Abstract

The weathered profile over the Posse gold deposit results from the modification of a complete lateritic profile related to the Velhas surface of lower Pleistocene age. Lateritic and postlateritic weathering has affected the distribution of gold in the profile. Mass balance studies show that during postlateritic weathering all levels of the profile have been absolutely enriched in gold and that the gold-bearing overburden has been eroded from the upper part of the profile. Gold enrichment has taken place by chemical and mechanical processes. In the saprolite, chemical reprecipitation of gold by reduction of complexes below the water table has resulted in homogenization of the gold grades and supergene enrichment. Gold is also chemically reprecipitated onto the surfaces of the coarse ferruginous quartz particles, probably by iron adsorption. This process is very important in the stone line, resulting in a strong gold enrichment there. Downward mechanical accumulation of the coarse gold, facilitated by pedogenetic processes, has also contributed to this enrichment. Lateral dispersion is mostly related to soil creep and is more pronounced in the top soil, from where the smaller gold particles have been preferentially eroded. Low gold concentrations in pisoliths suggest that the gold concentration in the ancient iron crust was low. A model is presented to describe the weathering evolution and the mechanisms of gold redistribution in the Posse profile. This is compared to other areas in Western Australia and west Africa. The implications for gold exploration in lateritic terrains are also discussed.

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