Dolomite Carbonatite Research Articles

Post-Magmatic Fluids Dominate the Mineralization of Dolomite Carbonatitic Dykes Next to the Giant Bayan Obo REE Deposit, Northern China

The Bayan Obo rare earth element (REE) deposit in Inner Mongolia, northern China, is the largest REE deposit in the world, whose mineralization process remains controversial. There are dozens of carbonatite dykes that are tightly related to the deposit. Here we report the petrological and mineralogical characteristics of a typical dolomite carbonatite dyke near the deposit. The dolomite within the dyke experienced intense post-emplacement fluids metasomatism as evidenced by the widespread hydrothermal REE-bearing minerals occurring along the carbonate mineral grains. REE contents of bulk rocks and constituent dolomite minerals (>90 vol.%) are 1407–4184 ppm and 63–152 ppm, respectively, indicating that dolomite is not the dominant mineral controlling the REE budgets of the dyke. There are three types of apatite in the dyke: Type 1 apatite is the primary apatite and contains REE2O3 at 2.35–4.20 wt.% and SrO at 1.75–2.19 wt.%; Type 2 and Type 3 apatites are the products of replacement of primary apatite. The REE2O3 (6.10–8.21 wt.%) and SrO (2.83–3.63 wt.%) contents of Type 2 apatite are significantly elevated for overprinting of REE and Sr-rich fluids derived from the carbonatite. Conversely, Type 3 apatite has decreased REE2O3 (1.17–2.35 wt.%) and SrO (1.51–1.99 wt.%) contents, resulting from infiltration of fluids with low REE and Na concentrations. Our results on the dyke suggest that post-magmatic fluids expelled from the carbonatitic melts dominated the REE mineralization of the Bayan Obo deposit, and a significant fluid disturbance occurred but probably provided no extra REEs to the deposit.

Geology and ore genesis of the carbonatite-associated Yangibana REE district, Gascoyne Province, Western Australia

The Yangibana rare earth element (REE) district consists of multiple mineral deposits/prospects hosted within the Mesoproterozoic Gifford Creek Carbonatite Complex (GCCC), Western Australia, which comprises a range of rock types including calcite carbonatite, dolomite carbonatite, ankerite–siderite carbonatite, magnetite–biotite dykes, silica-rich alkaline veins, fenite, glimmerites and what have historically been called “ironstones”. The dykes/sills were emplaced during a period of extension and/or transtension, likely utilising existing structures. The Yangibana REE deposits/prospects are located along many of these structures, particularly along the prominent Bald Hill Lineament. The primary ore mineral at Yangibana is monazite, which is contained within ankerite–siderite carbonatite, magnetite–biotite dykes and ironstone units. The ironstones comprise boxwork-textured Fe oxides/hydroxides, quartz, chalcedony and minor monazite and subordinate rhabdophane. Carbonate mineral-shaped cavities in ironstone, fenite and glimmerite alteration mantling the ironstone units, and ankerite–siderite carbonatite dykes altering to ironstone-like assemblages in drill core indicate that the ironstones are derived from ankerite–siderite carbonatite. This premise is further supported by similar bulk–rock Nd isotope composition of ironstone and other alkaline igneous rocks of the GCCC. Mass balance evaluation shows that the ironstones can be derived from the ankerite–siderite carbonatites via significant mass removal, which has resulted in passive REE concentration by ~ 2 to ~ 10 times. This mass removal and ore tenor upgrade is attributed to extensive carbonate breakdown and weathering of ankerite–siderite carbonatite by near-surface meteoric water. Monazite from the ironstones has strong positive and negative correlations between Pr and Nd, and Nd and La, respectively. These relationships are reflected in the bulk–rock drill assays, which display substantial variation in the La/Nd throughout the GCCC. The changes in La/Nd are attributed to variations in primary magmatic composition, shifts in the magmatic-hydrothermal systems related to CO2 versus water-dominated fluid phases, and changes in temperature.

Petrogenesis of the Gifford Creek Carbonatite Complex, Western Australia

The 1370 Ma Gifford Creek Carbonatite Complex (GCCC) comprises a diverse suite of alkaline dyke and sill complexes that cover an area of ~ 250 km2 in the Gascoyne Province, Western Australia. Most carbonatite types are interpreted to be related products of fractional crystallisation, with calcite carbonatites representing cumulate rocks and dolomite carbonatites representing crystallised products of the derivative liquids. Genetic relationships between these carbonatites and other alkaline igneous units are less clear. The ankerite–siderite carbonatites and magnetite-biotite dykes are likely of related magmatic origin as both have distinctly high LREE and low HFSE contents. The ankerite–siderite carbonatites have mantle-like δ13C isotope values of − 6.1 to − 7.1‰ and similar geochemistry to other known magmatic ferrocarbonatites. Silica-rich alkaline veins found near the centre of the complex have trace element signatures that are antithetic to the magnetite–biotite dykes, so these veins are interpreted to represent products of alkali- and F-rich magmatic-hydrothermal fluids exsolved from the magnetite–biotite dykes during their emplacement. Carbon, O, Sr, and Nd isotope data are consistent with an enriched mantle source for the origin of the GCCC, with mantle enrichment likely caused by plate convergence processes associated with the c. 2.0 Ga Glenburgh Orogeny. There is no evidence to link mantle plume activity with formation of the GCCC; rather, alkaline magmatism is interpreted to result from low degree melting of the metasomatised mantle during reactivation of the crustal suture zone at 1370 Ma. The carbonatitic magmas utilised the Lyons River Fault to traverse the crust to be emplaced as the GCCC. Post magmatic alteration has variably modified the O and Sr isotope compositions of carbonates from these rocks. We therefore appeal for careful evaluation of isotopic data from ancient carbonatites, as isotopic resetting may be more common than currently recognised.

Carbon–Strontium Isotope Decoupling in Carbonatites from Caotan (Qinling, China): Implications for the Origin of Calcite Carbonatite in Orogenic Settings

Abstract Mantle-derived carbonatites emplaced in orogenic belts and some extensional settings are hypothesized to contain recycled crustal material. However, these carbonatites are typically composed of calcite showing a typical mantle range of C–O isotopic values devoid of recognizable sedimentary fingerprints. Here, we report the first known instance of C–Sr isotope decoupling between intimately associated dolomite carbonatites and magnetite–forsterite–calcite carbonatites from the northern Qinling orogen, central China. The calcite-dominant variety is developed at the contact between the dolomite carbonatite and metasomatized wall-rock gneiss. The two types of carbonatites have similar δ18OVSMOW (6·98‰ to 9·96‰), εNd(i) (-3·01 to -6·47) and Pb (206Pb/204Pb(i) = 17·369–17·584, 207Pb/204Pb(i) = 15·443–15·466) isotopic compositions, but significantly different C and Sr isotopic signatures (δ13CVPDB = -3·09 to -3·58‰ and -6·11 to -7·19‰; 87Sr/86Sr(i) = 0·70373 to 0·70565 vs 0·70565 to 0·70624 for the dolomite and calcite rocks, respectively). The relative enrichment of the early-crystallizing dolomite carbonatite in 13C and its depletion in 87Sr are primary isotopic characteristics inherited from its mantle source. The observed field relations, petrographic and geochemical characteristics of the Caotan dolomite and calcite carbonatites imply that the strong C–Sr isotopic decoupling between them could not result from mixing of different mantle reservoirs (e.g. HIMU and EM1), or from magma fractionation processes. We propose that the calcite carbonatites were a by-product of metasomatic reactions between primary dolomitic melts and felsic wall-rock. These reactions involved the loss of Mg and CO2 from the magma, leading to depletion of the evolved calcite-saturated liquid in 13C and its enrichment in radiogenic Sr. We conclude that calcite carbonatites in plate-collision zones may not represent primary melts even if their isotopic signature is recognizably ‘mantle-like’.

Extremely Al-Depleted Chlorites From Dolomite Carbonatites of the Kovdor Ultramafic-Alkaline Complex, Kola Peninsula, Russia

AbstractThe problem to be solved is whether Al is a necessary component of Fe-Mg chlorites. Very unusual Al-depleted and Fe-enriched trioctahedral chlorites with the empirical formulae Na0.05Ca0.05(Fe2+3.01Mg2.01Ti0.14Fe3+0.04)Σ6.00[(Si3.53Fe3+0.41Al0.06)Σ4.00O10](OH)8·nH2O (Sample 1) and Na0.05Ca0.01(Fe2+3.26Mg1.97Fe3+0.75Mn0.01Ti0.01)Σ6.00[(Si3.16Fe3+0.75Al0.09)Σ4.00O10](OH)8 (Sample 2) have been discovered in Al-depleted dolomite carbonatites of the Kovdor complex of ultramafic, alkaline rocks and carbonatites, Kola Peninsula, Russia. The presence of substantial amounts of Ti in Sample 1 is another unusual feature of this mineral. In both samples, chlorites are intimately intergrown with cronstedtite-1T which is an indication of a low stability of chlorite structure in the absence of aluminum in the tetrahedral sheet. The crystal structure of chlorite in Sample 1 was solved by the Rietveld method. The mineral is triclinic (IIb-4-module), space group C-1, a = 5.4153(4), b = 9.3805(7), c = 14.5743(12) Å, α = 90.137(5)°, β = 96.928(5)°, γ = 90.043(6)°, V = 734.95(10) Å3, and Z = 2. A problem to be solved is how stable are Al-free chlorites belonging to the clinochlore–chamosite solid-solution series and whether their existence in natural mineral assemblages is possible. The results obtained indicate that even though Al-depleted chlorites belonging to the clinochlore–chamosite solid-solution series exist in Nature as metastable phases, these minerals are extremely rare and much less stable than Al-poor serpentines.

A genetic story of olivine crystallisation in the Mark kimberlite (Canada) revealed by zoning and melt inclusions

Elucidating the composition of primary kimberlite melts is essential to understanding the nature of their source, petrogenesis, rheology, transport and ultimately the origin of diamonds. Kimberlite rocks are typically comprised of abundant olivine (~25–60 vol%), which occurs as individual grains of variable size and morphology, and includes xenocrysts and zoned phenocrysts. Zoning patterns and inclusions in olivine can be used to decipher the petrogenetic history of kimberlites, starting from their generation in the mantle through to emplacement in the crust. This study examines well-preserved, euhedral, zoned olivine crystals from the Mark kimberlite (Lac de Gras, Canada). Olivine typically consists of xenocrystic cores, which are homogeneous in composition but vary widely between grains (Fo88.1–93.6). These cores are in turn surrounded by (in order of crystallisation) magmatic rims and Mg-rich rinds (Fo95.3–98.1). In addition, we document a new type of olivine zone (‘outmost rind’) that overgrows Mg-rich rinds.Crystal and melt/fluid inclusions are abundant in olivine and preserve a record of kimberlite melt evolution. For the first time in the studies of kimberlite olivine, we report primary melt inclusions hosted in Mg-rich olivine rinds. In addition, we observe that pseudosecondary melt/fluid inclusions are restricted to interior olivine zones (cores, rims) and are considered to have formed prior to rind formation. Pseudosecondary melt/fluid inclusions are inferred to have been entrapped at depth, as evidenced by measured densities in thermometric experiments of CO2 and decrepitation haloes, indicating a minimum entrapment pressure of ~200–450 MPa (or ~6–15 km). Both primary and pseudosecondary melt inclusions in olivine have daughter minerals dominated by CaMg and K-Na-Ba-Sr-bearing carbonates, K-Na-chlorides along with subordinate silicates (e.g., phlogopite, monticellite), Fe-Mg-Al-Ti-spinel, perovskite, phosphates and sulphates/sulphides and periclase. In addition to phases reported in primary melt inclusions, pseudosecondary melt inclusions contain more diverse and exotic daughter mineral assemblages, where they contain phases such as tetraferriphlogopite Ba- or K-sulphates, kalsilite and Na-phosphates. The daughter mineral assemblages are consistent with a silica-poor, alkali dolomitic carbonatite melt. We demonstrate that the different types of inclusions in olivine can assist in constraining the timing of multi-stage olivine growth and the composition of the crystallising melt.The large variance in olivine zoning patterns, morphologies and Ni distribution (i.e. both coupling with and decoupling from Fo) indicates that olivine in the studied Mark kimberlite samples represent an accumulation of olivine, where olivine was derived from successive stages of the ascending magma and/or from multiple, but related pulses of magma. Primary and pseudosecondary melt/fluid inclusions in olivine indicate that a variably differentiated silica-poor, halogen-bearing, alkali-dolomitic melt crystallised and transported olivine in the Mark kimberlite.

Multiphase carbonatite-related magmatic and metasomatic processes in the genesis of the ore-hosting dolomite in the giant Bayan Obo REE-Nb-Fe deposit

The origin of dolomite that hosts the Bayan Obo REE-Nb-Fe deposit (57.4 Mt.@6% REE2O3, 2.16 Mt.@0.13% Nb2O5, and >1500 Mt.@35% iron oxides) has been controversial for decades, but it is integral to understanding of the genesis of this giant deposit. In this study, based on the textures and in situ major and trace element composition of its carbonates, the dolomite was proved to be initially generated from magnesio-ferro‑carbonatite melts. It subsequently experienced magmatic-hydrothermal alteration and recrystallization in a low strain environment, caused by calcio‑carbonatitic fluids, with formation of finer-grained dolomite, interstitial calcite and increasing amounts of associated fluorocarbonates. Available stable isotope analyses indicate that the recrystallized ore-hosting dolomite has higher δ13C and δ18O ratios compared to its igneous coarse-grained precursor. Rayleigh fractionation during the recrystallization process, rather than crustal contamination, played a major role in the highly-variable stable isotope composition of carbonates in the dolomite. Low-T alteration increased variability with apparently random increases in δ18O within carbonates. The REE, Ba and Sr were added simultaneously with the elevated (La/Yb)cn from magnesio-ferro‑carbonatite melts to calcio‑carbonatitic fluids, and to carbonatite-derived aqueous fluids, through which extensive fluorine metasomatism and alkali alteration overlapped the recrystallization of the ore-hosting dolomite. Therefore, the multi-stage REE mineralization at Bayan Obo is closely related to metasomatism by calcio‑carbonatitic fluids of previously-emplaced intrusive magnesio-ferro‑carbonatite bodies during late evolution of the Bayan Obo carbonatite complex. Then, the ore-hosting dolomitic carbonatite was subjected to compressive tectonics during a Paleozoic subduction event, and suffered intense, largely brittle, deformation, which partially obscured the earlier recrystallization process. The complex, multi-stage evolution of the ore-hosting dolomite is responsible for the uniqueness, high grade and giant size of the Bayan Obo deposit, the world's largest single REE resource with million tonnes of REE oxides.

Genetic relationship between fenitized ores and hosting dolomite carbonatite of the Bayan Obo REE deposit, Inner Mongolia, China

The Bayan Obo deposit is the largest rare-earth element (REE) deposit in the world. In this deposit, ore bodies are hosted in fine-grained ore-bearing dolomite marbles, known as the H8 dolomite marble. The H8 dolomite marble has been considered as part of the Bayan Obo Group composed Paleoproterozoic sedimentary rocks. However, the H8 dolomite marble can be younger than the Bayan Obo Group, and thus, the genesis of the dolomite marble should be re-examined. In this paper, we describe petrography and compositions of the H8 dolomite marble. The dolomite marbles generally have high FeO, MnO and SrO contents, different from sedimentary limestone or dolostone but similar to dolomite carbonatites. In composition, they can be grouped into magnesio-carbonatite and ferro-carbonatite. Texturally they are porphyritic and comprise originally hypabyssal intrusions. It is thus proposed that carbonatitic magmas injected slowly into a magma chamber where they have undergone fractionation to produce evolved dolomite-carbonatite magmas rich in alkali, REE, Fe and volatile components. Such evolved, metal-rich magmas were consequently injected along the contact zone between the early dolomite carbonatite sill and other layer of the Bayan Obo Group, and produced an large fenitization aureole enclosing ores rich in Fe, Nb, REE, F and alkali metals. Changes in CO2/H2O ratios, oxygen fugacity and halogen content of fluids were responsible for the formation of different types of ore, including massive magnetite-rich, aegirine-rich and Na-amphibole-rich ores. Considering the fact that the ore bodies are located at the place where dolomite carbonatites are relatively thick and the fenitization is relatively extensive, we speculate that the ore formation was genetically related to extensive fenitization involving exsolution of large amounts of fluids and subsequently extensive metasomatism. It is concluded that carbonatitic magmas played a major role in the formation of the REE mineralization in the Bayan Obo deposit.

Ti-Nb Mineralization of Late Carbonatites and Role of Fluids in Its Formation: Petyayan-Vara Rare-Earth Carbonatites (Vuoriyarvi Massif, Russia)

This article is devoted to the geology of titanium-rich varieties of the Petyayan-Vara rare-earth dolomitic carbonatites in Vuoriyarvi, Northwest Russia. Analogues of these varieties are present in many carbonatite complexes. The aim of this study was to investigate the behavior of high field strength elements during the late stages of carbonatite formation. We conducted a multilateral study of titanium- and niobium-bearing minerals, including a petrographic study, Raman spectroscopy, microprobe determination of chemical composition, and electron backscatter diffraction. Three TiO2-polymorphs (anatase, brookite and rutile) and three pyrochlore group members (hydroxycalcio-, fluorcalcio-, and kenoplumbopyrochlore) were found to coexist in the studied rocks. The formation of these minerals occurred in several stages. First, Nb-poor Ti-oxides were formed in the fluid-permeable zones. The overprinting of this assemblage by residual fluids led to the generation of Nb-rich brookite (the main niobium concentrator in the Petyayan-Vara) and minerals of the pyrochlore group. This process also caused niobium enrichment with of early generations of Ti oxides. Our results indicate abrupt changes in the physicochemical parameters at the late hydro (carbo) thermal stage of the carbonatite formation and high migration capacity of Ti and Nb under these conditions. The metasomatism was accompanied by the separation of these elements.

Hydroxynatropyrochlore, (Na,Сa,Ce)2Nb2O6(OH), a new member of the pyrochlore group from the Kovdor phoscorite–carbonatite pipe, Kola Peninsula, Russia

AbstractHydroxynatropyrochlore, (Na,Сa,Ce)2Nb2O6(OH), is a new Na–Nb–OH-dominant member of the pyrochlore supergroup from the Kovdor phoscorite–carbonatite pipe, Kola Peninsula, Russia. It is cubic, Fd$\bar{3}$m, a = 10.3211(3) Å, V = 1099.46(8) Å3 and Z = 8 (from powder diffraction data) or a = 10.3276(5) Å, V = 1101.5(2) Å3 and Z = 8 (from single-crystal diffraction data). Hydroxynatropyrochlore is a characteristic accessory mineral of the low-carbonate phoscorite in the contact zone of the phoscorite–carbonatite pipe with host foidolite as well as in the carbonate-rich phoscorite and carbonatite of the pipe axial zone. It usually forms zonal cubic or cubooctahedral crystals (up to 0.5 mm in diameter) with irregularly shaped relics of amorphous U–Ta-rich hydroxykenopyrochlore inside. Characteristic associated minerals include rock-forming calcite, dolomite, forsterite, hydroxylapatite, magnetite and phlogopite, accessory baddeleyite, baryte, barytocalcite, chalcopyrite, chamosite–clinochlore, galena, gladiusite, juonniite, ilmenite, magnesite, pyrite, pyrrhotite, quintinite, spinel, strontianite, valleriite and zirconolite. Hydroxynatropyrochlore is pale brown, with an adamantine to greasy lustre and a white streak. The cleavage is average on {111} and the fracture is conchoidal. Mohs hardness is ~5. In transmitted light, the mineral is light brown, isotropic and n = 2.10(5) (λ = 589 nm). The calculated and measured densities are 4.77 and 4.60(5) g cm−3, respectively. The mean chemical composition determined by electron microprobe is: F 0.05, Na2O 7.97, CaO 10.38, TiO2 4.71, FeO 0.42, Nb2O5 56.44, Ce2O3 3.56, Ta2O5 4.73, ThO2 5.73, UO2 3.66, total 97.65 wt.%. The empirical formula calculated on the basis of Nb + Ta + Ti = 2 apfu is (Na1.02Ca0.73Ce0.09Th0.09 U0.05${\rm Fe}_{{\rm 0}{\rm. 02}}^{2 +} $)Σ2.00(Nb1.68Ti0.23Ta0.09)Σ2.00O6.03(OH1.04F0.01)Σ1.05. The simplified formula is (Na,Ca,Ce)2Nb2O6(OH). The mineral dissolves slowly in hot HCl. The strongest X-ray powder-diffraction lines [listed as (d in Å)(I)(hkl)] are as follows: 5.96(47)(111), 3.110(30)(311), 2.580(100)(222), 2.368(19)(400), 1.9875(6)(333), 1.8257(25)(440) and 1.5561(14)(622). The crystal structure of hydroxynatropyrochlore was refined to R1 = 0.026 on the basis of 80 unique observed reflections. The mineral belongs to the pyrochlore structure type A2B2O6Y1 with octahedral framework of corner-sharing BO6 octahedra with A cations and OH groups in the interstices. The Raman spectrum of hydroxynatropyrochlore contains characteristic bands of the lattice, BO6, B–O and O–H vibrations and no characteristic bands of the H2O vibrations. Within the Kovdor phoscorite–carbonatite pipe, hydroxynatropyrochlore is the latest hydrothermal mineral of the pyrochlore supergroup, which forms external rims around grains of earlier U-rich hydroxykenopyrochlore and separated crystals in voids of dolomite carbonatite veins. The mineral is named in accordance with the pyrochlore supergroup nomenclature.

Fenitized Wall Rock Geochemistry of the First Carbonatite Dyke at Bayan Obo, Inner Mongolia, China

AbstractThe first carbonatite dyke at Bayan Obo is well exposed on the surface for a length and width of approximately 60 m and 1.1–1.5 m, respectively. Along its strike, the fenitized H1 (Qs) and H2 (Cs) quartzite is replaced by Na‐amphiboles, aegirines, and alkali‐feldspars, intermittently stretching as far away as 800 m in length. Based on petrographical characteristics, the dyke's fenitized wall rocks are divisible into different zones: (1) outer, (2) middle, and (3) inner. The outer zone is 5–17 m from the NW margin of the dyke. The middle zone is located at 3.5–5 m from the NW margin of the dyke. The inner contact zone is located between direct contact with the dyke and 3.5 m from the dyke. In the outer zone, upon visual examination, no evidence of outcrop fenitization was found and the major elemental rock composition is nearly identical to the unaltered H1 and H2 lithologies. In the thin sections, however, small amounts of Na‐amphibole and phlogopite are present. Despite relatively poor development throughout the 5 m of fenitization, the wall rocks have retained at least a small geochemical signature comparable to the original sedimentary protolith. The fenites occurring in the inner zone exhibit distinct variations, not only for the sharp contact at the outcrop scale, but also for variations in major, rare earth elements (REE), and trace elements and Sm‐Nd isotope composition. The wall rocks within 3.5 m have undergone strong fenitization, inheriting the geochemical signature derived from the carbonatite dyke. Fenitization in the middle zone was not as strong, at least compared to the inner zone, but was stronger than the outer zone. Compared to some trace elements and REEs, the major elements are relatively immobile during fenitization. The Sm‐Nd isotope data for the carbonatite dyke and the adjacent fenitized wall rocks, where the Sm and Nd originate solely from the dyke, plots as a six‐point isochron with an age of 1308±56 Ma. This age is identical to that of ore‐bearing dolomite carbonatite and the related ore‐forming events, indicating that there may be a petrogenetic link between the two. Based on Sr and Nd isotope compositional data, the first carbonatite dyke may be derived from an enriched mantle.

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

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

Surface alteration of a melilitite-clan carbonatite and the potential for remote carbonatite detection

The varied lithologic facies and mineralogy resulting from emplacement of syngenetic alkaline, ultramafic and carbonatite (AUC) intrusions are made more diverse by variable weathering and alteration. Ultramafic-carbonatite intrusive complexes are a source for many valuable minerals including diamonds and rare earth element minerals. The intrusive bodies are often difficult to detect in the field due to their paucity, weathering, vegetation, and, in some instances, similarity to country rock, especially in the case of carbonatites among sedimentary carbonates. Remote spectroscopic detection is used extensively for geologic mapping yet has not been applied to differentiating sedimentary and igneous carbonate weathering profiles. Here we document the alteration mineralogy of a newly authenticated melilitite-clan carbonatite occurrence in the Avon Volcanic District in southeast Missouri, USA. The presence of lizardite, vermiculite, phlogopite, and andradite in the weathered crust of calcic and dolomitic carbonatites differentiate them from sedimentary dolomites. We apply field and laboratory spectral measurements to determine the feasibility of humid region AUC remote sensing and classification. Automated humid region detection and classification of carbonatites among sedimentary carbonates is shown to be possible using ratios of absorption features in the 2000–2400 nm range as well as features centered near 680, 900, and 1100 nm due transition metal charge transfer and crystal field splitting in garnet, sheet-silicates, and spinel.

Magnetite-apatite-dolomitic rocks of Ust-Chulman (Aldan shield, Russia): Seligdar-type carbonatites?

The Ust-Chulman apatite ore body is situated within the Nimnyrskaya apatite zone at the Aldan shield in Russia. The latest data confirm the carbonatitic origin of the Seligdar apatite deposit (Prokopyev et al. in Ore Geol Rev 81:296–308, 2017). The results of our investigations demonstrate that the magnetite-apatite-dolomitic rocks of the Ust-Chulman are highly similar to Seligdar-type dolomitic carbonatites in terms of the mineralogy and the fluid regime of formation. The ilmenite and spinel mineral phases occur as solid solutions with magnetite, and support the magmatic origin of the Ust-Chulman ores. The chemical composition of REE- and SO3-bearing apatite crystals and, specifically, monazite-(Ce) mineralisation and the formation of Nb-rutile, late hydrothermal sulphate minerals (barite, anhydrite) and haematite are typical for carbonatite complexes. The fluid inclusions study revealed similarities to the evolutionary trend of the Seligdar carbonatites that included changes of the hydrothermal solutions from highly concentrated chloride to medium–low concentrated chloride-sulphate and oxidized carbonate-ferrous.

Apatite in carbonatitic rocks: Compositional variation, zoning, element partitioning and petrogenetic significance

Apatite-group phosphates are nearly ubiquitous in carbonatites, but our understanding of these minerals is inadequate, particularly in the areas of element partitioning and petrogenetic interpretation of their compositional variation among spatially associated rocks and within individual crystals. In the present work, the mode of occurrence, and major- and trace-element chemistry of apatite (sensu lato) from calcite and dolomite carbonatites, their associated cumulate rocks (including phoscorites) and hydrothermal parageneses were studied using a set of 80 samples from 50 localities worldwide. The majority of this set represents material for which no analytical data are available in the literature. Electron-microprobe and laser-ablation mass-spectrometry data (~600 and 400 analyses, respectively), accompanied by back-scattered-electron and cathodoluminescence images and Raman spectra, were used to identify the key compositional characteristics and zoning patterns of carbonatitic apatite. These data are placed in the context of phosphorus geochemistry in carbonatitic systems and carbonatite evolution, and compared to the models proposed by previous workers. The documented variations in apatite morphology and zoning represent a detailed record of a wide range of evolutionary processes, both magmatic and fluid-driven. The majority of igneous apatite from the examined rocks is Cl-poor fluorapatite or F-rich hydroxylapatite (≥0.3apfu F) with 0.2–2.7wt.% SrO, 0–4.5wt.% LREE2O3, 0–0.8wt.% Na2O, and low levels of other cations accommodated in the Ca site (up to 1000ppm Mn, 2300ppm Fe, 200ppm Ba, 150ppm Pb, 700ppm Th and 150ppm U), none of which show meaningful correlation with the host-rock type. Silicate, (SO4)2− and (VO4)3− anions, substituting for (PO4)3−, tend to occur in greater abundance in crystals from calcite carbonatites (up to 4.2wt.% SiO2, 1.5wt.% SO3 and 660ppm V). Although (CO3)2− groups are very likely present in some samples, Raman micro-spectroscopy proved inconclusive for apatites with small P-site deficiencies and other substituent elements in this site. Indicator REE ratios sensitive to redox conditions (δCe, δEu) and hydrothermal overprint (δY) form a fairly tight cluster of values (0.8–1.3, 0.8–1.1 and 0.6–0.9, respectively) and may be used in combination with trace-element abundances for the development of geochemical exploration tools. Hydrothermal apatite forms in carbonatites as the product of replacement of primary apatite, or is deposited in fractures and interstices as euhedral crystals and aggregates associated with typical late-stage minerals (e.g., quartz and chlorite). Hydrothermal apatite is typically depleted in Sr, REE, Mn and Th, but enriched in F (up to 4.8wt.%) relative to its igneous precursor, and also differs from the latter in at least some of key REE ratios [e.g., shows (La/Yb)cn≤25, or a negative Ce anomaly]. The only significant exception is Sr(± REE,Na)-rich replacement zones and overgrowths on igneous apatite from some dolomite(-bearing) carbonatites. Their crystallization conditions and source fluid appear to be very different from the more common Sr-REE-depleted variety. Based on the new evidence presented in this work, trace-element partitioning between apatite and carbonatitic magmas, phosphate solubility in these magmas, and compositional variation of apatite-group minerals from spatially associated carbonatitic rocks are critically re-evaluated.

Insights into the late-stage differentiation processes of the Catalão I carbonatite complex in Brazil: New Sr–Nd and C–O isotopic data in minerals from niobium ores

The Late Cretaceous Catalão I carbonatite complex consists of ultramafic silicate rocks, phoscorites, nelsonites and carbonatites. The latest stages of the evolution of the complex are characterized by several nelsonite (magnetite–apatite rock) and carbonatite dykes, plugs and veins crosscutting earlier alkaline rocks. The interaction between the latter and late-stage carbonatites and/or carbo-hydrothermal fluids, converted the original dunites and bebedourites to metasomatic phlogopitites. Late-stage nelsonites (N1), pseudonelsonites (N2) and various types of dolomite carbonatites (DC) including norsethite-, magnesite- and/or monazite-bearing varieties show significant whole-rock Nd and Sr isotopic variations. To elucidate whether magmatic or metasomatic processes, or both, were responsible for these isotope variations we characterized the Nd and Sr isotope compositions of major mineral phases (i.e. apatite, dolomite, norsethite, pyrochlore and tetraferriphlogopite) in these late-stage rocks. Mineral isotope data recorded the same differences observed between N1 and N2 whole-rocks with N2 minerals showing more enriched isotopic signatures than minerals from N1. Sr isotopic disequilibrium among minerals from N2 pseudonelsonites and spatially related dolomite carbonatite pockets implies formation from batches of carbonate melts with distinct isotopic compositions. A detailed investigation of Nd and Sr isotopes from whole-rocks and minerals suggests that the most evolved rocks of the Catalão I complex probably derive from two different evolution paths. We propose that an earlier magmatic trend (path A) could be explained by several batches of immiscible and/or residual melts derived from carbonated-silicate parental magma (e.g. phlogopite picrite) contaminated with continental crust to a variable extent, in an AFCLI-like process. A second trend (path B) comprises highly variable 143Nd/144Ndi at nearly constant 87Sr/86Sri coupled with high δ18O in carbonates. This is interpreted here as the result of the interaction of previously-formed dolomite carbonatites with carbo-hydrothermal fluids.

Unusual shape of pyrrhotite inclusions in scapolite of igneous rocks from the southernern Urals

The unique igneous rock (scapolite–diopside gabbro) from the Ilmeny Mountains in the southern Urals is described. Gabbro fills a segment of dike 1.3 m thick that cuts through calcite–dolomite carbonatite. Medium-grain pyroxenite with scapolite that occurs at selvages gradually passes to scapolite-bearing gabbro in the central part of the dike. Scapolite crystals display surfaces of concurrent growth, which are evidence of their magmatic origin. Scapolite (Me 63–70%) contains numerous pyrrhotite inclusions as platelets 0.001 mm thick oriented parallel to the cleavage plane {100}. The calculated pyrrhotite formula is consistent with its stoichiometry (Fe1–xS). The morphology of the platelets (hexagonal sections) and their optical properties indicate a hexagonal symmetry of pyrrhotite. As follows from the insignificant difference between scapolite grains with and without pyrrhotite inclusions, scapolite and pyrrhotite should be regarded as products of synchronous magmatic melt crystallization.

Carbonatite magmatism of the southern Siberian Craton 1 Ga ago: Evidence for the beginning of breakup of Laurasia in the early Neoproterozoic

Apatite and biotite from dolomite‒ankerite and calcite‒dolomite carbonatite dikes emplaced into the Paleoproterozoic metamorphic rock complex in the southern part of the Siberian Craton are dated by the U–Pb (LA-ICP-MS) and 40Ar–39Ar methods, respectively. Proceeding from the lower intercept of discordia with concordia, the age of apatite from calcite‒dolomite carbonatite is estimated to be 972 ± 21 Ma and that for apatite from dolomite‒ankerite carbonatite, as 929 ± 37 Ma. Values derived from their upper intercept have no geological sense. The ages obtained for biotite by the 40Ar–39Ar method are 965 ± 9 and 975 ± 14 Ma. It means that the formation of carbonatites reflects the earliest phases of the Neoproterozoic stage in extension of the continental lithosphere.

Mineralogy, age and genesis of apatite-dolomite ores at the Seligdar apatite deposit (Central Aldan, Russia)

The Seligdar apatite deposit is located in the Aldan-Stanovoy shield of the Siberian platform in Russia. This deposit is a typical ore deposit of the Nimnyrskaya approximately N-S apatite zone, which is about 400km long. The genesis of the apatite-dolomite ores at the Seligdar deposit is a matter of debate. This article presents new evidence of the carbonatitic genesis of the apatite-dolomite rocks at the Seligdar deposit based on modern methods of mineralogical, geochronological, melt and fluid inclusion investigations. According to our data, the age of the apatite-dolomite ores is 1880±13Ma (U-Pb SHRIMP, zircon). Study of melt inclusions indicates that the ores were formed from a carbonate melt of dolomitic composition with alkali (sulphates, chlorides and fluorides of Na and K) and silica components (1–10wt.%) at a temperature of >1100°C. The dolomite carbonatites have been subsequently exposed to the intense processes of hydrothermal-metasomatic alteration and metamorphism. The evolution of mineral parageneses from the magmatic apatite-magnetite-dolomite carbonatite stage to the hydrothermal stages with quartz, calcite, monazite-Ce, xenotime-Y, haematite, thorite, thorianite, sulphates and sulphides mineralization agrees with the fluid inclusion regime evolution from the carbonate melt to the chloride brines, and the varying concentrations of the chloride solutions are also described in this article. The investigation of the apatite deposits within the Aldan shield not only allows us to take a new look at the question of their origin but also helps us to study the composition of the ancient mantle, as well as the specifics of apatite-dolomite carbonatite and related hydrothermal Fe and Th-REE mineralization in this region.

Zirconolite, zircon and monazite-(Ce) U-Th-Pb age constraints on the emplacement, deformation and alteration history of the Cummins Range Carbonatite Complex, Halls Creek Orogen, Kimberley region, Western Australia

In situ SHRIMP U-Pb dating of zirconolite in clinopyroxenite from the Cummins Range Carbonatite Complex, situated in the southern Halls Creek Orogen, Kimberley region, Western Australia, has provided a reliable 207Pb/206Pb age of emplacement of 1009 ± 16 Ma. Variably metamict and recrystallised zircons from co-magmatic carbonatites, including a megacryst ~1.5 cm long, gave a range of ages from ~1043–998 Ma, reflecting partial isotopic resetting during post-emplacement deformation and alteration. Monazite-(Ce) in a strongly foliated dolomite carbonatite produced U-Th-Pb dates ranging from ~900–590 Ma. Although the monazite-(Ce) data cannot give any definitive ages, they clearly reflect a long history of hydrothermal alteration/recrystallisation, over at least 300 million years. This is consistent with the apparent resetting of the Rb-Sr and K-Ar isotopic systems by a post-emplacement thermal event at ~900 Ma during the intracratonic Yampi Orogeny. The emplacement of the Cummins Range Carbonatite Complex probably resulted from the reactivation of a deep crustal structure within the Halls Creek Orogen during the amalgamation of Proterozoic Australia with Rodinia over the period ~1000–950 Ma. This may have allowed an alkaline carbonated silicate magma that was parental to the Cummins Range carbonatites, and generated by redox and/or decompression partial melting of the asthenospheric mantle, to ascend from the base of the continental lithosphere along the lithospheric discontinuity constituted by the southern edge of the Halls Creek Orogen. There is no evidence of a link between the emplacement of the Cummins Range Carbonatite Complex and mafic large igneous province magmatism indicative of mantle plume activity. Rather, patterns of Proterozoic alkaline magmatism in the Kimberley Craton may have been controlled by changing plate motions during the Nuna–Rodinia supercontinent cycles (~1200–800 Ma).