Award Date

May 2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geoscience

First Committee Member

Simon Jowitt

Second Committee Member

Eugene Smith

Third Committee Member

Rod Metcalf

Fourth Committee Member

Shichun Huang

Fifth Committee Member

Vernon Hodge

Number of Pages

491

Abstract

Critical metals are vital to the functionality of the modern world. The increased diversity and usage intensity of these commodities in defense-related and strategic technologies (e.g., jet engines, rocket assemblies), modern infrastructure and products (e.g., high-strength steel, cellular phones, laptop computers), and energy-related, low-emissions technologies (e.g., nuclear reactors, solar panels, wind turbines, hybrid and electric vehicles) has drastically increased our reliance on critical metals and, thus, the United States (and global) exposure to supply restrictions. Granitic pegmatites are unique mineral deposits that represent a past, present and future resource for many (approximately half) of these critical resources. Even with the significance of granitic pegmatites for critical metal resources and the plethora of research available on these systems, numerous controversies related to the formation of these deposits remain unresolved. This includes the petrogenetic origin of critical metal-mineralized pegmatite-forming melts, namely whether these melts are formed exclusively via extensive fractionation of a pluton-scale volume of magma, or also by low-degree anatexis of a critical metal-rich source. In addition, the importance of primary internal processes such as undercooling or high-temperature melt-melt-fluid immiscibility, which can induce critical metal mineralization at the regional scale within a pegmatite field and locally within a single pegmatite, remains actively debated. Understanding each of these controversies provides context for the exploration of critical metals in these deposits at both local and regional scales. Lastly, our lack of understanding of the material flow of critical metals in the anthropogenic cycle is a key factor that inhibits the resolution of supply chain restrictions. This is exemplified by the fact that we do not know how much of these metals we trade outside of raw material forms (i.e., embedded critical metals within the widely traded products generated by large-scale industries). The projects in this dissertation approach critical metals and granitic pegmatites from an economic geology perspective with consideration of igneous petrology, geochemistry, mineral economics, and industrial ecology to shed light on the above controversies that hinder our understanding of the concentration and flow of critical metals in a geologic and anthropogenic context, with the goal of outlining solutions of supply resilience for critical commodities.The first project in this dissertation (Chapter 3) focuses on the economic potential, characterization, and petrogenesis of the Virgin Mountains pegmatite field bordering Nevada and Arizona, an understudied, domestic (for the United States) critical metal-mineralized (Be, Nb) pegmatite field. Understanding the geologic processes involved in the petrogenesis of these critical metal pegmatites is necessary to determine the essential geologic criteria required to form these deposits, and to delineate areas of interest domestically and globally for the occurrence of potentially economic-grade granitic pegmatites. A combination of fieldwork and whole-rock geochemical data collected during this study highlights several indicators of critical metal-mineralized pegmatites, including the presence of green-tinted muscovite, elevated bulk rock critical metal anomalies and P2O5, and low bulk rock K/Rb, Zr/Hf, Ba, and K2O relative to barren pegmatites. These tools were used to outline a narrow pegmatite trend in the Virgin Mountains that is prospective for critical metals and as a potential domestic source of high-purity Be. In addition, field and petrographic observations undertaken during this project enable the division of critical metal pegmatites into beryl-columbite and chrysoberyl types, and barren pegmatites into multiple mineralogic varieties. This research demonstrates that chrysoberyl pegmatites are likely metamorphosed beryl-columbite pegmatites. Incomplete reactions within the critical metal pegmatites involving (1) beryl-chrysoberyl and (2) magmatic andalusite recrystallizing to subsolidus kyanite and crosscut by sillimanite constrain metamorphic pressure-temperature conditions to a sillimanite-grade metamorphic overprint. Trace element modeling undertaken during this study indicates that (1) the critical metal pegmatites in the study area are likely the result of a combination of equilibrium and fractional crystallization of a local biotite monzogranite, and (2) modal fractional melting, or non-modal muscovite fluid-fluxed or dehydration melting of local mica schist units, could not have produced these critical metal pegmatites. Importantly, the geochemical variability present in these samples indicates that other petrogenetic scenarios are possible, including the melting of a heterogenous, metasedimentary source, episodic melt extraction from a thermally maturing source, crustal contamination, or the presence of a blind pluton. An important implication of this study is that although recent and previous pegmatite classification schemes would have labeled this pegmatite field as the product of direct anatexis, our results indicate that these pegmatites may instead be pluton-related. Our results also suggest that critical metal pegmatites associated with direct anatexis urgently require a unified petrogenetic model with feasible geologic processes, rather than citing the lack of evidence for a pluton-related origin as evidence for direct anatexis. A more definitive working model would greatly assist in the exploration for these deposits, whether regionally within a pegmatite belt or locally within a pegmatite field, which may indeed be anatectic. Following this natural case study, I investigate the internal (intrapegmatite) processes responsible for the distribution of critical metal mineralization within granitic pegmatites and throughout a pegmatite field via the largest collated database of published crystallization temperatures (compiled for this study) from global pegmatite occurrences (Chapter 4). Thermometry results from different pegmatite classes and families show that decreasing temperature (increasing liquidus undercooling) is correlative with the mineralization of critical metals within granitic pegmatites. My assessment of this comprehensive dataset indicates that critical metal-mineralized pegmatite classes (i.e., Rare Element and Miarolitic classes) crystallize on average ~170°C or more below (i.e., subsolidus) the hydrous haplogranite liquidus, whereas barren pegmatite classes generally crystallize close to their liquidus temperature (i.e., suprasolidus). In addition, the different pegmatite families crystallize at different mean temperatures suggesting that a combination of melt composition and source controls may be responsible for the mineralization of the associated groups of commodities in each family. These results also are used to compare intrapegmatite crystallization temperatures, showing that undercooling is one important factor responsible for the development of complex internal zoning that results in the development of critical metal-mineralized zones that can be targeted for economic exploitation. In comparison, unzoned pegmatites typically crystallize at temperatures between their liquidus and solidus similar to typical granites. Temperature variability amongst classes and zones indicates that the magnitude of intrapegmatite temperature variation may be a factor in the development of world-class pegmatite deposits. These results, coupled with the similar textures and relatively low temperatures recorded in other economic-grade mineral deposits, strongly suggest that undercooling may also be a necessary process in the development of other types of critical mineral deposits. This chapter is currently under review for publication in Earth-Science Reviews. Finally, I examine the movement of critical metals after geologic extraction through the anthropogenic cycle (i.e., mineral concentrates to embedded forms within manufactured products) to determine the effect of the inclusion of these material flows on supply chain restrictions (Chapter 5). In particular, this project focuses on the material flow (e.g., imports and exports) of Nb in primary forms, partially derived from columbite in granitic pegmatites, and those embedded within fabricated and manufactured products, specifically the steel and automotive industries, for the United States and China. Both of these countries are large, primary consumers and high net importers of Nb. These results demonstrate that accounting for embedded Nb, which is typically not included in most critical material flow studies, significantly modifies apparent consumption estimates. The inclusion of the consideration of embedded Nb in U.S. and China imports and exports demonstrates that the United States on average actually consumes nearly double the amount of Nb that was previously reported, whereas China significantly decreases its apparent consumption of Nb because much of its imported primary Nb is re-exported within steels. In addition, the United States has a multi-stage Nb import dependence throughout its material flow cycle, while China is only import-dependent at the primary stage of the Nb flow cycle. This indicates that net import reliance of raw material or primary forms do not provide the entire picture necessary for outlining critical mineral commodity supply restrictions and that quantifying the embedded flows of critical mineral commodities would provide much-needed insights for crafting solutions to reducing the import dependence of these commodities. Solutions to reduce criticality or import dependence of critical minerals are commodity-specific and may require the discovery of domestic resources, securing non-domestic resources by incentivizing their acquisition, country-specific partnerships, or the development and expansion of domestic industries that utilize these commodities to reduce downstream imports. This chapter has been published in Resources, Conservation & Recycling. Together these projects provide insights into the controversies surrounding critical commodities, from the geologic processes that form these deposits, specifically in granitic pegmatites but possibly applicable to other critical metal deposits, to the pathway of one of these commodities in the anthropogenic cycle. Importantly, these projects illustrate that solutions to criticality require a multi-faceted approach with an understanding of the mechanisms responsible for the concentration and flow of these commodities in a geologic and anthropogenic context. A combination of the approaches undertaken in this dissertation are necessary if stated improvements to critical metal and mineral supply chain security and associated targets such as an energy-efficient, low-emissions future are to be realized.

Keywords

Criticality; Niobium; Pegmatite petrogenesis; Rare element pegmatites; Undercooling; Virgin Mountains

Disciplines

Geology

File Format

pdf

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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