GEOCHEMICAL CHARACTERIZATION OF ORGANIC-RICH BLACK SHALES AND THEIR RELATIONSHIP TO PB-ZN ORES IN THE MIDCONTINENT UNITED STATES

The source(s) of the metal ions during the formation of carbonate rock-hosted Mississippi Valley-type (MVT) lead-zinc ore deposits has remained unclear. To constrain the source(s), geochemical analyses of organic-rich black shales of the midcontinent US were carried out to assess their provenance, depositional environments, and to identify potential relationship to MVT ores. The MVT ores of the midcontinent US are thought to have formed in part due to the migration of warm, subsurface fluids during the Pennsylvanian Subperiod while the Ouachita Mountains were undergoing orogeny. Significantly concentrated in many black shales are U and Th, which decay into radiogenic Pb. Radiogenic Pb can be used as a proxy to determine their source, even if the Pb ions were transported from elsewhere. However, it is not clear if the Pb was transported from distant sources or if it originated from proximal sources or in-situ. Black shales can act as a sink for metal ions, such as Ni, V, and Co, which are associated with paleoproductivity and oxia of the depositional environment. Other metal ions, such as Ti and Zr, are associated with detrital sediment input. The shales that were sampled from across the midcontinent (Arkansas, Illinois, Indiana, Iowa, Kansas, Missouri, Nebraska, and Oklahoma) include: Collier Shale (Cambrian-Ordovician); Mazarn, Polk Creek, and Womble Shales (Ordovician); Chattanooga and New Albany Shales (Devonian-Mississippian); Fayetteville and Stanley Shales (Mississippian); Atoka, Eudora, Excello, Heebner, Hushpuckney, Jackfork, Little Osage, Muncie Creek, Stark, and Vilas Shales (Pennsylvanian). The analyses in this study include total organic carbon (TOC), x-ray diffraction (XRD), radiogenic isotope (Pb, Sr, Nd) signatures, and redox-sensitive trace element concentrations. The results indicate that these shales formed in a range of depositional environments, including dysoxic, anoxic, and euxinic waters. Pb, Zn, and U concentrations vary, but many Pennsylvanian shales exhibit higher concentrations, as well as higher TOC values, than the other shales. Inversely, Th concentrations were lower in the Pennsylvanian shales than in many of the other shales.