Origin and metal-enrichment mechanism of sedimentary rare earth element deposits in Yunnan and Guizhou provinces, western Yangtze Block, China

In southwest China, claystone layers occur in the lower part of the late Permian Xuanwei Formation, immediately overlying the Permian Emeishan flood basalts. The lowermost claystone layers are characterised by high contents of rare earth elements (REEs; 1100-16,000 ppm, average 2700 ppm), indicating a new type of REE deposit. However, the origin of this deposit and the enrichment mechanisms are unclear. We collected samples from 10 sections of the Xuanwei Formation, including REE-rich claystones, siltstones, iron claystones, and tuff. Detrital zircons gave U-Pb ages of 275-245 Ma, epsilon Hf(t) values of -16.7 to 8.9, and TDM2 model ages of 0.70-2.32 Ga. The field, petrological, geochemical, and isotopic data suggest: (i) the REE-rich claystones were deposited over 2.6 Myr in the period 257.22-254.62 Ma at a very low average rate of deposition (0.076-0.61 cm/ka), and (ii) the REE-rich claystones were derived from a region of denudation in the inner zone of the Emeishan Large Igneous Province, with the ore metals sourced mainly from syenite but with minor contributions from high-Ti basalts and felsic igneous rocks. The siltstones near the base of the Xuanwei Formation were derived mainly from felsic volcanic rocks with minor contributions from basalt. The iron-bearing claystones resulted from weathering of the Emeishan flood basalts. Our study shows a surface REE migration pattern of "weathering of REE-rich matrix -* fluid migration -* physical or chemical precipitation", which is consistent with the origin of the REE-rich claystones.