The heterogeneous distribution of REE-Nb in the claystone at the bottom of Xuanwei Formation, southwestern China: Insights from LA-ICP-MS mapping

The REE-Nb-Zr-Ga polymetallic mineralization layers are extensively developed at the bottom of the Xuanwei Formation (Late Permian) in southwestern China and regarded as a potentially significant resource. However, the distribution patterns of these elements across different layers and their association with clay minerals remain enigmatic. This study employs TIMA and LA-ICP-MS mapping analyses to investigate the mineralogical and geochemical characteristics of the Fe-rich claystone (lower part) and the overlying Al-rich claystone (upper part) of the Xuanwei Formation, revealing the distribution patterns of REE and Nb, as well as their spatial relationship with clay minerals. Our results demonstrate that REE and Nb mainly enrich in Al-rich claystone, with lower contents in Fe-rich claystone. The highest REE concentrations occur at the Al-rich claystone boundaries and at the top of the Fe-rich claystone. Nb is predominatly in Fe-Ti oxide phases and Ti-rich kaolinite. TIMA and LA-ICP-MS mapping reveals REE-Nb enrichment correlates with specific kaolinite types. REE and Nb are primarily concentrated in kaolinite matrix, which commonly coexists with numerous clay mineral fragments. However, these fragments themselves do not contain REE. Notably, Nb exhibits a coupled relationship with Ti rather than REE in Ti-rich kaolinite fragments. Especially in Fe-rich claystone, Ti-rich kaolinite fragments constitute the dominant host phase for Nb while being depleted in REE. Conversely, in Al-rich claystone, clay mineral fragments are mainly composed of boehmite, a phase depleted in REE, Nb and Zr, but it is the principal host for Ga. This study gives critical insights into the potential for industrial utilization of these resources and guide for future exploration and extraction. Moreover, high-temperature calcination significantly enhances the acid leaching efficiency of REEs, promoting green and efficient extraction.