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Chemical alteration index values and rare earth element data and expected ranges for regolith overlying the Stewartsville pluton, Virginia

July 29, 2022

This dataset contains whole major element geochemical data used to calculate values of the chemical alteration index (CIA), data for Nd, Sm, Y, and total REE and expected ranges for total REEY for samples of regolith overlying the Stewartsville pluton, Virginia. The southeastern United States was first identified as prospective for regolith-hosted REE deposits based on the recognition that the region has been subjected to a long history of intense differential chemical weathering and saprolitization, comparable to that which formed the REE clay deposits of South China and Southeast Asia since the break-up of Pangea (Foley and Ayuso, 2013). Foley et al. (2014) established that due to their inherent high concentrations of REE, anorogenic (A-type) and highly fractionated igneous (I-type) granitic rocks of southeastern United States were highly prospective source rocks for deposits of this type. More recently, additional studies investigated accumulation processes resulting in high concentrations of REE in granite-derived regolith deposits related to the Stewartsville pluton and other plutons in Virginia. The Stewartsville pluton was emplaced along the flank of the Blue Ridge province during regional crustal extension related to the opening of the Iapetus Ocean and breakup of the supercontinent Rodinia. The studied rock samples consist of medium- to coarse-grained biotite granite and are mineralogically complex. They contain phenocrysts of quartz, sericitized and albitized k-feldspar, sodic plagioclase, and mafic clots and stringers that are composed primarily of biotite and stilpnomelane and, less typically, include magnetite and remnant cores of green and green-brown hornblende. Feldspar contains inclusions of synchysite and fergusonite; other accessory minerals include abundant and diagnostic allanite and fluorite, as well as apatite, epidote, garnet, Nb-rutile, fergusonite, monazite, titanite, xenotime, gadolinite, and zircon (Foley and Ayuso, 2015 and references therein). Granite outcrop exposures in the Piedmont and Blue Ridge areas of Virginia tend to be intensely weathered, with overlying regoliths ranging from thin and discontinuous to meters thick and laterally extensive, and often with overlying B-horizon type soils. Saprolite can extend down to depths of tens of meters below the B-horizon. In the case of the Stewartsville Pluton, regolith is well developed in multiple exposures. The sampled section described in this data release is >20 meters high by >60 meters long. The profile includes nearly fresh rock, partially to highly weathered saprolite, indurated gravels and sands, and poorly delineated layers of subsoil and topsoil. Granite at the base of the profile is iron stained (mostly goethite) and weathered on exposed surfaces and along cracks. Partially weathered sections of the outcrop display a range of rock textures throughout, rather than systematic changes from base to surface. For example, in the lower parts, cobble and boulder-sized relics of spheroidally weathered granite knobs retain distinctive primary textures but are surrounded by nearly disaggregated granite that crumbles to sand and gravel-sized fragments when sampled. Subsoils, mainly B-horizon, comprise the uppermost meter of the section and contain a higher proportion of clay minerals (i.e. kaolinite-nontronite-iron-oxide mixtures) than the underlying saprolite.

Publication Year 2022
Title Chemical alteration index values and rare earth element data and expected ranges for regolith overlying the Stewartsville pluton, Virginia
DOI 10.5066/P9MJRRP4
Authors Nora K Foley
Product Type Data Release
Record Source USGS Digital Object Identifier Catalog
USGS Organization Geology, Energy & Minerals Science Center