Lead Isotopic Composition and Lead Source in the Bainiuchang Ag‐polymetallic Deposit, Yunnan Province, China

AbstractThe Bainiuchang deposit in Yunnan Province, China, is located geographically between the Gejiu ore field and the Dulong ore field. In addition to the >7000 t Ag reserves, the deposit also boasts of large‐scale Pb, Zn and Sn reserves with a lot of dispersed elements (In, Cd, Ge, Ga, etc.). We have determined systematically the Pb isotope composition of the deposit. The Pb isotope ratios of the ores that are of sea‐floor exhalative sedimentary origin in the northwest of the mining district, are 206Pb/204Pb = 17.758‐18.537, 207Pb/204Pb = 15.175‐15.862 and 208Pb/204Pb = 37.289‐39.424, while those of ores that are of magmatic hydrothermal superimposition origin in the southeast of the mining district, are 206Pb/204Pb = 17.264‐18.359, 207Pb/204Pb = 14.843‐15.683 and 208Pb/204Pb = 36.481‐38.838, respectively. In terms of the Pb isotope composition of feldspar in magmatic rocks or magmatic whole‐rock samples from the mining district, we have determined the Pb isotope composition and acquired the Pb isotope ratios as: 206Pb/204Pb = 18.224‐18.700, 207Pb/204Pb = 15.595‐15.797 and 208Pb/204Pb = 38.193‐39.608. Then, in the light of the Pb isotope composition of metamorphic rock samples from the Proterozoic basement exposed in the Dulong ore field, we have determined the Pb isotope composition and obtained the isotope ratios as: 206Pb/204Pb = 18.434‐19.119, 207Pb/204Pb = 15.644‐15.693, and 208Pb/204Pb = 38.514‐38.832. And the Pb isotope ratios of Cambrian sedimentary rocks, which are exposed in the Bainiuchang mining district, are 206Pb/204Pb = 18.307‐19.206, 207Pb/204Pb = 15.622‐15.809, and 208Pb/204Pb = 38.436‐39.932. By comparing the two types of ores with respect to their Pb isotope compositions, it is indicated that lead in the Bainiuchang deposit was derived largely from the lower‐crust granulite which is earlier than Neoproterozoic in age, but the Yanshanian magmatic hydrothermal fluids probably provided a part of ore‐forming elements such as Sn for the ore blocks in the south of the mining district.