Iron and magnesium isotope geochemistry in hydrothermal uranium ore systems: Insights from the Bong Deposit, Canada

Abstract

The basement-hosted Bong uranium deposit in the Kiggavik region is associated with a broad hydrothermal alteration halo characterized by an inner illite-rich zone and an outer chlorite-rich zone. The uranium-bearing zones characterized by intense illite alteration yield high δ56Fe and δ26Mg values relative to the host rocks, with average whole-rock values of δ56Fe = 0.58 ± 0.15‰ and δ26Mg = 0.61 ± 0.29‰, and clay-size fraction values of δ56Fe = 0.51 ± 0.20‰ and δ26Mg = 0.82 ± 0.09‰. These isotopic values correlate with a decrease in average Fe2+ concentrations (0.47 versus 2.30 mol% Fe2+ in least-altered hosts).The high δ26Mg values reflect intense illite alteration, which preferentially incorporated heavier magnesium isotopes during alteration of both the ore zones and an upper hematite-altered horizon. Although the clay alteration with high δ56Fe and δ26Mg values coincides with the main uranium-bearing zone, the δ56Fe signatures reflect multiple stages of alteration and mineralization. The high δ56Fe values and lower Fe2+ contents of the uranium-bearing zones support leaching of Fe2+(aq) enriched in lighter iron isotopes. Sequential leach δ56Fe experiments also indicate that formation of fine-grained iron oxides/hydroxides with positive δ56Fe values (+0.23 to +0.60‰) in late chemical fronts enhanced the primary positive δ56Fe signatures recorded in ore zones. Near surface samples with similar isotopic signatures provide clues to buried mineralization and support structural disruption of the systems after primary mineralization. Distal geochemical expressions of this system may include enrichment in iron contents with negative δ56Fe values and/or negative δ26Mg whole-rock or clay-size fractions values, reflecting precipitation of the isotopically light iron and magnesium leached during hydrothermal alteration. Although more costly to process, sequential leach δ26Mg or δ56Fe analyses produced greater variance in the ore zones relative to background values and removed anomalous values from carbonate minerals. Isotopic analyses of the clay-size fraction produced results comparable to the final digestion in the sequential leach process but this method in particular requires greater understanding the mineralogical controls on the isotopic signatures.