Formation of gold and silver sulfides from melts in the Ag-Au-S system: Experimental data

Abstract

as well as solid solutions Ag 2- xS (x → 0)-Au0.1Ag1.9S, Au0.1Ag1.9S-Au0.4Ag1.6S, and Au0.4Ag1.6S-Au1.8Ag0.2S (with facecentered, volumecentered, and primitive cubic cells) are stable in the threecomponent Ag- Au-S system (Fig. 1) (1, 2). Polymorphic transforma� tions of gold and silver sulfides have been studied insufficiently (3). Uytenbogaardtite (α�Ag 3AuS2) and petrovskaite (α�AgAuS ) discovered in the 1980s (4, 5) and already established in ores of more than 30 depos� its are related to the Ag-Au-S system. A review of ore objects, mineral associations with Au-Agsulfides, and compositions of uytenbogaardtite and petrovs� kaite is given in (6). Wide variations of Au and Ag con� centrations in these minerals allowed us to assume the possible existence of solid solutions of gold- silver sulfides in natural processes. Since the transition α�Ag 2S β�Ag 2S is enantiotropic and acanthite par� amorphs after argentite are considered as geother� mometers, one of the aims of this investigation was to understand the type of polymorphic transformations for more complex gold and silver sulfides, namely uytenbogaardtite and petrovskaite. The genesis of Au- Agsulfides is not completely clear as well. The physi� cochemical models of the formation of Au-Agsul� fides in the oxidation zone and lowtemperature hydrothermal processes (6-8) supporting the "hyper� gene" and "hypogene" hypotheses were worked out on the example of the Ulakhan, Krutoe, and Yunoe deposits (Northeastern Russia) (2, 4, 5, 9, 10). In this study we consider the possibility of the "magmato� gene" hypothesis of the formation of these minerals 1 Not found in nature. from melts in the twocomponent 2 Ag2S-Au2S sys� tem. This work is aimed at investigation of Au and Ag behavior during crystallization from melts without excess of sulfur, with an initial ratio of (Au + Ag)/S = 2 and revealing of the conditions of the formation of uyten� bogaardtite, petrovskaite, and solid solutions Ag 2- хAuхS.