The alleged kimberlite-carbonatite relationship: evidence from ilmenite and spinel from Premier and Wesselton mines and the Benfontein sill, South Africa
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Carbonatite
Ilmenite
Baddeleyite
Carbonatites are rare igneous carbonate-rich rocks. Most carbonatites contain a large number of accessory oxide, sulfide, and silicate minerals. Baddeleyite (ZrO2) and zircon (ZrSiO4) are common accessory minerals in carbonatites and because these minerals host high concentrations of U and Th, they are often used to determine the ages of formation of the carbonatite. In an experimental study, we constrain the stability fields of baddeleyite and zircon in Ca-rich carbonate melts with different silica concentrations. Our results show that SiO2-free and low silica carbonate melts crystallize baddeleyite, whereas zircon only crystallizes in melts with higher concentration of SiO2. We also find that the zirconsilicate baghdadite (Ca3ZrSi2O9) crystallizes in intermediate compositions. Our experiments indicate that zircon may not be a primary mineral in a low-silica carbonatite melt and care must be taken when interpreting zircon ages from low-silica carbonatite rocks.
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Despite extensive industrial application, global scandium resources are uncertain. Although Sc is mainly supplied as a by-product of mining of rare earth metals, uranium or aluminum, it is also concentrated by carbonatite process, both magmatic and post-magmatic. In this paper, we report data on Sc distribution within the Kovdor baddeleyite–apatite–magnetite deposit (phoscorite–carbonatite pipe) in the Murmansk Region of Russia, which seems to be a significant reservoir of this "strategic" metal. We show that baddeleyite is the main Sc-concentrating mineral, and reveal the spatial distribution of Sc-bearing baddeleyite within the Kovdor phoscorite–carbonatite pipe. The scandium content in baddeleyite differs according to the petrographic zonation of the pipe culminating in the inner (axial) zone of the pipe and in dolomite carbonatites cutting the pipe. We have estimated the amount of Sc2O3 in the Kovdor deposit as amounting to 420 t at average content in baddeleyite of 0.078 wt.%, and revealed the economic potential for Sc recovery as a by-product. Other Sc-bearing minerals – pyrochlore and ilmenite groups, zirconolite, and juonniite – have been described and opportunity of Sc recovery was examined, too.
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Carbonatite
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Ilmenite populations (megacrysts and macrocrysts) from 26 kimberlites in North America have been characterized by electron microprobe analysis to assist in the understanding of the origin and significance of ilmenite in kimberlites worldwide. Most belong to the Cr-poor megacryst suite. Geochemical trends in Cr-poor-suite ilmenites are consistent with a mantle fractional crystallization origin, with ilmenite forming only a minor proportion of the crystallizing assemblage. Coprecipitating magnesite is inferred to be an important host for Mg, with its crystallization causing Mg depletion in coexisting ilmenite. Decrepitation of magnesite megacrysts during kimberlite ascent may have enriched kimberlite hosts in Mg, contributing to the Mg increase characteristic of ilmenite rims. Ilmenite rims commonly have lower hematite contents than do cores, suggesting that the oxidation state of the kimberlite, and thus its potential for diamond resorption, can be overestimated if core compositions alone are considered. No evidence has been found to support the hypothesis that oxidized ilmenite populations correlate with increased potential for diamond resorption in a given kimberlite.
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