Epigenetic gold-platinum-group element deposits: examples from Brazil and Australia

摘要

Recent research into anomalous gold-platinum-group element (PGE) occurrences (+- uranium) in Brazil and Australia has identified an unusual class of epigenetic Au-PGE deposit, previously poorly understood and little documented in the scientific literature. These epigenetic, sediment-hosted deposits display a strong structural control and are hosted entirely or partly within reducing (graphitic or carbonaceous) rocks. Metal transport in these deposits is interpreted to have been in low-temperature (<300 deg C), saline and acidic fluids as chloride complexes, as deduced from the common metasomatic alteration haloes rich in hematite and sericite. Few, if any, sulphides (pyrite +- chalcopyrite) are found disseminated within the orebodies and their enveloping host rocks. Within the ore there is a distinct Au>Pd>Pt abundance association, other PGE displaying only minor enrichment (<0.5 ppm). The precious-metal mineralization tends to occur as Au-Pd or Au phases, which contain variable base-metal concentrations, and as more distinct phases of Pd-Pt or Pt. Some ore minerals contain anomalous Se and occur, in places, as complex precious-metal selenide phases. Additionally, Co, Ni, U and LREE, among other trace elements, commonly reach anomalously high concentrations (>20 ppm). These metal associations resemble those of the Fe-oxide Cu-Au deposit group.