The oxidized profile of BIF-associated Pb-Zn mineralization: Pegmont, Northwest Queensland, Australia

Abstract Lead-zinc mineralization with a low Fe-sulphide content occurs in association with a quartz-magnetite-garnet-apatite banded iron formation (BIF) at Pegmont, northwest Queensland. A study of the ore horizon and wall rocks at different levels in the oxidized profile has revealed considerable depletion of Mg, Ca, Na, K, S, Ag, Cd and Zn during weathering. The least mobile elements have undergone residual concentration up to the profile, whereas concentrations of other elements are either unaffected by weathering or vary irregularly, in many cases reflecting local variations within the ore horizon. On oxidation, galena changes either through a pyromorphite ± cerussite assemblage to plumbogummite/corkite or directly to coronadite. The Pb content of outcropping gossan is the same as, or slightly higher than that of unweathered ore. Although Zn is strongly depleted at the surface (×10), its greater abundance (×2 depletion) only 15 m below the surface reflects the truncated profile and immaturity of the gossan. Comparisons with gossans overlying similar mineralization at Broken Hill, New South Wales and at Gamsberg and Aggeneys, North West Cape Province, South Africa, indicate that Pb, Zn, Ag and Cu contents can be used to evaluate BIF-associated ironstone outcrop.