A solution model for high-temperature PbS-AgSbS2-AgBiS2 galena

Significant quantities of silver can be accommodated in galena as a coupled substitution with Ag + Sb and/or Ag + Bi for 2 Pb atoms. The limitations of this solubility are quantified experimentally for the cubic (Fm (3) over barm) alpha-galena phase. Above 441.7 degrees C, the alpha-galena phase has complete solid solution across the Pb2S2-AgSbS2-AgBiS2 (galena-miargyrite-matildite) ternary. Dry sinter experiments at 400, 375, and 350 degrees C extend the known miscibility gap on the Pb2S2-AgSbS2 binary into ternary compositions. Phase equilibrium and miscibility gap data and self-consistent thermodynamic data for other high-temperature phases in this system combined with data for other sulfides and sulfosalts are used to constrain an asymmetric regular solution model for the alpha-galena phase (Pb-2,AgSb,AgBi) S-2. Phase relations in this system suggest that simple retrograde re-equilibration during cooling can produce phase assemblages and textures commonly observed in Ag-bearing ore deposits. Bulk metal ratios of deposits in conjunction with the application of this thermodynamic model may be used to understand district Ag-distribution or as an exploration tool.