CONTRIBUTIONS OF HYDROTHERMAL ACTIVITY TO MIDCONTINENT PENNSYLVANIAN STRATA

Pennsylvanian strata of Kansas City contain small-scale occurrences of barite and zinc mineralization and associated gangue minerals, similar to the principal carbonate-hosted lead-zinc-barite ores of the Mississippi Valley region. Barite and sphalerite both occur in dolomite-calcite gangue although the two ore minerals are rarely found together. Sphalerite crystals from limestones and shales contain primary brine inclusions whose homogenization temperatures range from 83 to 100 degrees Celsius. Melting temperatures range from -17.5 to -22.5 degrees C, corresponding to salinities equal to or greater than 20.6% equivalent NaCl. Fluid inclusion values are substantially the same as those from major mines including the world-class, but now inactive, Tri-State Zn-Pb district, which is centered about 160 km to the south of Kansas City. Hydrothermal activity is also implied by the widespread occurrence of coarsely crystalline kaolinite and dickite in altered limestone beds in Kansas City. Metal-rich black shales, such as the Hushpuckney and Stark members, have probably derived a substantial fraction of their metal contents which average 2000 ppm Zn throughout Kansas City from hydrothermal fluids as well. The opportunity for mineralization existed during sedimentation, but also after deposition when these extremely thin and jointed black shale beds were bathed in the migrating basinal brines that were responsible for mineralization in the main ore districts of the Mississippi Valley. Available radiometric dates for mineralization in the Kansas City region (39-251 Ma) are in accord with late hydrothermal mineralization, but leave open the possibility of a syngenetic contribution during sedimentation of the black shales.