Study On Mineralization Of The Cuonadong Be-Sn-W Polymetallic Deposit,Tibet,China

The North Himalayan metallogenic belt has long been considered as a Pb-Zn-Ag-Au-Sb metallogenic belt characterized by low-to medium mineralization temperatures.Although significant attention has been paid to rare-metal mineralization in this belt,there has been no progress in prospecting.In 2016,on the basis of years of exploration and research,the Chengdu Center of China Geological Survey discovered the Cuonadong Be-Sn-W polymetallic deposit in the Cuonadong gneiss dome,center of the Zhaxikang ore cluster area.This deposit with super-large metallogenic potential is the first beryllium polymetallic deposit found in the Tethys Himalayan Pb-Zn-Ag-Sb-Au metallogenic belt.Thus,the discovery of this deposit opens a window for the exploration of rare-metal deposits in the Himalayan metallogenic belt.However,systematic study on the ore-forming process of this deposit has not been done so far.A mineralization and metallogenic model of the Cuonadong Be-Sn-W polymetallic deposit has been developed based on detailed field geological investigation and petrography,isotopic geochronology,stable isotope geochemistry,major and trace elemental geochemistry of whole rocks and individual minerals,and fluid inclusion microthermometry.The major achievements and understandings of this thesis are as follows:?1?Three episodes of leucogranites occurred in the Cuonadong dome.The monazite U-Th-Pb isotopic dating,together with available ages in literature,indicate that they were emplaced at 34-20 Ma?deformed two-mica granite?,20-18 Ma?garnet-bearing two-mica granite?and 16-15 Ma?garnet-bearing muscovite granite?,respectively.These leucogranites display a whole-rock Sr-Nd isotope similar to that of metapelite of the High Himalayan crystalline sequence and the boron isotope of magmatic tourmaline resembles that of continental crust.Geochemically,these magmatic rocks have high Si O₂ and Al₂O₃,low Mg O,Mn O and Fe₂O₃^T contents with aluminium saturation index?A/CNK?equal to or greater than 1.1,and are enriched in large-ion lithophile elements,while depleted in high field-strength element with a negative correlation between Rb/Sr ratio and Ba content.Besides,these rocks contain a large number of aluminum-rich minerals?such as muscovite,garnet,tourmaline,etc.?.These characteristics collectively indicate that the Cuonadong leucogranites are S-type peraluminous granite formed by muscovite-dehydration melting of metapelite of High Himalayan crystalline basement.The Cuonadong leucogranites have significant negative anomalies of Eu,Sr and Ti and tetrad effect of rare earth elements.The anomalies of Eu,Sr and Ti,the tetrad effect of rare earth elements increase while the content of dark-colored minerals,Ba and Sr decrease significantly from the weakly oriented two-mica granite,through garnet-bearing two-mica granite,to the garnet-bearing muscovite granite.These features,together with the fractional crystallization calculations,indicate that these leucogranites experienced sequentially enhanced fractional crystallization of feldspar,biotite,Ti-bearing minerals during their formation.The Cuonadong leucogranites have high Be,Sn and W contents,and change into Be-Sn-W-rich granite with evolution,together with their B-and F-rich and reductive features,which is indicative of rare metal mineralization potential.?2?The detailed field geological survey indicates that the Cuonadong Be-Sn-W polymetallic deposit is completely controlled by the Cuonadong gneiss dome,and has a close spatial relationship with the Cuonadong Cenozoic leucogranites.The deposit consists of four ore types:greisen-type Sn,pegmatite-type Be,skarn-type Be-W-Sn and hydrothermal vein-type Be-Sn-W.The cassiterite U-Pb and mica ⁴⁰Ar-³⁹Ar dating indicates that two metallogenic periods have occurred in the Cuonadong dome,including a 17-18 Ma-aged greisen-type Sn metallogenic stage and a 14-15 Ma-aged pegmatite-type Be,skarn-type Be-W-Sn and hydrothermal vein-type Be-Sn-W metallogenic stage.These ages are similar to the garnet-bearing two-mica granite?18?20 Ma?and garnet-bearing muscovite granite?16?15 Ma?,respectively.C-H-O isotopes of ore and gangue minerals,in-situ trace elements of scheelite,and Sr-Nd isotopes of scheelite and fluorite,collectively indicate that the ore-forming fluids of main mineralization stages of the Cuonadong Be-Sn-W deposit are magmatic in origin.S isotopes of sulfides from the Cuonadong Be-Sn-W polymetallic deposit are similar to those of the Cuonadong leucogranites,implying that leucogranites are the main source of ore-forming materials.Above data demonstrate that the earlier and later mineralizations were genetically related to the garnet-bearing two-mica granite and garnet-bearing muscovite granite,respectively.Microthermometry of fluid inclusions shows that boiling of the ore-forming fluid is the key factor to Sn mineralization of the greisen,the phase separation of magma during late stage of evolution controlled the Be mineralization of the pegmatite,intense water-rock interaction between ore-forming fluid and marble controlled the Be-W-Sn mineralization of the skarn,while cooling and addition of formation water caused the Be-Sn-W mineralization of the hydrothermal veins.