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

At Sمo Bartolomeu, central Brazil, chemical weathering of auriferous sulphide-rich quartz veins has produced a gold-bearing gossan. In the gossan, gold is present both as a residual primary mineral and as a neoformed secondary mineral. Primary gold particles contain varying amounts of silver. They are always more or less weathered, presenting rough surfaces pitted by numerous micrometric cavities. Other dissolution features identified in primary particles are indented shapes and jagged rims. Secondary gold occurs as aggregates of minute pseudohexagonal particles strongly depleted in silver. The presence of sulphur-containing complexes formed during the oxidation of the sulphides appears adequate to explain both the dissolution features on primary gold particles and the formation of pure supergene gold.