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Eagle’s Nest: A Magmatic Ni-Sulfide Deposit in the James Bay Lowlands, Ontario, Canada

By
James E Mungall
James E Mungall
1
Department of Geology, University of Toronto, 22 Russell St, Toronto, Ontario, Canada M5S 3B1
2
Noront Resources Ltd, 105 Adelaide St West, Ste 110, Toronto, Ontario, Canada M5H 1P9
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John D Harvey
John D Harvey
2
Noront Resources Ltd, 105 Adelaide St West, Ste 110, Toronto, Ontario, Canada M5H 1P9
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Steven J Balch
Steven J Balch
3
Canadian Mining Geophysics Ltd, 11500 Fifth Ln, Rockwood, Ontario, Canada N0B 2K0
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Bronwyn Azar
Bronwyn Azar
1
Department of Geology, University of Toronto, 22 Russell St, Toronto, Ontario, Canada M5S 3B1
2
Noront Resources Ltd, 105 Adelaide St West, Ste 110, Toronto, Ontario, Canada M5H 1P9
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James Atkinson
James Atkinson
2
Noront Resources Ltd, 105 Adelaide St West, Ste 110, Toronto, Ontario, Canada M5H 1P9
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Michael A Hamilton
Michael A Hamilton
1
Department of Geology, University of Toronto, 22 Russell St, Toronto, Ontario, Canada M5S 3B1
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Published:
January 01, 2010

Abstract

The Eagle’s Nest Ni-Cu-PGE deposit was discovered in the McFaulds Lake area of the James Bay lowlands of northern Ontario, Canada, in 2007 by Noront Resources Ltd. It is a magmatic sulfide deposit hosted by mafic and ultramafic rocks interpreted to be a feeder conduit beneath an extensive complex of sills and related volcanic rocks, which range in composition from dunite through ferrogabbro to rhyolite. The complex, called the Ring of Fire, has been dated at 2734.5 ± 1.0 Ma and it was emplaced into 2773.37 ± 0.9 Ma felsic plutonic rocks. The felsic rocks form a sill complex structurally beneath metasedimentary and metavolcanic rocks considered to have formed along a passive margin at ca. 2800 Ma within the Oxford-Stull domain of the North Caribou superterrane in the Archean Superior province.

In its original configuration, the Eagle’s Nest deposit formed in a shallowly plunging or subhorizontal keel structure at the base of a dike-like chonolith, but subsequent deformation has turned it into a vertically plunging rod of sulfide mineralization along the northwestern margin of a north-south–striking dike.

The most magnesian chilled margin is a picrite with MgO content near 18 wt percent, placing it at the boundary between komatiite sensu stricto and komatiitic basalt. Modeling suggests that the parental magma contained at least 22 percent MgO and was derived from previously melt depleted mantle. Sulfide saturation was attained following extensive contamination of the magma, resulting in the accumulation of a slurry of olivine crystals with variable amounts of interstitial sulfide melt and postcumulus orthopyroxene at the base of the conduit, locally producing significant pools of massive sulfide at or near the lower contact. The sulfide segregation occurred at moderate degrees of sulfide supersaturation from a magma rich in chalcophile elements, leading to high base and precious metal tenors in the resulting deposit. Minor fractionation of the sulfide magma is evidenced by the dispersion of massive and net-textured sulfide compositions along a tie-line between Ni-rich monosulfide and Cu-rich intermediate sulfide solid solutions, as well as by minor quantities of vein-hosted massive sulfide with extremely enriched base and precious metal tenors throughout the deposit. The former are interpreted as sulfide cumulates, whereas the latter are possible remnants of highly evolved sulfide liquids. Extensive metamorphic remobilization of Pt is considered to be responsible for wholesale depletion of Pt in much of the massive sulfide and for the local generation of sulfide veins carrying >1,100 ppm Pt.

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Contents

Special Publications of the Society of Economic Geologists

The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries

Richard J. Goldfarb
Richard J. Goldfarb
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Erin E. Marsh
Erin E. Marsh
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Thomas Monecke
Thomas Monecke
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Society of Economic Geologists
Volume
15
ISBN electronic:
9781629490403
Publication date:
January 01, 2010

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