ORDOVICIAN AND DEVONIAN BLACK SHALE REE PHOSPHORITES OF THE US MIDCONTINENT: ACTIVE EARTH MRI PHASE 2 TARGETS
The targeted REE phosphorites interfinger black shales and cover large areas. The primary Ordovician black shale REE phosphorite occurs over > 200 mi2 (~360 km2) and is on average 10 ft-thick (3.3 m) within the Maquoketa Group of northeastern Iowa and adjacent states. The other major deposit of interest is the Devonian “blue-rock” phosphorite in the Chattanooga Shale of west-central Tennessee that occurs over 400 mi2 (~720 km2), though with a thickness that rarely exceeds 3 ft (~1 m). The REE concentrations within the Ordovician phosphorite are nearly 10 times higher than associated black shales and the Devonian phosphorite is nearly 10 times more enriched than the Ordovician deposit.
The ultimate source of REEs in these deposits was enriched sea water. Traditional models of phosphogenesis that invoke “condensation” and “upwelling” are inconsistent with the detailed sedimentary geochemistry and mass balance estimates for these units. Geochemical and regional studies to better constrain the age, depositional setting, and black shale-associated environmental perturbations are being used to evaluate if changes in the redox-cycling of elements during oceanic anoxic events may, ultimately, explain the abundance of both phosphorous and REEs.