LINKING CORED REGOLITH FABRIC TO PARENT ROCKS IN THE FALL ZONE PLACER FOCUS AREA FOR REE EXPLORATION, NORTH CAROLINA

The EARTH MRI program funded the NC Geological Survey to 1) map and define the stratigraphy of updip Coastal Plain deposits in the Fall Zone, and 2) characterize the geochemistry of granitoid-derived regolith that may be enriched in rare earth elements (REE). The Fall Zone is host to placers associated with Pliocene and Pleistocene paleoshorelines, and this Coastal Plain section is adjacent to and underlain by clay-rich regolith derived from weathered granitoid and metasedimentary/metavolcanic bedrock. During transgressive/regressive cycles, materials from these unconsolidated, weathered ‘source’ terranes are disaggregated, reworked, and redeposited; sand-sized clasts may be incorporated into coastal placers; silt and clay-sized particles are transported offshore.

The regolith interval is basically ‘no-man’s land’ with respect to mapping. Bedrock mapping requires hard-rock outcrops usually in drainages; regolith is not mapped. To map Coastal Plain units, cores are collected with a Geoprobe, with profiles that start on interfluves and extend across terraces down to valley bottoms. Differentiating severely weathered Coastal Plain units from regolith is difficult: coring proceeds downhole until unambiguous fabrics confirm basement. As a result, regolith sequences (<20 m) were intercepted that exhibit a variety of textural and deformation fabrics that may represent several types of parent bedrock. Some of these identified regolith sequences will be resampled for detailed geochemistry.

Most known REE-bearing ion adsorption clay deposits form over granitoid rocks, but, REE-bearing clays weathered from a range of rock compositions are documented in China. Research on volcanic rock-derived weathering profiles is limited in southeastern U.S., but a study suggests that weathering profiles over high-alumina volcanic rocks may provide low grade REE resources. Metasedimentary rock weathering profiles could be potential sources of REE if the sediment was derived from felsic igneous rocks and contains REE-bearing minerals. Although the current study will focus on the geochemical signature of granitoid derived regolith, similar baseline studies of other clay-rich, weathering profiles would help characterize the resource potential of non-granitoid igneous, metamorphic and sedimentary rocks.