Investigation of the formation mechanism of gold nanoparticles in arsenian pyrite crystal structure of the Zarshuran deposit by advanced electronic technologies

A significant characteristic distinguishing sediment-hosted disseminated gold deposits (i.e. Carlin-style) from other Au deposits is the abundance of invisible Au-nanoparticles in the Fe-As-S ores (e.g., arsenian pyrite). In this study, we used the focused ion beam combined with scanning electron microscope (FIB-SEM) techniques, and a high-resolution transmission electron microscope (HR-TEM) to examine invisible Au and how it evolved through later geologic events that eventually led to the formation of Zarshuran gold deposit. This electron examination was performed on auriferous arsenian pyrite (Py4) of Zarshuran deposit with colloform texture and average content of Au (21 ppm) and As (2.4 wt%). Results concluded that the post-ore magmatic-hydrothermal events after configuration of main orebody of the Zarshuran initiated the annealing of the ionic Au-bearing arsenian pyrite, leading to the redistribution of trace elements and specially formation of Au-bearing nanoparticles. High-angle annular dark field (HAADF) images show gold ions present in a undersaturated hydrothermal fluid, resulting in a gradual annealing process of gold nanoparticles (<100 nm in size) in the nanopores and stacking faults of arsenian pyrite crystals, which have taken place and the occurrence of this continuous cycle has led to the formation of Zarshuran world-class ore deposit.