Remobilization and transferring of rare earth elements in the formation of regolith-hosted REE deposits

Rare earth elements (REEs) are indispensable for high-tech industries, such as clean energy, national defense, and military industries, rendering them strategically critical minerals. In China, regolith-hosted REE deposits constitute one of the most important REE resources, supplying over 90% global heavy rare earth elements (HREE). Understanding the formation of such REE deposits can provide a theoretical basis for their efficient utilization.

This paper summarizes the recent research results on the two key processes of REE remobilization and transferring and puts forward prospects for future research to deepen the knowledge and understanding of the formation of regolith-hosted REE deposit.

These deposits developed primarily in the weathering crusts of REE-rich granitic rocks, with the REE distribution patterns largely reflecting those of the underlying bedrock. The granitoid weathering crusts are primarily developed by chemical and biological weathering. Clay minerals and Fe–Mn (hydr) oxides, resulting from the weathering of major rock-forming minerals, such as feldspar, mica, and amphibole, serve as the primary hosts for REE ions in weathered crusts. These REE ions originate from the weathering and decomposition of REE-bearing accessory minerals in the bedrock, which exhibit varying degrees of susceptibility to weathering. Furthermore, metabolites such as microbial organic acids can breakdown refractory minerals like monazite and xenotime, facilitating REE remobilization. Simultaneously, microbial action can cause significant REE fractionation, and gram-positive bacteria are significantly more selective for HREE than for LREE. During weathering and leaching processes, REE primarily form REE complex ions within weathering crusts and are then transferred by meteoric water or groundwater. This process is primarily controlled by factors such as pH, secondary mineral formation, and the weathering environment. Notably, in addition to inorganic ligands, such as F⁻ and ${\mathrm{CO}}_3^{2-} $, organic matter can directly interact with REE, acting as organic ligands that aid in REE transfer.

Consequently, the REE remobilization and transferring mechanisms in regolith-hosted REE deposits are predominantly controlled by chemical and biological weathering processes, which result from interactions between inorganic and organic agents. However, the quantitative impact of these processes on the formation of these deposits requires further evaluation.