| Abstract: | Gold deposits overlain by allochthonous (transported) sediments more than 10 m thick give very weak geochemical anomalies in the soils. Hence, information on the mode of occurrence of Au in overlying soils and sediments is important for the interpretation of these weak anomalies. The different forms of Au in the overburden are zonally distributed around subcropping deposits. Lateral hypergene (supergene) zoning in the overlying sediments and soils may appear as increasing contents of native Au particles, and as increasing amounts of water-soluble Au complexes which may change in composition with approach to the Au deposits. In regional background soils, allochthonous sediments, and the weathered crust Au is predominantly concentrated in the insoluble mineral residue, in fulvic acids, and to some extent in insoluble organic residues and humic acids. Only 4–7% of the Au occurs as free native particles. In unconsolidated sediments of auriferous fields native Au, Au in insoluble residues, and Au in fulvic complexes predominate. Au is sporadically contained in pore solutions and can be removed by potassium iodide and aqueous extracts. High concentrations of native Au particles in combination with the soluble complexes, Au in pore solutions, and Au determined by potassium iodide and aqueous extracts are extremely significant indicators of the proximity to buried Au deposits.Based on thermodynamic calculations derived from the results of electrodialysis and from fractionating studies utilizing exchange resins and from major ligand concentrations in experimental solutions, it has been shown that an increase of sulphate and a decrease of Au in complex forms occurs in oxidizing environments outward from buried deposits, toward neutral and alkaline conditions. The sequence of changes of the forms of dissolved Au upward in hypergene (supergene) zones is controlled by the change of conditions from reducing to oxidizing. The variation of the mode of occurrence of Au in unconsolidated sediments, as well as the presence of soluble complexes of Au in low-contrast total-gold anomalies, serve as important criteria of the proximity to Au deposits. |
