ASSESSING CONCEALED PHOSPHATE OCCURRENCES IN VIRGINIA'S COASTAL PLAIN

The Virginia Division of Geology and Mineral Resources, with support from the United State Geologic Survey, is investigating phosphate occurrences in Virginia’s Coastal Plain. Phosphate is essential to modern agriculture, and international markets have become volatile. This study focuses on the Chesapeake Group, sedimentary deposits laid down during the extensive Miocene/Pliocene marine excursion. Because thickness of overburden is an important consideration for mining, and because Virginia’s Coastal Plain formations dip to the east, the study is confined to the area extending from the fall zone eastward about thirty miles, where Chesapeake Group formations occur within a hundred feet of the surface. Outcrops on the Coastal Plain are generally confined to river cut-banks, and initial sampling has been undertaken using the Division’s Boston Whaler. Phosphate occurrences often exhibit a radioactive “kick” and gamma logs from existing wells are being employed to direct exploratory drilling using the Division’s truck-mounted, 4-inch mechanical auger. Preliminary data and field observations indicate that phosphate tends to accumulate at the unconformable basal contacts of the Yorktown, Eastover, and Calvert Formations. Lab results thus far show correlations between phosphate and uranium enrichment. This study also has potential for locating concentrations of rare earth elements, which are frequently associated with phosphates.