Petrogenesis And Metallogenic Significance Of Azhalang Intrusion In Songdo Area,Tibet

Porphyry-type precious metals,non-ferrous metals deposits and related porphyry intrusions in the Gangdise metallogenic belt have always been the focus of research on the Qinghai-Tibet Plateau.On the basis of geological survey and geological profile measure in Songduo area of southern Tibet,the Azhalang porphyry intrusion and adjacent Demingding copper-molybdenum deposit are studied.By zircon U-Pb dating,whole-rock geochemical analysis and zircon Hf isotope analysis,the petrogenesis,material source area and tectonic background of Azhalang porphyry are discussed.Some properties of the Demingding copper-molybdenum deposit have been involed,including geological characteristics,fluid inclusion and Re-Os isotope dating,as well as its genetic type of the deposit,the space-time and genetic relationship with the Azhalang porphyry.The study results show that the lithology of the Azhalang porphyry intrusion is mainly quartz monzonite porphyry and granite porphyry,and its formation age is Miocene?quartz monzonite porphyry:17.9±0.2 Ma;granite porphyry:18.4±0.3 Ma?.the quartz monzonite porphyry geochemistry shows high Sr?1052¹150 ppm?,low Y?8.51⁹.04 ppm?and Yb?0.85⁰.94 ppm?,high Sr/Y?118¹28?and La/Yb?30.9⁴0.8?respectively,and there is no obvious Eu anomaly.It has higher content of K₂O?3.17³.84%?,lower content of Cr?6.46⁷.78 ppm?and Ni?5.41⁷.45 ppm?,higher Rb/Sr and Mg^#?43.8⁴9.8?,and Rb,Ba,Th and U are obviously higher than LREE.They have typical geochemical characteristics of adakitic rocks.These characteristics can be used to judge the genesis of rocks that may be formed by partial melting of the thickened lower crust.The magma source area mainly comes from the components of partial melting of the new crust,and there may be some mantle-derived materials involved.The Azhalang porphyry intrusion formed in the post-collisional tectonic setting of the Indo-Eurasian continental collision orogeny.The Demingding copper-molybdenum deposit occurs within and contact zone of porphyry.The main metallic minerals in the ore are molybdenite,pyrite,chalcopyrite,malachite and bluecopper.The ore structure is mainly veinlet disseminated,veinlet and chrysanthemum.There are three main types of fluid inclusions in quartz associated with metal minerals:liquid-rich?L-type?,gas-rich?V-type?and multiphase inclusions?S-type?.The homogenization temperatures of inclusions are mainly concentrated in140²80?,260³40?and 380⁴60?,which have three distinct stages of hydrothermal mineralization.The ore-forming fluids are NaCl-H₂O system with medium-high temperature,medium-high salinity and rich ore-forming elements.From early to late stage,the homogenization temperature,salinity,density and oxygen fugacity of inclusions tend to decrease gradually.During the early-middle metallogenic stage?Main metallogenic stage?,there is a distinct boiling phenomenon in the fluid.The ore-forming pressure is 553.3¹369.1 MPa,and the corresponding depth is 1.8⁴.6 Km.The isochron age of molybdenite?20.41±0.55 Ma?is consistent with the weighted average age of model age?20.51±0.14 Ma?,which is similar to the zircon U-Pb age of ore-bearing granite porphyry?18.4±0.3 Ma?.Based on comprehensive study of space-time relationship of deposit and porphyry intrusion and fluid inclusions in quartz veins,the Demingding copper-molybdenum deposit is determined to be a porphyry type deposit,and the granite porphyry is the metallogenic porphyry.The data and results of this study provide important constraints for revealing the genesis and metallogenic geological background of the Miocene adakite in the Gangdise metallogenic belt.