Preliminary Earth MRI results from Devonian-Mississippian rare earth element (REE) enriched sedimentary phosphate across the Eastern US

Previous USGS research identified sedimentary phosphate as a potential REE resource. Through a collaborative Earth MRI study, we conducted geochemical reconnaissance of U.S. Devonian to early Mississippian age phosphate across the Appalachian and midcontinent basins with preliminary results showing high REE concentrations. The Appalachian basin contains several Lower to Middle Devonian intervals of phosphate that are enriched in REEs. Notably, nodular and concretionary phosphate is abundant in the Pragian where it is associated with localized oolitic ironstone and overlain by phosphate concretions. However, nodular phosphate in the upper Givetian, though less concentrated, contains the highest REE enrichment. Importantly, there is a gradation of shale-normalized REE patterns from relatively low and flat at the base of the Devonian to distinct middle rare earth-enrichment toward the top of the Givetian. In the midcontinent basin the Upper Devonian to lower Mississippian contains a thick phosphorite and concretionary phosphate horizons. In western Tennessee the previously mined "blue-rock" phosphorite occurs over ca. 20,000 km2 with a maximum thickness approaching 1.5 m. Our new data indicate that this deposit extends into Kentucky, Illinois, and Iowa. Locally it is immediately overlain by an interval of reworked lower Mississippian phosphate concretions. These concretions extend westward to Oklahoma and as far north as central Illinois (ca. 370,000 km2). Overall phosphate REE patterns in the Upper Devonian to lower Mississippian of the midcontinent show a decrease in middle rare earth enrichment and a consistent negative Ce anomaly. The consistency of REE patterns across vast areas suggest the ultimate source of REEs was sea water. Improved constraints on the age, depositional setting, and environmental perturbations associated with these deposits taken together indicate that redox-cycling of elements during oceanic anoxic events may, ultimately, explain the episodic increases of both phosphorous and REEs. These events may also have had a long-term cumulative impact on global seawater chemistry. Our results confirm the high REE potential of sedimentary phosphate and are providing new insights for their exploration.