Invisible gold in arsenian pyrite and arsenopyrite from a multistage Archaean gold deposit: Sunrise Dam, Eastern Goldfields Province, Western Australia

The Sunrise Dam gold mine (11.1 Moz Au) is the largest deposit in the Archaean Laverton Greenstone Belt (Eastern Goldfields Province, Yilgarn Craton, Western Australia). The deposit is characterized by multiple events of fluid flow leading to repeated alteration and mineralization next to a major crustal-scale structure. The Au content of arsenian pyrite and arsenopyrite from four mineralizing stages (D-1, D-3, D-4a, and D-4b) and from different structural and lithostratigraphic environments was measured using in situ laser ablation inductively coupled plasma mass spectrometry. Pyrite contains up to 3,067 ppm Au (n = 224), whereas arsenopyrite contains up to 5,767 ppm (n = 19). Gold in arsenopyrite (D-4a stage) was coprecipitated and remained as invisible gold (nanoparticles and/or lattice-bound) during subsequent deformation events. In contrast, gold in pyrite is present not only as invisible gold but also as micrometer-size inclusions of native gold, electrum, and Au(Ag)-tellurides. Pristine D-1 and D-3 arsenian pyrite contains relatively low Au concentrations (a parts per thousand currency sign26 ppm). The highest Au concentrations occur in D-4a arsenian-rich pyrite that has recrystallized from D-3 pyrite. Textures show that this recrystallization proceeded via a coupled dissolution-reprecipitation process, and this process may have contributed to upgrading Au grades during D-4a. In contrast, Au in D-4b pyrite shows grain-scale redistribution of invisible gold resulting in the formation of micrometer-scale inclusions of Au minerals. The speciation of Au at Sunrise Dam and the exceptional size of the deposit at province scale result from multiple fluid flow and multiple Au-precipitating mechanisms within a single plumbing system.