Carbonatitic Magma Fractionation and Contamination Generate Rare Earth Element Enrichment and Mineralization in the Maoniuping Giant REE Deposit, SW China
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Abstract Carbonatite intrusions host the world’s most important light rare earth element (LREE) deposits, and their formation generally requires extraordinary fertile sources, magmatic evolution, and hydrothermal events. However, carbonatitic magma evolution, particularly the role of fractional crystallization and contamination from silicate rocks in REE enrichment, remains enigmatic. The Maoniuping world-class REE deposit in southwestern China, is an ideal target to decipher magmatic evolution and related REE enrichment as it shows continuous textual evolution from medium- to coarse-grained calcite carbonatite (carbonatite I) at depth, to progressively pegmatoidal calcite carbonatite (carbonatite II) at shallow levels. In both types of calcite carbonatites, four generations of calcite can be classified according to petrographic and geochemical characteristics. Early-crystalizing calcite (Cal-I and Cal-II) are found in carbonatite I and exhibit equigranular and a polygonal mosaic textures, while late calcites (Cal-III and Cal-IV) in carbonatite II are large-size oikocrysts (>0.5 mm in length) with strain-induced undulatory extinction and bent twinning lamellae. All these generations of calcite yield similar, near-chondritic, Y/Ho ratios (26.6–28.1) and are inferred to be of magmatic origin. Remarkably, gradual enrichment of MgO, FeO and MnO from Cal-I to Cal-IV is coupled with a significant increase in REE contents (~800 to 2000 ppm), with LREE-rich and gentle-to-steep chondrite-normalized REE patterns ((La/Yb)N = 3.1–26.8 and (La/Sm)N = 0.9–3.9, respectively). Such significant REE enrichment is ascribed to protracted magma fractional crystallization with initial low degree of fractional crystallization (fraction of melt remining (F) = ~0.95) evolving to late stage (F = 0.5–0.6) by formation of abundant calcite cumulates. Differential LREE and HREE behavior during magma evolution largely depend on separation of phlogopite, amphibole, and clinopyroxene from the carbonatitic melt, which is indicated by progressively elevated (La/Yb)N ratios ranging from 3.1 to 26.8. The four generations of calcite have significantly different C and Sr isotopic compositions with δ13CV-PDB decreasing from −3.28 to −9.97‰ and 87Sr/86Sr increasing from 0.70613 to 0.70670. According to spatial relations and petrographic observations, the relative enrichment of δ13C and depletion in 87Sr/86Sr ratios of Cal-I and Cal-II show primary isotopic characteristics inherited from initial carbonatitic magma. By contrast, the variable Sr and C isotopic compositions of Cal-III and Cal-IV are interpreted as the results of contamination by components derived from silicate wall rocks and loss of CO2 by decarbonation reactions. To model such contamination processes, Raleigh volatilization and Monte Carlo simulation have been invoked and the model results reveal that carbonatitic melt-wall rock interaction requires 40% radiogenic Sr contamination from silicate rocks and 35% CO2 degassing from carbonatitic melt. Moreover, positive correlations between decreasing δ13C values and increasing REE contents, together with bastnäsite-(Ce) precipitation, indicate further REE accumulation during the contamination processes. In summary, alongside REE-rich magma sources, the extent of fractional crystallization and contamination during carbonatitic magma evolution are inferred to be important mechanisms in terms of REE enrichment and mineralization in carbonatite-related REE deposits worldwide.Keywords:
Carbonatite
Rare-earth element
Fractional crystallization (geology)
Trace element
Carbonatite
Rare-earth element
Fluorite
Gangue
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Carbonatite
Peralkaline rock
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Abstract The Twyfelskupje carbonatite complex, Southern Namibia, exhibits the typical igneous emplacement structures of carbonatites, including plugs, cone sheets and dyke stockworks. The excellent exposure allows for detailed studies of the high-level geochemical and structural evolution of the carbonatite, and the nature of the concomitant rare earth element mineralization. Radiogenic isotope analyses (Sr, Nd, Pb) reveal that, in common with many other carbonatites, the Twyfelskupje carbonatite complex appears to be predominantly derived from mixing between HIMU and EM1 mantle end-members. Following partial melting of these mantle sources, the geochemical and structural evolution of the Twyfelskupje carbonatite complex proceeded by a staged process involving separate magma pulses, a series of emplacement structures, sub-solidus crystallization, fractionation and low-temperature hydrothermal alteration. The dominant rare earth element minerals in the Twyfelskupje carbonatite complex are fluorcarbonates and monazite, and are characterized by variable Ca, high F and light rare earth elements in the order Ce>La>Nd. Comparison between the rare earth element concentrations of the whole rocks, dominant rare earth element minerals and carbonates suggests that ∼95 % of the total rare earth element abundance of the Twyfelskupje carbonatite complex is contained within fluorcarbonates and monazite. Overall, the early calcio-carbonatite plugs are rare earth element enriched (mean 4.47 wt % rare earth oxides) relative to the magnesio-carbonatite cone sheets (mean 2.51 wt % rare earth oxides).
Carbonatite
Rare-earth element
Nepheline syenite
Metasomatism
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Abstract Rare earth elements are critical constituents for modern technologies, and some of their largest natural resource deposits are related to carbonatite systems. However, the mechanisms leading to rare earth element mineralization and the role of magmatic fluids in carbonatite systems remain poorly understood. Here, we present the first in situ characterization of fluids and their trace-element compositions in natural carbonatite systems by studying secondary quartz-hosted fluid inclusions from Oldoinyo Lengai volcano. By comparing our data to other fluids and melts from various carbonatite systems, we constructed a model for fluid-mediated rare earth element transport and mineralization. We show that carbonatite-related fluids are rich in alkali-carbonate + sulfate + chloride and CO2, but poor in H2O, and they can be significant carriers of rare earth elements (>1600 ppm). We argue that fluid CO2 contents are essential to preclude or slow down the interaction with wall rock during migration and that fluid-mediated rare earth element mineralization occurs when partial pressure of CO2 decreases in the fluid (i.e., during degassing).
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Trace element
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The Mt Vulture carbonatites are the only carbonatite occurrence in the southern Apennines. We present new trace element data for these rocks in order to evaluate the factors influencing rare earth element (REE) and other trace element fractionations and their REE grade. This study focuses on massive hyalo-alvikites from two lava flows and one dike, which have different relative abundances of silicate and carbonate (i.e. Si/Ca). These differences are also evident from CaO/(CaO + MgO + FeO(T) + MnO) and Sr/Ba ratios. The REE grade of the Mt Vulture carbonatites is very similar to that of the global average for calcio-carbonatites. R-mode factor analysis shows that most of the trace element variance reflects the relative roles of carbonate and silicate minerals in influencing trace element distributions. Silicates largely control heavy rare earth element (HREE), transition metal, Zr, and Th abundances, whereas carbonate minerals control light rare earth element (LREE), Ba, and Pb abundances. In addition, apatite influences LREE concentrations. Increasing silica contents are accompanied by decreases in (La/Yb)N and (La/Sm)N ratios and less marked LREE enrichment. In contrast, higher carbonate contents are associated with increases in (La/Yb)N and (La/Sm)N. The Si/Ca ratio has little influence on Eu anomalies and middle rare earth element (MREE) to HREE fractionations. Apatite has a negligible effect on inter-REE fractionations amongst the carbonatites.
Carbonatite
Trace element
Rare-earth element
Silicate minerals
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Carbonatite
Rare-earth element
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Carbonatite
Rare-earth element
Trace element
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Carbonatite
Rare-earth element
Inner mongolia
Texture (cosmology)
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Carbonatite
Extrusive
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Rare earth elements (REE) have been a focus of global interest because of their irreplaceable role in developing “low carbon” technologies. The Bayan Obo is the world’s largest REE deposit, but its genesis is still highly debated. It is considered to have a close genetic association with carbonatite due to the presence of the carbonatite dykes around the orefield, as well as the geochemical similarities between these dykes and the orebody. However, the evolution of the carbonatite dykes and their REE mineralization are still poorly understood, hindering the interpretation of the genesis of the deposit. More than 100 carbonatite dykes have been found within the area of 0-3.5km nearby the orebodies of the deposit. These dykes show significant variations in mineralogy and geochemistry and were classified into dolomite (DC) and calcite carbonatite (CC). The rocks show an evolutionary sequence from DC to CC, and their corresponding REE contents increased remarkably, with the latter having very high REE content (REE2O3 up to 20 wt. %). The DC is composed of coarse-grained dolomite, magnetite, calcite, and apatite without apparent REE mineralization. The medium-grained calcites, and significant amounts of REE minerals, such as monazite, bastnäsite, and synchysite, make up CC. The REE minerals have a close relationship with barite, quartz, and aegirine. The REE patterns of dolomite and calcite in DC showed a steep negative slope with a strong LREE enrichment. In contrast, the calcite from CC has a near-flat REE pattern enriched in both LREE and HREE. Besides, apatite and magnetite in CC are characterized by strong REE enrichment compared to those from DC. Based on detailed petrology, mineralogy, and element geochemistry, we propose that strong fractional crystallization of initial carbonatitic melts led the REE enriched in the residual melt/fluid to form REE mineralization. In addition, sulfate, alkalis, and silica components play an important role in REE transportation and precipitation.
Carbonatite
Rare-earth element
Trace element
Metasomatism
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