Multiple sources of selenium in ancient seafloor hydrothermal systems: Compositional and Se, S, and Pb isotopic evidence from volcanic-hosted and volcanic-sediment-hosted massive sulfide deposits of the Finlayson Lake District, Yukon, Canada

Volcanic-hosted massive sulfide (VHMS) and volcanic-sediment-hosted massive sulfide (VSHMS; i.e., hosted by both volcanic and sedimentary rocks) deposits in the Finlayson Lake District, Yukon, Canada, provide a unique opportunity to study the influence of seafloor and sub-seafloor hydrothermal processes on the formation of Se-poor (GP4F VHMS deposit; 7ppm Se average), intermediate (Kudz Ze Kayah—KZK VHMS deposit; 200ppm Se average), and Se-enriched (Wolverine VSHMS deposit; 1100ppm Se average) mineralization. All three deposits are hosted by mid-Paleozoic (∼360–346Ma) felsic volcanic rocks, but only the Wolverine deposit has voluminous coeval carbonaceous argillites (black shales) in the host rock package. Here we report the first application of Se isotope analyses to ancient seafloor mineralization and use these data, in conjunction with Pb and S isotope analyses, to better understand the source(s) and depositional process(es) of Se within VHMS and VSHMS systems. The wide range of δ⁸²Se (−10.2‰ to 1.3‰, relative to NIST 3149), δ³⁴S (+2.0‰ to +12.8‰ CDT), and elevated Se contents (up to 5865ppm) within the Wolverine deposit contrast with the narrower range of δ⁸²Se (−3.8‰ to −0.5‰), δ³⁴S (9.8‰ to 13.0‰), and lower Se contents (200ppm average) of the KZK deposit. The Wolverine and KZK deposits have similar sulfide depositional histories (i.e., deposition at the seafloor, with concomitant zone refining). The Se in the KZK deposit is magmatic (leaching or degassing) in origin, whereas the Wolverine deposit requires an additional large isotopically negative Se source (i.e. ∼−15‰ δ⁸²Se). The negative δ⁸²Se values for the Wolverine deposit are at the extreme light end for measured terrestrial samples, and the lightest observed for hypogene sulfide minerals, but are within calculated equilibrium values of δ⁸²Se relative to NIST 3149 (∼30‰ at 25°C between SeO₄ and Se²⁻). We propose that the most negative Se isotope values at the Wolverine deposit record the δ⁸²Se of the Se-source, and that the wide range in δ⁸²Se values results from the combined effects of thermal and chemical degradation and Se-loss from the carbonaceous argillite source to a hydrothermal fluid (including magmatic Se i.e., leached and/or magmatic-hydrothermal) with deposition at or near the paleoseafloor. Pristine unaltered black shales show little variation in δ⁸²Se relative to bulk earth; Se accumulation and fractionation to more negative isotopic values is interpreted to have been produced by post-sediment deposition, but pre-ore stage, reduction of seawater Se within the black shales.