The Wood River lead-silver deposits, near Bellevue in south-central Idaho, occur in veins in sheared zones in contact metamorphosed siliceous and limy argillites of Mississippian age near contacts with quartz diorite. Galena and sphalerite are the principal ore minerals, siderite the principal gangue mineral; the ore averages 1.25 ounces silver for each percent lead.Seven purified mineral separates of galena contain 2,100 to 5,000 ppm silver and 3,100 to 4,700 ppm antimony; antimony and silver are in an atomic proportion of approximately 1:1. Electron microprobe analyses show that slightly less than one half of the antimony and silver is in solid solution in the galena. The rest is present in minute, high Ag-Sb inclusions of five distinct compostions, though principally as diaphorite (Pb 2 Sb 3 Ag 3 S 8 ). Precise analyses of these inclusions were obtained by electron microprobe using a series of synthesized complex sulfosalts as standards. Some limitations on thermal stability of the mineral assemblages were obtained by these syntheses and by annealing natural materials at temperatures between 251 degrees and 399 degrees C.