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 Potosí 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 Potosí 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 Potosí ores, and the isotopic homogeneity of the Potosí ores also argues against mixing of lead from diverse sources in the hydrothermal system. Lead from the sedimentary rocks may have been incorporated by magmatic assimilation followed by extraction of ore metals from the resulting magma.

Lead isotope ratios of San Cristobal ores are different from those of Pulacayo, Potosí, and other deposits to the east, but resemble the compositions of ores and volcanic rocks in western Bolivia. On this basis we identify a new ore lead isotope province extending from San Cristobal northward across the eastern Altiplano and into southern Perú. This province is coincident with but smaller than the extent of the proposed Arequipa-Antofalla metamorphic basement craton. The degree of incorporation of ore metals from the metamorphic basement appears to depend on the timing and/or location of the mineralizing event. Ore deposits in the northern part of province IV formed before the thickening of Andean crust, beginning around 20 Ma, and incorporated minor amounts of metals from the metamorphic basement. Younger deposits farther to the south contain major to dominant components of basement lead.

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