Investigation of low-grade REE offshore sands from North and South Carolina, and Georgia, USA, using automated mineralogy

摘要

Forty-two mineral sand samples from vibracores, collected offshore of Georgia (GA), South Carolina (SC), and North Carolina (NC), were investigated to determine the rare earth element (REE) concentration and corresponding rare earth minerals (REM). Geochemical analyses indicate that REE are present in low concentrations in the samples. For example, cerium ranges from 8 to 100 ppm, but it is generally < 50 ppm in most samples, and yttrium is < 30 ppm. Cerium shows a strong correlation with lanthanum, dysprosium, samarium, and neodymium reflecting the presence of mainly monazite. QEMSCAN analysis indicates that the main REM is indeed monazite and occurs in trace amounts in agreement with the REE geochemistry. Zircon, apatite, Fe-oxides, rutile, and ilmenite are also present in varied concentrations. QEMSCAN analysis of mineral concentrates, using heavy liquids, of selected samples indicates a significant concentration of monazite, zircon, and rutile. Monazite is mainly sub-rounded and rounded to well-rounded in the GA group samples, sub-rounded to angular in the SC group, and ranges widely in shape in the NC group. The liberation % of monazite, calculated with the QEMSCAN, is variable due to its low-grade mass in the samples, but it illustrates its potential for concentration, and thus recovery. Electron probe micro-analysis (EPMA) of monazite shows that it has a similar Ce2O3, La2O3, and Nd2O3 concentration across the region, while Y2O3, Gd2O3, and Dy2O3 show slightly wider variations. The average ThO2 concentration ranges from 3.97 to 5.05 wt% and UO2 from 0.31 to 0.92 wt%, in general agreement with previously reported chemistry of monazite in the region. REE were not detected in zircon which contains an average of 0.11 to 0.15 wt% Y2O3. Rutile consists mainly of TiO2 (96.66 to 99.14 wt%), minor Nb2O5 (0.30 to 1.36 wt%), Fe2O3 (below 1 wt%) and Cr2O3 (0.06 to 0.40 wt%). The QEMSCAN data provide an accurate estimation of not only the mass of the minerals, but also mineralogical parameters, such the morphology, liberation, and association % of the minerals, and grain size. The data, coupled with geochemical analyses and mineral chemistry, provide mineralogical attributes which can be used to evaluate the economic potential of the sands and the provenance of the minerals in the region.