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The Eagle magmatic nickel-copper deposit is located 35 km north west of Marquette on the Upper Peninsula of Michigan, USA.
The ore deposit is hosted in the westernmost of two small peridotite bodies known as the Yellow Dog Peridotite associated with the Marquette-Baraga dyke swarm, which intrude metamorphosed sediments of the Palaeoproterozoic Baraga sedimentary basin. These intrusions, which lack penetrative foliations and truncate Penokean tectonic fabrics in the surrounding meta-sediments, represent picritic magmatism in a dynamic magma conduit formed during the early stages in the development of the ~1.1 Ga Midcontinent Rift System.
The deposit occurs as a thickened section of a narrow (<10 m thick) dyke of restriced strike extent, that enlarges to thicknesses of 40 to 90 m over a strike length of ~400 m, and vertical extent of ~250 m (Ding et al., 2012).
Three major rock types have been recognized in the Eagle intrusion:
Feldspathic peridotite, the most olivine-rich unit in the intrusion, occurs as a sheet at the top of the sequence, and contains 30 to 60% olivine, 5 to 15% clinopyroxene, 15 to 40% orthopyroxene and 15 to 25% plagioclase;
Melatroctolite melagabbro, which contains 30 to 45% olivine, 5 to 10% clinopyroxene, 20 to 25% orthopyroxene and 25 to 35% plagioclase and
Olivine melagabbro, which occurs as a core wrapped by melatroctolite, and comprises 10 to 35% olivine, 5 to 15% clinopyroxene, 20 to 45% orthopyroxene and 25 to 40% plagioclase.
In addition, minor feldspathic pyroxenite is present locally as small pods within the intrusion. It is free of olivine and contains 30 to 45% orthopyroxene, 25 to 30% clinopyroxene and 15 to 25% plagioclase.
Plagioclase is more common as an interstitial mineral in the feldspathic peridotite, with lesser amounts of interstitial plagioclase found in the melatroctolite and olivine melagabbro. Hornblende, biotite, titaniferous magnetite, and sulphide (up to 5%) occur as interstitial minerals in all units.
Four main types of sulphide mineralization have been recognized within the Eagle deposit:
i). disseminated sulphides (0.2 to 6 wt.% S) occuring as irregular-shaped, poly-minerallic blebs that are found interstitially to silicate minerals assemblages (plus sulphide globules within olivines) and comprise pyrrhotite, pentlandite and chalcopyrite, generally in a proportion of 6:2:2, that are found in feldspathic peridotite, feldspathic pyroxenite, melatroctolite and olivine melagabbro;
ii). rocks with semi-massive sulphides (11 to 30 wt.% S), occur as a net-textured matrix enclosing olivine, pyroxene, and rare plagioclase, and is composed of pyrrhotite (70 to 80%), pentlandite, and chalcopyrite in decreasing abundance. Semi-massive sulphides occur in two separate zones that are separated by a zone of massive sulphides, and are bounded above and below by low-sulphide intrusive rocks. The upper zone also contains cubanite. The transition between the disseminated and semi-massive sulphide mineralisation is typically abrupt and occurs within a few centimeters. The boundaries between semi-massive and massive sulphide ores are also sharp, occuring within less than a 1 cm interface;
iii). massive sulphides (30 to 35 wt.% S), mainly composed of pyrrhotite (65 to 85%) with lesser amounts of pentlandite (10 to 25%) and chalcopyrite (5 to 15%), form a relatively large, coherent body that crosscuts the semi-massive mineralisation. In a number of localities, massive and semi-massive sulphides are separated by olivine melagabbro and sedimentary country rocks. Locally, massive sulphides also extend for 10 to 20 m out into the sedimentary country rocks; and
iv). sulphide veins occur within the Proterozoic sedimentary country rocks located 5 to 10 m NW of the semi-massive and massive mineralisation. Sulphide veins have sharp boundaries with sedimentary rocks, with transitions within <1 mm.
The semi-massive and massive sulphide masses occur as a near vertical pipe-like mass with a diameter of ~75 m, and vertical extent of ~200 m, with the massive sulphide zone concentrated near the bottom, close to the abrupt thickening of the intrusive mass from the feeder dyke. Lesser semi-massive sulphide occurs below part of the massive sulphide lateral to the main feeder dyke.
Although the nickel contents of the semi-massive and massive sulphides are relatively uniform throughout the deposit, copper contents vary significantly. Platinum group metals (PGM) and gold values are significantly higher in the copper rich massive sulphides (Ding et al., 2012).
The deposit was discovered by Kennecott Exploration in 2002 with and intersection of 84.2 m of massive sulphide mineralization averaging 6.3% Ni and 4.0% Cu
The estimated reserves and resource for the deposit at the end of 2011 (Rio Tinto, 2012) were:
Total reserves (probable) - 4.3 Mt @ 3.16% Ni, 2.69% Cu, 0.27 g/t Au plus
Total resources (indicated + inferred) - 0.2 Mt @ 3.48% Ni, 2.37% Cu, 0.24 g/t Au
The JORC compliant resources for the deposit in January 2013 (Tech Report for Lundin Mining) were:
Indicated resource - 4.83 Mt @ 3.52% Ni, 2.94% Cu, 0.29 g/t Au, 0.75 g/t Pt, 0.51 g/t Pd, 0.10% Co;
Inferred resources - 0.17 Mt @ 1.01% Ni, 0.97% Cu, 0.13 g/t Au, 0.26 g/t Pt, 0.17 g/t Pd, 0.03% Co;
The mineral resources are inclusive of the ore reserves.
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 20/10/2012
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
References to this deposit in the PGC Literature Collection:
Keays, R.R. and Lightfoot, P.C., 2015 - Geochemical Stratigraphy of the Keweenawan Midcontinent Rift Volcanic Rocks with Regional Implications for the Genesis of Associated Ni, Cu, Co, and Platinum Group Element Sulfide Mineralization : in Econ. Geol. v.110 pp. 1235-1267|
Ripley E M, 2014 - Ni-Cu-PGE Mineralization in the Partridge River, South Kawishiwi, and Eagle Intrusions: A Review of Contrasting Styles of Sulfide-Rich Occurrences in the Midcontinent Rift System: in Econ. Geol. v.109 pp. 309-324|
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