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In the Quadrilátero Ferrífero, the world-class iron district of Minas Gerais, Brazil, numerous manganese oxide deposits occur in the Palaeoproterozoic Itabira Iron Formation (Itabira Group). Although some deposits have been investigated, 5,23,24,25,31,33,36,41,42 systematic geochemical studies are lacking. Reconnaissance geochemical data on the manganese ore of the Conta História manganese oxide deposit, which are presented here, indicate important contents of mercury, thallium, barium and strontium (Table 1), suggesting an event of hydrothermal manganese oxide mineralization in the Quadrilátero Ferrífero that has hitherto gone unrecognized owing to the deep weathering imprint. Geological setting and Conta História depositSituated in the eastern part of the Quadrilátero Ferrífero (Fig. 1), the Conta História deposit comprises rocks of the Palaeoproterozoic Itabira Iron Formation metamorphosed under amphibolite-facies conditions. The Itabira Iron Formation, 25 or the Itabira Group, 20 consists predominantly of itabirites (Cauê Formation) and dolomitic rocks (Gandarela Formation). With a minimum Pb/Pb age of 2400 m.y., 2 the Itabira Iron Formation is the middle unit of the Minas Supergroup, which includes the Caraça Group, the B123

New data on the Gongo Soco palladium-oxygenated compounds are presented for the first time since the work of Johnson and Lampadius (1837). A variety of PdO species is documented, always in association with Cu, Fe and Mn, and includes Pd(OH)2or PdO.H2O, and phases with metal excess in relation to oxygen, interpreted as metastable. Their remarkable occurrence in ‘ouro preto’ (black gold) and jacutinga (auriferous itabirite/iron ore) is the result of mertieite-II alteration. It involves leaching of Sb and As, limited removal of Pd, and incorporation of Cu, Mn and Hg into PdO species. The locus of Hg-bearing PdO species in microfractures parallel to the enclosing gold-mineralized shear band suggests at least one low-temperature hydrothermal generation. The presence of a halite crystal attached to gold testifies to the involvement of Cl-saturated hydrothermal fluids in the oxidizing environment of jacutinga formation.