Trace element geochemistry of the Mt Vulture carbonatites, southern Italy
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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.Keywords:
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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).
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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.
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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.
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