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
Most of the Proterozoic gneiss in the area has been mapped as hornblende granite. This slightly variable rock type is commonly composed of quartz, potassium feldspar, sodium feldspar, and hornblende. Accessories include titanite, monazite, zircon, epidote, magnetite, allanite, and biotite. Permeability in the amphibolite is moderate. In the unsheared gneiss, soil radon concentration is moderate to low as is the indoor radon concentration. Uranium concentration in the rock varies between 2 and 8 ppm (parts per million).

A small body of hornblende amphibolite flanks the mylonite zone to the northeast. Hornblende, biotite, quartz, and sodic plagioclase are the principal minerals present. Abundant titanite, zircon, and magnetite occur as accessories. Permeability is slow in soil derived from the amphibolite. Soil radon is low to moderate and indoor radon is low. Uranium concentration in the rocks varies between 1 to 3 ppm.

Magnetite Ore
Three abandoned magnetite pits are located along the northern edge of the mylonite zone. The magnetite ore is made up of magnetite and pyrrhotite interlayered with sillimanite and quartz gangue. Uranium concentration varies from 2 to 5 ppm in the magnetite ore, and radon concentrations over the ore were low to moderate. The immediate host of the orebody is a more amphibolitic facies of the hornblende granite and a biotite-sillimanite gneiss containing local layers of monazite. The biotite-sillimanite gneiss yields moderate to high radon concentrations and has uranium concentrations as high as 28 ppm where it has been sheared. Soil derived from the magnetite varies in permeability from slow to moderate.

Deformed hornblende granite, minor hornblende gneiss, and minor magnetite ore are found in the main mylonite zone. Where mylonite is developed in the hornblende granite, iron oxides are common on the surface of the rock, and the mylonite is composed of quartz and sericitized feldspar. Foliations are composed of minor biotite, epidote, hematite, and muscovite. Soil derived from the mylonite is moderately permeable. Uranium appears to be amorphous in association with hematite in the foliation and varies in concentration from 5 to 20 ppm. Processes similar to those that occurred in the Boyertown mylonite are the reason that uranium and radon concentrations in the mylonite are high here.

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13 October 1995