MINERALOGICAL AND GEOCHEMICAL APPLICATIONS TO EVALUATE CRITICAL MINERALS CONTENT IN MARINE SEDIMENTS OFFSHORE OF VIRGINIA

Heavy mineral concentrates (HMC) from twenty marine sand and three clay-rich sediment samples collected in two locations offshore of Virginia were assessed for critical minerals, geochemistry, mineral chemistry, and mineral liberation potential. Sandbridge Shoal is an active federal lease area for coastal re-nourishment sand. The Atlantic Ocean Federal Navigation Channel is dredged to maintain marine navigation at the mouth of the Chesapeake Bay. Both areas potentially contain untapped resources of critical minerals. The HMC fractions were extracted from bulk core samples by gravity spiraling followed by heavy liquid separation. In the sand samples, the HMC ranged from 0.39 wt% to 1.93 wt% of the bulk sediment mass; in the clay-rich sediments, HMC accounted for 0.1 wt% to 0.2 wt% of the total mass. Analysis of the HMC fractions by TIMA-X (Tescan Integrated Mineral Analyzer) indicated the presence of economic minerals with the following mean abundances: ilmenite (33.7 wt%), altered ilmenite (6.3 wt%), rutile (4.5 wt%), zircon (6.4 wt%), monazite (0.31 wt%), and xenotime (0.02 wt%). Liberation of these minerals is greater than 80%, indicating they occur mostly in pure form.

The total rare earth element (REE) plus yttrium content in the HMC ranged from 744.7 ppm to 3,089.4 ppm. Electron probe micro-analyses of monazite grains indicated that it is the primary carrier of LREE. The analyses also showed that ThO₂ concentrations ranged from 4.48 wt% to 7.34 wt%, UO₂ from 0.29 wt% to 0.71 wt%, and Y₂O₃ from 0.95 wt% to 1.49 wt%. Xenotime consisted of Y₂O₃ that ranged from 38.87 wt% to 43.03 wt%, and minor concentrations of Gd, Er, and Dy. LA-ICP-MS analyses of zircon indicated the presence of Y (560 ppm to 1,341 ppm), Yb (140 ppm to 374 ppm), and Er (81 ppm to 206 ppm). Chondrite normalized REE+Y plots for zircon show depletion in LREE and enrichment in HREE.

TIMA-X can speciate the type of minerals (e.g., monazite, xenotime), and determine their abundance, liberation and association, and grain size. The data can aid in the assessment of methods for their potential recovery and separation of critical minerals. They can also be used to define geometallurgical domains based on the type of critical minerals for mining purposes. Furthermore, particle morphology, e.g., sphericity, and mineral chemistry can assist in the provenance of the minerals.