Re-Os isotopic ratios of the ore plot above a chondritic 1.1 Ga reference isochron with γOs values between 18 and 72. Achaean gneiss fall along a chondritic 2.3 Ga reference isochron, however, 187Re/188Os ratios are < 10. Two of four Proterozoic samples fall on a chondritic 1.85 Ga reference isochron; whereas the remaining samples fall below, suggesting local hydrothermal processes may have resulted in Re gain or Os loss. Elevated Os isotopic ratios of ore from Eagle suggest that the parental magmas were contaminated via assimilation of country rocks. However, the S abundance of Achaean samples is low (< 1000 ppm) and mass balance considerations indicate that excessive amounts of assimilation are required to explain the Re-Os isotopic compositions of the ore. Proterozoic metasedimentary rocks may be a more obvious source of S, but their 187Re/188Os ratios are high (up to ~400). Thus, the amount of contamination required to account for the elevated 187Os/188Os ratios of the ore should also lead to 187Re/188Os ratios that are considerably higher than those that we observe. Although our data confirm that crustal contamination of mafic magmas associated with the Eagle deposit has occurred, the mechanism of mass transfer appears to be complex. Some of the Re-Os isotopic variability may be related to late stage hydrothermal processes that produced widespread serpentinization.