Toward the application of molybdenum and thallium isotopes as indicators of paleoredox conditions and genesis of hyper-enriched black shale (HEBS) deposits, Peel River, Yukon

Abstract

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 (10 000s km2) stratiform and stratabound mineralized layer that is hyper-enriched in Ni-Mo-Zn-PGE. The genesis of such deposits and the ambient paleoenvironment in which they formed are the subject of vigorous debate. Non-traditional stable isotopes, particularly molybdenum and thallium, are shown to be robust paleoredox indicators. Systematic sampling with molybdenum and thallium 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 localities give delta-98Mo values of -1.1 to -0.53 permille and epsilon-205Tl values of -8.1 to -5.2 for the mineralization and 0.05 to 0.60 permille and -4.8 to -4.4 for the unmineralized strata, respectively. These values, together with bulk geochemical redox indicators and rare earth elements, indicate that the Peel River HEBS mineralization formed from seawater in a quiescent, euxinic basinal paleoenvironment.