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Mineralogic observations of naturally occurring Au-Ag nuggets from the Yilgarn Block, Western Australia, indicate that dissolution of Ag and/or Au is a relatively frequent occurrence along the rim of nuggets, especially when there are adjoining iron oxides. Small gold crystals of very high purity (0.4 % Ag) have been found intimately associated with iron oxide in laterite profiles. These also suggest hat Au and Ag have been dissolved, transported, and redeposited uring lateritization. Experimental evidence suggests hat very acid chloride solutions generated in lateritic profiles by ferrolysis (oxidation and hydrolysis of iron) are responsible for the dissolution of gold and silver. Au is subsequently reprecipitated by reduction of the AuCI•- ion with Fe +•. Reduction of the AgC1 ø complex does not similarly occur because of the relative redox potentials of Fe+•/Fe +a and Ag/AgC1 ø. Thus chemical refinement of gold occurs during lateritization. The development of low pH and the redistribution of gold and silver in lateritic weathering profiles appear to be more common over granitic and gneissic basement and may be inhibited by the presence of carbonate in the weathering zone of basic rock sequences.
A.W. Mann (Wed,) studied this question.