Formation of gold and silver sulfides in the system Ag–Au–S

Gold and silver sulfides were synthesized from melts Ag2-χAuχS (χ = 0.1, 0.4, 0.8, 1.6) in the system Ag–Au–S with Au/Ag = 0.1–7.4. Optical microscopy, X-raying, electron probe microanalysis, and scanning electron microscopy were used to identify and analyze the solid products. It has been established that acanthite, solid solutions Ag2-χAuχS (χ < 0.5), and electrum are produced in the Ag-rich binary system Ag2S–Au2S. Petrovskaite, high-fineness gold, and sulfur are produced in the systems with low Ag contents (χ > 1). The systems with an intermediate composition produce petrovskaite, uytenbogaardtite, and solid solutions Ag2-χAuχS (0.5 < χ < 1). The results of experimental studies together with data on the compositions of natural gold and silver sulfides suggest that the wide variations in Au and Ag contents in uytenbogaardtite and petrovskaite from different deposits are, most likely, due to the inhomogeneous structure of mineral grains and the presence of solid solutions Ag2-χAuχS. It is assumed that gold and silver sulfides crystallizing from Au-Ag-containing sulfide melts might be of magmatic genesis. Poorly extractable gold in sulfide ores might be present as microinclusions of Au-Ag sulfides.