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Title: Enrichment of Rare Earth Elements in Cretaceous Clays of the lower Dakota Formation in Northwest Iowa

Abstract: This study investigates rare earth element (REE) enrichment in clay-rich intervals of the lower Dakota Formation (Cretaceous) in northwest Iowa. Whole-rock geochemical data from two cores, Larson Lake and Cherokee, and 58 well chip samples were analyzed using ICP-MS to quantify REE concentrations and evaluate enrichment mechanisms. Results identify a regionally consistent REE-enriched interval (30–35 ft thick) at the base of the Cretaceous directly above a major unconformity, with total REEY concentrations ranging from 400 to 2,000 ppm which are well above average upper continental crust values and reach resource-grade enrichment.

Geochemical signatures indicate substantial light REE (LREE) enrichment with variable medium and heavy REE (MREE and HREE) contributions. The Larson Lake core exhibits enrichment consistent with ion adsorption onto clay minerals and iron oxyhydroxides within a well-developed paleosol, reflecting intense chemical weathering under humid Cretaceous conditions. In contrast, the basal Cherokee interval shows more complex behavior, including strong correlations between REEs and P₂O₅, suggesting an additional control from phosphate minerals. SEM-EDS data support the presence of clay minerals, Fe–Mn oxyhydroxides, and minor phosphate phases as key REE hosts.

A secondary, stratigraphically higher enrichment zone is unique to the Cherokee core and is dominated by adsorption-controlled processes with minimal phosphate influence. Collectively, results support a model of REE mobilization from Precambrian source rocks, followed by concentration via weathering, transport, and adsorption in clay-rich paleosols and fluvial deposits. These findings highlight the Dakota Formation in northwest Iowa as a promising target for REE resources in the midcontinent and warrants further exploration.