Abstract

The complex mineralogical and chemical system of a tropical rain forest weathering profile at the Ashanti mine, Ghana, was characterized to evaluate the role of inorganic and organic processes with the potential to mobilize gold in the rain forest soils. The concentration of gold has been measured in bulk saprolite, soil samples, and in pore-water extractions from the saprolite and soil horizons from the Sansu prospect at Ashanti. Gold concentration in the ground waters varied from below the detection limit (0.01 mu g/l) to a maximum of 84.4 mu g/l at the water table. Average gold concentration in the soil pore waters is 3.6 mu g/l. The gold in the pore waters is present as either soluble gold or as suspended particles less than 0.45 mu m in size.Despite the presence of significant gold concentrations in the soil pore waters, gold concentration in soils over the auriferous shear zones is up to 400 times greater than over the nonauriferous country rocks suggesting a limited gold mobility. Similarly, a close correlation between gold held in the solid soil fraction and in soil pore waters reveals very little in the way of supergene gold enrichment. Conversely, the gold is dissolved and reprecipitated over a range of a few centimeters. The high concentration of gold in the soil pore waters at Ashanti is a reflection both of the high gold content of the hypogene ore and of the efficiency of supergene fluids in the tropical rain forest environment to leach the gold but not retain it in a mobile form. Solubility calculations indicate that gold complexing in the soils appears to be dominated by AuOH(H 2 O), Au(CN) 2 (super -) , Au(S 2 O 3 ) 2 (super -3) , Au(NH 3 ) 2 (super +) , and a gold fulvate complex. Selective extraction of gold from organic and inorganic soil fractions reveals that the element is largely concentrated as native gold. A significant proportion (up to 28%) is concentrated in the light organic fraction of the near-surface soils which is dominated by fulvic acid. The role of gold adsorption by clays and hydrous Fe-Mn oxides appears to be small with less than 1 percent of the gold being present in an easily exchangeable form.Gold appears relatively immobile in the lower saprolite with gold values in solution being below the detection limit (0.01 mu g/l). Higher up in the saprolite, gold appears to be dissolved by hydroxide and thiosulfate complexes in the upper saprolite and by fulvate, cyanide, and hydroxide complexes in the soils. Due to the in situ nature of gold dissolution and reprecipitation, even short-lived species such as thiosulfate and cyanide can play an important role in the supergene chemistry of gold in the Ashanti soils. Halide complexes are not considered important owing to their low concentration in the ground waters (Cl, <9 mg/l) and the unrealistically high Eh-low pH conditions required. Humic acids are also not considered important in the dissolution of gold as they are only sparingly soluble in the pH range of the Ashanti soils (4.2-6.8).

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