Date of Graduation

5-2022

Document Type

Thesis

Degree Name

Master of Science in Geology (MS)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Paradise, Thomas R.

Committee Member

Dumond, Gregory

Second Committee Member

Potra, Adriana

Keywords

lead isotopes; Mona Lisa Mine; Raman spectroscopy; strontium isotopes; turquoise; x-ray diffraction

Abstract

The presence of turquoise in Arkansas has long been disputed by geologists and gem collectors. Despite the prevalence of other phosphate minerals in Arkansas, such as planerite, wavellite, and variscite, the absence of an obvious copper (Cu) source required for turquoise formation was considered too great of an obstacle for turquoise mineralization. In this study, geologic, mineralogical, and geochemical techniques provide overwhelming evidence that the novaculite-hosted Mona Lisa Mine in Arkansas does produce turquoise. X-ray diffraction and Raman spectroscopy have confirmed the structural nature of the material and identified other phosphate minerals, such as crandallite, often found in assemblage with turquoise. Elemental concentrations of Cu up to 5.82 wt.% analyzed on a Thermo Scientific™ iCAP Q ICP-MS clearly show that Cu is a major chemical component. Lead and strontium isotope signatures have been used in studies of the southwestern U.S.A. turquoise to determine the origin of archaeological turquoise. This study applies Lead (Pb) isotope analyses determined on a Nu Plasma multi-collector ICP-MS to constrain the sources of Cu and its potential path prior to mineralization. The determined 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios indicate that the Pb was derived from the host Arkansas Novaculite. However, the novaculite does not contain enough Cu to be the primary Cu source for turquoise. Therefore, an additional, yet unknown Cu source is required which has negligible lead content. Sr isotope analyses (87Sr/86Sr ratios between 0.71229 and 0.71916) indicate the crustal (basement) source of the material and combined with Pb ratios to provide a unique isotopic signature of Mona Lisa turquoise that can be used as a database for future artifact provenance study. Studying the petrogenesis of Mona Lisa turquoise provided the necessary background for future assessment of potential turquoise sites in the southeastern U.S.A. By combining these analytical techniques with a gemological evaluation, the current study takes an innovative and inclusive look at each aspect of the Mona Lisa Mine and its potential as a source of gem-quality American turquoise in the future.

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