Lead isotopes as a tool for gossan assessment in base metal exploration

Lead isotope measurements on gossans may be used as an exploration tool for Pb-Zn-Cu deposits of the “stratiform” type. The method is based on the homogeneous isotopic composition of stratiform orebodies and the close fit of their ratios to the so-called “growth curve”. These features also characterize oxidized outcrops. (gossans) of the primary sulphide ore. The retention of homogeneous Pb isotope ratios during oxidation was tested and proved for true gossans derived from known mineralization in which there was a variation in vertical depth from gossan to primary sulphide, different ages and different geological environments. The deposits included Pb-Zn-Cu (Woodlawn), Pb-Zn (Broken Hill), Zn-Pb (Dugald River), Zn-Cu-Pb (Currawang).The method has been extended to distinguish true gossans from false (pseudo) gossans where the base metals and associated trace elements have been scavenged from the surrounding rocks. It has also been applied to a company exploration program in which a number of ironstone cappings had been drilled and other geochemical information was available. Diamond drilling indicated that five prospects were barren, i.e. underlain by iron sulphides devoid of significant base metal mineralization. On the Pb isotope data, three of the prospects would be rejected; a fourth shows a particularly high potential and is worthy of further exploration, whilst the fifth initially showed high potential but a comparison of gossan and sulphide Pb isotope data after drilling suggests that the drill hole was sited on the fringes of economic mineralization.The Pb isotope method may be utilized further in exploration at the drilling stage. If sulphides are intersected, Pb isotopes may be used to distinguish barren sulphides from those with economic potential, even though they both may contain similar base metal concentrations.