WERE CENTRAL ANDEAN CRUSTALLY-DERIVED ORE METALS ASSIMILATED OR LEACHED? THE EXAMPLE OF POTOSI

Ore lead isotope ratios from the eastern Altiplano and Eastern Cordillera of Bolivia reflect compositions of shallow crustal rocks having high time-integrated U/Pb and Th/Pb. Geochemical and isotopic studies of volcanic rocks from this region also indicate substantial incorporation of continental crust. Lead isotope ratios of Eastern Cordillera polymetallic deposits, including the giant Potosí Ag-Sn deposit, generally increase with decreasing deposit age. This variation has been interpreted to reflect the episodic extraction of ore metals from an evolving, fairly homogenous source, thought to be the thick Paleozoic shale-dominated sedimentary sequence which immediately underlies Potosí and many other deposits in this region. A few published whole-rock lead isotope measurements of Ordovician-Silurian shales have supported this idea, with ratios near those of mid-to-late Tertiary ores. Whether metals were incorporated from the Paleozoic rocks by magmatic assimilation or by hydrothermal leaching has been an open question. Powders of the Paleozoic sedimentary rocks were leached using 0.5N HCl and 15% NaCl at 200°C. Lead isotope ratios of whole-rock powders, leachates and some residua were compared with each other, and with those of ores from the Eastern Cordillera. Whole-rock lead isotope data for the Paleozoic sedimentary rocks resemble the few published values and the compositions of mid-to-late Cenozoic ores, which have 206Pb/204Pb > 18.55. Lead isotope ratios of NaCl and HCl leachates from individual samples are very similar, often within error of each other. Both leachate types contain less radiogenic lead than the whole-rock powders, with 206Pb/204Pb ranging from 18.131 to 18.623. Leachates also have much lower µ than the whole rock samples, and leaching of black shale samples resulted in scavenging of blank-level U by the residue. Leachates of samples of the Paleozoic sedimentary section in the Bolivian Eastern Cordillera have lead isotope ratios which appear to rule out leaching as a significant source of metal in late Cenozoic ores, including Potosí. Incorporation of ore lead from the Paleozoic rocks probably occurred by magmatic assimilation.