Colour and cash : the exquisite minerals of the oxidized zone

The geochemistry and chemical mineralogy of the oxidized zones of orebodies are extraordinarily complex, much more so than the primary deposits from which they are derived. The zones themselves are of great economic importance, especially to Australia. Many of the free-milling gold ores exploited over the last two decades are of supergene origin (secondary gold) and cobalt, nickel and copper deposits of the same kind are of increasing importance. We know much of the equilibrium chemistry of such mineral settings for some elements and something of the general processes that play a part in the development of ore grades in the oxidized zone. For other elements the situation is somewhat more sketchy and of certain aspects, including kinetics of crystallization, temperatures of mineral formation and solid-solution phenomena, we know very little indeed. Oxidized zone mineralization, some of which is exquisitely beautiful, is described in terms of its origin and the associated chemistry necessary to extract metals from these sorts of deposits. Further research directions are highlighted, together with problems that remain to be solved.