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TitleIron and magnesium isotope geochemistry in hydrothermal uranium ore systems: Insights from the Bong Deposit, Canada
AuthorPotter, EORCID logo; Gammon, P; Girard, IORCID logo; Sharpe, R; Fayek, M
SourceJournal of Geochemical Exploration vol. 229, 106843, 2021 p. 1-19,
Alt SeriesNatural Resources Canada, Contribution Series 20210492
Mediaon-line; digital
File formatpdf
NTS66A/05; 66A/06; 66A/11; 66A/12; 66B/08; 66B/09
Lat/Long WENS -98.5000 -97.0000 64.7500 64.2500
Subjectsgeochemistry; Science and Technology; uranium; iron; magnesium; isotopes; unconformities; unconformity-type deposit
Illustrationslocation maps; cross-sections; photographs; photomicrographs; geochemical plots; plots; tables
ProgramTargeted Geoscience Initiative (TGI-5) Uranium ore systems - fluid pathways
Released2021 06 16
AbstractThe 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 d56Fe and d26Mg values relative to the host rocks, with average whole-rock values of d56Fe = 0.58 ± 0.15 per mil and d26Mg = 0.61 ± 0.29 per mil, and clay-size fraction values of d56Fe = 0.51 ± 0.20 per mil and d26Mg = 0.82 ± 0.09 per mil. These isotopic values correlate with a decrease in average Fe2+ concentrations (0.47 versus 2.30 mol% Fe2+ in least-altered hosts). The high d26Mg 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 d56Fe and d26Mg values coincides with the main uranium-bearing zone, the d56Fe signatures reflect multiple stages of alteration and mineralization. The high d56Fe values and lower Fe2+ contents of the uranium-bearing zones support leaching of Fe2+(aq) enriched in lighter iron isotopes. Sequential leach d56Fe experiments also indicate that formation of fine-grained iron oxides/hydroxides with positive d56Fe values (+0.23 to +0.60 per mil) in late chemical fronts enhanced the primary positive d56Fe 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 d56Fe values and/or negative d26Mg 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 d26Mg or d56Fe 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.
Summary(Plain Language Summary, not published)
The Targeted Geoscience Initiative (TGI) is a collaborative federal geoscience program that provides industry with the next generation of geoscience knowledge and innovative techniques to better detect buried mineral deposits, thereby reducing some of the risks of exploration. This contribution uses iron and magnesium isotopes to understand the formation of a hydrothermal uranium deposit in Nunavut and examines whether or not these signatures can be used in mineral exploration. The results indicate that the composition of the hydrothermal fluids coupled with reduction-oxidation (redox) reactions generated significant changes in iron and magnesium isotopic compositions in the ore and surrounding alteration zones. While yielding promising results, further work is required to test whether or not these signatures can be extrapolated to the present-day surface for regional mineral exploration.

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