Episodic Nature of Magmatic Ascent in a Dynamic Conduit System: Evidence From a Late Gabbroic Intrusion Associated with the Eagle Ni-Cu Sulfide Deposit in Northern Michigan, USA

Joyashish Thakurta, Kevin Rupp, Beau Haag

Research output: Contribution to journalArticlepeer-review

Abstract

New exploration studies have revealed a near-vertical, plug-like gabbroic intrusive body emplaced along the feeder conduit of the funnel-shaped Eagle East ultramafic intrusion in the Upper Peninsula of Michigan. U-Pb baddeleyite analysis on this gabbro yielded an age of 1103.4 ± 1.2 Ma. Although a sulfide-deficient gabbro is the principal rock unit, peripheral zones of the intrusion contain feldspathic pyroxenite and melagabbro with minor blebs of chalcopyrite and pyrrhotite. A small pocket-like body of peridotite with significant quantities of sulfide minerals occurs along the edge of the main gabbroic unit. The modes of occurrence of sulfide minerals in the pocket peridotite are similar to the larger Eagle East and Eagle intrusions nearby. The composition of olivine in the main gabbroic rock is forsterite (Fo)24-30, but olivine in pocket peridotite shows a composition of ~Fo74. The Ni-concentration in the olivine of the main gabbro unit is below 100 ppm, but in the Mg-rich olivine of the pocket peridotite, it is between 2,100 and 2,800 ppm. Although δ34S values of sulfide minerals in the pyroxenite rim and in the pocket peridotite range from –0.6 to 7.0‰, the average value is higher than those reported from sulfides in the Eagle East and Eagle intrusions. The origin of the gabbro unit could be explained by a closed-system crystallization of a fractionated late derivative of an original high-MgO magma, while the peridotite pocket is a xenolithic representative of the old peridotitic intrusion dislodged and entrained in a late magmatic pulse.

Original languageEnglish (US)
Pages (from-to)1105-1130
Number of pages26
JournalEconomic Geology
Volume117
Issue number5
DOIs
StatePublished - Aug 2022

Bibliographical note

Funding Information:
We are thankful to all members of the exploration team at the Eagle mine, particularly Bob Mahin, Espree Essig, Mark DeHoog, Cody Suits, and Guillermo Berridi for providing us access to drill cores, sharing petrological and geochemical data, and assisting in the preparation of 3-D models using Leapfrog. Part of this project was funded by the Lundin Mining Company. Mike Hamilton and Sandra Kano from Jack Satterly Geochronology Lab at the University of Toronto did the analytical work for geochronology. Owen Neil from the University of Michigan assisted us with electron microprobe analysis. Alex Koerber and Nick Moleski from Western Michigan University helped with sample collection, while Robb Gillespie and Peter Voice provided input in the preparation of a previous draft of the article. Constructive reviews by R. Ernst and Peter C. Lightfoot have enabled us to improve the quality of the manuscript.

Publisher Copyright:
© 2022 Society of Economic Geologists, Inc.

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