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Fig. 4.8 a Pb isotopic ratios measured within Ordovician, Mesozoic and Cretaceous rocks versus
those measured in pre-mining (and uncontaminated) terrace deposits ( purple dashed line ). Pre-
mining terrace deposits appear to be composed of sediment from Mesozoic rocks with relatively
high Pb ratios and Ordovician rocks with low ratios (from Miller et al. 2007 ); b Pb Isotopic data
collected in 2000 from the modern channel bed ( red circles and brown squares ). High ratio end
member is consistent with isotopic values measured in ore deposits mined and milled tailings
from Cerro Rico, indicating Pb is derived from the mines and the underlying rocks (particularly
Ordovician Rocks) (from Miller et al. 2002 )
within the channel bed sediment downstream of the mills at Cerro Rico rapidly
changed through time (from 2000 to 2004). The alterations were attributed, in part,
to changes in the isotopic signatures of the ores that were mined and milled at Cerro
Rico. In 2000, the channel bed samples downstream of the mills exhibit 206 Pb/ 207 Pb
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