Abstract

Optical petrography, fission track U mapping, and electron microprobe analysis of granitoid rocks from the Liberty Hill pluton, South Carolina, show a nonuniform distribution of U in the rocks. In order of abundance, U occurs: (1) in accessory minerals, with U concentrations decreasing in the order: uraninite, coffinite, uranoan thorite, zircon, monazite, titanite, allanite, apatite, zeolites; (2) on altered surfaces of Fe (super +2) - and S-bearing minerals; (3) at microcracks filled with carbonate, zeolite, and Fe-Mn oxyhydroxide minerals; (4) on clay minerals of altered feldspars; and (5) in major minerals as a widely dispersed minor component. Thorium has been identified in the accessory minerals thorite, thoroan uraninite, coffinite, allanite, and monazite.Uranium and Th distribution is believed to be of magmatic origin within the major U-Th-Zr phase assemblage: uranoan thorite + zircon + or - uraninite. Granitoids containing uraninite have Th/U ratios less than 3 and the coexisting thorites contain nearly 20 mole percent USiO 4 . Mineralogical and textural evidence suggests that the remaining occurrences of U in coffinite and indiscernible U-bearing phases result from postmagmatic fixation of U migrating by way of fractures and microcracks. Fixation of the U in the granitoids can be attributed to either: (1) reactions involving Fe (super +2) , Mn, S, or Ca (super +2) which destroy the oxidizing or complexing agents of U, or (2) ion exchange or sorption of U by zeolite, clay, or Fe-Mn oxyhydroxides. The U is believed to be derived from the granitoid itself. These mechanisms of U retention, which demonstrate the ability of the rocks on a microscale to buffer chemical conditions and prevent U loss, are similar to those found in sedimentary U deposits. The U-Th-Pb isotope studies of the Liberty Hill pluton show that these mechanisms are effective in retaining the initial concentrations of U.

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