The Paleo-Proterozoic High Heat Production Richardson Granite, Great Bear Magmatic Zone, Northwest Territories, Canada : Source of U for Port Radium?

Airborne radiometric survey and field studies outlined a large, elongate, high-level plutonic suite within the Richardson pluton south of the Contact Lake Belt in the Great Bear Magmatic Zone, Northwest Territories, Canada. In terms of content of radioactive elements, the Richardson pluton is composed of two distinct granite types, low heat production (LHP) and high heat production (HHP). Uranium content in the LHP and HHP granites ranges from 3.0 to 4.9 ppm and 6.5 to 24.6 ppm, respectively, showing similarity of the LHP granite to average granites. Geochemical studies indicate that there is a genetic relationship between these two types of granite; the LHP granite was the early product of magma crystallization, whereas the HHP granite is the result of extensive crystal fractionation of biotite, plagioclase and apatite. The presence of magmatic fluorite in granite suggests that high fluorine content lowered the liquidus temperature of magma causing lower temperature fractionation during ascent to high crustal levels, which increased U and Th concentrations in the resultant HHP granite. Weak U mineralization occurs locally as discontinuous quartz +/- hematite +/- pitchblende veins and veinlets within the HHP granite. Stronger U mineralization (U +/- Ag +/- Ni +/- Co +/- Cu) occurred in the past-producing Contact Lake and Port Radium deposits. It appears that such mineralization may have had a spatial and temporal genetic-paragenetic relationship with the HHP granite.