The Duluth Complex in northeastern Minnesota has interested explorationists for many years. This mafic layered intrusion is known to host numerous large deposits of low-grade, disseminated copper and nickel (some of which are locally enriched with platinum-group elements) similar to such world-class deposits as those at Sudbury, Ontario, Noril'sk in Russia, Canada's Voisey Bay, and Jinchuan in China. All of these are magmatic sulphide deposits, a unique class of orebody which, globally, contain an estimated 40% of the world's nickel reserves and currently provide around 60% of the world's production of nickel. Moreover, PGMs are produced almost entirely from magmatic sulphide deposits.
However, the last round of exploration and development along the Duluth Complex ended in the early 1980s because of metallurgical difficulties associated with the mineralisation, coupled with low metal prices. But advances in hydrometallurgical processing technology have given Colorado-based junior, PolyMet Mining Corp. (formerly Fleck Resources), sufficient reason to re-assess one particular lowgrade Cu-Ni deposit, NorthMet, that was first identified back in the 1960s.
The Duluth Complex is comprised of a large series of mafic to felsic intrusive bodies covering some 5,000 [km.sup.2] of northeastern Minnesota. The complex contains many individual mafic layered intrusions of tholeiitic affinity and structurally complex coeval anorthositic and felsic instrusions. Recent age dating of volcanic rocks throughout the Lake Superior region show episodic magmatic activity occurring over a 21 million year period, although the main stage of activity, resulting in the formation of the Duluth Complex intrusions and the extensive flood basalts of the region, spans only 11 million years.
It is believed that as the magma slowly cooled and crystallised, droplets of immiscible sulphides formed within the silicate melt. The ease with which these sulphide droplets bond to such metals as Fe, Co, Ni, Pd, Pt, Rh, Ru, Ir and Os (the so-called Group VII transition metals) contained within the silicate magma results in a concentration effect, as these metal laden droplets, which are more dense than the enclosing silicate magma, gravity settle towards the base of the cooling magma body. At the base of the intrusion, the sulphide droplets cool and solidify, creating sulphide minerals that remain interstitial or intercumulate to the crystals of silicate minerals. It is this concentrating mechanism that allows the Group VII metals to become important targets for mining.
Several types of mineralised bodies occur within the Duluth Complex. The most extensive type of mineralisation, which is found at the NorthMet deposit, is low grade, disseminated to locally massive Cu-Ni sulphide bodies located along the basal portion and northwestern margin of the complex. More than 1,400 exploration holes drilled previously over the Duluth Complex have delineated nine Cu-Ni sulphide bodies with a combined tonnage of 4,400 Mt averaging 0.66% Cu and 0.20% Ni. The Cu:Ni ratio of the mineralisation varies from 3:1 to 5:1.
The NorthMet deposit, located almost 100 km north of Duluth in the Mesabi Range district, was discovered by US Steel Corp. (USX) during exploratory drilling of the Duluth Complex in the late 1960s. The deposit lies within the western edge of the complex at the contact between the Partridge River intrusion and the shallow dipping, older metasedimentary Virginia formation. The Partridge River intrusion, the lowermost of three intrusions making up the complex in this area, is subdivided into eight rock units. Drilling on the NorthMet project has intersected seven of these units. Generally, mineralised zones that contain high copper values also contain PGMs, gold and silver. Sulphide concentrations average 2-3% of the rock.
USX completed a 112 hole wide-spaced (183 m centres) drilling programme at the site to an average depth of 366 m, encountering Cu-Ni mineralisation up to 91 m thick along a 4.8 km strike length, and estimated, for an underground mining scenario, the presence of 100 Mt of ore averaging 0.77% Cu and 0.24% Ni. A number of metallurgical processes available at that time were tested by USX, but none were sufficiently successful to make economic underground development viable. Minor analyses for PGMs were run by USX in 1976, but the project remained uneconomic.
In 1989, two years after Minnesota's Natural Resource Research Institute (NRRI) published PGM data collected from a number of the Duluth Complex deposits, PolyMet secured a 20-year renewable lease agreement with USX (subject to a 3-5% NSR) covering 1,685 ha over the NorthMet deposit. During the year, the company began a relogging and assay project in conjunction with the NRRI, reassaying much of USX original drill core for precious metals. This assay data was analysed for PGMs on a routine basis for the first time at NorthMet and allowed for the recognition of stratigraphic controls of PGM mineralisation.
In July 1990, Nerco Exploration Co. leased the property from PolyMet with an option to earn a 60% interest in NorthMet by spending a minimum of US$1.1 million within four years and completing a feasibility study prior to the end of the fourth year. Nerco contracted Fluor Daniel Wright to undertake a technical and economic evaluation of the property based on data from 102 drill holes and 4,200 assayed samples. Fluor determined a new open pittable resource of 808 Mt averaging 0.432% Cu, 0.109% Ni, 0.006% Co, 0.061 g/t Au, 1.5 g/t Ag, 0.437 g/t Pd and 0.16 g/t Pt, contained within a mineral resource of 1,450 Mt at similar metal grades.
Financial difficulties, however, forced Nerco to allow their option to lapse in 1991 and three years later, Argosy Mining Corp. leased the property from PolyMet, contracting with the NRRI to conduct GPS controlled geological mapping and continued metallurgical investigations. Argosy's lease expired on the NorthMet project in 1996.
Since June 1998, PolyMet has sharpened its project focus, and work currently underway is aimed at taking the NorthMet project to a bankable feasibility stage for mine financing. A 14-hole RC drilling programme over 1,940 m was completed in September 1998 to collect sufficient material for metallurgical testing purposes. 'Mine-grade' samples from the 152 mm RC holes were sampled every 1.5 m and submitted for assay. Representative intercepts include 73 m at 1.79% Cu equivalent, 72 m at 2.51% Cu-eq., 17 m at 1.41% Cu-eq., 10 m at 1.84% Cu-eq., 12 m at 1.35% Cu-eq., and 48 m at 1.76% Cu-eq (using March 1999 metal prices).. PolyMet anticipates that these results will be incorporated into the new resource calculations later this year.
During December last year, a total of 26 tof drill cuttings was processed through Lakefield Research's flotation pilot plant on a continuous basis over a 42 h duration. Milling and flotation testwork carried out concluded that a bulk sulphide flotation concentrate could be produced at a concentration ratio of 40:1, with the mass of concentrate approximately 2.5% of the original ore:
Crucial to the project's development has been trying to solve the metallurgy of the ores of the NorthMet deposit which, like other polymetallic deposits of the Duluth Complex, presented significant metal recovery problems where the use of smelters was considered. The more recent development of bio-oxidation (BIOX) and pressure leaching technologies have vastly improved the potential metallurgical viability of the project. Special attention was given to the platinum-palladium metals at NorthMet since this deposit contains one of the largest known resources of these metals in North America.
BIOX vs POX
Two downstream processing options for the bulk concentrate have been evaluated. In January this year, BacTech Metallurgical Solutions of Australia was selected to conduct Phase 1 of the bioleaching testwork on the NorthMet sulphide bulk concentrate. Responsiveness of the concentrate to both bioleaching and ferric leaching was carried out, coupled with some preliminary process design.
Lakefield Research, meanwhile, was selected as the lead laboratory for the rigorous pressure oxidation testing programme. The test programme involved pressure oxidation leaching of the bulk concentrate at 220[degrees]C and 100 lb/[in.sup.2] oxygen overpressure.
Both processing techniques oxidise and solubilise the desired metals. The base metals can be selectively recovered by standard solvent extraction/electrowinning to produce saleable copper and nickel cathodes, and a cobalt salt. In POX, the precious metals are precipitated first, then the base metals solution is sent to the SX-EW circuit.
At the end of April, PolyMet determined pressure oxidation to be a successful metallurgical process to treat its NorthMet ores. Based on the results from the Lakefield tests, the following metal recoveries from the bulk concentrate were reported: 97.5% Cu, 92.5% Ni, 92.5% Co, 94% Au, 94% Pt and 94% Pd. Also, pressure oxidation results indicate the recovery of rhodium and ruthenium, both of which are also high value metals.
Final results from the bio-leaching testwork, carried out by BacTech Metallurgical Solutions at their laboratories in Australia, showed base metal (copper, nickel and cobalt) recoveries similar to those achieved with the pressure oxidation process. However, platinum, palladium and gold recoveries were somewhat lower.
The company also evaluated a hybrid process, combining bio-leaching and POX in order to improve PGM recovery. Although the alternative was economically competitive with POX, the company chose the POX route for pilot plant testing after considering all NorthMet project factors.
PolyMet has optimised the POX process developed through the testwork at Lakefield Research, and plans to further refine the process during continuous mini-pilot plant testing in December or January. Results of the pilot plant tests will be incorporated in the pre-feasibility study, expected in the first quarter of 2000.
PolyMet is hoping to develop the NorthMet deposit into an open pit operation utilising large state of the art surface equipment. The mine is anticipated to produce around 50,000 t/d of ore at start up, doubling to 100,000 tin production year two or three. Initial mine start up is likely to occur at more than one location on the NorthMet property. Priority will be given to an open pit mine start up that maximises ore grade and minimises strip mine ratios in the early years, in keeping with overall project economics and rational mine planning.
Infrastructure is already in place for continued exploration and future mine development as a direct result of the project's location just 3 km south of the large Mesabi Range open pit iron ore operations. NorthMet is easily accessible via state and country roads, whilst a 19 km all-season gravel road links the NorthMet project to the town of Babbitt. Erie Mining Co. Railroad, a private railroad, crosses the property immediately south of the deposit, whilst two other railway lines are located within 16 km and connect with three ports on Lake Superior. There is ready access to under-utilised industrial electric power.
In late 1999, PolyMet has and will embark on two major activities associated with the project. First was an intensive, multi-rig drilling programme involving 15,250 m of infill drilling to upgrade Fluor Daniel Wright's 1991 geological resource estimate to the reserve category. Information obtained from this drilling programme will be used also for the purposes of mine planning, open pit design, and preparation of the pre-feasibility study. The second will be the operation of a mini-continuous pilot plant to further refine and fine tune the POX metallurgical process.
Many of the drill cuttings will be used to make a bulk concentrate to run through the continuous mini-pilot plant, scheduled to run in December or January at Lakefield Research laboratories in Ontario. The drilling will also provide environmental and geotechnical data to be collected for environmental and mine planning. At the same time, PolyMet will begin the process of performing the necessary environmental studies, and planning the environmental permitting process for the NorthMet project.
An assaying and relogging programme by PolyMet and Minnesota's Natural Resources Research Institute was also initiated in September. Previously unassayed drill core from holes drilled in the NorthMet deposit in the 1970s are being relogged and sampled, and the results will also be used in the ore reserve calculations.
PolyMet was also awarded a US$58,000 grant from Minnesota Technology Inc., an agency of the State of Minnesota, which, together with US$93,000 from PolyMet will be used to complete work that is part of the NorthMet pre-feasibility study. The Natural Resources Research Institute, together with the Coleraine Minerals Research Laboratory, are participating in four tasks proposed by PolyMet;
* Analyse approximately 2,000-2,500 previously unassayed samples of core from holes drilled in the 1970s using PolyMet's sampling and assaying protocol -- this may lead to an expansion of the ore zones as previously identified and will lead to a better understanding of metal distribution within the NorthMet deposit;
* Take numerous measurements of the specific gravity of ore and waste rock in order to convert measurements of volume to mass -- once tonnage factors have been determined, they will be used to calculate reserves, stripping ratios and other parameters associated with mine planning and equipment selection;
* Detailed geological logging of rocks below the ore zones of the NorthMet deposit in order to characterise the rock types, mineralogy, alteration and structure; and
* Compile a master computerised database of assay and other pertinent data.
PolyMet intends to raise, primarily through private placements, approximately US$6 million in 1999 to complete ongoing development work. In keeping with its intention of focusing solely on the NorthMet project, the company will also selectively dispose of its interests in certain properties in Quebec, British Columbia and Ontario, proceeds from which will also help finance the 1999 programme.
The US$6.7 million budget for this year's programme includes US$2.4 million for drilling and assaying, US$2.3 million for metallurgical testing and US$2.0 million for permitting and engineering design/analysis. The company is aiming to complete its pre-feasibility study, and advance the indicated resource to a proven reserve, by March next year, with completion of the final feasibility by mid-2001.
Grade Feed Concentrate Recovery (%) Copper (%) 0.43 15.5 93.7 Nickel (%) 0.12 3.69 77.1 Cobalt (%) 0.009 0.149 46.4 Platinum (g/t) 0.08 2.49 76.4 Palladium (glt) 0.37 11.1 75.8 Gold (g/t) 0.06 2.80 76.6 Silver (g/t) 1.48 41.5 69.9 Copper Nickel Cobalt Platinum Palladium Ore (millfeed) 0.43% 0.12% 0.0096% 0.143 g/t 0.389 g/t Recovery to flot. conc. 94.6% 77.2% 46.0% 76.0% 76.0% POX Process Recovery 97.5% 92.5% 92.5% 94.0% 94.0% Overall Recovery 92.2% 73.0% 42.6% 71.4% 71.4% Gold Ore (millfeed) 0.088 g/t Recovery to flot. conc. 77.0% POX Process Recovery 94.0% Overall Recovery 72.4%
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|Date:||Nov 1, 1999|
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