Novel Applications of Stable Metal Isotope Geochemistry to Understand Ore Formation

Abstract

Metals including molybdenum (Mo) and iron (Fe) are critical resources for modern society. These metals are extracted from various types of ore deposits including porphyry (Mo) and skarn (Fe and Mo) and are predominantly used in steel production and contribute to technological, industrial, and energy production. However, as population, industrialization, and technology continue to grow and advance, the demand for metals like Mo and Fe will increase, requiring the discovery of new resources. Understanding geochemical behavior and the geological processes that transport and concentrate these metals in the crust can help scientists better develop ore deposit models and discover additional resources. One way to achieve this is to develop new geochemical tools to elucidate the ore-forming processes recorded in rocks and minerals. This dissertation utilizes the stable isotope systems of Mo and sulfur (S) in molybdenite (MoS2) and Fe in garnets to develop and refine geochemical techniques that can be used to understand ore deposit formation. Chapter 2 applies the isotope pairing technique using Mo and S isotopes in MoS2 to develop a new method to illuminate Mo isotope variation in magmatic-hydrothermal systems. The results indicate that Mo-S isotope pairs can distinguish deposit types, which was not possible using Mo isotopes alone. When paired with Mo isotopes, S isotope ratios reveal information about the source fluid while Mo isotope ratios provide insight into processes during ore formation. More work is needed to further develop this technique. Chapter 3 presents the first experimental work to preliminarily quantify the Mo isotope fractionation factor between MoS2 and an aqueous fluid at magmatic-hydrothermal conditions (800°C, 150 MPa). The data confirm that Mo isotope fractionation is controlled by redox and Mo coordination; MoS2 preferentially incorporates the lighter Mo isotopes while the fluid retains the heavier Mo isotopes. These results also support the hypothesis proposed by previous studies that Rayleigh fractionation is the main factor controlling Mo isotope variation observed among magmatic-hydrothermal ore deposits. Chapter 4 focuses on the Tibes Fe skarn in Puerto Rico as a case study to test the use of Fe isotope geochemistry in garnets to elucidate fluid evolution during skarn formation. The data from this work reveal that the bulk Fe isotope compositions measured across the garnet populations capture a subtle change in fluid redox conditions over time. The oldest population has the highest isotope ratios while the youngest population has the lowest Fe isotope ratios, suggesting that fluid conditions evolved from relatively oxidizing to reducing. This trend was also observed on the crystal scale with the garnet cores having isotopically heavier compositions compared to garnet rims. These results demonstrate the promise of Fe isotopes in garnets as effective tracers of fluid evolution during skarn formation.