Hou, and 4 co-authors, reported in 2015 that carbonatite-associated rare-earth-element (REE) deposits are the most significant source of the world’s REEs; however, their genesis remains unclear. The authors presented new Sr-Nd-Pb and C-O isotopic data for Cenozoic carbonatite-hosted giant REE deposits in southwest China.
These REE deposits are located along the western margin of the Yangtze Craton that experienced Proterozoic lithospheric accretion, and controlled by Cenozoic strike-slip faults related to Indo-Asian continental collision. The Cenozoic carbonatites were emplaced as stocks or dykes with associated syenites, and tend to be extremely enriched in Ba, Sr, and REEs and have high 87Sr/86Sr ratios (>0.7055). These carbonatites were likely formed by melting of the sub-continental lithospheric mantle (SCLM), which had been previously metasomatized by high-flux REE- and CO2-rich fluids derived from subducted marine sediments. The fertility of these carbonatites depends on the release of REEs from recycled marine sediments and on the intensity of metasomatic REE refertilization of the SCLM.
The authors suggested that cratonic edges, particularly along ancient convergent margins, possess the optimal configuration for generating giant REE deposits; therefore, areas of metamorphic basement bounded or cut by translithospheric faults along cratonic edges have a high potential for such deposits. Metasomatic refertilization of the SCLM and formation of fertile carbonatites and carbonatite-associated REE deposits (CARDs) are involved in:
a) Upwelling of asthenosphere triggers the melting of refertilized SCLM that was previously metasomatized by CO2-rich fluids derived from marine sediments associated with “fossil” subduction zones. The subducted sediments released their REEs into CO2-rich fluids that metasomatized old depleted or enriched SCLM to form an unusually REE-rich, carbonated mantle source, which then produced carbonatite melts or CO2-rich silicate melts. The margins of the craton experience low degrees of partial melting, and the melts ascend through fracture zones into the overriding crust.
b) The models of CARD formation, includes a variety of orebodies formed by fluids exsolved from REE-rich carbonatitic magmas emplaced at shallow crustal levels. Lateral migration, replacement, open-space filling, and focused discharges of ore-forming fluids produced semi-stratabound (Bayan Obo-style), disseminated (Lizhuang or Mountain Pass-style), stringer-stockwork (Maoniuping-style) and breccia pipe (Dalucao-style) orebodies with associated fenitization and K-silicate alterations, respectively.
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