Isotopic composition of Pb in Central Andean ore deposits
Published:January 01, 1981
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George R. Tilton, Robert J. Pollak, Alan H. Clark, Ronald C. R. Robertson, 1981. "Isotopic composition of Pb in Central Andean ore deposits", Nazca Plate: Crustal Formation and Andean Convergence, La Verne D. Kulm, Jack Dymond, E. Julius Dasch, Donald M. Hussong, Roxanne Roderick
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Pb-isotope ratios are reported for 21 specimens of sulfides from 17 hydrothermal ore deposits, representative of several richly mineralized areas of the ensialic Central Andes. The deposits range in age from Middle or Late Triassic to mid-Miocene—embracing much of the time span of Andean metallogenesis—and include members of the copper deposit-dominated Cordillera Occidental and of the northern domains of the tin belt of the Bolivian Cordillera Oriental.
Pb from five deposits of Paleocene-Eocene age in northern Chile (lat 26° to 28°S) yields positive correlations of 207Pb/204Pb and 208Pb/204Pb with 206Pb/204Pb. The slope of the regression line in the 207Pb/204Pb versus 206Pb/204Pb diagram is too great to correspond to a meaningful Pb-Pb age; instead, this slope is ascribed to mixing between two dominant sources of Pb. There is no relationship between the Pb-isotope ratios in ore and the age or rock type of the local, pre-Andean, basement terrane. In isotope correlation diagrams, published Pb data from coeval igneous rocks of the area generally plot in the same field as the Pb from ores, but show greater scatter. These observations suggest that the ore Pb and rock Pb are cogenetic and that ore Pb has not been affected by interaction between hydrothermal fluids and the underlying basement rocks.
In isotope correlation diagrams, both ore Pb and rock Pb plot distinctly above the regression lines for Pb from oceanic volcanic rocks; hence, models in which such volcanic rocks are the sole source of Pb must be eliminated, Pb from pelagic sediments of the Pacific Ocean and Nazca plate are systematically displaced below the ore regression line and therefore cannot account for the ore Pb. Mixtures of sediment and volcanic rock Pb likewise cannot produce the Pb-isotope ratios of the ore. The ore Pb is markedly enriched in radiogenic 207Pb relative to comparable assemblages in most other circum-Pacific segments, which seems to require significant contributions of Pb from old sialic crust. The crustal component may contain Pb from subducted pelagic sediments, but an additional component enriched in 207Pb is required to account completely for all of the ore Pb data. The Pb-isotope relations for the northern Chile ores are ascribed to mixing of a mantle component, which could be derived from subducted oceanic basalt, and a more radiogenic Pb derived from upper or intermediate continental crust. In the case of the igneous rocks, parameters such as the bulk chemical composition require that the crustal Pb component was introduced by selective contamination rather than by bulk mixing of the parent materials. Pb is therefore inferred to have behaved as a relatively mobile element during magma genesis.
Parallel experiments are reported from the northern Bolivian tin belt, where tin and tungsten mineralization is associated, in part, with granites displaying strong evidence of interaction with metapelitic crustal rocks. Two subprovinces are distinguished on the basis of ore Pb isotopes, as well as age. The Oligocene-Miocene ores in both the tin belt and the contiguous Altiplano have Pb-isotope ratios that plot in isotope correlation diagrams approximately along the same regression lines defined by the Chilean ores. The Triassic-Cretaceous ores, however, define regression lines that are displaced toward lower 206Pb/204Pb. In the 207Pb/204Pb versus 206Pb/204Pb diagram, the separation of the regression lines corresponds closely to the age difference. No rock Pb data are available from the tin belt. The Bolivian trends are explained in the same manner as those of the Chilean ores, that is, by mixing of a mantle and a crustal Pb component. Although metallogenetically distinctive, the tin belt is not delimited by a specific compositional range of Pb-isotope ratios. In particular, Pb-rich ores associated with mafic to intermediate igneous rocks in the tin belt have Pb-isotope ratios that are similar to those of the approximately coeval Sn-bearing deposits. Pb-isotope data obtained directly on the cassiterite in the tin ores may help to determine whether the Sn and Pb are derived from separate sources.