Origin and mineralization processes of REE during magmatic-hydrothermal evolution: Insights from in-situ geochemistry of calcite from the Weishan REE deposit, eastern North China Craton

Abstract

The Weishan REE deposit, hosting more than one million tons of light REE oxides (LREE2O3), is one of the largest LREE deposits in China. It has been considered to be a carbonatite-related REE deposit, but whether carbonatites occurred or not is still controversial, leading to the confusions of how the LREE originated and mineralized. Here, we conducted in-situ elemental and Sr-Nd isotopic analyses on different stages of calcite from the Weishan REE deposit, combined with petrographic and bulk C-O isotopic studies, with the aim to unravel the origin and mineralization processes of REE. Four stages of calcite (Cal-1, Cal-2, Cal-3 and Cal-4) were identified, of which the Cal-1 typically coexists with K-feldspar and quartz while the Cal-2 and Cal-3 coexist with sulfates and REE minerals (e.g., bastnäsite and parisite), respectively. The Cal-4 occurs in the post-ore veinlets and coexists with sulfides. The Cal-1 is characterized by exsolution of carbocernaite along its cleavage planes with highest Na, K, Sr, Ba and LREE contents. The other stages of calcite show continuous decrease of the above elements. Combined with petrographic features and elemental discrimination diagrams, it is concluded that the Cal-1 is magmatic while the others are hydrothermal. These calcites thus indicate a continuous magmatic-hydrothermal evolution. The four stages of calcite show similar isotopic compositions, which are consistent with those of the alkaline silicate rocks in the ore district, suggesting an uniform source or magma chamber for their derivation. Considering the magmatic features of Cal-1, it is inferred that carbonate melts coupled with abundant alkaline fluids were exsolved from the alkaline magmas. Alkalis (e. g., K and Na) were enriched in the magmatic-hydrothermal system with potassic metasomatism being more prevalent, which might play an important role in REE exsolution and transportation. Focused REE deposition occurred shortly after massive deposition of sulfates, indicating that REE-sulfate complexes might be the dominant transporting ligands of REE. The deposition of REE was mainly induced by fluid boiling, as indicated by the boiling/unmixing features of fluid inclusions. The Sr-Nd isotopic compositions of ores and associated alkaline rocks show affinities to the enriched lithospheric mantle (EM) beneath the study region, but are more depleted. Because subduction of the Paleo-Pacific oceanic plate beneath the eastern Asian occurred since the Jurassic, it is inferred that CO2- and REE-rich melts/fluids released from the subducted oceanic slab metasomatized the EM, leading to a REE-rich carbonated mantle. Rollback of the subducted slab occurred during the early Cretaceous, resulting in partial melting of the EM and thus generating the CO2- and REE-rich alkaline magmas. This deposit shows significant hydrothermal features with poorly-developed carbonatites, distinct from the typical carbonatite-related REE deposits in the other places. The REE mineralization is thus considered to be basically controlled by the alkaline magmas rather than the carbonatites.