Isotope Geochemistry and Mineralogy Applied to Energy-Critical Mineral Deposit Geology

Abstract

This dissertation broadly addresses issues at the intersection of minerals and society. Mineral resources such as lithium and copper are crucial to renewable energy technology and global decarbonization goals. Chapter 2 investigates metal and fluid sources of the Ernest Henry iron oxide copper gold (IOCG) deposit using iron, oxygen, and titanium isotopes in magnetite. The results indicate a major leached iron component, contrasting with other IOCG systems and suggesting that one metal source hypothesis cannot explain all IOCGs. The results also showed that titanium isotopes can fractionate during ore formation, which was not anticipated for a hydrothermal system—prior to this work, significant titanium isotope fractionation on Earth was believed to exclusively originate from equilibrium melt-mineral fractionation producing values within a limited range.

Chapter 3 explores the potential for fluid-mineral titanium isotope fractionation through computational modeling of dissolved titanium complexes and leaching experiments to determine if fluid-mineral interaction can fractionate titanium isotopes, determining that fluid interaction with most Earth materials would result in a fluid enriched in heavy titanium isotopes. Kinetic isotope effects are interpreted to have affected the leaching experiments, resulting in preferential leaching of light titanium isotopes. This suggests that the counterintuitive fractionation in natural systems may be explained by kinetic effects.

Chapter 4 builds upon these ideas by investigating titanium isotope compositions in numerous additional mineral deposits, including igneous layered mafic intrusions as well as iron oxide apatite systems that are believed to form through a combination of igneous and hydrothermal processes. Extensive fractionation was observed in all three deposit types, including values that are too high to be explained by equilibrium melt-mineral fractionation, as well as other values that are too low to be explained by this mechanism. New fractionation models incorporating both equilibrium and kinetic effects were proposed to explain the fractionation observed in mineral deposits. During magma mixing, titanium concentration gradients can yield kinetic isotope effects that can be preserved if magma mixing triggers mineral formation. In hydrothermal systems, kinetic effects can preferentially leach light titanium isotopes, whereas heavy titanium isotopes are lost during equilibrium fractionation. Magnetite may also lose light titanium during the exsolution of titanium-rich phases like ilmenite.

Chapter 5 investigates the origin of the Thacker Pass lithium deposit in Nevada through clay chemistry, X-ray diffraction, stable isotope geochemistry, and computational modeling to resolve the mechanism responsible for converting primary smectite claystone with lithium grades of c. 0.3 weight percent to illite claystone with lithium grades above c. 0.6 weight percent. The results are consistent with a hydrothermal model and underscore the importance of illitization and F availability for enabling increased Li grades due to crystal chemical considerations.

Chapter 6 investigates how gender is represented in mineral names. More than half of all minerals are named for people, mostly those involved in the geosciences and mining industry. Women comprise about 6.1% of people with minerals named for them globally. Representation is disparate across different countries, with Russian women representing about 43% of women eponyms, but only 14.8% of all eponyms. Women scientists were also found to be older on average than their male colleagues when honored with a mineral name. While women’s representation among mineral namesakes has improved in the last sixty years, annual representation among new mineral namesakes is not expected to exceed around 10.5% without unprecedented changes to naming trends.