Silver-bearing zinc-iron tetrahedrite-tennantite and freibergit fahlores approximating the simplified formula (Ag,Cu)₁₀(Fe,Zn)₂(As,Sb)₄S₁₃ have been equilibrated with excess electrum (Au_xAg_1−x) and chalcopyrite + pyrite + iron-bearing sphalerite (CuFeS₂ + FeS₂ + Fe_0.05Zn_0.95S) in evacuated silica tubes at 300 °C, in reversed silver-copper exchange experiments with less than 0.1 mg NH₄Cl added as a transport medium. A thermodynamic formulation and parameters describing As-Ag incompatibility at 400 °C (Ebel and Sack, 1989), which incorporate large temperature dependencies of standard-state properties and composition-ordering systematics, are shown to apply equally well to these 300 °C results. A generalised graphical model for this mineral assemblage is presented, describing fahlore composition as a function of temperature and the compositions of coexisting electrum and (Fe,Zn)S, which define the Ag(Cu)₋₁ and Fe(Zn)−1 exchange properties controlling fahlore compositions.