Could Black Shales be the Only Source of Metals in the Mississippi Valley-type Pb-Zn Deposits in the Mid-continent US?

Mississippi Valley-type (MVT) Pb-Zn deposits are major economic sources of Zn and Pb. Even though the Mississippi River drainage hosts world-class MVT ores, the metal source(s) and the formation models of the ores are not well-constrained. Lead isotopes are excellent tools to study the genesis of these hydrothermal deposits, which are exceptionally enriched in radiogenic Pb. According to isotope data, two end-member sources of Pb exist: one with isotope ratios higher than the highest value for the measured ores, and another with isotope ratios equal to or lower than the lowest value recorded for ores. Of the varieties of studied potential source rocks, only a few units of the lower Chattanooga Shale and the shale units of the Jackfork Sandstone show isotopic signatures comparable to the less radiogenic endmember of the ores. However, the source of the more radiogenic component is yet to be identified. Our hypothesis is that the metal-rich black shales are the only source of ore metals. We analyze the inorganic and the organic (kerogen and bitumen) fractions of the shale units from the Appalachian, the southeastern Illinois, the Forest City, and the western Illinois basins separately for redox-sensitive trace element concentrations and Pb isotope compositions. Each of these fractions is expected to have a unique geochemical signature and match those of the two end-member components that supplied the ore metals. We propose that such geochemical approaches supported by further analyses will enable us to understand the genetic relationship between the black shales and the mineralization in carbonates and shales, U and Th enrichments, and particularly the link to hydrocarbon fields.