Lead isotope geochemical characteristics of Pb-Zn-Cu deposits on the southwestern margin of Tarim, and their significance

The polymetallic （Pb, Zn, Cu, etc） ore belt on the southwestern margin of Tarim is one of the major re- gions with the greatest prospecting potential in Xinjiang. Reported in this paper are the lead isotope data for 66 sul- fide samples （including 50 galena samples, 15 chalcopyrite samples and 1 pyrite sample） collected from such repre- sentative deposits as Tamu, Tiekelike, Kalangu, Abalieke, etc. in this ore belt. The Pb isotopic ratios of 206pb/204pb, 207pb/20apb and 20spb/204pb in the galena samples range from 17.931 to 18.176, 15.609 to 15.818 and 38.197 to 38.944, with the average values of 18.017, 15.684 and 38.462, respectively. Those in the chalcopyrite samples range from 17.926 to 18.144, 15.598 to 15.628 and 38.171 to 38.583, with the average values of 18.020, 15.606 and 38.262, respectively. The pyrite sample has the Pb isotopic ratios of 206pb/204pb, 207Tpb/204apb and 208spb/204npb to be 17.980, 15.604 and 38.145, respectively. In combination with the previous Pb isotope data for sulfides, it is found that there is only a slight variation in the Pb isotopic composition of galena, chalcopyrite, sphalerite and pyrite in the ore belt. However, there is some difference in Pb isotopic characteristics between galena and chalcopyrite, especially the Pb isotopic composition of galena shows an obvious linear correlation with some other relevant parameters （e.g. AI3 and AT）. The comprehensive analysis suggested that lead in galena （maybe including sphalerite and pyrite） was derived principally from wall rocks and underlying basement, and that in chalcopyrite only originated from the basement. The single-stage model ages of these sulfides couldn＇t indicate the time limit of metallogenesis （Pb, Zn, Cu, etc.）, and the positive linear correlations for the Pb isotopic composition of galena are of no single-stage and two-stage Pb-Pb isochron significance. Furthermore, there are significant differences in Pb isotopic composition characteristics between the genetic type of deposits in this polymetallic ore belt and the Mississippi Valley type （MVT）. In addition, the authors also pointed out that there is a phenomenon of differentiation （not paragenesis） for lead and copper elements during the process of metallogenesis in this ore belt.