NorthMet is the first copper-nickel-platinum group metals (PGMs) mine planned to be developed in Minnesota, US. Poly Met Mining, a wholly-owned subsidiary of Canada based PolyMet Mining Corporation, is the owner and developer of the project.
Located within the Duluth Complex in northeastern Minnesota, the NorthMet project represents one of the biggest untapped copper, nickel and PGMs deposits in the world.
An updated definitive feasibility study (DFS) for the project was completed in March 2018 and the company received the final key permit to construct and operate the project in March 2019.
The polymetallic open-pit mine is planned to be developed in two phases. The capital expenditure on the phase one development of the project is estimated to be £665m ($945m) while phase two is estimated to incur an additional investment of £182m ($259m).
Construction works are expected to be started after the settlement of on-going legal challenges contesting the environmental permits for the project.
The NorthMet project is expected to produce up to 57.7 million pounds (Mlbs) of copper, 8.7Mlbs of nickel, 311,000 pounds of cobalt, 14,000 ounces (oz) of platinum, 59,000oz of palladium, 4,000oz of gold and 48,000oz of silver annually over an estimated mine life of 20 years.
Location and site details
The NorthMet project comprises two components, the greenfield development of the NorthMet non-ferrous polymetallic deposit and the brownfield development at the nearby Erie Plant, the former LTV Steel Mining Company’s iron ore processing facility that was acquired by Poly Met Mining from Cleveland-Cliffs in 2005.
The NorthMet deposit occupies approximately 4,300 acres in St Louis County, in northeastern Minnesota, approximately 112km north of Duluth. The Erie plant site is spread over 12,300 acres, approximately 10km to the west of the NorthMet deposit.
Geology and mineralisation
The NorthMet copper, nickel, cobalt, and precious metals deposit lies in the Partridge River Intrusion along the northern margin of the Duluth Complex, which is a vast geological formation near the eastern edge of the Mesabi Iron Range in Minnesota.
NorthMet is a massive disseminated sulphide deposit hosting copper, nickel, cobalt, platinum, palladium, gold, and silver mineralisation in chalcopyrite, cubanite, pentlandite, and pyrrhotite.
Ore reserves at NorthMet
The proven and probable ore reserves for the NorthMet project are estimated to be 290Mt grading 0.288% copper, 0.083% nickel, 75 particles per billion (ppb) platinum, 264ppb palladium, 39ppb gold, 73.95 particles per million (ppm) cobalt and 1.06ppm silver.
The measured and indicated resources are estimated to be 795Mt grading 0.234% copper and 0.07% nickel.
NorthMet project development phases
The phase one development of the project involves the start of a 32,000 tonnes per day (tpd) open-pit mining and processing operation with the refurbishment of the existing primary crushing circuit and the replacement of the existing rod and ball mill circuits with a new, modern semi-autogenous grinding (SAG) mill, ball mill and flotation circuit at the Erie plant.
Phase one also includes rail upgrades, new mining equipment, and a wastewater treatment facility.
In phase two, a new hydrometallurgical plant is planned to be constructed in order to treat nickel sulphide concentrates into upgraded nickel-cobalt hydroxide and produce additional copper and PGMs.
Mining and ore processing
The conventional open-pit mining method involving the drill, blast, truck, and shovel operations will be employed to extract ore from the NorthMet deposit. The mine will involve three pits, namely the East Pit, West Pit, and the Central Pit.
The run of mine (ROM) ore will be transported by a private railroad to the Erie processing plant located approximately 10km away from the mine site.
The polymetallic NorthMet ore will undergo two-stage crushing and two-stage grinding before passing through the rougher and cleaner floatation circuits for the production of copper, nickel and PGM concentrates.
Infrastructure facilities for the NorthMet project
The electricity for the project will be sourced through 138kV overhead transmission lines from the Minnesota Power grid.
The raw water for the project will be sourced from the Colby Lake located approximately 8km south of the Erie processing plant.
The other infrastructure facilities will include a rail loadout facility at the mine site, two 5MW emergency power plants (one for the mine site and one for the processing plant), and a wastewater treatment plant at the processing plant.
The flotation tailings will be pumped into the existing tailings basin located near the processing plant.
M3 Engineering & Technology Corporation (M3) prepared the updated technical report for the NorthMet project in 2018.
The other consultants engaged for the feasibility studies for the project were Independent Mining Consultants (IMC), Senet, Hard Rock Consulting (HRC), Barr Engineering Company (Barr), and Bateman Engineering.