Fe and Mg isotopic compositions of clay minerals from the environs of the McArthur River uranium deposit, Saskatchewan, Canada

Abstract

Iron and Mg isotopic composition of alteration minerals from the environs of the McArthur River uranium deposit Zone 4 in the Athabasca Basin are the main focus of this project. Iron isotopic signatures can be used to discriminate areas where significant redox reactions have occurred and thus, may provide indications of areas fertile for uranium mineralization. Therefore, this study investigates the premise that Fe isotopic values can be used to indicate how and where uranium ore deposition occurred. Fe has been known to play an important role as a reductant for U during the formation of U-deposits, acting as a reducing agent during their formation as follows: U6+(aq) + 2Fe2+(aq) + 2H2O ↔ U4+O2(uraninite) + 2Fe3+(chlorite or hematite) + 4H+(aq) Using Mg isotope ratios, this study also aims to ascertain whether δ26Mg values are useful in differentiating alteration minerals associated with diagenetic, hydrothermal, and retrograde alteration origins. Mineral phases that control the isotopic composition for both Fe and Mg are illite (I1, I1bsmt), chlorite (C1, C1bsmt, C2 and C3), and illite-chlorite mixed-layer (ICML). There is a minor contribution from dravite (T1) on Mg isotopic values. Both the Mg and Fe isotopic compositions indicate that samples above the silicified zone (up to 300m) were affected by the primary dispersion of the mineralizing system. The initial δ56Fe and δ26Mg values at McArthur River deposit are constrained based on available literature. Fe is sourced from the basement rocks in the McArthur River area with δ56Fe values near 0‰, similar to most igneous and metamorphic rocks. Magnesium is mainly sourced from evaporated seawater, with background δ26Mg values ranging from -2 to -0.23‰, typical of evaporates and continental crust. This study could become the stepping stone in exploration of unconformity related U deposits. The results obtained in this study support the role of Fe as a reductant for U in deposits in the Athabasca Basin. If δ56Fe values >0.5‰ are in fact reflecting hydrothermal alteration associated to the ore-stage alteration, then Fe isotopic signatures would prove useful vectors for mineralization.