Geochemical Characteristics And Genesis Of Granite Porphyry In Tangba Gold Deposit,Wudu,Gansu

The qinling orogenic belt is located between the Yangtze plate and the north China plate and is an important part of the central orogenic belt of the Chinese mainland.Along the mian-lue suture zone at the southern edge of the qinling orogenic belt and on both sides,a400km long Indochina granite belt with east-west distribution was developed.The xiqinling orogenic belt belongs to the western segment of the qinling orogenic belt,and Tangba gold mining area is located in the eastern segment of the southern metallogenic subbelt of the west qinling gold belt,and is located to the south of the xinguan-hejiaba-liping fault and to the north of the wenxian-kangxian-liuyang fault.Gold ore bodies are mainly distributed in fault structures near the footwall of granite porphyry,or in the interior of granite porphyry.Therefore,the study of granitic magmatism and geotectonic setting of vein rocks in the study area can provide the basis for the genesis,metallogenic era and metallogenic geological background of the deposit.Based on the detailed field geological survey of the granite porphyry in Tangba gold mining area,this paper conducted a comprehensive analysis and research on rock mineralogy,rock geochemistry and zircon u-pb chronology,and discussed the formation age,genetic type,material source and diagenetic dynamic background of granite porphyry.The following cognition results are obtained:(1)Through U-Pb dating of LA-ICP-MS zircon,the age of granoporphyry formation in Tangba gold deposit was 212.8Ma±2.7Ma,which was consistent with the main age of granoporphyry formation in the west qinling mountains(220?200Ma)and was the product of magmatism at the end of Indo-chinese.(2)Granite porphyry in the study area is generally a calc-alkaline series of over-aluminiferous rocks.In the A/CNK-A/NK discriminant diagram,Zr-Ti O₂ discriminant diagram and Si O₂-P₂O₅ diagram,it is shown that granite porphyry has the characteristics of s-type granite.Specific manifestations:high silicon(w(Si O₂)is 70.84%?75.23%),rich aluminum(w(Al₂O₃)is 14.59%?16.08%),and the average value of A/CNK is 1.16,greater than 1.1.The distribution model of rare earth is right inclined,and the light rare earth fractionation is obvious.LREE/HREE is between 3.98 and 11.98,and(La/Yb)N is between3.77 and 18.6,with obvious negative europium anomaly,and obvious deficit of Ta,Nb,P and Ti elements with high field strength.(3)The Mg^#value of granite porphyry in the study area is between 24.95 and 35.90(mean 30.47)and less than 45,indicating shell source characteristics.The Nb/Ta value between 8.90 and 12.31(mean 10.29)was consistent with the continental crust mean value(11),and the Sm/Nd mean value was 0.21 less than 0.3,indicating that the diagenetic material may have originated from the continental crust.The mean value of Ca O/Na₂O was 0.42,greater than 0.3,indicating that the composition of the source area might be related to the melting of the complex sandstone.The discriminant diagrams of Rb/Sr-Rb/Ba and A/MF-C/MF source areas also show that the granitic porphyry source rocks may be metamorphic complex sandstones.Granitic porphyry is formed by partial melting from metamorphic sandstones.(4)Granitic porphyry in the study area is characterized by post-collision magmatic activity.Under the mechanism dominated by extensional decompression and melting after the collision and extrusion of the north China plate and the Yangtze plate in the Indochina period,the magma with mixed sandstone as the main part of melting was rapidly and highly embedded and formed at the stage of transformation from the extruded environment to the extensional environment.