POTENTIAL SOURCES OF RADIONUCLIDES IN THE WAITS RIVER FORMATION, CENTRAL VERMONT

In the Barre-Montpelier area of central Vermont, the Silurian-Devonian Waits River Formation consists of greenschist facies (biotite- to garnet-zone) interbedded phyllite and metalimestone. Ground-based gamma-ray spectrometry shows that the phyllites, not the interbedded metalimestones, are the main host rocks for radionuclides in the Waits River Formation. Backscattered electron (BSE) images confirm the presence of monazite, apatite, and pyrite as metamorphic minerals in these rocks. Aligned monazite grew in the plane of the dominant S1 schistosity during peak metamorphic conditions at the same time as garnet.

Chemical compositions of eight dark-gray, sulfidic, graphitic, phyllosilicate-rich phyllite samples included 44 major and trace elements by X-ray fluorescence, instrumental neutral activation, and carbon/sulfur analysis. Lab-based gamma-ray spectrometry shows positive correlations between total alpha and U, and total alpha and Th, indicating that elevated levels of gross alpha radiation in the phyllites are a product of U and Th decay. Strong positive correlations of U and Th with phosphate suggest that most of the U and Th occur in a phosphate mineral. Weak correlations of U and Th with Ce suggest that monazite is not the only phosphate host for U and Th. Positive correlations between P and Ca suggest apatite is a secondary host. Positive correlations between U and S, but not Th and S, suggest pyrite is another secondary host for U. Negative correlations with As suggest that arsonides are unimportant. Lack of a correlation of Th with S or C suggests that deposition of Th occurred as a detrital grain in the original sediment, while a positive correlation of U with S suggests that U precipitated inorganically in a reducing environment. These findings point to a primary mineral host for U and Th of monazite with secondary hosts of apatite and pyrite. Weathering and dissolution of these minerals can account for locally elevated levels of gross alpha in the ground water.