The supergene zone of the Peak Hill gold mine, New South Wales

The oxidized zone of the Peak Hill gold mine developed under hyper-acidic conditions that essentially stripped it of transition and other heavy metals. It is characterized by abundant jarosite, itself an acid indicator. A notable feature of the oxidized zone is the abundance of secondary barite remobilized from primary material at pH values near zero. Secondary barite is often perched on iridescent goethite that displays a remarkable play of colour. Secondary gold of high fineness fills voids in goethite and ultra-thin foils often generate lamellar structures between layers of goethite. An irregular, thin supergene-enriched zone is present at the base of oxidation. Covellite, CuS, and grey secondary copper sulfides ranging in composition between those of digenite and yarrowite are present in this zone also as a result of prevailing acidic conditions. Some other secondary phases present in the oxidized zone are described.