Eagle Mine is an underground high-grade nickel and copper mine located in the US state of Michigan. The mine commenced commercial operations in 2014.
The property has two active mineralised zones, Eagle and Eagle East. A third zone, Keel, is under development and is expected to start producing mill feed in 2024.
Eagle Mine, an indirect wholly owned subsidiary of Canadian base metals mining company Lundin Mining, is the 100% owner and operator of the mine.
An updated technical report on the Eagle Mine was filed by Lundin Mining in February 2023.
The mine is expected to produce 440 million pounds of nickel, 429 million pounds of copper, and small quantities of other minerals over its estimated mine life to 2027.
Location and site details
The Eagle Mine is located in the Michigan Upper Peninsula in Michigamme Township, Marquette County, covering an area of approximately 0.63km2.
The Humboldt Mill, which processes the ore, is located 61km west of Marquette and approximately 105km by road from the mine site, covering approximately 1.42km2 area.
The mine has a geological field office in Negaunee and an information centre in Marquette.
Overall, Eagle Mine holds around 4,565hectares (ha) of mineral rights and approximately 3,080ha of surface rights in the form of leases, agreements, or ownership.
Ownership history of the Eagle Mine
KEX commenced working in the area in 1991 and nickel exploration started near the Eagle property in 1995.
Rio Tinto drilled the deposit in 2002 and two separate high grade sulphide zones were identified by the end of 2003. Following this, a magnitude study, and an extensive resource and geotechnical drill programme were completed in 2004.
The construction of Eagle Mine began in 2010 and underground development commenced in September 2011. The Humboldt Mill was also refurbished.
In 2013, the mine was acquired by Lundin Mining from Rio Tinto for $325m.
The state leases were renewed for ten years in 2022, while the private leases have various expiry dates and they can be extended via continued payments or production.
Geology and Mineralisation
The Eagle property is situated in the Baraga Basin on the south side of Lake Superior.
The mine’s geology consists of the Eagle intrusion, Eagle East intrusion, and the Keel zone of the Eagle East, which are part of the same ultramafic intrusive system.
All three host high grade primary magmatic Ni-Cu sulphide mineralisation and are related to the feeder system for the Keweenawan flood basalts.
The mineralisation is similar in Eagle and Eagle East comprising mineralised peridotite bodies with concentrations of semi-massive sulphide in the centre and massive sulphides at the base of intrusions. Keel is a lower grade version of Eagle East ore.
Mineral reserves
Effective December 2022, Eagle Mine’s Measured and Indicated Mineral Resources were reported to be 3,855kt at a grading of 1.88% nickel (Ni), 1.44% copper (Cu), 0.05% cobalt (Co), 0.15g/t gold (Au), 6.06g/t silver (Ag), 0.38g/t platinum (Pt), and 0.27g/t palladium (Pd).
The proven mineral reserves estimate is 303kt containing 5.7kt Ni at a grading of 1.89%, 4.7kt Cu at a grading of 1.54%, 1.26koz Au at a grading of 0.13g/t, and 93.6koz Ag at a grading of 9.62g/t.
The probable mineral reserves are 3,127kt containing 50.6kt Ni at a grading of 1.62%, 39.2kt Cu at a grading of 1.25%, 13.15koz Au at a grading of 0.14g/t, and 516.7koz Ag at a grading of 5.14g/t.
The Mineral Resource estimates are inclusive of Mineral Reserves, however, it excludes mineralisation within previously mined (depleted) areas.
Mining and ore processing
The mine uses three mining methods- Transverse Sublevel Open Stoping (TSLOS), Longitudinal Sublevel Open Stoping (LSLOS), and Drift and Fill Mining (D&F).
Under TSLOS, a portion between two sublevels is mined by dividing the ore into primary and secondary stopes alternately, extending parallel from the footwall to the hanging wall.
In LSLOS method, the longitudinal stopes, typically 6m wide, are mined along strike in panels up to 45m long.
D&F is like overhand cut-and-fill method but in this case lifts are mined one drift at one time instead of excavating the ore from footwall to hanging wall.
The ore from the Eagle Mine is transported to the Humboldt Mill using special highway haul trucks and delivered to the Coarse Ore Storage Area (COSA).
In the mill, the ore undergoes a conventional three-stage crushing process and then stored in bins.
Subsequently, the ore undergoes through a single stage ball milling process and bulk flotation followed by separation flotation to deliver nickel and copper concentrates separately.
Nickel and copper concentrates are stored in a covered concentrate building on site. from where the material is transported via rail car directly to smelter facilities within North America or ports for export.
The metallurgical recoveries of Ni and Cu average 85% and 97% for the ore.
The mill’s average throughput was 59,380 tonnes per month during the period from January 2021 to September 2022.
Tailings from the plant are dumped sub-aqueously in the nearby Humboldt iron ore mine open pit, now known as Humboldt Tailings Deposit Facility (HTDF).
The underground mine produces 2,000 tonnes per day (tpd) of high-grade nickel-copper ore. Production is expected to continue until 2027, following which, the mine is scheduled for closure.
Eagle Mine has produced 143.7kt of nickel and 142.3kt of copper until the end of 2021. It is expected to produce approximately 755,000tpa of ore from 2023 to 2026 and reduce it to half of that tonnage in 2027.
Eagle Mine Infrastructure and Power supply
The infrastructure of the mine includes the HTDF. Its key components are tailings delivery system, tailings vault and deposition system, pit lake, water reclaim system, cut-off wall and water treatment plant (WTF).
The infrastructure also includes site roads, non-potable and potable water wells, and ancillary facilities such as powerhouses, storage basins, mine and mill dry facilities, and warehouses among others.
Alger Delta Electric Co-operative (ADEC) provides grid power to the site for mining operations. A diesel generator of 4.16kV, 2,500kVA power provides emergency backup power.
Contractors involved
WSP Golder prepared the technical report, effective December 2022, on the Eagle Mine for Lundin Mining.
Another technical report, that disclosed Eagle East Mineral Reserves, was prepared by Roscoe Postle Associates (RPA) with effective date of December 2016.
Cementation USA was selected by Lundin Mining as the mining contractor.
