Nonsulfide Zn–Pb mineralization and vertical zoning in the world-class Angouran deposit, NW Iran: linked to water table-controlled supergene processes

Abstract

This study reevaluates the previously proposed hypogene hypothesis and redefines the origin of smithsonite mineralization in the Angouran deposit, Iran (4.7 Mt of sulfide ore at 27.7% Zn; 14.6 Mt of carbonate ore at 22% Zn). Geological mapping and petrological observations reveal a distinct vertical mineralized zoning: a lower primary sulfide zone containing minor amounts of smithsonite, an intermediate transition zone characterized by smithsonite and newly formed sulfides, and an upper oxidation zone comprising smithsonite and Fe–Mn oxides, which is widely recognized as supergene origin under atmospheric oxygen fugacity conditions. This zoning pattern is analogous to classic supergene weathering profiles observed in porphyry Cu deposits. Morphological and sulfur isotopic analyses (³⁴S-depleted signatures) confirm that the newly formed sulfides within the transition zone are of supergene origin, primarily induced by bacterial sulfate reduction processes and partly formed by the direct replacement of primary sphalerite immediately below the paleo-water table. Smithsonite from all three zones exhibits similar C–O isotopic compositions that fall between those of the host marble and travertines, further supporting a consistent supergene origin. Reaction path modeling demonstrates that smithsonite cannot be generated by the direct replacement of sphalerite by CO₂-rich spring water unless under atmospheric oxygen fugacity conditions. The development of the vertical zoning with distinct mineral assemblages is primarily controlled by the position of the paleo-water table. Near it, rapid fluctuations in oxygen fugacity and sulfur fugacity occur, allowing sulfides formed under reducing conditions to coexist with smithsonite precipitated in relatively oxidative environments. The occurrence of supergene sulfides provides valuable insights into the origin of nonsulfide Zn–Pb mineralization and aid in locating deeply buried primary sulfide orebodies.