Uranium and Thorium Geochemistry in the Elberton Batholith of the Southern Appalachians, USA

David B. Wenner and James D. Spaulding
Department of Geology and Center for Applied Isotope Studies, University of Georgia, Athens, Georgia 30602, USA
Center for Applied Isotope Studies, University of Georgia, Athens, Georgia 30602, USA

Abstract: U and Th data have been obtained by γ-ray spectrometry on samples from thirty quarry sites in the Elberton batholith, a large (500 km2) late orogenic (320 Ma) mesozonal biotite granite in the Southern Piedmont, USA. The granite is enriched in the Th (X¯=42ppm), but U contents (X¯=3ppm) are close to the average abundance for granites (18 ppm Th; 4 ppm U). Th/U ratios vary widely throughout the batholith, ranging from 3 to 58, with an unusually high mean of 18±10 (1). At some localities, lower Th/U ratios, high U contents of zircons (1500 ppm), and similarities to two-mica granites which have low Th/U ratios of ∼ 1 indicate that the granite once had a much higher U content. Pegmatites are enriched in U (up to a factor of 8), but Th contents are low (by a factor of 3) compared with the granite suggesting loss of U from the granite by an early magmatic vapour-phase. Significant U loss may also have occurred as a result of groundwater leaching, possibly associated with uplift and weathering of the intrusion 200 Ma ago. The highest Th/U values occur in the upper portions of the batholith, consistent with the hypothesis of U loss by these processes. The U content is highly variable throughout the batholith but Th is more uniform, both regionally and at outcrop. The distribution of Th does not correlate with patterns shown using previously published, 18O/16O, 87Sr/86Sr, or trace element (Ba, Rb. Sr, Mn) data, which are thought to reflect inhomogeneity in the protolith of the granite although the possibility that at least some of the isotopic ratios were disturbed by late alteration processes cannot be eliminated.

Mineralogical Magazine; June 1982 v. 46; no. 339; p. 227-231; DOI: 10.1180/minmag.1982.046.339.09
© 1982, The Mineralogical Society
Mineralogical Society (www.minersoc.org)