GEOCHEMICAL CHARACTERIZATION OF EASTERN PIEDMONT ROCKS AND ATLANTIC COASTAL PLAIN SEDIMENTS ALONG THE FALL ZONE IN SOUTHEASTERN VIRGINIA

Collaborative 1:100,000-scale mapping through the USGS NCGMP and Earth MRI Programs aim to interpret the geological setting of heavy mineral (HM) sand paleoplacer deposits and source rocks for economic HM in southeastern Virginia. Potential source terranes from west to east include felsic plutonic bodies within the Triplet terrane (Tt), low to medium-grade metavolcanic and plutonic rocks of the Roanoke Rapids terrane (RRt) and foliated to massive granite of the Dinwiddie terrane (Dt). The Gaston Dam and Nottoway River fault zones juxtapose the Tt and RRt, and the RRt and Dt, respectively, and are identified by sheared bedrock, fault gouge, cataclasite, and linear magnetic anomalies from 2021 airborne geophysical data. Eastward through Miocene to Quaternary Coastal Plain (CP) sedimentary units, mapping and aeroradiometric Th anomalies highlight deposits containing economic HM sands. Research along the Fall Zone shows ilmenite, zircon, rutile, and monazite (from most to least abundant) in CP units, with the mined 'Old Hickory' deposit as an example.

Geochemical analysis of 40 samples (18 from Piedmont rock, 15 from CP sediment, 5 from granitic saprolite, and 2 from in-situ soil) were conducted via inductively coupled plasma - optical emission spectroscopy (ICP-OES) to provide elemental and total rare earth element (REE) abundance. These data indicate that granite-derived soil and saprolite, and massive granite from Dt, exhibit elevated levels of REEs compared to post-Archean upper continental crust. REE-bearing minerals are likely not sourced from low-grade metamorphic rocks in RRt or from silicified cataclasite in Tt. Thorium content shows a strong positive correlation to total REE content for Tt felsic rocks, and a moderate correlation to Dt saprolite, indicating the potential presence of monazite in these settings as Th and REEs can be proxies for monazite. Dt, RRt, and Tt are potential sources for Zr (proxy for zircon), except for the mafic and cataclasite rocks. Rock Ti content ranges from 0.1-0.6 wt % in all but the cataclasite. The behavior of trace elements such as Ta and Nb may help differentiate sources for Ti-bearing minerals rutile and ilmenite, given that Dt samples have higher Ta and Nb content than RRt and Tt. Pending HM sand analyses will help determine contribution of Ti-bearing minerals from Dt, RRt and Tt.