Naturally occurring Au nanoparticles associated with high-grade mineralization at the world-class Callie deposit, Northern Territory, Australia

A significant amount of the gold extracted today is sourced from orogenic deposits where it is concentrated in very high-grade quartz veins. The local elevated gold concentration in the veins appears to exceed the gold solubility expected in hydrothermal fluid in mid-crustal conditions. Accordingly, the gold transport mechanism and/or deposition processes associated to the formation of the ultra-high grade veins remains contentious. Here, we present Transmitted Electron Microscopy (TEM) analyses conducted on a high-grade quartz vein (>100 g/t Au) from the world-class Callie orogenic gold deposit (14.6 Moz Au). The TEM analyses are focused on a silica inclusion (similar to 2.5 mu m wide) enclosed within a gold grain and the results reveal that the inclusion is composed of amorphous silica that contains numerous Au nanoparticles which vary in size between 0.5 to 11 nm. The preservation of amorphous silica and Au nanoparticles indicates that the formation of colloidal Au and Si might have played a significant role in the formation of high-grade gold veins.