Geochemical Characteristics And Genetic Mechanism Of Weathering Crust Profile Of Xiangshan,Xiajialing Rare Earth Deposit In Jiangxi Facies

Xiangshan,Xiajialing rare earth deposit is a newly discovered weathering crust eluvial rare earth deposit located in Xiangshan basin of Jiangxi province.Based on detailed field geological investigation,the weathering crust section is divided into yellow soil layer,iron disk layer,red soil layer,spherical weathering layer and parent rock layer from top to bottom.First of all,in order to explore the lithology and characteristics of the original rocks of Xiangshan,Xiajialing rare earth deposit,the lithology,elemental geochemistry,chronology and Hf isotope geochemistry of the parent rock samples were studied by means of microscopy,XRF,ICP-MS,La-ICP-MS,MC-ICP-MS and other research methods,and the obtained research results were compared with the fragmentary porphyry lava of Xiangshan,Xiajialing.Results show:The characteristics of the major elements,microelement and rare earth elements in the original rocks of Xiangshan,Xiajialing rare earth mine are highly consistent with those in Xiangshan,Xiajialing crushed porphyry lava.The weighted average age of zircon 206Pb/238 U in the original rocks was 133.64±0.84 Ma(MSWD=1.20)and the Hf isotope two-stage model age(T2DM between 1680 and 1800 Ma)were consistent with the error range of the facies mountain porphyroclastic lava,indicating that the original rocks were rhyolitic porphyroclastic lava.Secondly,by means of microscope,XRD,ICP-MS and other research means,mineralogical and elemental geochemistry analysis of samples in each layer of weathering crust profile is conducted to find out the distribution characteristics of each element on this weathering crust profile and discuss the causes of its distribution.The results show that:(1)Different strata on the weathering crust profile of the lower Xiajialing facies have different geological and geochemical characteristics.The weathering degree of the rock gradually decreases from top to bottom,and the content of plagioclase,potash feldspar and biotite increases.In the weathering process,these minerals first form illite and then decompose to form kaolinite.(2)The rare earth minerals in Xiangshan,Xiajialing facies are classified as light rare earth elements,which mainly concentrate La,Ce,Nd and Y.In the crust of weathering profile,rare earth element content from yellow soil layer to the spherical weathering,in turn,increases,the main enrichment in spherical weathering,analysts say because of yellow soil layer,iron plate,clay minerals in the red soil layer and Fe(OH)3 gel combination,make its adsorption amount of rare earth elements greatly reduced resulting in rare earth elements in the spherical weathering enrichment.However,Ce is mainly enriched in the upper part of the weathering crust.Ce is obviously enriched in the yellow soil,iron disk and red soil,while it is deficient in the spherical weathering layer.The anomaly is not obvious in the parent rock.Finally,in order to study the enrichment mechanism of rare earth elements in the spherical regolith,the fine mineralogy of the spherical regolith was studied by scanning electron microscopy and electron probe.The enrichment mechanism is as follows:when the rare earth elements on the upper part of the weathering crust migrate to the spherical weathering layer along the cracks,most of the Ce elements have been fixed,and la-rich,relatively poor Ce and f-containing fluids are formed.The fluid enters the epidote-bearing minerals,in which the rare earth elements are fixed,causing the epidote-bearing minerals to be changed into hydroxycerium ore and hydroxylanthanum ore,and forming the rare earth mineral ring,resulting in the increase of the content of rare earth elements.With the deepening of weathering,the rare earth minerals are broken and corroded by melting.