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The supergene gold deposits occurring in the lateritic regolith of the Yilgarn block, Western Australia, are examples of a distinctive type of mineralization. The deposits are mostly small (<1.5 million metric tons) and of low grade (1.5-5.0 g/metric ton) but can be exploited because of the relative ease of mining and treatment. The lateritic gold deposits are more or less flat-lying zones contiguous with the ferruginous horizon and underlying mottled zone of the weathering profile. They contain residual, Ag-bearing, primary gold as fine-grained particles and rare nuggets, and Ag-poor secondary gold, commonly as very fine grained particles but with some euhedral crystals, typically associated with iron oxides. The deposits developed mainly during tropical, humid conditions in the early Tertiary by relative enrichment of primary gold and absolute enrichment of secondary gold. Gold mobilization was probably as organic complexes, reduced and precipitated by the oxidation of ferrous iron. The saprolitic gold deposits are enrichments deeper in the profile, either confined to the lode system or laterally dispersed into weathered wall rocks, commonly as one or more subhorizontal zones. They consist principally of secondary gold, with residual primary gold confined to the weathered lode system. The deposits formed after the change to more arid climates caused a gradual lowering of water tables and the development of salinity. Gold was probably mobilized as chloride complexes and reprecipitated following reduction by ferrous iron. Similar concentrations of gold in the regolith are known from other deeply weathered and lateritic terranes, particularly in Africa and South America. Regional differences between the types of deposit formed can be attributed mainly to differences in climatic history.

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