Miocene-Pliocene phosphate strata

Region East, Southeast
Mineral systems
Deposit types
Critical minerals
Other minerals

Information leading to the delineation of this focus area

Basis for focus area The focus area includes past producers and district outlines from De Voto and Stevens (1979) and Cathcart (1980).
Identified resources Identified resources and historical production of phosphate, REE, and uranium.
Production Longstanding phosphate district producing approximately 18,000,000 t/yr of francolite from the Miocene Hawthorn Group and age-equivalent formations. About 44 million lbs of U3O8 has been produced from the central Florida phosphate district (Dahlkamp, 2010).
Status Past and current mining.
Estimated resources Estimated phosphate resources 14,600 Mt with average 30% P2O5 (Notholt and others, 1989) with annual production of 18,000,000 t of francolite (Jasinski, 2011). Estimated total REE resource 17 Mt ΣREE and 7.6 Mt ΣHREE. Estimated yearly byproduct potential 16,200 t ΣREE of which 7,200 t is ΣHREE (Emsbo and others (2016b). Total U resources could be as high as 1.2 billion lbs of U3O8, but those that are mineable in conjunction with phosphate production may be closer to 250 to 500 million lbs of U3O8 (Dahlkamp, 2010).
Geologic maps Horton and others (2017), scale 1:500,000; North Carolina Geological Survey (1985), scale 1:1,000,000; Richmond and others (1987a, b), scale 1:1,000,000; Johnson and Peebles (1986), scale 1:1,000,000.
Geophysical data Inadequate Rank 4 aeromagnetic and Rank 5 aeroradiometric coverage.
Favorable rocks and structures Miocene Hawthorn Group and age-equivalent formations; North Carolina: Miocene Hawthorn Formation, Miocene Pungo River Formation; Florida and Georgia: Pliocene Bone Valley Formation; Georgia: Pleistocene Ladson Formation.
Deposits Deposits and occurrences from Chernoff and Orris (2002) and MRDS. More than 25 mines have produced some U as a byproduct of phosphate production in the southeastern United States. The Aurora Phosphate district is in North Carolina.
Evidence from mineral occurrences MRDS; Chernoff and Orris (2002); International Atomic Energy Agency (2020b); U.S. Environmental Protection Agency (2006).
Geochemical evidence The Miocene Hawthorn Group and age-equivalent formations are major phosphate producers. Estimated mean REE in francolite is 900 ppm ΣREE and 400 ppm ΣHREE (Eu through Lu and Y) (Emsbo and others, 2015, 2016b). Economic F-bearing phosphate deposits in the eastern United States are confined to marine sedimentary phosphorites of Miocene age in North Carolina, Georgia, and South Carolina, and of Miocene and Pliocene ages in Florida. Fluorine content of the rock is about 3-4%, and billions of tons of phosphate resources are known. The U content of phosphate is generally low (50-200 ppm), and as such U is economic as a byproduct of phosphate production (De Voto and Stevens, 1979). In the Land Pebble district in central Florida, U is concentrated in a nodular horizon in the Pliocene Bone Valley Formation (Dahlkamp, 2010).
Geophysical evidence North Carolina: Natural gamma-ray log response patterns; downhole logs in boreholes.
Evidence from other sources North Carolina boreholes.
Comments Equivalent units in North and South Carolina are not mapped at state geologic scale. In Georgia some of the units are lumped into 'Neogene undifferentiated'. North Carolina says polygons might not be connected over Cape Fear Arch (between North and South Carolina) although districts might be connected offshore.
Cover thickness and description Interested in partially exposed (zero cover) phosphate units and currently mined units in deposits. In North Carolina, deposits are subsurface (covered) except for Aurora district mines.
Authors Poul Emsbo, John D. Horton, Susan Hall, Nora K. Foley, Bernard Hubbard, K.M. Farrell, W.R. Doar, III, Mary E. Lupo.
New data needs Geochemical analysis of REE content of francolite and P2O5 content of phosphate units to document spatial and stratigraphic variability. Also, analysis of beneficiated ore, product phosphoric acid, and waste streams from deposits to quantify byproduct REE potential. Need geologic mapping with subsurface analysis.
Geologic mapping and modeling needs No detailed maps of ore bodies and framework geology; need isopach and structure contour maps of Miocene; probably drilling and/or high resolution seismic reflection profiles needed to characterize subsurface without drilling many boreholes.
Geophysical survey and modeling needs High resolution aeromagnetic and aeroradiometric coverage. Radiometric surveys and anomaly maps for U-bearing phosphorite. Aeromagnetic data can assist with framework geology such as structural features and contacts.
Digital elevation data needs Lidar variable over large focus area; mostly in progress with some areas complete.