Date of Graduation
12-2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Geosciences (PhD)
Degree Level
Graduate
Department
Geosciences
Advisor/Mentor
Adriana Potra
Committee Member
Dumond, Gregory
Second Committee Member
Sharman, Glenn
Third Committee Member
McGilvery, Thomas "Mac"
Keywords
Black shales, Isotope Geochemistry, Lead-zinc (Pb-Zn), Neodymium (Nd), Ore deposits, Trace elements
Abstract
The formation of Mississippi Valley-type (MVT) lead-zinc (Pb-Zn) ores is complex, involving an interaction of tectonic compression, hydrothermal fluid migration, and geochemical processes within the subsurface. Fundamental questions regarding this interaction remain unclear, including the source(s) of the Pb and Zn ions that make up the ores. A genetic link between black shales and MVT ores has been thought to exist because of (1) the presence of mature hydrocarbon and brine fluid inclusions within many MVT ores, (2) the association of the hydrothermal fluids that are linked to MVT ore formation with organic compounds and oil-like droplets, and (3) the highly radiogenic nature of the ores. Many Mid-continent MVT deposits also form along basin margins interstratified with organic-rich black shales. These shales can form laterally extensive deposits which can accumulate a variety of metals and acquire high concentrations of U and Th, which decay into radiogenic Pb. The goal of this study is to explore the geochemistry of organic-rich black shales from the Mid-continent region to constrain their depositional environments, sediment source(s), and, most importantly, to uncover any possible genetic relationship with MVT ores. Recognition and proof that these shales may be a major source of metals in ores may cause a paradigm shift of the basic concepts of how and where these ores form and may lead to identification of yet-to-be-found deposits. Sixty-nine shales from 21 Cambrian to Pennsylvanian stratigraphic units across the Mid-continent have been analyzed for total organic carbon (TOC), elemental concentrations, mineralogical composition, and isotope ratios (Nd and Pb). The formation of these shales is constrained using trace element paleoenvironmental proxies, which serve as critical indicators of the prevailing depositional conditions under which the shales formed. The results show a wide spectrum of environments, spanning from Cambrian samples in the northern Mid-continent influenced by upwelling-driven oxic marine conditions, to Ordovician and Pennsylvanian shales characterized by anoxic, open marine environments. Additionally, the proxies reveal an escalation of anoxic conditions during the deposition of the Ordovician strata, as denoted by changes in Sr/Ba ratios during this interval indicating decreasing salinity. The isotopic signatures of Nd alongside the relative proportions of Rare Earth Elements (REEs) are also investigated to unravel insights into the source(s) of the sediment that comprises the shales. The findings demonstrate that the preeminent source of sedimentary constituents for these Mid-continent shales predominantly consists of felsic detrital minerals originating from the craton. However, a discernible shift in sediment source transpires during the Ordovician, which coincides with the uplift of the Appalachian Mountains during the Taconic orogeny. The role of these black shales in the genesis of Mississippi Valley-type lead-zinc ore deposits is examined to address the origin of the markedly radiogenic Pb isotope signatures in these deposits. The Pb isotope ratios of most black shales of the Mid-continent U.S. analyzed in this study are lower than those of the Mid-continent Pb-Zn ores, and only select shales exhibit Pb isotope ratios that more closely resemble those of the MVT ores. While none of the shales analyzed in this study match the highly radiogenic end-member that contributed to the formation of the Pb-Zn ores, the linear pattern of the ores does not preclude the possibility of select shales acting as the lesser radiogenic source that mixed with a highly radiogenic source, creating the isotopic signature pattern seen in the regional ores. A potential source for this highly radiogenic Pb, hypothesized in several studies, may be deep basinal migration of brines transporting Pb from the basement via faults. However, recent Pb isotope data from basement rocks of the Southern Granite-Rhyolite Province do not support this hypothesis, causing the source of this highly radiogenic Pb to lay further afield.
Citation
Morris, N. S. (2023). Geochemistry of Metal-Rich Black Shales and Their Genetic Relationship to Mississippi Valley-Type Pb-Zn Ore Deposits, Midcontinent U.S.. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/5123