Gold remobilisation from Arsenopyrite: crystal-plasticity and dissolution-reprecipitation reactions

Denis Fougerouse, Steven Micklethwaite, Angela Halfpenny, Matthew Kilburn, Stanislav Ulrich

Research output: Chapter in Book/Report/Conference proceedingConference PaperResearch

Abstract

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.
Original languageEnglish
Title of host publicationMineral Resources in a Sustainable World
Subtitle of host publicationProceedings Volume 4
PublisherSociety for Geology Applied to Mineral Deposits
Pages1611-1614
Number of pages4
Volume4
ISBN (Electronic)978-2-85555-065-7
Publication statusPublished - 2015
Externally publishedYes

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