Document Type

Honors Thesis

Publication Date

Spring 5-8-2026

Abstract

The Phosphoria Formation is a world-class phosphate-producing paleobasin in the Rocky Mountains. It has produced over 230 Mt of commercial phosphate since the 1920s. Rare earth element (REE) mineralization in phosphorite was observed historically, but commercial recovery of REEs has not been reported. Increasing demand for domestic REE supply has made resource characterization a priority. This study investigates the distribution of REEs within the Phosphoria Formation across southwest Montana. Five sections of Retort Member from the Phosphoria were exposed to measure stratigraphy and collect samples for geochemical analysis. Historic mine sites and prospects were also visited to collect and analyze surface mine waste and collect grab samples. Thin sections and polished plugs of phosphorite samples were analyzed by petrographic microscopy, automated SEM-EDS, EPMA, and LA-ICP-MS. Assays indicate ΣREY concentrations up to 3480 ppm with an average of 1360 ± 615 ppm ΣREY and 25.5 ± 8.25 wt.% P2O5 across all phosphorite lithofacies (n=133). Heavy REEs (HREEs) comprise 48.3 ± 5.00% of total REE content. REEs are hosted in carbonate-fluorapatite (francolite), although correlations between phosphate and REE concentrations are variable. Lithologic interpretation of the stratigraphy indicates a cyclical pattern of enrichment at local scales, as well as regional variation in grade. Phosphoria strata in the Garrison and Philipsburg districts along the northern paleobasin margin exhibit elevated grades averaging 1670 ± 587 ppm ΣREY and 26.4 ± 8.50 wt.% P2O5 (n=60), compared to strata from North Big Hole Canyon near the paleobasin center. Those strata average 681 ± 258 ppm ΣREY and 25.6 ± 5.60 wt.% P2O5 (n=15). Locally, phosphorite beds at the top and base of the Retort Member exhibit elevated ΣREY:P2O5 ratios, whereas middle beds are commonly depleted. Concavo-convex pressure solution textures on allochems in phosphorite are correlated with REE-enriched zones at lithologic facies transitions. These textures are interpreted to enhance REE enrichment by remobilizing phosphate into porewaters, facilitating interaction with dissolved REEs and promoting re-precipitation within phosphatic sediments. This interpretation can be further evaluated through detailed geologic mapping of phosphorite packstone facies, as well as core drilling and expanded stratigraphic sampling along the northern margins of the Phosphoria paleobasin.

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