U.S. Geological Survey

Radon in Sheared Rocks

The Brookneal mylonite zone near Brookneal, Va.bounds the western edge of the Danville basin in the southern Virginia Piedmont (Gates and others, 1986). The mylonite has developed in a homogeneous, Cambrian granodiorite composed of quartz, sodic plagioclase, minor hornblende, biotite, and titanite. Soil radon concentrations show a positive correlation with the amount of shear measured in the underlying mylonitic outcrop (Gates and Gundersen, 1988). In the Brookneal zone, the amount of shear was measured by determining the angle between the two foliation bands developed during deformation. These are known as C and S foliation bands (Berthe and others, 1979; Simpson and Schmid, 1983; Simpson, 1984). The smaller the angle between the C and S bands, the greater the amount of shear strain.

The C band forms initially at an angle of 45 degrees to the S band. This angle becomes increasingly smaller with progressive shearing. Therefore, the smaller the C/S angle the greater the shear strain. Figure 5 is a summary map of the area showing the geology, the angle between C and S bands, and soil radon concentrations. The angle between the C and S bands was measured on the outcrop exposed along the railroad track. Radon in soil was measured in the moderately permeable soil directly overlying the outcrop and along a road on top of the ridge that is above the outcrop. Uranium was measured in 10 of the rocks for which the C and S angles were measured. The correlation among uranium, soil radon, and the C/S angle is shown in Figure 6 and Figure 7. These results clearly show that soil radon and uranium concentration in the rock increases with increasing shear. Uranium and radon both increase as the C/S angle becomes smaller. The progressive breakdown of the rock is directly linked to changes in chemical and physical composition with increasing shear.

The Brookneal zone is a good example of the relative enrichment of uranium by volume loss. Figure 6A shows that the uranium-thorium ratios change little with increasing shear. Thorium is considered an immobile element, thus it appears that new uranium has not moved into the system to cause the increase in uranium concentration. Instead, other elements have moved out, especially silica and sodium crreating a relative enrichment of uranium in the mylonite as a result of volume loss.

The Glen Gardner area was originally mapped by Markowitz (1975) and shows a number of mylonite zones occurring at the contacts between different rock types or crossing several different rock types. Indoor radon and soil radon concentrations are high within the mylonite zone shown in Figure 8 and Figure 9. Four major lithologies are associated with the mylonite.

Hornblende Granite

Amphibolite

Magnetite Ore

Mylonite