The Bayhorse base and precious metal district is situated east of the Idaho batholith in south-central Idaho. The ores occur near the Nevada Mountain granitic stock as veins cutting the lower Paleozoic Ramshorn Slate and the Garden Creek Phyllite, and as fillings around breccia fragments within the Bayhorse Dolomite. The veins are dominated by siderite and tetrahedrite, with lesser quartz and galena, whereas the breccia ores dominantly comprise only quartz and galena. Mineralization and intrusive activity were contemporaneous during Cretaceous time. Fluid inclusion and stable isotope data indicate that mineralization formed from hot (ca. 375 degrees -225 degrees C), CO ₂ -rich (< or =8.3+ or -1.4 mole %) brines (5-20 wt % NaCl equiv) at confining pressures between 1.1 and 1.7 kbars. Fluid cooling and the resulting CO ₂ effervescence were the most important causes of ore deposition.Whole-rock delta ¹⁸ O and delta D values from the Garden Creek Phyllite define an isotopically depleted zone (60 km ² ) around the Nevada Mountain stock and are the result of high-temperature interactions with ancient meteoric waters at water/rock ratios (mass fractions) ranging from 0.002 to 0.09. The delta ¹⁸ O (sub H ₂ O) and delta D (sub H ₂ O) values of the ore fluids ranged from 11.2 to 3.9 per rail and from -55 to -146 per mil, respectively. Comparison of the ore fluid delta ¹⁸ O (sub H ₂ O) and delta D (sub H ₂ O) values with hypothetical waters equilibrated with the Garden Creek Phyllite intricates that the hydrothermal fluids must have also interacted with the basal dolomite of Bayhorse Creek, which underlies the phyllite. Early delta ¹³ C (sub CO ₂ ) values calculated in equilibrium with siderite ranged from -9.1 to -7.4 per mil, whereas later delta ¹³ C (sub CO ₂ ) values obtained from quartz-hosted fluid inclusion extracts range from -4.0 to -1.8 per mil. The delta ¹³ C (sub CO ₂ ) values for the hydrothermal fluids thus also record a transition from early water/rock interactions that were dominated by the Garden Creek Phyllite (organic matter delta ¹³ C = -23.6 to -20.9ppm) to later interactions that were influenced significantly by the basal dolomite of Bayhorse Creek (delta ¹³ C = -2.3 to -2.2ppm). The delta ³⁴ S values of main-stage hydrothermal sulfides range from 3.0 to 28.3 per mil and display a distinct mode at approximately 10 per mil. The range of values may be interpreted as either a heterogeneous sedimentary source or mixed sedimentary-magmatic sources. On the basis of mass balance considerations, it is likely that all of the fluid salinity and the dissolved metals were derived from the Garden Creek Phyllite. The paragenetic sequence developed in the ores appears to be related to: (1) processes at the site of deposition, most notably cooling and CO ₂ effervescence, and (2) water-rock interactions within the Garden Creek Phyllite in the 60-km ² isotopically altered area surrounding the Nevada Mountain stock.