The role of carbonatite-silicate liquid immiscibility in the global large carbonatite-associated REE deposits (CARDs): New insight from the trace element

Abstract

More than 600 carbonatites worldwide are spatially and temporally associated with silicate rocks, which contributes to the formation of carbonatite-silicate complexes originating from mantle. However, only a minority of these carbonatites evolve into large to giant rare earth element (REE) deposits. The formation mechanisms of carbonatite-associated REE deposits (CARDs) are primarily attributed to liquid immiscibility. Nonetheless, experimental petrology has discussed the REE distribution coefficients (D^Carb/Sil) during liquid immiscibility, yet lacks quantification of the difference of the natural deposits. Moreover, the magma source of carbonatites and silicate rocks from these deposits is derived from the mantle, and the geochemical indexes can support further evidence. This study focuses on the globally significant large carbonatite-associated REE deposits, specifically six deposits involving Mianning-Dechang REE belt (Maoniuping, Dalucao, Muluozhai, Lizhuang), Bayan Obo, and Mountain Pass, to investigate the characteristics of the magma source and origin of carbonatite-silicate complexes through bulk-rock geochemistry. The Y/Ho values of carbonatites and silicate rocks from these deposits, except for Lizhaung, are similar to mantle value, indicating a mantle-derived origin. Except for Mountain Pass and Maoniuping carbonatites, high Nb/Ta ratios in the other samples indicate geochemical decoupling during liquid immiscibility, although they have similar charge and ionic radius. To verify the liquid immiscibility, we utilized the discernable geochemical indexes (e.g., Ba/Mn, Ba/La) based on their well-constrained partitioning behaviors in different magma compositions. The results show that the compositions of carbonatites and silicate rocks in Dalucao are strictly accordant with those confirmed by experiments, whereas others deviate more or less. This further suggests that subsequent magma fractionation or fluid-mediated works on the other five deposits and compositionally covers up the evidence of liquid immiscibility. To trace carbonatite-silicate liquid immiscibility in these deposits, we propose the deviation coefficient Ƙ via a comparison on the difference of REE distribution coefficients (D^Carb/Sil) between experimental results and natural deposits. The examined results show that Ƙ values for Maoniuping, Dalucao, Muluozhai, Bayan Obo Mg-carbonatite and Ca-carbonatite are close to 0, while the Ƙ values for Lizhuang, Bayan Obo Fe-carbonatite and Mountain Pass show the opposite trend. Utilizing the C-O isotopic composition from previous studies and immiscible deviation coefficient Ƙ, the occurrence of immiscibility in Maoniuping, Dalucao, Muluozhai, Bayan Obo Mg-carbonatite and Ca-carbonatite has been confirmed. Based on previous research, we believe that liquid immiscibility not only facilitates pre-enrichment of rare earth elements but also contributes to subsequent REE mineralization.