Conditions of formation of gold-bearing arsenopyrite: a comparison of synthetic crystals with samples from Le Chatelet gold deposit, Creuse, France

In order to understand the formation mechanisms of gold-bearing arsenopyrites, hydrothermal experiments have been performed. Needle-shaped gold-rich (up to 1.7 wt % as compared to 1.6 wt % in some natural occurrences) zoned arsenopyrites were obtained at 400 °C and 500 °C under 1 and 2 kbar PH₂O. Comparisons with natural needle-like gold-rich arsenopyrite from the Le Châtelet deposit (Creuse, France) revealed analogous zoning of As/S ratio, with similar gold content and zonations. Fluid inclusion studies carried out in the host quartz of the latter indicate high formation temperatures and variable redox conditions. Compared with the neighbouring Villeranges gold deposit, where arsenopyrite was formed at about 200 °C with gold being mainly trapped within its structure, it appears that temperature is not likely to be the major factor for such an incorporation. Alternatively, it is suggested that non-equilibrium rapid crystallization may induce gold trapping under an extended range of T-P-fO₂ conditions. The practical importance of the needle-like habit of arsenopyrite in areas where geochemical gold anomalies are known without visible gold being detected, is emphasized.