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postgraduate thesis: Origin of the regolith-hosted HREE deposits in South China

TitleOrigin of the regolith-hosted HREE deposits in South China
Authors
Advisors
Advisor(s):Zhou, MFChan, LS
Issue Date2019
PublisherThe University of Hong Kong (Pokfulam, Hong Kong)
Citation
Li, Y. [李欣禧]. (2019). Origin of the regolith-hosted HREE deposits in South China. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR.
AbstractRare 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.
DegreeDoctor of Philosophy
SubjectRare earth metals - China
Dept/ProgramEarth Sciences
Persistent Identifierhttp://hdl.handle.net/10722/283210

 

DC FieldValueLanguage
dc.contributor.advisorZhou, MF-
dc.contributor.advisorChan, LS-
dc.contributor.authorLi, Yan-hei-
dc.contributor.author李欣禧-
dc.date.accessioned2020-06-19T00:45:54Z-
dc.date.available2020-06-19T00:45:54Z-
dc.date.issued2019-
dc.identifier.citationLi, Y. [李欣禧]. (2019). Origin of the regolith-hosted HREE deposits in South China. (Thesis). University of Hong Kong, Pokfulam, Hong Kong SAR.-
dc.identifier.urihttp://hdl.handle.net/10722/283210-
dc.description.abstractRare 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.-
dc.languageeng-
dc.publisherThe University of Hong Kong (Pokfulam, Hong Kong)-
dc.relation.ispartofHKU Theses Online (HKUTO)-
dc.rightsThe author retains all proprietary rights, (such as patent rights) and the right to use in future works.-
dc.rightsThis work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.-
dc.subject.lcshRare earth metals - China-
dc.titleOrigin of the regolith-hosted HREE deposits in South China-
dc.typePG_Thesis-
dc.description.thesisnameDoctor of Philosophy-
dc.description.thesislevelDoctoral-
dc.description.thesisdisciplineEarth Sciences-
dc.description.naturepublished_or_final_version-
dc.date.hkucongregation2019-
dc.identifier.mmsid991044139572503414-

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