Bacterial oxidation is often attributed with the advantage that a high level of gold recovery can be obtained at a relatively low level of sulphur oxidation. This is sometimes attributed to the bacteria "tunneling" towards gold inclusions, however it is often due to the presence of two or more sulphides of differing gold content which leach at different rates. Gold is liberated from the individual sul- phides linearly with oxidation. The distribution of gold between pyrite and arsenopyrite in two samples taken from a prospective gold resource was examined._x000D_
The two samples showed differing responses to bacterial oxida tion and it was shown that this was due to differing intrinsic gold grades in the mineral phases and differing proportions of the minerals in the two samples.