Chemical weathering of S-type granite and formation of Rare Earth Element (REE)-rich regolith in South China: Critical control of lithology

Abstract

REE-rich granite regolith is a significant host for the ion-adsorption type Rare Earth Elements (REE) ore resources in South China. The issue of why and how such specific regolith was generated has attracted great attentions for both scientific and economic interests. To improve the understanding of critical factors that control the formation of REE-rich granite regolith, especially for those related to weathering of the S-type granite, an investigation of granite and its overlying regolith is conducted in South China's largest S-type granite terrain (Darongshan, Guangxi). The Hercynian and Indosinian granites outcrop as neighboring lithofaces, but their overlying regolith show significant differential chemical weathering. Examinations of their representative regolith profiles found that the profile from the Hercynian granite is thick and REE-rich, whereas that from the Indosinian granite is relative thin and REE-poor (12 m vs. 6 m in depth, 929 ppm vs. 226 ppm in REE concentration). Given similar climatic, topographic, and vegetative conditions, the difference between two profiles can be principally traced down to their parent granite lithology. Formation of the thick and REE-rich profile is closely associated with some specific lithological factors of its parent granite. Contrastingly, it has coarser grain size, wider microcracks, as well as higher biotite and plagioclase contents, which is expected to be more conductive to enhance water-rock interaction and drive deep weathering. Meanwhile, it contains higher initial REE concentration (342 ppm vs. 132 ppm) and, more importantly, richer REE-bearing accessory minerals (monazite, apatite and zircon), which offered dominant REE source to regolith. Especially, apatite (REE = 1549–4413 ppm) is an critical REE source mineral in supplying mobile REE to the regolith, through which REE have access to be fixed by clay minerals (mainly kaolinite and illite) and then developed ion-exchangeable form enrichment in the regolith. These evidences indicate that the granite lithology exerts a principal influence on the formation of thick and REE-rich regolith through fundamental control of chemical weathering and REE input. Moreover, this study find that, for the S-type granite particularly, the high P2O5 content (>0.08 wt%) granite seems more optimistic in generating ion-adsorption type REE ore (especially LREE) by weathering than previously thought.