Palladiferous placer gold from the Paraiba do Sul River in northern Rio de Janeiro (Brazil) and its geological implications

Placer gold recovered from the Paraiba do Sul River, which flows along a high-grade shear zone in northern Rio de Janeiro, Brazil, has a distinctive component of palladiferous gold. Its finding, reported for the first time in the state of Rio de Janeiro, implies a hydrothermal system of high salinity at relatively low temperatures, in which metal transport is facilitated by hematite-rich rocks and generally precipitated at redox fronts. Such a hydrothermal system is not geologically manifested in the currently exposed high-grade shear zone. The finding of palladiferous gold suggests that its source was eroded during periods of increased denudation rates since the Lower Cretaceous, as indicated by apatite fission-track (AFT) data that have been reported for the region.