Petroleum-bearing fluid inclusions occur in sphalerite, calcite, dolomite, and barite at the Jumbo mine, a Mississippi Valley-type deposit in eastern Kansas. In addition to petroleum, Na-Ca-Mg-Fe chloride brines were present during deposition of calcite and sphalerite in which primary inclusions contain ≳23 equivalent wt.% NaCl. Dolomite- and barite-hosted inclusions are more dilute, possibly because of mixing between hydrothermal fluids and groundwater during mineralization. Primary oil inclusions in sphalerite have homogenization temperatures ( Th) between 85 and 95°C. Aqueous inclusions have Th values ranging from ~90 to 130°C for sphalerite to below ~50°C for barite. Primary brine inclusions in calcite at the Jumbo mine contain goethite, apparently as a daughter mineral (although it is possible that the goethite formed as a result of diffusion of H 2 from the inclusions like the hematite daughter described by Roedder and Skinner, 1968, from Bingham, Utah). Goethite has also been tentatively identified in inclusions from the Fletcher mine of Missouri. If goethite is a true daughter phase, it implies the presence of oxidized fluids during mineralization. This suggests that ore deposition resulted from interactions between hydrothermal fluids and dilute groundwater ( e.g., by dilution or cooling of ore fluids, or oxidation of organometallic complexes that may have transported metal constituents).