Re-Os Isotopic Study of the Eagle Ni-Cu Sulfide Deposit, Northern Michigan

The Eagle Ni-Cu-PGE deposit occurs in melagabbro, melatroctolite, and feldspathic peridotite units that were emplaced during the early stage development of the ~1.1 Ga Midcontinent Rift. Mineralization is composed of massive, semi-massive, and disseminated sulfide zones containing pyrrhotite, pentlandite, and chalcopyrite. Country rocks surrounding the Eagle deposit include Archean gneiss and Proterozoic metasedimentary packages. Re-Os isotope ratios of ore material, along with country rocks, have been analyzed in order to evaluate the importance of country rock contamination in the ore-forming process.

Re-Os isotopic ratios of the ore plot above a chondritic 1.1 Ga reference isochron with γ_Os 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.