| Summary: | Hyper-enriched black shale (HEBS) deposits, such as those hosted in Devonian black shales of northern Yukon, are a globally significant source of a variety of economically important metals, particularly nickel, copper, zinc and platinum group elements (PGE). The Yukon HEBS occurs as a thin (<10 cm) but laterally extensive (1000s km) stratiform and stratabound mineralized horizon that is enriched in a number of metals, significantly, Ni-Mo-Zn-PGE. The genesis of such deposits and the ambient paleoenvironment in which they formed are the subject of vigorous debate. Two genetic models, seafloor hydrothermal alteration and seawater scavenging have the most traction. These models invoke euxinic conditions for the formation and preservation of semi-massive sulfides. Non-traditional isotopes, particularly molybdenum and thallium, are robust paleoredox indicators, especially where bulk geochemical paleoredox indicators exhibit ambiguities. Systematic sampling and Mo and Tl isotopic analysis of a 200 m stratigraphic section through the Yukon HEBS mineralization and the footwall and hanging-wall strata at the Peel River north and south bank showings give δ98Mo -1.24 to -0.53‰ and ε205Tl -8.1 to -5.2 for the mineralization, and 0.05 to 0.60‰ and -4.8 to -4.4 for the unmineralized strata, respectively. These values preclude a hydrothermal origin, but, together with rare earth elements (REE) and several bulk geochemical redox indicators, indicate that the mineralization formed from seawater in a quiescent, euxinic basinal paleoenvironment. M.Sc.
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