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 (AuxAg1−x) and chalcopyrite + pyrite + iron-bearing sphalerite (CuFeS2 + FeS2 + Fe0.05Zn0.95S) 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.

Gold-silver vein deposits in the Taebaeksan district, Korea, coexist in time and space with a variety of other deposit types such as skarns, hydrothermal carbonate replacement bodies and Carlin-like deposits and reflect proximity to a magmatic source. The seven gold-silver deposits within the district show common features such as multiple complex veins, an abundance (up to 30% in ore) of base-metal sulphides, a wide range of Ag/Au ratios and the common occurrence of carbonate. Quartz-vein textures indicate open-space-filling at shallow crustal levels. On the basis of Ag/Au ratios of ore, mode of occurrence, and associated mineral assemblages, the seven deposits studied can be classified as follows; Au-dominant type (Group I), Au-Ag type (Group II), Ag-dominant type (Group IIIA) and base-metal and Ag-dominant type (Group IIIB). Group I is characterized by paragenetically early Pb-Zn basemetal sulphides with electrum and late, rare Ag-sulphides and Ag-sulphosalts, whereas Group III contains more Ag-sulphides and/or Ag-sulphosalts. Group II is gold-rich, but transitional to Group III. The Au contents and FeS contents of electrum and sphalerite, respectively, from all of the deposits decreased as mineralization proceeded. Temperature and log fS2 conditions of gold-silver mineralization tend to decrease from Group I and II to Group III deposits (i.e. 340–270°C, –9.3 to –11.8 bar, 320–240°C, –9.5 to –10.3 bar, 250–160°C, –12.5 to –16.9 bar, respectively) as well as from the main to late stages of mineralization in each deposit. The systematic mineralogy and variation of physicochemical conditions in Groups I, II and III are thought to be due to their relative positions with respect to a magma source that is genetically related to a low-to-intermediate-sulphidation porphyry system. Au-rich deposits are proximal to a magmatic source, whereas Ag-rich deposits are more distal.