PolyMet Mining’s NorthMet copper nickel platinum group metals (PGMs) project, located in northeastern Minnesota, US, will be the first ever mining project to commercially extract metals from Duluth Complex.
Duluth Complex is one of the world’s largest known undeveloped deposit containing copper, nickel and other precious metals.
The definitive feasibility study for the polymetallic mine was completed in September 2006. The project is still in permitting stage with construction works anticipated to begin in 2015.
The mine is expected to produce 72 million pounds (mlbs) of copper, 15.4mlbs of nickel, 720,000lbs of cobalt and 106,000 troy ounces of precious metals a year over an estimated mine life of 20 years. It is expected to create 360 jobs during operational phase.
NorthMet project location, geology and reserves
Owned by Duluth Metals, the deposit is listed as one of the biggest combined base and precious metal resources to be developed in North America.
NorthMet project is located within the Partridge River intrusion along the northern margin of Duluth Complex, a geological formation near the Mesabi Iron Range, which is known to contain abundant amounts of copper nickel platinum group metals (PGMs).
Polymetallic deposit is known to be a disseminated sulphide deposit in heterogeneous troctolitic rocks and is associated with the Midcontinent Rift, which is believed to have formed more than one billion years ago.
Four types of metallic minerals including chalcopyrite, cubanite, pentlandite and pyrrhotite are widely found at this deposit.
The project area also contains troctolitic to anorthositic rock units dipping southeast at 20° to 25°, with basal ultramafic units defining the boundaries of these units.
Mineralisation at NorthMet occurs mostly in four broadly defined horizons or zones. Sulphide mineralisation at the deposit includes minor quantities of bornite, violarite, pyrite, sphalerite, galena, talnakhite, mackinawite and valerite.
The minerals mainly occur as blebs interstitial with plagioclase, olivine and augite grains.
The mine is estimated to contain 249.2 million tonnes (Mt) of proven and probable reserves grading 0.284% copper, 0.082% nickel and 0.38 gain-to-noise-temperature (g/t) precious metals.
Mining and processing of ore from the NorthMet project
Conventional drill and blast mining method followed by truck and shovel operation will be employed for the NorthMet mining project.
The mine is divided into three open pits known as West Pit, East Pit and the Central Pit. Mining will start in the East Pit, followed by the West and the Central Pits.
Approximately 32,000t of ore a day will be extracted from the mine, which will be loaded onto diesel-powered haul trucks and transported to a nearby rail transfer hopper.
The ore from the hopper will be loaded onto 100t side dumping railcars and transported to the processing plant located at former Erie processing facility, which was bought by PolyMet in 2005.
NorthMet ore is planned to be processed in two phases. The first phase will involve the production and marketing of copper, nickel, cobalt and precious metal concentrates.
Under the second phase, the company plans to process the nickel concentrate through a single autoclave, and produce high-grade saleable product.
In the first phase, the material will pass through four stages of crushing in the fine crusher building. It will be first crushed in a primary crusher, followed by the second stage of crushing using three gyratory crushers.
From the fine crusher building, the ore will be transported to the concentrator building, where it will be grinded using a series of rod and ball mills. The grinded material, after undergoing flotation, will be filtered and dewatered, resulting in a powdery concentrate containing 28% copper.
Second phase involves the installation of a hydrometallurgical facility for further refining nickel concentrate from the flotation circuit.
Sale of concentrate produced at NorthMet
Glencore will purchase concentrates, metal or intermediate products produced at the NorthMet project for a minimum period of five years, under an agreement made in September 2008.
Glencore agreed to invest $50m in the project through a convertible loan, of which $25m will be available immediately with the additional $25m to be provided upon publication of the final environmental impact statement (EIS).
Infrastructure facilities and construction details
Major construction works for the project include a rail hopper, 10,600ft of track including the installation of 1,600 new ties and 3,000ft of new rail on the existing track.
The mine site facilities include a field service and refuelling facility comprising storage tanks, mine mobile equipment facilities, offices, change house facilities and other buildings.
Power supply for the mine will be sourced from the nearby Minnesota power grid via 138kV transmission lines.
The definitive feasibility study (DFS) for the multi-metal project was prepared by Bateman Engineering. BNP Paribas was engaged to provide advisory services to PolyMet for NorthMet project’s construction finance.