RADIOACTIVITY OF THE LUCERNE PLUTON, MAINE: EVIDENCE FOR POST-INTRUSIVE URANIUM REDISTRIBUTION

The Devonian Lucerne Granite in mid coastal Maine is an extensive (672 km²) pluton that is uranium- and thorium-rich and as such poses a health hazard because it can result in high radon levels in buildings and high uranium in well water. We used a portable Gamma Spectrometer (RS230) to map the Lucerne Granite for radioactivity and to understand the relative distribution of K, U, and Th. Our group has made over 250 measurements of the radioactivity of surface exposures, faults, and fractures of the granite and about 10 from surrounding country rock. This study focusses on the variability of ordinary surface exposures of the granite, and also on characterizing the radioactivity of faults and fractures. Typical glaciated surface exposures of the Lucerne Granite have gamma radioactivity of about 20 uR/h defined by potassium (~4.3%), uranium (8 ppm), and thorium (28 ppm), and thus the total contribution to radioactivity is typically 34% from K, 26% from U, and 41% from Th. Thorium enrichment occurs in large-scale fault zones adjacent to the Norumbega fault (along the Airline Rd - Rt 9) and also in mafic clots in uncommon biotite-rich schlieren. In these exposures the average radioactivity of about 36 uR/h defined by potassium (~4.6%), uranium (13 ppm), and thorium (68 ppm), and thus the total contribution of radioactivity is typically 21% from K, 24% from U, and 56% from Th. Of note are those exposures with highly localized uranium enrichment, and it would appear that all of these occurrences are along what we interpret as discreet brittle faults and fractures best exposed in the central and southern part of the pluton. These exposures have an average gamma radioactivity of about 72 uR/h defined by potassium (~4.9%), uranium (84 ppm), and thorium (72 ppm), and thus the total contribution of radioactivity is typically 16% from K, 61% from U, and 23% from Th. But measurements only on high-uranium cm-scale faults show that the contribution to radioactivity is in the range of 4% K, 95% U, and 6% Th where total radioactivity is >200 uR/h and uranium is between 150 and 650 ppm. We can demonstrate that locally these latter features are open, and contribute significantly to fracture porosity, and hence these rocks with mobilized uranium are in intimate contact with groundwater and thus are a key target for addressing the health hazard of these rocks.