MINERALOGICAL AND GEOCHEMICAL CONSTRAINTS OF THE ZUDONG REGOLITH-HOSTED HREE DEPOSIT IN SOUTH CHINA

The Zudong heavy rare earth element (HREE) deposit in South China is the largest regolith-hosted HREE deposit in the world with a resource of ~17,600 t of REO, at an average grade of ~0.1 wt. % REO. The deposit develops from subtropical weathering of an A-type alkaline granite. The parent granite contains quartz, albite, K-feldspar, and muscovite, and has an accessory mineral assemblage of synchysite-(Y), yttrian fluorite, gadolinite-(Y), hingganite-(Y), yttrialite-(Y), zircon, euxenite-(Y), fergusonite-(Y), polycrase-(Y), xenotime-(Y), columbite, and thorite. Major REE minerals, such as synchysite-(Y) and yttrialite-(Y), as well as muscovite are of hydrothermal origin. Albite, K-feldspar and muscovite progressively weathered in the soil profiles forming kaolinite and halloysite. Smectite and illite are minor and their occurrence are limited to the C horizon. Several supergene REE minerals occur in the weathering crust, including chernovite [YAsO₄], and cerianite [CeO₂]. Remaining REE minerals in the weathering profiles comprises residual euxenite-(Y), fergusonite-(Y), polycrase-(Y), xenotime-(Y), zircon, and thorite.

In term of geochemistry, REE contents of the parent granite vary from ~200 to ~450 ppm. The parent granite is also HREE-enriched. In the weathering crust, rare earth element concentrations increase from the A horizon to the maximum in the lower B to upper C horizons and decrease with further depth in the profile. The entire soil profile is HREE-enriched with (La/Yb)_N values <1, but the REE-enriched lower B horizon is less HREE-enriched (i.e. higher (La/Yb)_N values) than the underlying C horizon. Exchangeable REE, representing REE that are adsorbed in the deposit, comprises 25 to 75% of the bulk REE contents. The main minerals adsorbing the REE are kaolinite and halloysite.

During weathering, decomposition of synchysite-(Y) and gadolinite-(Y) in the parent rock released the REE, whereas kaolinite and halloysite formed to adsorb the REE. In upper parts of the soil profile, REE tends to remain in dissolved forms under acidic conditions. Deeper in the profile, conditions become less acidic leading to adsorption of the REE on the surface of various clay minerals. Ongoing operation of this eluviation-illuviation process at Zudong gradually enriched the soils in HREE to form such a giant deposit.