The Paleozoic strata of the U.S. mid-continent host several major Mississippi Valley-type (MVT) ore deposits and many trace occurrences of MVT mineralization. The present study sought to test the hypothesis that systematic differences in fluid composition led to the differences in scale of mineralization. To conduct this test, fluid inclusions in trace MVT occurrences were characterized and compared to previously characterized fluid inclusions in mid-continent MVT ore deposits.Primary sphalerite-hosted fluid inclusions in trace MVT occurrences from across the U.S. mid-continent were analyzed using microthermometry, Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometry. Interquartile ranges (25th to 75th percentile) of trace MVT fluid inclusion properties were found to be as follows: homogenization temperature = 78 to 107 °C, salinity = 13.8 to 23.9 equivalent weight percent NaCl, atomic ratios of K/Na = 0.017 to 0.038, Ca/Na = 0.18 to 0.31, Mg/Na = 0.0081 to 0.013, Sr/Na = 0.0093 to 0.015, Ba/Na = 0.00013 to 0.00041. Compared to MVT ore deposits, fluid inclusions in trace MVT occurrences have lower homogenization temperatures and K/Na and Ba/Na ratios, similar salinities and Ca/Na and Sr/Na ratios, and slightly higher Mg/Na ratios (excluding Mg-rich fluid inclusions from the Upper Mississippi Valley district). In addition, fluid inclusions in trace MVT occurrences have low to undetectable methane contents whereas MVT ore deposits have high methane concentrations on the order of 100's to 1000's of ppm. Sphalerite matrix trace metal concentrations of trace MVT occurrences generally overlap those of MVT ore deposits.Differences in elemental concentrations and temperature between trace MVT and MVT ore-forming fluids are insufficient to account for the differences in deposit size. This suggests that differences in the local availability of sulfide, which could not be measured in this study but may correspond with the large observed differences in methane content, may have governed MVT deposit size.