Gold distribution and mobility in the surficial environment, Carajas region, Brazil

Increasing evidence indicates gold is mobile in the surficial environment. In arid regions this is attributable to complexing by chloride and near oxidizing sulfides to thiosulfate or related ligands. In this exploratory study, the extent and mechanism of gold mobility in a deeply weathered tropical rainforest environment has been investigated by analysis of waters, vegetation and stream sediment at the Salobo Cu-Au and Bahia Au-Cu deposits in the Carajas Mineral Province, Para Province, Brazil. Waters from drill holes, an adit and a stream draining the Salobo deposit contain 11 to 73 ng/L dissolved Au, compared to background levels of 2 to 3 ng/L. Elevated concentrations of dissolved Au are tentatively attributed to complexing by thiosulfate generated by accelerated weathering of sulfides beneath the steep slopes at the deposit. At the Bahia deposit, which occurs beneath an ancient deeply weathered plateau surface, the highest level of dissolved Au in small streams draining the deposit is 3 ng/L. Vegetation (multiple species) over ore at both deposits contains elevated concentrations of gold (65-400 ng/g of ash). The data suggest that vegetation may be a useful medium for gold exploration, but that in view of the results at Bahia, waters are of questionable value in deeply weathered low-relief tropical rainforest areas. The mobilization of gold by vegetation on ancient surfaces over 10⁶-10⁷ year time periods appears adequate to explain near-surface enrichment of Au and its lateral dispersion, thereby contributing to lateritic gold deposits.