Pyrrhotite is a common inclusion in several phenocrystal phases from the Fish Canyon Tuff, a voluminous, homogeneous ash flow erupted from the Central San Juan field. The fugacities of various gaseous species have been calculated using the composition of the pyrrhotite, iron-titanium oxides, and biotite. Sulfur and oxygen fugacities fall very near the sulfur condensation curve, implying that liquid sulfur may have been present in the parent magma. High sulfur fugacities during melt formation can cause high oxygen fugacity in calc-alkaline magmas due to the breakdown of pyrite at high temperatures. During cooling of such magmas, separation of SO2-rich gases causes a reduction in oxygen fugacity. Significant amounts of sulfur are available during degassing and represent a major source of sulfur for related ore deposits. The ultimate source of the high sulfur and oxygen activities is probably pyrite and magnetite deposited during the waning stages of hydrothermal circulation at active oceanic ridges. Other ash flows and calc-alkalic volcanic rocks should be carefully examined for similar sulfide phases.