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Gold occurrences have been reported in the northeastern part of Goiás State since the beginning of the 18th Century. The main mineralization is associated with Paleoproterozoic peraluminous, syntectonic granites of the Aurumina Suite and associated metasedimentary, graphite-bearing country rocks of the Ticunzal Formation. In the Buraco do Ouro gold mine, the mineralization is hosted in muscovite-quartz mylonite in a silicified shear zone near the contact between biotite-muscovite granite and paragneiss of the Ticunzal Formation. The ore mineralogy consists of gold, paraguanajuatite (Bi2Se3), kalungaite (PdAsSe), isomertieite [Pd11Sb2As2], mertieite II [Pd8(Sb,As)3], sperrylite (PtAs2), padmaite (PdBiSe), bohdanowiczite (AgBiSe2), clausthalite (PbSe), krutaite (CuSe2), ferroselite (FeSe2), uraninite (UO2) and unnamedAg-Pb-Bi-Se minerals. Local magnetite concentrations and rare chalcopyrite and pyrite are also associated with both mineralized and barrenmylonites in a gangue consisting of muscovite, quartz and rare tourmaline. High TiO2 muscovite clasts in the ore are interpreted as the magmatic muscovite of the original granite, and the mineralization is considered to be synchronous with the syntectonic granite intrusion during syn-emplacement shearing and alteration. The association between granitic rocks and platinum-group element (PGE)-bearing gold mineralization observed in the Buraco do Ouro mine is uncommon and unique in the context of the Aurumina Suite and the Ticunzal Formation, where gold deposits and occurrences are gold-only. The chemical data suggest the possibility of a solid solution between paraguanajuatite and bohdanowiczite. In addition, a complex intergrowth occurs between paraguanajuatite, clausthalite and Ag-Pb-Bi-Se phases, one of which, a Pb-Bi-Se phase could represent a new mineral. Uraninite is identified for the first time in this mineral assemblage and its concentration in the ore seems important, as revealed by high gamma spectrometric measurements in the samples collected in the mine. The association between gold and uranium constitutes a regional signature, observed in both gold and uranium deposits in the Cavalcante region. [ABSTRACT FROM AUTHOR]

Low-temperature Pt enrichment is illustrated with reference to two hypogene platiniferous minerals: (i) a Pd–Cu-bearing ‘Pt2HgSe3’, an unnamed phase; and (ii) hongshiite, PtCu, with up to 0.7 wt% Hg. The former has patches oxidized to a Pt–O species with curvilinear microcracks originated by desiccation. The latter displays parallel sets of Cu-depleted, Pt–Hg-enriched patches, along which microfractures occur. Such microstructures are interpreted to arise from two processes: (i) oxidation and formation of a hydrated Pt–O compound, which tends to succumb to a Pt-rich alloy by deoxygenation and dehydration, leaving behind curvilinear desiccation cracks; (ii) metal leaching conducive to native Pt by removal of Cu from hongshiite via micropores that coalesce along crystallographic planes to result in reticulated microcracks. Platinum enrichment either by oxidation or metal leaching ideally produces characteristic microcracks, which may be indicative of the origin, i.e. detrital vs. supergene, of alluvial and eluvial Pt-rich alloys. [ABSTRACT FROM AUTHOR]

• The Serra Pelada deposit was formed by mixing between a reduced and an oxidizing, metal-carrier hydrothermal fluid. • Gold contents of the mineralizing fluids are between 50 and 500 ppb. • Pressure and temperature for the mineralization are estimated at 250–270 °C and 1.2–1.7 kbar. Serra Pelada is a high-grade hydrothermal Au-Pd-Pt deposit within the Carajás mineral province, eastern Amazon craton, which shows an exceptional metal enrichment at carbon-enriched hosted ores (>1000's ppm Au + Pt + Pd). The main orebody is structurally hosted at the hinge of a recumbent syncline, consisting of dolomitic metasandstones at the outer part of the fold, a layer of carbonaceous metasiltstone in which most orebodies are located, and red siltstone in the core of the fold. Distal hydrothermal alteration comprises spatially separated chlorite – carbon (reduced) and hematite (oxidized) alteration zones. The mineralized zones include silicification and argillic (carbon- kaolinite-, and hematite-rich) alteration zones. Fluid inclusions hosted by quartz and dolomite crystals from distal alteration zones and silicified portions within the mineralized zone record a wide range of salinity (5–22 wt% NaCl eqv.) but relatively constant homogenization temperatures (170 ± 25 °C). Pressure and temperature estimations based in the fluid inclusion microthermometry and selenide thermometry constrain the formation of the deposit at 250–270 °C and 1.2–1.7 kbar. Element concentrations of alkali metals, Pb, Cs and to a lesser extent Ba correlate with salinity, whereas redox-sensitive elements including U and Ce vary widely. Gold concentrations from 50 to 500 ppb detected in some inclusions are consistent with the highly oxidizing conditions defined by selenide mineralogy. The presence of uranium and gold within fluid inclusions of the distal hematite alteration zone confirms the oxidizing, metal carrier fluids. The occurrence of hydrothermally precipitated carbon implies that a reducing CH 4 -bearing fluid was also present, and the structural relationships suggest that mixing between this reducing fluid (and interaction with carbon-upgraded rocks) with an acidic, oxidizing and metal-carrying fluids was the main process driving ore metal precipitation. The genesis of the Serra Pelada Au–Pd-Pt deposit shows similarities with sediment-hosted copper deposits and unconformity-related U ± Au ± PGE deposits, having in common the downflow of highly oxidizing fluids into the hydrothermal domain of the Earth's upper crust. Major differences in metal proportions of these broad classes of ore deposits and other sediment-hosted systems in different ore provinces are primarily caused by differences in source rocks available for oxidative leaching and variations in the geometry of the hydrothermal system. [ABSTRACT FROM AUTHOR]