HEAVY-MINERAL SANDS AS SOURCES OF RARE EARTH ELEMENTS: MONAZITE IN THE SOUTHEASTERN UNITED STATES

Rivers and streams carry sediments derived from igneous and metamorphic rocks to coastal areas where they are deposited in near-shore environments. The sediments carry variable amounts of heavy, dense minerals that are naturally sorted and concentrated through actions of waves, longshore currents, wind (sand dunes), and tides. Large deposits of this type, referred to as “heavy-mineral sands” or placer deposits, have been, and continue to be, the principal source of several industrial minerals, in particular Ti-bearing minerals (ilmenite, rutile, and leucoxene) and zircon. Coexisting heavy minerals, such as sillimanite/kyanite, staurolite, monazite, and garnet, are often recovered. Economic deposits encompass Cretaceous to Holocene sediments. Examples are mined on every continent except Antarctica. Mining of heavy-mineral sands has several advantages relative to most typical ore deposit types; they are relatively easy to excavate and process, they can encompass large areas and tonnages, and they can supply several salable minerals as co-products.

In the southeastern U.S., weakly consolidated strata comprised of Late Cretaceous and Tertiary heavy-mineral sands follow the contact of the Piedmont region with the Atlantic Coastal Plain, along and extending east of the Atlantic “Fall Zone”. Heavy-mineral sands that formed along Quaternary strandlines also occur along the southern coastline. Past and current mining/processing of these sand-silt deposits in the southeastern U.S. has recovered ilmenite, leucoxene, and rutile for Ti feedstock, as well as zircon. The heavy-mineral content of the deposits is typically dominated by ilmenite, followed by zircon, staurolite, kyanite/sillimanite, rutile, and leuxocene. Monazite, a REE- and Th-bearing phosphate, commonly comprises less than 2% of the heavy-mineral suite; it is not recovered by present operations. This monazite is principally sourced from high-grade metamorphic sources in the Piedmont, such as sillimanite gneiss, and was transported by rivers and streams to the ancient coasts. Monazite, the highest density mineral in the consolidated sands, could be recovered during ore processing as a source of mainly light REEs. Xenotime, a Y-bearing phosphate, is rarely reported, but Y anomalies in regional stream sediments suggest it may be widespread.