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

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, Ag2Pd3Se4, a phase analogous to tischendorfite, Pd8Hg3Se9, an argentiferous palladseite-like phase, empirical Pd5(Hg,Sb,Ag)2Se6 and (Pd,Sb,Ag,Hg)5Se4, at Gongo Soco; palladseite, ideally Pd17Se15, and sudovikovite, PtSe2, at Itabira; and, exclusively to Serra Pelada, empirical Pd9PtSe2, Pd9Se2, Pd3(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 Pd9Se2 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.