Carbonatite Magma Research Articles

Wall rock assimilation in carbonatite magmas: Textural, mineral and whole-rock geochemical signatures in the jacupiranga complex, Brazil

Carbonatite magmas, with their unique low SiO2 and high volatile compositions, can be extremely reactive with silicate country rocks, driving large chemical exchanges through processes such as magmatic assimilation and fenitization. In this study, we investigate the assimilation process between Jacupiranga carbonatite and clinopyroxenite xenoliths, focusing on the textural, mineralogical, and whole-rock geochemical features of the resulting reaction rocks. These data are compared with those from fluid-derived phlogopitites from the Alto Paranaíba Igneous Province, to evaluate the processes behind the reaction between carbonatite magma and ultramafic rocks. We report the first trace element data on phlogopites from Brazilian carbonatites and associated reaction rocks. Textural, mineralogical and whole-rock geochemical features can be interpreted as the effect of carbonatite magma assimilation of ultramafic rocks, as seen in the Jacupiranga complex. Thermal and chemical quenching promote magma undercooling, allowing the comb-layering formation. Xenolith boundaries act as orbicule cores, favoring nucleation and the crystallization of the reaction assemblage. Phlogopites from Jacupiranga reaction rocks exhibit a wide range of estimated temperatures (550 up to >1000 °C), although diffusive re-equilibration during magmatic and post-magmatic cooling cannot be ruled out. Reaction phlogopites are enriched in Ni and Co near the clinopyroxenite, with depletion of these elements towards the carbonatite front. Conversely, reaction phlogopites near the carbonatite are enriched in Ba, highlighting the effect of the carbonatite melt in the generation of the reaction rocks, which gradually decreases towards the clinopyroxenite contact. The reaction rocks have whole-rock major and trace element contents intermediate between those of the clinopyroxenites and the carbonatite front. Partition coefficients between calcite and apatite in the reaction rocks have flat, somewhat U-shaped patterns for REE, typical signatures of primary igneous calcite and consistent with an origin by magmatic assimilation. This study shows that several imprints of assimilation of ultramafic wall rocks by carbonatite magmas are preserved in the resulting reaction rocks, allowing such a mechanism to be traced by using textural, mineralogical, and whole-rock geochemical evidence.

Involvement of syn–, para– and post–magmatic hydrothermal fluids in the alteration of the Kamthai carbonatite complex (India): Insights from in-situ measured 87Sr/86Sr isotope and trace element composition of calcite

The major-trace element and C-O-Sr isotopic composition of calcite constitute important proxies for reconstructing the magmatic and hydrothermal evolution of carbonatites. This study reports new 87Sr/86Sr isotope data measured in-situ from six textural types of calcite in carbonatite and fenitized phonolites of the Kamthai carbonatite complex in western India. Textural relations, along with major-trace element chemistry, and Sr isotope composition helps to discriminate four types of calcite in carbonatites—magmatic (CalM), partially reequilibrated magmatic (CalPR), and two varieties of secondary (CalSK and CalS). CalM has high concentrations of ƩREE + Y, Sr, Ba and Mn, restricted 87Sr/86Sr (0.70437 ± 0.00005), and lacks Ce or Y anomaly. It crystallized from late-stage brine-melt together with primary carbocernaite after prolonged fractional crystallization of the carbonatite magma. CalPR has identical 87Sr/86Sr (0.70425 ± 0.00024) as CalM, and formed by its partial reequilibration, accompanied by significant loss of REE, and Sr during interaction with synmagmatic fluid. The two secondary varieties of calcite in the carbonatites have overlapping but marginally higher 87Sr/86Sr (CalSK = 0.70469 ± 0.00041; CalS = 0.70478 ± 0.00025) compared to CalM, which indicates that they were altered by syn– to para– magmatic fluids with minor contribution from post-magmatic fluid (e.g., external crust-derived fluid). Both types are highly porous and have low-Z contrast in back scattered electron images, which is the result of expulsion of most of its original Sr and REE budget during fluid-induced reequilibration. Two types of secondary calcite (CalS1 and CalS2) are identified in three veins within fenitized phonolite. These contain the lowest abundance of ΣHREE + Y, Sr, Ba and Mn. One type (CalS2) occurs in calcite-quartz-pyrite veins and is characterized by LREE-depleted REE patterns and igneous-calcite-like 87Sr/86Sr (0.70434 ± 0.00073). It possibly crystallized from evolved late-stage para-magmatic fluids from the carbonatite magma after the brine-melt stage of primary LREE mineralization. The other type of vein calcite (CalS1) is characterized by strong negative Ce (δCe*: ≪0.034) anomalies, which indicates that it crystallized from oxidized fluids from which Ce was removed as Ce (IV). The 87Sr/86Sr of some of these calcites are significantly more radiogenic (0.70768 ± 0.00011) than magmatic calcite. This indicates that post-magmatic fluid played a significant role in their formation. Two component mixing calculations show that up to 40–70 % post-magmatic fluid mixed with syn-magmatic fluid can account for their Sr isotope composition. Magmatic calcite and carbocernaite were the major sources of REE and Sr during secondary redistribution.

MAGMATIC SOURCES OF THE EVOLVING CONTINENTAL TECTONOSPHERE IN INDIA: GENERATION OF ALKALINE IGNEOUS COMPLEXES WITH CARBONATITES IN THE SAMALPATTI (SOUTHERN INDIA) AND AMBA DONGAR (WESTERN INDIA) MASSIFS

Neoproterozoic Samalpatti and Cretaceous-Paleogene Amba Dongar massifs are considered to evaluate the change of sources of carbonatite magmatism in evolving tectonosphere of India. The inherited character of Ba and Sr distribution components is traced in heterogeneous carbonatites of India, whereas the independent trends in Ba and Sr enrichment are traced in carbonatites of North Asia. It is inferred that in the process of activity in the tectonosphere of the Indian subcontinent ~800 Ma ago, carbonatite melts were generated in the mantle whose photolith has an age of 4.26 Ga. This protomantle reservoir was remarkably different from the primordial mantle reservoir in the solidified magma ocean in terms of low initial μ and high initial Nd isotope ratio. After the Indian subcontinent had separated from Gondwana 130–100 Ma ago and began to join Asia ~66 Ma ago, ELMU source was activiated in the tectonosphere. The Amba Dongar carbonatites might have been derived from the 4.40 Ga mantle protoliths. Silicate melts of the Deccan LIP were simultaneously generated from the protoliths with Pb-Pb age of ~2 Ga.

New discovery of the Early Paleozoic carbonatite in Bayan Obo (China): Insights into the giant REE accumulation

The Bayan Obo deposit in China hosts the world’s largest rare earth element (REE) resource. Age dating results for Bayan Obo range across approximately one billion years (from ∼1.4 to 0.3 Ga), with three age groups (∼1.3 Ga, ∼0.45–0.40 Ga, and 0.28–0.26 Ga). Carbonatite dated to around 1.3 Ga and granitoids to approximately 270 Ma have been identified. The Early Paleozoic vein-type mineralization is widespread in the deposit. However, magmatic activity related to the highest age peak at ∼0.4 Ga has not well been identified in the deposit, complicating the interpretation of the ore genesis. In this study, we report a zircon 232Th-208Pb age of 435.2 ± 4.6 Ma (MSWD = 1.9, N = 26) from carbonatite in Bayan Obo. The zircon morphology, characterized by bipyramidal crystal forms, low U concentrations (mostly < 1 ppm), high Th/U ratio, and carbonatite-like oxygen isotopes (5.92 ± 0.31 ‰), demonstrates that the zircon crystallized from carbonatitic magmas, thus their ages represent the timing of magmatic activity. Although remelting of the Mesoproterozoic carbonatite during Paleozoic carbonatite has been previously documented, the newly identified Early Paleozoic carbonatite derives from a different mantle source, as indicated by their more radiogenic Hf isotope compositions. This suggests that the Bayan Obo deposit experienced multiple carbonatitic activities. The Early Paleozoic carbonatitic activity may have facilitated element mobilization and mineral recrystallization, improving the exploitable levels of the world’s largest REE deposit.

The newly discovered Attu carbonatite of West Greenland: A Mesoproterozoic dyke intrusion enriched in primary Sr-Ba-REE minerals

A newly discovered ca. 1.5 Ga old carbonatite dyke in central West Greenland is characterized by very high contents of rare earth elements (REE), Sr and Ba (up to 12.4 wt% total REE, 14.6 wt% SrO and 12.4 wt% BaO). The dyke occurrence is named ‘Attu carbonatite’ after the nearby village Attu. The carbonatite is primarily composed of carbonate minerals such as Sr-rich calcite (CaCO3) and dolomite (CaMg(CO3)2), huntite (Mg3Ca(CO3)4), strontianite (SrCO3), alstonite (CaBa(CO3)2), burbankite ((Na,Ca)3(Sr,Ba, Ce)3(CO3)5) and daqingshanite ((Sr,Ca,Ba)3(Ce, La)(PO4)(CO3)3−x(OH, F)x). In addition to the wide range of carbonate minerals, the carbonatite contains coarse-grained monazite(Ce), apatite and magnetite. Barite occurs as discrete crystals together with the rock-forming carbonates. Texturally, the carbonatite rock displays abundant intimately intergrown fine- to medium-grained Ca, Sr, Ba and REE carbonate minerals, which exhibit prominent exsolution textures within calcite, burbankite, strontianite and dolomite hosts, as well as in the apatite crystals. Several different exsolution textures are observed: 1) alstonite in calcite; 2) daqingshanite in calcite; 3) daqingshanite in burbankite; 4) MgBa carbonate in strontianite; 5) MgBa carbonate in calcite and strontianite in dolomite; 6) strontianite in apatite; and 7) monazite(Ce) in apatite. The carbonatite dyke is foliated and exsolution textures are observed internally in the foliation-defining minerals indicating that exsolution occurred after the main deformation event.The carbonatite magma intruded into Archaean basement gneisses that had been affected by the Nagssugtoqidian tectono-metamorphic event at approximately 1850 Ma. Magmatic monazite(Ce) crystals from the carbonatite yield a UPb age of 1565 ± 53 Ma, which is the current best estimate of dyke emplacement. Monazite(Ce) that exsolved from apatite and calcite yields a UPb age of 1492 ± 33 Ma, which is within the analytical uncertainty of the primary magmatic monazite UPb age. However, the UPb age determinations suggest that mineral exsolution occurred a few million years after carbonatite magma emplacement, in response to further cooling of the deep-seated dyke (tectonically-induced uplift?). Dyke emplacement may have occurred within an active ductile shear zone, which helps to explain the foliation of the carbonatite rock, predating cooling-related mineral exsolution. Country rock fenitization by fluids that emanated from the carbonatite dyke intrusion is recorded by the increasing abundance of mafic silicates such as Ba-rich phlogopite at the contact zone.

The composition of apatite in the Archean Siilinjärvi glimmerite-carbonatite complex in eastern Finland

We present a geochemical dataset and cathodoluminescence images of apatite from the Neoarchean (2610 Ma) glimmerite-carbonatite rocks from the Siilinjärvi complex, Eastern Finland. The subhedral, tapered prismatic grains are compositionally fluorapatite, with limited substitution of Ca by Sr, Na and low REE and Si contents. The Sr/Y ratios are among the highest in a global apatite comparison, comparable to those from other calcite carbonatites, dolomite carbonatites, and phoscorites. Some grains show evidence of late- or post-magmatic interaction with a carbonatite magma or a hydrothermal fluid, resulting in REE-rich overgrowth rims or recrystallized grains with abundant fluid inclusions. We interpret the high Sr/Y ratios combined with low REE contents and depleted heavy REE+Y to represent crystallization from a mantle-derived carbonatite parental magma. We show that the Siilinjärvi apatite is chemically heterogeneous but with a limited range in compositions. There are noticeable compositional differences on all spatial scales from micrometer to tens of meters, i.e., within a single crystal, between crystals in a sample, between samples and the two individual sampling locations. We conclude that the intra-crystal geochemical variability in apatite is a suitable tracer of the magmatic and post-magmatic evolution of carbonatite complexes.

Configuration of Carbonatite Constrained in Preintrusion Transpositional Foliation in the Bayan Obo Giant Rare Earth Element Deposit, China

Abstract The Bayan Obo ore deposit is the largest rare earth element (REE) deposit in the world and has been assumed to be hosted in dolomite that was folded in a syncline. This has been challenged by results from drill holes and low-resistivity, controlled-source audio-frequency magnetotellurics (CSAMT) survey. In this paper, we present structural analysis of clastic sedimentary rocks from around the deposit, their relationship with carbonatite, and the orientations of the carbonatite bodies and dikes to constrain the possible configuration of carbonatite. Clastic sedimentary rocks underwent mylonitic deformation to slate, metasandstone, and metaconglomerate, displaying dramatic changes of thickness along strike. Slates locally preserve intrafolial folds and hook folds bounded by foliations; metasandstones have parallel layers of recrystallized and preferred-orientated quartz aggregations; metaconglomerates contain flattened pebbles with Flinn k values of 0.01 to 0.05 and 0. The above structures and foliations were crosscut and intruded by carbonatite and associated fenite, demonstrating preintrusion transposition of original bedding to steep foliation. Preintrusion foliation provides zones of weakness that were exploited by upwelling of carbonatite magma. Northeast-SW–striking left-stepping en echelon carbonatite dikes and E-W–striking carbonatite bodies indicate that the carbonatite was emplaced in a sinistral transtensional zone. The northern and southern segments of the carbonatite bodies are parallel to the steep foliation at shallow depths and merge together at depth, constraining a Y-shaped configuration, consistent with the low resistivity result of the CSAMT survey. The newly recognized Y-shaped morphology indicates that the carbonatite extends deeper than 1,775.4 m, more than twice the previously inferred maximum depth of the syncline model, and contributes to a significant &amp;gt;1.78 times increase in global potential RE2O3 resources than previously estimated in 2021.

Niobium ore genesis in a capsule

Experiments were conducted to test the hypothesis that carbonatite-hosted Nb deposits owe their origin to the metasomatic alteration of K-feldspar-rich fenite by carbonatitic magma. This involved reacting K-feldspar with a synthetic mixture of CaCO3, MgCO3, Mg(OH)2, CaF2, and Nb2O5. At the experimental conditions, a phlogopite-rich calcite-bearing metasomatic rind containing pyrochlore formed on the K-feldspar. This supports the above hypothesis and also explains the origin of fenite-associated glimmerite, the association of glimmerite with calcite carbonatite, and the genesis of the carbonatite-hosted ores that supply the bulk of the world’s niobium.

Genesis of the Hujiazhuang apatite-rich carbonatite complex in North China and its implication for the REE fertility of the carbonatite system

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

The first finding of a highly evolved Mg-rich carbonatite intrusion in NW Anatolia (Arıklı-Çanakkale)

The carbonatite-alkaline rock suites are quite unique for hosting high grade critical metals including rare earth elements (REE), niobium (Nb), tantalum (Ta), thorium (Th) and uranium (U). Most of these rock suites are generally associated with continental rifting in overthickened Precambrian cratonic environments. In Turkey, despite majority of magmatism is quite young, a few number of carbonatite intrusions were identified in Central Anatolian Crystalline block. However, the scientific literature which focused on the magmatic activities of NW Anatolia with its numerous articles involving magmatic intrusions and their extrusive counterparts lacked genuine magmatic carbonatite data until the present day. The Arıklı carbonatite intrusion is the first magmatic carbonatite identified in NW Anatolia, and also interesting for its unusual mineralogical and geochemical features. The carbonatite body is generally composed of multiple phases of calcite and apatite mineralizations. Carbonatitic rocks of Arıklı show a highly evolved nature with considerable Mg and Ca contents accompanied by Th (up to 0.36 wt%), U (up to 373.6 ppm) and heavy rare earth elements (HREE) (up to 194.7 ppm). The δ13C (PDB) and δ18O (SMOW) values show that fractional crystalization and hydrothermal activities effected the composition of initial carbonatitic melt. The metasomatic halo of Arıklı carbonatites are enriched in terms of K (up to 12 wt%), P (up to 9 wt%), Th (248 ppm), and U (up to 0.27 wt%). However, the LREE values of both carbonatites and metasomatic rocks are pretty low compared to other carbonatite-fenite complexes. The effects of magmatic and post-magmatic events (partial melting, fractional crystalization, liquid immiscibility, and fenitization) played an important role in the formation of Arıklı carbonatite intrusion.

Giant Rare Earth Element Accumulation Related to Voluminous, Highly Evolved Carbonatite: A Microanalytical Study of Carbonate Minerals From the Bayan Obo Deposit, China

Abstract The giant Bayan Obo deposit in China represents the largest rare earth element (REE) resource in the world, but the mechanisms for its highly anomalous REE enrichment have long been controversial. The central debate concerns the nature and origin of the ore-hosting dolomite. In this study, a texturally constrained microanalytical study of carbonate minerals from the Bayan Obo ore-hosting dolomite is presented to resolve this controversy. The dolomite rocks contain two major components: the early-stage coarse-grained dolomite (CD) and the late-stage fine-grained dolomite (FD). The CD dolomite grains have C-O isotope ratios (δ13CVienna-PeeDee Belemnite (V-PDB): –4.8 to –3.3‰; δ18OVienna-standard mean ocean water (V-SMOW): 7.1 to 11.9‰) plotting in or adjacent to the primary igneous carbonatite field, with a narrow range of low 87Sr/86Sr ratios (0.70262–0.70327). The mantle-like C-O and Sr isotopes indicate that the coarse-grained dolomite rocks are magmatic in origin. Dolomite grains from the FD have experienced extensive hydrothermal alteration related to both REE mineralization and post-ore metamorphism of the Bayan Obo deposit. The domains of primary unaltered dolomite have high SrO and MnO contents that clearly distinguish them from sedimentary carbonates. The 87Sr/86Sr ratios of unaltered dolomite domains range from 0.70271 to 0.70473, with the majority lower than 0.7035, contrasting with higher 87Sr/86Sr ratios of Mesoproterozoic sedimentary carbonates globally. Thus, the primary unaltered fine-grained dolomite, the precursor to the FD, is also proposed to be a carbonatite. Compared with dolomite grains from the CD, most of the unaltered dolomite domains within the FD are more enriched in FeO and MnO and have higher δ13CV-PDB (-4.9 to 0.3 ‰) and δ18OV-SMOW (9.4 to 17.1 ‰) values. In addition, the FD contains abundant REE- and volatile-rich hydrothermal minerals and Fe-Mg carbonates, which are rare in the CD. The geochemical and mineralogical data in conjunction indicate that the melts forming the late-stage FD were much more evolved than those forming the early-stage CD. It is noteworthy that the unaltered dolomite domains within the FD have a wider range of 87Sr/86Sr ratios than those within the CD, which implies that the CD and FD, at least a proportion of them, are unlikely to have crystallized from the same progenitor magmas. Some FD was possibly the product of fractionation of less-evolved carbonatitic magma that generated the CD, whereas other FD crystallized from new pulses of magmas that were highly fractionated at depth. When compared with other carbonatite complexes, the Bayan Obo carbonatite suite is notable for having a large surface area (~48 km2 in outcrop) and containing an anomalously large proportion of highly evolved components. The voluminous evolved carbonatite clearly provided a basis for the accumulation of significant ore metals. Therefore, the giant-sized REE deposit is proposed to be associated with large-volume, highly evolved carbonatite at Bayan Obo.

Three-stage niobium mineralization at Bayan Obo, China.

The Chinese Bayan Obo deposit is a world-class rare earth element (REE) deposit with considerable niobium (Nb) and iron (Fe) resources. A complete genetic understanding on all metals is fundamental for establishing genetic models at Bayan Obo. With extensive research being focused on REE enrichment, the timing and controls of Nb enrichment remain unresolved at Bayan Obo, which is mainly due to the challenges in dating, i.e. multistage thermal events, fine-grained minerals with complex textures and the rare occurrence of uranium-enriched minerals with mature dating methods. Based on robust geological and petrographic frameworks, here we conducted ion probe uranium-lead (U-Pb) dating of ferrocolumbite to unravel the timing, hence the genesis of Nb mineralization. Three types of hydrothermal ferrocolumbites-key Nb-bearing minerals-are identified based on their textures and mineral assemblages. They yield U-Pb ages of 1312±47 Ma (n=99), 438±7 Ma (n=93), and 268±5 Ma (n=19), respectively. In line with deposit geology, we tentatively link the first, second and third stage Nb mineralization to Mesoproterozoic carbonatite magmatism, ubiquitous early Paleozoic hydrothermal activity, and Permian granitic magmatism, respectively. While quantifying the contribution of metal endowment from each stage requires further investigation, our new dates highlight that multi-stage mineralization is critical for Nb enrichment at Bayan Obo, which may also have implications for the enrichment mechanism of Nb in REE deposits in general.

Detail mineralogical study and geochronological framework of Bayan Obo (China) Nb mineralization recorded by in situ U-Pb dating of columbite

The Bayan Obo deposit is world-famous for its giant rare earth reserves, and also accompanied with 2.2 Mt Nb2O5 resources at an average grade of 0.13 %, renowned as the second largest Nb deposit globally. Previous studies have focused on the origin of carbonatite (igneous/sedimentary), and variation composition during evolution, as well as its super-enrichment of light rare earth elements (LREE), while the metallogenic mechanism of niobium is being overlooked despite the identification of plenty Nb-bearing minerals. In this study, we identify four types of columbite as major Nb-bearing minerals, instead of pyrochlore in the west pit of Bayan Obo. Detailed mineralogical study has revealed large primitive columbite (Clb1-I, 100–200 μm) is hosted in the coarse-grained Fe-dolomite carbonatite (FeO = 5.46–10.47 wt%), being an ideal target for future mineral beneficiation. Those columbites (Clb1-I) are featured by lowest Sr, Y, and ∑REE but enriched in iron in composition, revealing its magmatic origin. Sieve-textured columbite (Clb1-II, ∼100 μm) dispersed in the fluorination and chloritization dolomite carbonatite, with a U–Pb age of 1359 ± 31 Ma, contemporary with ages of the regional carbonatite magma intrusion and rift-related magmatism, indicating niobium being sourced from mantle-derived carbonatite. The occurrence of fluorite as inclusions in Clb1-I indicates the solubility of Nb was enhanced by fluxes, and the columbite saturation was easily achieved during the carbonatite magma evolution into ferroan carbonatite. Strong fenitization metasomatism further locally concentrated columbite either within the mineral assemblages of pyrite + phlogopite + allanite (Clb2) or in the mica-dominated fenite (Clb3), and crystallized at 346 ± 18 Ma and 284 ± 16 Ma, respectively. The latter metalogenetic epoch is accordant with late Permian granitic magmatism event (ca. 280 Ma) while the former age may attribute to its occurrences related to the early Paleozoic arc-continent collision. Differ from pristine columbite (Clb1-I and Clb1-II), the metamorphic columbite (Clb2 and Cl3, especially Clb3) are enriched in Ta (1803–4092 ppm), Y (560–2029 ppm) and ∑REE (720–6317 ppm) content, probably suggesting the highest degree of evolution. The similar metallogenesis framework between Nb and REE, combined with the coexistence of columbite and REE minerals assemblages, implies the spatial and temporal coupling of Nb and REE mineralization. This first innovative and systematic mineralogical and geochronological work directly constrains the nature of Mesoproterozoic mantle-derived carbonatite with fertile Fe and fluxes, and the Paleozoic and Permian fenitization metasomatism, are the major factors contribute to the metallogenic specialization of niobium in Bayan Obo.

Mafic Alkaline Magmatism and Rare Earth Element Mineralization in the Mojave Desert, California: The Bobcat Hills Connection to Mountain Pass

AbstractOccurrences of alkaline and carbonatite rocks with high concentrations of rare earth elements (REE) are a defining feature of Precambrian geology in the Mojave Desert of southeastern California. The most economically important occurrence is the carbonatite stock at Mountain Pass, which constitutes the largest REE deposit in the United States. A central scientific goal is to understand the genesis of the carbonatite ore body in the context of widespread REE‐rich igneous activity. A swarm of mafic alkaline (shonkinite) dikes has been mapped and sampled at Bobcat Hills, 65 km southeast of the Mountain Pass mine. Whole‐rock geochemistry and zircon geochronology demonstrate a clear affinity to the ca. 1.4 Ga Mountain Pass intrusive system. Bobcat Hills dikes have comparably high REE concentrations (La ∼1,000× chondritic) and an error‐weighted mean 207Pb/206Pb zircon crystallization age of 1,426 ± 2 Ma (2σ). Unlike the alkaline intrusions at Mountain Pass, which have abundant inherited zircon from Paleoproterozoic basement rocks and crustally influenced oxygen isotope compositions (δ18Ozircon = 6.5–7.5‰), the Bobcat Hills dikes lack any evidence of crustal assimilation and have oxygen isotope values that overlap a mantle range (Bobcat Hills average δ18Ozircon = 5.6 ± 0.3‰). The dikes were a high‐temperature, early center of mafic alkaline magmatism in the Mojave Desert that serve as a snapshot of melt generation from a spatially extensive, metasomatized mantle source. We propose that modification of the crust over many tens of Myr at Mountain Pass created an environment that favored crustal assimilation and enabled ascent of late‐stage, REE‐rich carbonatite magmas.

Main minerals of the Araxá alkali-carbonatite complex, Minas Gerais State, Brazil

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

Origin of carbonatite-related niobium deposits: Insights from pyrochlore geochemistry

The carbonatite-related Nb deposits of the Alto Paranaíba Igneous Province (APIP) in central Brazil, currently account for ∼92 % of the global Nb production. In the APIP, pyrochlore is abundant in magnetite–apatite–tetraferriphlogopite ± carbonate rocks or phoscorites, occurring as interbedded layers with carbonatites in the lower hypogene zone, feeding dike swarms of phoscorite and calcite carbonatite, and late-stage carbothermal veins in the upper hypogene zone. The origin of the phoscorite-carbonatite association can be explained by three hypotheses: (1) crystal segregation from fractional crystallization, (2) liquid immiscibility, and/or (3) phoscoritic magma formation after basement metasomatism (fenitization). However, it is not well understood whether pyrochlore formation is limited to a carbonatitic event, carbohydrothermal, or both, and this gap of knowledge is addressed in this work. To investigate the petrogenesis of pyrochlore-rich phoscorite, cathodoluminescence (CL) images, chemical maps, and LA-ICP-MS data were acquired of pyrochlore crystals from magmatic and carbothermal rocks from the Boa Vista Nb mine, Catalão II Complex. In the Boa Vista mine, oscillatory and patchy zoning were identified as primary pyrochlore textures commonly recorded at the lower hypogene zone, while secondary dissolution, skeletal and zonation-free textures are registered at shallower depths in the upper hypogene zone. Calciopyrochlore is the dominant Nb phase at the Boa Vista mine, with only two kenopyrochlore outliers. The pyrochlore CI chondrite-normalized REE distribution is consistent with geochemical results of the carbonatite and phoscorite rocks, indicating a magmatic origin for pyrochlore and the presence of pyrochlore antecrysts in carbothermal veins. The Sr/Y vs La and Na vs Ce diagrams in pyrochlore indicate a continuous fractionation pattern, with some mixtures of antecrysts and primary phases. An examination of intercumulus calcite using CL provide evidence of carbonatitic magma residues within tetraferriphlogopite phoscorite dikes and suggests that alkaline–carbonate-rich fluids played a role in transporting heavy minerals (i.e., magnetite, apatite, pyrochlore). Consequently, the textural and chemical evidence in the Boa Vista Nb mine indicates that the origin of pyrochlore-rich phoscorites is the result of physical segregation of heavy minerals from a carbonatite magma by fractional crystallization, leading to the emplacement of pyrochlore-rich carbonatite and phoscorite dikes. The implications at Catalão II may extend to other APIP alkaline-carbonatite complexes, as they share a genetic connection, and should motivate further studies focusing on pyrochlore geochemistry in other carbonatite-related Nb deposits, which will be crucial for advancing our knowledge of global Nb metallogenesis.

Petrology and source characteristics of the Arbarastakh alkaline ultramafic carbonatite-phoscorite complex, the Aldan-Stanovoy Shield

Late Neoproterozoic Arbarastakh alkaline ultramafic carbonatite-phoscorite complex in the southern margin of the Siberian craton (Aldan-Stanovoy shield), includes carbonatites and phoscorites closely associated with pyroxenites-ijolites and ultramafic lamprophyres. Major and trace elements data, Sr, Nd and Pb isotope compositions for the Arbarastakh rocks have been obtained to characterize the sources involved in their formation, primary melt composition and to build the petrogenetic model.All rock varieties, excluding nepheline syenites, are characterized by incompatible elements enrichments, including light rare earth elements, and strong fractionation of REEs. The initial isotope ratios of the analyzed samples, calculated at 645 Ma, display limited variations: εNd from +5.9 to +6.9 and 87Sr/86Sr from 0.70225 to 0.70272, excluding nepheline syenite with εNd +5.4. The initial Pb isotope ratios for the most studied samples overlap with each other within uncertainties. They yield an age of 642 ± 5 Ma. The Nd and Sr isotope data of the Arbarastakh rocks generally fit the patterns of the other Neoproterozoic alkaline ultramafic carbonatite complexes of the southwestern and southern margins of the Siberian craton. Observed Sr, Nd and Pb isotope variations indicate mixing of the asthenosphere and the depleted mantle components.It is supposed that the primary melts for the Arbarastakh rocks were generated directly by low-degree melting of metasomatic phlogopite‑carbonate veins with apatite and Ti-oxides in garnet peridotite, which formed shortly before the onset of melting. Ultramafic lamprophyre (aillikite) is closest to the primary melt composition in terms of high #MgO, Cr and Ni. The aillikite and latter pyroxenite crystallized from primitive melt by fractionating the olivine, phlogopite and clinopyroxene dominated mineral assemblage that was free of feldspar. This fractionation forced Na-enrichment in the magmas resulted in liquid silicate‑carbonate immiscibility. Major and trace element and isotope data indicate that the nepheline syenites are unlikely to be related with other alkaline silicate rocks through fractional crystallization and could have been formed through pre-emplacement interaction with the ambient crustal materials. Emplacement of calcite carbonatites resulted in metasomatism of surrounding pyroxenites with formation of clinopyroxene-phlogopite-calcite carbonatites. Subsequent portions of Fe-P-enriched carbonatite magmas crystallized phoscorites and apatite-dolomitic carbonatites.

Genesis of Carbonatite at Oldoinyo Lengai (Tanzania) from Olivine Nephelinite: Protracted Melt Evolution and Reactive Porous Flow in Deep Crustal Mushes

Abstract Carbonatites, carbon-rich magmatic rocks, are thought to form by low-degree partial melting of a relatively carbon-poor mantle followed by protracted differentiation and immiscibility. However, the nature of parental magmas and the characteristics of the early stages of differentiation that shape the subsequent crystal and liquid lines of descent remain poorly constrained. To provide new constraints, deep crustal cumulative xenoliths from Oldoinyo Lengai (East African Rift), the only active volcano erupting carbonatite magmas, were studied. We use major and volatile elements in primitive olivine-hosted melt inclusions, as well as major and trace elements in crystals, to reconstruct the conditions of formation and evolution of cumulates (pressure, temperature, composition). Xenoliths are composed of olivine, diopside, phlogopite, amphibole and accessory minerals. One remarkable feature is the presence of diopside and phlogopite oikocrysts enclosing roundish olivine chadacrysts. Melt inclusions do not have vapor bubble and have major element compositions resembling olivine nephelinite (7–10 wt % MgO after corrections for post-entrapment crystallization). The absence of vapor bubbles implies that the concentrations of volatile components (i.e. CO2, H2O, S) were not compromised by well-known post-entrapment volatile loss into the vapor bubble. Based on the melt inclusion study by SIMS, the volatile concentrations in olivine nephelinite magmas (early stage of differentiation) at Oldoinyo Lengai were 20–130 ppm S, 390–4500 ppm F, 50–540 ppm Cl, up to 6074 ppm CO2 and up to 1.5 wt % H2O. According to the calculated CO2-H2O saturation pressures and geophysical data, xenoliths from Embalulu Oltatwa document a mushy reservoir in the lower crust. Primitive olivine nephelinite melt inclusions have higher H2O contents than olivine nephelinite lavas from other further South volcanoes from the North Tanzanian Divergence (0.2–0.5 wt % H2O), suggesting that the lithospheric mantle source beneath the Oldoinyo Lengai is more hydrated than the mantle beneath the rest of North Tanzanian Divergence. We present a model in which resorption features observed in olivine chadacrysts, together with the LREE enrichments in olivine grains, are the consequences of reactive porous flows in a deep crustal mushy reservoir. We provide constraints on the major, trace and volatile element composition of the parental magmas of carbonatite series and demonstrate with Rhyolite-MELTS models that phonolites and related natrocarbonatites from Oldoinyo Lengai can be produced by protracted differentiation of olivine nephelinite melts.

Mantle Metasomatism and REE Enrichment in the Genetic Source of the Dalucao Carbonatite Complex (Sichuan, China): Insights from Elemental Geochemistry and In-Situ Sr Isotopes of Two Calcite Types

Abstract Carbonatites possess the highest rare earth element (REE) concentrations among all magmatic rocks, yet the mechanisms governing the enrichment of REEs in carbonatites remain enigmatic. Carbonatite-hosted calcites provide crucial mineralogical and geochemical insights into addressing this matter. In this study, we present a dataset comprising major and trace elemental geochemistry, as well as in-situ Sr isotopic data, for two types of calcites from the Dalucao carbonatite complex in Sichuan Province, China. Our primary objective is to shed light on the origin of fertile carbonatites. These calcites are believed to have originated at different stages of carbonatitic evolution. Type I calcites, characterized by a euhedral granular texture and a homogeneous surface, crystallized at the early stage of carbonatite magmatism. In contrast, type II calcites, closely associated with bastnäsite in a paragenesis, formed within a fluid setting of hydrothermal overprinting. Both calcite types exhibit highly radiogenic (87Sr/86Sr)i ratios, ranging from 0.7059 to 0.7060 for type I calcites and 0.7059 to 0.7068 for type II calcites. The high Sr concentrations (3646–7315 µg/g for type I calcites and 6566–13,427 µg/g for type II calcites) and uniform Pb isotopic compositions (206Pb/204Pb ratios of 18.23–18.27) in the Dalucao calcites refute the hypothesis of crustal assimilation as their genetic source. Instead, the radiogenic Sr isotopic compositions suggest that subducted marine sediments have contaminated the lithospheric mantle that generated the carbonatitic magma. Mineralogical evidence indicating the presence of abundant silicate minerals in the Dalucao carbonatites, combined with a compilation of published C-O isotopic data, suggests that these carbonatites may have resulted from low-degree partial melting (melting proportion &amp;lt;1%) of a metasomatized lithospheric mantle. Finally, we propose a comprehensive model for REE enrichment in the Dalucao carbonatite complex and explore the significance of tectonism in the genesis of fertile carbonatites.

Study on the complex magmatic and hydrothermal metallogenic processes in the giant Huayangchuan uranium (U)-polymetallic ore district: Constraints from geochemical and geochronological evidences

The supergiant Huayangchuan uranium (U)-polymetallic deposit is situated in the west part of Lesser Qinling Orogen, Central China, which is renowned for its unique metallogenic type and complex mineralization processes. This newly-discovered deposit contains economic endowments of U, Nb, Pb, Se, Sr, Ba and REEs. The genesis of this deposit remains hugely controversial and the related magmatic and metallogenic framework have not been well established. In our study, we present newly obtained zircon U-Pb age data, whole-rock trace elements, detailed SEM observation, carbon and oxygen isotope determination and titanite U-Pb age result to provide geochemical and geochronological constraints on the magmatic and hydrothermal metallogenic processes in Huayangchuan ore district and give insight into the REE, Nb and U mineralization and remobilization mechanism. Carbonatite in Huayangchuan deposit is rich in Ba, Sr, Nb, U and REE and the δ18OSMOW and δ13CPDB (‰) values of carbonatite vary between + 7.30 ‰ to + 9.80 ‰ and − 6.00 ‰ to − 7.20 ‰, respectively, which is the unique characteristic of igneous carbonatite. The total REE contents of carbonatite range from 1319.5 to 2535.8 ppm, significantly higher than that of the granite pegmatite dykes and Taihua Group granitic gneiss. Two zircon grains from the Huayangchuan carbonatite yield the 206 Pb / 238 U age of 223 ± 5.6 Ma and 217 ± 8.6 Ma. Uranium and niobium are mainly hosted in the primary mineral of betafite, minorly in uraninite and fergusonite. REE are remained in various REE carbonates, phosphates and silicates, e.g. allanite, parite, monazite, apatite, yttrialite and xenotime. Betafite contains numerous mineral inclusions of calcite, allanite and apatite, which displays as vermicular relicts with metasomatic corrosion texture. The intimate relationship between betafite, calcite, apatite, allanite and REE carbonates implies that U and Nb mineralization occurred during carbonatite-related magmatic stage. Late Triassic carbonatite magmatism (∼220 Ma) was responsible for the world-class U-Nb-Mo-REE mineralization in the Qinling Orogen. Whereas, LA-ICP-MS U-Pb data of titanite collected from Huayangchuan carbonatite reveal two sets of ages in the Huayangchuan deposit: during the Late Triassic and the Early Cretaceous. LA-ICP-MS data of the Triassic titanite and Cretaceous titanite in Huayangchuan deposit yield the lower intercept age of 223.1 ± 9.4 Ma and 141.9 ± 4.7 Ma respectively in the Tera-Wasserburg (238U/206Pb - 207Pb/206Pb) diagram. Cretaceous hydrothermal titanites show enrichment in Y, HREEs and depletion in TiO2 and LREEs relative to the Triassic magmatic titanites. Primary betafite and titanite were subjected to later hydrothermal alteration. The close genetic relation between altered titanite and Ti-Fe-Nb-Pb-U mineral inclusions demonstrates that Early Cretaceous hydrothermal fluids contributed to the Nb, U and Ti remobilization. Early Cretaceous regional granitic magmatic activity brought hydrothermal fluids to alter the early carbonatite magmatic pyrochlore and titanite. U, Nb, Ti, Y, Pb and HREEs were transferred and subsequently re-precipitated into uraninite, and hydrothermal titanite.