Formation of gold-silver sulfides and native gold in Fe-Ag-Au-S system

摘要

We carried out experiments on crystallization of Fe-containing melts FeS_2Ag_(0.1-0.1)xAu_(0.1x) (x = 0.05, 0.2, 0.4, and 0.8) with Ag/Au weight ratios from 10 to 0.1. Mixtures prepared from elements in corresponding proportions were heated in evacuated quartz ampoules to 1050 oC and kept at this temperature for 12 h; then they were cooled to 150 oC, annealed for 30 days, and cooled to room temperature. The solid-phase products were studied by optical and electron microscopy and X-ray spectroscopy. The crystallization products were mainly from iron sulfides: monoclinic pyrrhotite (Fe_(0.47)S_(0.53) or Fe_7S_8) and pyrite (Fe_(0.99)S_(2.01)). Gold-silver sulfides (low-temperature modifications) are present in all synthesized samples. Depending on Ag/Au, the following sulfides are produced: acanthite (Ag/Au = 10), solid solutions Ag_(2-x)Au_xS (Ag/Au = 10, 2), uytenbogaardtite (Ag/Au = 2, 0.75), and petrovskaite (Ag/Au = 0.75, 0.12). They contain iron impurities (up to 3.3 wt.%). Xenomorphic micro- (<1-5 μm) and macrograins (5-50 μm) of Au-Ag sulfides are localized in pyrite or between the grains of pyrite and pyrrhotite. High-fineness gold was detected in the samples with initial ratio Ag/Au ≤ 2. It is present as fine and large rounded microinclusions or as intergrowths with Au-Ag sulfides in pyrite or, more seldom, at the boundary of pyrite and pyrrhotite grains. This gold contains up to 5.7 wt.% Fe. Based on the sample textures and phase relations, a sequence of their crystallization was determined. At ~1050 oC, there are probably iron sulfide melt L_1 (Fe,S ? Ag,Au), gold-silver sulfide melt L_2 (Au,Ag,S ? Fe), and liquid sulfur L_S. On cooling, melt L_1 produces pyrrhotite; further cooling leads to the crystallization of high-fineness gold (macrograins from L_1 and micrograins from L_2) and Au-Ag sulfides (micrograins from L_1 and macrograins from L_2). Pyrite crystallizes after gold-silver sulfides by the peritectic reaction FeS + L_S = FeS_2 at ~743 oC. Elemental sulfur is the last to crystallize. Gold-silver sulfides are stable and dominate over native gold and silver, especially in pyrite-containing ores with high Ag/Au ratios.