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

[Objective] Rare earth elements (REE) are indispensable for high-tech industries such as clean technologies, national defense and military industry, rendering them strategically critical minerals. In China, regolith-hosted REE deposits constitute one of the most important REE resources, supplying over 90% of the global heavy rare earth elements (HREE). Understanding the formation of such REE deposits can provide theoretical basis for the search and better utilization of these REE resources. [Methods] This paper summarizes the latest research results in recent years from the two key processes of remobilization and transferring of REE, and puts forward prospects for future research, so as to deepen the knowledge and understanding of the formation of regolith-hosted REE deposits. [Results] These deposits are primarily developed in weathering crusts of REE-rich granitic rocks, with REE distribution patterns in these crusts largely reflecting those of the underlying bedrock. The development of granitoid weathering crusts is mainly induced 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 bedrock, which exhibit varying degrees of susceptibility to weathering. Furthermore, metabolites such as organic acids produced by microbes can benefit the breakdown of refractory minerals like monazite and xenotime, facilitating the remobilization of REE. At the same time, microbial action can lead to significant fractionation of REE, and Gram-positive bacteria are significantly more selective to HREE than light REE (LREE). During weathering and leaching processes, REE primarily form REE-complexes within weathering crusts and then transferring by meteoric or ground water. This process is primarily controlled by factors like pH, the formation of secondary minerals, and the weathering environment. Notably, in addition to inorganic ligands such as F^- and (CO₃)^2-, organic matter can also directly interact with REE, acting as organic ligands that aid in REE transferring. [Conclusion] Consequently, the remobilization and transferring mechanism of REE in regolith-hosted REE deposits is predominantly controlled by both chemical and biological weathering processes, which result from the interactions of inorganic and organic agents. Nonetheless, the quantitative impact of these processes on the formation of such deposits remains to be further evaluated.