Zircon U-Pb Ages,Geochemistry of the Porphyries from the Duobuza Porphyry Cu-Au Deposit,Tibet and Their Metallogenic Significance

The Duobuza porphyry Cu-Au deposit is the first discovered porphyry copper deposit in the middle of the Tibet plateau.Two ganodiorite porphyry phases and a late diorite porphyry phase were emplaced in the Duobuza porphyry deposit,with the second granodiorite porphyry phase being the main mineralized porphyry.Zircon U-Pb dating,bulk rock geochemical compositions and Sr-Nd-Hf isotopic compositions of these porphyries were analyzed.Zircon U-Pb dating results of the second ganodiorite porphyry phase and the late diorite porphyry phase show that these porphyries were emplaced at 120 Ma.The former two porphyry phases are enriched in Light Rare Earth Elements and Large Ion Lithophile Elements,and depleted in Heavy Rare Earth Elements and High Field-Strength Elements,with weak Eu anomalies,indicating that these granodiorite porphyries resemble arc magmas in composition.These granodiorite porphyries have high SiO2,Al2O3 and low MgO content,together with the high Sr and low Y content,showing the adakitic affinities.These two granodiorite porphyry phases have(87Sr/86Sr)ivalues of 0.7057~0.7062 and 0.7059~0.7064,andeNd(t)(0.2~3.3)andeHf(t)values of-2.5~-1.7and-6.2~-4.0,together with theeHf(t)varies of the second granodiorite porphyry vary within 3.7~7.5,indicating that the former two granodiorite porphyry phases were derived from juvenile lower crust and some mantle-derived materials were mixed in the source region.The late diorite porphyry show similar petrological features with these granodiorite porphyry phases,but have higher Na2 O,MgO contents and Y,Yb concentrations,suggesting that the diorite porphyry perhaps have the same source region to these granodiorite porphyries,but more mantle-derived materials were mixed in.These geochemical data indicate that the formation of the Duobuza deposit might have close relationship with the northward subduction of the Neo-Tethys Ocean.Several porphyry Cu deposits were formed within ca.118~120 Ma in the Duolong metallogenic district,the epoch of ca.118~120 Ma should be an important porphyry CuAu mineralization stage in the middle of Tibet plateau,this epoch is very close to the extrusive age of the continental andesite(the Meiriqiecuo Group),thus these porphyry deposits should be produced in the continental arc setting.