“Invisible” Gold in Synthetic and Natural Arsenopyrite Crystals, Vorontsovka Deposit, Northern Urals

“Invisible” gold in hydrothermal ores is frequently scattered in the most abundant minerals of the Fe–As–S system. It is assumed that “invisible” gold does not incorporate into the mineral structure (nanoscale inclusions of the metal or its compounds) or is chemically bound (isomorphous substitution). The aim of this study is to determine the concentration range of “invisible” gold, species of its occurrence in arsenopyrite, and conditions facilitating the formation of gold-bearing arsenopyrite using synthetic crystals and natural samples from the Vorontsovka Carlin-type deposit in the Northern Urals. Arsenopyrite crystals have been synthesized using the ampoule method in a eutectic melt of alkali metal chlorides and Al at a stationary thermal gradient and 400–500°C at the cold ampoule end. The chemical composition of arsenopyrite has been measured by electron probe microanalysis. The chemical composition of synthesized arsenopyrite is, at %: 32.6–34.4 Fe, 30–36.5 As, and 29.4–36.0 As. The gold concentration ranges from below the detection limit (<45 ppm) to 3 wt %. The obtained chemical data for synthetic crystals are compared with theoretical trends calculated for various gold species. It has been established that the slope of the trends of the average arsenopyrite compositions is very close to that of the theoretical line of isomorphous substitution Au ↔ Fe. It allows the assumption that the isomorphous solid solution in which Au occupies the Fe site formed during experiment. In general, all our data on synthetic and natural arsenopyrite show a strong negative correlation between Au and Fe, which supports the formation of the solid solution with Au at the cation site. In addition, a weak positive correlation between Au and As is observed: the higher As concentration is characteristic of As-rich (As/S > 1 at %) arsenopyrite and is close to stoichiometry, whereas in the S-rich variety, the Au content is as low as 0.25 wt %. This dependence is not only within individual grains, but also at the deposit in general: later As-rich arsenopyrite formed at lower temperature and sulfur fugacity (T = 250–370°C, log f  S₂ = –12 to –17) is enriched in Au compared to early arsenopyrite (T = 270–400°C, log f  S₂ = –7 to –9). Comparison of our data with the literature shows that an increasing Au content with increasing As concentration and decreasing Fe content in arsenopyrite is a common feature of Carlin-type deposits. We believe that in contrast to the negative correlation between Au and Fe, the correlation between Au and As is not obvious and may be caused by external factors, different composition of hydrothermal fluids, and sulfur fugacity.