Isotopic (H, Sr, Pb, Ar) and fluid inclusion data for hydrothermal fluids associated with potassic alteration from three Late Cretaceous porphyry Cu occurrences, west central Yukon, suggest a nonmagmatic fluid component was present in these hydrothermal fluids. Potassic stage quartz veins contain a dominant assemblage of saline and vapor-rich fluid inclusions that have #D values between #120 and #180o/oo. Phyllic stage quartz veins are dominated by vapor-rich fluid inclusions and have #D values that overlap with but are, on average, heavier (#117 to #132o/oo) than those in potassic stage quartz veins. These #D values are significantly lower than those from plutonic quartz phenocrysts (#91 to #113o/oo), and from values typically reported for primary fluids from porphyry-style mineralization (#40 to #100o/oo). The initial Sr ( 87 Sr/ 86 Sr i ) isotopic values for the plutons are 0.7055 (Casino), 0.7048 (Mt. Nansen), and 0.7055 (Cash). The 87 Sr/ 86 Sr i compositions of hydrothermal K-feldspar ranges from magmatic Sr values to more radiogenic compositions (Casino: 0.70551--0.70834, n # 8; Mt. Nansen: 0.7063--0.7070, n # 4; Cash: 0.7058, n # 1). The fluid inclusion waters from potassic quartz veins have 87 Sr/ 86 Sr i values that are similar to those of co-existing hydrothermal K-feldspar. The Pb isotopic compositions of hydrothermal K-feldspar show a weak positive correlation with Sr i for identical samples. Fluid inclusion waters of phyllic quartz veins also have Sr i compositions more radiogenic than the plutons. The Pb isotopic composition of pyrite and bornite from phyllic alteration veins are similar to, or more radiogenic than, hydrothermal K-feldspar Pb isotopic values.