The Supergene Alteration of Pyrrhotite-Pentlandite Ore at Kambalda, Western Australia

Abstract

The mineralogical and chemical changes that occur as a result of the supergene alteration of massive pyrrhotite-pentlandite ore at Kambalda are described in detail. In the lowermost Transition Zone, pentlandite is progressively replaced by violarite, accompanied by the release of iron and nickel. This nickel reacts with the pyrrhotite, converting some of it to a second type of violarite. When the pentlandite has been completely replaced, the pyrrhotite is dissolved and the iron and sulfur, together with some Ni (super 2+) and sulfur species derived from the Oxide Zone, are redeposited as nickel-rich pyrite and marcasite. The new assemblage violarite-pyrite (and/or marcasite) remains stable throughout a considerable part of the supergene profile, here called the Violarite-Pyrite Zone. The violarite becomes richer in nickel toward the top of this zone, presumably through exchange with the nickel-rich solutions migrating downward from the overlying Oxide Zone. The base of the Oxide Zone coincides approximately with the water table and marks the point where atmospheric oxygen has access to the sulfides. Here violarite appears to be unstable and is replaced by nickel carbonates and iron oxides over a vertical interval of one to two m. Pyrite and marcasite are oxidized to goethite, although relict pyrite and marcasite may persist upwards, in decreasing abundance, through much of the Oxide Zone.The supergene process can be explained in terms of an electrochemical model in which the driving force is the gradient in oxidation potential resulting from access of atmospheric oxygen to the suboutcrop of massive sulfides. Cathodic reduction of oxygen at the water table (electron consuming) results in anodic oxidation (electron producing) of sulfides at depth and upward conduction of electrons. Above the water table, where the sulfides are no longer protected by cathodic reactions, they decompose, releasing nickel and ionic sulfur species that are transported downwards by aqueous solutions and redeposited, resulting in secondary enrichment below.