Lead Isotopic Composition and Genesis of Phanerozoic Metal Deposits in China

<正> Lead isotopic compositions of more than 130 galena specimens from Phanerozoic metaldeposits in China have been analyzed with a standard deviation of less than 0.5%.The rangesof isotopic composition are from 17.8 to 19.3 for Pb206/Pb204,15.2 to 16.0 for Pb207/Pb204and37 to 40.8 for Pb208/Pb204.These values overlap those of MORB,pelagic chemical sedimentsand intra-oceanic ares of western Pacific in plots of Pb207/Pb204vs.Pb206/Pb204and Pb208/Pb204vs.Pb206/Pb204.This indicates that mixing between mantle and crustal materials due topartial melting of the Pacific subduction slab is the principal mechanism for ore genesis.Onthe basis of global Precambrian conformable leads,together with that of Guanmenshan,Peng-jiapu and Baiyun deposits in China,it is suggested that mantle lead was developed in asingle stage with a(U238/Pb204)value of 7.8.If the ages of igneous rocks associated withthese deposits are adopted as formation ages,the mean Th/U of the second stage of crust-mantle evolution can be calculated with a lead mixing model.The mean μ value of Chinacontinent is 9.98±0.78,which is in agreement with that of global conformable leads,9.81±0.58.The mean Th/U ratio of 4.27 is distinctly larger than the global value（3.81）,indicat-ing that the China continent is rich in Th.Simultaneously,mean formation ages have also-been obtained of the continental crusts in various regions,ranging from 3,000 to 3,800 m.y-and from 2,500 to 2,700 m.y.which are comparable with the age distribution of global Ar-chaean cratons.