The source of uranium for the Lac Cinquante uranium deposit, Nunavut, Canada

Abstract

The Lac Cinquante uranium deposit is hosted in an Archean greenstone belt below the Paleoproterozoic Baker Lake Basin, Nunavut, and is currently characterized as a vein-type uranium deposit. Vein-type uranium deposits are usually spatially associated with source granitoids and consist of uranium mineralization concentrated in fractures, shear zones, and stockworks. The source of uranium in the Lac Cinquante is unknown and is determined in this study through petrographic work and trace element analysis of uranium minerals. We hypothesize that the uranium was sourced in one of two ways: either uranium was leached from apatite, zircon, or monazite, or sourced from exsolved fluids, from nearby c. 1.84 Ga Hudsonian granites or the uranium was sourced from glass, or fluids related to, the c. 1.83 – 1.81 Ga potassic volcanic rocks (Christopher Island Formation; CIF) of the Baker Lake Group. Petrographic work including micro-XRF mapping and scanning electron microscopy confirm the complete paragenetic history of the Lac Cinquante uranium deposit: i) primary minerals of the host rock including plagioclase and quartz, ii) albitization of plagioclase, iii) formation of uraninite, brannerite, hydrothermal zircon, apatite, (± barite, pyrite, chalcopyrite, molybdenite, sphalerite, and galena) in calcite and albite veins, and iv) hematite, carbonate, and chlorite alteration. Pervasive albitization of the host rocks, the complex mineralogy (brannerite, hydrothermal zircon, apatite), and the distinct geochemistry (high Ti, Ba, Zr) indicate reclassification of this deposit as Na-metasomatic uranium deposit is warranted. Furthermore, trace element data of uraninite and brannerite reveals four distinct chondrite normalized REE patterns with flat (i.e. none) to positive Eu anomalies and enrichments in LREE/HREE. These profiles are inconsistent with a granite source but are similar to the CIF. The ore minerals are also high in Ti, Ba, Zr, and Sr, consistent with CIF geochemistry that is more enriched in these components than the Hudson granites.