An estimate of the lead isotopic compositions of upper mantle and upper crust and implications for the source of lead in the Jinding Pb-Zn deposit in Western Yunnan, China

Abstract

The Jinding Pb-Zn deposit occurs in the Tertiary red sedimentary formation of the Cenozoic continental fault depression basin at Lanping, western Yunnan. The orebodies are stratiform and stratoid in form and giant in size, with the associated Sr and Cd being of high grade. In the past thirty years the source of lead in this deposit has been a long-standing controversial problem. Some scholars considered that lead came from the upper mantle while others shed doubt on that viewpoint. However, no conclusive evidence has been obtained to support their respective viewpoints, and to find a more definite answer to the above question. We have re-examined the lead isotopic composition of the Jinding Pb-Zn deposit. The lead isotopic ratios of the striped ore are ²⁰⁶Pb/²⁰⁴Pb = 18.138–18.552, ²⁰⁷Pb/²⁰⁴Pb = 15.345–15.552, and ²⁰⁸Pb/²⁰⁴Pb = 37.803–38.443, and the lead isotopic ratios of the massive ore are ²⁰⁶Pb/²⁰⁴Pb = 18.394–18.600, ²⁰⁷Pb/²⁰⁴Pb = 15.583–15.767, and ²⁰⁸Pb/²⁰⁴Pb = 38.266–39.046. In terms of the lead isotopic composition of minerals and whole rock samples from basic-ultrabasic rocks and alkaline rockbodies in the region of western Yunnan, we have determined the lead isotopic composition of the upper mantle in this region at Meso- and Cenozoic times, as ²⁰⁶Pb/²⁰⁴Pb = 17.850–18.350, ²⁰⁷Pb/²⁰⁴Pb = 15.430–15.540, and ²⁰⁸Pb/²⁰⁴Pb = 37.800–38.410. Then in the light of the lead isotopic composition of metamorphic rocks and sedimentary rocks from Precambrian to Tertiary, which are exposed in the Lanping Basin and its adjacent areas, we have determined the lead isotopic composition of the upper crust, as ²⁰⁶Pb/²⁰⁴Pb = 18.420–19.200, ²⁰⁷Pb/²⁰⁴Pb = 15.500–16.010, and ²⁰⁸Pb/²⁰⁴Pb = 38.450–39.500, and several mineral samples from granulite and eclogite xenoliths yielded the lead isotopic information of the lower crust, as ²⁰⁶Pb/²⁰⁴Pb = 18.135–18.187, ²⁰⁷Pb/²⁰⁴Pb = 15.335–15.388 and ²⁰⁸Pb/²⁰⁴Pb = 37.836–37.899. A comparison has been made with the lead isotopic composition of the ore, and it is indicated that lead in the Jinding Pb-Zn deposit was not derived from the upper mantle, and there are two sources of lead: one is the upper crust and the other is the lower crust.