The world-class Idrija mercury deposit (western Slovenia) is hosted by highly deformed Permocarboniferous to Middle Triassic sedimentary rocks within a complex tectonic structure at the transition between the External Dinarides and the Southern Alps. Concordant and discordant mineralization formed concomitant with Middle Triassic bimodal volcanism in an aborted rift. A multiple isotopic (C, O, S) investigation of host rocks and ore minerals was performed to put constraints on the source and composition of the fluid, and the hydrothermal alteration. The distributions of the δ^sup 13^C and δ^sup 18^O values of host and gangue carbonates are indicative of a fracture-controlled hydrothermal system, with locally high fluid-rock ratios. Quantitative modeling of the δ^sup 13^C and δ^sup 18^O covariation for host carbonates during temperature dependent fluid-rock interaction, and concomitant precipitation of void-filling dolomites points to a slightly acidic hydrothermal fluid (δ^sup 13^Casymptotically =-4per thousand and δ^sup 18^Oasymptotically =+10per thousand), which most likely evolved during isotopic exchange with carbonates under low fluid/rock ratios. The δ^sup 34^S values of hydrothermal and sedimentary sulfur minerals were used to re-evaluate the previously proposed magmatic and evaporitic sulfur sources for the mineralization, and to assess the importance of other possible sulfur sources such as the contemporaneous seawater sulfate, sedimentary pyrite, and organic sulfur compounds. The δ^sup 34^S values of the sulfides show a large variation at deposit down to hand-specimen scale. They range for cinnabar and pyrite from -19.1 to +22.8per thousand, and from -22.4 to +59.6per thousand, respectively, suggesting mixing of sulfur from different sources. The peak of δ^sup 34^S values of cinnabar and pyrite close to 0per thousand is compatible with ore sulfur derived dominantly from a magmatic fluid and/or from hydrothermal leaching of basement rocks. The similar stratigraphic trends of the δ^sup 34^S values of both cinnabar and pyrite suggest a minor contribution of sedimentary sulfur (pyrite and organic sulfur) to the ore formation. Some of the positive δ^sup 34^S values are probably derived from thermochemical reduction of evaporitic and contemporaneous seawater sulfates.PUBLICATION ABSTRACT