The present-day Gongo Soco iron ore deposit was one of the most famous gold mines in Brazil in the nineteenth century. The iron ore mining has exposed crosscutting, quartz–specularite auriferous ore shoots within Palaeoproterozoic banded iron formation. The coarse-grained gold particles are characteristically palladium-bearing and commonly have inclusions of palladium arsenide-antimonides, whose compositions are close to mertieite-II and isomertieite. Composite crystals with a mertieite-II core and isomertieite rim are interpreted as precipitates from a hydrothermal fluid of variable Sb : As ratio. The palladium content in gold appears to be correlated with the platinum content in the associated palladium arsenide-antimonide. Its platinum content, up to about 1 wt% Pt, seems to be related to the amount of arsenic. A myrmekitic intergrowth of an Fe-rich, goethite-like phase in a mertieite-II-bearing palladian gold nugget is recorded and is interpreted to be of low-temperature hydrothermal origin. A late-stage event, possibly of a low-temperature hydrothermal nature, deposited Pt–Pd alloy locally.
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