Hydrothermal alteration of allanite promotes the generation of ion-adsorption LREE deposits in South China
Xu Zhao, Ningbo Li, He‐Cai Niu, Yuhang Jiang, Shuang Yan, Yu-Yuan Yang, Rui-Xin Fu
Abstract
Most ion adsorption rare earth element (REE) deposits (>85%) are light REE (LREE) enriched, but it is unclear whether the magmatic and hydrothermal activity have a control to the LREE mineralization, and why only some granitoids generate ion adsorption LREE deposits despite similar weathering conditions and REE contents. In this study, we investigated the main LREE-bearing mineral (i.e., allanite) in the bedrocks of the large Bachi ion adsorption LREE deposit (REE oxide reserves of 42,589 t) in South China. The allanite hosts at least 80% of the LREE in the bedrocks of the Bachi deposit. The allanite comprises primary (Aln-1) and altered (Aln-2) allanite. Aln-2 shows larger lattice parameters, smaller diagnostic Raman spectroscopy peaks, with extensive micro-pores and -fractures infilled by synchysite-(Ce), indicating a lower crystallinity and less stable mineral structure. In addition, Aln-2 has lower LREE contents, higher Sr, Ba, and Th contents, more negative Eu anomalies, and less positive Ce anomalies than Aln-1, but both types of allanite have similar Nd isotopic compositions with those of the whole-rock samples. These morphological and geochemical features suggest that F−, CO2, and Th4+-rich, REE-undersaturated, medium–low-temperature, and oxidizing fluids interacted with the allanite, leached LREE from the allanite, and then formed synchysite-(Ce). The LREE leached from the allanite and precipitated as synchysite-(Ce) on a micron scale, and external materials carried in the fluid did not make a significant contribution to the REE budget and Sm–Nd isotopic compositions of the allanite. The structural and geochemical changes from Aln-1 to Aln-2 made it easier for the allanite to be weathered, and synchysite-(Ce) was also easily weathered, which released LREE3+ ions and generated the ion adsorption LREE deposits. This study highlights that the LREE could be migrated from allanite to synchysite-(Ce) during hydrothermal alteration, which contributed to the generation of ion adsorption REE deposits. The LREE mobilization during hydrothermal alteration may explain why only some granitoids generated ion adsorption LREE mineralization in South China.