Origin of the heterogeneous enrichment of critical elements (Zr(Hf)-Nb(Ta)-Ga-REY) at the base of late permian tuffs, southwestern China

The regionally extensive basal Upper Permian tuff in southwestern China exhibits a unique genesis and is significantly enriched in Zr(Hf)-Nb(Ta)-Ga-REY, but show substantial spatial heterogeneity, with differential enrichment mechanisms not fully understood. This study investigates their geochemistry and mineral compositions. The results indicate that Zr(Hf)-Nb(Ta) contents are relatively stable in eastern Yunnan and northern Guizhou, while significant variations in critical element concentrations occur in western Guizhou. Notably, some tuff layers in this region display high Zr(Hf) concentrations but low Nb(Ta) abundances. Layers enriched in REY are typically localized at either the base or top of the tuff layers across different regions. Provenance analysis indicates that the basal Upper Permian tuffs originated from a complex mixture of sources, consisting of both mafic and intermediate-felsic volcaniclastic materials. Vertically, the sequence is characterized by a transition from mafic-dominated material in the lower sections to intermediate-felsic compositions in the upper sections. These intermediate-felsic tuffs are notably enriched in critical elements, including Zr(Hf)-Nb(Ta)-Ga-REY. Subsequent sedimentary and diagenetic processes, including leaching and interaction with low-temperature hydrothermal fluids, remobilize critical elements such as Nb(Ta), Ga, and REY. Intensive leaching results in the dissolution and downward migration of these elements, particularly from tuffs situated above the water table.