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Abstract:
One alluvial grain of gold, recovered from a placer deposit in the platiniferous Au-Pd belt of Minas Gerais, at Fazenda Limeira, is distinguished by inclusions, between approximately 2 and 10 µm across, of sperrylite. Field-emission electron-microprobe analyses indicate amounts of Au of up to about 3.4 wt.% at the expense of As, without affecting Pt, the amount of which remains stoichiometric, i.e., 1.0 atom per formula unit (1.0 apfu Pt). Transmission electron microscopy of sperrylite foils, extracted by focused ion beam (FIB), shows no gold nanoparticles and thus provides evidence for Au-bearing sperrylite. The finding of Au-bearing sperrylite represents a Au-rich example of invisible Au in sulfide and arsenide minerals. Regionally, the occurrence of gold-hosted inclusions of Au-bearing sperrylite is in line with the highly oxidized fluid environment of the auriferous lodes that characterize the platiniferous Au-Pd belt of Minas Gerais.
