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The Bayhorse base and precious metal district situated east of the Idaho batholiths 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 breccias fragments within the Bayhorse Dolomite. The veins are dominated by siderite and tetrahedrite, with lesser quartz and galena, whereas the breccias 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° -225°C), CO2-rich (≤8.3 ± 1.4 mole %) brines (5-20 wt% NaCl equiv) at confining pressures between 1.1 and 1.7 kbars. Fluid cooling and the resulting CO2 effervescence were the most important causes of ore deposition.