Seleniferous minerals of palladium and platinum from ouro preto-bearing mineralisation in Brazil

Abstract

Selenium is a trace, but characteristic element in the bonanza-style mineralisation of palladiferous gold that triggered two major gold rushes in Brazil (Minas Gerais and Serra Pelada). The auriferous mineralisations of Gongo Soco and Itabira, Minas Gerais, and Serra Pelada, Pará, have Pd–O-bearing, dark-coloured ferruginous coatings on gold ( ouro preto). Depending upon the amount of palladium alloyed with gold, a number of seleniferous minerals of palladium and platinum can be found: chrisstanleyite, Ag 2Pd 3Se 4, a phase analogous to tischendorfite, Pd 8Hg 3Se 9, an argentiferous palladseite-like phase, empirical Pd 5(Hg,Sb,Ag) 2Se 6 and (Pd,Sb,Ag,Hg) 5Se 4, at Gongo Soco; palladseite, ideally Pd 17Se 15, and sudovikovite, PtSe 2, at Itabira; and, exclusively to Serra Pelada, empirical Pd 9PtSe 2, Pd 9Se 2, Pd 3(Se,Bi) and (Pd,Hg,Pb) 3Se. This mineralogical signature of selenium is typical of hydrothermal vein-type deposits formed at relatively low temperatures. Although the synthetic Pd 9Se 2 equivalent is stable at temperatures over 390 °C, fluid inclusion data suggest lower temperatures of formation (< 150 °C). The exclusiveness of Se-bearing, alloy-like phases of palladium to Serra Pelada testifies to its very Pd-rich hydrothermal system.