HIGH AND LOW CONCENTRATIONS OF RADIONUCLIDES IN PHYLLITES AND METALIMESTONES OF THE WAITS RIVER FORMATION, CENTRAL VERMONT, BY GROUND BASED GAMMA RAY SPECTROMETRY

Detailed ground based gamma ray spectrometry with a scintillation probe shows above and below average radioactivity in the Silurian-Devonian Waits River Formation in Berlin, Vermont.  The study included 43 measurements in a roadcut on Interstate 89 that exposes subvertical, 018° trending, interbedded phyllite and metalimestone, with bed thickness ranging from 0.15 to 8 m.  Spectrometer data include direct measurements of ⁴⁰K (%), ²³⁸U (ppm), ²³²Th (ppm), and total counts expressed as uranium-equivalent (Ur) concentration (ppm).  Spectrometer measurements in the center of each alternating phyllite and metalimestone bed over a 60-m-thick section show consistently higher radioactivity in the phyllite beds.  The phyllite beds (n=22) average 2.9% ⁴⁰K, 3.1 ppm ²³⁸U, 11.3 ppm ²³²Th, and 22.9 ppm Ur.  The metalimestone beds (n=21) average 1.3% ⁴⁰K, 1.5 ppm ²³⁸U, 5.2 ppm ²³²Th, and 10.7 ppm Ur.  These results indicate that the phyllite is above, and the metalimestone is below the average background concentration of continental crust. 

The data also indicate that measured concentrations are dependent on bed thickness because the probe's 30 cm diameter zone of detection exceeds the thickness of some beds.  Thus measurements of beds thicker than 30 cm represent the concentrations of the radionuclides in each rock type, while thinner (<30 cm) phyllite beds yield lower values due to the detection of adjacent, less radioactive metalimestone beds.  The results of this study will help predict potential sources of elevated radionuclides in the local ground water.  Future work includes plans to test whether the monazite found by J.M. Pyle (RPI, oral commun., 2003) in garnet-grade Waits River Formation is also the mineral host of these radionuclides at biotite grade.