Geologic and metallogenic significance of the isotopic composition of lead in galenas of the Chilean Andes

Serv. Nac. Geol. y Min., Santiago, Chile

Eighty galena samples from different mineral deposits within the Chilean Andes from the Mesozoic and Cenozoic eras were analyzed for lead isotopes. The results show a restricted dispersion between 18.65 to 18.28, 15.68 to 15.56, and 38.72 to 38.11 on ²⁰⁶ Pb/ ²⁰⁴ Pb, ²⁰⁷ Pb/ ²⁰⁴ Pb, and ²⁰⁸ Pb/ ²⁰⁴ Pb ratios, respectively. Galenas from deposits hosted by pre-Jurassic to Jurassic rocks are relatively enriched in radiogenic Pb, reflecting the involvement of old crustal Pb. Galenas from Lower Cretaceous-hosted deposits are less radiogenic and may reflect a mantlelike component. Those from Tertiary rocks are isotopically more variable, reflecting the influence of a more evolved crust. There is no obvious correlation between the isotopic composition of the galena leads and the age of the hydrothermal event that gave rise to the particular mineral deposit (when such ages can be determined from independent geologic evidence). It is concluded that the galena leads were derived from a mixture of different lead reservoirs (as defined in Zartman and Doe, 1981). The correlation of isotopic composition with age of host rocks suggests either that the concentration of lead to form galena was more or less contemporaneous with the formation of the host rock or that the galena lead was essentially derived from the country rocks and inherited their original isotopic signature. Other workers have reported an overall similarity between the isotopic composition of galena leads and whole-rock leads from the Chilean Andes. However, the isotopic compositions of galena leads from the Argentinian Andes show different characteristics from those of the Chilean Andes, but the data are insufficient to allow firm conclusions.

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