We report the first discovery of magmatic anhydrite-sulfide assemblages in a subvolcanic intrusion associated with the Siberian Traps. The δ34S values of anhydrite and coexisting sulfide crystals analyzed by ion probe are 18‰–22‰ and 9‰–11‰, respectively. More than 50% of the total sulfur in the intrusion is estimated to derive from marine evaporites in the footwall strata. The contaminated magma was highly oxidized and able to dissolve up to one order of magnitude more sulfur than pure mantle-derived basaltic magma. Such contaminated magma, if erupted, would have released far more SO2 into the atmosphere than is generally appreciated.