Lithogeochemical and sulphur isotope indicators of environment of formation and genesis of the Moss hyper-enriched black shale showing, Yukon

Abstract

The Moss Ni-Mo-Zn-Pt-Pd-Re-Au hyper-enriched black shale (HEBS) showing is located in the western Richardson Mountains and is one of several in northern Yukon. The mineralization consists of a thin, stratiform semi-massive Fe-Ni-Mo-Zn sulphide horizon that occurs at the stratigraphic contact between the Road River Group and Canol Formation. This study evaluates the ambient paleoenvironmental conditions using several robust lithogeochemical proxies. Prior to HEBS formation, terrigenous clastic sedimentation predominated, whereas chemical sedimentation pre-dominated during and immediately after HEBS formation. Rare earth element-Y data indicate that the water column was (weakly) oxygenated (Ce/Ce*SN < 1), that hydrothermal activity was absent (Eu/Eu*SN almost equal to 1), and that there was a significant seawater influence on the sedimentary environment (Y/Ho > 28) throughout the deposition interval, even during HEBS mineralization. High (>10) authigenic Mo/U ratios suggest that a ferromanganese particulate shuttle delivered metals sourced from seawater to the seafloor. Negative bulk delta-34S values (-19.3 to -23 permille) in the HEBS indicate that microbially reduced seawater sulphate was the source of reduced sulphur for the mineralization. Collectively, these data signify a basinal environment that experienced varying degrees of restriction and stratification, but fresh (i.e. unfractionated) marine waters delivered metals, metalloids, and sulphur. This type of geological setting is considered critical for the formation and preservation of HEBS mineralization.