Origin of the regolith-hosted HREE deposits in South China

Abstract

Rare earth elements (REEs) become critical to our modern society due to their numerous high-technology applications. Currently, China is the dominant producer of the global REEs, especially for the more demanding and economically valuable heavy REEs (HREEs). The HREEs are mainly from regolith-hosted HREE deposits in South China. In these deposits, the HREEs are liberated from the parent rocks, mostly granites, during sub-tropical weathering and progressively accumulate in the weathering crusts through a series of eluviation and illuviation processes to form the economically valuable ore deposits. Regolith-hosted HREE deposits are only related to HREE-enriched protoliths, mostly alkaline granites. Specifically, these granites have been subjected to extensive late-stage or later hydrothermal overprinting, which re-distributed and concentrated the REEs in several weathering susceptible minerals, including synchysite-(Y), gadolinite-(Y), hingganite-(Y), and yttrialite-(Y). Thermodynamic calculation shows that these minerals are susceptible to weathering and would efficiently release the HREEs they host during weathering. Therefore, pre-weathering metasomatism is essential for pre-concentration of the HREEs and thus for subsequent ore formation. Exemplified by the largest deposit of this kind, the Zudong deposit in Jiangxi province, South China, the HREEs are largely enriched through adsorption on clay minerals, mainly kaolinite and halloysite. Also, precipitation of supergene chernovite-(Y) [REEAsO4] contributes to a certain amount of the enrichment. Contributions from Fe-Mn oxyhydroxides and organic matter are negligible. During incipient weathering, rapid decomposition of albite produced abundant short and stubby halloysite and microcrystalline kaolinite with low crystallinity, high specific surface area, and high cation exchange capacity. Therefore, these halloysite and kaolinite crystals have high adsorption capacity to adsorb the HREEs, probably as carbonate complexes at the interlayer positions. Under progressive weathering, halloysite and microcrystalline kaolinite are progressively transformed into large, well-crystallized, vermicular kaolinite “booklets”. In this process, dramatic decrease in the adsorption capacity occurred and hence, the adsorbed REEs would be desorbed in the shallow soils and transported by the soil solutions, likely as carbonate complexes based on thermodynamic circulation, to the deeper parts of the soil profiles, where abundant halloysite and microcrystalline kaolinite facilitate the REE adsorption. Arsenic behaves similarly to the HREEs and therefore, can be combined with the HREEs at the lower part of the soil profiles to form the supergene chernovite-(Y). Both REE adsorption and precipitation of chernovite-(Y) enrich the lower part of the soil profiles in the HREEs. Continuous operation of this eluviation-illuviation process eventually causes the formation of world-class regolith-hosted HREE deposits in South China.