Detailed mineralogical and chemical study of sphalerite-rich lead ores from Flat River, Mo., confirms the presence of anomalous amounts of silver in the sphalerite. Although silver is closely associated with chlorine and no silver sulfide minerals were identified, geochemical considerations indicate the silver may be in the form of discrete submicron-size grains of sulfide rather than chloride. However, the close correlation between abundance variations in silver, chlorine, cadmium, iron, and minor lead suggests the possible existence of submicron-size grains of a complex chloride or oxychloride. Results from detailed chemical study of sphalerite show: (1) chlorine is present in areas containing silver and changes in abundance of both elements are similar; (2) the mole ratio of chlorine to silver varies from approximately 3:1 to 6:1 with an approximate average of 5:1 and the total content of each element varies widely between grains and within the same grain; (3) taking total silver in any analysis and calculating the amount of chlorine required for silver chloride leaves an excess of chlorine; (4) in chlorine-deficient areas the amounts of iron and cadmium substituting for zinc in the sphalerite structure vary widely; and (5) in areas containing chlorine, changes in abundance of iron and cadmium vary directly with that of chlorine. The fine-grained dusty opaque inclusions characterize sphalerite from this area; they were identified as mainly galena with lesser amounts of pyrite and cassiterite. Galena is silver-free (<0.02 weight percent Ag) and is essentially pure lead sulfide. Other sulfides identified include pyrite, a cobalt-nickel-iron sulfide of composition Co (sub 0.55) Ni (sub 0.25) Fe (sub 0.20) S (sub 2.00), and an iron-cobalt-nickel sulfide containing 22-28 weight percent Fe, 10-15 weight percent Co, and 3-5 weight percent Ni. Anomalous amounts of tin present as cassiterite are associated with the gangue minerals quartz, potash feldspar, a kaolinite-type clay, and trace amounts of cuprite. Carbonate gangue minerals include calcite, dolomite, ankerite, magnesite, and minor amounts of cerussite. Chemical analyses were made of many minerals and physical and textural relationships examined.