GSA Connects 2022 meeting in Denver, Colorado

Paper No. 4-12
Presentation Time: 11:20 AM

ECONOMIC MINERAL ASSEMBLAGES OF HEAVY MINERAL SANDS: INSIGHTS FROM THE EARTH MRI VIRGINIA FALL ZONE PLACER FOCUS AREA


HAWKINS, David W., LASSETTER, William and NELSON, Michelle S., Virginia Department of Energy, Geology and Mineral Resources Program, 900 Natural Resources Dr., Suite 500, Charlottesville, VA 22903

Heavy mineral sand placer deposits containing economic resources of titanium and zirconium are well documented in southeastern Virginia. The surficial geology of Virginia’s Coastal Plain is a result of multiple marine transgressive and regressive sea-level cycles since the Pliocene, which may have preserved other undiscovered placer deposits within Neogene and Quaternary sedimentary units. Utilizing a multi-method top down approach, we assessed available airborne radiometric and stream sediment geochemical data from the National Uranium Resource Evaluation (NURE) datasets to refine target areas for geologic map compilations and new mapping at the 1:100,000-scale, and to complete additional sampling of surficial geologic units to evaluate the presence for titanium minerals, zircon, and monazite. Heavy mineral concentrates indicate higher total heavy mineral (THM) fractions are most common along the Fall Zone in sediments of the Pliocene Cold Harbor formation (Tch, informal unit) and from stream sediment samples draining Neoproterozoic metasedimentary and metavolcanic rocks (Zmsv) of the Roanoke Rapids terrane. The THM content ranges up to 13 percent by weight (wt %) of the bulk sample, and on average, the economic mineral assemblage of the THM fractions is ~81 wt %. Ilmenite, rutile, and zircon are the primary economic minerals present. Monazite was detected at concentrations of less than 2 wt % of the THM fraction in the 20 analyzed mineral concentrates. Rare earth element (REE) trends observed in analyzed samples suggest that other unidentified REE-bearing minerals may be present. Higher percentages of zircon are present in THM concentrates collected in the Fall Zone from the following formations: Zmsv, Tch, and the Bacons Castle Formation (Qbc). Titanium-bearing mineral percentages of the THM concentrates are more variable across the project areas. Geologic maps, cross-sections and a GeMS-compliant geodatabase were created for this project and are publicly available. A second phase of work focusing on the easternmost Piedmont just west of the project area is underway, aiming to understand mineral provenance and critical mineral occurrences within regolith on source rock.