THERMODYNAMIC PROPERTIES OF SILVER AND BISMUTH SULFOSALT MINERALS, PAVONITE (AgBi_3S_5) AND MATILDITE (AgBiS_2) AND IMPLICATIONS FOR ORE DEPOSITS

摘要

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 (AgBi_3S_5) and matildite (AgBiS_2), 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 and 1 bar of the Gibbs free energy of formation from the elements, enthalpy and entropy are -227.12 4.33 kJmol~(-1), -223.05 4.03 kJmol~(-1), and 386.67 5.84 Jmol~(-1) K~(-1), respectively, for pavonite, and -90.88 1.73 kJmol~(-1), -86.47 1.63 kJmol~(-1), and 178.16 2.40 Jmol~(-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 (f_(S_2)) of ore deposition. It is shown that the stability field of AgBiS_2 (matildite) in T-f_(S_2) 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.