The Mascot-Jefferson City zinc district of East Tennessee is the major source of zinc in the United States. Sphalerite mineralization which occurs as a breccia infilling in the Lower Ordovician Knox Group carbonates has been studied in detail at the Young Mine, located in the central portion of the district. The sphalerite occurs as fracture fillings and as rosettes between carbonate breccia blocks and displays well developed growth zoning characterized by variations in color, opacity, inclusions, and composition. Ore emplacement was preceded by two periods of solution brecciation. The first episode was related to a paleokarst terrain developed atop the Knox unconf orrni ty. The second episode was probably related to tectonic activity and expulsion of fluids from a sedimentary basin to the southeast. Episodic dolomitization in the vicinity of the ore bodies both predated and accompanied sphalerite deposition. Dolomite and other minor gangue phases deposited concurrently with sphalerite. Fluorite, calcite and quartz grew as postore, vug-filling phases. Sphalerite fluid inclusions reveal wide ranges in homogenization and freezing temperatures ( 81° to 199° C; -11.2° to -37.0° C) which may represent the mixing of two fluids--a hypersaline metal-rich brine with a less saline fluid resident in the host rocks. A geochemical model shows that Mg/Ca ratios and temperatures were such that dolomite was stable during ore deposition. Inclusions in post-ore gangue phases display an overall decrease in homogenization temperatures, are less saline, and show a narrower range of salinities. / M.S.