Occurrence and provenance of ion-adsorbed rare earth elements mineralization in alumina-rich rocks of the Permian Heshan formation, Western Guangxi, China

Abstract

The enrichment of rare earth elements (REEs) in alumina-rich rocks (ARR) is widely observed. However, REEs in ARR are primarily present in mineral forms, which poses significant challenges to their practical utilization. Recently, the Permian Heshan Formation in Shangsi County, western Guangxi, China, has attracted attention due to the discovery of an unusual occurrence of ion-adsorbed REEs mineralization within ARR. In this study, four ARR samples and an underlying limestone were collected from a single geological profile to investigate the occurrence of REEs through chemical sequential extraction, scanning electron microscopy (SEM), X-ray diffraction (XRD). Furthermore, the provenance of REEs was analyzed based on U–Pb geochronology and chemical compositions in detrital zircons. The results show that REEs content in four ARR ranges from 897 to 2753 ppm, with an average of 1507 ppm, exceeding the industrial grade (1000 ppm) for ion-adsorbed REEs deposits. Notably, the ion-exchangeable phase (F2, average 58.97 %) constitutes the dominant REEs fraction, followed by the residual phase (F7, average 17.98 %), Fe–Mn oxide phase (F5, average 7.01 %), humic acid phase (F4, average 5.58 %), organic-matter phase (F6, average 5.50 %), water soluble phase (F1, average 3.19 %), and carbonate-bound phase (F3, average 0.52 %). Moreover, distinct REEs fractionation patterns are evident across different phases. Light REEs (LREEs) are predominantly adsorbed by clay minerals, whereas heavy REEs (HREEs) primarily retained in the residual phase. The morphology and U–Pb geochronology of detrital zircons provide evidence that the provenance of REEs underwent a significant change from the bottom to the top of the stratigraphic profile. Additionally, mineralogical compositions, chondrite-normalized REE patterns, and zircon geochemical characteristics further support that the variation in REE sources within the profile. Geochemical plots and tectonic discrimination diagrams suggest that REEs in the upper section of the profile primarily originated from the Permian Paleo-Tethys magmatic arc, whereas the lower section predominantly inherited its REEs from the underlying limestone. The introduction of foreign acidic magmatic materials is presumed to have contributed to the formation of the ion-adsorbed REEs enrichment zone within the ARR of the Heshan Formation. The present study contributes to an enhanced and more comprehensive understanding of the mechanism governing REEs enrichment in paleo-weathered horizons.