The mobility of gold in tropical rain forest soils

Bowell, Robert John (1991) The mobility of gold in tropical rain forest soils. University of Southampton, Doctoral Thesis.

Abstract

At the Ashanti concession, Ghana, narrow (1-3 m) auriferous shear zones comprise gold-bearing quartz veins and disseminated auriferous sulphides in altered phyllite and metabasalt host rocks. The mineralization is concealed by up to 10m of kaolinite-mica forest ochrosol soils, beneath which is a saprolite zone of leached rock extending down 60-70m to the hypogene ore zone. The mineralization is evident as a Au-Ag-As-Cu-Zn-Pb signature within the soil which closely mimics element dispersion patterns in the underlying lithologies due to the steep angle to dip (65^o-90^o) of the shear zones. Anomalously high values of As and Cu define regions of sulphide formation while high values of K, Rb, and Al reflect potassic alteration and high Ca and Mg reflect carbonate alteration of the wallrocks. Gold occurs as free grains, as inclusions in the disseminated arsenopyrite, and as a minor gold-telluride component in some quartz veins. Identification of primary gold mineralization in the soils at Ashanti can readily be achieved by direct analytical determination of gold. By discriminant function analysis the gold-bearing shear zones can be characterized by a multivariate anomaly of Au-Ag-As-Cu-Pb-Zn while the more extensive zones of wallrock alteration are characterized in the soils by a Au-As-Cu-Rb-K₂O halo. Gold concentration in the ground waters and soil waters at Ashanti show an increase up the profile with a maximum concentration of 84.4μg/1 in soil waters at the water-table. Gold is relatively immobile in the lower saprolite, however with an increasing oxidation of the orebody in the upper saprolite gold dissolution occurs by supergene fluids involving hydroxo- and thiosulphate complexing. However, due to the low stability of these complexes mobility is limited and dissolution and re-precipitation occur in-situ. In the soils, gold is released into the weathering cycle by mechanical disaggregation and chemical dissolution of the hypogene ore involving fulvate, cyanide, thiosulphate, and hydroxy complexing. From experimental studies, gold-fulvate complexing is dominated by a colloidal mechanism of adsorption at acidic pH (2-5). With increasing pH, the fulvic acid is deprotonated, leading to a more open fulvic acid structure and with less competition for binding sites by hydrogen and the more open structure, gold is complexed by a chelate mechanism. The gold is bound to S-, N- and O- donor functional groups in the fulvic acid with the affinity of S-donors> > N-donors> O-donors. The presence of R-COOH groups, adjacent to the chelating functional group, is considered important as it acts as an oxidant for gold although does not itself complex gold. The reaction has first-order rate kinetics. Humic acid is not considered as it is insoluble in acidic to neutral-pH Ashanti soil-water. Gold-mobility, in the soils, appears to be limited with migration of the soluble gold complexes over a few metres or even centimetres due to low stability of the complexes. This would explain the lack of lateral re-distribution of the gold and a series of dissolution-re-precipitation cycles appears to have taken place, in the soils, largely in situ. The geochemical patterns noted at Ashanti can therefore be interpreted largely in terms of the underlying lithology, topography and hypogene element dispersion.

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