Abstract
Pb-isotopes have been proposed as pathfinders for sandstone-hosted unconformity-related U deposits, with isotope ratios providing information on mineralization timing and element remobilization and migration. Pb-isotopes proximal to mineralization display radiogenic signatures, often with ‘excess Pb’ suggestive of derivation from greater U concentrations than are currently present. The U deposits in the Kiggavik project area (west of Baker Lake, NU, Canada) are basement-hosted, contain several generations of pitchblende mineralization, display a strong structural control, and are located in fault-related fracture systems and foliation-parallel veinlets. Drill core samples were analysed by Inductively-Coupled Plasma-Mass Spectrometer (ICP-MS) for Pb isotopes following multi-acid total-digestion, reverse Aqua Regia partial-digestion, and weak-acid-leach attacks, to evaluate the utility of the respective dissolution methods in Pb-isotope pathfinder geochemistry. Partial-digestion results are similar to weak-acid-leach results, indicating that interpretation of Pb-isotope signatures can be carried out from partial-digestion data if weak-acid-leach data are unavailable. Application of this pathfinder method at Kiggavik shows that Pb-isotope ratios display systematic trends useful for exploration vectoring. Uranium-content-adjusted 206Pb/204Pb ratios and 206Pb/204Pb ‘excess-lead’ data highlight anomalous isotopic values. 207Pb/206Pb ratios display downhole trends complementary to location of mineralization. Three-dimensional (3D) distributions of Pb-isotope data at the Contact U prospect show systematic trends and form halos around the mineralization. Isotopic footprints are limited to <50 m from the mineralization outline, reflecting host-rock and structural control, but indicate areas with elevated potential for U mineralization and provide vectoring information within basement lithologies.
Highlights
The goal of this study is to evaluate the utility of Pb-isotope dispersion, Pb-isotope pathfinder geochemistry, in exploration for basement-hosted unconformity-related U deposits [1,2,3]
Uranium mineralization characteristically contains very high Pb/204 Pb and Pb/204 Pb ratios, and U mineralization, such as in Mesoproterozoic unconformity-related deposits, will display low 207 Pb/206 Pb ratios, due to the original ratio of the two parent isotopes in the mineralization (235 U/238 U = 0.720%/99.274% = 0.0073)
The lithogeochemical analyses used by the exploration industry typically follow total digestion (TAD) or partial digestion (PAD)
Summary
The goal of this study is to evaluate the utility of Pb-isotope dispersion, Pb-isotope pathfinder geochemistry, in exploration for basement-hosted unconformity-related U deposits [1,2,3].This geochemical method is based on the fact that Pb is the natural end product of the decay ofU, the target element of the exploration, and that several radiogenic Pb-isotopes can be used to identify the presence of nearby U mineralization as a result of dispersion of radiogenic Pb into the environment around the mineralization [4,5]. The goal of this study is to evaluate the utility of Pb-isotope dispersion, Pb-isotope pathfinder geochemistry, in exploration for basement-hosted unconformity-related U deposits [1,2,3]. This geochemical method is based on the fact that Pb is the natural end product of the decay of. Large quantities of U produce enrichments in 206 Pb and 207 Pb, through radiogenic decay, relative to common lead 204 Pb. As a consequence, uranium mineralization characteristically contains very high Pb/204 Pb and Pb/204 Pb ratios, and U mineralization, such as in Mesoproterozoic unconformity-related deposits, will display low 207 Pb/206 Pb ratios, due to the original ratio of the two parent isotopes in the mineralization (235 U/238 U = 0.720%/99.274% = 0.0073)
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