In central Missouri, many small high-grade barite deposits occur as open-space fillings in solution collapse structures in carbonate rocks. In southeast Missouri, extensive low-grade residual deposits have been derived from the weathering and dissolution of carbonate host rocks. Fluid inclusion studies show that the barite in both districts was precipitated from solutions having salinities in the range of 4 to 10 weight percent salts and at temperatures less than 40 degrees to 50 degrees C. The salinities of different primary and sets of pseudosecondary inclusions may vary within a single barite crystal--showing that the salinity of the mineralizing fluids changed during crystal growth.Electron microprobe analysis shows calcium and strontium to be the only trace elements detected in barite. Most barite shows an increase in strontium concentration toward the younger parts of the crystal. Some barite crystals have small-scale, strontium compositional banding which suggests that the crystals responded to regular fluctuations in physiochemical conditions during mineralization. This systematic compositional banding suggests that the depositional sites were influenced by near-surface processes, specifically seasonal changes.Paragenetic relationships indicate that the period of barite mineralization was distinctly later than that of the lead-zinc mineralization in the Central and Southeast Missouri barite districts. Fluid inclusion data show that sphalerite was deposited by brines, generally greater than 22 weight percent salts, at temperatures ranging from 80 degrees to 110 degrees C. The lack of any mineral deposition in the barite deposits in the temperature range from 80 degrees to 40 degrees C and salinity range of 21 to 10 weight percent salts suggests the possibility of two distinct and perhaps completely unrelated episodes of mineralization. The most geologically plausible model for the large amount of barite deposited requires the mixing of a barium-rich fluid with a sulfate-bearing surface fluid.