Author: Martin Yan, Hei Li, Mei-Fu Zhou
Abstract
Rare earth elements (REEs) have become increasingly important to ourmodern society due to their strategical significance and numeroushigh technological applications. Regolith-hosted heavy rare earthelement (HREE) deposits in South China are currently the main sourceof the HREEs, but the ore-forming processes are poorly understood. Inthese deposits, the REEs are postulated to ac- cumulate in regoliththrough adsorption on clay minerals. In the Zudong deposit, theworld’s largest regolith-hosted HREE deposit, clay minerals aredominated by short, stubby, nanometer-scale halloysite tubes (either10 or 7 Å) and microcrystalline kaolinite in the saprolite and lowerpedolith and micrometer-sized vermicular kaolinite in the humic layerand upper pedolith. A critical transformation of the clay minerals inthe upper pedolith is coalescence and unrolling of halloysite to formvermicular kaolinite. Microcrystalline kaolinite also transformed tolarge, well-crystalline vermicular kaolinite. This transformationcould result in significant changes in different physicochemicalproperties of the clay assemblages. Halloysite-abundant clayassemblages in the deep regolith have specific surface area andporosity significantly higher than the kaolinite-dominant clayassemblages in the shallow soils. The crystallinity of clay mineralsalso increased, exemplified by decrease in Fe contents of thekaolinite group minerals (from ~1.2 wt% in the lower saprolite to~0.35 wt% in the upper pedolith), thereby indicative of lessavailability of various types of adsorption sites. Hence,halloysite-abundant clay minerals of high adsorption capacity in deepregolith could efficiently retain the REEs released from weatheringof the parent granite. Reduction in adsorption capacity during theclay transformation in shallow depth partially leads to REEdesorption, and the released REEs would be subsequently transportedto and adsorbed at deeper part of the soil profile. Hence, theclay-adsorbed REE concentration in the lower pedolith and saprolite(~2500 ppm on average) is much higher than the uppermost soils (~400ppm on average). Therefore, weathering environments that favor therelease of the REEs in the shallow soils but preservation ofhalloysite in the deep regolith can continuously adsorb REEs inthe clay minerals to form economically valuable deposits.
Keywords: Rare earth elements (REEs); REE adsorption; halloysite; kaolinite; regolith-hosted REE deposits; weathering