The mineralogical distribution of pathfinder elements in gossans derived from dolomitic shale-hosted Pb-Zn deposits, northwest Queensland, Australia

Abstract

The gossans formed on Pb-Zn mineralization in northwest Queensland contain potential pathfinder and indicator elements such as Ag, Ba, Cl, Cu, Pb, Sr and Zn in variable amounts. Studies of the secondary minerals in these gossans reveal that mineralogy strongly influences elemental abundances in the whole rock. An understanding of the distribution of the pathfinder elements within the secondary minerals should therefore aid assessment of the economic potential of a gossan. For example, because of the ready incorporation of Pb and the more mobile Cu and Zn in alunite-jarosite minerals, the absence of Pb in these minerals can be considered to reflect barren Fe-sulfides at depth. Pb and Ba form simple sulfates which incorporate minor amounts of Zn and Cu. Significant Zn and Ag may occur in cerussite but other elements are incorporated only in minor amounts. Secondary carbonates indicate more alkaline conditions at the time of formation than do secondary sulfates. Cl occurs in trace amounts in many secondary minerals but only forms simple chlorides with Ag and Na late in the development of a gossan. Hemimorphite forms under alkaline conditions and does not preferentially incorporate other base metals. Of the secondary non-oxide minerals, only the alunite-jarosite minerals are both good hosts to pathfinder elements and relatively abundant in outcrop. Thus although the Fe-oxides are quantitatively the most important hosts to pathfinder elements, a significant proportion of such elements may be stabilized by alunite-jarosite minerals.