ISOTOPIC AND CHEMICAL STUDIES OF THE VIBURNUM TREND LEAD ORES OF SOUTHEAST MISSOURI (MISSISSIPPI, VALLEY, TYPE, DEPOSITS)

Lead isotope analyses of galena determined on the ion microprobe were combined with sulfur isotope analyses done by conventional mass spectrometry and fluid inclusion leachate analyses in an effort to determine the origin of the Viburnum Trend lead ores.;A rough correlation between the isotopic composition of lead and sulfur in galena persists throughout the Viburnum Trend. The isotopic data can be explained mostly in terms of the mixing of two end members. The normal lead component in most of the galena leads has a composition that is similar to that of lead in basinal brines. The radiogenic lead component has a composition that is consistent with a source 1.0 - 1.2 b.y. in age. The composition of lead in the galena cubes may reflect the addition of dissolved lead from earlier generations of octahedral galena. The sulfur of both end members appears to have a marine (evaporite) sulfate origin. The age of the mineralizing event, the physical requirements for the source basin, and the chemical similarities of the ore forming fluids to present day Arkoma Basin brines indicate that the source of the ore forming brines was the Ouachita-Arkoma Basin during the later Carboniferous to Early Permian.;The correlation of the isotopic composition of lead and sulfur in the galenas strongly suggests that both elements were transported together in solution. It is proposed that the sulfur was transported in the oxidized state (i.e. SO(,4)('=)) and that sulfate reduction via the oxidation of organic carbon was the mechanism of ore precipitation.;Fluid inclusion leachate analyses indicate that the composition of the ore forming fluids of the Viburnum Trend is similar to that of typical oil field brines. The relatively elevated K('+)/Na('+) ratio of the ore forming brines may be due to interaction with the granitic basement or with arkosic sediments. The Br('-)/Cl('-) ratio of the fluid inclusions suggests that the brines from which the octahedral galena was deposited originated as evaporated seawater, and that the brines from which the cubic galena was deposited derived much of their salinity from the dissolution of halite.