Ore characterization and textural relationships among gold, selenides, platinum-group minerals and uraninite at the granite-related Buraco do Ouro goldmine, Cavalcante, Central Brazil

摘要

Gold occurrences have been reported in the northeastern part of Goias 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 [Pd-8(Sb,As)(3)], sperrylite (PtAs2), padmaite (PdBiSe), bohdanowiczite (AgBiSe2), clausthalite (PbSe), krutaite (CuSe2), ferroselite (FeSe2), uraninite (UO2) and unnamed Ag-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.