Fluid Inclusions Features And Genesis Of Beryls From Cuonadong Gneiss Dome,Southern Tibet

In recent years,a large number of beryl mineralization traces have been found near domes widely distributed in southern Tibet,of which the Cuonadong Gneiss Dome is a typical example.The Cuonadong Gneiss Dome is located in the eastern part of the Tethys Himalaya Belt.The core of the dome is mainly made up of leucogranite,with large amounts of intruded pegmatite veins.Beryl is abundantly distributed in pegmatite.The main rock-forming minerals in the rock mass are quartz,albite,dolomite,in addition,there are a small amount of biotite,potassium feldspar.The accessory minerals are mainly zircon,monazite.By scanning thin sections of tourmaline with SEM-EDS,it can be seen that tourmaline has obvious element zoning phenomenon.It means that it has undergone several stages of magmatic evolution.Considering the scheelite,cassiterite and other minerals related to W-Sn mineralization in this area,we concluded that the ore-forming body is highly fractionated.In this study,we identified a large number of gas-liquid two-phase inclusions and mineral inclusions in beryl.Raman spectroscopy of the gas-liquid inclusions of beryl shows that the gas phase of the inclusions is mainly composed of CO₂ and N₂,and the liquid phase is mainly composed of CO₃^2-?HCO₃^-?K⁺?SO₄^2-,which indicates that the ore-forming fluid is rich in many kinds of volatiles.The existence of these volatiles reduces the crystallization temperature and carries huge heat,which promotes the long-term slow crystallization of melts and the highly differentiated evolution of magma.Based on the study of beryl deposits in Xinjiang,South China,western Sichuan and Yunnan,it is found that these beryls also contain a large amount of volatile such as CO₂ and H₂O.In addition,these mineralizationrelated rock masses have undergone a high degree of crystallization differentiation,which is a highly differentiated granite.Zircon U-Pb dating and oxygen isotope studies of the Dala granite,which does not occur rare metal mineralization nearby,show that the Dala granite was formed in the main collision orogenic stage?about 43 Ma?,which is different from the Cuonadong leucogranite.The Cuonadong leucogranite was formed in the post-collision stage?about 20 Ma?and related to crust extension.Dala granite is formed by amphibolite melting in the process of crustal thickening,and undergoes partial melting of meta-argillaceous rocks to varying degrees.The Dala granite is not highly fractionated,so there is no beryl and other rare metals mineralization.In summary,beryl mineralization has two important factors,namely,highly differentiated magmatic system and volatile-rich ore-forming fluids,which have guiding significance for further prospecting.