The significance of gold remobilisation in ore deposits is a contentious issue. The microstructural and geochemical evolutions of arsenopyrite ores from the 62 Moz Obuasi deposit, Ghana, reveal the mechanisms for how such a process may operate. The arsenopyrite ores were coeval with high strain deformation and metamorphism in the country rocks (400±50ºC and 2 kbar). The majority of the arsenopyrite crystals are zoned with a gold-poor core and epitaxial rims (A-rims). The epitaxial rims are characterised by oscillatory zoning and high concentrations of gold (up to 1000 ppm) in the crystal lattice. Commonly, cores and A-rims are dissected by gold-depleted zones (B-rims) associated with microfractures, grain boundaries and subgrain boundaries formed during recrystallisation. B-rims are interpreted as alteration zones with sharp reaction fronts, which formed post-peak metamorphism and deformation. We develop a model whereby grain-scale microfractures and crystal-plastic microstructures facilitate a dissolutionreprecipitation replacement reaction and the release of gold from the arsenopyrite crystal lattice. At Obuasi, this gold appears to have contributed to a significant upgrade in the form of ultra-high grade ore shoots with visible gold.
|Title of host publication||Mineral Resources in a Sustainable World|
|Subtitle of host publication||Proceedings Volume 4|
|Publisher||Society for Geology Applied to Mineral Deposits|
|Number of pages||4|
|Publication status||Published - 2015|