URANIUM IN SMALL-SCALE FAULTS THAT CUT THE UPPER DEVONIAN LUCERNE PLUTON, MAINE

The Lucerne Pluton in Down East Maine has been shown to support high concentrations of uranium and radon in ground water and high radon in homes. Gamma spectrometry across the pluton shows that the Lucerne Granite has an average radioactivity of about 20 μR/h defined by potassium (~4.3%), uranium (~8 ppm), and thorium (~28 ppm) (using a BGO RS230). Some small-scale faults and fractures have high levels of radioactivity (70-600 μR/h), and uranium enrichment (50 to 600 ppm). The age of these structures is unknown, but are likely related to movement of the Norumbega fault, which cuts the northern end of the pluton. Fault rock mineralization at Bald Mtn, and Upper Patten Pond is dominated by the greenschist facies assemblage chlorite, epidote, and quartz, with accessory minerals that record reducing conditions followed by later oxidation. In addition to U, the fault rock is high in As, with several arsenic and uranium bearing minerals present and characterized with BSE and EDS elemental mapping. Euhedral arsenopyrite is rimmed with arsenic oxide that also fills brittle fractures within the sulfides. Uranothorite and pitchblende/uraninite, identified by EDS, are commonly found within fractures or along grain boundaries. The source of uranium-rich secondary minerals may include uranium rich, metamict zircons. Our work on zircon crystallinity using Raman Spectroscopy combined with LA-ICPMS laser ablation confirms that the least crystalline zircons are nearly structureless and have high [U] of 1000 to 5000 ppm. U/Pb isotopic measurements of zircon from three locations are similar in that most zircons are discordant with excess common lead, suggesting the zircons have been affected by meteoritic water. The interaction of meteoric water with the damaged zircons has possibly resulted in the mobilization and redistribution of uranium and lead. The high damage zircons, commonly penetrated by microfractures, probably have a higher than normal radon emanation rate. The uranium minerals in these small-scale faults and fractures are an important health concern, and future efforts should be directed at understanding the extent and locations of these mineralized structures.