LEAD ISOTOPE TRENDS AND METAL SOURCES IN SW MEXICAN DEPOSITS

New Pb isotope analyses of three Cenozoic ores from the La Verde porphyry copper deposit located in the Guerrero composite terrane are presented and the metal sources are evaluated. Lead isotope ratios of three Cenozoic ores from the El Malacate and La Esmeralda porphyry copper deposits from the Guerrero terrane and of fourteen ores from the Zimapan and La Negra skarn deposits from the adjoining Sierra Madre terrane are also presented to look for systematic differences in the lead isotope trends and ore metal sources among the proposed exotic tectonostratigraphic terranes of southern Mexico.

Comparison among the isotopic signatures of ores from the Sierra Madre terrane and distinct subterranes of the Guerrero terrane supports the idea that there is no direct correlation between the distinct suspect terranes of Mexico and the isotopic signatures of the associated Cenozoic ores. Rather, these Pb isotope patterns are interpreted to reflect increasing crustal contribution to mantle-derived magmas as the arc advanced eastward onto a progressively thicker continental crust.

The lead isotope trend observed in Cenozoic ores is not recognized in the ores from Mesozoic VMS and SEDEX deposits. The Mesozoic ores formed prior to the amalgamation of the Guerrero composite terrane to the continental margin, which took place during the Late Cretaceous, in intraoceanic island arc and intracontinental marginal basin settings, while the Tertiary deposits formed after this event in a continental arc setting. Lead isotope ratios of the Mesozoic and Cenozoic ores appear to reflect these differences in tectonic setting of ore formation.

Most Pb isotope values of ores from the La Verde deposit (²⁰⁶Pb/²⁰⁴Pb = 18.674-18.719) are less radiogenic than those of the host igneous rocks, but plot within the field defined by the Huetamo Sequence, suggesting that these ores may also contain metals from the sedimentary rocks. The Pb isotope ratios of ore samples from the Zimapan deposit (²⁰⁶Pb/²⁰⁴Pb = 18.771-18.848) are substantially higher than the whole-rock Pb isotope compositions of the basement rocks. The similarity of ore Pb to igneous rock Pb in the Zimapan district (²⁰⁶Pb/²⁰⁴Pb = 18.800-18.968) may indicate that the proximal source of ore metals in the hydrothermal system was the igneous activity.