Abstract

Lead isotope values of ore and host-rock samples from 18 deposits of nine mining districts in an Andean transect between latitudes 9 degrees and 14 degrees S are reported. The ore deposits discussed in this paper comprise a relatively narrow age span of about 200 m.y. (Upper Triassic to early Pliocene). The lead isotope ratios from different ore deposits correlate closely with their geotectonic settings. There is a systematic increase of radiogenic lead isotope ratios from west to east, i.e., from ore deposits located near the coast (in the Lower Cretaceous marginal basin of central Peru) to ore deposits in the carbonate platform at the margin of the Brazilian shield. For the coastal deposits Ral, Condestable, and Los Icas the lead may be mantle derived and was supplied via the Casma volcanics. The more radiogenic lead of the Leonila Graciela-Santa Cecilia ores may be related to lateral facies variations within the marginal basin. It is assumed that here a mantle-derived component was mixed with a crustal component. The lead isotope ratios from ore deposits in the Western Cordillera mainly indicate a contribution from the orogen reservoir. Leaching of the Paleozoic basement is probably the main source. Igneous activity may have facilitated remobilization processes during younger tectonic and magmatic cycles. Mixing between upper crust and orogen reservoirs with participation of magmatic lead can explain the lead isotope ratios of the ore deposits in the Atacocha district. The lead isotope signature of the Shalipayco ores is interpreted in relation to the Permo-Triassic Mitu volcanism. Leaching of Permian rift-related volcanics could be responsible for the less radiogenic lead in this deposit. The ores of the San Vicente deposit are characterized by highly radiogenic values and negative model ages, typical of upper crustal leads in cratonic regions. The ore lead of San Vicente could have been derived by leaching clastic sediments derived from Precambrian upper crust (Brazilian shield).

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