U.S. Geological Survey

Radon in Sheared Rocks


Radon concentrations in soil gas and their relation to geology were studied in Colebrookdale Township on the west side of Boyertown, Pa. The Boyertown occurrence was one of the first well-publicized discoveries of severe levels of indoor radon in the United States. The Proterozoic geology of Boyertown is described in detail by Gundersen and others (1987) and is only summarized here. Three major rock types underlie the area of interest shown on the maps in Figure 3 and Figure 4. In the rock descriptions below, minerals are listed in order of decreasing abundance.

Hornblende Gneiss
This rock is composed of quartz, plagioclase feldspar, and at least 20 percent hornblende. Pyroxene, biotite, and iron-sulfides occur in variable amounts. The maximum uranium concentration analyzed in this rock was 5 ppm. Soil-gas and indoor radon values are low. Soils derived from this rock type are slowly permeable.

Interlayered Quartz-Feldspar and Biotite Gneiss (QFB)
Quartz-feldspar gneiss is composed of quartz, potassium feldspar, and sodium plagioclase. Biotite, titanite, magnetite, allanite, monazite, and rare garnet are accessories. Uranium concentrations as high as 25 ppm were measured. Biotite gneiss contains quartz, sodium plagioclase, and biotite and has potassium feldspar, titanite, magnetite, and monazite as common accessories. Uranium concentrations can be as high as 10 ppm. In both units, uranium-bearing titanite, allanite, monazite, and uraninite are sometimes concentrated in migmatite. These anatectic zones are marked by very large crystals of quartz and feldspar and locally produce high soil radon concentrations. In general, radon levels in soil gas, and indoor radon levels are low to moderate for this rock unit. Soils derived from this rock type are moderately permeable.

Part of the QFB has been mylonitized near its contact with the hornblende gneiss. The mylonite differs mineralogically from the unsheared QFB by its abundant garnet. The mylonite's texture, defined by two foliation bands, is characterized by hematite and dark, fine-grained, highly uraniferous material. Late brittle shear or cataclasis is evident. Uranium concentrations as high as 50 ppm have been measured. Soils derived from this rock type are rapidly permeable.

The highest indoor radon and soil radon measurements are found over the mylonite developed in the QFB. Uranium in the QFB is located in titanite, monazite, zircon, and allanite. These minerals have low radon emanation; much of the radon produced does not escape the mineral or surrounding grains. When the QFB was mylonitized, the uraniferous minerals were broken down and uranium was made available for reaction with hot, oxidizing fluids present in the shear zone. During deformation, the uranium was redistributed into the developing foliation along with hematite. Uranium is a much more effective producer of mobile radon when it is in the foliation. The mylonite weathers primarily along the foliation, exposing the uraniferous surfaces. Chemical analyses of the Boyertown rocks show uranium and radium to be in equilibrium (Agard and Gundersen, 1991). Which is due in part to radium being scavenged by iron-oxides as it is produced by uranium in the foliation. Radon emanation also is increased by this process. The Boyertown mylonite zone is structurally complex and the source of the uranium enrichment in the zone is equivocal. The uranium/thorium ratio is highly variable in fresh rock samples of the Boyertown mylonite, and because veinlets of quartz are common in small, sheared fractures throughout the zone, it is suggested that fluid has been introduced into the zone, perhaps bearing uranium.

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13 October 1995