Geologic Feature And Discussion On Genesis Of The Cu-Zn-Mo Skarn Deposit In Chengbapu,Tibet

The Chengbapu deposit is the first Cu-Zn-Mo mineralization discovered in the Northern Gangdese Pb-Zn-Ag-Fe-Mo-W polymetallic belt(NGPB),which is located in the continental margin volcanic island arc tectonic zone in the eastern part of the middle section of the Gangdese tectonic magma belt.The Cu-Zn-Mo ore body were mainly developed at contact zones between limestone in Duodigou formation and granodiorite.Ore bodies are mainly occurred as layer,stratoid,lenticular and vein.The skarn,marbleization and hornstone alteration are generally developed in the surrounding rocks.Garnet in skarn is mainly composed of a series of andradite-grossularite solid solution.Ore minerals are mainly composed of chalcopyrite,sphalerite,molybdenite,pyrrhotite and so on.Granodiorite and granodiorite-porphyry asssociated with skarn mineralization were emplaced at 58.69±0.26Ma and 58.41 ±0.23Ma,respectively.This result coincides with the Re-Os dating result(Isochronous age:59.02±0.83Ma)of ore minerals molybdenite in skarm within the error range,indicating the diagenesis and mineralization of the Chengbapu deposit occurred in the main collision stage of the India-Asian collision.Granodiorite porphyry is a product of granodiorite undergone mineral differentiation of plagioclase and amphibole,according to mineral composition,main trace and isotopic compositions of the whole rock,and zircon Hf isotopes.The rock belong to quasi aluminous calc alkaline I-type granite.The rock enrich large ion lithophile elements such as Rb,Th,U,Pb and deplet high field strength elements such as Nb,Ta,Nd,Ti and so on.The rock have a significant REE fractionation.Compared to other contemporaneous ore deposits in the metallogenic belt,two sets of rock have relatively lower 87Sr/86Sr(i)value(0.7058?0.7063)and relatively higher ?Nd(t)value(-0.08?0.48)and ?Hf(t)value(+0.7-+8.0).There is a large amount of juvenile mantle material in the magma.Based on the calculation results of Hf isotopic,the contribution rate of mantle material in the magma source is up to 71%and the contribution rate of ancient crustal material in the magma source is less than 30%.Based on the calculation results of Ti content of zircon,its crystallization temperature is about 700?.Based on the calculation results of Ce anomalies of zircon,the average oxygen fugacity(fO2)is-12,and the average value of the ?FMQ is +5.18.And those show the diagenetic magma has characteristics of the high oxygen fugacity.The study of skarn minerals also coincides with this conclusion.In the deposit area,the end member of garnet changes from the mineralization crentre to the surrounding rock,the content of andradite decreasing and grossular increasing,and showing that the initial mineralization fluid have low pH and high oxygen fugacity,and with the distance increasing,the oxygen fugacity decreases,the pH increase.The high mantle contribution suggests that the magma source has a high content of chalcophlie element.The high oxygen fugacity is favor of extraction and migration of copper.The high oxygen fugacity magma can release relatively high oxygen fugacity fluid,which can carry the metallogenic material to the contact area of rock and limestone.The skarnization were formed and the metal components precipitated and mineralized,following.Combining the published data of predecessors and other data obtained from the author's participation in the project,the following conclusions can be obtained.The mineralization types of the northern Gangdese metallogenic belt are mainly controlled by crustal structure,magma source and magma nature.The continental collision of India and Asia caused the upwelling of asthenospheric material,which brought a lot of mantle material and heat to the Lhasa terrane.The juvenile lower crust of Southern Lhasa terrane and the ancient crust of Centre Lhasa terrane partially melt,and the ancient lower crust release metal Mo,the ancient The release of metal Mo in the ancient lower upper crust release metal Pb and Zn,the juvenile lower crust and mantle source release metal Cu and Fe.Those metal formed different metal deposits at different positions in the metallogenic belt.Meanwhile,the location of the juvenile and ancient crust junction like Chengbapu deposit is an important area for further prospecting of Cu-Zn deposits.