Genetic Relationship Between The Neoproterozoic Granite And Tin Deposit In Baotan Of Northern Guangxi, South China

The Baotan tin deposit was located in the Jiuwandashan-Yuanbaoshan tin-polymetallic area in southeastern Yangtze Block and western end of Jiangnan Orogenic Belt.It is one of the Neoproterozoic large-scale tin deposits in South China,spatially associated with the Pingying granitic pluton.Based on the previous studies,we carried out detailed works on field geological investigation,petrography,lithogeochemistry,zircon U-Pb chronology and Hf isotopic composition.This dissertation discussed the emplacement age,genetic type,source and ore-forming potential of the Pingying granite.Meanwhile,based on the studies of cassiterite U-Pb dating,and the chemical composition and boron isotopic of tourmaline,we discussed the timing of tin mineralization and the source of ore-forming materials and fluid in the Baotan deposit.Finally,we combined with the characteristics of the biotite granite in the Jiuwandashan-Yuanbaoshan area,explore the potential of tin mineralization in this area.The Pingying granite can be divided into three facies:the central facies of coarse-grained biotite granite,the transitional facies of medium-grained biotite granite and the margin facies of fine-grained biotite granite.In-situ LA-MC-ICP-MS zircon U-Pb dating gives a weighted mean 206Pb/238U age of 834±5 Ma for coarse-grained biotite granite in the central facies,indicating that it formed in the Neoproterozoic.The Pingying granite is peraluminous with high silica,alkali contents.From the coarse-grained biotite granite,to medium-grained biotite granite,then to fine-grained biotite granite,the contents of TiO2?FeOT?MgO??EREE?Eu/Eu*were gradually decrease;in the primitive mantle normalizaed spider diagrams,the positive anomalies of Cs,Rb,U and Ta and negative anomalies of Ba,Sr and Ti are increased;the Rb/Sr ratios increased,while the zircon saturation temperature gradually reduced.The ?Hf(t)values of zircon grains vary from-12.6?-1.6 with the peak at-4.3?-3.0,and two-stage Hf model ages range from 1.83 to 2.51 Ga with the peak at 1.9?2.0 Ga.These characteristics indicate that the Pingying granite was formed by partial melting of the Paleoproterozoic crustal basement,and experienced highly differentiation.According to the occurrence and spatial distribution of the ore bodies,the tin mineralization in the Baotan deposit can be divided into four stages:(1)the greisen stage;(2)the cassiterite-tourmaline-quartz-vein stage;(3)cassiterite-quartz-vein stage and(4)cassiterite-sulfide stage.In situ LA-ICP-MS cassiterite U-Pb dating on two samples from the cassiterite-tourmaline-quartz-vein and cassiterite-quartz-vein yield two tight 206Pb/238U weighted mean ages of 832±5 Ma and 834±4 Ma,respectively.These dating results are well consistent with the zircon U-Pb age of Pingying granite(?834 Ma)within errors.Three types of tourmaline occurrences have been distinguished in the Baotan tin district:(1)The stratiform tourmaline-quartz layers in the metasedimentary rock of Sibao Group(ST type);(2)Tourmaline-quartz nodules in the Pingying granite(NT type);(3)tourmaline-quartz veins in the hydrothermal mineralization stages(VT type),composed of tourmaline-quartz veins in the mafic rocks(VT-1 subtype)and in the metasedimentary(VT-2 subtype),respectively.Detailed electron microprobe chemical analyses(EMPA)for the above three types of tourmaline indicate that the ST type tourmaline belongs to dravite,which may origin from the sedimentary exhalative processes in the early Neoproterozoic,while the NT-and VT type tourmaline belong to schorl.The NT type tourmaline has higher ratios of Fe/(Fe+Mg)and Na/(Na+Ca)and F contents with values of 0.86,0.94 and 0.31 wt%,respectively.Combined with the petrographic characteristics of tourmaline,we suggest that the NT type tourmaline formed in the magma to hydrothermal stage.By contrast,the VT type tourmaline has lower ratios of Fe/(Fe+Mg)and Na/(Na+Ca)and F contents,so we suggested that it formed in the hydrothermal mineralization stage with higher oxygen fugacity.While the different ratios of Fe/(Fe+Mg)in the VT-1 subtype(mean of 0.74)and VT-2 subtype(mean of 0.57)mainly controlled by the different surrounding rocks.In addition,the boron isotope analyses of NT-and VT type tourmaline by in-situ LA-MC-ICP-MS methods reveal that they have similar ?11B values,range from-14.14 ‰ to-8.44 ‰,most of in a narrow range between-13 ‰ to-12 ‰,indicating that both of them mainly derived from the granitic magma.While,the fine-grained tourmaline with zonal texture from VT type has higher values of the ?11B,from-10.71 ‰ to-8.44 ‰,indicating that the boron isotope took place a slight fractionation in the later hydrothermal stage.It may controlled by the lower temperature in the system.Detailed study on the Pingying granite and the Baotan tin deposit shows that the Baotan tin deposit has a close spatio,temporal and genetical relationship with with the emplacement of the Pingying granite.It is also suggest that other Neoproterozoic biotite granites with high degrees of differentiation in the Jiuwandashan-Yuanbaoshan area probably has a good potential of tin mineralization.