Arsenic-silver incompatibility in fahlore

Silver-bearing zinc-iron tetrahedrite-tennantite and freibergit fahlores approximating the simplified formula (Ag,Cu)m(Fe,Zn)2(As,Sb)4S~3 have been equilibrated with excess electrum (AuxAgl_~) and chalcopyrite + pyrite + iron-bearing sphalerite (CuFeS2 + FeS2 + Fe0.05Zn0.95S) in evacuated silica tubes at 300 ~ in reversed silver-copper exchange experiments with less than 0.1 mg NH4CI added as a transport medium. A thermodynamic formulation and parameters describing A s A g incompatibility at 400 ~ (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~ 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)_l and Fe(Zn)_l exchange properties controlling fahlore compositions.