7-34 Rare Earth Element Concentrations in the Basal Mount Simon Sandstone Formation, Iowa

Session: Undergraduate Research, Part I (Posters)

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Abstract:

In Iowa, a major regional unconformity separates deeply eroded Precambrian (pC) rocks from younger Phanerozoic sedimentary sequences. This unconformity signifies a period of extensive physical and chemical weathering, representing as much as 1.3 Ga. This study focuses on the basal facies of the Cambrian Mount Simon Sandstone (MSs) Formation, which may contain rare earth element (REE)-bearing minerals (apatite, monazite, zircon, etc.) as well as other critical minerals (Ni-Cu sulfides, Fe-Mn-Ti-Zn oxides, etc.) that have been concentrated through sedimentary and diagenetic processes and are often associated with weathered regolith on top of crystalline basement rocks. We test the hypothesis that the basal Mount Simon is richer in REEs when sourced from granitic basement rather than mafic crystalline rocks. New whole rock geochemical analysis was acquired for 70 samples from 11 drill cores from across Iowa, which are in repository at the Iowa Geological Survey. Samples were collected from unweathered and weathered Precambrian basement and the Mount Simon Sandstone. A subset of these cores were analyzed with a portable X-ray fluorescence spectrometer to determine elemental composition profiles spanning the unconformity.

The results demonstrate higher concentrations of common rare earth elements in drill cores with a silicic basement compared to a mafic one. Elevated total REE values measured from granitic rocks are dominated by light REE, with an average LREE fraction of 0.77. Mafic units are REE depleted but have slight to moderate transition metal enrichment, with as much as 1,000 ppm Cu, 1,500 ppm Ni, and 1,300 ppm V. The basal MSs also shows elevated transition metal enrichment where overlying mafic pC rock. Additional insights into the differential weathering intensities across this unconformity are also evaluated for influencing the concentration of heavy minerals in the basal MSs. Our results indicate that the basal Mount Simon in Iowa is prospective for REE deposits. However, this is only one small aspect to consider when determining whether domestic sedimentary sources of REEs are economically-viable and a possible substitute for foreign ones.

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