Date of Award

6-2019

Degree Name

Master of Science

Department

Geological and Environmental Sciences

Department

Geosciences

First Advisor

Dr. Joyashish Thakurta

Second Advisor

Dr. Robb Gillespie

Third Advisor

Dr. Peter Voice

Keywords

Eagle East, Gabbro, sulfide, midcontinent, nickel

Access Setting

Masters Thesis-Open Access

Abstract

The Eagle East gabbroic intrusion in Michigan’s Upper Peninsula, is a subvertical pluglike intrusion found near the base of the Eagle East Ni-Cu-PGE deposit. U-Pb baddeleyite dating of the gabbro yields an age of 1103.4 ± 1.2 Ma, suggesting formation during the early stages of the Midcontinent Rift System (MRS). Drill core observations show that the intrusion cross-cuts the eastern most portion of the Eagle East ultramafic conduit. The internal portion of the intrusion is made up of two main rock types, gabbro and gabbro norite, which consist of plagioclase (50-60%), clinopyroxene (15-25%), orthopyroxene (0-15%), olivine (0-10%), and are generally sulfide-poor. Surrounding the gabbro is a 3-5-meter sleeve of melagabbro and feldspathic pyroxenites containing minor blebs of chalcopyrite and pyrrhotite. In addition, a peridotite-bearing unit containing semi-massive and massive textured sulfides is located within the gabbroic intrusion. Olivine compositions (Fo74) in this peridotite are similar the olivine hosted in the semi-massive sulfide-bearing units reported from the Eagle and Eagle East intrusions. Olivine in the gabbro is more enriched in iron (Fo24-30) than olivine grains from the peridotitic units from the Eagle and Eagle East intrusions. In addition, nickel concentrations in these iron-rich olivine are below the detection limit (100 ppm), indicating crystallization from a magma depleted in nickel. MELTS modelling shows that Eagle East gabbro could be explained by extensive fractional crystallization of a high-MgO picritic parent magma similar to the one that has been proposed for the Eagle and Eagle East intrusions.

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