Atom probe microscopy could unlock hidden gold resources

Using the Geoscience Atom Probe Facility at Curtin University, scientists have found metallic gold nanoparticles only a few nanometres in diameter within the common mineral arsenopyrite. Their study challenges our current understanding of nanoparticle formation and allowed the team to establish the main controls on gold incorporation in sulfides.

According to Curtin research associate Dr Denis Fougerouse, the application of atom probe microscopy in geosciences is relatively new. “The technique is based on field evaporation of atoms from tiny, needle-shaped specimens to provide three-dimensional, subnanometre-scale information of the position and type of individual atoms in the specimen in the mineral,” he explained.

“Typically, the amount of material analysed is really, really small — a single grain of salt is over a billion times larger than a typical analysis.”

Dr Fougerouse said that large resources of gold nanoparticles are ‘locked’ in arsenopyrite, an iron arsenic sulfide, which can be found in mines across the world. “Although not every arsenopyrite contains gold, it is common to find gold locked inside this mineral,” he said.

While previous models have linked gold distribution to gold concentration, the researchers found that gold incorporation in arsenopyrite was controlled by the rate of crystal growth, with slow growth rate promoting the formation of gold clusters and rapid growth rate leading to homogeneous gold distribution.

“Our results show that gold can be hosted either as nanoparticles or as individual atoms in different parts of the crystal structure, and the different types of gold yield important information about the controls on gold deposition as the ore body forms,” Dr Fougerouse said.

Dr Fougerouse said the study, published in the journal American Mineralogist, demonstrates the capability of atom probe microscopy in geosciences.

“Our research shows the Geoscience Atom Probe has potential to characterise gold deposition processes at the atomic level,” he said. “In turn, this could help unlock hidden gold resources in known deposits and will enhance gold recovery.”

Image caption: Synchrotron XFM RGB images of an arsenopyrite aggregate (yellow = Au; blue = Ni). The trace element distribution shows epitaxial gold zonation surrounding gold-poor cores. The region of interest for the atom probe study is shown. Modified after Fisher et al (2015).