Geochemical background and dispersion pattern of the world's largest REE deposit of Bayan Obo, China

The Bayan Obo deposit is the largest deposit of rare earth elements (REEs) in the world, with its light REE (LREE) reserve constituting approximately 45% of the world's total LREEs. This study identifies the geochemical background of REEs in the Bayan Obo region and investigates REE dispersion pattern along a stream, the fractionation between parent rocks and soils, as well as REE distribution in a weathering profile, to reveal the geochemical behavior of REEs during hypergene processes in semi-arid grasslands. Results show that the REE content in the slates of the Bilute, Jianshan, and Dulahala Formations, which are surrounding rocks of the Bayan Obo deposit, is far higher than those observed in ordinary slate and the continental crust of the North China Craton. These surrounding rocks may have provided sufficient REE sources to form the current, huge REE deposit. The total REE content of stream sediments in the Bayan Obo region ranges from 46.9 to 799.8 mu g/g, with a median of 170.8 mu g/g. Two large-scale REE anomalies were discovered to the west of the Bayan Obo region. The REE content in soil is primarily determined by the parent rock, and no fractionation occurs between heavy REEs (HREEs) and LREEs after the parent rock weathers in semi-arid grasslands. Across the soil profile in Bayan Obo region, REEs typically enrich the surface and deep layers; the middle layer typically contains low REE content owing to the existence of caliche. Along streams, REEs primarily migrate with fine-grained particles over long distances (7 km or longer) via mechanical means. In contrast, coarse-grained particles mainly remain in situ. Affected by aeolian sand, the REE content is relatively low in the 40-80 mesh fraction.