Geochemistry And REE Occurrence In Weathering Profile Of Ion-adsorption REE Deposits In Southern Jiangxi Province

The ion-adsorbed rare earth deposits developed in the seven southern provinces of my country are the most important source of heavy rare earths in the world.Studies have shown that the chemical weathering process may be a key element in controlling the enrichment of rare earths.However,the constraints of mineralogical evolution of parent rocks and changes in geochemical conditions on the formation of ion-adsorbed deposits during granite weathering are still the weak links in current research.The mechanism of the decoupling and enrichment of light and heavy rare earths in the chemical weathering process is unknown,which restricts the understanding of the formation mechanism of this type of deposit.In this study,we selects the in-situ weathered Dabu heavy rare earth-rich granite weathering profile and Qingxi light rare-earth-rich granite weathering profile developed under tropical-subtropical climate conditions in Gan County,Jiangxi Province as the research objects.The comprehensive study of iron-manganese oxides and other mineralogy,element and lithium isotopic geochemistry,clarifies the occurrence characteristics of rare earth elements,and clarifies the activation-migration-enrichment behavior of rare earth elements during the weathering of granite.The main findings obtained are listed as follows:1.The heavy rainfall under tropical-subtropical climate conditions and the acidic leaching environment at the top of the profile promoted elemental remobilization and leaching in the chemical weathering.The immobile elements Th,Nb,Zr,Hf and Ti have experienced varying degrees of leaching loss or enrichment behavior in the chemical weathering of granite;among these elements,Ti is the most stable.Alkali metal and alkaline earth metal elements such as K,Na,Ca,Rb and Sr,which are highly mobile,are lost strongly by leaching.Major elements such as Al,Fe and Mn are depleted relative to the parent rock in the weathering profile of Dabu granite,but are enriched in the Qingxi profile.2.The geochemical behaviors of Li and Li isotope in the weathering profile of granite are closely related to primary minerals and secondary clay minerals.The granite parent rock usually has a lighter Li isotope composition than primary mineral(Quartz,biotite,etc.),and its physical characteristics,containing small pores,few cracks and infiltration,are conducive to the ~7Li-rich weathering fluid remaining at the bottom of the semi-weathered layer.The inconsistent dissolution of primary minerals is conducive to the enrichment of primary minerals with strong resistance to weathering in the upper part of the profile,which leads to an increase in the~7Li of the weathered residual soil.The secondary clay minerals containing Mg(Fe or Mn)preferentially adsorb the ~6Li in the weathering fluid,and then dissolve and release the previously adsorbed~6Li under extreme weathering conditions.In addition,atmospheric precipitation and ancient groundwater control the external source input of Li and ~7Li in the horizon where the surface soil and ancient groundwater are located,respectively.3.The activation-migration-accumulation behavior of rare earth elements in granite weathering profile is controlled by the adsorption of secondary clay minerals,iron-manganese(hydrogen)oxides and p H.Primary rare earth minerals are weathered and dissolved,and a large number of rare earth elements are reactivated into weathered fluids by ionic phase.The adsorption capacity of secondary clay minerals and iron-manganese(hydrogen)oxides to rare earth elements increases with the increase of p H value.Long-term leaching and adsorption make rare earth elements gradually enriched and mineralized in the layer between p H 5.73 and 6.84,and the highest total content of rare earth in the ore body can reach 5 times of the granite parent rock.4.The rare earth element ore bodies in the weathered profiles of Dabu and Qingxi rare earth-rich granites all show relatively enrichment of light rare earth in the upper part and higher content of heavy rare earth in the lower part.The preferential adsorption of light rare earth elements by iron-manganese(hydrogen)oxides and the existence of weathered residual rare earth minerals in the chemical weathering process of rare earth-rich granites jointly control the decoupling of light and heavy rare earths,facilitate the formation of a combined deposit of light rare earth mineralization in the upper part and heavy rare earth mineralization in the lower part of the ion-adsorption type light(or heavy)rare earth deposits.5.Rare earth elements mainly exist in three occurrences in granite ion adsorption rare earth deposits:(1)Secondary clay minerals as ion adsorption state.Compared with 1:1type clay minerals,the 2:1 clay minerals have stronger adsorption capacity for rare earth elements.(2)The nano-scale iron oxide as iron oxide bound state.(3)Weathered residual rare earth minerals as residual state,such as monazite and xenotime.