THE ROLE OF FULVIC-ACID IN THE SUPERGENE MIGRATION OF GOLD IN TROPICAL RAIN-FOREST SOILS

In tropical terrains, gold is released during lateritic weathering from the breakdown of hypogene ore. Soil organic matter provides potential ligands for gold complexing, of which fulvic acid (FA) is the dominant mobile component. At the Ashanti mine, southwest Ghana, a strong correlation between gold and soil organic matter exists. Dissolution studies show that Au, Au-Ag alloy, calaverite, and K AuCl4 are dissolved by soil fulvic acid. Fulvic acid dissolution increases with pH with a 25 mgL-1 fulvic acid solution leaching 156 mugL-1 Au from K AuCl4 over twenty-eight days at pH 3 and 477 mugL-1 Au at pH 12. Fulvic acid composition is shown to influence the potential to complex gold. High-sulphur fulvic acid (4.2%S, 2.7%N, 41.1%0) solubilizes more gold than high-nitrogen fulvic acid (0.9%S, 4.9%N, 46.7%O), and more than high-oxygen, low-sulphur fulvic acid (0.7%S, 4.4%N, 53.9%O) at the same pH. From Fourier transform-infrared (FTIR) spectroscopy and titration studies, gold is suggested to be incorporated into fulvic acid in a pseudocrystalline form, probably as a colloid at low pHs. The binding mechanism involves initial formation of a gold complex, possibly a hydroxy complex, by an ion-exchange mechanism. This intermediate complex is then slowly reduced by fulvic acid to a gold-fulvate colloid. In alkaline solutions, gold is fixed by reduction of an intermediate species and chelation with gold bound to S-ligands. Given the low ratio of gold in natural soil waters at Ashanti (0.01-84.4 mug L-1) to fulvic acid (15 mgL-1) the greatest control on gold-fulvate complexing is the availability of gold and not thermodynamic considerations of the gold-complex stability.