A new kind of invisible gold in pyrite hosted in deformation-related dislocations

Abstract

Mining of “invisible gold” associated with sulfides in gold ores represents a significant proportion of gold production worldwide. Gold hosted in sulfide minerals has been proposed to be structurally bound in the crystal lattice as a sulfide-gold alloy and/or to occur as discrete metallic nanoparticles. Using a combination of microstructural quantification and nanoscale geochemical analyses on a pyrite crystal from an orogenic gold deposit, we show that dislocations hosted in a deformation low-angle boundary can be enriched in Ni, Cu, As, Pb, Sb, Bi, and Au. The cumulative trace-element enrichment in the dislocations is 3.2 at% higher compared to the bulk crystal. We propose that trace elements were segregated during the migration of the dislocation following the dislocation-impurity pair model. The gold hosted in nanoscale dislocations represents a new style of invisible gold.

Structural and geochemical ore-forming processes in deformed gold deposits: towards a multiscale and multimethod approach

“…In situ analytical methods, such as electron microprobe (EMPA), SIMS, nanoSIMS or LA-ICP-MS, allow to couple major, minor and trace element geochemistry of sulphide minerals with their textural evolution at the µm-scale (Reich et al, 2017;Fougerouse et al, 2016bFougerouse et al, , 2021Román et al, 2019). Recent advances in atom probe tomography even enable the study of mineral geochemistry at the sub-nanometric atomic scale (e.g.…”

“…Recent advances in atom probe tomography even enable the study of mineral geochemistry at the sub-nanometric atomic scale (e.g. Fougerouse et al, 2016cFougerouse et al, , 2021Gamal El Dien et al, 2019;Wu et al, 2019b;Reddy et al, 2020).…”

“…Cugerone et al, 2020) or nanoSIMS (e.g. Fougerouse et al, 2016bFougerouse et al, , 2021. To go further, constraining deformation-related ore-forming processes could even be extended up to the nanometric and sub-nanometric, atomic scales.…”

“…To go further, constraining deformation-related ore-forming processes could even be extended up to the nanometric and sub-nanometric, atomic scales. This is possible thanks to the emergence of techniques such as the atom probe tomography that can image mineral defects, dislocations, etc… and better constrain micro-deformation processes (Reddy et al, 2020;Fougerouse et al, 2021). Systematically highlighting the relationships between microstructures and geochemical signatures of ore-related minerals would therefore represent a step forward towards a holistic laboratory approach for the study of deformed hydrothermal ore deposits.…”

“…Sulphide minerals, most commonly pyrite and arsenopyrite, can host a variable amount of the gold stock in such deposits, either as free gold inclusions or fracture infilling, nano-inclusions and invisible-refractory gold structurally bound in the sulphide lattice or segregated along crystal defects during sulphide plastic deformation (e.g. Cook and Chryssoulis, 1990;Reich et al, 2005;Hough et al, 2011;Deditius et al, 2014;Pokrovski et al, 2019;Fougerouse et al, 2021). In a given deposit, sulphide mineral texture and geochemical signature may thus indicate gold input and crystallisation-remobilisation stages (e.g.…”