Hydrothermally altered country rocks surrounding hydrothermal ore deposits are commonly characterized by altered δ18O and δ13C values relative to distal, unaltered rocks. Stable isotopes have been recognized as a powerful tool to complement traditional geochemical and geophysical approaches during mineral exploration, particularly in carbonate replacement and Carlin-type deposits. However, wall-rock alteration and carbonate veining during ore formation is often accompanied by the precipitation of sulfide minerals, complicating stable isotope analysis. The classic phosphoric acid extraction technique for carbonate minerals results in the release of additional H2S gas, interfering with isotope ratio mass spectrometry (IRMS), and laser absorption spectrometry (LAS) analysis of CO2. Here we present the first results from a novel off-axis integrated cavity output spectrometer (OA-ICOS) instrument, capable of determining the stable isotope composition (δ13C, δ18O) of mixed CO2-H2S gas evolved from carbonate-sulfide mixtures. Instrument calibration was performed by analyzing in-house and commercially available CaCO3 stable isotope reference materials. Analyses of mixed CO2-H2S tank gas and sulfide-spiked stable isotope reference materials containing up to 40 wt % chalcopyrite and sphalerite demonstrate that the method outlined here is capable of effectively analyzing previously problematic high sulfide samples. The analytical capabilities, together with the robust nature of the OA-ICOS instrument, may be of direct practical use for mineral exploration in carbonate-hosted hydrothermal systems including ore deposits.