Eagle’s Nest: A Magmatic Ni-Sulfide Deposit in the James Bay Lowlands, Ontario, Canada
James E Mungall, John D Harvey, Steven J Balch, Bronwyn Azar, James Atkinson, Michael A Hamilton, 2010. "Eagle’s Nest: A Magmatic Ni-Sulfide Deposit in the James Bay Lowlands, Ontario, Canada", The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Richard J. Goldfarb, Erin E. Marsh, Thomas Monecke
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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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The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
VOLCANIC-ASSOCIATED and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world's total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth's surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific "high-grade" area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area.