Palladian gold and palladium arsenide-antimonide minerals from Gongo Soco iron ore mine

摘要

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 percent 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.