U-PB DETRITAL ZIRCON PROVENANCE AND MINERALOGY OF HEAVY MINERAL SANDS FROM THE ATLANTIC COASTAL PLAIN ADJACENT TO THE FALL LINE OF THE SOUTHEASTERN US

Heavy mineral sand (HMS) deposits are a primary source of industrial minerals including critical elements Ti0₂ (ilmenite and rutile) and Zr0₂ (zircon). Monazite and xenotime are sometimes recovered as byproduct and are potential resources of rare Earth elements.

Understanding processes associated with concentrating HMS are fundamental to resource exploration and assessment. Here we used zircon, a common mineral in HMS and a common accessory mineral in most rocks, as a geochronometer to determine resource provenance. We also used automated mineralogy to determine HMS compositions. Together, these are powerful tools for better understanding sediment transport over regional scales.

Here we present U-Pb zircon and automated mineralogy results from Cretaceous to Pliocene stream sediments that were collected proximal to the southeastern U.S. Fall Line (maximum transgression along passive Appalachian margin) in the Atlantic Coastal Plain (ACP) from VA to AL. New detrital zircon analyses of >20 samples show distinct zircon age populations along the Fall Line margin. In the north, Grenville-age (ca. 1.2-1.0 Ga) zircon is the dominant mode or is mixed with Appalachian-age (ca. 460-300 Ma) zircon. Approaching southern NC, through SC, and into GA, Grenville-age zircon is nearly absent and unimodal or bimodal peak ages of ca. 440-410 Ma and 360-310 Ma are most common. Samples into AL have dominant Grenville-age peaks.

Changes in detrital zircon age populations likely reflect near-source changes of the Appalachian terrane collage. In addition, apparently abrupt changes suggest very limited longshore drift and mixing in the Cretaceous ACP units compared to Cenozoic HMS in the eastern ACP.

Mineralogy of HMS samples is highly variable. Total HMS % mass of zircon ranges from 44% to <1%; Ilmenite >90% to 8%; rutile 45% to 3%; monazite 9% to <1%; xenotime typically <1%. Other abundant HMS species include staurolite, kyanite, and garnet. Wide ranging modalities likely reflect very localized differences in source rocks.

These data suggest that near-Fall Line HMS occurrences are highly dependent on local source materials as mixing appears to be limited. This supports the idea of an immature coastline as a depositional environment and is reflected in the geographically sporadic HMS deposits proximal to the Fall Line in the ACP.