REE Enrichment And Fractionation Mechanism Of The Renju Ion Adsorption Type REE Deposit In Guangdong Province

Ion adsorption type rare earth element(REE)deposit in South China is one of the most important REE resources in China,characterized by large reserves,wide distribution,high content of heavy REEs(HREE)and low radioactivity,and at present this type of REE deposits provide approximately 20% of the global REE production and more than 90% of the global HREE production.In recent years,the mechanism of REE enrichment and fractionation of ion adsorption type REE deposit has attracted widespread attention due to the increasing demand of HREE in new energy materials,electronic technology,and aerospace fields.Recent studies show that the mineralization process of ion adsorption type REE deposit is related to the transformation of clay minerals and the erosion-denudation rate of weathering crust.However,compared with the endogenic mineralization,the geological environment and process where the weathering crust are formed are much more complicated.The evolution of the weathering crust is not only affected by properties of the bedrock such as micro-cracks,granularity,mineralogical composition and weathering resistance,but also controlled by geological conditions such as climate,landform and geological structure,microbial activity and groundwater seepage.The REE mineralization type and economic value of ion adsorption type REE deposit is profoundly determined by the weathering sequence and REE distribution pattern of the significant source of REEs,the REE-bearing accessory minerals and rockforming minerals.However,there are few investigations on the contribution of different REE-bearing accessory minerals to the mineralization of ion adsorption type REE deposits.Fe-Mn(hydr)oxides are widely distributed in weathering crust with a large specific surface area and many surficial active sites.They can effectively fix REEs via adsorption,isomorphous substitution,and co-precipitation.However,compared with the ion exchangeable REEs,which are mainly mined and utilized in this type of deposit at present,there is barely research on the Fe-Mn(hydr)oxide fraction,resulting in the role of Fe-Mn(hydro)oxides playing on the enrichment and fractionation of REEs in weathering crust remains unclear.The groundwater of weathering crust is not only the essential material of rock weathering,but also the important driving force of element migration,and additionally it could constrain the redox environment of the regolith.Therefore,it is very necessary to further investigate the influence of groundwater on the migration,enrichment and fractionation of REEs in the weathering crust.Herein,to address these issues,we choose two complete weathering profiles developed on granites in the Renju REE mining area,northeastern Guangdong province,China to study their mineralogical topography and geochemistry through TIMA,EPMA,SEM and LA-ICP-MS.The mineral weathering sequence is clarified and the REE source in the weathering crust is identified.The mineralogy and geochemistry of the weathering crust are studied by XRD,TEM,XRF,ICP-MS and sequence extraction of REE speciation.The mineral composition and REE distribution pattern of the weathering profile are clarified,as well as the REE speciation and their variation.The mineral evolution and groundwater geochemistry of the weathering profile are carefully investigated.The favorable position of water table during the REE mineralization and the model of groundwater constraining the REE mineralization are proposed.Through summarizing the above three results,the source of REEs and mechanism of REE enrichment and differentiation of the ion adsorption type REE deposits are revealed.The main conclusions and understandings obtained in this paper are as follows:(1)The mineral weathering sequence during the weathering process of the Renju quartz diorite is expounded,and the REE source of ion adsorption type REE deposit is clarified.Based on lithofacies observation,the mineral weathering sequence is as follows: parasite ?(titanite ? apatite)/(biotite/amphibole ? plagioclase)? Kfeldspar ? magnetite ? quartz ? zircon.It is indicated that biotite and plagioclase might have transformed into kaolinite when the weathering of titanite begins.The new formed kaolinite could become intense adsorption carrier for the released REEs.Therefore,titanite,which are moderately resistant to weathering,are the main REE sources in the weathering crust.The light REEs(LREE)enrichment characteristic in the weathering crust mostly inherits its REE distribution pattern,and eventually forms the ion adsorption type REE deposit.(2)The REE speciation and their variation rule in weathering profile are clarified,which aids in the REE mining and utilization.The result of the chemical sequence extraction shows that the REEs released from the dissolution of primary minerals in the parent rock occur mainly as the ion exchangeable and Fe-Mn(hydr)oxide fractions,with a few amounts of the organic and carbonate binding fractions.(3)The REE distribution pattern and fractionation mechanism of REE speciation are expounded.In the process of mineral weathering,the HREE enrichment degree in the residual fraction gradually increases with progressive weathering.During mineralization of REE,the REE speciation mutually effect.Fe(hydr)oxides,as analysed via transmission electron microscopy,usually occur in the form of goethite and hematite in the lower and upper parts of regoliths,respectively,while Mn is usually coexist with Fe(hydr)oxides.Fe-Mn(hydr)oxides preferentially adsorb HREE via inner-sphere complexation,resulting in that the LREE tends to be adsorbed clay minerals and mainly fixed by ion exchangeable fraction,thus forming the LREE enriched Renju ion adsorption type REE deposit.(4)The mineralogical and geochemical proxies of the groundwater table during the formation of ion adsorption type REE deposits are proposed.The result suggest that the abrupt change of cerium(Ce)anomaly values can refllect the sharp change of redox environment in weathering crusts;the prominent variation of hematite and goethite contents is an indicator of the rapid change of dry and wet environment in weathering crusts;the lamellar Fe-REE oxide colloids is a product of the fluctuation of groundwater table in weathering crusts.It is proposed that three indicators can be used as effective proxies for the groundwater table in the regolith.(5)In this study,we establish a mineralization model for groundwater controls REE mineralization,which highlights the distinctive roles of groundwater in the vadose and saturated zones with regard to the activation,migration,and enrichment of REEs in the regolith of South China.The groundwater system in the regolith includes a vadose zone above the groundwater table and a saturated zone below the groundwater table.The distribution and geochemical behaviour of REEs reveal that the highly permeable vadose zone facilitates the downward migration of REEs liberated via dissolution of REE-bearing minerals by seepage water,whereas the relatively stable and high-p H saturated zone facilitates the adsorption of REE ions by kaolinite and halloysite.The long-term eluviation–illuviation processes led to the continuous accumulation of REEs in the saturated zone and,ultimately,the formation of lenticular-shaped REE ore bodies.