CHARACTERIZATION OF REE-ENRICHED MINE WASTE AT THE BROWNS BOTTOM QUARRY IN DUBUQUE, IOWA, USA

Rare earth element (REE)-enriched phosphorites commonly overly limestone quarries throughout the central U.S. Unknowingly, these rocks are stripped and stockpiled as mine waste despite, in some cases, being worth several times more than the aggregate itself. An excellent example of this type of REE-enriched mine waste was discovered at the Browns Bottom Quarry (BBQ) site in Dubuque, Iowa. For the last half-century phosphatic shales and dolostones of the Elgin Member of the Ordovician Maquoketa Formation have been stripped and stockpiled in an abandoned pit up to 100 feet deep and used to construct berms around settling ponds. This two-year project was funded by the USGS Earth MRI – Mine Waste Characterization program to assess the resource potential of this already mined REE accumulation.

The BBQ mine waste assessment is an outgrowth of our ongoing EMRI regional geochemical reconnaissance project of Ordovician phosphorites across the midcontinent that is being conducted by a coalition of six midwestern states. Of the 250 samples collected from Iowa during the first year of the study, 15 were from the Elgin Member at the BBQ site. Results indicate that up to eight feet of phosphorite, averaging 21% P₂O₅, occurs in the Elgin over the footprint of the BBQ site. As expected, REE concentrations correlate closely with PO₄ grades with high ∑REE values of 1,270 ppm (870 ppm average) and ∑HREE of 670 ppm (415 ppm average).

We estimate eight feet of phosphate (~20% P₂O₅) and an additional eight feet of interbedded black shale were stripped from an area of 0.15 miles² (~4 million feet²) at the BBQ site and accumulated in waste piles. Based on these rough estimates, the mine waste stockpiled at the BBQ site may contain as much as 1,000 metric tons of HREE or 20% of the contained resource in the Bokan Mountain prospect in southern Alaska. The BBQ mine waste project will include drilling 10 boreholes through the material stockpiled in the abandoned pit for bulk composite sampling. Composite sampling of the berms has already been completed. Analytical results from this sampling campaign will help to verify these estimates and lead to a resource evaluation of the mine waste at the BBQ site. Critical mineral resources contained in mine waste provide a possible opportunity to fast-track a domestic supply of HREEs.