The morphology of gold grains associated with oxidation of sulphide-bearing quartz veins at Sao Bartolomeu, central Brazil

At Sao Bartolomeu, central Brazil. chemical weathering of auriferous sulphi de-rich quartz veins has produced a gold-bearing gossan. In the gossan, gol d is present both as a residual primary mineral and as a neoformed secondar y mineral. Primary gold particles contain varying amounts of silver. They a re always more or less weathered, presenting rough surfaces pitted by numer ous micrometric cavities. Other dissolution features identified in primary particles are indented shapes and jagged rims. Secondary gold occurs as agg regates of minute pseudohexagonal particles strongly depleted in silver. Th e presence of sulphur-containing complexes formed during the oxidation of t he sulphides appears adequate to explain both the dissolution features on p rimary Sold particles and the formation of pure supergene gold. (C) 2000 El sevier Science Ltd. All rights reserved.