Abstract
Carbonatites, usually occurring within intra-continental rift-related settings, have strong light rare earth element (LREE) enrichment; they rarely contain economic heavy REE (HREE). Here, we report the identification of Late Triassic HREE-Mo-rich carbonatites in the northernmost Qinling orogen. The rocks contain abundant primary HREE minerals and molybdenite. Calcite-hosted fluid inclusions, inferred to represent a magmatic-derived aqueous fluid phase, contain significant concentrations of Mo (~17 ppm), reinforcing the inference that these carbonatitic magmas had high Mo concentrations. By contrast, Late Triassic carbonatites in southernmost Qinling have economic LREE concentrations, but are depleted in HREE and Mo. Both of these carbonatite types have low δ26Mg values (−1.89 to −1.07‰), similar to sedimentary carbonates, suggesting a recycled sediment contribution for REE enrichment in their mantle sources. We propose that the carbonatites in the Qinling orogen were formed, at least in part, by the melting of a subducted carbonate-bearing slab, and that 10 Ma younger carbonatite magmas in the northernmost Qinling metasomatized the thickened eclogitic lower crust to produce high levels of HREE and Mo.
Highlights
China currently supplies 85% of the World’s rare earth elements (REE)[1,2], this supply is expected to decrease as China reduces REE exports
Experiments have suggested that low REE concentrations in magma are dispersed among the major rock-forming constituents of carbonate minerals, and REE mineralization does not develop[47]
Statistical data[3] have shown that there are less than 10 mines with economic REE values among the known 527 carbonatites worldwide
Summary
China currently supplies 85% of the World’s rare earth elements (REE)[1,2], this supply is expected to decrease as China reduces REE exports. Carbonatites are important hosts for LREE resources worldwide[3] They rarely contain HREE minerals, with the main example derived from alteration of early-formed HREE-rich zircon[6]. The carbonatites in the northernmost Qinling (NQ) orogenic belt (Fig. 1) contain quartz and economic molybdenite which are quite rare, and they show relatively flat REE patterns with high HREE contents (e.g., Yb > 30 ppm)[7,8]. These mineralogical and geochemical characteristics distinguish them from almost all reported carbonatites. The carbonatites contain abundant molybdenite and form a large Mo deposit[7]
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