of HREE-bearing minerals and further precipitation of xenotime and huttonite/thorite. At a maximum Y content of 3,100 μg/g, HREE concentrations in the presented example are not sufficient to constitute ore, but the similar composition and texture of these rocks to other cases of HREE enrichment related to carbonatite suggests that all form via a common mechanism linked to fenitisation. Precipitation of HREE minerals only occurs where a pre-existing structure provides a focussing conduit for fenitising fluids, reducing fluid-country rock interaction. Enrichment of HREE and Th in fenite breccia serves as an indicator of fluid expulsion from a carbonatite, and may indicate the presence of LREE mineralisation within the source carbonatite body at depth.Natural Environment Research Council (NERC)European Union Horizon 202

Carbonatites and Alkaline Igneous Rocks in Post-Collisional Settings: Storehouses of Rare Earth Elements

This is the final version. Available on open access from Springer via the DOI in this recordThe rare earth elements (REE) are critical raw materials for much of modern technology, particularly renewable energy infrastructure and electric vehicles that are vital for the energy transition. Many of the world’s largest REE deposits occur in alkaline rocks and carbonatites, which are found in intracontinental, rift-related settings, and also in syn- to post-collisional settings. Post-collisional settings host significant REE deposits, such as those of the Mianning-Dechang belt in China. This paper reviews REE mineralisation in syn- to post-collisional alkaline-carbonatite complexes worldwide, in order to demonstrate some of the key physical and chemical features of these deposits. We use three examples, in Scotland, Namibia, and Turkey, to illustrate the structure of these systems. We review published geochemical data and use these to build up a broad model for the REE mineral system in post-collisional alkaline-carbonatite complexes. It is evident that immiscibility of carbonate-rich magmas and fluids plays an important part in generating mineralisation in these settings, with REE, Ba and F partitioning into the carbonate-rich phase. The most significant REE mineralisation in post-collisional alkaline-carbonatite complexes occurs in shallow-level, carbothermal or carbonatite intrusions, but deeper carbonatite bodies and associated alteration zones may also have REE enrichment.European Union Horizon 2020Natural Environment Research Council (NERC

Key process mineralogy parameters for rare earth fluorcarbonate-bearing carbonatite deposits: the example of Songwe Hill, Malawi

This is the final version. Available on open access from Elsevier via the DOI in this recordRare earth element (REE)-bearing carbonatite deposits commonly contain a wide range of different REE- and REE-bearing minerals associated with various gangue matrices. In order to select the most-suitable mineral processing technique for these deposits, it is essential to identify and quantify the minerals of interest, including their liberation, associations and grain size distribution, along with whole rock compositions. These data are also vital for ore feed optimisation and metallurgical troubleshooting during and after designing a mineral processing flowsheet. This paper summarises the key mineralogical parameters needed before conducting metallurgical beneficiation tests, using the Songwe Hill carbonatite deposit as an example. This REE ore deposit consists of poorly-liberated synchysite-(Ce), which hosts the light rare earth elements including Nd plus some heavy rare earths and well-liberated apatite, which hosts 50% of Gd, 63% of Dy and 71% of Y (heavy rare earth elements) in the deposit. For all REE heavier than Gd, apatite is the most important REE host, however, for the two REE where data are available in both synchysite-(Ce) and apatite (Dy and Y), synchysite27 (Ce) still accommodates >25% of the whole-rock HREE content. Both of these ore minerals are associated with ankerite, calcite, and to a lesser extent with iron oxides/carbonates, K-feldspar, strontianite and baryte. According to the quantitative mineralogical data, the possibility of using gravity separation, magnetic separation, froth flotation and leaching to process Songwe Hill carbonatite ore is discussed and a potential beneficiation flowsheet is presented.Mkango Resources LtdHigher Committee of Education Development in Iraq (HCED)Natural Environment Research Council (NERC)European Union Horizon 202

Deducing the source and composition of rare earth mineralising fluids in carbonatites: insights from isotopic (C, O, 87Sr/86Sr) data from Kangankunde, Malawi

This is the final version of the article. Available from Springer Verlag via the DOI in this record.Carbonatites host some of the largest and highest grade rare earth element (REE) deposits but the composition and source of their REE-mineralising fluids remains enigmatic. Using C, O and 87Sr/86Sr isotope data together with major and trace element compositions for the REE-rich Kangankunde carbonatite (Malawi), we show that the commonly observed, dark brown, Fe-rich carbonatite that hosts REE minerals in many carbonatites is decoupled from the REE mineral assemblage. REE-rich ferroan dolomite carbonatites, containing 8–15 wt% REE2O3, comprise assemblages of monazite-(Ce), strontianite and baryte forming hexagonal pseudomorphs after probable burbankite. The 87Sr/86Sr values (0.70302–0.70307) affirm a carbonatitic origin for these pseudomorph-forming fluids. Carbon and oxygen isotope ratios of strontianite, representing the REE mineral assemblage, indicate equilibrium between these assemblages and a carbonatite-derived, deuteric fluid between 250 and 400 °C (δ18O + 3 to + 5‰VSMOW and δ13C − 3.5 to − 3.2‰VPDB). In contrast, dolomite in the same samples has similar δ13C values but much higher δ18O, corresponding to increasing degrees of exchange with low-temperature fluids (< 125 °C), causing exsolution of Fe oxides resulting in the dark colour of these rocks. REE-rich quartz rocks, which occur outside of the intrusion, have similar δ18O and 87Sr/86Sr to those of the main complex, indicating both are carbonatite-derived and, locally, REE mineralisation can extend up to 1.5 km away from the intrusion. Early, REE-poor apatite-bearing dolomite carbonatite (beforsite: δ18O + 7.7 to + 10.3‰ and δ13C −5.2 to −6.0‰; 87Sr/86Sr 0.70296–0.70298) is not directly linked with the REE mineralisation.This project was funded by the UK Natural Environment Research Council (NERC) SoS RARE project (NE/M011429/1) and by NIGL (NERC Isotope Geoscience Laboratory) Project number 20135

The origin and composition of carbonatite-derived carbonate-bearing fluorapatite deposits

Carbonate-bearing fluorapatite rocks occur at over 30 globally distributed carbonatite complexes and represent a substantial potential supply of phosphorus for the fertiliser industry. However, the process(es) involved in forming carbonate-bearing fluorapatite at some carbonatites remain equivocal, with both hydrothermal and weathering mechanisms inferred. In this contribution, we compare the paragenesis and trace element contents of carbonate-bearing fluorapatite rocks from the Kovdor, Sokli, Bukusu, Catalão I and Glenover carbonatites in order to further understand their origin, as well as to comment upon the concentration of elements that may be deleterious to fertiliser production. The paragenesis of apatite from each deposit is broadly equivalent, comprising residual magmatic grains overgrown by several different stages of carbonate-bearing fluorapatite. The first forms epitactic overgrowths on residual magmatic grains, followed by the formation of massive apatite which, in turn, is cross-cut by late euhedral and colloform apatite generations. Compositionally, the paragenetic sequence corresponds to a substantial decrease in the concentration of rare earth elements (REE), Sr, Na and Th, with an increase in U and Cd. The carbonate-bearing fluorapatite exhibits a negative Ce anomaly, attributed to oxic conditions in a surficial environment and, in combination with the textural and compositional commonality, supports a weathering origin for these rocks. Carbonate-bearing fluorapatite has Th contents which are several orders of magnitude lower than magmatic apatite grains, potentially making such apatite a more environmentally attractive feedstock for the fertiliser industry. Uranium and cadmium contents are higher in carbonate-bearing fluorapatite than magmatic carbonatite apatite, but are much lower than most marine phosphorites