PROVENANCE OF HEAVY-MINERAL SANDS THROUGH RUTILE GEOCHEMISTRY: AN EARTH MRI PROJECT FROM THE UPPER PART OF THE ATLANTIC COASTAL PLAIN, SOUTH CAROLINA

Heavy-mineral sands can represent significant resources for titanium (rutile), zirconium (zircon) and rare-earth elements (monazite, xenotime). These elements are all listed in the 2022 list of “critical minerals”, and thus have been deemed vital to the United States’ national security and economic prosperity. The South Carolina Department of Natural Resources – Geological Survey has partnered with the USGS and Furman University through an Earth Mapping Resource Initiative (EarthMRI) project to analyse heavy-mineral sand deposits along the Fall Line, the boundary between the Piedmont and the Atlantic Coastal Plain in South Carolina.

Siliciclastic sediments, from Cretaceous to Eocene in age, were collected near Orangeburg, SC in the upper part of the Atlantic Coastal Plain. While these deposits are typically accepted to be fluvial to marine in origin, their provenance is uncertain. Following crushing and heavy liquid separation (at 3.1 g.cc^-3 then 2.9 g.cc^-3) the heavy mineral concentrate included: ilmenite, zircon, rutile, staurolite, quartz, monazite, kyanite, spinel and xenotime (from largest to smallest fraction). The presence of large amounts of rutile is an indicator that the source materials were most likely metamorphic rocks. Analysis of rutile compositions performed by electron probe microanalyzer were used to further investigate their origin. All rutile Fe_TOT compositions were >1000 ppm, indicating a metamorphic origin (Meinhold et al., 2010). The comparison of Cr (274 to 684 ppm) to Nb (684 to 10486 ppm) and calculation of negative log(Cr/Nb) values (-1.79 to -3.22) indicate that the source rocks were medium-grade metapelites (Meinhold et al., 2010). However, grains with the higher Nb concentrations (>3000 ppm) better correspond to felsic granulite facies rocks (Meinhold et al., 2010). This indicates the bulk of the rutile grains may be sourced from Appalachian rocks in the upstate of South Carolina with potentially the grains from granulite facies rocks being sourced from preserved bands of Grenville-aged rocks in North Carolina or Tennessee.

References:

Meinhold, G., Anders, B., Kostopoulos, D., & Reischmann, T., 2008, Rutile chemistry and thermometry as provenance indicator: An example from Chios Island, Greece. Sedimentary Geology, 203, 98–111, DOI: 10.1016/j.sedgeo.2007.11.004.