Carbonatite Research Articles

Minerals of the apatite group from ultramafic lamprophyres of the Zima alkaline-ultramafic carbonate complex (Urik-Iya graben, Eastern Sayan region)

Research subject. This paper presents the results of a study of minerals of the apatite group from aillikites of the Zima alkaline-ultramafic carbonatite complex. Aim. To determine the composition of apatites to characterize the studied rocks and to elucidate the patterns of element distribution in the magma of aillikites in the late crystallization stages. Materials and Methods. Minerals of the apatite group from 6 samples (4 from dikes of the Bol'shetagninsky massif, 1 from the Bushkanai dike, 1 from the Beloziminskaya pipe) were studied. The samples showed similar textural-structural features, differing in the mineral composition of the groundmass. Results. Minerals of the apatite group are widespread in the groundmass of all the studied rocks. They are characterized by idiomorphic, subidiomorphic crystals of homogeneous composition, 10-100 microns in size. Minerals of the apatite group were identified as fluorapatite and hydroxyapatite characterized by a significant SiO2 content (up to 5 wt %), low SrO content (up to 1.5 wt %) and REE2O3 (up to 2 wt %). The studied apatites also include MgO, FeO, Na2O, SO4, and CO3. Fluorapatites from dyke aillikites are characterized by a higher silicon content as compared to apatites from the Beloziminskiy pipe. Apatites from fresh aillikites of the Bushkanai dyke have a relatively higher content of Sr, REE, and F than minerals from serpentinized samples. Conclusions. The composition of the studied apatites has a lower Sr and Ba content than orangeite and lamproite apatites. In terms of Sr, Si and REE contents, the studied apatites are similar to kimberlite and ailikite apatites. Such a difference allows minerals of the apatite group to be used as an indicator mineral for classification of kimberlites and related rocks, but only in combination with petrography and composition of other minerals of the groundmass.

Okorusu tailings – Part II: REE mineralisation, and oxygen and carbon isotope variations

Abstract The Okorusu carbonatite complex and the mine’s tailings dump constitute a potential resource of rare earth elements meaning lanthanides and yttrium (REY). Small (≤100 μm), hydrothermal REE fluorcarbonates (bastnäsite, parisite, röntgenite and synchysite) are the principal light REE carriers while the heavy REE and Y are primarily hosted in magmatic apatite, magmatic-hydrothermal carbonates (calcite, dolomite, ankerite) and hydrothermal fluorite. Bastnäsite and röntgenite occur as individual, liberated crystals in the tailings albeit syntaxial intergrowths of acicular parisite-synchysite-röntgenite are more common. There is a clear trend of decreasing REY contents in the magmatic carbonates over the early hydrothermal carbonates to the late hydrothermal carbonates. The lower total REY abundance in the hydrothermal carbonates is interpreted to be mainly a consequence of their limited redistribution during hydrothermal activity following the breakdown of the primary magmatic minerals and precipitation of REE fluorcarbonates. During this process hydrothermal fluids changed the isotopic composition of the Okorusu carbonatites by shifting δ13C-δ18O pairs of magmatic carbonatite (δ13C: -6.5‰ to -5.4‰; δ18O: +6.4‰ to +11.2‰) to higher values of hydrothermally modified carbonatite (δ13C: -4.2 to -2.2; δ18O: +12.0‰ to +17.5‰). The preferential presence of REY in the magmatic carbonates and to a minor degree in the hydrothermal carbonates in the Okorusu carbonatite and tailings is an important exploration tool for primary magmatic REY mineralisations. The recovery of REE fluorcarbonates can be considered in the development of the flowsheet for the planned recycling of the tailings material as a potential REY by-product.

Magmatic and post-magmatic evolution of the Newania carbonatite complex, Rajasthan, north-western India

AbstractThis work describes the mineralogy of dolomite carbonatite occurring at the Newania carbonatite complex, Rajasthan, north-western India. The mineralogy records the textural and compositional features of magmatic and post-magmatic stages of carbonatite evolution. Ferroan dolomite is the principal constituent and displays variable degrees of deformation, ranging from brittle-to-ductile deformation regimes. Apatite exhibits textural and compositional evolutionary trends from early-to-late stages of carbonatite evolution. Two varieties of amphibole are reported for the first time from this complex, ferri-winchite and cummingtonite; the former is magmatic and the latter is metamorphic in origin. The columbite–tantalite-series minerals are columbite-(Fe), and their paragenesis evolves from composite grains with pyrochlore to individual crystals. Pyrochlore is magmatic with U–Ta–Ti-rich compositions and shows evolution from calciopyrochlore to kenopyrochlore, followed by alteration during late-stages of carbonatite evolution. Monazite and baryte constitute the post-magmatic mineral assemblage; the former is hydrothermal and crystallised after precursor apatite, whereas the latter is associated exclusively with columbite–pyrochlore composites. On the basis of the mineralogy of the carbonatite, it is concluded that the parent magma was generated by low-degree partial melting of magnesite–phlogopite-bearing peridotite.

Geochronology, geochemistry, and Sr–Nd–C–O isotopic composition of the Qieganbulake carbonatites at the northeastern margin of the Tarim Craton, northwest China

Carbonatites are the predominant source of rare earth element (REE) resources globally, but their origins and petrogenesis remain unclear. Currently, >600 carbonatites have been found worldwide, but only a few (<14%) of these carbonatites contain REE deposits. Some carbonatites are rich in REEs but do not host high-grade REE deposits. The Qieganbulake carbonatites at the northeastern margin of the Tarim Craton, China, are ideal for investigating why some carbonatites do not host REE mineralization. We undertook a detailed petrographic, geochronological, and isotopic study of the Qieganbulake carbonatites. In previous studies, a carbonatite at Qieganbulake was dated to ca. 811 Ma. However, in this study, ages of ca. 926 Ma were obtained for phlogopite and baddeleyite in carbonatite dykes rather than pyroxenites or phlogopitelites-bearing carbonatite complex. Such ages of ca. 926 Ma are consistent with a previously published apatite U–Pb age. 40Ar/39Ar dating of phlogopite yielded a plateau age of 928 ± 17 Ma and secondary ion mass spectrometry U–Pb dating of baddeleyite yielded a concordia age of 926 ± 8 Ma, implying the carbonatite was emplaced at ca. 926 Ma. Laser ablation–inductively coupled plasma–mass spectrometry U–Pb dating of monazite yielded a 206Pb/238U intercept age of 811 ± 27 Ma, indicating that the main REE mineralization stage was coeval with later carbonatite emplacement at ca. 811 Ma. The studied Qieganbulake calcio-carbonatite samples are REE-rich (864–1495 ppm) and consist of calcite (62.18–93.92 vol%), apatite (1.10–24.82 vol%), dolomite (0.37–10.48 vol%), and lesser amounts of phlogopite, olivine, monazite, magnetite, and vermiculite. The calcite, apatite, and monazite (<1 vol%) have REE contents of 757–1111 ppm, 5127–11,556 ppm, and 45.35–59.88 wt%, respectively, and account for the high REE contents of the carbonatites (883–1539 ppm). The small amount of monazite in this carbonatite suggests it is a prospective rather than a confirmed ore deposit. Petrographic studies, oxygen values of 5.74 and 7.51 ‰ and characteristics of trace element for apatites with less monazite around suggest they have magmatic origin with less alteration. The 926 Ma Qieganbulake carbonatite has high Sr isotope ratios (0.70580–0.70639) and negative εNd(t) values (−7.6 to −11.8), indicative of an enriched mantle source. δ13CPDB and δ18OVSMOW values of the carbonatites vary from –3.3‰ to –4.1‰ and 9.7‰ to 12.0‰, respectively, indicative of a contribution from recycled crustal C that was probably derived from a subducted oceanic slab. Above characteristics of carbonatite with ∼926 Ma are different with those of carbonatite occurred at ∼811 Ma, suggesting two carbonatite emplacements in Qieganbulake. Based on our results and the regional geology, we infer that the 926 Ma Qieganbulake carbonatite was derived by partial melting of a subcontinental lithospheric mantle source that had been metasomatized by early Neoproterozoic subduction during circum-Rodinia subduction in the northern Tarim Craton. Early crystallization of apatite sequestered the REEs, resulting in limited REE mineralization. In addition, the limited hydrothermal alteration and ligands available, such as F− and SO42−, hindered REE transportation and precipitation.

Early-stage application of process mineralogy methodologies for mineral tracking in flotation of rare earth elements (REE)-bearing minerals from a deposit in Norway

The Fen carbonatite complex, situated in Vestfold and Telemark County, South Norway, is one of the largest known rare earth elements (REE) deposits in Europe with ongoing exploration drilling. Literature regarding the complex processing of the Fen ore is lacking, and thus, the present work investigates for the first time its beneficiation potential and applies process mineralogy studies to understand in a sound manner the performance variations during the processing of REE ore. Flotation kinetics studies revealed that (i) high concentration of total REE-oxide (TREO) can be achieved during rougher flotation; (ii) the high concentration of sulphur (in form of pyrite as main sulphide) can be achieved at the beginning of flotation; (iii) the highest concentration of TREO can be reached after flotation of S. Further processing (i.e., by the addition of cleaning stages and grinding) has a potential to improve the overall recovery of TREO and S. However, the decision of further processing will depend, among other factors, on the definition of following hydrometallurgical routes (e.g., leaching, solvent extraction) and the economic aspects of the overall process. So far, the reported results show that a rather flexible process can be achieved for the material from the Fen deposit.

The genesis of calcite and dolomite carbonatite-forming magma by liquid immiscibility: a critical appraisal

AbstractLiquid immiscibility has become the preferred mode of genesis for the carbonatite rocks, which commonly, but not exclusively, accompany silicate rocks in alkaline-rock complexes. This concept has been universally based on the presumption that nephelinitic and phonolitic magmas can evolve to a stage where two conjugate immiscible liquids separate. It is assumed that these two liquids separate quickly, or even instantaneously, into discrete bodies of magma capable of being intruded or extruded with subsequent independent crystallization. Supporting evidence generally given is: alleged consanguinity as discrete occurrence of the two rock types; similarity of radiogenic isotope ratios; trace element contents similar to those predicted from experimentally derived partition coefficients. We do not accept that a general case for liquid immiscibility has been demonstrated; although we do accept that silicate and carbonate liquids are inherently immiscible, we maintain that they are not conjugate in a petrogenetic context. We have reviewed and critically examined the experimental data purporting to establish liquid immiscibility and find that when applied to natural rocks, they are based on inappropriate experimental designs, which are not relevant to the genesis of calcite or dolomite carbonatites, although they might have some relevance to Oldoinyo Lengai nyerereite–gregoryite lavas. The design of these experiments guarantees immiscibility and ensures that the carbonate liquids formed will be calcitic or sodium-rich. We dispute the validity of comparing the trace element contents of natural rocks, which in many instances do not represent liquid compositions, to experimentally determine partition coefficients. We consider that experimental design inadequacies, principally assuming but not proving, that the liquids involved are conjugate, indicate that these coefficients are merely an expression of the preference of certain elements for particular liquids, regardless of how the liquids formed. Proof of consanguinity in alkaline complexes requires more accurate age determinations on the relevant rock types than has generally been the case, and in most complexes, consanguinity can be discounted. We dispute the contention that melt inclusions represent parental melts, although they might elucidate the character of magmas undergoing fractional crystallization from magmatic to carbothermal stages. Radiogenic isotope data are shown to be too widely variable to support a case for liquid immiscibility. We address the contention that calcite cannot crystallize from a dolomitic liquid formed by direct mantle melting, and must therefore have crystallized from a calcite carbonate liquid generated by liquid immiscibility, and demonstrate that it is an unsupported hypothesis as calcite can readily crystallize from dolomitic liquids. We observe that, because immiscible dolomite liquids have never been produced experimentally, the liquid immiscibility proposition could at best be applied only to calcite carbonatites, thus leaving unexplained the large number of dolomite carbonatites and those of either type, which are not accompanied by alkaline silicate rocks. The assumed bimodality of alkaline-rock carbonatite complexes is considered to be fallacious and no actual geological or petrographic evidence for immiscibility processes is evident in these complexes. Several examples of alkaline rock carbonatite complexes for which immiscibility has been proposed are evaluated critically and shown to fail in attempts to establish them as exemplifying immiscibility. We conclude that no actual geological or experimental data exist to establish liquid immiscibility being involved in the genesis of calcite or dolomite carbonatite-forming magmas.

Paleoproterozoic alkaline-carbonatite magmatism in the convergent tectonic setting: Evidences from 2.07 Ga Dubravinsky complex in the Eastern Sarmatia

The Dubravinsky alkaline-carbonatite complex (ACC) include two relatively large plutons: the arcuate shape Dubravinka pluton and linear Chernyanka pluton intruding the Paleoarchean TTG of the Kursk block of Sarmatia and consisting of three main lithologies: alkaline pyroxenites, carbonatites (together with silicocarbonatites and phoscorites) and syenites (including alkaline granites). The carbonatites and silicocarbonatites are enriched in trace and rare earth elements, which contents vary significantly.The source for the alkaline pyroxenites and carbonatites could be enriched protoliths from the subcontinental lithospheric mantle, produced by melting and release of fluids from the subducted oceanic slab at c. 2.1 Ga. The primary igneous C and O isotope composition is preserved in the carbonatite: δ13C (‰ VPDB) = (−4.9) - (−6.4), δ18O (‰ VSMOW) = (+8.0) – (+9.8). The alkaline granites display well preserved isotope markers of a long-lasting crustal prehistory indicating Paleoarchean source. The syenites have a lesser contribution of Paleoarchean crustal material.The alkaline pyroxenites and carbonatites are undoubtedly initially igneous rocks that have undergone c. 2.07 Ga-old high-temperature metamorphism. The Dubravinsky ACC is most likely 2.07–2.08 Ga-old, not much older than the metamorphic event. The Dubravinsky ACC has been formed in a suprasubduction setting from both mantle and crustal sources. It is the earliest known carbonatite complex in the World resulted from subduction-collision processes, and the first sign of transition to deep subduction of the modern style.

Indian carbonatites in the global tectonic context

Chrono-tectonic settings of the carbonatite occurrences of India are reviewed with a focus on the “big picture” of carbonatite emplacements in the Indian plate in relation to global tectonic events associated with the amalgamation and breakup of supercontinents. Four chrono-tectonic domains, namely, Southern domain, Southeastern domain, Northeastern domain and Northwestern domain, are delineated based on the geographical distribution, tectonic settings and temporal relationships amongst the carbonatite complexes. The Southern domain comprises two sub-domains – Paleoproterozic and Neoproterozoic. The Paleoproterozoic sub-domain is related to extension due to relaxation after the Southern Granulite Terrain-Dharwar accretion, while the Neoproterozoic sub-domain is related to rifting related to the fragmentation of Rodinia. The Southeastern domain is related to the Mesoproterozoic fragmentation of Columbia. The Northeastern domain is related to the Mid-Cretaceous breakup of Greater India from Australia-Antarctica driven by the Kerguelen mantle plume that also produced the Rajmahal-Sylhet Large Igneous Province (LIP). The Northwestern domain is related to the Late-Cretaceous Indo-Seychelles-Madagascar split and the passage of Greater India over the reunion hotspot, which also produced the Deccan LIP.

Cold season dose rate contributions from gamma, radon, thoron or progeny in legacy mines with high natural background radiation.

In areas with high natural background radiation, underground cavities tend to have high levels of airborne radionuclides. Within mines, occupancy may involve significant exposure to airborne radionuclides like radon (222Rn), thoron (220Rn) and progeny. The Fen carbonatite complex in Norway has legacy mines going through bedrock with significantly elevated levels of uranium (238U) and especially thorium (232Th), and significant levels of their progeny 222Rn and 220Rn. There are also significantly elevated levels of gamma radiation in these mines. These mines are naturally chimney ventilated and release large volumes of air to the outdoors giving a large local outdoor impact. We placed alpha track detectors at several localities within these mines to measure airborne radionuclides and measured gamma radiation of bedrock at each locality. The bedrock within the mines shows levels up to 1900Bqkg-1 for 238U, 12000Bqkg-1 for 232Th and gamma dose rates up to 11μSvh-1. Maximum levels of airborne radionuclides were 45000Bqm-3 for 220Rn and 6900Bqm-3 for 222Rn. In addition, we measured levels of thoron progeny (TnP). In order to estimate radiation dose contribution, TnP should be assessed rather than 220Rn, but deposition-based detectors may be biased by the airflow of mine-draft. We present dose rate contributions using UNSCEAR dose conversion factors, and correcting for airflow bias, finding a combined cold season dose rate within these mines of 17-24μSvh-1. Interestingly, fractional dose rate contributions vary from 0.02 to 0.6 for gamma, 0.33 to 0.95 for radon and 0.1 to 0.25 for TnP.

Study of the Distribution of Rare-Earth Elements and Strontium in Apatite from Rocks of the Vuoriyarvi Carbonatite Complex by Total-Reflection X-Ray Fluorescence Spectrometry (TXRF): First Results and Prospects

Abstract —We have studied the trace element composition of apatite from several varieties of carbonatites and associated apatite ores of the Vuoriyarvi alkaline-ultrabasic carbonatite complex (Kola region, northwestern Russia), which hosts several commercial deposits of apatite–magnetite and pyrochlore ores and large-scale unexplored rare-earth mineral occurrences. The composition of apatite was analyzed by total-reflection X-ray fluorescence spectrometry (hereafter, TXRF). Verification of the obtained results was carried out by ICP-MS analysis of weighed portions of apatite monomineral samples. It is shown that during the evolution of the Vuoriyarvi carbonatites and associated apatite–magnetite ores, the contents of Sr and rare-earth elements (REE) and the degree of REE fractionation in apatite synchronously increased. The obtained TXRF data on the composition of apatite in the carbonatite complex demonstrate the efficiency of TXRF and its high potential in both applied and fundamental research concerned with apatite of various geologic objects.

A new model for the origin of pyrochlore: Evidence from the St Honoré Carbonatite, Canada

The Neoproterozoic St Honoré carbonatite complex is one of three currently mined niobium deposits. In these and most other carbonatite-hosted niobium deposits, pyrochlore is the principal ore mineral. Four major pyrochlore-bearing rock types are observed in the St Honoré complex, namely, biotitite, magnetite-biotite rock, apatitite and carbonatite. The textural relationships among the different rock types and within each rock type are extremely complex, as are the textures and crystal chemistry of the pyrochlore. Compositionally, there are two major types of pyrochlore. Type-1 is enriched in Ta, U, Zr, Sr, Th, Fe, REE and Cl and Type-2 is a Ca-Na-F-rich pyrochlore, with proportions of other elements close to or below their detection limits. Type-1 pyrochlore is yellow- to red-brown and, in many cases, displays oscillatory zoning. This variety occurs most commonly in the cores of crystals in apatitite, where it has its highest concentrations of the elements listed above. It also is observed in biotitite, but the proportions of the above elements are much lower. Type-2 pyrochlore is light to dark brown and, generally, does not exhibit oscillatory zoning. It forms overgrowths on Type-1 pyrochlore in apatitite and biotitite, and occurs as large crystals in the magnetite-biotite rock (Type-1 pyrochlore is not present in this rock). We propose that Type-1 pyrochlore crystallised from a highly evolved, fluid-undersaturated carbonatitic magma, whereas Type-2 pyrochlore crystallised from a carbonatitic magma that had undergone fluid exsolution, which depleted the magma in elements that tend to partition preferentially into aqueous fluids, i.e., U, Sr, Fe, REE, Ba and Cl.The processes that governed the evolution of the carbonatitic magma (which was mantle-derived and introduced in numerous small batches) and crystallisation of pyrochlore at St Honoré were biotitisation (by the magma), crystallisation of calcite and aqueous fluid exsolution. Biotitisation of the host syenite consumed MgO, H2O and CO2 from each magma batch (the CO2 was released as a gas), leading to the crystallisation of Type-1 pyrochlore in biotitite. This process also locally increased the CaO content of the magma to a level sufficient to saturate it with calcite, leaving a residual phoscoritic magma that crystallised the most Ta-U-enriched variety of Type-1 pyrochlore. Exsolution of the aqueous fluid occurred as a result of prolonged crystallisation and/or pressure release and led to the crystallisation of Type-2 pyrochlore. The evolution of each of the magma batches was different and depended on the extent of biotitisation, on whether calcite crystallised, and on the timing of fluid exsolution. As the two types of pyrochlore and their temporal relationship have been described for many carbonatite complexes, the hypotheses presented in this paper are likely also applicable to the genesis of pyrochlore in these other complexes.

The Evolution of the REE-Bearing Özvatan Nepheline Syenite-Carbonatite Complex, Central Turkey: Mineralogical, Geochemical, and Stable Isotopic Approaches

Carbonatite complexes and associated fenite zones are famous for their high-grade rare metal ores. The carbonatite–fenite complexes generally contain high concentrations of light rare earth elements (LREE), thorium (Th), and uranium (U). While most carbonatites are closely related to continental rift zones, some complexes can be observed in post-collisional tectonic environments. The Özvatan nepheline syenite–carbonatite complex is an example of post-collisional carbonatitic magmatism in Central Anatolia, Turkey. The magmatic suite is generally composed of silica-undersaturated ultra-alkaline rocks and carbonatite dikes accompanied by high-intensity fenite zones. The carbonatites of the complex are generally dominated by coarse-grained calcite minerals accompanied by fluorite phenocrysts and may also contain minor amounts of rock-forming silicate minerals. The metasomatic aureole zones (fenites) are mainly composed of euhedral nephelines, K-feldspars, aegirines, augites, and garnets. Carbonatites of the Özvatan complex show enrichments in Ca and F with depletion of alkaline (K and Na) elements. Carbonatites and fenite zones of the Özvatan complex host a variety of incompatible elements, including La, Ce, Nd, Th, U, and Nb. The isotopic composition and general geochemical properties of carbonatites in the study area represent mantle-derived carbonatites rather than crustal limestones/skarns.

Brine–Melts and Fluids of the Fe-F-P-(Ba)-(Sr)-REE Central Asian Carbonatite Province (Southern Siberia and Mongolia): The Petrogenetic Aspects

The carbonatite complexes of the Central Asian carbonatite province comprise the Siberian carbonatites of the Western Transbaikalia and the Central Tuva regions, as well as those from the Mushugai-Khudag complex in Southern Mongolia. They are confined to Late Mesozoic rift structures and have endured considerable tectono-magmatic processes caused by intense plume activity, which also accompanied their formation. A systematic study of melt and fluid inclusions revealed that these carbonatites formed as a result of immiscibility processes in silicate–carbonate (salt) melts, as well as fractional crystallization. Alkaline–carbonatite rocks crystallized in the presence of brine–melts with different compositions, i.e., alkaline–fluorine, carbonate, sulfate, phosphate, and chloride. These melts are responsible for mineralization during the orthomagmatic stage and the primary phase of Fe-F-P-(Ba)-(Sr)-REE ore formation at temperature ranges of 850–830 °C, 650–610 °C, and 560–440 °C and pressures between 290 and 350 MPa. At a later stage, the brine–melts evolved into saline hydrothermal fluids, which are considered to be the source of the second stage of F-(Ba)-(Sr)-REE ore mineralization. The saline crystal–fluid inclusions consist mainly of fluorine–sulfate–carbonate–chloride and bicarbonate–chloride compositions, with temperatures of approximately 480–250 °C and pressures below 250 MPa. The shift from melt to fluid in carbonatite complexes could occur more frequently in nature than previously believed and could also apply to other F-REE carbonatite complexes that are linked to rifting and plume activity in mountain-building zones.

Crust–mantle interaction beneath the Pamir orogenic belt: Insights from the Miocene Dunkeldik carbonatite–syenite complex in Tajikistan

Only some carbonatite–syenite complexes in the world could form carbonatite-related rare earth element (REE) deposits, which is regarded as the main REE origin. The Miocene Dunkeldik carbonatite–syenite complex is located in the Dunkeldik magmatic field in Tajikistan between the Pamir and Tibetan plateaus. So far, the origin, tectonic setting, and REE mineralization potential of the Dunkeldik carbonatite–syenite complex remain unclear due to the limited isotopic values, which inhibit the further understanding the genesis of such carbonatite complex. This study presents zircon U–Pb age, whole-rock geochemical, and Sr–Nd–Pb isotope, and zircon Hf–O isotope data for the Dunkeldik carbonatite–syenite complex on the Pamir Plateau. Zircon U–Pb dating of syenite yielded two concordant ages of 10.27 ± 0.16 and 10.48 ± 0.21 Ma, indicative of formation in a post-collisional extensional setting related to India–Asia collision. Both the carbonatite and syenite exhibit similar enrichments in large-ion lithophile elements (e.g., Sr and Ba) and light REEs, depletions in high-field-strength elements (e.g., Nb and Ta) and heavy REEs, and small Eu anomalies. Whole-rock Sr–Nd–Pb isotopic compositions are highly enriched, with high (87Sr/86Sr)i (0.70971 to 0.70979), low εNd(t) (–8.9 to –9.2), and radiogenic Pb isotope composition (206Pb/204Pb (18.322–18.358), 207Pb/204Pb (15.689–15.695), and 208Pb/204Pb (39.091–39.149)). Zircons Hf-O isotope analyses yield low εHf(t) values (–9.5 to –19.0) and high δ18O value (7.4 ‰–9.5 ‰), respectively. These geochemical and isotopic features indicate that the complex was formed by partial melting of enriched lithospheric mantle during the Miocene India–Asia collision and was assimilated by upper crustal material during magma ascent. The carbonatite and syenite in the complex formed by melt immiscibility. The involvement of crustal material in the magma and less development of fenitization may responsible for the limited REE mineralization potential as compared with other large REE deposits that formed during coeval India–Asia collision.

Reconstruction of hydrothermal fluid composition of the Kamthai carbonatite complex, India using lattice strain model: Implications for LREE/HREE fractionation

The composition of the fluid phase is often a poorly constrained variable in many ore deposits. In this study, we used calcite-fluid partition coefficients extrapolated to the P-T conditions of mineralization using the lattice strain model to reconstruct the composition of the hydrothermal fluids associated with different stages of the hydrothermal alteration and secondary rare earth element (REE) mineralization in the Kamthai carbonatite complex of western India. We identified three generations of calcite including magmatic, two generations of secondary/hydrothermal, as well as partially reequilibrated magmatic grains. The magmatic and partially reequilibrated calcite are unusually enriched in the REE, particularly the LREE, and Sr, which were incorporated via coupled cationic substitutions from highly differentiated brine- and incompatible element-enriched carbonatitic melts. Calcite and carbocernaite were the important primary phases that sequestered the REE from the carbonatite magma. Textural and geochemical evidence suggest that magmatic calcite and carbocernaite underwent extensive partial to complete reequilibration through dissolution and reprecipitation during hydrothermal alteration. The prominent chemical changes during the alteration of calcite involved loss of substantial REE, Sr, Fe, and Mn, and transition from LREE-dominated to LREE-poor compositions. The REE released during hydrothermal alteration were incorporated in secondary REE minerals such as REE-(fluor)carbonates, REE-(hydroxyl)carbonates, ancylite, and cerianite. The reconstructed chemistry of the carbonatite melt and the hydrothermal fluid, together with the trace element chemistry of secondary calcite, point towards progressive decrease in the REE contents. However, the HREE were conserved and ƒO2 increased from the most primitive to the most evolved fluid. The REE were redistributed as alkali‑carbonate-hydroxyl complexes, which can explain the LREE/HREE fractionation observed between the early and the late-stage hydrothermal fluid. Precipitation of the LREE-bearing secondary minerals is attributed to a decrease in temperature of the hydrothermal fluid as a consequence of mixing of the carbonatite-derived fluid with a relatively cooler and oxidized fluid. Such mixing explains the observed negative Ce-anomaly in hydrothermal calcite.

Petrogenesis and metallogenesis of the Qieganbulake carbonatite-related phosphate deposit associated with the mafic–ultramafic–carbonatite complex in the Kuluketage block, northeastern Tarim Craton

AbstractThe Qieganbulake deposit associated with a mafic–ultramafic–carbonatite complex in the Kuluketage block is not only the world’s second-largest vermiculite deposit, but also a medium-size carbonatite-related phosphate deposit. Field observations, radiometric dating results and Sr–Nd–Hf isotopes reveal that the parental magmas of the carbonatite and mafic–ultramafic rocks are cogenetic and formed synchronously at c. 810 Ma. Geochemical characteristics and Sr–Nd–Hf–S isotopes ((87Sr/86Sr)i = 0.70581–0.70710; ϵNd(t) = −0.20 to −11.80; ϵHf(t) = −7.5 to −10.3; δ34S = +0.7 ‰ to +3.0 ‰ (some sulfides with high δ34S values (+3.2 to +6.6) were formed by late hydrothermal sulfur)), in combination with mineral compositions and previous research, strongly indicate that the Qieganbulake mafic–ultramafic–carbonatite complex formed via extensive crystal fractionation/cumulation and liquid immiscibility of a carbonated tholeiitic magma, possibly derived from partial melting of an enriched subcontinental lithospheric mantle previously modified by slab-released fluids and sediment input in a continental rift setting. The coupled enriched Sr–Nd isotopic signatures, in combination with previous research, suggest that the enriched subcontinental lithospheric mantle could have been metasomatized by asthenospheric mantle melts to different degrees. The Qieganbulake carbonatite-related phosphate ores were the products of normal fractional crystallization/cumulation of P–Fe3+ complex enriched carbonatite magma in high oxygen fugacity conditions, which was generated by liquid immiscibility of CO2–Fe–Ti–P-rich residual magma undergoing high differentiation.

A preliminary study of the rare earth element-enriched Twyfelskupje carbonatite complex, southern Namibia

AbstractThe Twyfelskupje carbonatite complex, Southern Namibia, exhibits the typical igneous emplacement structures of carbonatites, including plugs, cone sheets and dyke stockworks. The excellent exposure allows for detailed studies of the high-level geochemical and structural evolution of the carbonatite, and the nature of the concomitant rare earth element mineralization. Radiogenic isotope analyses (Sr, Nd, Pb) reveal that, in common with many other carbonatites, the Twyfelskupje carbonatite complex appears to be predominantly derived from mixing between HIMU and EM1 mantle end-members. Following partial melting of these mantle sources, the geochemical and structural evolution of the Twyfelskupje carbonatite complex proceeded by a staged process involving separate magma pulses, a series of emplacement structures, sub-solidus crystallization, fractionation and low-temperature hydrothermal alteration. The dominant rare earth element minerals in the Twyfelskupje carbonatite complex are fluorcarbonates and monazite, and are characterized by variable Ca, high F and light rare earth elements in the order Ce&gt;La&gt;Nd. Comparison between the rare earth element concentrations of the whole rocks, dominant rare earth element minerals and carbonates suggests that ∼95 % of the total rare earth element abundance of the Twyfelskupje carbonatite complex is contained within fluorcarbonates and monazite. Overall, the early calcio-carbonatite plugs are rare earth element enriched (mean 4.47 wt % rare earth oxides) relative to the magnesio-carbonatite cone sheets (mean 2.51 wt % rare earth oxides).

Petrogenetic history and melt inclusion characteristics of mantle plume-derived ijolites from NE India: implications for multistage crystallization and occurrence of “nano-calciocarbonatites”

The Sung Valley ultramafic–alkaline–carbonatite complex (UACC) of Meghalaya, NE, India, is a result of magmatic activity related to the Kerguelen mantle plume spanning from 101 to 115 Ma. In the present study, an integrated crystal size distribution (CSD), mineral chemistry, and melt inclusion analysis are carried out on the ijolites present within this UACC. The CSD analysis shows that these ijolites were formed in multiple stages through changes in the crystallization environment, such as cooling and nucleation rates. Raman spectroscopy of mineral inclusions of rutile, aphthitalite, apatite, carbonate–silicate melt inclusions, and disordered graphite within clinopyroxene and titanite, respectively, indicates a heterogeneous composition of the parental magma. These mineral and melt inclusion phases further suggest localized changes in oxygen fugacity (fO2) due to redox reactions in the lower crust. SEM–EDX analysis of the exposed melt inclusions reveals the presence of alkali-bearing diopside, phlogopite, and andradite, along with an unidentified carbonated silicate daughter phase. The studied melt inclusions are dominated by carbonate, whereas silicates are subordinate. The presence of this fully crystallized carbonate–silicate melt as calcite, diopside, phlogopite, magnetite, apatite, and andradite suggests the presence of “nano-calciocarbonatites” in these ijolites. Our study provides insights into different mechanisms of the loss of alkalies from initially entrapped alkaline carbonate melt in clinopyroxenes. The predominant occurrence of calcite as the only carbonate phase in the studied melt inclusions is a result of silicate–carbonate melt immiscibility, calcite-normative system in these inclusions, dealkalization of the alkaline carbonates in the presence of external fluid, and/or redistribution of the alkalies to the daughter alkali-bearing silicates.

A comparison of the fenites at the Chilwa Island and Kangankunde carbonatite complexes, Malawi

AbstractCarbonatites are igneous carbonate rocks. They are the main source of the rare earth elements (REE) that are essential in low carbon and high technology applications. Exploration targeting and mine planning would both benefit from a better understanding of the processes that create the almost ubiquitous alkaline and REE-bearing metasomatic aureoles in the surrounding country rocks.Using scanning electron microscopy and whole-rock geochemistry, we investigated the composition and mineralogy of the fenite aureoles developed around the REE-poor Chilwa Island carbonatite and the REE-rich Kangankunde carbonatite, which intrude similar country rocks in the Chilwa Alkaline Province of Southern Malawi. Although common characteristics and trends in their mineralogy and composition may be typical of fenites in general, there are significant differences in their petrography and petrogenesis. For example, the mineralogically diverse breccia at Kangankunde contrasts with the intensely altered potassic breccia of Chilwa Island. This might be caused by differing sequences of fluids expelled from the carbonatites into the aureoles. The main REE-bearing mineral in fenite is different at each complex, and reflects the characteristic REE-bearing mineral of the main carbonatite: fluorapatite at Chilwa Island; and monazite at Kangankunde. Each fenite has distinctive mineral assemblages, in which the relative abundance of the REE-bearing minerals appears to be determined by the mineralogy of their respective host carbonatites.At both localities, the REE minerals in fenite are less enriched in lanthanum and cerium than their equivalents in carbonatite, a characteristic that we attribute to REE fractionation within fluids in the aureole.Identifying the mineral assemblages present in fenite and understanding the sequence of alkaline and mineralising fluid events could therefore be useful in predicting whether a fenite is associated with a REE-rich carbonatite. Detailed studies of other aureoles would be required to assess the reliability of these characteristics.

Mapping critical mineral resources using airborne geophysics, 3D joint inversion and geology differentiation: A case study of a buried niobium deposit in the Elk Creek carbonatite, Nebraska, USA

ABSTRACTCritical minerals are an integral part of the energy transition because of their important and, sometimes irreplaceable, uses in solar panels, wind turbines, electrical vehicles, etc. Mapping critical mineral resources is, therefore, essential to achieving the net‐zero emission goal by 2050. We present a case study on using airborne geophysical data, borehole and physical property measurements to characterize the Elk Creek Carbonatite complex located in the southeast of Nebraska, USA. It hosts the largest known niobium deposit in the United States and contains a high level of rare earth element mineralization. Our goal is to develop a better understanding of the three‐dimensional structures and composition of the Elk Creek Carbonatite complex as well as its critical mineral resource potential. We performed three‐dimensional joint inversion of the airborne gravity gradiometry and magnetic data to produce structurally similar density and susceptibility models. We carried out geology differentiation, a process of classifying recovered physical property values into distinct units, and obtained a three‐dimensional quasi‐geology model for the Elk Creek Carbonatite complex. We identified 12 geologic units, each of which is characterized by a distinct range of physical property values. Our quasi‐geology model, especially Units 9 and 11, shows a remarkable consistency with the borehole assay measurements. Unit 9 is spatially coincident with the known niobium ore zone. Unit 11 represents a significant volume of highly dense and magnetic materials below the deepest boreholes. These materials are likely associated with unexplored niobium mineralization. Our study demonstrates the added value of three‐dimensional geophysical joint inversions and geology differentiation in the context of critical mineral exploration under a thick sedimentary overburden.