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Genesis of porphyries and tectonic controls on the Narigongma porphyry Mo(-Cu)deposit,southern Qinghai.

YANG ZhiMing~1 HOU ZengQian~1 YANG ZhuShen~2 WANG ShuXian~3 WANG GuiRen~4 TIAN ShiHong~2 WEN DeYin~3 WANG ZhaoLin~1 LIU YingChao~2 1.Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China 2.Institute of Mineral Resources,Chinese Academy of Geological Sciences,Beijing 100037,China 3.National Research Center for Geoanalysis,Beijing 100037,China 4.Qinghai Geological Survey Institute,Xining 810012,China  
A large porphyry (-skarn) type mineralization belt,located at Southern Qinghai,is manifested by recent preliminary exploration.Narigongma porphyry Mo (-Cu) deposit is the most important deposit in the belt,so characteristics and origin of the porphyries related to the deposit,mainly composed of granite-diorite porphyry,is pivotal to understanding the relationship between the belt and the Yulong porphyry belt.For this purpose,detailed petrological,element geochemical,Sr-Nd-Pb isotopic and radioactive dating data were carried out for the porphyries.The SHRIMP U-Pb zircon dating result defines emplacement of the porphyries occurred at 43.3 Ma ago,which indicates that the deposit is the northern extension of Yulong copper belt.Geochemical data indicate that these ore-bearing porphyries are high K calc-alkaline,with highly evolutional as shoshonitic,and exhibit ealc-alkaline composition with high Sr/Y and La/Y coupled with low Y and HREE,showing some affinities with the adakite.Sr-Nd-Pb isotopic data indicates that the porphyries were derived by partial melting of crust-mantle transition belt at a depth between 50 and 80 kin.In comparison with porphyries at the Yulong belt,porphyries at Narigongma are charactered by much lower K_2O content and more depleted Sr-Nd-Pb isotopic,reflecting more and more addition of the crust constituent in the source from northwest to southeast.Additionally,from Narigongma to Yulong,formation ages of ore-bearing porphyries decrease systematically with a duration of magmatism of about 7 Ma, which indicates that formation of the porphyries at Narigongman,as well at Yulong belt,was controlled by a uniform dynamic mechanism.An Eocene regional-scale dextral strike-slip fault system caused by the India-Asia continental collision may be the key factor triggered melting of the crust-mantle transition belt.Metal assemblages at Narigongma are dominated by Mo-Cu,which is different from Cu-Au assemblages at Yulong belt.This difference of metal assemblages may be caused by deep magma process,as well as shallow crystalline process.The later process is evident by more and more facts.If it is fact,Narigongma should be a deeply denudated deposit,and porphyry Mo and skarn type deposits should be focused in the next exploration in the area.
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