| The Dayishan pluton is located in the middle part of Nanling metallogenic belt,which is one of the important tin ore prospective areas in South China.Wujiaping tin ore is a newly discovered tin deposit in the Dayishan area in recent years,and is hosted within a pluton with vein（belt）output,which is strictly controlled by NE-oriented fractures.In order to determine the rock-forming and ore-forming age,magma source area,evolution and deposit genesis of the Wujiaping tin deposit,this paper conducted chronological,petrological,petrographic and isotopic geochemical studies on two types of granites and ore-bearing quartz veins in the mine area based on extensive field investigations and the combined use of various analytical and testing techniques.We have precisely determined the age of tin ore formation and mineralization,discussed the composition of the magma source area,the genesis of the deposit and the tectonic background,and pointed out the direction of ore search in this area.A wealth of basic information on mineralogy is reported,providing an important reference basis for summarizing regional metallogenic regularity.The zircon LA-ICP-MS U-Pb ages of the middle to coarse-grained porphyritic biotite monzogranite and middle to fine-grained porphyritic biotite monzogranite are157.1±1.1～155.5±0.5Ma and 154.5±1.6～153.7±1.2Ma,respectively.The 40Ar-39Ar plateau age of sericite in tin ore is 150.4±0.9 Ma,and the Rb-Sr isochron age of quartz inclusions is148.1±2.1 Ma.It is determined that the rock-forming and ore-forming age of the area are both early Yanshanian,and the late medium to fine grained porphyritic biotite monzonite has genetic relationship with the Wujiaping tin deposit,providing a new idea for regional ore prospecting.Petrogeochemical analysis shows that the middle to fine-grained porphyritic biotite monzogranite and middle to coarse-grained porphyritic biotite monzogranite have similar geochemical characteristics:（1）Plutons as a whole are characterized by high silica and alkali,poor calcium and magnesium,and overall high aluminum saturation index A/CNK values.（2）The Ga/Al（×10~4）ratios of the two types of granites are 3.01～3.77 and 2.74～2.97,respectively,and the Zr+Nb+Ce+Y values are 138.7×10-6～209.4×10-6and177.7×10-6～214.3×10-6,respectively,enriched with large ion lithophile elements such as Rb,Th,U and Nd,and deficient in Ba,Sr,Nb,Ti,and other high-field strength elements.（3）The CIPW standard mineral corundum content is 2.46%～3.11%,2.30%～2.30%,respectively,and the zircon saturation temperature is concentrated between 702.1°C～723.3°C and688.1°C～722.2°C.（4）The rare earth element content,LREE/HREE ratio and（La/Yb）N ratio are 94.9～151.1×10-6,133.9～165.0×10-6,2.83～3.67,3.12～3.93,1.68～2.41 and 2.04～2.19,respectively,and theδEu values are both very low,0.018～0.025,0.017～0.032,respectively.,all of which are europium-deficient.Combining with isotope and existing data,it is determined that both types of granites in the area are S-type granites.Isotopic studies show that the initial values of Sr isotopes of the two types of granites in the Wujiaping mine range from 0.70251 to 0.71713,and theεNd（t）values range from-5.62 to-7.94,both of which are negative values.The Hf isotope analysis results of the two types of granitesεHf（t）values are 0.37～8.50 and-6.24～3.35 intervals respectively,indicating that both originated from the melting of metamorphic supracrustal rocks in the Mesoproterozoic of the Cathaysia block with a minor contribution from the juvenile crust.The Hf isotopic TDM2values of the two types of granites range between 662～1180 Ma and 1431～1599 Ma,respectively,showing a significant gap reflecting the proportion of contribution from the juvenile crust.The Nd isotope TDM2values are 1491～1540 Ma and 1537～1566 Ma respectively,implying that the source rocks were extracted from the mantle reservoir in the Mesoproterozoic.Combining the tectonic evolution and magmatic activity sequence in the Dayishan area,this thesis suggests that the Wujiaping tin deposit was formed in an extensional intracontinental environment associated with the subduction of the Pacific plate. |
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