Three significant base metal deposits were recently discovered in the Finlayson Lake district, Yukon Territory, Canada; however, widely varying selenium contents (e.g., 1,100 ppm avg Se at Wolverine, 200 ppm avg Se at Kudz Ze Kayah, and 7 ppm avg Se at GP4F) have hindered their rapid exploitation. Observations and data from the deposits in the Finlayson Lake district show a strong positive correlation between high-temperature (>300°C), copper-rich sulfide assemblages at the base of volcanic-hosted massive sulfide (VHMS) and the selenium content of all sulfide minerals. Thermodynamic calculations for selenide and sulfide minerals indicate significantly higher temperatures of formation and lower solubilities of selenide versus sulfide minerals, which is consistent with petrographic observations and can explain the distribution of selenium in zone-refined VHMS and volcanic sediment-hosted massive sulfide deposits. Volumetric mass-balance calculations indicate that some selenium could be sourced from contemporaneous volatile degassing of SeO4, its rapid reduction to H2Se, and subsequent entrainment into hydrothermal ore fluid. However, estimates for the Kudz Ze Kayah and Wolverine deposits suggest an additional large Se reservoir. Volumetric mass-balance estimates indicate that contemporaneous volcanic rocks and black shales are likely selenium reservoirs for the Kudz Ze Kayah and Wolverine deposits, respectively.