BEHAVIOR OF GOLD IN THE LATERITIC EQUATORIAL ENVIRONMENT - WEATHERING AND SURFACE DISPERSION OF RESIDUAL GOLD PARTICLES, AT DONDO MOBI, GABON

A supergene gold dispersion halo has developed at Dondo Mobi, southern Gabon, from the weathering of an auriferous lisvenite under equatorial humid rain forest conditions. Morphological and chemical studies were performed on gold particles extracted from both fresh and weathered rocks from different parts of the dispersion halo. The results demonstrate that the gold particles are residual and are subjected to increasing weathering, upward through the profile, from the protore to the surface in the central part of the halo, and at the surface both laterally upslope and downslope. Weathering continues downward into the formerly barren saprolites which were derived from Proterozoic schist and Archean gneiss (rim of the halo). Thus, the Au content progressively decreases as does the Ag content of gold particles with distance from the center of the ore body. Gold particles subjected to increased weathering are smaller with more rounded shapes and surfaces pitted by dissolution. The dispersion pattern is controlled by Au dissolution and translocation of residual gold particles. These processes involve an autochthonous evolution in the central part of the halo and a para-autochthonous evolution at the rim of the halo. At Dondo Mobi, chloride and humic substances are the only ligands present in significant concentrations that are capable of complexing Au. Thermodynamic calculations demonstrate that under such acidic conditions (pH ranges from 4 to 6), gold stability increases with increasing silver content. The dominant Au complexes formed have been calculated to be an aurons hydroxichloride complex and an organometallic complex, both of which are stable at surficial conditions. Thus, both chemical and translocation processes control the mobility of Au and gold particles in equatorial lateritic weathering profiles.