CHARACTERISTICS OF THE PARENTAL MAGMA OF THE EAGLE NI-CU SULFIDE DEPOSIT, MIDCONTINENT RIFT SYSTEM, NORTHERN MICHIGAN

The Eagle Ni-Cu sulfide deposit is located in the Baraga basin in northern Michigan, to the south of the axis of the ~1.1 Ga Midcontinent Rift System. Massive and semi-massive sulfide ores of the Eagle deposit are hosted by a small intrusion comprised primarily of feldspathic peridotite and melagabbro. Olivine is a primocryst mineral whereas clinopyroxene and plagioclase are interstitial minerals. Minor Cr-spinel is present as inclusions in olivine crystals. Sulfide mineral assemblages are composed of pyrrhotite, chalcopyrite, and pentlandite. The U-Pb baddeleyite age of the Eagle intrusion is 1107±4 Ma, corresponding to the earliest phase of basaltic magmatism associated with the Midcontinent Rift System. Whole-rock Mg-numbers range from 83 to 74, significantly higher than typical values in the coeval, but slightly younger (1099 Ma) Duluth Complex. The contents of Fo and Ni in olivine from the Eagle intrusion are 76-85 mole % and 1426-3520 ppm, respectively. These values are also significantly higher than those of olivine found in the Duluth Complex. The Cr/(Cr+Al) ratios of Cr-spinel from the Eagle intrusion vary between 57 and 79. The FeO/MgO ratio and Al₂O₃ content of the parental magma for the Eagle deposit calculated using the compositions of olivine and Cr-spinel in the rocks are similar to those of Group 1, olivine-phyric picrites from Mamainse Point, located near the base of the volcanic sequence of the Midcontinent Rift System. Modeled crystallization sequence, olivine compositions, and trace element variations of high-MgO basalts from Mamainse Point, calculated using the MELTS program, are consistent with observations in the Eagle intrusion. Our data indicate that the Eagle sulfide-bearing intrusion formed from high-MgO basaltic magma generated during the early stages of the development of the Midcontinent Rift System.