Pb isotopic compositions of some Zn–Pb deposits and occurrences from Urumieh–Dokhtar and Sanandaj–Sirjan zones in Iran

摘要

This study reports Pb isotope ratios of galena (PbS) sampled from some Zn–Pb deposits and occurrences (n=18) within two major structural provinces of Iran, the Sanandaj–Sirjan zone (SSZ) and Urumieh–Dokhtar zone (UDZ). Regardless of the nature of the host rock and ore type, the Pb isotopic data plot in distinct isotopic fields according to their geographic situation, which suggest that the structural terranes had an important control on the lead isotope compositions. The Zn–Pb deposits in the UDZ record distinctly higher ~(206)Pb/~(204)Pb and ~(208)Pb/~(204)Pb ratios compared to those within the SSZ. Both zones, however, have radiogenic ~(207)Pb/~(204)Pb ratios (N15.55), suggesting a dominant input of Pb fromcrustal sources. In thorogenic (~(208)Pb/~(204)Pb vs. ~(206)Pb/~(204)Pb) and uranogenic (~(207)Pb/~(204)Pb vs. ~(206)Pb/~(204)Pb) plots, the Pb isotope data define two linear arrays, which may be attributed to mixing betweenmantle-like endmembers and at least two distinct crustal sources characterized by high U/Pb and Th/U ratios. In ‘Plumbotectonic’ discrimination diagrams, the samples plot proximal to the orogenic curve, which again supports interaction between crustal and mantle reservoirs. However, both Neo-Tethys subduction and in particular Mesozoic and Tertiary orogenic activities seem to have played important roles in remobilizing Pb and forming Zn–Pb deposits in SSZ and UDZ, respectively. The similarity of Pb isotopic compositions between UDZ deposits and those for previously reported Eocene volcanics demonstrates that long-lived andwidespread hydrothermal activity promoted remobilization of lead fromthe crust; the latter occurred subsequent to Neo-Tethys lithosphere subduction in the Tertiary and continued subsequent the collision of the Iranian and Arabian plates in the Miocene.