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 delta(13)C and delta(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
delta(13)C and delta(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 (delta(13)Capproximate to-4parts per thousand and
delta(18)Oapproximate to+10parts per thousand), which most likely
evolved during isotopic exchange with carbonates under low fluid/rock
ratios. The delta(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
delta(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
+22.8parts per thousand, and from -22.4 to +59.6parts per thousand,
respectively, suggesting mixing of sulfur from different sources. The
peak of delta(34)S values of cinnabar and pyrite close to 0parts per
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 delta(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
delta(34)S values are probably derived from thermochemical reduction of
evaporitic and contemporaneous seawater sulfates.