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Unconventional Resources of Rare Elements:

The Bearing of Source and Process on the Genesis of Residual Deposits

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Statement of the problem:

Residual deposits, including laterites, bauxites, clays, weathered crusts and soils, comprise unconventional resources for rare elements; however, residual deposits are, in general, much less well-understood compared to more conventional resources. These resources are found in residual deposits:

rare earth elements (REE),
yttrium (Y),
gallium (Ga),
indium (In),
tin (Sn), and
tungsten (W).

Maps of SE USA and China, comparing REE deposits

This project will investigate (image, left: The southeastern United States has extensive suites of granitic rocks that are deeply weathered and show mobility of REE (Foley et al., 2013). Our studies are aimed at understanding whether there is significant potential for economic REE ion-adsorption clay deposits in weathered metaluminous-to-alkaline igneous rocks of the Southeast US. REE ion-adsorption clay deposits of China have grades from ~500 to over 3000 ppm total REO; some are enriched in heavy REE and these now supply virtually all of the heavy REE to global markets (Yang et al., 2013).)

The investigation results will be used to develop models for these unconventional rare element resources. This research will focus on the rare earth element-Ga-Sn-In-W geochemistry of weathered crusts, soils, and clay deposits (e.g., Foley et al., 2002; Foley, 2012) associated with fractionated A-type igneous systems of the Southern Appalachians Mountains of Virginia, North Carolina, South Carolina and Georgia, (e.g., Tollo et al., 2004) with an emphasis on identifying potential HREE(heavy rare earth elements)-dominated deposits that resemble the ion-adsorption clay deposits of South China (Wu et al., 1996, Wu and Ishihara, 1994). The rare earth elements Ga, In, Sn, and W contents of weathered equivalents are thought to reflect a combination of parent lithochemistry and surficial weathering processes. Features that are expected to exert control on the contents of trace elements in residual deposits include source rock composition, mineralogy, solution chemistry, and weathering history (e.g., Foley and Ayuso, 2008).

The Appalachian Mountains contain voluminous, deeply weathered Neoproterozoic-Phanerozoic anorogenic metaluminous-to-peralkaline granitoids (e.g., Robertson River batholith, Irish Creek pluton, Virginia). A-type granites typically have highly silicic compositions with high contents of Be, F, Ga, In, Nb, Zr, rare earth elements, Sn, Ta, and Y. Granitic rocks of the southeastern US have been subjected to a long history of intense differential chemical weathering and saprolitization, comparable to that of South China and Southeast Asia -- areas which contain important examples of residual deposits (see Wilson, 2004; Wu et al. 1996), e.g., heavy-rare-earth-element clay deposits at Longnan, China. Little is currently known about the occurrence of associated rare metals, such as Ga, In, and W, in any residual deposits formed from highly weathered granite.

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