Sources of Lead in the San Cristobal, Pulacayo, and Potosi Mining Districts, Bolivia, and a Reevaluation of Regional Ore Lead Isotope Provinces

Abstract

New lead isotope data on ores, crustal rocks, and leachates of crustal rocks, combined with data in the literature, provide important new constraints on the sources of ore metals in southwest to south-central Bolivia, including the very large recently discovered silver-zinc deposit at San Cristobal, the Pulacayo polymetallic district, and the giant Potosi silver-tin-base metal deposit. Lead isotope ratios of ores and igneous rocks from the San Cristobal deposit and from Paleozoic and Cretaceous sedimentary rocks are compared with published data on high-grade Middle Proterozoic metamorphic basement rocks. These data constrain the major source of lead, and by inference of other ore metals, at San Cristobal to be the metamorphic basement rocks. Leaching experiments on samples of Paleozoic and Cretaceous sedimentary rocks show that the easily leachable lead from these rocks is much less radiogenic than the whole-rock compositions. However, lead isotope ratios of both whole rocks and leachates of these upper crustal rocks are too radiogenic for them to be major sources of ore lead at San Cristobal. Lead isotope ratios of ores from Pulacayo and Potosi are similar to each other and lie within the range of Paleozoic and Cretaceous sedimentary whole-rock compositions. Leaching of Pb from the sedimentary rocks cannot explain the isotopic compositions of the Pulacayo and Potosi ores, and the isotopic homogeneity of the Potosi ores also argues against mixing of lead from diverse sources…