Oral Presentation 24th Australian Conference on Microscopy and Microanalysis 2016

Correlative atom probe tomography, NanoSIMS and Maia Mapping of gold in arsenopyrite (FeAsS) (#122)

Denis Fougerouse 1 2 , Steven M Reddy 1 2 , David W Saxey 1 3 , William DA Rickard 1 3 , Steven Micklethwaite 4 , Matt R Kilburn 5 , Louise A Fisher 6 , Paul Guagliardo 5
  1. Geoscience Atom Probe, Advanced Resource Characterisation Facility & John de Laeter Centre, Curtin University, Perth, WA, Australia
  2. Department of Applied Geology, Curtin University, Perth, WA, Australia
  3. Department of Physics and Astronomy, Curtin University, Perth, WA, Australia
  4. School of Earth, Atmosphere & Environment, Monash University, Melbourne, Vic, Australia
  5. Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Crawley, WA, Australia
  6. CSIRO Mineral Resources Flagship, Bentley, WA, Australia

Arsenopyrite (FeAsS) is found in many precious metal deposits and commonly contains gold up to several thousand parts per million. The distribution of the gold in such minerals is critical in understanding the geological development of the deposit and optimising gold extraction during mineral processing. However, the primary distribution of gold remains controversial with competing models including gold homogenously distributed within the crystal lattice or hosted as nanoparticles. To address this controversy, arsenopyrite from the giant Obuasi gold deposit, Ghana, has been studied by correlative atom probe tomography (CAMECA LEAP 4000X HR), sub-micron resolution ion microprobe imaging (CAMECA NanoSIMS 50) and quantified synchrotron X-ray fluorescence element mapping using the Maia detector array at the Australian Synchrotron. Each technique provides different, nonetheless complementary information at various scales and resolution. The large synchrotron maps (several mm2) indicate that the arsenopyrites are zoned, with gold-rich rims hosting up to 2000 ppm of gold and nickel-rich overgrowths, dissecting the auriferous domains. The high-resolution NanoSIMS data yield 100x100um2 reveal that gold is distributed in alternating, submicron-scale concentric bands (up to 100 in individual grains) with sharp boundaries. Atom probe tomography of these different bands reveals the distribution of gold at the nanoscale and provides the data required to differentiate between competing models of gold distribution in arsenopyrite. The workflow developed in this study allows to investigate the microchemistry of ore minerals at the smallest atom probe scale within a well characterised framework.