A geochemical evaluation of the genetic relationship between Ouachita Mountains Paleozoic rocks and the Mississippi Valley-type mineralization in the southern Ozark Region, USA

Abstract

The Pb and/or Zn deposits of the Tri-State and the Northern Arkansas Mississippi Valley-type (MVT) districts in the southern Ozark Region, located north of the Arkoma Basin and the Ouachita fold-thrust belt in North America, are genetically connected to the Pennsylvanian-Permian Ouachita orogeny. This orogenic event triggered a south-north topographic gradient flow of basinal brines, leaching metal-rich sediments en route. To establish whether the organic-rich shales of the Ouachita Mountains and the Ozark Region provided metals during the mineralization event, the Pb isotope compositions of the ores (sphalerite) have been compared to those of the sedimentary rocks (whole-rocks and leachates) of the Ouachita Mountains (Cambrian Collier Shale, Early Ordovician Mazarn Shale, Middle Ordovician Womble Shale, Late Ordovician Polk Creek Shale, Mississippian Stanley Shale, and Pennsylvanian Jackfork Sandstone) and the Ozark Plateau (Devonian/Mississippian Chattanooga Shale and Mississippian Fayetteville Shale). The Pb isotope compositions of ores in both districts show a broad and a linear range of isotopic values. This trend suggests mixing of Pb from two distinct end-member components. One end-member must be highly radiogenic, with Pb isotope ratios equal to or higher than the highest noticed value for ores. The other end-member must be less radiogenic, with Pb isotope ratios equal to or lower than the lowest value recorded for ores. The Pb isotope compositions of the analyzed whole rocks and leachates indicate that the best candidates for contributing the less radiogenic Pb to the ores are the sandstone members of the upper and middle Jackfork Sandstone and the lowermost section of the Chattanooga Shale.The depositional environment of the potential source rocks and their ability to sequester metals has been appraised using redox sensitive trace elements (Cd, Co, Cr, Cu, Mo, Ni, Pb, Th, U, V, Zn). Paleoredox proxies (e.g., enrichment factors of redox sensitive trace elements and V/V + Ni ratios) indicate deposition of the analyzed rocks mainly under anoxic and euxinic conditions, which are favorable for metal enrichment. However, oxic-suboxic conditions are also indicated by other geochemical proxies, suggesting a possible first order control of primary production that has resulted in an increased flux of organic matter. A disagreement in the redox conditions yielded by the paleoredox proxies has been noticed in several cases. This inconsistent behavior may be associated with several factors, including redox potential, pH, sulfur abundance, chemical availability, TOC values, and sediment provenance. The authigenic flux is the only fraction needed to unravel past redox conditions; however, the analyzed bulk sediments consist of authigenic, biogenic, and siliciclastic components, which may also have led to the observed disagreement in the redox conditions. Additional factors such as the basin geometry, the rates of sediment accumulation, the post depositional alteration processes associated with diagenesis and low-grade metamorphism during the Ouachita orogeny might have overprinted the depositional signatures.