USE OF SOIL PARENT MATERIAL DATA IN DRILL SITE SELECTION FOR A RARE EARTH ELEMENT CLAY ION ADSORPTION REGOLITH STUDY, NORTH CAROLINA

The NC Geological Survey (NCGS), in collaboration with the SC Geological Survey and Virginia Energy, is working on a USGS-funded Earth Mapping Resources Initiative (Earth MRI) study to characterize potential rare earth element (REE) deposits hosted in granitoid regolith. Neo-Acadian and Alleghanian granitoids in the Carolinas and Virginia have been delineated as potential sources of clay ion adsorption REE using the Earth MRI mineral systems approach. However, the large number of plutons must be culled to select representative drill sites. Though NCGS staff conducted an initial assessment of study area plutons in North Carolina based on radiometric data and geophysical weathering indices, further filtering is still needed prior to conducting field sampling. Site selection requires reviewing various geologic data, such as soil profile chemistry and mineralogy, topography, soil pH, depth to bedrock (thickness of regolith), local rock types, and whether the soil profile formed in-situ. The presence of late Cenozoic Coastal Plain sediment is also important because it impacts airborne radiometric data interpretations. Thicker profiles are targeted since they provide more regolith for clay ion adsorption, and thus more potential for economically viable REE deposits.

To select specific drill sites from a range of potential study areas, aggregation, reorganization, and re-analysis of existing data are pivotal steps to familiarize researchers with local geology and weathering profiles, as well as minimize time, resources, and labor costs. The U.S. Department of Agriculture (USDA-NRCS) soil survey data and NCGS STATEMAP field data, available as ArcGIS shapefiles, are the main focus of this data compilation effort. USDA ArcGIS soil survey polygon shapefiles only contain soil series names in the attribute tables. Thus, relevant information is obtained from county soil survey reports and supplementary online data and used to populate polygon attribute tables. The polygons are then classified and color-coded as bedrock, saprolite, Coastal Plain sediment, etc., based on observations at ~5 feet depth. The soil maps help eliminate areas with shallow bedrock or Coastal Plain deposits. Outcrop locations and Coastal Plain observation points from STATEMAP field data are used to ground truth soil data interpretations.