Comparative Geochemical Analysis of Ordovician and Mississippian Cherts in Relation to the Northern Arkansas and the Tri-State Mississippi Valley-type Ore Districts

It has been hypothesized that Ordovician, Mississippian, and younger carbonate and clastic formations found within the Ouachita Basin of west-central Arkansas and northward onto the Ozark Dome have experienced interaction with hydrothermal fluids due to tectonic forces produced by the Ouachita Orogeny. Lead (Pb) isotope analyses and resulting values produced through previous and concurrent geochemical studies of several formations indicate that the Mississippi Valley-type (MVT) ores were produced from a mixing and cooling of these sedimentary basinal brines by meteoric or connate waters. Reduction of sulfate to sulfide is one of the possible mechanisms by which MVT ore precipitation may be recognized. Organics sequestered in shales result in general enrichment of uranium (U) through their decay, as evidenced by the U-enriched Devonian Chattanooga Shale. The Chattanooga Shale and early Pennsylvanian Jackfork Sandstone of the Ouachitas have been shown to be prominent source rocks through metal source evaluations (Bottoms et al., 2019 and Simbo et al., 2019). The linear trend defined by the Pb isotope ratios of the MVT ores suggests that two end-members were involved in the formation of the MVT ores. Of particular interest is the identity of source rocks that contributed the enriched radiogenic Pb component to the MVT ores. A highly radiogenic end-member appears to exist, as evidenced by the Pb isotope values of the Cotter Dolomite analyzed in this study. The Bigfork Chert, Boone tripolitic chert, and Hatton Tuff appear to be related through elemental concentration analyses, confirming volcanic ash contributions. Other studies cited herein suggest that metasomatic fluid pulses produced by Reelfoot Rift tectonics may have contributed genetically related, metalliferous fluids to these brines. Meteoric waters originally confined to the Ozark Aquifer may have also interacted with initial and subsequent pulses through fracturing and faulting following the uplift produced by the Ouachita Orogeny. Close stratigraphic position to Pre-Cambrian basement rocks may have also resulted in Pb enrichment by fluid-rock interactions. In addition, the briny Western Interior Plains Aquifer is in close proximity to the Tri-State MVT ore district and may have also supplied elemental additions to the fluids that resulted in deposition of these MVT ore deposits. Other chert formations, such as the Grand Falls, Penters, and Short Creek, or cherty dolomites, including the Jefferson City, Roubidoux, Gasconade, Eminence, and Potosi of southern Missouri could be sampled for Pb isotope values in future studies in order to further constrain the linear two end-member components that contributed Pb to the ores. Clastic formations, including the Blaylock and Hartshorne Sandstones within the Ouachita Mountains and Arkoma Basin of Arkansas may also have enriched radiogenic Pb signatures. The Upper Cambrian Davis Formation, as well as the Lamotte, Mt, Simon, and Reagan Sandstones, in southern Missouri may have also contributed elemental Pb to the MVT ore fluids through dissolution of potassium feldspars, exclusive of any primary U and Th or accumulation of lead through radiogenic decay