Gold-enriched rims on placer gold grains: an evaluation of formational processes

Abstract

Placer gold is frequently reported to assay at overall higher values of fineness than the gold in the rock from which it was liberated. A related phenomenon is the historical discovery of many extremely large gold nuggets (up to 28 lbs) in the southeastern United States that have no apparent source rock. Placer gold grains from the southeastem United States have been examined and found to frequently exhibit the development of nearly pure to pure gold rims around their borders. These gold rims are suggested as a possible cause of the high fineness placer deposits. Formation of very thick rims may also be the cause of the large nuggets. Formation of these gold-enriched rims by the often attributed mechanism of simple silver leaching is disputed on the basis of ineffective mechanisms for the removal of silver from the alloy. Diffusion of silver through the gold at low temperatures proceeds far too slowly to produce the chemical gradients observed in the placer gold grains. Comparison of the complexation capacities of 41 ligands with subsequent modelling of expected complex concentrations in natural stream and stream sediment waters indicates CN⁻ and S² as the most likely functional ligands for the transport and redeposition of supergene gold. Electrolytic refining of placer Au-Ag grains is also a process for forming gold-enriched rims that can operate together with secondary enrichment to produce the observed phenomena.