Quantitative approach to physical and chemical gold mobility in equatorial rainforest lateritic environment

Mobility of gold throughout the global weathering system of Dondo Mobi, South Gabon, has been quantatively addressed by calculating the gold mass transfer during lateritic weathering of auriferous lisvenite, Archean gneiss and Proterozoic schist, and by proposing a thermodynamic approach to gold solubility under equatorial rainforest acidic conditions. Both chemical and short-distance translocation processes govern the mobility of gold and gold particles during the long-term evolution of collapsed open weathering systems, which result from the combined interaction of rock, meteoric water and biological agents. Free gold particles are translocated from the lisvenite-derived soil towards formerly barren Archean gneiss and porous Proterozoic formations derived from schist, to form a 200 m wide supergene gold dispersion halo, which took shape over about 0.86-1.26 million years. Simultaneously with physical translocation of gold, dissolution of gold makes the total balance of gold transfer through the whole weathering system strongly negative, which is the converse of what is generally believed to occur in lateritic systems. Taking into account hydrolysis reactions, three main gold complexes are proposed as a function of corresponding inorganic and organic ligand concentrations. These are: Au(OH),(H,O)”, AuCIOH- and Au(OH),FA-. These gold complexes leave the Dondo Mobi weathering mantle and move towards the river system in the course of both physical and chemical processes involved in the weathering of the formation.