LEAD ISOTOPIC COMPOSITION AND GENESIS OF PHANEROZOIC METAL DEPOSITS IN CHINA
Chen Yuwei Mao Cunxiao Zhu Bingquan
Lead isotopic compositions of more than 130 galena specimens from Phanerozoic metal deposits in China have been analysed with a standard deviation of less than 0.5%. The ranges of isotopic composition are 17.8 to 19.3 for 206Pb/204Pb, 15.2 to 16.0 for 207Pb/204Pb and 37 to 40.8 for 208Pb/204Pb, which overlap those of MOEB, pelagic chemical sediments and intra-oceanic arcs of Western Pacific in plots of 207Pb/204Pb v.s. 206Pb/204Pb and 208Pb/204Pb v.s. 206Pb/204Pb This indicates that the mixture betweenmantle and crustal materials due to partial melting of the Pacific subduction slab is the principal mechanism for ore genesis. On the basis of global Precambrian conforma-ble lead, together with that of Guanmenshan, Pengjiabu and Baiyun deposits in China, it is suggested that mantle lead developed in a single stage with a (238U/204Pb) value of 7.8. If the ages of igneous rocks associated with these deposits are adopted as forma-tion ages, the mean and Th/U of the second stage of crust-mantle evolution can be calculated with a lead mixing model. The mean of China continent is 9.98 ± 0.78, which is in agreement with that of global conformable lead, 9.81 ± 0.58. The mean Th/U ratio of 4.27 is distinctly larger than the global value (3.81), indicating that China continent is rich in Th. Simultaneously, mean formation ages have also been obtained of the continental crust in various regions ranging from 3.0 to 3.8 AE and from 2.5 to 2.7 AE, which are comparable with the age distribution of global Archaean cratons.