The Mascot-Jefferson City Zn district, extending from about 15 to 30 mi. NE. of Knoxville, lies in central E. Tennessee, in the Appalachian Valley and Ridge province. The ore bodies, restricted to about 150 ft. of dolomitized limestone in the Lower Ordovician Kingsport and Longview formations are elongated and continuous with bedding but are irregular in detail. Distinction is made between coarse-grained and fine-grained dolomites. Both types are believed to have been formed penecontemporaneous with deposition. Ore has been deposited as replacements and open-space fillings in coarse dolomite, but only as fracture filling in fine dolomite. Sedimentary breccias of 2 principal types are distinguished: coarse-grained dolomite breccia located in dolomitized limestone beds and restricted to these beds; and, fine-grained dolomite breccias crosscutting the stratigraphic section. Reefs of coarse dolomite, possibly of algal origin, surrounded by limestone characteristically are capped with sphalerite and are good ore guides, grading into large shoots of replacement-type ore. Vug-filling laminated clastic dolomite with laminations parallel to country rock bedding is post-ore and indicates ore deposition while beds were flat. This, together with the marked stratigraphic restriction of the ore and its intimate association with sedimentary structures, permits an alternative to the customarily cited tectonic structure-hydrothermal theory of origin. Concentration and deposition from the sea contemporaneous with sedimentation is suggested. Later regeneration of such a deposit would explain the migration of mineralization into fractured fine-grained dolomite.