Date of Graduation

5-2019

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Manger, Walter L.

Committee Member

Potra, Adriana

Second Committee Member

McGilvery, T.A. "Mac"

Keywords

Arkansas; Chert; Novaculite; Radiogenic Isotopes; Trace Elements

Abstract

This study uses trace elements and radiogenic isotopes (Pb, Sr, and Nd) to investigate the origin and mode of formation for the siliceous deposits in the Lower Mississippian Boone Formation and the Devonian-Mississippian Arkansas Novaculite in the southern midcontinent. Mississippi Valley-type Pb-Zn ore deposits in the Tri-State District and the Northern Arkansas District were deposited by hydrothermal fluids, and highly radiogenic Pb isotope ratios suggest a genetic relationship between the Boone Formation chert (206Pb/204Pb ~ 21.59, 207Pb/204Pb ~ 15.87, 208Pb/204Pb ~ 40.10) and the MVT ores. Due to the very low concentration of Pb in the Boone chert (~2 ppm) and the close proximity of samples containing radiogenic Pb to the tripolitic chert interval, the Boone Formation is interpreted to have been contaminated with radiogenic Pb by the hydrothermal fluids that emplaced the ores. Calculated epsilon values for Nd reveal a small positive shift in the Boone samples (epsilon Nd ~ -5.5) relative to other measured stratigraphic intervals in the southern midcontinent (epsilon Nd ~ -15.2), potentially indicating volcanic contribution of Nd. Upper Boone cherts (epsilon Nd ~ -4.6) and Hatton Tuff (epsilon Nd ~ -3.8) values are remarkably similar, indicating the same Nd source. Sr isotope ratios of the Boone samples (87Sr/86Sr ~ 0.7089) largely reflect Mississippian seawater Sr (87Sr/86Sr ~ 0.7080), while the novaculite samples contain more radiogenic Sr (87Sr/86Sr ~ 0.7121), likely reflecting Sr exchange from the detrital component within the Arkansas Novaculite stratigraphic interval. Rare earth element (REE) concentrations normalized to average continental arcs plot near one for the upper Boone chert samples, indicating similar compositions, while the lower Boone and Arkansas Novaculite have lower normalized values. Rare earth element plus yttrium (REY) concentrations normalized to Post-Archean Australian Shales (PAAS) reveal a negative cerium (Ce) anomaly and a positive yttrium (Y) anomaly, which is a signature of seawater, indicating that the Boone cherts may retain a depositional seawater REY signature.

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