Carbonatite Dykes Research Articles

U‐Pb Ages and Hf Isotope of Zircons from a Carbonatite Dyke in the Bayan Obo Fe‐REE Deposit in Inner Mongolia: its Geological Significance

Detailed studies on U‐Pb ages and Hf isotope have been carried out in zircons from a carbonatite dyke associated with the Bayan Obo giant REE‐Nb‐Fe deposit, northern margin of the North China Craton (NCC), which provide insights into the plate tectonic in Paleoproterozoic. Analyses of small amounts of zircons extracted from a large sample of the Wu carbonatite dyke have yielded two ages of late Archaean and late Paleoproterozoic (with mean 207Pb/206Pb ages of 2521±25 Ma and 1921±14 Ma, respectively). Mineral inclusions in the zircon identified by Raman spectroscopy are all silicate minerals, and none of the zircon grains has the extremely high Th/U characteristic of carbonatite, which are consistent with crystallization of the zircon from silicate, and the zircon is suggested to be derived from trapped basement complex. Hf isotopes in the zircon from the studied carbonatite are different from grain to grain, suggesting the zircons were not all formed in one single process. Majority of ∊Hf(t) values are compatible with ancient crustal sources with limited juvenile component. The Hf data and their TDM2 values also suggest a juvenile continental growth in Paleoproterozoic during the period of 1940–1957 Ma. Our data demonstrate the major crustal growth during the Paleoproterozoic in the northern margin of the NCC, coeval with the assembly of the supercontinent Columbia, and provide insights into the plate tectonic of the NCC in Paleoproterozoic.

Kimberlite and carbonatite dykes within the Premier diatreme root (Cullinan Diamond Mine, South Africa): New insights to mineralogical-genetic classifications and magma CO2 degassing

The ca. 1153 Ma Premier kimberlite pipe on the Kaapvaal craton has been intruded by late-stage kimberlite and carbonatite magmas forming discrete 0.5 to 5 m wide dykes within the lower diatreme. On the basis of petrography and geochemistry, the fresh kimberlite dykes represent archetypal monticellite phlogopite kimberlite of Group-1 affinity. Their mineral compositions, however, show marked deviations from trends that are typically considered as diagnostic for Group-1 kimberlite in mineralogical-genetic classification schemes for volatile-rich ultramafic rocks. Groundmass spinel compositions are transitional between magnesian ulvöspinel (a Group-1 kimberlite hallmark feature) and titanomagnetite trends, the latter being more diagnostic for lamproite, orangeite (formerly Group-2 kimberlite), and aillikite. The Premier kimberlite dykes contain groundmass phlogopite that evolves by Al- and Ba-depletion to tetraferriphlogopite, a compositional trend that is more typical for orangeite and aillikite. Although high-pressure cognate and groundmass ilmenites from the Premier hypabyssal kimberlites are characteristically Mg-rich (up to 15 wt% MgO), they contain up to 5 wt% MnO, which is more typical for carbonate-rich magmatic systems such as aillikite and carbonatite. Manganese-rich groundmass ilmenite also occurs in the Premier carbonatite dykes, which are largely devoid of mantle-derived crystal cargo, suggesting a link to the kimberlite dykes by fractionation processes involving development of residual carbonate-rich melts and fluids. Although mineralogical-genetic classification schemes for kimberlites and related rocks may provide an elegant approach to circumvent common issues such as mantle debris entrainment, many of the key mineral compositional trends are not as robust for magma type identification as previously thought.Utilizing an experimentally constrained CO2-degassing model, it is suggested that the Premier kimberlite dykes have lost between 10 and 20 wt% CO2 during magma ascent through the cratonic lithosphere, prior to emplacement near the Earth's surface. Comparatively low fO2 values down to −5.6 ∆NNO are obtained for the kimberlite dykes when applying monticellite and perovskite oxybarometry, which probably reflects significant CO2 degassing during magma ascent rather than the original magma redox conditions and those of the deep upper mantle source. Thus, groundmass mineral oxybarometry may have little value for the prediction of the diamond preservation potential of ascending kimberlite magmas.After correction for olivine fractionation and CO2-loss, there remains a wide gap between the primitive kimberlite and carbonatite melt compositions at Premier, which suggests that these magma types cannot be linked by variably low degrees of partial melting of the same carbonated peridotite source in the deep upper mantle. Instead, fractionation processes produced carbonate-rich residual melts/fluids from ascending kimberlite magma, which led to the carbonatite dykes within Premier pipe.

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.

Carbonatitic dykes during Pangaea transtension (Pelagonian Zone, Greece)

Carbonatitic dykes surrounded by K-Na-fenites were discovered in the Pelagonian Zone in Greece. Their carbonate portions have an isotopic mantle signature of δ13C and δ18O ranging from −5.18 to −5.56 (‰ vs. VPDB) and from 10.68 to 11.59 (‰ vs. VSMOW) respectively, whereas their mafic silicate portions have high Nb, Ta and ɛNd values, typical of alkaline basalts. Textural relationships hint at a cogenetic intrusion of silicate and carbonate liquids that according to antithetic REE profiles segregated at shallow depths (<0.6 GPa) from a parental melt sourced deeper in the mantle. Fenites bear similar REE abundances to mafic rocks but with high Rb-Ba and low Nb-Ta values. SHRIMP II U-Pb analyses of magmatic zircon cores (δ18O = 7.21–7.51) from a carbonate-bearing syenitic amphibolite yielded a Permian intrusion age at 278 ± 2 Ma, considerably older than a Cretaceous (118 ± 4 Ma) greenschist overprint obtained from metamorphic zircon rims (δ18O = 6.78–7.02).From 300 to 175 Ma the ɛNd of the Pelagonian magmatism rose irregularly to more primitive values attesting to a higher increment of asthenosphere-derived melts. In this context, the carbonatite formed within a transtensional regime of an intra-Pangaea dextral transform fault that signalled the forthcoming penetrating breakoff of the supercontinent, manifested in the Permo-Triassic.

Fenitization in the giant Bayan Obo REE-Nb-Fe deposit: Implication for REE mineralization

Fenitization, a unique type of alkali metasomatism related to carbonatite or alkaline igneous rocks, is widely distributed around the Mesoproterozoic carbonatite dykes in the Bayan Obo giant REE-Nb-Fe deposit. Meanwhile, represented by alkali silicates (sodic amphibole or aegirine) enriched REE-(Fe) ores, there is extensive alkali alteration in ores at Bayan Obo, whose similarity to fenitization has been proposed. Fenitization associated with carbonatite is an appropriate indicator of REE mineralizing process. Alkali silicates (sodic amphibole, aegirine and phlogopite) from the East and Main open pits, and fenite around the Wu dyke at Bayan Obo were collected for EMPA and laser ablation ICPMS analyses. Aegirine and phlogopite from the open pits and fenite around the Wu dyke contain similar major components. Sodic amphiboles from those two localities contain comparable trace element composition but with different Cao, MgO and F contents. Alkali silicates from fenites around the Wu dyke show systematic enrichment of Zr, Hf and Ti over alkali silicates from the open pits. Considering different wallrocks and other factors influencing the composition, combined with previous results of geochronology, mineralogy and fluid inclusion, the alkali alteration in the open pits is essentially fenitization in ore-hosting dolomite. A two-stage fenitization process is supported by the zonal texture of sodic amphiboles from Wu carbonatite dyke. The early stage fenitization was Mg, Na, K, F rich and Fe, La, Ce depleted, distributed in alkali silicates from the banded REE-Fe ores and in the core of sodic amphiboles from the Wu dyke. The late stage was Fe, LREE rich and Mg, Na, K, F depleted, distributed in the massive REE-(Fe) ores and the rim of sodic amphiboles from the Wu dyke. The initial alkali fluids derived from carbonatite commonly exhibits LREE-rich patterns. Therefore, the LREE-depletion and low La/Nd ratio of the early stage silicates indicates intense LREE-bearing mineral precipitation and the REE mineralization gradually weakened in the late stage when alkali fluids kept the initial REE patterns unchanged.

The formation of the giant Bayan Obo REE-Nb-Fe deposit, North China, Mesoproterozoic carbonatite and overprinted Paleozoic dolomitization

The Bayan Obo ore deposit in Inner Mongolia, North China, the largest-known rare earth element (REE) deposit in the world, is closely associated with carbonatite dykes. Scarce zircon grains, with a wide range of ages and diverse origins, have been extracted from the Wu dyke, a REE-enriched calcitic carbonatite dyke 2 km from the East Ore Body of the Bayan Obo deposit. Three zircon populations were identified based on ages and trace element compositions: 1) Captured zircons with Paleoproterozoic and Archean ages. These zircons have REE patterns and moderate Th/U ratios similar to zircon with silicate inclusions from basement igneous rocks, which have been recognized as contaminants from wall rocks. 2) Carbonatite magmatic zircons with Mesoproterozoic ages. These zircons have high to extremely high Th/U ratios (13–1600), a characteristic signature of the Bayan Obo deposit. Two zircon grains yielded concordant 206Pb/238U ages (1.27 ± 0.11 Ga ∼ 1.42 ± 0.18 Ga) and 208Pb/232Th age (1.26 ± 0.20 Ga) with calcite inclusions, indicating that the Wu dyke was emplaced at ca. 1.34 Ga, which coincides with a worldwide generation of Mesoproterozoic kimberlites, lamprophyres, carbonatites, and anorogenic magmatism. 3) Hydrothermal zircons with Caledonian and Triassic ages. The Caledonian zircon has 206Pb/238U age of 381 ± 4 Ma and 208Pb/232Th age of 367 ± 14 Ma with dolomite inclusion. These evidences are consistent with multiple stages of mineralization, Mesoproterozoic calcite carbonatite magmatism interacted by protracted fluxing of subduction-released Caledonian fluids during the closure of the Palaeo-Asian Ocean, coupled with interaction with the mantle wedge and metasomatism of overlying sedimentary carbonate.

New SIMS U-Pb age on zircon from the Epembe carbonatite dyke, NW Namibia: Implications for Mesoproterozoic evolution of carbonatites at the southern margin of the Congo Craton

The Epembe carbonatite dyke is situated approximately 80 km north of Opuwo, NW Namibia. The 10 km long dyke is dominated by massive and banded sövitic carbonatite intrusions. Two types of sövite have been recognized: (1) coarse-grained light grey Sövite I which occurs in highly brecciated and fractured zones and (2) medium to fine-grained Sövite II which hosts significant concentrations of pyrochlore and apatite. The contact zone between the carbonatite and basement gneisses is marked by K-feldspar fenite. Zircons from Sövite II were dated with Secondary Ion Mass Spectroscopy (SIMS) yielding a concordia intercept age of 1184 ± 10 Ma (MSWD = 0.65, Pof = 0.42) corresponding with the weighted mean 207Pb/206Pb age of 1173 ± 12 Ma (N = 20, MSWD = 1.2) as the crystallization age for the Epembe carbonatite. Our result presents a slightly older age than previously published data for the Swartbooisdrif ferrocarbonatites about 35 km to the east of Epembe. As calcitic carbonatites usually represent the early magmatic phase during the evolution of carbonatite magmas, the calcitic and ferrocarbonatite veins linking the two carbonatite suites at the southern margin of the Kunene Intrusive Complex (KIC), suggests that the spatially associated sövitic and ferrocarbonatite dykes are genetically related and form part of carbonatite melts emplaced about 180 Ma after the KIC magmatic event.

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.

Remote Sensing Exploration of Nb-Ta-LREE-Enriched Carbonatite (Epembe/Namibia)

On the example of the Epembe carbonatite-hosted Nb-Ta-LREE deposit, we demonstrate the use of hyperspectral reflectance data and geomorphic indicators for improving the accuracy of remote sensing exploration data of structurally-controlled critical raw material deposits. The results further show how exploration can benefit from a combination of expert knowledge and remotely-sensed relief, as well as imaging data. In the first stage, multi-source remote sensing data were used in lithological mapping based on Kohonen Self-Organizing Maps (SOM). We exemplify that morphological indices, such as Topographic Position Index (TPI), and spatial coordinates are crucial parameters to improve the accuracy of carbonate classification as much as 10%. The resulting lithological map shows the spatial distribution of the ridge forming carbonatite dyke, the fenitization zone, syenite plugs and mafic intrusions. In a second step, the internal zones of the carbonatite complex were identified using the Multi-Range Spectral Feature Fitting (MRSFF) algorithm and a specific decision tree. This approach allowed detecting potential enrichment zones characterized by an abundance of fluorapatite and pyroxene, as well as dolomite-carbonatite (beforsite). Cross-validation of the mineral map with field observations and radiometric data confirms the accuracy of the proposed method.

Genesis of the Bayan Obo Fe–REE–Nb deposit: Evidences from Pb–Pb age and microanalysis of the H8 Formation in Inner Mongolia, North China Craton

The Bayan Obo Fe–REE–Nb deposit is a world-largest REE deposit in Inner Mongolia, North China Craton. It also contains large Fe and Nb reserves. The REE ore body is hosted in H8 dolomite of the Bayan Obo Group. Depositional time and genesis of H8 dolomite are still controversial in addressing the genesis of the REE deposit. In this study, Pb–Pb age and geochemical features of carbonate minerals in H8 dolomite have been studied, providing constraints to genesis of H8 dolomite and the REE deposit. Results of Pb–Pb dating from unmetamorphosed and non-mineralized domains of the H8 dolomite well constrain the depositional age as 1619±150Ma, which is earlier than both REE ore and carbonatite dykes in the Bayan Obo region. This indicates that REE mineralization has characteristics of epigenetic origin. Geochemical data of carbonate minerals in H8 dolomite have low REE contents, distinctly distinguished from those carbonatitic dykes. However, the fine-grained H8 dolomite whole rocks have high REE contents, similar to those carbonatitic dykes. Mineral analysis suggests a close relationship between REE mineralization and calcite carbonatite dykes and related-derived fluids, which could transport a large amounts of REE. Integrated with these new geochronological and geochemical data, we draw conclusion that the Bayan Obo Group was a Proterozoic depositional succession, REE mineralization is as result of sedimentary carbonate rocks once being metasomatised by fluids derived from regional REE-rich calcite carbonatitic magma at depth.

The giant Bayan Obo REE-Nb-Fe deposit, China: Controversy and ore genesis

Bayan Obo ore deposit is the largest rare-earth element (REE) resource, and the second largest niobium (Nb) resource in the world. Due to the complicated element/mineral compositions and involving several geological events, the REE enrichment mechanism and genesis of this giant deposit still remains intense debated. The deposit is hosted in the massive dolomite, and nearly one hundred carbonatite dykes occur in the vicinity of the deposit. The carbonatite dykes can be divided into three types from early to late: dolomite, co-existing dolomite-calcite and calcite type, corresponding to different evolutionary stages of carbonatite magmatism based on the REE and trace element data. The latter always has higher REE content. The origin of the ore-hosting dolomite at Bayan Obo has been addressed in various models, ranging from a normal sedimentary carbonate rocks to volcano-sedimentary sequence, and a large carbonatitic intrusion. More geochemical evidences show that the coarse-grained dolomite represents a Mesoproterozoic carbonatite pluton and the fine-grained dolomite resulted from the extensive REE mineralization and modification of the coarse-grained variety. The ore bodies, distributed along an E–W striking belt, occur as large lenses and underwent more intense fluoritization and fenitization. The first episode mineralization is characterized by disseminated mineralization in the dolomite. The second or main-episode is banded and/or massive mineralization, cut by the third episode consisting of aegirine-rich veins. Various dating methods gave different mineralization ages at Bayan Obo, resulting in long and hot debates. Compilation of available data suggests that the mineralization is rather variable with two peaks at ∼1400 and 440 Ma. The early mineralization peak closes in time to the intrusion of the carbonatite dykes. A significant thermal event at ca. 440 Ma resulted in the formation of late-stage veins with coarse crystals of REE minerals. Fluids involving in the REE-Nb-Fe mineralization at Bayan Obo might be REE-F-CO2-NaCl-H2O system. The presence of REE-carbonates as an abundant solid in the ores shows that the original ore-forming fluids are very rich in REE, and therefore, have the potential to produce economic REE ores at Bayan Obo. The Bayan Obo deposit is a product of mantle-derived carbonatitic magmatism at ca. 1400 Ma, which was likely related to the breakup of Columbia. Some remobilization of REE occurred due to subduction of the Palaeo-Asian oceanic plate during the Silurian, forming weak vein-like mineralization.

Structurally-controlled hydrothermal alteration in the syntectonic Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (Burundi): Implications for REE and HFSE mobilities

The Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (URAPC), Burundi, is located along the western branch of the East African Rift. It comprises oversaturated and undersaturated syenites and a shallow-level carbonatite body (the Matongo carbonatite) that does not outcrop but has been sampled by drill-cores. The elliptic map contour of the URAPC points to a syntectonic emplacement. Large shear zones that were active during magmatic emplacement have accommodated a regional NE-SW shortening. Mineralization features of late-magmatic to hydrothermal origin are associated with the carbonatite, which, by itself, contains a dense network of calcitic veins. HFSE mineralization occurring as zircon and ilmenite megacrysts can be found in an area of intense and extensive K-fenitization, which lead to the transformation of the surrounding syenite into a dominant K-feldspar+biotite mineral assemblage (Inamvumvu area). Carbonatitic dykes (overprinted by a hydrothermal alteration) are present a few kilometers north of the Matongo carbonatite, within highly deformed zones in the syenite. These dykes occur along with Na-fenites (resulting from the transformation of the feldspathoidal syenite into an albite-dominant paragenesis) and are enriched in REE-minerals (monazite and ancylite-(Ce)). Many magmatic (pegmatoid) dykes and hydrothermal (quartz+hematite) veins also occur in shear zones in the URAPC. Most of them can be interpreted as tension gashes. The chondrite-normalized REE patterns of some carbonatite whole rock samples are highly disturbed, in relation to post-magmatic hydrothermal alteration. The HFSE and REE distribution in the minerals from the hydrothermal veins/dykes (calcitic veins within the carbonatite, carbonatite dykes overprinted by a hydrothermal alteration in deformed zones, and zircon and ilmenite megacrysts) attests for a complex behavior of REE during alteration. Oxygen and carbon isotope compositions of the Matongo carbonatite and the carbonatitic dykes have a magmatic signature, with 7.2<δ18O (vs. SMOW) <8.5‰ and -4.7<δ13C (vs. PDB) <-5.4‰ in agreement with the Sr isotopic composition. The oxygen isotope composition of zircon and ilmenite megacrysts (δ18OZr=4−4.7‰, δ18OIlm=- 4.3−-1.5‰, respectively) also point to a magmato-hydrothermal origin of the forming fluids. Some samples of the Matongo carbonatite and the carbonatitic dykes, with high δ18O values (δ18O=8.6−21.8‰), show evidence of a medium- to low-temperature hydrothermal alteration event by an aqueous fluid. Calcitic veins in the carbonatite record another alteration event, outlined by the co-variation of δ18O and δ13C values (δ18O=16.3−24.7‰ and δ13C=-4.7−0.2‰), implying the involvement of a mixed H2O−CO2 fluid. As a whole, the circulation of fluids in the URAPC was initiated during magmatic emplacement and the geometry of this circulation was controlled by the syn-emplacement crustal scale shear zones. Element mobility, one expression of which being the mineralization features described here, follow the same scheme.

New data of the Bayan Obo Fe–REE–Nb deposit, Inner Mongolia: Implications for ore genesis

The Bayan Obo deposit in the Inner Mongolia, North China Craton, is the largest rare-earth element (REE) resource in the world. The deposit is hosted in Mesoproterozoic sediments of the Bayan Obo Group, the lithological sequence of which has been divided into seventeen units (H1–H17). The REE mineralization is mainly concentrated in the H8 dolomite unit, and therefore the depositional age of this unit is important in addressing the debate over the genesis of the REE–Fe–Nb ore deposit. Here we present U–Pb and Hf model ages of zircons from the adjacent H9 slate. Our data suggest that the H9 slate formed at 1505±12Ma and the H8 dolomite was deposited during the Mesoproterozoic. We also present Ar–Ar ages of riebeckite in veins cutting the H8 dolomite and biotite in the ore bodies which show 389.5±3.0Ma and 289.1±1.8Ma, respectively. The U–Pb measurements from overgrowth zircon in H9 slate give an age of 518.8±7.5Ma. These new age data indicate that the Bayan Obo deposit was disturbed during thermal events at 520Ma to 390Ma, generating a second phase of REE–Nb mineralization. The δ7Li values of the REE mineralized dolomite are similar to those of carbonatite dykes. Combined with ɛNd features, we provide geochemical evidences that REE mineralized dolomite has undergone fluid interaction. Results reveal that the carbonatite magmatism occurred within the Bayan Obo rift at ca. 1.2 Ga, provided the primary source for REE–Nb mineralization or to fertilize the Bayan Obo crust, and the later was remobilized during the Caledonian to generate the second phase of mineralization within a convergent plate margin regime.

Stable H–C–O isotope and trace element geochemistry of the Cummins Range Carbonatite Complex, Kimberley region, Western Australia: implications for hydrothermal REE mineralization, carbonatite evolution and mantle source regions

The Neoproterozoic Cummins Range Carbonatite Complex (CRCC) is situated in the southern Halls Creek Orogen adjacent to the Kimberley Craton in northern Western Australia. The CRCC is a composite, subvertical to vertical stock ∼2 km across with a rim of phlogopite–diopside clinopyroxenite surrounding a plug of calcite carbonatite and dolomite carbonatite dykes and veins that contain variable proportions of apatite–phlogopite–magnetite ± pyrochlore ± metasomatic Na–Ca amphiboles ± zircon. Early high-Sr calcite carbonatites (4,800–6,060 ppm Sr; La/YbCN = 31.6–41.5; δ13C = −4.2 to −4.0 ‰) possibly were derived from a carbonated silicate parental magma by fractional crystallization. Associated high-Sr dolomite carbonatites (4,090–6,310 ppm Sr; La/YbCN = 96.5–352) and a late-stage, narrow, high rare earth element (REE) dolomite carbonatite dyke (La/YbCN = 2756) define a shift in the C–O stable isotope data (δ18O = 7.5 to 12.6 ‰; δ13C = −4.2 to −2.2 ‰) from the primary carbonatite field that may have been produced by Rayleigh fractionation with magma crystallization and cooling or through crustal contamination via fluid infiltration. Past exploration has focussed primarily on the secondary monazite-(Ce)-rich REE and U mineralization in the oxidized zone overlying the carbonatite. However, high-grade primary hydrothermal REE mineralization also occurs in narrow (<1 m wide) shear-zone hosted lenses of apatite–monazite-(Ce) and foliated monazite-(Ce)–talc rocks (≤∼25.8 wt% total rare earth oxide (TREO); La/YbCN = 30,085), as well as in high-REE dolomite carbonatite dykes (3.43 wt% TREO), where calcite, parisite-(Ce) and synchysite-(Ce) replace monazite-(Ce) after apatite. Primary magmatic carbonatites were widely hydrothermally dolomitized to produce low-Sr dolomite carbonatite (38.5–282 ppm Sr; La/YbCN = 38.4–158.4; δ18O = 20.8 to 21.9 ‰; δ13C = −4.3 to −3.6 ‰) that contains weak REE mineralization in replacement textures, veins and coating vugs. The relatively high δD values (−54 to −34 ‰) of H2O derived from carbonatites from the CRCC indicate that the fluids associated with carbonate formation contained a significant amount of crustal component in accordance with the elevated δ13C values (∼−4 ‰). The high δD and δ13C signature of the carbonatites may have been produced by CO2–H2O metasomatism of the mantle source during Paleoproterozoic subduction beneath the eastern margin of the Kimberley Craton.

Triassic tectonics and mineral systems in the Qinling Orogen, central China

The Qinling Orogen in central China preserves the records of a complex geological and tectonic history, and also carries abundant mineral resources. Previous investigations on the Qinling Orogen have identified that the final transition from the Palaeo‐Tethys Ocean to a continental orogen occurred in the Triassic, although the timing of the oceanic closure and the tectonic affinity of the Triassic Qinling belt have been poorly constrained. In this contribution, we compile the palaeomagnetic data and geological evidence related to sedimentation, magmatism and mineral systems. The palaeomagnetic data show that the Palaeo‐Tethys Ocean in the Qinling Orogen gradually closed westward during the Triassic, terminating in the beginning of the Jurassic. Triassic strata are widely developed in the Qinling Orogen and conformably overlie the Permian strata. They are composed of carbonates and siliciclastic rocks that were strongly deformed and weakly metamorphosed during the Late Triassic to Late Jurassic. The Triassic magmatic rocks are dominated by I‐type granitoids, with subordinate S‐type and alkaline granites and minor carbonatite dykes, with a clear northward transition in lithology and geochemical features from the Mian‐Lue Suture. Different types of Triassic mineral systems have been reported from this region, including sedimentary‐hosted epizonogenic hydrothermal Hg–Sb, Pb–Zn ± Ag, Au deposits, metamorphic hydrothermal lodes of Au, Ag–Pb–Zn, Mo–Au and Mo, magmatic hydrothermal systems including porphyry Mo, breccia pipe Au, and carbonatite‐hosted Mo deposits. We infer that the Triassic Qinling Orogen witnessed synchronous oceanic plate subduction in the west and continental linkage in the east, with a transition from oceanic basin to continental orogen through gradual subduction‐related consumption of the oceanic plate. The Palaeo‐Tethys Ocean closed in a westward scissor‐like fashion in the Triassic and continental collision and extensive crustal shortening in the Jurassic, following which post‐collisional extension occurred in the Early Cretaceous. Copyright © 2014 John Wiley &amp; Sons, Ltd.

A review of the genesis of the world class Bayan Obo Fe–REE–Nb deposits, Inner Mongolia, China: Multistage processes and outstanding questions

The Bayan Obo Fe–REE–Nb deposit is the world's largest rare earth element (REE) resource and with the increasing focus on critical metal resources has become a focus of global interest. The deposit is hosted in the Palaeoproterozoic Bayan Obo Group, mainly concentrated in the H8 dolomite marble. The ores consist of light REE enriched monazite and bastnäsite, with a wide array of other REE minerals. Niobium mineralisation is hosted primarily in aeschynite and pyrochlore, although there are a wide range of other Nb-minerals. The origin of the host dolomite and ore bodies has been a subject of intense debate. The host dolomite has been proposed to be both of sedimentary origin and an igneous carbonatite. Carbonatite dykes do occur widely in the area, and consideration of the textural, geochemical and isotopic composition of the dolomite suggests an origin via intrusion of magmatic carbonatite into meta-sedimentary marble, accompanied by metasomatism. The origin of the ore bodies is complex, indicated most strongly by an ~1Ga range in radiometric age determinations. Compilation of available data suggests that the ores were originally formed around 1.3Ga (Sm–Nd isochron ages; Th–Pb ages of zircon), close in time to the intrusion of the carbonatite dykes. The ores were subsequently subjected to several stages of deformation and hydrothermal overprint, culminating in deformation, metamorphism and fluid flow related to the Caledonian subduction of the Mongolian Plate under the North China Craton from ~450 to 420Ma (Th–Pb ages of monazite). This stage resulted in the formation of the strong foliation (‘banding’) of the ore. The presence of undeformed veins with alkali mineral fills, and the overprinting of the foliation by Nb minerals suggest that secondary fluid flow events may also have contributed to the metal endowment of the deposits, as well as remobilising the original Fe and REE mineralisation. The alteration mineralogy and geochemistry of the ores are comparable to those of many REE mineralised carbonatites. Initial Nd isotope ratios at 450Ma, however, suggest crustal sources for the metals. These conflicting lines of evidence can be reconciled if a (at least) two stage isotopic evolution is accepted for the deposits, with an original mantle-sourced, carbonatite-related metal accumulation forming around 1.3Ga with εNd close to 0. The ore was remobilised, with associated re-equilibration of Th–Pb isotope systematics during deformation at ~450Ma. A further stage of alkaline hydrothermal fluid was responsible for Nb mineralisation at this stage. The complex geological history, with multiple stages of alkaline, high field strength element-rich, metasomatic fluid flow, is probably the main reason for the exceptional metal endowment of the Bayan Obo area.

Integrated U–Pb and Sm–Nd geochronology for a REE-rich carbonatite dyke at the giant Bayan Obo REE deposit, Northern China

The Bayan Obo deposit in North China contains the largest rare-earth element (REE) resources in the world, but its forming time remains controversial. Nearly one hundred carbonatite dykes occur around the Bayan Obo deposit, including dolomite, calcite and calcite–dolomite carbonatite varieties. Zircons from a REE-rich carbonatite dyke and wallrock quartz conglomerate at Bayan Obo have been analyzed for U–Pb to determine the age of the dyke. Zircon from the carbonatite dyke, analyzed by conventional isotope dilution thermal ionization mass spectrometry (ID-TIMS), yielded an upper intercept age of 1417±19Ma. This age is confirmed by SHRIMP U–Pb analysis of zircon from the same carbonatite dyke, which gave a 207Pb/206Pb weighted mean age of 1418±29Ma. In situ Nd isotope measurements of monazite collected from the carbonatite dyke gave an isochron age of 1275±87Ma. These results demonstrate that the dyke intruded ~1400Ma. In view of predecessor's results, it is clarified that the REE mineralization at Bayan Obo occurred at ca. 1400Ma, consistent with the timing of carbonatite dyke intrusion in the region. The youngest detrital zircons from the quartz conglomerate yielded a 207Pb/206Pb weighted mean age of 1941±7Ma using LA ICP-MS U–Pb method. Detrital zircons in the carbonatite dyke also gave a mean apparent age of 1932±3Ma using ID-TIMS U–Pb method and 1914±14Ma using SHRIMP U–Pb method. These ages constrain the beginning active time of the Zha'ertai–Bayan Obo rift in the northern margin of the North China Craton after ~1900Ma.

Carbonatite crystallization and alteration in the Tarr carbonatite–albitite complex, Sinai Peninsula, Egypt

Carbonate dykes occurring in the Arabian-Nubian Shield (ANS) are clearly intrusive in origin and carbonatites according to the IUGS classification, yet previous investigations refer to them as “intrusive carbonates”, due mainly to their low Sr, Ba, Nb, Y, Th and rare earth element (REE) contents. The Tarr carbonatite albitite complex (TCA) in SE Sinai, Egypt contains a series of small (<1.2km2) albitite intrusions surrounded by small veins and dykes of carbonatite, which occur predominantly in a narrow zone of brecciation surrounding the intrusions. Fennitic alteration surrounding TCA has been reported but there is little consensus on the extent and origin of this alteration. Fennitic alteration surrounding the TCA carbonatites is not abundant. Alteration is dominated by precipitation of carbonates in the breccia zone surrounding the albitite intrusion with associated actinolite, chlorite, sericite and epidote. Geochemical compositions are consistent with addition of carbonates and associated secondary minerals because the altered rocks contain higher CaO, MgO, Fe2O3 and MnO and lower SiO2, Al2O3, Na2O and K2O compared to their less altered rocks. Fluid inclusion investigations show that the carbonatite magma contained a high-salinity H2O–CO2–NaCl–CaCl2 fluid, although the lack of fennitic alteration implies that this fluid was not abundant. The crystallization conditions of the carbonatite dykes and carbonatite matrix in the breccia zones have been constrained using Zr-in-rutile thermometry and fluid inclusion microthermometry. Crystallization of the carbonatite in the dykes and in the breccia zone occurred between 565±38°C and 420–480°C, respectively and at 0.75–1.3kbar, which corresponds to a depth of 2.8–4.9km. Rutile hosted within the carbonatite crystallized earlier at high temperature and the carbonate matrix crystallized later after cooling. Immiscible fluid from carbonatite magma would have altered the surrounding country rocks at lower temperature (between 400°C and 150°C deduced from the fluid inclusion thermometry) after the intrusion of the carbonatite melt.

Geochemistry and petrogenesis of carbonatites from South Nam Xe, Lai Chau area, northwest Vietnam

This paper presents a study of the petrography, mineral chemistry, geochemistry, and Sr–Nd–Pb–C–O isotope systematics of carbonatite dykes and associated rocks from the northeastern part of the Song Da intracontinental rift in South Nam Xe (northwest Vietnam) aimed at constraining the origin of the carbonatite magmas. The carbonatites are characterized by SiO2 < 12.18 wt.% and by wide ranges in FeO, MgO and CaO content that define them as calciocarbonatite and ferrocarbonatite. On U–Th–Pb isochron diagrams, whole rocks and mineral separates from the ferrocarbonatites form linear arrays corresponding to ages of 30.2–31.6 Ma (Rupelian, Oligocene). The South Nam Xe carbonatites are extremely enriched in Sr, Ba, and light rare earth elements (LREE), and depleted in high field strength elements (HFSE) (e.g. Ti, Nb, Ta, Zr and Hf). The age–corrected Sr–Nd–Pb isotope ratios and C isotope data are relatively uniform (87Sr/86Sr(t) = 0.708193–0.708349; 143Nd/144Nd(t) = 0.512250–0.512267; eNd(t) = −6.46 to −6.80; 206Pb/204Pb(t) = 18.26–18.79; 207Pb/204Pb(t) = 15.62–15.64; 208Pb/204Pb(t) = 38.80–39.38; δ13CV-PDB = –2.7 ‰ to −4.1 ‰). These isotopic compositions indicate source contamination that occurred before the production of the carbonatite magmas, and did not change noticeably during or after emplacement. The variation in oxygen isotopes is consistent with the change in mineral compositions and trace element abundances: the lower δ18O values (9.1–11.0 ‰) coupled with Sr-rich, Mn-poor calcite, and igneous textures such as triple junctions among calcite grain boundaries, define a magmatic origin. However, the elevated δ18O values of the ferrocarbonatites (12.0–13.3 ‰) coupled with a volatile-bearing mineral assemblages (including REE fluorcarbonates, sulfates, sulfides and fluorite) may be due to interaction with meteoric water during low-temperature alteration. High δ13C values and Sr–Pb ratios, and low Rb/Sr (0.00014–0.00301), Sm/Nd (0.089–0.141) and 143Nd/144Nd ratios, coupled with very high Sr-Nd concentrations, suggest the involvement of an enriched mantle component, which probably resulted from metasomatism due to the migration of subducted material. Because of the lack of tectonic data and the limited number of samples studied, this conclusion is still ambiguous and requires further study.

Geochemical characteristics of the Bayan Obo giant REE–Nb–Fe deposit: Constraints on its genesis

Geochemical characteristics of different dolomites in the Bayan Obo giant REE–Nb–Fe deposit in Inner Mongolia have been studied. Intensively REE-mineralized dolomites (total REE over 800 ppm) show similar geochemical characteristics to associated carbonatite dykes, with Ba, Th, REE enrichments and Sr, Nb, Ti, Cu depletions, which is different from those of dolomites in the deposit with low REE contents (total REE less than 800 ppm). The low REE dolomites display some transitional characteristics between carbonatite dyke and sedimentary carbonate, with La depletion and Nb enrichment. This indicates that the genesis of the REE-mineralized dolomites might be related to both carbonatite magma and sedimentary carbonates. Sulfur isotope data indicates two sulfur sources, a mantle source (δ34S c.a. 0‰) and seawater (δ34S c.a. +25‰). It is proposed that mineralized dolomites in the Bayan Obo giant REE–Nb–Fe deposit are the product of sedimentary carbonate hydrothermally metasomatised by carbonatite magma and/or associated fluids. These dolomites formed the large-scale rare earth mineralization in the unique Bayan Obo REE–Nb–Fe deposit.