Geological And Geochemical Characteristics And Genesis Of The Xiaohuacha Uranium Deposit,Shanxi Province

The Xiaohuacha uranium deposit in ShanXi Province is a typical pegmatite-type uranium deposit,a newly discovered deposit type in recent years.It is located in the northeast of Danfeng uranium metallogenic region.Based on detailed geological mapping,the studies on the mineralogy,petrology,chronology and geochemistry of granitic rocks suggest the following conclusions:The deposit is hosted in the Lower Proterozoic Qinling Group and is controlled by the contact zone of the biotite granitic pegmatites and biotite gneiss.The uraninite in the pegmatite is mainly disseminated in the intergranular spaces between biotites peritectic,plagioclases,oligoclases and pyrites,forming symbiotic relationships with zircons,apatites retiles and monazites.The uraninite is normally surrounded by rutile and ilmenite,and some uraninite crystals occupy the center of host minerals.The intermediated-acidic magmatic rocks outcropped in the Xiaohuacha are of high potassic calc-alkaline and weak-peraluminous to peraluminous.These magmatic rocks are I-type granite according to the magmatic classification.The Huichizi and Gaoshangou granites yield LA-ICP-MS zircon U-Pb ages of444±4.0 Ma and 422±0.82 Ma,respectively.Uranium-bearing and non-mineralized biotite granitic pegmatites yield ages of 417±2.6 Ma and 413±1.8 Ma,respectively.The zircon Lu-Hf isotopic values of granitic rocks suggest a calculated two-stage model age(tDM2)of 1.4 Ga.The similar Hf isotopic characteristic values(?Hf(t)for granite and granitic pegmatite(-0.48 and-0.40,respectively)suggest a significant homology.Assimilation-contamination and element exchange between magma and wall rocks produced the high contents of basic components(Fe,Mg)and volatile matter(F)and the enrichments of LILE(Rb,Ba,K)and depleted HFSE(Nb,Ta)in uranium-baring granitic pegmatite.The composition differences between different parts of biotite granite pegmatites in the same dyke are mainly the products of the assimilation fractional crystallization of pegmatite magma.The element exchange within the contact zone between pegmatite and gneiss produced the decomposition ofU-F complex.Saturated U precipitated and formed uranium minerals(e.g.,uraninite)under favorable conditions.In summary,the formation of the Xiaohuacha uranium deposit was controlled by the combined effects of pegmatite magma,wall rocks and the volatile in magmatic hydrothermal fluid.