RADON POTENTIAL AND URANIUM REDISTRIBUTION IN THE LUCERNE AND GOULDSBORO GRANITES, MAINE

Radon is one of the primary causes of lung cancer, and radon activity is particularly high in several granites in Maine. The Devonian Lucerne pluton (~380 Ma) and the Silurian Gouldsboro pluton (~419 Ma) were studied to understand how the uranium distribution in these rocks is related to the radon hazard. Gamma radiation measurements were made in the field using and RS230 handheld gamma ray spectrometer at several different outcrops. The mean values for the Gouldsboro Granite are relatively low at ~10 µR/hr (3 ppm U; 13 ppm Th). The Lucerne granite has a mean of 27 µR/hr (14 ppm U; 34 ppm Th), but radioactivity is locally very high along fractures and joints with the means of six high measurements of 40 µR/hr (28 ppm U, and 42 ppm Th). The highest radioactivity occurs along major cracks and fractures that have obvious iron oxides, and display a preferential enrichment of U but not Th. The Lucerne pluton is biotite granite, with secondary chlorite alteration and this unit is of particular interest due to its relatively high radioactivity. An abundance of high U/Th minerals including zircon, allanite, xenotime, and a U-Th silicate were identified on the SEM using energy dispersive X-ray spectrometry (EDS). REE, particularly La and Ce, are clearly mobilized along cracks proximal to the allanite. The Lucerne pluton contains a remarkable abundance of zircons with pronounced radiation halos, showing a wide range of radiation damage: many are near amorphous. EDS mapping shows fractures within the zircons preferentially occur along U-rich and presumably more damaged zones. Raman spectroscopy on individual zircons allows quantification of the damage with the v₃SiO₄ peak ranging from 1005 (less damaged) to well under 1000 cm^-1 (high damage). Given the age since high-temperature cooling, this amount of damage likely corresponds to uranium concentrations in the zircon of of 500 ppm to well over 1000 ppm U, and thus they should be considered a potential source of liberated uranium. These field and laboratory observations suggest significant mobilization and redistribution of uranium has occurred in the Lucerne granite. The high uranium along fractures may facilitate transfer of radon to groundwater and dwellings built on or near bedrock.