MAPPING CRITICAL (HEAVY) MINERAL SOURCE ROCKS AND SINKS ALONG THE FALL ZONE IN VIRGINIA USING AIRBORNE MAGNETIC AND RADIOMETRIC DATA TO GUIDE GEOCHEMICAL AND MINERALOGICAL SAMPLING

The Fall Zone and Atlantic Coastal Plain in Virginia host economic heavy mineral sands that contain critical minerals rich in zirconium (Zr), titanium (Ti) and rare earth elements (REE). The Fall Zone marks the easternmost surface exposure of crystalline rocks of the Piedmont geological province. The onlapping Coastal Plain province has been a sink for easterly flowing fluvial and eolian sediments, reworked by marine processes since the middle Mesozoic. During the Pliocene, a global marine transgression juxtaposed the shoreline with source rocks and sediments that contain varying abundances of heavy minerals such as zircon, ilmenite, rutile, allanite, and monazite. Undulatory bedrock topography resulted in a rocky coastal environment, and as the sea regressed, emergent shoreline features prograded over older shallow marine sediments. As these sediments were reworked and sorted, heavy mineral grains were concentrated and preserved as paleoplacer marine shoreline deposits.

New (2021) airborne radiometric [potassium (K), thorium (eTh), uranium (eU)] and magnetic survey data provide detailed surficial and subsurface data for southeastern Virginia. We present a preliminary map based on geophysics, bedrock and surficial mapping, and portable x-ray fluorescence (XRF) results for rock, soil and sediment samples. Our 1:100K mapping effort utilizes the magnetic data to delineate the Paleozoic Dinwiddie terrane and metaplutonic and metavolcanic rocks of the Neoproterozoic Roanoke Rapids terrane. Radiometric K anomalies are used as a guide for geochemical sampling of rocks and overlying soils in these units, as they may indicate the presence of felsic rocks enriched in REE. Coastal Plain units near the Fall Zone are Pliocene to Pleistocene gravels, sands and muds (near-shore marine deposits) and are distinguished by paleoscarps, elevation ranges, and sedimentology. Fine-grain, high-density mineral sands are most common in the Pliocene units and associated with elevated radiometric eTh, indicating the possible presence of monazite and other heavy minerals such as Ti-bearing ilmenite and rutile. Our mapping and geochemical work along with the utilization of the new geophysical survey will serve as a tool for interpreting the geologic context for mineral deposits and source rocks in southeastern Virginia.