Abstract

A positive correlation exists among surface gamma radiation, uranium concentration, and the angles between C and S bands in mylonite and radon in the soils overlying the Brookneal zone of the southwest Virginia Piedmont. Part of the Brookneal zone lies within the compositionally homogeneous Melrose granite, which is undeformed to the west of the zone. The angle between C and S bands in mylonitic Melrose granite varies from 43° in the westernmost protomylonite to near 0° in the easternmost ultramylonite. Radon in the overlying soils increases from 450 picocuries/litre (pCi/L) in undeformed granite to as much as 4100 pCi/L in ultramylonite. In undeformed granite, uranium is hosted in titanite and zircon. During mylonitization, uranium was redistributed into the foliation and is associated with hematite and fine-grained monazite. Uranium, however, is in constant proportion to thorium, indicating little introduction of new uranium into the mylonite zone. An increase in the uranium concentration from 1 to 2 ppm in undeformed granite to 7 to 8 ppm in ultramylonite may be the result of volume loss during shearing.

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