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Abstract The Lugiin Gol nepheline syenite intrusion, Mongolia, hosts a range of carbonatite dikes mineralized in rare-earth elements (REE). Both carbonatites and nepheline syenite-fluorite-calcite veinlets are host to a previously unreported macroscale texture involving pseudo-graphic intergrowths of fluorite and calcite. The inclusions within calcite occur as either pure fluorite, with associated REE minerals within the surrounding calcite, or as mixed calcite-fluorite inclusions, with associated zirconosilicate minerals. Consideration of the nature of the texture, and the proportions of fluorite and calcite present (∼29 and 71 mol%, respectively), indicates that these textures most likely formed either through the immiscible separation of carbonate and fluoride melts, or from cotectic crystallization of a carbonate-fluoride melt. Laser ablation ICP-MS analyses show the pure fluorite inclusions to be depleted in REE relative to the calcite. A model is proposed, in which a carbonate-fluoride melt phase enriched in Zr and the REE, separated from a phonolitic melt, and then either unmixed or underwent cotectic crystallization to generate an REE-rich carbonate melt and an REE-poor fluoride phase. The separation of the fluoride phase (either solid or melt) may have contributed to the enrichment of the carbonate melt in REE, and ultimately its saturation with REE minerals. Previous data have suggested that carbonate melts separated from silicate melts are relatively depleted in the REE, and thus melt immiscibility cannot result in the formation of REE-enriched carbonatites. The observations presented here provide a mechanism by which this could occur, as under either model the textures imply initial separation of a mixed carbonate-fluoride melt from a silicate magma. The separation of an REE-enriched carbonate-fluoride melt from phonolitic magma is a hitherto unrecognized mechanism for REE-enrichment in carbonatites, and may play an important role in the formation of shallow magmatic REE deposits. Graphical abstract Highlights • The Lugiin Gol nepheline syenite intrusion hosts a range of carbonatite dikes mineralized in rare earth elements (REE). • Both these dikes, and composite nepheline syenite-fluorite-calcite veinlets, are host to a unique macroscale intergrowths of fluorite and calcite. • All results indicate that these textures formed through the immiscible separation of carbonate and fluoride melts, or from cotectic crystallization of a carbonate-fluoride melt. • Multistage silicate-carbonate-fluoride melt immiscibility may be a previously unrecognized, but important process in the generation of REE-mineralized carbonatites. [ABSTRACT FROM AUTHOR]

Zoned crystals of carbocernaite occur in hydrothermally reworked burbankite-fluorapatite-bearing calcite carbonatite at Bear Lodge, Wyoming. The mineral is paragenetically associated with pyrite, strontianite, barite, ancylite-(Ce), and late-stage calcite, and is interpreted to have precipitated from sulfate-bearing fluids derived from an external source and enriched in Na, Ca, Sr, Ba, and rare-earth elements (REE) through dissolution of the primary calcite and burbankite. The crystals of carbocernaite show a complex juxtaposition of core-rim, sectoral, and oscillatory zoning patterns arising from significant variations in the content of all major cations, which can be expressed by the empirical formula (Ca0.43-0.91Sr0.40-0.69REE0.18-0.59Na0.18-0.53Ba0-0.08)Σ1.96-2.00(CO3)2. Interelement correlations indicate that the examined crystals can be viewed as a solid solution between two hypothetical end-members, CaSr(CO3)2 and NaREE(CO3)2, with the most Na-REE-rich areas in pyramidal (morphologically speaking) growth sectors representing a probable new mineral species. Although the Bear Lodge carbocernaite is consistently enriched in light REE relative to heavy REE and Y (chondrite-normalized La/Er = 500-4200), the pyramidal sectors exhibit a greater degree of fractionation between these two groups of elements relative to their associated prismatic sectors. A sample approaching the solid-solution midline [(Ca0.57Na0.42)Σ0.99(Sr0.50REE0.47Ba0.01)Σ0.98(CO3)2] was studied by single-crystal X-ray diffraction and shown to have a monoclinic symmetry [space group P11 m, a = 6.434(4), b = 7.266(5), c = 5.220(3) Å, γ = 89.979(17)°, Z = 2] as opposed to the orthorhombic symmetry (space group Pb21 m) proposed in earlier studies. The symmetry reduction is due to partial cation order in sevenfold-coordinated sites occupied predominantly by Ca and Na, and in tenfold-coordinated sites hosting Sr, REE, and Ba. The ordering also causes splitting of carbonate vibrational modes at 690-740 and 1080-1100 cm−1 in Raman spectra. Using Raman micro-spectroscopy, carbocernaite can be readily distinguished from burbankite- and ancylite-group carbonates characterized by similar energy-dispersive spectra. [ABSTRACT FROM AUTHOR]

The Bear Lodge alkaline complex in northeastern Wyoming (USA) is host to potentially economic rare-earth mineralization in carbonatite and carbonatite-related veins and dikes that intrude heterolithic diatreme breccias in the Bull Hill area of the Bear Lodge Mountains. The deposit is zoned and consists of pervasively oxidized material at and near the surface, which passes through a thin transitional zone at a depth of ~ 120–183 m, and grades into unaltered carbonatites at depths greater than ~ 183–190 m. Carbonatites in the unoxidized zone consist of coarse and fine-grained calcite that is Sr-, Mn- and inclusion-rich and are characterized by the presence of primary burbankite, early-stage parisite and synchysite with minor bastnäsite that have high (La/Nd) cn and (La/Ce) cn values. The early minerals are replaced with polycrystalline pseudomorphs consisting of secondary rare-earth fluorocarbonates and ancylite with minor monazite. Different secondary parageneses can be distinguished on the basis of the relative abundances and composition of individual minerals. Variations in key element ratios, such as (La/Nd) cn , and chondrite-normalized profiles of the rare-earth minerals and calcite record multiple stages of hydrothermal deposition involving fluids of different chemistry. A single sample of primary calcite shows mantle-like δ 18 O V-SMOW and δ 13 C V-PDB values, whereas most other samples are somewhat depleted in 13 C (δ 13 C V-PDB ≈ − 8 to − 10‰) and show a small positive shift in δ 18 O V-SMOW due to degassing and wall-rock interaction. Isotopic re-equilibration is more pronounced in the transitional and oxidized zones; large shifts in δ 18 O V-SMOW (to ~ 18‰) reflect the input of meteoric water during pervasive hydrothermal reworking and supergene oxidation. The textural relations, mineral chemistry and C and O stable-isotopic variations record a polygenetic sequence of rare-earth mineralization in the deposit. With the exception of one Pb-poor sample showing an appreciable positive shift in 208 Pb/ 204 Pb value (~ 39.2), the Bear Lodge carbonatites are remarkably uniform in their Nd, Sr and Pb isotopic composition: 143 Nd/ 144 Nd t = 0.512591–0.512608; εNd t = 0.2–0.6; 87 Sr/ 86 Sr t = 0.704555–0.704639; εSr t = − 1.5–2.7; 206 Pb/ 204 Pb t = 18.071–18.320; 207 Pb/ 204 Pb t = 15.543–15.593; and 208 Pb/ 204 Pb t = 38.045–39.165. These isotopic characteristics indicate that the source of the carbonatitic magma was in the subcontinental lithospheric mantle, and modified by subduction-related metasomatism. Carbonatites are interpreted to be generated from small degrees of partial melt that may have been produced via interaction of upwelling asthenosphere giving a small depleted MORB component, with an EM1 component likely derived from subducted Farallon crust. [ABSTRACT FROM AUTHOR]

The Aley Nb deposit in northern British Columbia, Canada, is hosted by metamorphosed calcite and dolomite carbonatites of anorogenic affinity emplaced in Lower Paleozoic sedimentary carbonate rocks in the Devonian. Primary Nb mineralization consists of pyrochlore (commonly comprising a U–Ta-rich and F-poor core) and ferrocolumbite developed as discrete crystals and replacement products after the pyrochlore. These phases and associated heavy minerals (apatite ± magnetite ± zircon ± baddeleyite) precipitated early in the magmatic history and probably formed laterally extensive cumulate layers up to at least 1.5 m in thickness. Fractionation of copious amounts of pyrochlore is reflected in the chemical composition of the carbonatites and their constituent minerals, which show large variations in Nb/Ta value, but a near-chondritic Zr/Hf ratio. Alkali-rich metasomatic rocks (in particular, fenites and glimmerites) associated with the carbonatites are barren; the bulk of Nb in these rocks is contained in rutile, phlogopite and, to a much lesser extent, amphibole. When the passive margin of North America became the zone of plate convergence in the Cretaceous, the host carbonatites were strongly deformed, which is manifested in structures and textures indicative of grain comminution, ductile flow, folding and, locally, brecciation. The structure and continuity of the cumulate units enriched in Nb minerals were profoundly affected by these processes. Interaction of the carbonatites with crustal fluids of complex chemistry resulted in extensive dolomitization, replacement of the pyrochlore and ferrocolumbite by fersmite, and development of hydrothermal parageneses consistent with the lower greenschist-facies conditions. At these late evolutionary stages, Nb was mobilized only to a very limited extent and sequestered in a variety of minerals (fersmite, euxenite, Mg-rich ferrocolumbite and Nb-bearing rutile) typically occurring as scarce minute crystals associated with hydrothermal dolomite, quartz and chlorite. Progressive enrichment of the deformed dolomite carbonatites in heavy C and O isotopes relative to primary calcite, coupled with changes in the trace-element composition of Nb phases, indicate that the fluids were equilibrated with the wall-rock sedimentary rocks hosting the Aley deposit and were capable of transporting F − , (PO 4 ) 3 − , U, Th and rare-earth elements, but not Nb. [ABSTRACT FROM AUTHOR]

Abstract: The present work is a first comprehensive study of the trace-element composition and zoning in clinopyroxene- and amphibole-group minerals from carbonatites, incorporating samples from 14 localities worldwide (Afrikanda, Aley, Alnö, Blue River, Eden Lake, Huayangchuan, Murun, Oka, Ozernaya Varaka, Ozernyi, Paint Lake, Pinghe, Prairie Lake, Turiy Mys). The new electron-microprobe data presented here significantly extend the known compositional range of clinopyroxenes and amphiboles from carbonatites. These data confirm that calcic and sodic clinopyroxenes from carbonatites are not separated by a compositional gap, instead forming an arcuate trend from nearly pure diopside through intermediate aegirine–augite compositions confined to a limited range of CaFeSi2O6 contents (15–45mol%) to aegirine with <25mol% of CaMgSi2O6 and a negligible proportion of CaFeSi2O6. A large set of LA-ICPMS data shows that the clinopyroxenes of different composition are characterized by relatively low levels of Cr, Co and Ni (≤40ppm) and manifold variations in the concentration of trivalent lithophile and some incompatible elements (1–150ppm Sc, 26–6870ppmV, 5–550ppm Sr, 90–2360ppm Zr, and nil to 150ppm REE), recorded in some cases within a single crystal. The relative contribution of clinopyroxenes to the whole-rock Rb, Nb, Ta, Th and U budget is negligible. The major-element compositional range of amphiboles spans from alkali- and Al-poor members (tremolite) to Na–Al-rich Mg- or, less commonly, Fe-dominant members (magnesiohastingsite, hastingsite and pargasite), to calcic–sodic, sodic and potassic–sodic compositions intermediate between magnesio-ferrikatophorite, richterite, magnesioriebeckite, ferri-nyböite and (potassic-)magnesio-arfvedsonite. In comparison with the clinopyroxenes, the amphiboles contain similar levels of tetravalent high-field-strength elements (Ti, Zr and Hf) and compatible transition elements (Cr, Co and Ni), but are capable of incorporating much higher concentrations of Sc and incompatible elements (up to 500ppm Sc, 43ppm Rb, 1470ppm Sr, 1230ppm Ba, 80ppm Pb, 1070ppm REE, 140ppm Y, and 180ppm Nb). In some carbonatites, amphiboles contribute as much as 25% of the Zr+Hf, 15% of the Sr and 35% of the Rb+Ba whole-rock budget. Both clinopyroxenes and amphiboles may also host a significant share (~10%) of the bulk heavy-REE content. Our trace-element data show that the partitioning of REE between clinopyroxene (and, in some samples, amphibole) and the melt is clearly bimodal and requires a revision of the existing models assuming single-site REE partitioning. Clinopyroxenes and amphiboles from carbonatites exhibit a diversity of zoning patterns that cannot be explained exclusively on the basis of crystal chemistry and relative compatibility of different trace-element in these minerals. Paragenetic analysis indicates that in most cases, the observed zoning patterns develop in response to removal of selected trace elements by phases co-precipitating with clinopyroxene and amphibole (especially magnetite, fluorapatite, phlogopite and pyrochlore). With the exception of magnesiohastingsite–richterite sample from Afrikanda, the invariability of trace-element ratios in the majority of zoned clinopyroxene and amphibole crystals implies that fluids are not involved in the development of zoning in these minerals. The implications of the new trace-element data for mineral exploration targeting REE, Nb and other types of carbonatite-hosted rare-metal mineralization are discussed. [Copyright &y& Elsevier]

Many carbonatites are associated both spatially and temporally with large igneous provinces (LIPs), and considered to originate from a mantle plume source lacking any contribution from recycled crustal materials. Here, we report an occurrence of carbonatite enriched in rare-earth elements (REE) and associated with the Tarim LIP in northwestern China. The Tarim LIP comprises intrusive and volcanic products of mantle plume activity spanning from ~ 300 to 280 Ma. The carbonatites at Wajilitage in the northwestern part of Tarim are dominated by calcite and dolomite varieties, and contain abundant REE minerals (principally, monazite and REE-fluorcarbonates). Th–Pb age determination of monazite yielded an emplacement age of 266 ± 5.3 Ma, i.e. appreciably younger than the eruption age of flood basalts at ~ 290 Ma. The carbonatites show low initial 87 Sr/ 86 Sr (0.7037–0.7041) and high ε Nd(t) (1.2–4) values, which depart from the isotopic characteristics of plume-derived basalts and high-Mg picrites from the same area. This indicates that the Wajilitage carbonatites derived from a mantle source isotopically distinct from the one responsible for the voluminous (ultra)mafic volcanism at Tarim. The carbonatites show δ 26 Mg DSM3 values (− 0.99 to − 0.65‰) that are significantly lower than those in typical mantle-derived rocks and rift carbonatites, but close to marine sediments and orogenic carbonatites. We propose that the carbonatites in the Tarim LIP formed by decompressional melting of recycled sediments mixed with the ambient mantle peridotite. The enriched components in the Tarim plume could be accounted for by the presence of recycled sedimentary components in the subcontinental mantle. [ABSTRACT FROM AUTHOR]

The South Qinling orogen in central China hosts carbonatites occurring as stocks associated with syenites and collectively regarded as the Miaoya intrusive complex. The complex hosts economic resources of rare-earth elements (REE) and Nb. The Miaoya syenites are strongly metasomatized at the contact with the carbonatites and cross-cut by carbonate and felsic veinlets. Small oscillatory-zoned crystals of zircons from the syenites give a concordant U–Pb age of 147 ± 0.5 Ma, which differs significantly from the ages of both large magmatic zircon grains from the syenites and primary monazite from the carbonatites (766 Ma and 234 Ma, respectively). To account for the possibility that the Miaoya syenites are coeval and cogenetic with the carbonatites, the trace-element budget of both rock types was examined in detail. The Miaoya carbonatite contains primary REE-rich fluorapatite and monazite, which precipitated earlier than the rock-forming REE-poor calcite, indicating that the primary carbonatitic magma was rich in REE. The compositions of the parental syenitic and carbonatitic magmas, calculated on the basis of the trace-element composition of primary fluorapatite in the two rock suites, show that the carbonatitic magma contained higher Sr and REE (La–Tb), but lower Ba, Pb, Th, U, Nb and Ta levels in comparison with the syenitic melt. These differences are inconsistent with derivation of the Miaoya rocks from a homogeneous carbonate–silicate melt by immiscibility or crystal fractionation. It is therefore concluded that the carbonatitic magma at Miaoya was generated directly in the mantle. Emplacement of the carbonatites in the South Qinling orogen marked transition to a postorogenic regime, and was preceded by oceanic crust subduction and closure of the Mianlue Ocean in the Triassic. Our models show that melting of the Mianlue crust and up to 10 wt.% of sediments cannot produce the levels of REE enrichment observed in the Miaoya carbonatites. More complex models, involving recycling of the Mianlue oceanic crust and a REE-rich carbonate liquid from an old deep-seated mantle source are required to explain the observed trace-element characteristics of the Miaoya carbonatites. [ABSTRACT FROM AUTHOR]

Abstract: Carbonatites are known to contain the highest concentrations of rare-earth elements (REE) among all igneous rocks. The REE distribution of carbonatites is commonly believed to be controlled by that of the rock-forming Ca minerals (i.e., calcite, dolomite, and ankerite) and apatite because of their high modal content and tolerance for the substitution of Ca by light REE (LREE). Contrary to this conjecture, calcite from the Miaoya carbonatite (China), analyzed in situ by laser-ablation inductively-coupled-plasma mass-spectrometry, is characterized by low REE contents (100–260ppm) and relatively flat chondrite-normalized REE distribution patterns [average (La/Yb)CN =1.6]. The carbonatite contains abundant REE-rich minerals, including monazite and fluorapatite, both precipitated earlier than the REE-poor calcite, and REE-fluorocarbonates that postdated the calcite. Hydrothermal REE-bearing fluorite and barite veins are not observed at Miaoya. The textural and analytical evidence indicates that the initially high concentrations of REE and P in the carbonatitic magma facilitated early precipitation of REE-rich phosphates. Subsequent crystallization of REE-poor calcite led to enrichment of the residual liquid in REE, particularly LREE. This implies that REE are generally incompatible with respect to calcite and the calcite/melt partition coefficients for heavy REE (HREE) are significantly greater than those for LREE. Precipitation of REE-fluorocarbonates late in the evolutionary history resulted in depletion of the residual liquid in LREE, as manifested by the development of HREE-enriched late-stage calcite [(La/Yb)CN ≈0.7] in syenites associated with the carbonatite. The observed variations of REE distribution between calcite and whole rocks are interpreted to arise from multistage fractional crystallization (phosphates⇒calcite⇒REE-fluorocarbonates) from an initially REE-rich carbonatitic liquid. [Copyright &y& Elsevier]

Abstract: The Qinling molybdenum belt is a prominent metallogenic structure in central China hosting several significant porphyry- and porphyry–skarn-type deposits. The Huanglongpu Mo deposit in the north-western part of the belt is unique in that it is associated with carbonatite dykes, rather than felsic magmatism. The carbonatites are composed largely of Sr–Mn-rich calcite and characterized by high concentrations of Sr and rare-earth elements (REE), and stable-isotope values indicative of a mantle source (δ 13CPDB =−6.7±0.2‰ and δ 18OSMOW =8.2±1.0‰). Molybdenite is associated with galena and REE minerals (parisite, bastnäsite and monazite). Both molybdenite and galena are characterized by high Re contents (up to 0.4 and 0.2wt.%, respectively) and Re/(Mo, Pb) ratios approaching the primitive-mantle values. In contrast to the rock-forming calcite, the REE minerals are enriched in light REE, whose relative proportion increases from parisite-(Ce) [average (La/Nd)n =2.1] to bastnäsite-(Ce) and monazite-(Ce) [average (La/Nd)n =3.1, 4.6, respectively]. The whole-rock compositions are characterized by some of the highest Mo and heavy REE abundances reported for carbonatites to date: up to 1010ppm Mo, 1130ppm Y+Gd…Lu and (La/Yb)n =1.2–2.7. The unusual trace-element geochemistry of the Huanglongpu rocks may ultimately reflect the composition of their mantle source, but their enrichment in Mo+Re was undoubtedly enhanced through preferential partitioning of these elements into a light REE–Pb–S-rich fluid derived from the carbonatitic magma modified by calcite fractionation. The present work shows that Mo can be retained, transported and deposited by carbonatitic fluids capable of generating economic Mo deposits. [Copyright &y& Elsevier]