Volcanic-hosted massive sulfide (VHMS) and volcanic and sedimentary-hosted massive sulfide (VSHMS) deposits recently discovered in the Finlayson Lake District (FLD), Yukon, Canada have provided a unique opportunity to study the influence of seafloor and sub-seafloor processes leading to the formation of Se-poor (GP4F VHMS deposit;​ 7 ppm Se average), intermediate Se (Kudz Ze Kayah -KZK VHMS deposit;​ 200 ppm Se average), and Se-rich (Wolverine VSHMS deposit;​ 1100 ppm Se average) mineralization. Here, a new instrumental technique for the determination of Se isotopic ratios (m/z:​ 76, 78, 80, and 82) and the first application of Se isotope analyses to ancient seafloor mineralization are coupled with trace element contents, Pb isotope, and S isotope analyses to elucidate the source(s) and depositional process(es) of Se within the VHMS and VSHMS systems. The wide ranges of δ^82/76Se_MERCK (-11.8 to -0.3%₀) and δ³⁴S_CDF (+2.0 to +12.8%o), and high Se contents (1100 ppm average;​ up to 5865 ppm) within the Wolverine deposit contrasts with the narrow range of δ^82/76Se_MERCK (-5.4 to -2.2%₀), narrow range of δ³⁴S_CDT (9.8 to 13.0%₀) and lower Se contents (200 ppm average) of the KZK deposit. These data suggest similar sulfide mineral saturation histories for the fluids from which the three deposits had formed (i.e. seafloor deposition, zone-refining) but also indicate that the KZK and GP4F deposits had a magmatic (leach or contemporaneous exsolution) Se source whereas the Wolverine had an additional large negative δ^82/76Se_MERCK selenium source. Creation of large variations of δ^82/76Se_MERCK during shallow-path circulation (e.g., abiogenic Fe(II/III) hydroxysulfate reactions) do not seem probable, as the presence of abundant coeval carbonaceous argillite host rocks indicate bottom water anoxia. The very negative δ^82/76Se_MERCK (-11.8%o) data for the Wolverine deposit are the most negative reported for any natural system;​ however, they are well within equilibrium calculations (~30 Δ^82/76Se). It is proposed that the negative δ^82/76Se_MERCK ratios reflect the δ^82/76Se_MERCK ratio of the Se-source. Thermal and chemical degradation and Se-loss from the carbonaceous argillite (perhaps as negative as -15%o δ^82/76Se_MERCK) to a hydrothermal fluid, and subsequent mixing with seawater (likely close to -1.4%o δ^82/76Se_MERCK) at or near the seafloor interface resulted in the wide range of δ^82/76Se_MERCK.