Lead isotopic compositions of Paleozoic to Miocene ore deposits in the Western Tethyan Belt

Abstract

The Romanian Carpathians host some of the richest base and precious metal deposits in Europe. The existing lead isotopic data for Romania covered almost exclusively Miocene epithermal and porphyry deposits in the Baia Mare area and the South Apuseni Mountains. There is, therefore, an evident lack of isotopic and chronological constraints which have limited the metallogenic interpretation of the metal sources and hindered data-supported comparisons with the neighboring metallogenic units within the Western Tethyan Belt. New lead isotopic analyses were carried out on ore samples selected from Cambrian to Miocene magmatic sulfide, porphyry, skarn-related, replacement, epithermal, and metamorphosed and unmetamorphosed volcanogenic massive sulfide deposits located in the Apuseni Mountains (North and South), Banat, Southern Carpathians, and Dobrogea. The range of the analyzed ores is 17.926 to 19.083 for ²⁰⁶Pb/²⁰⁴Pb, 15.550 to 15.741 for ²⁰⁷Pb/²⁰⁴Pb, and 38.062 to 39.224 for ²⁰⁸Pb/²⁰⁴Pb. It turns out that the lead isotopic composition of the ores clusters by age, i.e., Paleozoic, Triassic-Jurassic, and Cretaceous-Miocene. The average of lead isotopic values of Paleozoic ores is 18.168 for ²⁰⁶Pb/²⁰⁴Pb, 15.681 for ²⁰⁷Pb/²⁰⁴Pb, and 38.216 for ²⁰⁸Pb/²⁰⁴Pb; of Triassic-Jurassic ores is 18.442 for ²⁰⁶Pb/²⁰⁴Pb, 15.606 for ²⁰⁷Pb/²⁰⁴Pb, and 38.324 for ²⁰⁸Pb/²⁰⁴Pb; and of Cretaceous and Miocene ores is 18.677 for ²⁰⁶Pb/²⁰⁴Pb, 15.662 for ²⁰⁷Pb/²⁰⁴Pb, 38.726 for ²⁰⁸Pb/²⁰⁴Pb. The wider age range and the broader geological coverage of the analyzed ore deposits reveal that the radiogenic lead isotopic composition of the ores increases with time but always overlaps with the isotopic ranges defined by the host rocks. Since the Paleozoic, except a Late Jurassic magmatic sulfide deposit related to tholeiitic magmatic rocks where the upper mantle is the main source of lead, the lead incorporated in Carpathian ores has a typical crustal signature with a model µ value (²³⁸U/²⁰⁴Pb) of about 10 and a time-integrated Th/U ratio of about 4.0. The calculated model ages of the ores are generally older than the ore deposition ages demonstrating that older crustal material contributed to the lead within the ores. Our results significantly increase the available lead isotopic data for Romanian ores, and allows for the first comprehensive overview of the lead isotopic signatures of the ore deposits in the Western Tethyan Belt through geological time.