Date of Award

Spring 6-14-2026

Document Type

Thesis (Undergraduate)

Department

Earth Sciences

First Advisor

C. Brenhin Keller

Abstract

Banded iron formations (BIF) are redox-dependent chemical sedimentary rocks that have served as proxies for marine and atmospheric oxygen and iron conditions throughout Earth's history. While abundant in the Archean and early Proterozoic, the widespread deposition of BIFs declined following the Great Oxidation Event, as oxygenated surface waters prevented the accumulation of ferrous iron. However, the Needle Mountains, located in southwestern Colorado, house an anomalously young BIF deposited between 1786±10 Ma - 1801±6 Ma given existing geochronology. Here, we use rare earth element (REE) systematics, bulk chemistry, and petrographic analysis to determine the depositional conditions under which this BIF was formed. We hypothesize that this BIF was deposited in a restricted or partially restricted stratified basin, with mafic hydrothermal Fe input and felsic continental REE input. Bulk geochemical analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to determine elemental compositions in magnetically separated mineral fractions of primarily chert and magnetite. We determine that, when chondrite-normalized, there is a distinct negative Eu/Eu* anomaly (0.67-0.84), a weakly positive Ce/Ce* anomaly (1.12) in the magnetic fraction, a strong LREE/HREE enrichment, and suprachondritic Y/Ho values. These observations support our hypothesis and are interpreted within the tectonic context of the back-arc basin that developed between the assembling of the Yavapai and Mazatzal Provinces during the formation of the supercontinent Columbia, providing a restricted marine setting for the formation of the Needle Mountains BIF. These findings ultimately support the view that ferruginous anoxic basins suitable for the precipitation of BIF continued to persist locally into at least the Statherian.

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