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Molybdenite Re-Os and SHRIMP zircon U-Pb dating of Duobuza porphyry copper deposit in Tibet and its geological implications

SHE HongQuan1,LI JinWen1,MA DongFang2,LI GuangMing2,ZHANG DeQuan1,FENG ChengYou1,QU WenJun3 and PAN GuiTang2 (1 MLR Key Laboratory of Metallogeny and Mineral Assessment,Institute of Mineral Resources,CAGS,Beijing 100037, China;2 Chengdu Institute of Geology and Mineral Resources,China Geological Survey,Chengdu 610082,Sichuan,China;3 National Research Center of Geoanalysis,Beijing 100037,China)  
The Duobuza porphyry copper deposit,the first large-size porphyry copper deposit discovered along the Bangong-Nujiang suture zone,is situated in the Duobuza tectonic-magmatic zone on the southern margin of the Qiangtang-Sanjiang massif.The main mineralized porphyry in the ore deposit is a granodiorite porphyry,whose SiO2 content varies from 61.37% to 67.73%,65.16% on the average.Its REE content is (41.4~94)×10-6,and the LREE content is higher than the HREE content,suggesting the enrichment of LREE.As for trace elements,the porphyry shows enrichment of Rb,K,Th,Ba,La,Ce,Sr and depletion of Nb,Ta,P,Ti.The major,rare earth and trace elements of the mineralized porphyry have characteristics of volcanic rocks formed in the island arc.The zircon SHRIMP dating (14 samples) of Duobuza granodiorite porphyry gave 206Pb/238U concordant ages from (116.5±4.6) Ma to (125.3±4.7) Ma,with a weighted mean age of (120.9±2.4) Ma,which indicates the age of the copper-mineralized porphyry rock.Meanwhile,the 187Re and 187Os content of molybdenite from the ore deposit was determined by Inductively Coupled Plasma Mass Sperctrometry (ICP-MS),and the results show that the Re-Os model ages of the deposit are from (117.6±1.3) to (118.5±1.4) Ma,and its isochron age (118.0±1.5) Ma,which have accurately determined the timing of mineralization.The age of the ore deposit is consistent with the evolution of the Bange-Nujiang archipelagic arc-basin system in Early Cretaceous,implying that the Duobuza copper deposit was formed in an archipelagic are-basin system similar to the system of southeast Asia.
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