Geochemistry and Geochronology of the Andrew Lake Deposit, Thelon Basin, Nunavut, Canada

English Abstract:

The Thelon Basin is an intracratonic Paleoproterozoic basin that shares many similarities with the U producing Athabasca Basin, Canada. However, there are striking geological differences that highlight the need to better characterize the Thelon U systems. The Kiggavik project area, located near the northeastern edge of the basin, comprises a series of deposits and showings along an ~18km long NE-SW structural trend. At present, the basement-hosted Andrew Lake deposit is the southernmost end-member along this trend, located near the intersection with the Scissons North fault. Mineralization is hosted within Neoarchean metasedimentary rocks informally termed the Woodburn Lake group. The objectives of this study are to: (a) characterize the deposit mineralogy, (b) determine the ages of U and alteration minerals, and (c) develop a genetic model for the Andrew Lake deposit for comparison to other deposits along the trend. Three generations of U minerals have been identified: (1) uraninite, (2) coffinite, and (3) boltwoodite. Stage 1 uraninite occurs as nodules, veins and fracture-filling grains interstitial to quartz, and as colloform uraninite. Stage 1 uraninite has relatively high and variable SiO₂ and CaO contents, ranging from 0.63 to 7.23 wt.% and 1.94 to 8.14 wt.%, respectively. Alteration of nodular and colloform uraninite formed stage 2 coffinite, which is characterized by elevated and variable SiO₂ contents ranging from 9.35 to 20.97 wt.%. Stage 3 boltwoodite occurs clusters and disseminated halos around coffinite. Apatite occurs in several units, but is not associated with U minerals. Based on major element chemistry and U-Pb isotopic analysis of U minerals, both U and Pb were remobilized by numerous hydrothermal fluid events. Therefore, the oldest age obtained for U minerals, 578 Ma, is clearly not primary. Further work is required to establish how these stages of U correlate with hydrothermal fluid events recorded in other deposits in the Kiggavik area.