RARE EARTH ELEMENT DISTRIBUTION AND MOBILITY IN REGOLITH DEPOSITS FORMED ON ALTERED A-TYPE GRANITES OF THE SOUTHEASTERN UNITED STATES

The Southeastern United States (SE-US) contains numerous anorogenic (A-type) granites that constitute favorable source rocks for regolith-hosted REE deposits. Compositional ranges of large igneous suites of the SE-US generally match those of A-type or highly fractionated I-type granitic rocks associated with South China REE clay deposits. These deposits currently supply all heavy REEs, Y, and a major portion of light REEs to global markets.

Neoproterozoic granites of SE-US (e.g., Robertson River, Suck Mountain, Stewartsville, Striped Rock, Beach, Crossnore) have high contents of Ga, F, Nb, Sn, Ta, Y, Zr, and REEs. In common with granites of South China, granites of the SE-US have undergone a long history of chemical weathering. Detailed analyses of regolith formed on Virginia plutons indicate that REEs are mobile and locally attain grades comparable to deposits of China. Accumulation processes include late-magmatic to deuteric alteration and deep lateritic weathering. The REEs occur as adsorbed ions and in soluble secondary minerals. Regolith developed on the Stewartsville and Striped Rock plutons highlight the importance of allanite weathering in generating REE-enriched clays; regolith developed on the Suck Mountain pluton likely formed by weathering of allanite and REE-fluoro-carbonate minerals that formed in pegmatitic melt fractions. One prominent feature in REE-mineralized regoliths and source granites is the tetrad behavior displayed in REECN patterns. Tetrads result from processes that promote redistribution, enrichment, and fractionation of REEs. Favorable processes include late- to post-magmatic alteration of granite and silicate hydrolysis in regolith. Thus, REE patterns showing tetrad effects provide likely exploration indices for identifying highly prospective source rocks and associated regoliths.

Establishing the extent of the resource potential for regolith-related REE deposits in the SE-US will depend on three main factors: 1) identifying age- and compositionally-appropriate plutons, 2) recognizing shallow intrusions that are allanite or apatite-rich, 3) detecting plutons having regoliths formed by intense weathering under humid conditions. These factors enable granites to produce significant volumes of regolith with high amounts of extractable, high-value REEs.