Origin Of Carbonatites Research Articles

Mesoproterozoic and Paleozoic hydrothermal metasomatism in the giant Bayan Obo REE-Nb-Fe deposit: Constrains from trace elements and Sr-Nd isotope of fluorite and preliminary thermodynamic calculation

Fluorite of various occurrences could portray the hydrothermal metasomatism occurred in the giant Bayan Obo REE-Nb-Fe ore deposit. Two Mesoproterozoic and Paleozoic hydrothermal events with distinguished fluid origin and geochemical properties have been studied. In Mesoproterozoic, discrete fluorite from the dolomite-dominating REE-Nb ores and banded fluorite aggregates from the banded REE-Nb-Fe ores are extremely enriched in LREE and depleted in HFSE compared with the primitive mantle. In Paleozoic, fluorite from veined-breccia REE-Nb ores varies significantly in trace element components, ranging from LREE-enriched HFSE-depleted fluorite to fluorite with “flat” REE pattern and extremely depletion of Zr. The veined fluorites have variable and higher radiogenic Sr isotopic components (initial 87Sr/86Sr ratios: 0.703464–0.708208) than the discrete and banded fluorite (0.703183–0.705047), indicating crustal contamination in the Paleozoic hydrothermal fluids from Precambrian basement rocks and Bayan Obo Group. Fluorites of different occurrences and associated bastnäsite and monazite have similar Nd isotopic components and Nd evolution history, which is mostly consistent with whole rock of the ore-hosting dolomite. The Sm-Nd system of some fluorite and bastnäsite were reset during the Paleozoic alteration triggered by the closure of Paleo-Asian Ocean. In such geological setting, according to the δ34S of the dominating sulfur-bearing mineral, Paleozoic veined sulfides (∼0 ± 3‰), the prime sulfur source was oceanic crust/MORB. Relatively high δ34S of barite (∼13–14‰) was caused by fractionation of sulfur isotopes at an equilibrium temperature of 400–430 °C, rather than contribution from ancient seawater. The trace element, Sr-Nd isotope of fluorite and sulfur isotope characters of sulfides indicate carbonatitic origin of the Mesoproterozoic hydrothermal fluids, while the Paleozoic fluids, derived from subducted Paleo-Asian oceanic crust, carried sulfur components from MORB, scavenged high radiogenic Sr components from Precambrian basement rocks or Bayan Obo group, may leached a deep-seated carbonatite stock and finally altered the Mesoproterozoic mineralized ore-hosting dolomite.Both the Mesoproterozoic and Paleozoic hydrothermal fluids had initial temperature of ∼400–500 °C, and gangue minerals (fluorite, sulfide and sulfate) precipitated at the temperature of ∼320–440 °C. Preliminary thermodynamic calculations of the REE-F-C-Ca system constrained that the pressure of both fluids would not exceed 2 kbar. The drop of temperature and increase of pH are two critical factors to REE mineralization. Paragenesis of Fe-bearing minerals indicates an increase of fO2 in the Mesoproterozoic hydrothermal fluids and a drop of fS2 in the Paleozoic fluids in the Fe-O-S system. Furthermore, in the Ca-F-C-O system, the Mesoproterozoic hydrothermal fluids evolved with increasing aCa2+ and decreasing aF−, while the Paleozoic fluids present characteristic of mixing origin again with significantly variable aCa2+ and aF−.

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.

Recycling of subducted carbonates: Formation of the Taohuala Mountain carbonatite, North China Craton

Carbonatitic magmatism plays a significant role in Earth's carbon cycle, which is also a lithoprobe of crust-mantle interaction, mantle metasomatism and partial melting. Due to different mineral assemblages and geochemical compositions, and diverse tectonic settings, the origin of carbonatite has long been debated. At subduction zones, sediments (including carbonates) are subducted into the mantle with the downgoing oceanic slab. However, the detailed mechanism of how subducted carbonates contribute to carbonatitic magmatism remains unclear. Here we present geochronological, geochemical and isotopic study on the Taohuala Mountain carbonatite at the southern margin of the Alxa Block, North China Craton. The classification of carbonatite from the Taohuala Mountain relies strongly on the observations of obvious intrusion contact relationships and flow structures in field outcrop. The Taohuala Mountain carbonatite has SiO2 ranging from 2.37wt.% to 11.45wt%, high CaO (45.93–53.86wt%) and low MgO (0.51–4.39wt%), and is characterized by enrichment of LILE (Ba, Sr), depletion of HFSE (Nb, Ta, Zr, Hf), and slightly negative Ce and Eu anomalies. Carbonates in the samples have high 87Sr/86Sr (0.70686–0.70694) and low 143Nd/144Nd (0.511635–0.511924). Remarkably, the highly fractionated δ18OVSMOW (11.83–25.92‰) indicates components of both sedimentary and mantle origin. Detailed zircon in situ U-Pb dating and oxygen isotope analysis exhibit contrast ages and δ18OVSMOW from core to rim, i.e., old ages (mainly>800Ma), high Th/U (mainly>0.5) and low δ18OVSMOW (6.37–11.44‰) in cores (inherited), whereas young ages (~400Ma), low Th/U (mainly<0.01) and high δ18OVSMOW (20.04–24.54‰) in rims, suggesting that the Taohuala Mountain carbonatite may have been generated from melting of subducted sedimentary carbonates. Considering all these evidences, and that the collision along Qilian Mountains was older than the carbonatite, we propose that a large volume of sedimentary carbonates subducted and remained in the lithospheric mantle under the Alxa block during the closure of the Paleo-Qilian Ocean. Subsequently, the carbonatite was formed by melting of carbonates with minor contributions from the mantle during the breakoff or rollback of the Paleo-Asian oceanic slab.

REE mineralization in the Bayan Obo deposit, China: Evidence from mineral paragenesis

Preliminary mineralogical and geochemical studies have been carried out on dolomite marble drill cores from the Bayan Obo REE deposit in China. Three types of apatites and four types of monazites have been identified based on textural features: Type 1 apatite occurs as grains with minor monazite (Type 1 monazite) on its border; Type 2 apatite veinlet shows clusters of assemblages with abundant bastnäsite and parisite at the rim; Type 3 apatite has a linear array associated with fluorite and bastnäsite veinlets. Type 2 monazite occurs as clusters intergrowing with parisite and fluorite. Type 3 and 4 monazites occur as polymineralic (fluorite and bastnäsite) and monomineralic veinlets, respectively. These four types of monazites have similar LREE composition but variable Y content (Y2O3 ranging from below determination limits to 0.7 wt%). The three types of apatites also show different REE content and distribution patterns, ranging from high REE abundance (∑REE + Y: 27243–251789 ppm) and strong LREE enrichment [(La/Yb)CN ∼101] in Type 1, less LREE enrichment [(La/Yb)CN ∼8] in Type 2 to relatively low REE abundance (∑REE + Y: 4323–11175 ppm) but high REE fractionation [(La/Yb)CN ∼58] in Type 3. The primary apatite has high Sr (5461–6892 ppm) and REE content, implying a carbonatite origin. The late-stage apatites (Types 2 and 3) show different Sr and REE abundances. Significant differences in their Sr composition (6189 ± 573, 6041 ± 549 and 3492 ± 802 for Types 1–3 samples, respectively) and Y/Ho ratio (20.9 ± 0.11, 19.5 ± 0.17 and 17.4 ± 0.37, respectively) indicate that the three types of apatites may have crystallized from different metasomatic fluids. Multi-stage metasomatism resulted in remobilization and redeposition of primary REE minerals to form the Bayan Obo REE deposit.

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.

Trace-element composition of minerals and rocks in the Belaya Zima carbonatite complex (Russia): Implications for the mechanisms of magma evolution and carbonatite formation

The nature of petrogenetic links between carbonatites and associated silicate rocks is a matter of discussion for several decades and still remains controversial among igneous petrologists. The Belaya Zima plutonic complex in southern Siberia is a typical intrusion, in which carbonatites are spatially and temporally associated with the ijolite series rocks and nepheline syenites. In this study we use whole-rock compositions and trace element characteristics of the major and accessory minerals for reconstructing the magmatic evolution of the complex and clarifying the origin of carbonatites. We conclude that the observed gradual increase of Zr, Nb and REE concentrations in the magma is consistent with the process of extensive fractional crystallization and gradual transition from silicate rocks to carbonatites, and inconsistent with the formation of carbonatites by liquid immiscibility. Magma differentiation by fractional crystallization continued during the carbonatite formation. The textural and analytical evidence indicates that the early calcitic carbonatites evolved to dolomitic and ankeritic carbonatites. In addition, maximum Nb and Zr concentrations occur in the calcitic carbonatite, whereas the REE content increases in the dolomitic and ankeritic facies.

Decoupling of Mg–C and Sr–Nd–O isotopes traces the role of recycled carbon in magnesiocarbonatites from the Tarim Large Igneous Province

The Tarim Large Igneous Province in NW China hosts numerous magmatic carbonatite dikes along its northern margin. The carbonatites are composed mainly of dolomite (90vol.%) and minor calcite (5vol.%), with apatite, barite, celestine, aegirine, monazite and bastnaesite as accessory minerals. The rocks correspond to magnesiocarbonatites with a compositional range of 13.73–19.59wt.% MgO, and 20.03–30.11wt.% CaO, along with 1.65–3.31wt.% total Fe2O3, 0.02–2.39wt.% SiO2 and other minor elements, such as P2O5, Na2O and K2O. These magnesiocarbonatites are characterized by extreme enrichment in incompatible elements with high total rare earth element (REE) contents of 372–36965ppm. The strontium [(87Sr/86Sr)i=0.70378–0.70386], neodymium [εNd(t)=+2.51–+3.59] and oxygen (δ18OV-SMOW=5.9‰–8.0‰) isotope values of these rocks are consistent with a mantle origin, whereas the magnesium (δ26Mg=−1.09‰ to −0.85‰) and carbon (δ13CV-PDB=−4.1‰ to −5.9‰) isotopes are decoupled from mantle values and reflect signature of recycled sedimentary carbonates. Global plate tectonic models predict that sedimentary carbonates in convergent margins are subducted to deep domains in the mantle, with phase transitions from calcite/dolomite to magnesite, and eventually to periclase/perovskite. The involvement of a mantle plume enhances the normal mantle geotherms and promotes decomposition reactions of magnesite. The decoupling of Mg–C and Sr–Nd–O isotopes in the mangesiocarbonatites provides insights on the origin of carbonatites, and also illustrates a case of interaction between mantle plume and subduction-related components.

Recycling of subducted crustal components into carbonatite melts revealed by boron isotopes

The global boron geochemical cycle is closely linked to recycling of geologic material via subduction processes that have occurred over billions of years of Earth’s history. The origin of carbonatites, unique melts derived from carbon-rich and carbonate-rich regions of the upper mantle, has been linked to a variety of mantle-related processes, including subduction and plume–lithosphere interaction. Here we present boron isotope (δ11B) compositions for carbonatites from locations worldwide that span a wide range of emplacement ages (between ∼40 and ∼2,600 Ma). Hence, they provide insight into the temporal evolution of their mantle sources for ∼2.6 billion years of Earth’s history. Boron isotope values are highly variable and range between −8.6‰ and +5.5‰, with all of the young ( −4.0‰), whereas most of the older carbonatite samples record lower B isotope values. Given the δ11B value for asthenospheric mantle of −7 ± 1‰, the B isotope compositions for young carbonatites require the involvement of an enriched (crustal) component. Recycled crustal components may be sampled by carbonatite melts associated with mantle plume activity coincident with major tectonic events, and linked to past episodes of significant subduction associated with supercontinent formation. The origin of carbon-rich magmas is unclear. Boron isotopic analysis of carbonatite magmas that formed over the past 2.6 billion years reveals a link to carbon recycled during tectonic plate subduction.

Comprehensive mineralogical and technological characterisation of the Araxá (SE Brazil) complex REE (Nb-P) ore, and the fate of its processing

The rare earth elements (REE) are essential for a wide range of applications, from strategic assets (e.g. petroleum cracking, magnets for wind turbines) to popular merchandise, as smartphones. Since 2010, when China, the worlds close to exclusive REE supplier, imposed export quotas, several old and new deposits have been evaluated to compensate market shortage, taking advantage of significant price rises. The Araxá rare earth elements prospect boast a large reserve (6.34Mt @ 5.01% REO), as well as phosphate and niobium, in a deeply weathered ore of carbonatitic origin. The mineralogy and the ore properties are unconventional for rare earth elements, and require a detailed mineralogical and technological characterisation as starting point to develop a feasible processing route.Rare earths are predominantly carried by monazite (over 70%), and by a solid solution of the plumbogummite group minerals where the barium-rich term gorceixite predominates, while cerianite and bastnaesite account for less than 1% each. Minerals of the pyrochlore supergroup are the main Nb carriers, but phosphate is also due to monazite and the plumbogummite group minerals, as apatite has barely been detected. Goethite, high-Al hematite and quartz are the main gangue minerals, and goethite is thoroughly intergrown with the other phases. Fine particle size (P50 close to 45μm) and 47.4% of the REE in the −20μm size fraction is another feature typical of this kind of ore. The mineralogical and textural complexity of the ore required a comprehensive technological characterisation in order to evaluate processing options.Based on textural measurements, the concentration of monazite, the concentration of the REE carrying minerals and the reverse removal of quartz, as processing option for this ore, have been simulated.Incomplete liberation of monazite does limit its grade in an ideal concentrate to 80%, and its recovery to 70%. The low monazite recovery must be added to the loss of REE carried by other phases, reducing the overall REE recovery to below 45%. Monazite has also a very limited exposition of the mineral on the particle's surfaces, supposed to impair process efficiency enough to keep experimental results significantly far from the simulated ones.The concentration of the REE-bearing minerals might be efficient from the liberation point of view, and over 90% of the REE carriers can be recovered to a 97% grade concentrate. Due to the low REE grade of predominant gorceixite (3.3%), however, the concentrate's grade of 14% REE is just slightly above the double of the ore's grade. For the REE-bearing minerals taken together, the process efficiency might be hampered by selectivity due to the complex mineralogy.The major gangue minerals, goethite and hematite, are strongly intergrown with the other minerals of the assemblage, to an extent that evaluating reverse processing considering these phases was not feasible. The removal of quartz by reverse processing is quite straightforward, and 95% of the mineral might be removed to a high-grade quartz concentrate of 93%, with loss of REE of only 0.14%. The mass discharge of 8.7%, however, rises the grade of the concentrate only to 7.3% REE.Complex mineralogy and the fine crystals and particles with strong intergrowth that characterise the ore hamper efficient concentration for the Araxá REE ore, and direct hydrometallurgical processing of the whole was adopted. The results are in agreement with the few other published attempts to concentrate the rare earth minerals from residual lateritic ores related to carbonatites.

Origin of carbonatites in the South Qinling orogen: Implications for crustal recycling and timing of collision between the South and North China Blocks

Most studies of compositional heterogeneities in the mantle, related to recycling of crustal sediments or delaminated subcontinental lithosphere, come from oceanic setting basalts. In this work, we present direct geochronological and geochemical evidence for the incorporation of recycled crustal materials in collision-related carbonatites of the South Qinling orogenic belt (SQ), which merges with the Lesser Qinling orogen (LQ) to separate the South and North China Blocks. The SQ carbonatites occur mainly as stock associated with syenites. The data presented here show that zircon from the syenites yields an age of 766±25Ma, which differs significantly from the age of primary monazite from the carbonatites (233.6±1.7Ma). The syenites contain lower initial 87Sr/86Sr and higher εNd values. This indicates that the carbonatites do not have genetically related with the silicate rocks, and were directly derived from a primary carbonate magma generated in the mantle. The carbonatites show a Sr–Nd isotopic signature similar to that of the chondritic uniform reservoir (CHUR), and parallel Sm–Nd model ages (TCHUR) of 190–300Ma. However, the rocks have extremely variable Pb isotopic values straddling between the HIMU and EM1 mantle end-members. Most carbon and oxygen isotopic compositions of the SQ carbonatites plot outside the field for primary igneous carbonates. Their δ13C shows higher value than a ‘normal’ mantle, which implies an incorporation of recycled inorganic carbon. The carbonatites were emplaced close to the Mianlue suture, and followed the closure of the Mianlue ocean and Triassic collision of the South and North China Blocks. However, direct melting of the subducted Mianlue oceanic crust characterized by high εNd and low (EM1-like) 206Pb/204Pb values cannot explain the CHUR-like Nd signature and the Pb isotopic trend toward HIMU in the SQ carbonatites. We conclude that their parental magma was derived from a source incorporating the Mianlue oceanic crust mixed with an asthenospheric (or deeper) material characterized by high Pb and low Nd isotopic values. This material represents a deep-seated Proterozoic carbonate component recycled via mantle convection or localized upwelling. Notably, this model cannot explain the isotopic compositions of the Late Triassic (209–221Ma) carbonatites in the LQ, characterized by a mantle-derived δ13C, but EM1-like Sr–Nd–Pb isotopic compositions. This signature is best explained in terms of delamination of the lower continental crust thickened during the collision of the South and North China Blocks, and partial incorporation of the delaminated material into the LQ mantle source. Modeling of the measured Sr–Nd–Pb isotopic variations suggests that the source of the LQ carbonatites could be produced by mixing of 80–85% of mantle material and 15–20% of delaminated lower continental crust. The emplacement of the SQ and LQ carbonatites marked a gradual transition from a compressional tectonic regime, brought about by the collision of the South and North China Blocks to intra-orogenic extension in the waning stages of the Triassic orogeny.

Carbonatitic melts in olivine and magnetite from rare-metal carbonatite of the Belaya Zima alkaline carbonatite complex (East Sayan, Russia)

The problem of the origin of carbonatite is still a priority in Russian and foreign publications. Attention to these rocks is mostly explained by their raremetal specialization and links of Nb, Ta, Zr, Sr, Ba, and REE deposits to them. In addition to the hypothesis of the magmatic origin of carbonatites, currently some authors consider these rocks as products of hydrother� mal-metasomatic activity. This contradiction may be solved by study of the inclusions of mineralforming media in minerals, which provide direct evidence for natural melts or fluids captured in host minerals. In spite of the active investigation of melt inclu� sions in minerals from rocks of the alkaline carbonatite complexes over the last decade, there are still some data including the quantitative characteristics of the chemical composition of melts for carbonatites, as well as on other associated nonsilicate (fluorite, apa� tite, etc.) rocks (1-6). Only a few publications con� cern carbonatites properly (3, 5, 6). However, estima� tion of the role of melts and their compositions is of great importance for understanding of the origin of alkaline carbonatite complexes including the pro� cesses and conditions of accumulation of commercial minerals in them. In this paper, we report the results of the study of the melt and crystalline inclusions coexisting in min� erals of orebearing forsterite-calcite carbonatite from the Belaya Zima massif (East Sayan), which allowed us to distinguish the carbonatitic melts responsible for the formation of these rocks for the first time. A number of complex (Nb, Ta, REE, U, Pb, Zn, and P) deposits including the largest Belaya Zima nio� bium deposit related to the carbonatite massif of the same name are distinguished within the East Sayan province of ultrabasic alkaline rocks and carbonatites. Based on the data of geochronological U-Pb study of zircons, the age of this deposit is 643 Ma (7). The Belaya Zima (Nizhnesayanskii) massif is located in the northern foothill of East Sayan, in the upper reaches of the Belaya Zima River. The massif occurs in the zone of large faults of northwestern orientation, in the area of the junction of the Siberian Platform and folded complexes of East Sayan. In plan, it has a pearlike shape and spreads to the northwest for 8.5 km includ� ing a branch elongated in the same direction (8). The central part is composed of carbonatites of various types. Ultrabasic alkaline rocks form a semiring zone surrounding the stock along the periphery.

Origin of carbonatites of eastern taimyr deduced from an isotopic and geochemical study of zircons

Injection carbonatites of volcanic-plutonic origin with Au-bearing base-metal sulfide mineralization were established in 2006–2007 along the southern slope of the Byrranga Range in the Russian continental sector of the Arctic. According to thermobarogeochemical data, carbonatites were formed at a temperature not lower than 380–410°C from boiling gas-liquid H2O-CO2-salt fluids under decompression. The REE concentrations and patterns of sedimentary and magmatic carbonates are sharply distinct. Carbonatites are enriched in REE as compared to carbonates from limestone and are characterized by an REE distribution close to MORB. The geochronological and isotopic geochemical study of zircons has shown that carbonatite bodies (lavas?) were formed 238–219 Ma ago (Middle-Late Triassic); zircon crystallized at a high temperature. Detrital zircons from the Lower Carboniferous crinoid limestone are not younger than 594 Ma. The absolute predominance of Precambrian zircons, which are variable in age, indicates that they were captured from the crust, when carbonatitic magma ascended from subcrustal depths.

Carbonatites in China: A review for genesis and mineralization

Carbonatites are commonly related to the accumulation of economically valuable substances such as REE, Cu, and P. The debate over the origin of carbonatites and their relationship to associated silicate rocks has been ongoing for about 45 years. Worldwide, the rocks characteristically display more geochemical enrichments in Ba, Sr and REE than sedimentary carbonate rocks. However, carbonatite’s geochemical features are disputed because of secondary mineral effects. Rock-forming carbonates from carbonatites at Qinling, Panxi region, and Bayan Obo in China show REE distribution patterns ranging from LREE enrichment to flat patterns. They are characterized by a Sr content more than 10 times higher than that of secondary carbonates. The coarse- and fine-grained dolomites from Bayan Obo H8 dolomite marbles also show similar high Sr abundance, indicating that they are of igneous origin. Some carbonates in Chinese carbonatites show REE (especially HREE) contents and distribution patterns similar to those of the whole rocks. These intrusive carbonatites display lower platinum group elements and stronger fractionation between Pt and Ir relative to high-Si extrusive carbonatite. This indicates that most intrusive carbonatites may be carbonate cumulates. Maoniuping and Daluxiang in Panxi region are large REE deposits. Hydrothermal fluorite ore veins occur outside of the carbonatite bodies and are emplaced in wallrock syenite. The fluorite in Maoniuping has Sr and Nd isotopes similar to carbonatite. The Daluxiang fluorite shows Sr and REE compositions different from those in Maoniuping. The difference is reflected by both the carbonatites and rock-forming carbonates, indicating that REE mineralization is related to carbonatites. The cumulate processes of carbonate minerals make fractionated fluids rich in volatiles and LREE as a result of low partition coefficients for REE between carbonate and carbonatite melt and an increase from LREE to HREE. The carbonatite-derived fluid has interacted with wallrock to form REE ore veins. The amount of carbonatite dykes occurring near the Bayan Obo orebodies may support the same mineralization model, i.e. that fluids evolved from the carbonatite dykes reacted with H8 dolomite marble, and thus the different REE and isotope compositions of coarse- and fine-grained dolomite may be related to reaction processes.

Upper-mantle volatile chemistry at Oldoinyo Lengai volcano and the origin of carbonatites

Carbonatite lavas are highly unusual in that they contain almost no SiO(2) and are >50 per cent carbonate minerals. Although carbonatite magmatism has occurred throughout Earth's history, Oldoinyo Lengai, in Tanzania, is the only currently active volcano producing these exotic rocks. Here we show that volcanic gases captured during an eruptive episode at Oldoinyo Lengai are indistinguishable from those emitted along mid-ocean ridges, despite the fact that Oldoinyo Lengai carbonatites occur in a setting far removed from oceanic spreading centres. In contrast to lithophile trace elements, which are highly fractionated by the immiscible phase separation that produces these carbonatites, volatiles (CO(2), He, N(2) and Ar) are little affected by this process. Our results demonstrate that a globally homogenous reservoir exists in the upper mantle and supplies volatiles to both mid-ocean ridges and continental rifts. This argues against an unusually C-rich mantle being responsible for the genesis of Na-rich carbonatite and its nephelinite source magma at Oldoinyo Lengai. Rather, these carbonatites are formed in the shallow crust by immiscibility from silicate magmas (nephelinite), and are stable under eruption conditions as a result of their high Na contents.

The first finding of carbonatite tuffs in Cenozoic basaltic volcano of Southeastern Primorye

The origin of carbonatites and carbonatite-type rocks remains a debatable petrologic problem. Therefore, any information on findings of these rocks is valuable for the better understanding of key mechanisms of their formation. We found carbonatite tuffs among eruption products of the early Neogene Amba Volcano in southwestern Primorye (Fig. 1). The volcanic edifice occupies an area of ~20 km 2 in the middle reaches of the Amba River at a distance of 28 km from Amur Bay. The basalts of the stratovolcano base overlie the Permian carbonate and volcanosedimentary rocks and the Triassic arkose sandstones. The basalts are, in turn, overlapped by the upper Miocene volcanosedimentary rocks of the Ust-Suifun Formation and basalts of the Shufan Plateau. The central vent of the Amba Volcano is filled with explosion breccia and basaltic lavas. Pyroclastic rocks, including layers of carbonatite tuffs, compose the peripheral facies of the volcano. The eruption is characterized by a sequential transition from olivine‐clinopyroxene to spinel‐Ti-augite‐ plagioclase varieties. Spinel and clinopyroxene megacrysts (3‐8 mm across) in basalts contain rounded carbonate inclusions. In addition, the basalt groundmass contains numerous late carbonate segregations in gas cavities. The segregations are especially abundant in the vent facies, indicating predominance of carbon dioxide among volcanic gases. The basalts are characterized by subalkali, high-Ti, high-Mg, and high-Ca composition with low content of silica, high contents of Sr, Ba, Nb, and Zr, and less common Ce, Nd, Sm, and Th (Table 1). According to their HFSE and REE contents, they correspond to withinplate alkali basalts with negative slope of PM-normalized REE pattern. The carbonatite tuff layers were found in the southwestern part of the volcanic edifice on the right wall of the Vtoroi Creek, the right tributary of the Amba River. The welded carbonatite tuffs occur as two layers (2.0‐ 2.5 m thick) at the base and top of the visible section. They are separated by interlayers of tuffaceous basaltic sandstones and gravelstones with a variable carbonate admixture. The tuffs are dark gray massive, severely welded rocks with psammitic and psephitic dimensions of crystal fragments and lithic clasts. The high degree of tuff welding is related to enrichment of the natural carbonate‐silicate cement in calcareous component. Tuffs contain fragments of tree trunks and xenoliths of intrusive and sedimentary rocks from the basement. The geological observations indicate that the bedded volcaniclastic member of carbonatite tuffs was formed as a result of multiple directed volcanic explosions with formation of gas-saturated pyroclastic flows deposited in depressions (often waterlogged) around the volcano. The welded tuffs in the section are supplemented with tufflavas represented by more compact carbonate‐ silicate aggregates (20‐25 cm in size) with large spinel and clinopyroxene phenocrysts. Walls of numerous gas cavities and pores in this rock are coated with pale crystalline carbonate. The carbonatite tuffs are composed of crystal and lithic clasts. The white or light gray lithic clasts (0.2‐ 0.5 cm in diameter) have irregular or rounded (ocellar) shape. They are largely composed of carbonate material with spinel and pyroxene phenocrysts and plagioclase laths (Figs. 2, 3). The lithic clasts have an unusually high content (up to 3‐5 vol %) of rather large (up to 5‐ 12 mm) crystals of aluminous spinel and their fragments. The fassaite clinopyroxene is less abundant (Table 2). They also contain relatively large (up to 0.5 cm) orange (less frequently, transparent white or cherrycolored) euhedral and rhombohedral Fe‐Mg-calcite crystals incorporated into the cement (Fig. 3, Table 2). Crystals of spinel (with faces of dissolution), clinopyroxene, and calcite occur in both the carbonatite clasts

Geochemical and petrological characteristics of Eppawala phosphate deposits, Sri Lanka

The apatite-bearing carbonate rocks at Eppawala, Sri Lanka occur as massive, discontinuous bodies in a Precambrian, high-grade metamorphic terrain, which weather to form economically important phosphate deposits. The ore bodies at Eppawala contain ≤42% P2O5, and citric acid solubility of different components varies from 4 to 6%. The parent rocks are mainly made up of calcite, dolomite and apatite, with lesser amounts of ilmenite, magnetite, pyrite, forsterite, phlogopite, enstatite, magnesite, diopside, tremolite and spinel. Most of minerals show an euhedral habit, with a wide range of crystal sizes (from a few millimetres to several decimetres). The Eppawala rocks are characterised by low silica (≤0.41%), high phosphorous (≤10.58%) and high strontium content (2,960–6,819 ppm). Concentrations of light rare-earth elements in these rocks are comparably higher than those of marbles. The REE fractionation of these rocks is pronounced, and La/Yb ratios vary between 14 and 43. Both apatite and calcite show markedly elevated strontium levels (≤0.6%). The δ 13CPDB and δ 18OSMOW values of the carbonates are in the range of –3.4 to –2.2 and 7.7 to 16.4‰ respectively. The euhedral habit, as well as the presence of major quantities of apatite and considerable amounts of iron-bearing minerals suggest that the ore host rock has genetic links to an igneous source rather than to an intensely metamorphosed limestone. The higher light REE contents of the rocks, compared to marbles, also argue against a metamorphic or sedimentary origin. The Sr/Mn and Ce/La ratios in the apatite are ~40 and ~2 respectively, suggesting that they were formed in a carbonatite magma. The markedly increased REE concentrations in the bulk chemistry of the rocks have been shown to be mainly controlled by the content of phosphate minerals. Compared to most carbonatites, the Eppawala rocks are generally depleted in selected trace elements, particularly Ba, Nb, Th, V, U and Zr. This depletion may be due to either a primary infertility of the parent magma with regard to such trace elements, or it is a result of fractional crystallisation during the rock formation. The stable isotope ratios do not plot within the defined "mantle carbonatite box", but still lie within the broader range of carbonatitic rocks. With these data at hand, it can be readily argued that the mode of occurrence, petrography and geochemistry of the Eppawala apatite-bearing carbonates provide conclusive evidence of their carbonatitic origin.

Intervention of carbonate components in petrogenesis of the pyroxene nephelinites from the Jbel Saghro (Anti-Atlas, Morocco)

The Jbel Saghro alkaline complex was emplaced close to the eastern edge of the Moroccan Anti-Atlas. Within the northern part, two types of nephelinite were recognized [Ibhi and Nachit, 1999 and Ibhi, 2000]. The first type (olivine-rich nephelinite) constitutes the main volcanic mass south of the Bou Gafer granit (fig. 1), where the volcanism had been active at least during 2 Ma, between 9.6 and 7.5 ± 0.1 Ma [Berrahma et al., 1993]. The second group outcrops in the north (Foum El Kouss). It consists of pyroxene nephelinites which are younger (2.9 ± 0.1 Ma) [Berrahma et al., 1993], and bears carbonatitic xenoliths, melteigitic pyroxenites and metasomatised peridotite xenoliths. Geochemically, the pyroxene nephelinite is highly enriched in LILE compared with the first one. The mineralogical and geochemical characteristics may be explained by the incorporation of carbonatitic and melteigitic pyroxenite segregates of carbonatitic affinity.

Melt inclusion record of immiscibility between silicate, hydrosaline, and carbonate melts: Applications to skarn genesis at Mount Vesuvius

Research Article| November 01, 2001 Melt inclusion record of immiscibility between silicate, hydrosaline, and carbonate melts: Applications to skarn genesis at Mount Vesuvius Paolo Fulignati; Paolo Fulignati 1Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Search for other works by this author on: GSW Google Scholar Vadim S. Kamenetsky; Vadim S. Kamenetsky 2School of Earth Sciences and Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001, Australia Search for other works by this author on: GSW Google Scholar Paola Marianelli; Paola Marianelli 3Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Search for other works by this author on: GSW Google Scholar Alessandro Sbrana; Alessandro Sbrana 3Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Search for other works by this author on: GSW Google Scholar Terrence P. Mernagh Terrence P. Mernagh 4Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia Search for other works by this author on: GSW Google Scholar Author and Article Information Paolo Fulignati 1Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Vadim S. Kamenetsky 2School of Earth Sciences and Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, Tasmania 7001, Australia Paola Marianelli 3Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Alessandro Sbrana 3Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria 53, 56126 Pisa, Italy Terrence P. Mernagh 4Australian Geological Survey Organisation, GPO Box 378, Canberra, ACT 2601, Australia Publisher: Geological Society of America Received: 07 Mar 2001 Revision Received: 28 Jun 2001 Accepted: 20 Jul 2001 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2001) 29 (11): 1043–1046. https://doi.org/10.1130/0091-7613(2001)029<1043:MIROIB>2.0.CO;2 Article history Received: 07 Mar 2001 Revision Received: 28 Jun 2001 Accepted: 20 Jul 2001 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Paolo Fulignati, Vadim S. Kamenetsky, Paola Marianelli, Alessandro Sbrana, Terrence P. Mernagh; Melt inclusion record of immiscibility between silicate, hydrosaline, and carbonate melts: Applications to skarn genesis at Mount Vesuvius. Geology 2001;; 29 (11): 1043–1046. doi: https://doi.org/10.1130/0091-7613(2001)029<1043:MIROIB>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Foid-bearing syenites and endoskarn xenoliths of the A.D. 472 Vesuvius eruption represent the magma chamber–carbonate wall-rock interface. Melt inclusions hosted in crystals from these rocks offer a rare opportunity to depict the formation and the composition of metasomatic skarn-forming fluids at the peripheral part of a growing K-alkaline magma chamber disrupted by an explosive eruption. Four principal types of melt inclusions represent highly differentiated phonolite (type 1), hydrosaline melt (type 3), unmixed silicate– salt melts (type 2), and a complex chloride-carbonate melt with minor sulfates (type 4). The high-temperature (700–800 °C) magmatic-derived hydrosaline melt is considered to be the main metasomatic agent for the skarn-forming reactions. The interaction between this melt (fluid) and carbonate wall rocks produces a Na-K-Ca carbonate-chloride melt that shows immiscibility between carbonate and chloride constituents at ∼700 °C in 1 atm experiments. This unmixing can be viewed as a possible mechanism for the origin of carbonatites associated with intrusion-related skarn systems. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Trace Element Partitioning in Immiscible Silicate-Carbonate Liquid Systems: an Initial Experimental Study Using a Centrifuge Autoclave

The origin of carbonatites remains a contentious topic. However, INTRODUCTION an important role for liquid immiscibility between silicate and Silicate–carbonate liquid immiscibility has often been carbonate liquids has often been proposed. To understand and proposed as a possible mechanism for the origin of constrain the role this process may play, it is important to have carbonatites associated with ultramafic alkaline rocks in trace element partitioning data available. Few experimental studies composite volcanic and subvolcanic complexes. Field on trace element partitioning between silicate and carbonate liquids observations (Ferguson & Currie, 1971; Kjarsgaard & have been undertaken, reflecting both analytical and experimental Peterson, 1991; Church & Jones, 1995) and data on difficulties. To achieve better phase separation new two-liquid melt inclusions in minerals (Rankin & Le Bas, 1974; experiments have been performed utilizing the rotating centrifuge Romanchev & Sokolov, 1979) support the role of liquid autoclave. Trace elements in the run products were analysed in situ immiscibility in carbonatite petrogenesis. Since the piusing laser ablation microprobe–inductively coupled plasma mass oneering work of Koster van Groos & Wyllie (1963) spectrometry. Partition coefficients (D) have been determined for the separation of carbonatitic immiscible liquids from selected rare earth elements (La, Nd, Sm, Tb, Er, Tm), high field carbonated silicate melts has been demonstrated exstrength elements (Zr, Hf, Nb, Ta), and for Sr, Ba and Y. Most perimentally in many synthetic silicate–carbonate systems of the rare earth elements partition preferentially into the silicate and for natural rock compositions [e.g. see review by liquid. La, Sr and Ba, however, strongly partition into the carbonate Kjarsgaard & Hamilton (1989)]. However, there is no liquid. The high field strength elements, although all preferentially consensus about the role of liquid immiscibility in the partitioning into the silicate liquid, are characterized by a wide development of carbonatite magmas (see Gittins, 1989). range of D values. Zr and Hf have similar D values, which are In this connection, quantitative experimental data on one to two orders of magnitude lower than those of Nb, Ta and trace element partitioning between the immiscible liquids Ti. Ti and Nb behave similarly, whereas Ta demonstrates behaviour are important for constraining the origin of carbonatites. intermediate to that of Zr and Hf. Nb/Ta ratios are strongly Experimental studies in silicate–carbonate systems are fractionated by two-liquid partitioning. hampered by the rapid crystallization of carbonate-rich

The origin of carbonatites and related rocks from the Igaliko Dyke Swarm, Gardar Province, South Greenland: field, geochemical and C-O-Sr-Nd isotope evidence

The Igaliko Nepheline-Syenite Complex of Gardar age (1300-1120 Ma) includes carbonatite, lamprophyre, basalt, trachybasalt, phonolite and rhyolite dykes. The relationship of carbonatite magma to the other magma types in this area is not immediately obvious. Field, geochemical and petrological studies suggest three equally acceptable models for the evolution of the carbonatites. Here we present petrological, geochemical and isotopic (C, O, Sr and Nd) data for a series of dykes from the Igaliko Nepheline-Syenite Complex which are pertinent to the origin of these carbonatites. δ 13C and δ 18O ratios show that the isotope evolution of the dykes is consistent with closely related carbonatite and ultramafic lamprophyre magma genesis, prior to the production of more voluminous alkaline lamprophyre and basalt magma during repeated rifting episodes. The associated phonolite melts, produced by fractional crystallisation of basic magmas late in this evolution, had lengthy residence times in the crust during which they exchanged C, O and Sr isotopes with crustal rocks. Despite geochemical evidence suggesting a genetic link between phonolites and carbonatites, the C and O isotopic evidence shows that the phonolites are not related directly to the carbonatites by liquid immiscibility, whilst combined field, geochemical and isotopic data strongly point to an origin of carbonatites by liquid immiscibility from ultramafic lamprophyres.