Abstract

Although Bi-Ag sulfides often occur in many types of hydrothermal Au-Ag ore deposits, their stability and phase relationships are insufficiently quantified. We employed the electromotive force (emf) method to measure the thermodynamic properties of pavonite (AgBi3S5) and matildite (AgBiS2), which are the two main ternary mineral phases in the Ag-Bi-S system. These new results, coupled with a critical analysis of literature data, allow generation of a consistent set of thermodynamic values in the Ag-Bi-S system. The recommended standard values at 25°C and 1 bar of the Gibbs free energy of formation from the elements, enthalpy and entropy are −227.12 ± 4.33 kJ·mol−1, −223.05 ± 4.03 kJ·mol−1, and 386.67 ± 5.84 J·mol−1·K−1, respectively, for pavonite, and −90.88 ± 1.73 kJ·mol−1, −86.47 ± 1.63 kJ·mol−1, and 178.16 ± 2.40 J·mol−1·K−1, respectively, for matildite. The new thermodynamic data can be used for quantifying phase equilibria in the Ag-Bi-S system, predicting the solubilities of Bi and Ag in the ore-bearing hydrothermal fluids and constraining temperature (T) and sulfur fugacity (fS2 of ore deposition. It is shown that the stability field of AgBiS2 (matildite) in T- fS2) coordinates of the Fe-Ag-Bi-S system is narrow and thus can be used for constraining the temperature and sulfur fugacity of formation of Ag-Au-Bi ore mineral assemblages involving matildite.

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