Date of Graduation

5-2024

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Potra, Adriana

Committee Member

Dumond, Gregory

Second Committee Member

McGilvery, T.A. "Mac"

Third Committee Member

Shaulis, Barry

Keywords

Economic Geology; Geochemistry; Pb-Zn Ores; Precambrian Geology; Radiogenic Isotopes; Trace Elements

Abstract

Mississippi Valley-Type (MVT) ores are epigenetic, sedimentary hydrothermal base metal deposits that exist worldwide with type localities in the midcontinent United States. These Permian-aged sulfide ores represent an extraordinary concentration of economic metals, Pb and Zn. Ore deposits occur in vast districts that have a rich mining history. The Pb and Zn from these deposits account for 24% of worldwide reserves. The geologic processes responsible for MVT ore genesis are not fully understood, especially the sourcing of ore constituents. In the midcontinent, the Ouachita Orogeny expelled saline hydrothermal brines in response to high rates of subsidence. These fluids migrated through Arkoma Basin carrier beds and faults to deposit MVT ores up-dip within basin flank carbonate hosts. Base metals were leached into solution from surrounding wall rock, and metals traveled within the fluid to the ore deposition site. Which lithologies had a greater metal contribution is unconstrained. Lead (Pb) isotopes can be used to investigate potential relationships between geologic features. Southern midcontinent ores show highly radiogenic Pb signatures, so a lithologic metal source must have a similar isotopic nature. Precambrian basement rock has been a long-hypothesized metal source due to generally higher concentrations of radiogenic Th and U, evidence of hydrothermal alteration, and ore Pb source model ages yielding a 1.01 and 1.05 Ga Pb source. Fifty-three basement samples from the Central Plains Orogen (1.65-1.8 Ga) and Southern Granite Rhyolite Province (1.3-1.4 Ga) were collected from cores and cuttings retrieved during drilling operations. The full suite was analyzed for trace element concentrations, and thirty-nine samples were analyzed for radiogenic isotopes Pb, Sr, and Nd. Present-day Pb isotope ratios of basement rocks create linear trends similar to MVT ore trends and three samples are enriched in radiogenic Pb. Age-corrected Pb isotope ratios reflect the sample Pb signatures at the time of MVT ore deposition, and these ratios plot less similarly to ore Pb signatures. Age-corrected Pb isotope ratios alone suggest the basement rocks and the MVT ores are not genetically related. Present-day ratios are only slightly promising. The three highly radiogenic samples are unique from the rest not only because of their Pb signature but also their enrichment in 87Sr/86Sr and unique enrichment in heavy rare earth elements (HREEs). These data suggest that the basement as a whole did not supply metals to midcontinent MVT ores, but perhaps localized features within the basement such as younger intrusions or igneous dikes supplied the highly radiogenic Pb. The three anomalously enriched samples may represent one of these more local features. This would be consistent with the 1.01 and 1.05 Ga model Pb source ages because the expected age of midcontinent basement is 1.3-1.8 Ga. These new data provide valuable geochemical results to the limited database of southern midcontinent basement, and they have implications for understanding the formation of economic MVT deposits of the southern midcontinent.

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