THE DISTRIBUTION AND COMPOSITION OF REE-PHOSPHATES IN THE ATLANTIC AND GULF COASTAL PLAINS, USA

Rare earth element (REE) resources are of great interest as raw materials for high technology manufacturing. The REE-phosphates monazite (light REE enriched) and xenotime (heavy REE enriched) resist weathering and can accumulate in placer deposits as part of the heavy-mineral assemblage. The Atlantic and Gulf coastal plains of the southern United States are known to host economic concentrations of the heavy minerals zircon, ilmenite and rutile. This study provides a perspective on the distribution and composition of REE phosphate minerals in the region. The elemental chemistry and mineralogy of sands and heavy-mineral assemblages from across the coastal plains were examined, along with phase specific compositions of monazite, xenotime and zircon. Both monazite and xenotime are present across the coastal plains. The phase specific compositions allowed monazite content to be estimated using La as a geochemical proxy. Similarly, both Y and Yb are geochemical proxies for xenotime, but their additional presence within zircon and monazite grains requires a correction to prevent overestimation of xenotime content. Applying this correction, maps of monazite and xenotime content across the coastal plains were generated using sample coverage from the National Geochemical Database (NGS) and National Uranium Resource Evaluation (NURE). The results show an approximately 40 km-wide band of primarily Cretaceous, marine sediments bordering the Piedmont province from North Carolina to Alabama in which monazite and xenotime content are relatively high (up to 4.4 % in <150 µm bulk sediment). Strong correlations between concentrations of the two phases were found, with estimated monazite:xenotime ratios ranging approximately 6:1 to 12:1 depending upon the dataset analyzed. From a resource perspective, xenotime correlation with monazite indicates a heavy REE potential in coastal plain placers, and exploration may be warranted within coastal plain sediments along the boundary of the Piedmont region.