Award Date

12-1-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geoscience

First Committee Member

Simon Jowitt

Second Committee Member

Shichun Huang

Third Committee Member

Arya Udry

Fourth Committee Member

Andrew Martin

Fifth Committee Member

Stephen Lepp

Abstract

A-type rhyolites and granites often contain elevated concentrations of the rare earth elements (REE) and many other critical trace elements relative to I- and S-type rhyolites and granites. Their trace element-enriched nature makes them potentially prospective bulk tonnage, low-grade resources of the REE and many other critical trace elements, including Li, Be, Mo, Sn, and W, but little work has been performed to delineate their economic potential. The 22.1 Ma Red Beryl Rhyolite (RBR) and the 18.3 Ma Tetons Rhyolite (TR) units of the Blawn Formation represent two A-type rhyolite domes located in the southern Wah Wah Mountains of Utah. The RBR is subdivided into two units, the RBR Lower and RBR Upper, based on notable differences in the accessory mineral assemblages they contain and REE concentrations. Source discrimination diagrams and fractionation modeling suggest that the parental melts of the Blawn Formation were generated by partial melting of the upper mantle or mantle-hybridized crust that underwent 70+ % total fractional crystallization before erupting the RBR Lower and RBR Upper. The prolonged fractionation that the parental melt of the RBR underwent caused formerly incompatible elements to begin to behave compatibly after eruption of the RBR Lower unit, causing the subsequently erupted RBR Upper unit to be strongly depleted in the REE and F. These elements were removed by crystallization of fluorite, cerianite-Ce, allanite-La, monazite-Ce, and xenotime-Y from the evolving melt, all of which are present in the RBR Lower but are absent except for fluorite, cerianite-Ce, and allanite-La in the RBR Upper. The geochemical and mineralogical variations between the RBR units demonstrate that evolved rhyolites can reach a point of maximum REE concentrations during the later stages of fractional crystallization and can become subsequently removed with further fractional crystallization. The presence of both primary and secondary examples of REE-bearing accessory phases in hydrothermally altered samples, combined with geochemical data that suggests alteration did not result in the concentration or removal of the REE contained in these phases suggests that the trace element depletions within the RBR Upper unit are a result of igneous evolutionary processes, not from alteration. Alteration mobilized much of the REE- and critical metal-bearing accessory mineral assemblage and locally redistributed it within the rock, but did not affect the overall concentrations of the REE and most trace elements.

Keywords

A-type Magmatism; Critical Element Resources; Evolved Rhyolite; Hydrothermal Alteration; Igneous Evolution; Rare Earth Element Resources

Disciplines

Geochemistry | Geology

File Format

pdf

File Size

7400 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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