Aggregation of REE-bearing nanoparticles leads to REE enrichment in Late Permian Claystones from SW China

Abstract Rare earth elements (REEs) are indispensable to modern technologies, with global supply dominated by carbonatite-related and ion-adsorption-type deposits. Recent exploration has identified significant REE enrichment in Late Permian claystones of the Xuanwei Formation in southwestern China, representing a newly recognized type of REE resource. However, the mechanisms governing the occurrence and enrichment of REEs in these claystones remain poorly understood. Mineralogical and microstructural analyses reveal that REEs are predominantly hosted in discrete mineral phases, including florencite-(Ce), parisite-(Nd), bastnäsite-(Ce), cerianite-(Ce), apatite, xenotime, and Y-rich zircon, with florencite-(Ce) being the most abundant. At the nanoscale, REE-bearing clusters nucleated heterogeneously, commonly forming aggregates cemented by kaolinite/halloysite or precipitating within anatase dissolution pits. Subsequent aggregation and localized recrystallization produced discrete REE minerals. We propose a three-stage genetic model for REE enrichment involving input from basaltic volcanism and associated pyroclastic materials, mobilization during oxidative weathering, and subsequent fixation as authigenic minerals during sedimentary diagenesis and leaching. Weathering of primary REE-bearing phases in the basalt, notably apatite, titanite, and basaltic glass, released REE3+ into the weathering profile. These liberated REEs were subsequently redistributed by sedimentary reworking and chemical leaching within low-lying lacustrine–swamp depocenters intermittently influenced by marine incursions. Progressive dissolution of residual Fe-oxyhydroxides promoted development of aluminous claystones dominated by kaolinite, halloysite, chamosite, gibbsite, illite–smectite, anatase, and brookite. Collectively, these features are supportive of a dynamic diagenetic environment controlled by episodic fluid infiltration, redox fluctuations, and evolving pore-water chemistry. This study therefore links mineral evolution and REE enrichment to coupled volcanic–weathering–sedimentary processes and establishes a metallogenic framework applicable to analogous basalt-derived settings worldwide.