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Complex gold-iridium-osmium placer deposits associated with the Guli massif of ultramafic, alkaline rocks and carbonatites are located within the Maimecha-Kotui province in the northern part of the Siberian Platform. Unlike natural Os-Ir alloys, which are genetically related to ultramafic rocks, the bedrock source of gold remains controversial. We report, for the first time, morphological and compositional features of gold mineralization from calcite-dolomite carbonatite of the Guli massif. Gold minerals are represented by natural Au-Ag alloys containing gold (69.64–88.57 wt %) and silver (11.73–30.83 wt %) with low concentrations of copper (up to 0.18 wt %), which predominate over minerals of the system Au–Cu–Ag (e.g., natural alloys Au0.74Cu0.14Ag0.12 and Ag0.51Au0.47Cu0.02, tetra-auricupride (AuCu) and auricupride (Cu3Au)). Gold minerals are characterized by an early pentlandite-troilite-chalcopyrite association that form polyphase euhedral inclusions. Later galena (PbS) occurring in the form of subhedral monophase inclusions is replaced by cerussite (PbCO3). The revealed similarity of gold minerals and mineral assemblages in gold from calcite-dolomite carbonatite with those of gold minerals from placer deposits of the Guli massif indicates that derivatives of ijolite-carbonatite magmatism played a significant role in gold ore formation. [ABSTRACT FROM AUTHOR]

Carbonatites are rare igneous rocks that host the largest resources of REE and Nb, yet, their genesis and evolution are far from clear. The leading models of carbonatite formation are the direct melting of carbonate-bearing peridotites, silicate-carbonatite liquid immiscibility, and fractionation of carbonated silicate melts. The validity of these models has never been robustly addressed through combining the available experimental results with the natural rock record. We thus re-evaluate the presently 633 alleged carbonatite occurrences including carbonatite type, bulk composition, mineralogy, and field exposure, followed by a review of experimental data pertinent to carbonatite genesis and evolution. Based on the available data, 454 carbonatite occurrences are magmatic, of which 87 without and 338 with spatially associated alkaline magma, 9 with kimberlites, and 20 with ultramafic cumulates only. Eighty-four percent of the magmatic occurrences contain calcite carbonatite (of which 1/3 also contain dolomite carbonatite), only 9% have dolomite but not calcite carbonatite, the incidence of dolomite carbonatite being similar for occurrences with or without associated silicate magmas. Available experimental data show that crystallization of calcite, dolomite, ankerite, and siderite at crustal conditions requires moderately alkaline and/or hydrous carbonate melts with ≥20 to 25 wt % (Na,K)2CO3 + H2O. It follows that carbonatite rocks, poor in these elements, are at best magmatic cumulates (if not carbo- or hydrothermal) that lost these ephemeral components. Carbonatitic melts could form in the lithospheric mantle, but these are always dolomitic and cannot deviate from close-to-minimum compositions when rising, their strong adiabatic cooling keeping them on the solidus until they decompose to olivine, clinopyroxene (cpx), and CO2 when reaching <2.1 GPa, i.e. the carbonated peridotite solidus ledge, which renders their extraction from the mantle highly unlikely. Furthermore, dolomitic carbonate melts crystallize periclase + calcite at crustal conditions. Only when containing ≥15 wt % (Na,K)2O + H2O they may crystallize dolomite and form dolomitic carbonatites. This value is far above the 2 to 5 wt % (Na,K)2O of mantle-derived carbonatitic melts. Liquid immiscibility from CO2-bearing close-to-natural melilititic, nephelinitic, and phonolitic melts requires 10 to 15 wt % Na2O + K2O in the silicate melt, increasing with SiO2. Extensive differentiation of primitive alkaline melilititic or basanitic parents is hence required to achieve immiscibility. The experimental data show that evolved nephelinites and phonolites unmix calcic carbonatitic melts, while melilitites and undifferentiated nephelinites with >4 wt % MgO may also unmix dolomitic carbonatitic melts. The latter may hence arise from liquid immiscibility or develop through fractionation from calcic carbonatitic melts. Finally, carbonatites may also derive through fractionation of CO2-rich ultramafic melts, but a continuous increase in dissolved CO2 from a carbonated silicate melt to a carbonatitic melt requires ≥3 GPa. We conclude that the combination of the natural rock record with melting and crystallization phase relations excludes a direct mantle origin for almost all carbonatites found in the crust. Instead, their vast majority forms through immiscibility from an alkali-rich differentiated silicate melt that stems from a mantle-derived alkaline parent, consistent with the common spatial association with alkaline complexes and similar isotopic compositions of carbonatite and alkaline silicate rocks. Direct fractionation from silicate melts may occur for kimberlitic or ultramafic lamprophyric melts, but only at ≥3 GPa, i.e. within the lithospheric mantle. To make progress in this field, we suggest a more rigorous distinction of magmatic and carbo- or hydrothermal carbonatite rocks in each occurrence, and to focus on mineral compositions in the carbonatite and associated silicate magmas, as bulk rocks are at best cumulative in nature. Additional experimental work to understand the role of alkalis and H2O in the formation and evolution of carbonatites, in particular crystallization and fluid saturation at crustal conditions, will be essential to provide a more complete understanding of carbonatite petrogenesis. [ABSTRACT FROM AUTHOR]

The Neoproterozoic Sevattur carbonatite complex (India) consists of calcite carbonatite, silicate-poor- and silicate-rich (banded and blue) dolomite carbonatite. The trace element composition of the calcite and ferroan dolomite shows that most of them have magmatic characteristics. The predominance of dolomite carbonatite over calcite carbonatite and the absence of a genetically related silicate suite at Sevattur suggest a dolomitic parental magma derived by the direct melting of the mantle. Regardless of the prevalent magmatic character of the Sevattur carbonatites they host a plethora of Ba–Sr–REE minerals formed throughout the orthomagmatic-to-carbothermal stages under varying activities of |${\mathrm{F}}^{-}$|⁠ , |${\mathrm{CO}}_3^{2-}$|⁠ , |${\mathrm{PO}}_4^{3-},\kern0.5em$| and |${\mathrm{SO}}_4^{2-}$|⁠. The orthomagmatic stage begins with the crystallization of calcite carbonatite containing xenocrystic diopside, antecrystic Y-bearing calcite (Cal-I), and primary benstonite, apatite, and richterite in a Sr–Mg-rich calcite (Cal-II) matrix. Fractionation of apatite, carbocernaite, and cordylite-(Ce) together with the enrichment of Mg, Fe, Na, and F in late-magmatic stages, forms the dolomite carbonatite. The decomposition of benstonite to norsethite, Ba–Ca carbonate, and strontianite occurs within dolomite carbonatite at the same stage with minimal wall-rock assimilation. Interaction between the xenocrystal clinopyroxenite and the primary magma during late-magmatic stages has resulted in the formation of aegirine and phlogopite in calcite carbonatite. Similarly, the prolonged assimilation of clinopyroxenite by the evolving magma leads to the formation of magnesio-riebeckite and magnesio-arfvedsonite within the banded- and blue carbonatites. The increasing activities of |${\mathrm{PO}}_4^{3-},{\mathrm{SO}}_4^{2-}$|⁠ , and H2O during the carbothermal stage, destabilizes precursor carbocernaite and cordylite-(Ce), resulting in an assemblage consisting of synchysite-(Ce), ancylite-(Ce), daqingshanite, hydroxylbastnäsite-(Ce), ferriallanite-(Ce), monazite-(Ce), and baryte in dolomite carbonatite. In the banded carbonatite, the carbothermal stage is characterized by ferri-winchite, tremolite-actinolite, and phlogopite crystallization. Further contamination and differentiation of the carbothermal fluid have resulted in the crystallization of quartz and albite in blue carbonatite. Phlogopitization within the blue carbonatite and clinopyroxenite results from late K enrichment in the carbothermal fluids. [ABSTRACT FROM AUTHOR]

Over 30 small, discontinuous, tabular carbonatite bodies are located in the Sheep Creek area, Ravalli County, southwest Montana. The age and origin of these REE-Nb-rich deposits are currently being investigated. The purpose of this paper is to document the occurrence of several rare minerals, including daqingshanite, törnebohmite, biraite, sahamalite, and ferriperbøeite, in two of the carbonatite bodies. These minerals are found in association with monazite, hydroxylbastnäsite, ferriallanite, calcite, dolomite, baryte, quartz, actinolite, apatite, celsian, and Sr-rich aragonite. Automated SEM-EDS was used to target the areas of interest in polished specimens for more detailed spot SEM-EDS and electron probe microanalysis. Raman spectra were also acquired for each of the rare minerals. The complex mineralogy of the Sheep Creek carbonatites is most likely due to several overlapping thermal events, including primary magmatic, overprinting hydrothermal, and supergene weathering stages. The rare minerals described in this study are believed to be hydrothermal and/or carbothermal in origin, although no estimates of temperature are available at this time. [ABSTRACT FROM AUTHOR]

The He isotopic composition and the He and Ne ratio of fluid inclusions in magnetite and pyrite from carbonatite breccias of the Sallanlatva alkali-ultramafic complex of the Kola alkaline province (Northwest Russia) are studied using the step-crushing method. The results indicate the highly likely involvement of fluids from several sources trapped in various proportions during the formation of the Sallanlatva explosive carbonatite breccias. The R/Ra ratio, where R is the measured 3He/4He ratio and Ra of 1.382 × 10–6 is the same ratio of atmospheric air, reaches 2.3, which is a reliable indicator of the presence of mantle gases. A low (1–44) 4He/20Ne ratio allows us to suggest the involvement of atmospheric gases dissolved in paleometeoric waters. A combination of these two facts favors a hypothesis of the phreatomagmatic nature of the studied breccias, i.e., their formation at the expense of interaction between the intruding hot orthomagmatic fluids and meteoric waters with dissolved atmospheric gases. [ABSTRACT FROM AUTHOR]

Carbonatites are the primary geological sources for rare earth elements (REEs) and niobium (Nb). This study applies machine learning techniques to generate national-scale prospectivity models and support mineral exploration targeting of Canadian carbonatite-hosted REE +/− Nb deposits. Extreme target feature label imbalance, diverse geological settings hosting these deposits throughout Canada, selecting negative labels, and issues regarding the interpretability of some machine learning models are major challenges impeding data-driven prospectivity modeling of carbonatite-hosted REE +/− Nb deposits. A multi-stage framework, exploiting global hierarchical tessellation model systems, data-space similarity measures, ensemble modeling, and Shapley additive explanations was coupled with convolutional neural networks (CNN) and random forest to meet the objectives of this work. A risk–return analysis was further implemented to assist with model interpretation and visualization. Multiple models were compared in terms of their predictive ability and their capability of reducing the search space for mineral exploration. The best-performing model, derived using a CNN that incorporates public geoscience datasets, exhibits an area under the curve for receiver operating characteristics plot of 0.96 for the testing labels, reducing the search area by 80%, while predicting all known carbonatite-hosted REE +/− Nb occurrences. The framework used in our study allows for an explicit definition of input vectors and provides a clear interpretation of outcomes generated by prospectivity models. [ABSTRACT FROM AUTHOR]

The exact geological processes involved in the formation of subduction zone-related carbonatites remain ambiguous, along with their implications for crustal/carbon recycling in carbonatite melt generation. This study provides new geochemical and stable (C, O) and radiogenic (Sr, Nd, Pb) isotope data for Huangshuian carbonatite, located within the Lesser Qinling Orogen, with the aim to decipher its complex petrogenetic history. The carbonatites display elevated CaO, low MgO and alkali contents, and significant enrichments of Pb, Mo, and HREEs compared to typical carbonatites. The δ13CPDB (−4.6 to −4.9‰) and δ18OSMOW (+6.6 to +7.8‰) values plot within the field of primary igneous carbonatites. The carbonatites are characterized by consistent radiogenic isotopic compositions [(87Sr/86Sr)i = 0.70599–0.70603; εNd = −10.4 to −12.8; 206Pb/204Pb =16.24–17.74]. These combined results suggest that the carbonatites represent late-stage differentiation products of a parental, mantle-derived carbonatite melt. Their corresponding Sr-Nd-Pb isotopic compositions support the hypothesis that the Lesser Qinling carbonatites originate from a heterogeneous upper mantle source involving an EMI-like mantle component coupled with minor assimilation of the basement rocks. The parental carbonatite melt was derived by the melting of carbonate-bearing subcontinental lithospheric mantle metasomatized as the result of Early Triassic subduction of the Mianlue Ocean. [ABSTRACT FROM AUTHOR]

Natural carbonatites exhibit a wide range of high field strength elements (HFSEs) and the highest Nb/Ta and Zr/Hf ratios among various rock types. However, primitive carbonatitic melts derived from carbonated peridotite do not display significant fractionation of Nb-Ta and Zr-Hf. To investigate this further, we conducted liquid immiscibility experiments to comprehend the differentiation of these HFSEs. Our experiments revealed substantial changes in partition coefficients for Nb, Ta, Zr, and Hf between carbonatite and silicate melts. We identified a positive correlation between the partition coefficients of these elements and Si, indicating that Si determines the differentiation of Nb-Ta and Zr-Hf during liquid immiscibility. The partition coefficients of Si increase as temperature increases and pressure decreases, resulting in higher HFSE concentrations during the early stages of liquid immiscibility. Liquid immiscibility is crucial in differentiating HFSEs in carbonatitic melts, explaining the association between super large carbonatite-related Nb deposits and Si-undersaturated silicate rocks. [ABSTRACT FROM AUTHOR]

The physio-chemical framework that generates carbonatites and, ultimately, the associated rare earth element deposits remains contentious. This primarily reflects the diverse tectonic settings in which carbonatites occur: large igneous provinces, continental rifts and major extensional terranes, syn- to post-collisional settings, or ocean islands. There is, however, a broad consensus that carbonatites (or their parental melts) originate in the mantle. These exotic melts have small volumes that make them ideal probes of conditions in their underlying source regions. We combine the carbonatite locations with global maps of lithospheric thickness, derived from seismic tomography, and show that post-Neoproterozoic carbonatites occur preferentially above the margins of thick cratonic lithosphere (e.g., adjacent to the South Atlantic and Indian Oceans or in North America, Greenland, and Asia) and where once thick lithosphere has undergone stretching (e.g., eastern Asia). Our thermal modeling reveals that lateral and vertical heat conduction on rifted craton margins, or rapid stretching of cratonic lithosphere, can mobilize carbonated peridotite at the temperatures (950–1250 °C) and pressures (2–3 GPa) required to form primary carbonatites or their parental alkali silicate melts. Importantly, our models show that heat conduction from upwelling mantle plumes or ambient mantle on rifted cratonic margins may sufficiently modify the temperature of the lithospheric mantle to cause melting of carbonated peridotite, settling the long-standing debate on the role of rifting and heating in the generation of carbonatites. [ABSTRACT FROM AUTHOR]

The Mount Grace metamorphosed carbonatites (Late Devonian) outcrop as thin (0.5–4 m), laterally discontinuous, strata-bound mappable lenses within the Monashee complex of the southeastern Canadian Cordillera. The host stratigraphic sequence (Monashee cover gneiss) was metamorphosed and deformed in the Late Cretaceous to early Eocene followed immediately by exhumation of the Frenchman Cap and Thor Odin domes. We present seven stratigraphic logs for Mount Grace carbonatites including new and previously described outcroppings spanning ∼30 km. The Mount Grace carbonatite units were deposited regionally within or near the top of a shallow marine sedimentary sequence within miogeoclinal strata of the western margin of paleo-North America (Laurentia). The distribution of the Mount Grace carbonatite lithofacies and the preserved depositional structures and textures suggest that these are pyroclastic deposits resulting from phreatomagmatic eruptions. Our new data enhance the volcanological story with an eruption scenario involving phreatomagmatic reactions and deposition from pyroclastic density currents, sourced from multiple centers within a field of monogenetic maar volcanoes. The distribution of the Mount Grace carbonatites parallel to the western margin of the paleo-North American continent correlates well with regional Late Devonian alkaline magmatism associated with development of an extensional back-arc basin. [ABSTRACT FROM AUTHOR]

A statistical regression analysis was performed of a summary of borrowed literary iso- topic data on 176 carbonatite occurrences worldwide (a total of 1829 paired δ18О-δ13С values in primary carbonatites). For most of the occurrences, distinct linear trends in δ18О-δ13С values were revealed, approximated in diagrams by vectors using the regression formula y = kx + b. Trends in three directions are distinguished. The dominating trend possesses the angular coefficient k in the range of 0.0÷+0.6, averaging at + 0.31 (direct correlation δ18О-δ13С). In the literature, it is associated with the Rayleigh high-temperature fractionation of carbonatite melts or with their sedimentary crustal contamination. The second trend with k from 0.0 to -0.7 (inverse correlation δ18О-δ13С) is associated with CO2 degassing from the melts. The third trend with k from +0.6 to +1.5 is rare. In other carbonatite occurrences, no linear trends in δ18О-δ13С values are detected. This may be a consequence of a combined action of the multidirectional linear trends. In this regard, the initial point of the trend vectors is accepted as a universal isotope indicator for all the carbonatite occurrences. In the δ18О-δ13С diagram, the box PC-90 is outlined in the field of the initial points, including 90 % of the primary carbonatite occurrences. The coordinates of the corner points of the box in δ18О-δ13С ‰ are (+5÷-8), (+5÷-5), (+5.5÷-2.5), (+12÷-2.5), (+12÷-7), (+9÷-8.5). The box PC-90 is subdivided into two parts by the carbon isotope composition, using the value δ13C = -5.2 ‰. This suggests that the deep sources differ in carbon isotope composition. No correlation is found in the oxygen–carbon and strontium isotope compositions in the carbonatites, which suggests that the deep sources are also heterogeneous in the strontium isotope composition.

New petrological, geochemical, and P–T modelling results from igneous samples clarify how carbonatite‐lamprophyre magmatism, fluorite and rare earth element (REE) enrichment are petrogenetically related in southern Illinois. P–T modelling reveals that igneous rocks derive from a deep mantle carbonated source, that is consistent with trace element signatures for a fluorine‐rich transition zone origin. Major element systematics suggests liquid‐immiscibility with lamprophyric melts as the origin for Ca‐carbonatites. Heavy REE (HREE) enrichments in Hicks Dome breccias likely formed through preferential partitioning and transport of HREE by brine‐melts, exsolved from a deep carbonatite body. Brine‐melts redistributed HREEs throughout the system along brecciated pathways where they reprecipitated as HREE‐rich phosphate/fluorcarbonate minerals (e.g. xenotime, florencite, synchesite) in host bedrock. The diversity of igneous rocks in southern Illinois highlights the area as an excellent natural laboratory to study carbonated melt petrogenesis and evolution. [ABSTRACT FROM AUTHOR]

Whereas the genesis of carbonatitic rare earth element (REE) deposits has long been a focus of study, the controls on mobilization and mineralization of REEs during magmatic-hydrothermal processes still remain open to debate. Here, we present our investigation of the dissolution and crystallization of REE (fluor)carbonate minerals in alkaline carbonate brine-melts up to 850°C and 11.6 kbar. Our results show that REEs are soluble in Na2CO3 brine-melts, achieving concentrations exceeding 8 weight % at temperatures above 650°C. The addition of calcium and/or fluoride has minimal impact on REE mobilization, whereas introduction of silica suppresses REE solubilities by half, due to britholite formation above 550°C. Upon cooling, sodium and REEs combine to crystallize in burbankite or carbocernaite in sodium-enriched brine-melts, even at fluoride saturation. However, while the brine-melts contain substantial ferro-or aluminosilicate, REE mineralization in fluorcarbonates occurs after sufficient sodium precipitation in alkaline silicate minerals, hence revealing how silicate and sodium carbonate govern REE mineralization. [ABSTRACT FROM AUTHOR]

Rare earth elements (REE) are vital for powerful permanent magnets used in electric motors and wind turbines. These elements are chiefly sourced from carbonatites and their weathering products. The economic attractiveness of carbonatites is explained by the 10,000‐fold enrichment of REE in their mineralized portions relative to the average continental crust. Carbonatites form from mantle‐derived melts, but the ultimate origin of their REE is not completely clear. One widely cited model invokes subduction of marine sediments which accumulate REE‐rich material, priming the mantle to produce REE‐rich carbonatite melts which subsequently form deposits in the upper crust. Here we examine a global marine sediment compilation, revealing a wide variety in REE abundances and patterns. We use the sensitive lambda method that separates REE pattern curvature from redox‐related element anomalies to examine both marine sediments and presumably derived carbonatite rocks. We find that the most REE‐rich marine sediments are characterized by strongly negative Ce anomalies, which if recycled via subduction, mineralized carbonatites are expected to inherit. In contrast, we find that mineralized carbonatite rocks do not contain Ce anomalies. This indicates that the REE from the most REE‐rich marine sediments are not recycled into carbonatite deposits, and a different REE source is needed to explain carbonatite fertilities. We also find evidence that raises questions on whether any sediment‐derived REE are present in carbonatite deposits to a significant amount. We suggest that a REE‐rich source may not be required and REE enrichment occurs primarily during crustal magmatic differentiation. Plain Language Summary: Rare earth elements (REE) such as neodymium are important components of permanent magnets used in energy generation technologies including wind power turbines and electric motors. These metals are extracted from carbonatite rocks, but how the REE got to carbonatites in the first place is contentious. The seafloor contains accumulations of exceptionally REE‐rich sediments, and a popular model proposes that these REE are recycled to the mantle during subduction. Once melting occurs in the mantle, these same REE are suggested to be transported in the carbonatite melts and eventually reach the upper crust where they contribute to economic mineralization. However, the REE‐rich sediments contain a ubiquitous chemical signature in the form of a redox‐sensitive negative cerium anomaly. We show that the anomaly does not occur in carbonatite rocks, with the implication that recycled REE‐rich sediments cannot necessarily be the source for carbonatite‐related ore deposits. Key Points: Subducted REE‐rich sediments with negative Ce anomalies are thought to supply carbonatites with their REECarbonatites have no statistically significant negative Ce anomalyRecycling of particularly REE‐rich marine sediments not required for mineralization [ABSTRACT FROM AUTHOR]

This study presents the sulfur isotopic characteristics in baryte from carbonatites of the Sallanlatva massif and sulfides (mainly pyrite and pyrrhotite) from carbonatites, phoscorites and products of their contact interaction with the host silicate rocks of most carbonatite-bearing complexes of the Devonian Kola Alkaline Province (KAP). For some complexes (Ozernaya Varaka, Kontozero), these characteristics are reported for the first time. The determined range of δ34S variations of sulfides in one complex does not exceed 4‰, but reaches 20‰ for the entire Kola Alkaline Province. This may be related to the evolution style of carbonatites and associated rocks. It is shown that the δ34S value in sulfides decreases from (1) the least evolved volcanic carbonatites of the Kontozero complex (δ34Savg. = –1.3‰) through (2) carbonatites and phoscorites of the Kovdor, Ozernaya Varaka, Sokli, and Salmagora massifs towards (3) the rocks of Seblyavr, Vuoriyarvi, and, finally, the carbonatites of Sallanlatva (δ34Savg. = –14.7‰) massifs, where sulfides differ from those of other KAP carbonatites in their exceptionally low δ34S values. The carbonatite volcanics of Kontozero are almost barren of REE mineralization; carbonatites of the second group contain accessory amounts of REE minerals; the third group is peculiar in the abundance of late carbonatites, where REE carbonates are frequently major minerals. Thus, the greater the volume of REE minerals in carbonatites of the complex, the lower the δ34S value in sulfides from its carbonatites and associated rocks. For the first time in the KAP, the sulfur isotopic composition of associated baryte–pyrite pairs was studied in the Sallanlatva carbonatites. The sulfur isotopic characteristics are shown to correspond to the final low-temperature (250–350°C) stage of carbonatite evolution in oxidized conditions, which satisfies the parameters of baryte crystallization. Since the studied samples of the Sallanlatva carbonatites are explosive breccias, the oxidized composition of fluids may indicate their phreatomagmatic nature, i.e., formation due to the interaction of intruded hot matter (melt/fluid) with meteoric waters. [ABSTRACT FROM AUTHOR]

Aillikite dikes of carbonate–biotite composition, intersecting Paleoproterozoic banded iron formations, were discovered in the Kursk block of Sarmatia for the first time. Their age is 2.10–2.07 Ga. The aillikite dikes underwent epidote–amphibolite facies metamorphism (550°С, 2–3 kbar), and they are deformed and foliated. Primary magmatic olivine and pyroxene were not preserved. In terms of geochemistry, ultramafic lamprophyres of the Kursk block are similar to petrotypical aillikites. They are depleted in SiO2 and Al2O3 and enriched in MgO, TiO2, K2O, Cr, Ni, and Nb with highly fractionated LREE and HREE. The highly radiogenic neodymium isotopic composition is indicative of a juvenile enriched (metasomatized) mantle source of aillikites. Positive Nb and Ti anomalies suggest metasomatic alterations of the depleted lithosphere mantle by OIB melts under opening of the Tim back-arc basin. The aillikite dikes and carbonatites of the Dubravinsky complex could have been formed at different melting degrees of a single deep, lithospheric source, enriched in lithophile elements shortly before the melting episode, possibly in a suprasubduction setting about 2.1 Ga. [ABSTRACT FROM AUTHOR]

The Fuerteventura Basal Complex comprises Oligocene and Miocene ultra-alkaline-carbonatitic magmatic pulses with outcrops that extend across kilometer-scale areas in some specific sectors of this oceanic island. Additionally, there is evidence of associated weathering materials that affect these magmatic lithologies. These alkaline magmatic rocks (including trachytes, phonolites, syenites, melteigites, and ijolites), carbonatites, and their associated weathering products underwent a preliminary evaluation of rare Earth element (REE) contents based on mineralogical and geochemical studies. REE concentrations in carbonatites of about 10 300 ppm REY (REEs plus yttrium) have been detected in volumes comparable to other locations hosting significant deposits of these critical elements worldwide. Conversely, alkaline magmatic rocks and the resulting weathering products display limited REE contents. Notably, REEs in carbonatites are associated with primary accessory phases such as REE-bearing pyrochlore and britholite and secondary monazite. The results obtained in the carbonatites of Fuerteventura underscore the interest in studying the concentrations of critical elements, such as REEs, within a non-conventional geological setting like oceanic islands. However, due to intricate structural attributes, the irregular distribution of these mineralizations, and possible land use and environmental constraints, additional future detailed investigations are imperative to ascertain the real potential of these REE concentrations. [ABSTRACT FROM AUTHOR]

Fluorite-dominated rocks are occasionally found in association with carbonatites, but their geologic and petrologic relations are rarely reported. The Dunkeldyk area of the Pamir mountains in south-eastern Tajikistan contains dikes of distinctive rocks composed of calcite, fluorite, celestine-barite, sulfides, apatite, with minor quartz, biotite, and REE fluorcarbonates. The dikes have sharp contacts with the host (meta-)sedimentary rocks and layering with ribbons, ranging from fluorite-bearing calcite carbonatites to fluoritites (rocks with >50% fluorite). The fluoritites are characterized by high Ca, F, Ba, Sr, REE, and S coupled with anomalously low O. The geologic relations and textures suggest a magmatic origin of the dikes from melts close to calcite–fluorite eutectic that experienced nucleation-controlled differentiation during the crystallization of dikes and the formation of fluoritite cumulates in larger intrusions. The Dunkeldyk dikes demonstrate that sizable geological bodies of fluorite-dominated rocks could form from carbonate–fluoride melts originating from the differentiation of alkaline silicate magmas. [ABSTRACT FROM AUTHOR]

To gain insight into the source of ore material, this study presents new Re–Os and Pt–Os isotopic and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data for chromitite and native osmium from the Guli massif of ultramafic alkaline rocks and carbonatites located in the Maimecha–Kotui province, Polar Siberia. The study utilized a number of analytical techniques, including electron probe microanalysis (EPMA), negative thermal ionization mass spectrometry (N-TIMS), and high pressure asher digestion and isotope dilution inductively-coupled plasma mass-spectrometry (ID ICP-MS). The HSE concentrations in chromitite samples range from 191 to 866 ppb with a predominance of Ir-group platinum-group elements (PGE) (Os, Ir, and Ru) over Pt-group PGE (Rh, Pt, and Pd) and Re, which is consistent with the platinum-group mineral control (i.e., Os–Ir alloys and laurite, RuS2) within the chromitite. The Re–Os and Pt–Os isotope data indicate that the HSE budget of the chromitite and native osmium from the Guli massif was largely controlled by that of their mantle source, which evolved with long-term near-chondritic Re/Os and Pt/Os ratios; this source had time-integrated Re/Os and Pt/Os within the range of those for the majority of komatiites and abyssal peridotites worldwide. [ABSTRACT FROM AUTHOR]

The origin of the carbonatites that appear on Earth is one of the most controversial current topics in the petrogenesis of igneous rocks. Situated in the northern sector of the Basal Complex of Fuerteventura (Canary Islands), the Miocene Esquinzo ultra-alkaline plutonic rock complex is composed of pyroxenites, melteigites-ijolites-urtites, malignites-nepheline syenites, carbonatites, silicocarbonatites, nephelinites and nepheline phonolites. This work tries to establish the genesis of this massif of ultra-alkaline rocks with associated carbonatites from Fuerteventura (which are very rare in the oceans). The geochemical characteristics of these rocks and the minerals that are included in them have allowed us to establish their origin. This complex was generated by three successive magmatic events associated with differentiation of melanephelinite magmas emplaced in the oceanic crust. Silicocarbonatite and calciocarbonatite (sövites) dykes are related to the first magmatic event and were formed by fractional crystallization of H2O- and CO2-rich ijolite magmas. The melanephelinite magmas that formed these plutonic ultra-alkaline rocks were apparently generated as partial melts of asthenospheric mantle, which assimilated enriched lithospheric mantle material as they ascended. The upwelling of this large body of anomalous asthenospheric Miocene material exceeded the deformation associated with plate motions and led to an oceanic rifting event in Fuerteventura. [ABSTRACT FROM AUTHOR]

The Huayangchuan U-polymetallic deposit in the Qinling Orogen is a newly verified carbonatite-hosted deposit on the southern margin of the North China Craton (NCC) in Central China. Granitic magmatism is extensively developed in the Huayangchuan deposit area and is lacking analysis on the reasons for these situations; however, its ages, petrogenesis, and relationship with uranium mineralization are not well constrained. Zircon U–Pb ages for the hornblende-bearing granite porphyry and medium-fine-grained biotite granites in close proximity to carbonatite rocks are 229.8 ± 1.1 and 135.3 ± 0.6 Ma, respectively. High-K calc-alkaline series and weakly peraluminous Triassic hornblende-bearing granite porphyry are slightly enriched in light rare earth elements (LREE) with flat heavy rare earth element (HREE) patterns, enriched in Ba and Sr, and depleted in Nb, Ta, P, and Ti, i.e., geochemical characteristics similar to those of adakite-like rocks. The Early Cretaceous medium-fine-grained biotite granites are characterized by LREE enrichment and flat HREE patterns, which belong to high-K calc-alkaline series, and metaluminous belong to weakly peraluminous I-type granite, with U and large ion lithophile element (LILE) enrichment and high field strength element depletion. The high initial 87Sr/86Sr ratios and enriched Nd (εNd(t) = −10.7 to −9.5 and −19.9 to −18.9, respectively) and Hf (εHf(t) = −21.8 to −13.0 and −30.5 to −19.0, respectively) isotopes revealed that both granitic rocks from the Huayangchuan deposit mainly originated from lower crustal materials, generated by partial melting of the ancient basement materials of the Taihua Group. Triassic hornblende-bearing granite porphyry is significantly different from the mantle origin of the contemporaneous U-mineralization carbonatite. In combination with tectonic evolution, we argue that the Qinling Orogenic Belt was affected by the subduction of the North Mianlian Ocean during the Late Triassic. The ongoing northward subduction of the Yangtze Craton resulted in crustal thickening, forming large-scale Indosinian carbonatites, U-polymetallic mineralization, and contemporaneous intermediate-acid magmatism. Additionally, due to the tectonic system transformation caused by Paleo-Pacific Plate subduction, intracontinental lithosphere extension and lithospheric thinning occurred along the southern NCC margin in the Early Cretaceous. Intense magma underplating of the post-orogeny created a large number of magmatic rocks. The tremendous heat could have provided a thermal source and dynamic mechanism for the Yanshanian large-scale U-polymetallic mineralization events. [ABSTRACT FROM AUTHOR]

Rare earth elements (REEs) play a pivotal role in the ongoing energy and mobility transition challenges. Given their critical importance, governments worldwide and especially from the European Union, are actively promoting the exploration of REE resources. In this context, alkaline magmatic rocks (including trachytes, phonolites, syenites, melteigites and ijolites), carbonatites and their associated weathering products were subjected to a preliminary evaluation as potential targets for REE exploration on Fuerteventura Island (Canary Archipelago, Spain) based on mineralogical and geochemical studies. These lithologies show significant REE concentrations. However, only carbonatites exhibit the potential to host economically viable REE mineral deposits. REE concentrations in carbonatites of up to 10,301.83 ppm REY (REEs plus yttrium) have been detected, comparable to other locations hosting significant deposits of these critical elements worldwide. Conversely, alkaline magmatic rocks and the resulting weathering products display limited REE enrichment. Notably, REEs in carbonatites are associated with primary accessory phases such as REE-bearing pyrochlore and britholite, and secondary monazite. The carbonatites of Fuerteventura hold promise as prospective REE deposits within a non-conventional geological setting (oceanic island). However, due to intricate structural attributes and possible land use constraints, additional future detailed investigations are imperative to ascertain their genuine economic viability as substantial REE resources. [ABSTRACT FROM AUTHOR]

The processes by which lamprophyres and associated carbonatites are generated remain subject to debate. The Wase Basin on the SE Tibetan Plateau contains trachytes, rhyolites, and minor carbonatites that were emplaced at 37-36 Ma. Coeval lamprophyre dikes are widespread in the adjacent regions. Geochemically and petrographically, both the extrusive trachytes and dikes can be classified as lamprophyre. The numerous millimeter-sized SiO2-rich and calcite-rich ocelli that occur within the trachytes and dikes are solidified pseudomorphs of felsic and calcic melt drops, respectively. These ocelli combined with inherited granitic zircons suggest magma mixing between the calcic melt or felsic melt and basaltic melt. Petrographic evidence, mineral compositions, and zircon textures and U-Pb ages (827-682 Ma) suggest that the felsic melt and the calcic melt were generated by rapid H2O-saturated partial melting of Neoproterozoic granite and limestone, respectively, at >800 °C and <2 kbar, according to available experimental data. These melts were separated from each other and were stored at upper crustal depths, forming a zone of transient magma lenses or parcels. Melts mixed when an ascending basaltic magma intersected this zone. Reaction between the basaltic melt and the calcic melt occurred when they mixed, which formed clinopyroxene of predominantly diopsidehedenbergite solid solution. The observed heterogeneity of the upper crust combined with the variable degrees of magma mixing account for the great chemical diversity of the lamprophyres of SE Tibet. This model sheds new light on the petrogenesis of other lamprophyre- carbonatite associations elsewhere. [ABSTRACT FROM AUTHOR]

[Display omitted] • Strong metasomatism in Toro Ankole, due to altered oceanic crust or carbonates. • Variation from N to S, with less pronounced metasomatism and different paragenesis. • Low degree of melting of metasomes origins carbonatitic and ultrapotassic terms. The western branch of the East African Rift hosts four main Neogene-Quaternary volcanic provinces, characterized by products with wide chemical and mineralogical variability. This study focuses on the two northernmost volcanic provinces, Toro Ankole where ∼0.2 Ma-old carbonatites, melilitites and kamafugites, erupted together with foidites, and Virunga where ∼13–9 Ma-old strongly and mildly alkaline rocks, from nephelinite up to trachytes crop out. Petrographic and whole/rock geochemical characteristics (low SiO 2 , high CaO and K 2 O, coupled with LILE enrichment) of 49 samples from Toro Ankole and 5 samples from Virunga (tephrites, trachybasalts and trachyandesites) point out enriched and heterogeneous sub-lithospheric mantle sources. The nature of the ultramafic nodules occasionally associated to the investigated samples highlights the presence of metasomatic veins variably enriched in clinopyroxene, phlogopite, carbonate, apatite, Ti-magnetite and titanite. 143Nd/144Nd ratios of Toro Ankole rocks are below ChUR (0.51249–0.51260), and all but two carbonatite lavas have 87Sr/86Sr ˃ BSE (0.7046–0.7056). Virunga lavas display less radiogenic 143Nd/144Nd (0.51235–0.51249) and more radiogenic 87Sr/86Sr ratios (0.7058–0.7071). Lead isotopes are all above the NHRL (206Pb/204Pb mostly in the range 19.12–19.63). δ11B values cover the whole OIB range (−8.3 to −3.3 ‰), but with the exception of some Toro Ankole samples with notably heavier compositions (−1.9 to 6.6), here linked to the carbonate enrichment. The differences in the isotopic features observed between Toro Ankole and Virunga products may reflect a north to south change of the lithospheric mantle source in terms of composition, mineralogy and depth of melting. Toro Ankole phlogopite-bearing mantle is more intensely metasomatized by a 87Sr- and 11B-rich component (as altered oceanic crust or subducted carbonates) compare to that one beneath Virunga, in which the metasomes might contain amphibole rather than phlogopite. Low degree of partial melting of the metasomatized mantle produces magmas of carbonatitic and ultrabasic/ultrapotassic compositions of the western rift branch. [ABSTRACT FROM AUTHOR]

The data of mineralogical studies of carbonatite veins and exocontact fenites in the open pit near the Khlibodarivka occurrence (Donetsk region, Volnovaha district) are presented. The thickness of carbonatite veins is neglegible, rarely exceeding 30 cm, and are mainly composed of calcite, alkaline amphibole and albite are the most common silicate minerals. Accessory minerals are more often represented by monazite, apatite, columbite, minerals of pyrochlore group, zircon, and opaque (ilmenite, rutile, magnetite, sulfides). In addition to the above-mentioned minerals, except for the minerals of pyrochlore group, chlorannite, aegyrine, REE-carbonates, baryte, allanite-(Ce), REE-apatite were found in the fenite haloes and veinlets. The reactionary interaction of the carbonatite melt enriched in alkalis and volatiles with the hosted enderbites caused the appearance of fenite halos both around the veins of carbonatites and the thin "net" of veinlets without a visible connection with carbonatites. The significant thickness of the fenite halos (usually twice as much in comparison to the thickness of the carbonatite veins), albitization of the surrounding rocks and saturation of the alkaline femic minerals of the carbonatite veins and fenites indicate that the initial carbonatite melt contained a significant amount of Na and volatile components (H2O, F, CO2, possibly Cl), and also had a more ferruginous composition of carbonates due to the presence of siderite or ankerite components (in the melt). As a result of the dissociation of primary carbonates, iron, as well as alkalis, was concentrated in the fluid phase and subsequently included in aegirine and alkaline amphiboles. The fenitization of host rocks took place in several stages. At the initial stage, the alkali-enriched fluids, mainly Na, F, and possibly Cl, were separated, while the later ones fluids were Fe-enriched and existing in more oxidized conditions. The fluids genetically related to carbonatites were penetrating into the host rocks caused the redistribution of trace elements. Barium, Th, and partly LREE were carried out from carbonatites most intensively, while Sr, Nb and a significant part of REE remained immobile due to the early crystallization of their host minerals and lack of subsequent autometasomatic alteration. Geochemistry of carbonatite veins (high content of incompatible elements such as Sr (117 777 ppm), increased REE (1624 ppm), elevated Nb, chondrite-normalized pattern of REE, high (La/Yb)cn = 24.7 and absence of negative Eu anomaly (Eu* = 1.03) and endogenous ratios of isotopes of Sr (0.703), C and, partially, O) testify to their magmatic origin. It is possible that the insignificant thickness of the carbonatite veins and the intensive interaction of juvenile fluids with the host granitoids and meteoric fluids caused the enrichment of carbonatites with 18O (δ18O 8.4÷20), but with the preservation of primary δ13C (–8 ÷ –6.5). Currently, the increased concentrations of rare metals were not found in carbonatites of the Khlibodarivka village. However, the presence of carbonatite veins in association with fenite halo, spatial connection to the Oktyabrsky massif of alkaline rocks, as well as numerous fenite occurrences in this area, increase the prospects of this area for such mineralization. [ABSTRACT FROM AUTHOR]

The Srednyaya Zima alkaline-ultramafic carbonatite complex is located in the Eastern Sayan and is a part of the area of manifestation of Neoproterozoic rare-metal alkaline-carbonatite magmatism along the southern and southeastern margins of the Siberian craton. Mineralogical studies of calciocarbonatites of the Srednyaya Zima complex have shown the presence of primary magmatic mineral phases of calcite, biotite (annite-phlogopite), ilmenite, and fluorapatite. Pyrochlore, zircon, burbankite, magnetite, rutile, titanite, strontianite, and barite were identified of the accessory minerals. The chemical composition of the magmatic minerals of the Srednyaya Zima carbonatites is similar to the mineral composition of the closely aged carbonatite complexes Belaya Zima and Arbarastakh. The rare-element and structural analysis of zircon from carbonatites showed the presence of two zones – a magmatic core and areas of recrystallization. U-Pb dating of igneous zircon showed the age interval of its crystallization – 637±4 Ma, which coincides with the geochronology of the formation of alkali-ultramafic rare-metal complexes along the southern margin of the Siberian craton. The formation of Neoproterozoic alkaline-carbonatite complexes is associated with tectonic events of the breakup of the Rodinia supercontinent.

Deposits enriched in rare earth elements (REEs) are abundant and diverse in mineralogy, but those of current economic value have extremely limited geographic distribution. Due to rapidly increasing demand and concerns about supply chain security, new and improved methods of prospecting, beneficiation, separation, purification, and recycling are needed. By overviewing the types and geographic distribution of REE deposits globally, their petrogenesis and major mineralogic assemblages, this review aims to deepen understanding of different types of REE deposits and distribution characteristics at the global level. Our review also seeks to provide a concise summary of important background information that is accessible to researchers from a variety of disciplines who are seeking to engage this rapidly expanding field. • Comprehensively summarize types of REE deposits at the global level. • Carbonatite REE deposits and their weathered residuals are prominent REE resources. • Marine sedimentary phosphorites, coals, and mine tailings can serve as REE resources. • REE production is constrained by geography, mining techniques, and policies. • Enhancing REE production necessitates a comprehensive industry chain. [ABSTRACT FROM AUTHOR]

The annular Richat Structure, in Mauritania, is among the most striking geological features on Earth visible from space. Nicknamed "the Eye of Africa", its intriguing concentric shaping has drawn attention for several decades. Formerly hypothesized as an astrobleme, it is now consensually recognized as a complex igneous intrusion, involving two ring-like gabbroic bodies, a central breccia and carbonatite dykes. However, the coexistence of until now undated tholeiitic gabbros and 99 Ma carbonatites akin to alkaline magmatism remained enigmatic. We provide the first plagioclase 40Ar/39Ar age determination and new geochemical data on the gabbroic bodies. Although no robust age was obtained, numerical modelling of our results suggests that the gabbros were intruded between 230 and 200 Ma. This age bracket is compatible with the ∼200 Ma Central Atlantic Magmatic Province (CAMP). Moreover, the compositions of these tholeiitic gabbros match the two chemical groups of the CAMP the best represented in northwest Africa. Therefore, we argue that the Richat gabbros correspond to two CAMP sills intruded conformably into the Late Proterozoic/Lower Paleozoic sedimentary sequences of the Taoudenni basin. About 100 My later, the gabbros and their country rocks were locally uplifted by an alkaline intrusion (expressed now on surface by the carbonatites), resulting in a circular doming, 40 km in diameter. The subsequent erosion led to the actual flattened concentric structure. Therefore, the Richat Structure represents a two-stage igneous-history, separated by a time lapse of ∼100 My, resulting in two CAMP gabbroic sills mimicking ring-like bodies. • The annular Richat structure is one of the most striking geological features on Earth. • Magmatic rocks are present as carbonatites and tholeiitic gabbros. • We provide the first plagioclase 40Ar/39Ar age determination on gabbros. • Gabbros were intruded ≈ 200My ago, contemporaneously with the CAMP. • The Richat structure formed during two magmatic events separated by ≈100 My. [ABSTRACT FROM AUTHOR]

• The composition of carbonatite melts is poorly known. • Experiments are conducted to calibrate Apatite – carbonatite melt partition coefficient. • 750 apatites are used as a proxy to decipher carbonatite melt composition. • REE content in carbonatite melts varies over 3 order of magnitudes. • This suggests carbonatites formation by immiscibility. Carbonatites are fascinating magmatic rocks because of their anomalous compositions, including the fact that some of these rocks host major rare earth element (REE) deposits. Despite much recent work, our understanding of their genesis and the composition of initial carbonatite melts is hindered by two major obstacles: most carbonatites we see are intrusive rocks, and they are commonly affected by metasomatic overprint. Here, we report experimental data in which we use apatite as a geochemical proxy to see back through crystallization and metasomatic events, thus making it possible to decipher the composition of carbonatite melts. We determined partition coefficients between apatite and carbonatite melts for a broad range of elements. The Na-rich nature of carbonatite melts plays an important role in apatite-melt partition coefficients, which are in the range of 1–7 for Sr, Y and REE in carbonatite systems. Using our new experimental data combined with >700 apatite composition data from carbonatites in various geodynamic settings, we show that carbonatite melt REE contents vary by more than two orders of magnitude. This variation cannot be solely produced by crystal-melt fractionation, implying that some carbonatite melts must be REE-rich or REE-poor initially, and that they mostly remain so during differentiation. We conclude that the degree of REE enrichment reflects carbonatite melts produced by immiscibility from variably differentiated alkaline magmas. [ABSTRACT FROM AUTHOR]

Almost all igneous rocks are composed of silicate minerals; carbonatites are the main exception to this rule. They form only a minor proportion of the continental crust but are of fundamental scientific and economic importance. These rocks, originally described as a limestone (Kaiserstuhl, Germany) or magmatic limestone (Alnö, Sweden), were recognized in 1921 by W. C. Brøgger as a distinct magmatic rock type under the name "Karbonatite" in the Fen complex (Norway). Extensive field mapping in Africa, and particularly studies within the Chilwa Alkaline Province (Malawi), have led to the discovery of diverse intrusive and extrusive carbonatites. The latter, including the Fort Portal volcanic field in Uganda, Rufunsa Province in Zambia, and Oldoinyo Lengai volcano in Tanzania (the only volcano ever to have been seen to erupt carbonatites), have been exceptionally important in the recognition of carbonatites as truly magmatic rocks. The possibility of the existence of carbonate melts has been confirmed and shown by experimental studies of diverse carbonate systems with added volatile components (H 2 O, F) and alkali elements (Na, K). The study of the Oldoinyo Lengai gregoryite-nyerereite carbonatites, which are mineralogically and compositionally different from all known carbonatites worldwide, has led to long-lasting discussions about the origin of carbonatites. This includes composition of primary/parental carbonate melt, derivation of carbonatites by either liquid immiscibility or fractional crystallization, carbonatite evolution, and especially, the possible genetic relationships between alkali-rich and alkali-poor carbonatites. The rapid alteration of Oldoinyo Lengai carbonatites and their transformation to calcite carbonatite-like rocks has been proposed as the explanation for the absence of alkali-rich carbonatites in the geological past. Detailed mineralogical studies have shown that the occurrence of nyerereite is not restricted to Oldoinyo Lengai and that this mineral is now known to occur in other carbonatites (e.g., Guly, Kovdor, Oka, Kerimasi), alkaline rocks, and kimberlites (and even in diamond). This would suggest compositionally different mantle-derived melts enriched in alkali elements. In addition, carbonatite tephra has an important role in the preservation of some key paleontologic and anthropologic localities in East Africa. Despite these important discoveries, several problems related to carbonatite petrogenesis are not yet resolved. Future work is required, and carbonatites within Africa, with its key localities, may help to solve these problems. • The Kaiserstuhl volcano is the first locality where carbonatites were described in 1783. • Field observations are the key evidences for recognition of carbonatites as magmatic rocks. • Oldoinyo Lengai is the only active volcano that erupts gregoryite-nyerereite carbonatite. • Alkali-rich carbonatites are important for the understanding of carbonatite genesis and evolution. • Existence of compositionally different carbonatite melts is confirmed by experimental studies. [ABSTRACT FROM AUTHOR]

[Display omitted] • Bastnäsite formed in the Vuoriyarvi carbonatites during Triassic at ca. 235 Ma. • Bastnäsite is not related to the magmatic event that formed the Vuoriyarvi complex. • Bastnäsite of the Vuoriyarvi is not supergene. The Vuoriyarvi massif is a Devonian multistage alkaline-ultrabasic carbonatite complex within the Kola alkaline province. Dolomite carbonatites of the Vuoriyarvi massif contain abundant rare-earth mineralization mainly represented by ancylite-(Ce) and bastnäsite-(Ce). Ancylite was previously shown to have probably formed in the Devonian (ca. 365 Ma) during an early postmagmatic overprint. Previous geological observations have revealed a much later crystallization of bastnäsite but have not been able to specify the exact age of the mineralization. The in situ U-Pb dating of bastnäsite allowed us to constrain its genesis. Bastnäsite for this study was extracted from two varieties of dolomite carbonatite breccias cemented by (1) quartz-bastnäsite and (2) strontianite aggregates (hereafter bastnäsite-rich and strontianite-rich carbonatites – BRC and SRC, respectively). The obtained age estimations (237.7 ± 9.8 Ma and 239.9 ± 4.1 Ma, respectively) indicate that both studied rocks were formed during a single event. The revealed age difference (∼125 Ma) excludes the genetic link between the bastnäsite origin and regional alkaline magmatism, pointing out an additional source for the Vuoriyarvi bastnäsite-bearing rocks. Moreover, the obtained U-Pb ages provide strong evidence that a Triassic event is responsible for the occurrence of bastnäsite mineralization due to hydrothermal REE redistribution from the Devonian ancylite-rich carbonatites. Most of the REEs released during this process via dissolution of ancylite were precipitated in situ as bastnäsite, while strontium was transported and incorporated into strontianite. The Pb isotopic characteristics of bastnäsite (206Pb/204Pb = 18.1 ± 0.1, 207Pb/204Pb = 15.3 ± 0.1, and 207Pb/206Pb = 0.84 ± 0.01) are most probably inherited from the Devonian host rocks of the Vuoriyarvi massif involved in the Triassic overprint. Isotopic signatures of Pb, Sr, and Nd show that the depleted mantle and lower crust played the leading role in formation of the Vuoriyarvi alkaline complex. Taken together, the results of the present study negate the supergene origin of the Vuoriyarvi bastnäsite, implying that the bastnäsite mineralization is not confined to near-surface layers and, therefore, may be dispersed more broadly throughout the complex. These findings raise the question on underestimation of the probable REE reserves and lay the groundwork for a reassessment of the economic potential of the Vuoriyarvi complex. [ABSTRACT FROM AUTHOR]

• Calcite bodies (CBs) consistently appear within a marble band of the UHT granulite facies metamorphic terrain of the Highland Complex (HC) in Sri Lanka. • The trace element and isotopic compositions of the CBs show distinctions from typical mantle-derived carbonitites, but indicating crustal sources. • These findings suggest that the CBs should be derived from the crustal anatexis of marble. • The UHT metamorphism, along with the presence of CO 2 -rich fluids, likely facilitated the crustal anatexis of marble. The special calcite bodies (CBs) exposed within the ultrahigh temperature (UHT) granulite facies terrain in the Balangoda area of Sri Lanka preserve shreds of evidence for the generation of crust-derived carbonatite in the crust. The CBs are exclusively made up of massive calcites and appear as concordant bands extending tens of meters or as meso -scale isolated pockets hosting the massive dolomitic marble band. Various sizes of mafic and calc-silicate enclaves occur as rotated or tilted structures within the CBs. The contact between the CBs and the host marble is texturally and mineralogically gradational, while the contact between the enclaves and the CBs is sharp. The large-ion lithophile elements and rare earth element contents of the CBs show enrichment compared to the host marble, while depletion compared to typical carbonatites. Furthermore, the Sr content and C-O isotope values in CBs differ from those found in known carbonatites, hydrothermal carbonates, or metasomatic carbonates. We suggested that the crustal anatexis of marble should be hypothesized as the possible mechanism for the origin of the CBs. Microtexural evidence of the calcite grains shows indications of the melting of the host marble. The release of CO 2 -rich fluids from the collision and thrusting of HC over VC, or related metamorphic events, likely lowered the solidus of carbonates, triggering crustal anatexis of marble during UHT granulite facies metamorphism. The generated low viscous carbonate melt may have moved rapidly, resulting in a low degree of mixing of silicates and fragmentation and dislocation of enclaves. The results of the present study reflect the existence of anatexis of carbonates under extreme crustal conditions and provide a better understanding of the sources, migration paths and reservoirs of the carbon recycling processes. [ABSTRACT FROM AUTHOR]

• The Aketashitage carbonatites have features aligning with crustal-origin carbonate rocks. • The remarkable positive δ13C excursion sets them apart from conventional carbonates. • They might be formed by the anatexis of metasedimentary marbles during peak metamorphism. • They underscore the potential impact of the Lomagundi-Jatuli event on the Tarim Craton. The Paleoproterozoic Aketashitage carbonatite of northwest China is one of few high-δ13C carbonatites worldwide. Its absence of associated alkaline silicate rocks positions it as an intriguing case for probing the complex formation process of carbonatites. To provide further constraints on the genesis of these carbonatites, we have executed an exhaustive study encompassing petrography, geochemistry, geochronology, and radiogenic-stable isotope analysis. The Aketashitage carbonatites exhibit complex intrusive relationships with surrounding wall-rocks and house a diverse array of xenoliths, unmistakably signifying their igneous origin. Their rare earth elements (REE) diagrams are characterized by a nearly flat pattern and pronounced positive Eu anomalies, indicating a significant impact from hydrothermal fluids. These carbonatites are notably deficient in REEs, Ba, Sr, U, Th, Nb, and Ta. Their 143Nd/144Nd ratios and εNd (t) values range from 0.5109 to 0.5111 and −8.13 to −6.11, respectively. Remarkably, their δ13C V-PDB values (V-PDB, Vienna Pee Dee Belemnite) are exceptionally elevated, ranging from +13.31 to +15.48 ‰, thereby setting them apart from typical mantle-derived carbonatites, which exhibit δ13C values below 0 ‰. These comprehensive geochemical traits diverge significantly from those of mantle-derived carbonatites but share striking resemblances with crustal-origin carbonate rocks. Moreover, petrographic and mineralogical analyses reveal indicators of regional metamorphism within these carbonatites, proposing their possible for formation through the anatexis of high-grade marbles during the peak metamorphic event of the Paleoproterozoic era. Coupled with determined formation ages, these results imply a potential connection of Aketashitage carbonatites formation to the collision and amalgamation of the supercontinent Columbia. Moreover, the notable δ13C enrichment observed in these carbonatites presents substantial evidence for the impactful reach of the global Lomagundi-Jatuli event on the Tarim Craton. [ABSTRACT FROM AUTHOR]

• The Hujiazhuang apatite-rich carbonatite complex was derived from a metasomatized lithospheric mantle. • Magnetite-calcite-apatite carbonatites and calcite carbonatites were generated through fractional crystallization. • REE-rich apatite crystallization is unfavorable to REE mineralization in carbonatites. Apatite is a common REE-compatible mineral in carbonatites, so its formation can affect the REE ore-forming potential of carbonatite systems. In nature, there are numerous apatite-rich carbonatite complexes, but the petrogenesis of these complexes has long been debated. To resolve this controversy, this study conducts elemental and isotopic analyses on the Mesozoic (∼120 Ma) Hujiazhuang carbonatite complex in North China. The Hujiazhuang complex contains magnetite-calcite-apatite carbonatites and calcite carbonatites. Calcite grains from these two rock phases exhibit identical initial 87Sr/86Sr ratios (0.71027–0.71038), indicating a common parental magma derived from a metasomatized lithosphere mantle. Then, from magnetite-calcite-apatite carbonatites to calcite carbonatites, the Mg, Fe, and Mn contents of calcite continuously increase while the Sr and REE contents decrease, consistent with elemental variations of calcite during fractional crystallization of carbonatitic magma. Therefore, these two lithologies are proposed to be formed through fractional crystallization of a common magma. Notably, the Hujiazhuang carbonatite complex was derived from a metasomatized lithosphere mantle and experienced intensive fractionation, which potentially facilitate the formation of REE deposits. However, this complex cannot meet REE's economic standard. The early-stage magnetite-calcite-apatite carbonatites have high REE contents (3,675–11,075 ppm), and a positive correlation can be observed between REE and P contents in this lithology. It indicates that early, extensive crystallization of apatite led to REE dispersion, largely hindering REE enrichment in later stages. In general, this study provides important insights into the genesis of the apatite-rich carbonatite complex and advances our understanding of the REE dispersion or enrichment behaviors in carbonatite system. [ABSTRACT FROM AUTHOR]

Rare Earth Elements (REE) form critical raw materials in environment-friendly, high-technology devices and components, and therefore have been classified as "critical minerals and metals" by most countries. About 62% of the global resources of REE occur associated with carbonatite-alkaline complexes; however, the entire production of REE in India currently comes from secondary deposits, even though India contains a variety of REE-enriched primary source rocks, particularly carbonatites and alkaline complexes. There is, therefore, a significant potential in the county for new REE deposit discoveries associated with carbonatite-alkaline complexes. This research attempts to identify exploration targets for REE associated with carbonatite-alkaline complexes in northern India utilising a Self-Organising Maps (SOM)-driven workflow. This unsupervised machine-learning-based workflow eliminates the hand-crafting of input predictor features. The algorithm creates clusters of features directly from primary gridded geophysical and topographical datasets. The obtained clusters are then analysed based on available geological knowledge and empirical spatial associations with known occurrences in the study areas to identify prospective clusters and generate prospectivity maps. Nine new targets are identified across the Shillong plateau in northeastern and Western Rajasthan in northwestern India. These new targets, in addition to the known carbonatite-alkaline complexes, are recommended for further data collection and follow-up exploration. It is noteworthy that these targets conform to the targets identified by Aranha et al. (2022a, 2022b) using mineral systems-guided fuzzy inference systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • Calcite from two mineralization stages displays distinct REE enrichment, the early calcite more enriches the HREE, and the late calcite depletes HREE and relatively enriches LREE. • Xenotime is an independent mineral mainly enriched HREE in the Jialu REE deposit, especially the Y element. • The REE mineralization that occurred at 239.2–240.9 Ma was first reported in the Jialu deposit, which recorded an important metallogenic event in the Triassic period. The Jialu carbonatites (∼240 Ma) in the Qinling orogenic belt are characterized by the enrichment of heavy rare earth elements compared with typical global carbonatites. The carbonatites are dominantly comprised of calcite, quartz, sulfate, K-feldspar, minor sulfides, and rare earth minerals such as monazite, bastnäsite, parisite, and xenotime, with two mineralization stages including the early quartz-K-feldspar-calcite stage (Stage I) and the late sulfide-rare earth mineral-calcite stage (Stage II). The rare earth element contents of Stage I calcite are higher than those of the Stage II, especially heavy rare earth elements. Calcite from the different mineralization stages exhibits variable chondrite-normalized REE patterns, with the HREE and REE abundances (134–1023 ppm) decreasing from the early to the late stage owing to the crystallization of xenotime. The δ13C (−5.39 ‰ to −6.68 ‰) and slightly higher δ18O (10.77 ‰ to 12.60 ‰) values for calcite from the Jialu carbonatites generally deviate from the values observed in the primary carbonatite field, which may be a result of Rayleigh-type fractionation. LA–ICP–MS U–Pb dating shows the lower-intercept ages of 243.8±5 and 237.6±1.3 Ma for the monazite and xenotime, respectively, with the weighted average 206Pb/238U ages of 240.9±6 and 239.2±1.3 Ma. These results indicate that the Jialu REE deposit was concurrent with Triassic carbonatite magmatism and REE mineralization observed on the southern margin of the North China Craton. This implies that this region may have great REE mineralization potential. [ABSTRACT FROM AUTHOR]

Alkaline‑carbonatite complexes are the main source of critical raw materials such as niobium (Nb) and rare earth elements (REE), which concentrate through a combination of magmatic and carbohydrothermal processes. These systems typically occur in close spatial connection with altered country rocks resulting from metasomatic alteration by exsolved K-Na-rich fluids, a process known as fenitization. Thus, the association between Nb-REE-rich carbonatites and fenites provides a unique opportunity for the investigation of carbohydrothermal alteration processes leading to critical metal enrichment. In this study, we focused on K-rich fenites associated with a shallow dike swarm system in the Boa Vista niobium deposit, the second-largest global producer of Nb, which is hosted within the Catalão II alkaline‑carbonatite complex in Central Brazil. We used a combination of micro-analytical techniques including EBSD, EMPA, and μ-EDXRF to unravel the complex micro-textural features of the fenites. Our data suggest that alkaline fluids exsolved vigorously from the carbonatite melts upon dike emplacement and pervasively metasomatized the country rocks, forming a melanocratic proximal fenite (phlogopitite), and distal fine-grained orthoclase-phlogopite-calcite fenites. Fluid alteration mobilized soluble cations (K, Fe, Mg, Ba, and Sr) plus S, CO 2 and OH−, whereas Nb and REE were retained in the carbonatite dikes. The consistent dike-orthogonal orientation of metasomatic phlogopite and orthoclase grains in the proximal fenite, determined by EBSD, suggests that newly formed metasomatic minerals precipitated from the fenitizing fluids along the flow path. These orientations differ from the isotropic textures defined by primary (magmatic) minerals like pyrochlore, tetraferriphlogopite, dolomite, and calcite. However, and despite the microstructural differences between magmatic and carbohydrothermal phlogopite, their mineral chemistry is similar, suggesting that fenites are almost synchronous to the emplacement of carbonatites. These data indicate that fenites provide a rich archive of alkali-rich fluid infiltration around Nb-REE-rich carbonatite intrusions. • The Catalão II carbonatite complex produced a pervasive potassium-rich fenitization. • K-rich fenites include phlogopitite and phlogopite-orthoclase-calcite fenite. • Mineral chemistry of metasomatic phlogopite is similar to magmatic phlogopite cores. • Oriented basal plane of metasomatic phlogopite crystallized orthogonal to the dikes. • The metasomatic event was probably simultaneous with the carbonatite intrusion. [ABSTRACT FROM AUTHOR]

• Carbon isotope fractionation factor CO 2 -carbonate. • C-isotope fractionation for all species containing the carbonate-group almost identical. • Consistent set of 13C/12C fractionation functions of geologically important species. • Degassing of MORB dominated by H 2 CO 3 , hence same δ13C value as mantle. • High pressure fluids dominated by H 2 CO 3 , no 13C/12C fractionation during subduction. High-temperature experiments on the CO 2 - carbonate melt and carbonatite - nephelinite melt pairs are used to quantify equilibrium carbon isotope fractionation at conditions of 1 atm, 750–950 °C and 0.3–0.8 GPa, 1170 °C, respectively. Together with available experimental data, all results are within error consistent with a universal CO 2 – CO 3 2− fractionation function 10 3 ln α C O 2 / c a r b o n a t e = 6.41 (11) · 10 6 T 2 − 2.54 (8) · 10 12 T 4 (T in K, valid for >650 °C) that encompasses solid and molten carbonates and also carbonate components in silicate melts. The experiments hint towards a small fractionation of carbonatite vs. silicate melt of +0.39 ± 0.35 ‰ (1 σ), which however remains within error indistinguishable from zero. A generalized single fractionation function suggests that the nearest neighbours of carbon (i.e., O) dominate isotope fractionation while the next nearest neighbours only have a minor role, which greatly facilitates the understanding of carbon transfer in the deep Earth. If there is small to negligible 13C/12C-fractionation between solid carbonate, molten (ionic) carbonate liquid and covalent carbonated silicate melt, then H 2 CO 3 - and HCO 3 −-species in fluids or gases should also have low 13C/12C fractionation factors relative to the carbonate-group in depolymerized silicate melts or carbonates. Speciation models predict that the H 2 CO 3 - and HCO 3 −-species dominate over the CO 2 species in COH-fluids at high pressures or high temperatures, i.e. during MOR basalt degassing or subduction zone devolatilization. A consequent small 13C/12C fractionation (of 0.1–0.3 ‰) applies to (i) continuous degassing of COH-fluids from mid-ocean ridge basalts, consistent with an observed difference of δ13CMORB and δ13Cmantle ≤1 ‰ and (ii) C-transfer of a carbonatite or COH-fluid from the subducted crust to the mantle, which then has a near-identical δ13C as the source. Finally, a combination of our results with available experimental values allows calculation of a consistent set of high-temperature (>650 °C) carbon fractionation factors for CO 2 , CO 3 2− (carbonate, carbonatite or carbonated silicate melt), CH 4 , CO, and graphite/diamond. [ABSTRACT FROM AUTHOR]

The Araxá alkali-carbonatitic complex is a significant source of niobium and phosphate and accounts for approximately 75% of global niobium production. The complex contains magmatic niobium ore minerals, such as natrium-pyrochlore and calcium-pyrochlore, and hydrothermal niobium ore minerals in the form of kenopyrochlore and natrium-pyrochlore. The magmatic micas in the complex are tetraferriphlogopites that exhibit Mg-rich cores surrounded by one or two rims of tetraferriphlogopites formed by hydrothermal alteration. These rims have lower IVAl and Mg apfu (atom per formula unit) values but higher IVFe3+ and Fe2+ contents than the original phlogopite. Microphlogopitite dikes that formed during several episodes of magmatism between 83.48 and 77.4 Ma contain tetraferriphlogopites, phlogopite, eastonite, biotite, and annite-siderophyllite. These micas exhibit variations in Mg2+ and Fe2+ concentrations owing to the magmatic differentiation and subsequent hydrothermal alteration, resulting in rims with higher Mg and lower Fe2+ concentrations than the mica core. The Araxá complex also contains magmatic magnetite that has been further altered by hydrothermal processes, resulting in the formation of ilmenite and the oxidation and crystallization of titaniferous hematite. Magnetite within the microphlogopitites displays variations in the concentrations of Fe3+ and Ti, likely owing to the timing of the intrusion of the microphlogopitites into the Araxá complex. The degree of oxidation of the oxidized ilmenite of the Araxá complex varies based on the intrusions, which occurred before or during hydrothermal activity. Apatite is concentrated in carbonatites, which form a ring around the core of the complex, and disseminated in phlogopite-rich rocks hosting norsethite carbonatite veins in the center of the complex. The apatite in the core rocks exhibits complex zonation with unexpected changes in Ca, P, and Sr concentrations. Apatite in the carbonatite ring was generated during the differentiation of the carbonatite magma, whereas that in the core rocks exsolved from the phosphorus-saturated carbonatite magma (in press). The carbonate minerals dolomite, norsethite, and calcite exhibit compositions grouped around their end-members, suggesting that they formed during different stages of magmatic source differentiation. Strontium differentiation occurs in the magmas corresponding to each endmember, and carbocernaite is exsolved from the Sr-saturated crystals, thus forming micro-inclusions in some carbonate crystals that have been displaced by late-magmatic hydrothermal fluids. • The main minerals of the Araxá complex rocks are pyrochlore, apatite, carbonate, magnetite, ilmenite, and phlogopite. • Approximately 75% of global niobium production comes from pyrochlore mined in the Araxá alkali-carbonatite complex. • Araxá also produces significant amounts of apatite for phosphate production. • The Araxá complex is formed by a significant volume of Sr-rich norsethite (Ba) carbonatites. • Approximately 62 vol% of the core rocks of the Araxá complex are pyrochlore-rich phlogopitites and phlogopite-rich rocks. [ABSTRACT FROM AUTHOR]

The intraplate oceanic island of Fogo (Cape Verde islands) is in the shield-stage of its evolution. It experienced a major lateral collapse ca. 70 ka ago, after which a sequence of lavas about 2 km thick infilled the collapse scar, with eruptions continuing to the present day. Nearly 100 lavas from before and after the collapse have been sampled and analysed for bulk rock compositions. These lavas have been divided into five Formations (2 pre-collapse and 3 post-collapse), using stratigraphic principles. There is little difference in the major and trace element compositions of pre- and post-collapse lavas, and the main process that gives rise to their diversity is fractional crystallization of observed mineral phases. Pre-collapse lavas show a narrower range of bulk compositions than the post-collapse lavas, but this may be because the sampling strategy avoided these more altered and poorly exposed lavas. A group of high-P 2 O 5 lavas, erupted both before and after the collapse, may have been extensively contaminated by local seamount-stage intrusive carbonatites. A subset of 30 samples was analysed for Sr, Nd and Pb isotope ratios. They show a plume component which has interacted with two contaminants: (1) an increase in 87Sr/86Sr and decrease in 143Nd/144Nd with decreasing MgO, which indicates varying amounts of assimilation of an enriched lithospheric component coupled to fractionation (AFC), and (2) contamination of the more fractionated magmas, without further fractionation, involving the suspected carbonatite contaminant which originates from a more isotopically depleted source. Overall trends of increasing 87Sr/86Sr, and slight decreases in 143Nd/144Nd and all radiogenic Pb isotope ratios, with decreasing age of eruption, indicate a general increase in the assimilated lithospheric component throughout shield-stage activity. However, there is no clear evidence of any change in isotope compositions across the period of the collapse. All Formations show similar wide and overlapping ranges of isotope compositions, regardless of the timescales represented by each Formation. This implies that much short-term variation in the magma sources was superimposed on the overall trend of increasing lithospheric assimilation. • No evidence of major changes in geochemistry between pre- and post-collapse magmas on Fogo • Three separate sources of isotope variation in Fogo magmas: plume, enriched lithospheric component, local carbonatites • Evidence of long-term changes in magma source in Fogo, towards a more enriched source [ABSTRACT FROM AUTHOR]

• Ca isotopes do not fractionate during magmatic differentiation of carbonate-rich magmas. • Carbonated silicate melts have δ44/40Ca lower than MORB. • K-richterite and carbonate in mantle sources produce low δ44/40Ca values of magmas. • Ca isotopes are a robust tracer of volatile-induced mantle lithological variation. The volatile components CO 2 and H 2 O induce mantle melting and thus exert major controls on mantle heterogeneity. Primitive intraplate alkaline magmatic rocks are the closest analogues for incipient mantle melts and provide the most direct method to assess such mantle heterogeneity. Given the considerable Ca isotope differences among carbonate, clinopyroxene, garnet, and orthopyroxene in the mantle (up to 1 ‰ for δ44/40Ca), δ44/40Ca of alkaline rocks is a promising tracer of lithological heterogeneity. We present stable Ca isotope data for ca. 1.4 Ga lamproites, 590–555 Ma ultramafic lamprophyres and carbonatites, and 142 Ma nephelinites from Aillik Bay in Labrador, eastern Canada. These primitive alkaline rock suites are the products of three stages of magmatism that accompanied lithospheric thinning and rifting of the North Atlantic craton. The three discrete magmatic events formed by melting of different lithologies in a metasomatized lithospheric mantle column at various depths: (1) MARID-like components (mica-amphibole-rutile-ilmenite-diopside) in the source of the lamproites; (2) phlogopite-carbonate veins were an additional source component for ultramafic lamprophyres during the second event; and (3) wehrlites at shallower depths were an important source component for nephelinites during the final event. The Mesoproterozoic lamproites show lower δ44/40Ca values (0.58 to 0.66 ‰) than MORBs (0.84 ± 0.03 ‰, 2se). This cannot be explained by fractional crystallization or melting of the clinopyroxene-dominated source but can be attributed to a source enriched in the alkali amphibole K-richterite, which has characteristically low δ44/40Ca. The δ44/40Ca values of the ultramafic lamprophyre suite during the second rifting stage are remarkably uniform, with overlapping ranges for primary carbonated silicate melts (aillikite: 0.67 to 0.75 ‰), conjugate carbonatitic liquids (0.71 to 0.82 ‰) and silicate-dominated damtjernite liquid (primary damtjernite: 0.68 to 0.72 ‰). This suggests negligible Ca isotope fractionation during liquid immiscibility of carbonate-bearing magmas. Combined with previously reported δ44/40Ca values for carbonatites and kimberlites, our data suggest that carbonated silicate melts in Earth's mantle have δ44/40Ca compositions resolvably lower than those for MORBs (0.74 ± 0.02 ‰ versus 0.84 ± 0.03 ‰, 2se). The δ44/40Ca values of the Cretaceous nephelinites (0.72 to 0.78 ‰) are homogenous and similar to those of the 590–555 Ma ultramafic lamprophyres, suggesting that the wehrlitic source component for the nephelinites formed by mantle metasomatism during interaction with rising aillikite magmas during the second rifting stage. Our results highlight that both K-richterite and carbonate components in mantle sources can result in the systematically low δ44/40Ca values of alkaline magmas, which may explain previously reported low δ44/40Ca values of alkaline rocks and some carbonatites. Our study indicates that Ca isotopes are a robust tracer of lithological variation caused by volatiles in the Earth's upper mantle. [ABSTRACT FROM AUTHOR]

In the northwestern Mauritanian Taoudeni Basin, the famous Richat structure is the only occurrence known of carbonatite. Despite several studies on this unique geological feature, the mineralogy and petrogenesis of the Richat dome carbonatites are underdocumented.Major, trace element whole rocks and carbon-oxygen isotope data are reported for Cretaceous carbonatites from Richat dome in Mauritania. More than 45 carbonatite dykes have been mapped, they are oriented N°15-N°80. Based on color, four groups have been identified, dark, gray, red breccia, and brown carbonatites. They exhibit different mineralogical and texture characteristics. The studied carbonatites are mainly composed of dolomite, calcite, and ankerite, with lesser amounts of apatite and quartz. Accessory minerals such as pyrochlore, spinel, pyrite, chalcopyrite, magnetite, hematite, baryte, phlogopite, and zircon are also present. Other REE-bearing minerals (britholite, monazite, and bastnaesite) were revealed by X-ray diffraction.Three types of carbonatites are identified: magnesiocarbonatites (dominants), ferrocarbonatites and calciocarbonatite. The presence of spinel and pseudomorphed olivine, as well as high contents of Ni (380 ppm), Co (34 ppm), and Cr (840 ppm) in the dark and brown carbonatites, suggests that these rocks are the result of the low-degree partial melting of a spinel-bearing peridotite. Furthermore, the Zr/Hf (48–80) and Nb/Ta (26–48) ratios show significantly higher values than those observed in the primitive mantle, they are interpreted as formed by partial melting of the metasomatized mantle. The high values of δ18O in addition to the presence of baryte, ankerite, and quartz in the Richat carbonatites suggest that they are the result of secondary processes involving a low-temperature fluid-rock interaction.The carbonatites from the Richat dome are enriched in LREE (La = 258–1070 ppm) and LILE as Sr, Ba, Th, U, and depleted in HREE (Yb = 2–14 ppm) and HFSE such as Zr, Hf, Ti. These characteristics are like those of Cretaceous peri-atlantic alkaline carbonatites such as Barra do Itapirapua in Brazil, Ondurakorume and Kalkfeld in Namibian, lupangola in Angola, and Twihinate in Morocco. Reactivation of a deep pan-African NNE-SSW lineament would have been a fundamental factor in the formation of the Richat carbonatites. • The Cretaceous Richat carbonatites were generated by partial melting of the metasomatized mantle. • The partial melting occurred under significant structural influence. • The genesis of the Richat carbonatites was strongly influenced by hydrothermal activity. [ABSTRACT FROM AUTHOR]

Apatite is one of the most common minerals found in a wide variety of rocks. It has variations of minor and trace elements in its composition that allow the recognition of its host lithotype. In this study, apatite grains identified in rock samples and grains recovered from heavy mineral concentrates in the Angico dos Dias Carbonatite Complex region, located on the border between the State of Bahia and the south of the State of Piauí, Brazil, were analyzed using an electron probe microanalyzer. These concentrates come from alluvial sediments downstream of the carbonatitic body from drainage channels. Sampling was guided by regional geochemical analytical results for the chemical elements of apatite. Grains were detected in alluvial covers for up to 9.5 km downstream in all fraction including coarse sand (1.0 mm) in proximal samples and up to medium sand fractions (0.50 mm) in distal samples. The 96 analytical results on 48 apatite grains show that most have chemical signatures characteristic of metacarbonatitic lithotypes (SrO 0.88–1.7 wt%) and that some host lithotypes were affected by the substituition processes (Si + LREE (Ce+La+Nd) 0.15–0.59 apfu), which may be an alternative to metasomatic fenitization. Statistical analysis by principal component analysis (PCA) helped the visualization and interpretation of the data by constructing clusters of the different compositions of apatite. The apatite fields of carbonatitic and silicate source rocks were separated and associated with the results obtained for alluvial apatite grains, resulting in carbonatitic derivation. The results of using this prospecting tool were satisfactory. Thus, it was possible to classify and detect the sources of alluvial apatite grains relating them to the Angico dos Dias carbonatite body, in addition to proposing a new potential area for the occurrence of carbonatitic bodies northeast of the Angico dos Dias phosphate deposit. • Apatite is used to provide the chemical signature of the source. • Stream sediment data is useful to guide sampling of indicator heavy minerals. • PCA helps in the interpretation and visualization of mineral chemistry data. • Alluvial apatite grains found in a drainage channel. [ABSTRACT FROM AUTHOR]