Fu, Wei, Luo, Peng, Hu, Zuoying, Feng, Yangyang, Liu, Lei, Yang, Jingbao, Feng, Meng, Yu, Hongxia, and Zhou, Yongzhang
Indosinian felsic volcanic rocks occurring in the Chongzuo area, Guangxi, were subject to prolonged weathering, which resulted in development of thick regolith hosting valuable ion-adsorption type REE resources. The figure below demonstrates a quantified REE eluviation-illuviation model for delineating the ion-exchangeable REEs enrichment pathway from source to sink. The source of ion-exchangeable REEs is mainly traced to a titanite + allanite + apatite assemblage, which together with other igneous minerals contributed REE ions to the weathering solution. After liberation of REEs by weathering, a large fraction of the rare earths is fixed by clay minerals (kaolinite + halloysite + illite assemblage) and the ions are present in an exchangeable form. A small fraction is adsorbed by Fe-Mn oxides and organic matter. REEs in zircon, monazite, and ilmenite are conserved; these minerals are the source of residual REEs in the regolith. • iREEs enrichment pathway in rhyolite regolith is delineated from source to sink. • Accessory mineral assemblage (titanite + allanite + apatite) is main source of iREEs. • The affinity of clay minerals to iREEs follows a halloysite > kaolinite > illite order. • There is formation preference for the felsic volcanic rocks-related iREEs deposits. Felsic volcanic rocks are an important source for generating ion adsorption-type rare earth element (REE) resources. To better understand the supergene enrichment of REEs related to felsic volcanic rock weathering, this study conducted an investigation of Indosinian felsic volcanic rocks from Guangxi, southwest China, which have been subjected to deep weathering. The weathering has formed large areas of thick regolith with the potential to host ion adsorption-type REE mineralization. The felsic volcanic rocks belong to the dacite–rhyolite series and are peraluminous, high-K, calc-alkaline, and REE-rich (325–376 ppm). Abundant REE-bearing accessory minerals are present in the felsic volcanic rocks, including titanite (average ΣREE = 15.1 wt%), allanite (average ΣREE = 14.9 wt%), and apatite (average ΣREE = 1510 ppm), and these three minerals contain an estimated 88.7% of the whole-rock REEs content. Significant REE enrichment is present in the felsic volcanic rock-derived regolith. A typical rhyolite-derived regolith profile has REE contents that increase from 376 ppm in the rhyolite to 1737 ppm in the regolith, representing an almost five-fold enrichment due to weathering. The REEs in the regolith are present in an ion-exchangeable form (iREEs), which accounts for 52%–87% of the total REEs present (TREEs). The occurrence of iREEs is closely linked to clay minerals, showing an affinity in the order halloysite > kaolinite > illite. Continuous operations of REEs by an eluviation–illuviation process from the source minerals (titanite + allanite + apatite) to sink minerals (kaolinite + halloysite + illite) results in an iREE-enriched zone in the middle and lower parts of the rhyolitic regolith. Notably, besides the studied Indosinian felsic volcanic rocks, there are multi-epochs of such lithology outcropped in South China. Their related iREEs mineralizations, however, are preferentially developed in the weathered terrains of early Yanshanian and Indosinian felsic volcanic rocks, and not in those of the Yanshanian and late Yanshanian felsic volcanic rocks. A comparison of the mineralized and barren units indicates that the iREEs mineralization hosted in felsic volcanic rocks regolith is controlled by some key endogenic and exogenic ore-forming factors. High initial REE concentrations in the unaltered felsic volcanic rocks, a suitable climate, and a relatively quiescent tectonic setting are favorable for the formation and preservation of iREEs mineralization. [ABSTRACT FROM AUTHOR]