GOLD NANOPARTICLES IN ARSENIAN PYRITE FROM A CARLIN-TYPE DEPOSIT OBSERVED BY HRTEM

Gold mineralization has been shown to be associated with hydrothermal arsenian pyrite that forms as overgrowths on pyrite and as small porous grains in Carlin-type deposits. However, the location of gold within arsenian pyrite remains under debate, as evidence for both native gold particles (Au^o) and structurally bound Au⁺ have been established through SIMS and XANES. In samples from the Screamer zone of the Betze-Post-Screamer deposit, crystalline particles of native gold (1-50 nm) have been located by high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field (HAADF) - scanning TEM (STEM) and identified by electron diffraction and energy dispersive X-ray spectroscopy (EDS). The gold nanoparticles are dispersed throughout arsenian pyrite rims (1-10 mm thick, up to 10 wt% As), which have overgrown diagenetic As-poor pyrite. The gold nanoparticles are concentrated throughout arsenic-rich pyrite (up to 1 vol% Au, equivalent to 3 wt% Au estimated from HAADF-STEM images), but are not observed in associated As-poor pyrite. Elemental mapping of ~5 nm Au particles suggest that Sb and Hg are associated with the gold particles, while Fe, S and As are not detected. In the arsenian pyrite matrix surrounding the gold particles, As is heterogeneously distributed on a 5 nm scale, suggesting the possibility of an arsenopyrite phase. Gold was not detected in the arsenian pyrite matrix; however, due to the detection limits of EDS (~1 wt%), the existence of structurally bound Au⁺ at lower concentrations, as previously reported by XANES, could not be confirmed. The As vs. Au content measured by SIMS and electron microprobe in Carlin-type gold deposits suggest that the maximum gold content in arsenian pyrite is dependent upon the As concentration. On a plot of log As (ppm) vs. log Au (ppm), all data fall on or below the line: (log Au) = 1.56 * (log As) - 3.15, suggesting that the maximum gold content is dependent upon the arsenic concentration. The native gold observed in arsenian pyrite rims from the Screamer deposit may represent areas where the gold content has exceeded this limit for a given arsenic concentration. These results suggest that elemental gold may be a significant feature of Carlin-type mineralization.