The Genesis of Regolith-Hosted Heavy Rare Earth Element Deposits: Insights from the World-Class Zudong Deposit in Jiangxi Province, South China

摘要

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 similar to 17,600 tonnes (t) of rare earth oxides (REOs) at an average grade of similar to 0.1 wt % REOs. Despite more than 40 years of exploration and exploitation, the genesis of this deposit is poorly understood. Subtropical weathering of the parent A-type granite formed orebodies hosted mainly within the lower B to upper C horizons of the resulting soil, which is developed on the hillsides of a moderately incised landscape (relief similar to 150 m). The thickness of the orebodies varies from a few meters to up to 10 m. REE concentrations increase from similar to 300 ppm in the A horizon to similar to 1,500 ppm in the lower B to upper C horizons and decrease with further depth in the profile. The entire soil profile is enriched in HREEs, with (La/Yb)(N) ratios <1, but the REE-rich lower B horizon is less enriched in HREEs, with (La/Yb)(N) values up to 0.9, than the underlying upper C horizon. Exchangeable REEs, representing REEs that are adsorbed in the deposit, constitute similar to 65% of the bulk REE content; the light REEs (LREEs) are preferentially adsorbed. The main minerals adsorbing the REEs are kaolinite and halloysite; the proportions of REEs adsorbed by Fe-Mn oxyhydroxides and organic matter are negligible. The remaining REE content is hosted by supergene chernovite (YAsO4) and residual aeschynite/euxenite-group and fergusonite-group minerals, xenotime-(Y), zircon, and thorite.