Supergene nonsulfide Zn–Pb mineralization in the Mehdiabad world-class sub-seafloor replacement SEDEX-type deposit, Iran

Abstract

The world-class Mehdiabad nonsulfide deposit consists of several accumulations of Zn–Pb–Fe nonsulfide minerals, derived from supergene of a vent-proximal sub-seafloor replacement SEDEX-type deposit hosted in lower Cretaceous clastic–carbonate rocks of the Taft and Abkuh Formations. The main sulfides are galena, sphalerite, pyrite, and minor amounts of chalcopyrite, which have been partially or completely transformed into nonsulfide minerals by supergene processes. Currently, the Zn mineralization in the Abkuh Formation (ore horizon II) is almost fully oxidized: the Mehdiabad deposit which can be assigned to both direct replacement and wall-rock replacement types. The time of the formation of the nonsulfide deposit has not been determined, but it is probable that the main supergene metallogenesis happened during Tertiary, possibly between post-Cretaceous to present. The mineralogy of the nonsulfide mineralization is generally complex and consists of smithsonite, hydrozincite, and hemimorphite as the main economic minerals, accompanied by cerussite, anglesite, iron–manganese oxy-hydroxides, sauconite, and Zn-rich clays. Commonly, nonsulfide minerals in this deposit consist of two types of ore: red zinc ore (RZO), rich in Zn, Fe, Pb-(As), and white zinc ore (WZO), typically with very high zinc grades but low concentrations of iron and lead. The proposed three-step scenario for the deposition of nonsulfide mineralization in the Mehdiabad deposit comprises: (1) the deposition of primary sulfides contemporaneous with lower Cretaceous clastic–carbonate host rocks to form SEDEX-type mineralization; (2) during the Late Cretaceous to present, regional uplift leads to deformation, folding, and thrusting of sulfide-bearing lower Cretaceous sequence; and (3) as a result, Zn–Pb–Fe sulfides hosted in carbonates experienced oxidation under an arid-warm climate to form the supergene nonsulfide ore mineralizations.