Duluth Complex - NorthMet, Dunka Rd

Minnesota, USA

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The NorthMet copper-nickel deposit, formerly the Dunka Road Project of US Steel, is located ~110 km north of Duluth in Minnesota, USA, hosted by troctolitic rocks in the southern portion of the Duluth Complex. It is <5 km to the southwest of the Babbitt deposit (#Location: 47° 36' 49"N, 91° 57' 38"W).

For details of the setting and geology of the Duluth Complex, see the separate Duluth Complex - Overview record.

The deposit is hosted by the troctolitic Layered Series of the Partridge River Intrusion in the Hoyt Lake-Kawishiwi section of the Duluth Mafic Complex. The Partridge River Intrusion immediately overlies a wedge of sulphide bearing metamorphosed slates, argillites and greywackes of the Proterozoic Virginia Formation separating the Duluth Complex from the underlying, ~120 m thick, Biwabik Iron Formation. The Biwabik Iron Formation is progressively underlain by the quartzite, metasiltstone, argillite and localised quartz-pebble conglomerate of the Pokegama Quartzite, and then the Archaean Giants Range Batholith. The three Palaeoproterozoic units belong to the Animikie Group.

The Virginia Formation was contact metamorphosed by the Duluth Complex. Where not metamorphosed it consists of a thinly-bedded sequence of argillite and greywacke, with lesser siltstone, carbonaceous-sulphidic argillite/mudstone, cherty-limey layers, and possibly some tuffaceous material. In proximity to the Duluth Complex, metamorphic grade and local deformation progressively increases, with several superimposed metamorphic varieties and textures at an angle to the original stratigraphy. At least four distinctive metamorphic varieties of Virginia Formation are recognised at NorthMet, informally referred to as the cordieritic metasediments; disrupted unit; recrystallised unit; and graphitic argillite. The latter are near the base of the formation and locally contain up to 15% thinly laminated pyrrhotite.

The Partridge River Intrusion is predominantly composed of varied troctolitic, with lesser anorthositic troctolite, gabbro, olivine gabbro, norite and picrite. It is estimated to be nearly 2500 m thick (Miller and Ripley, 1996). The upper portion of the intrusion has a complex contact an assemblage of anorthositic, gabbroic and hornfelsic rocks, which is also found as large inclusions within the interior of the intrusive body (Severson and Miller, 1999). These inclusions are interpreted to represent earlier roof zone screens that were overprinted by later pulses Partridge River intrusion magmas.

The lower 900 m of the intrusion constitutes a marginal zone, consisting of varied troctolitic and gabbroic rock types, is subdivided into seven stratigraphic units (Severson and Hauck, 1990, 1997; Geerts, 1991; Severson, 1991, 1994) that can be correlated over a strike length of ~25 km. These igneous units generally dip at 10 to 25°SE. These units are, from the base upward:
Unit 1 - which averages 135 m in thickness, but is locally up to 300 m. It is the only unit containing significant, deposit-wide sulphide mineralisation. Sulphides occur principally occur as disseminated interstitial grains between a dominant silicate minerals and comprise chalcopyrite > pyrrhotite > cubanite > pentlandite. It is also the most complex, with internal ultramafic sub-units, varying degrees of mineralisation, complex textural relations and a range of grain sizes, as well as abundant metasedimentary inclusions. It is characteristically a fine- to coarse-grained heterogeneous rock ranging from anorthositic troctolite (more abundant in its upper half) to augite troctolite with lesser amounts of gabbro-norite and norite (increasing in abundance towards the basal contact) and numerous metasedimentary inclusions. The dominant rock type is medium-grained ophitic augite troctolite, though with widely variable texture. Two internal ultramafic sub-units averaging ~3 m in thickness occur in the SW of the deposit.
Unit 2 - which averages ~30 m in thickness, is characterised by homogeneous, medium to coarse-grained troctolite and pyroxene troctolite with a consistent basal ultramafic sub-unit. It is distinguished from the enclosing Units 1 and 3 by the continuity of the basal ultramafic sub-unit, as well as the relatively uniform grain size and homogeneity of the troctolite. The basal ultramafic sub-unit is the lowermost continuous ultramafic horizon at the NorthMet deposit, averaging 7.5 m thick, and comprising melatroctolite to peridotite and minor dunite. Based on the continuity of the basal ultramafic sub-unit, Unit 2 is interpreted to be a lower, more mafic, counterpart to Unit 3. The overall absence of footwall country rock inclusions in Unit 2 indicate an intrusive sequence of Unit 3 → 1 → 2. Though generally barren, mineralisation locally continues upward into Unit 2 in the western portion of the deposit.
Unit 3 - averages ~90 m in thickness and is the primary marker bed used as a reference to stratigraphic position. It is composed of fine to medium-grained, poikilitic and/or ophitic, troctolitic anorthosite to anorthositic troctolite. Poikilitic olivine gives the rock a characteristic overall mottled appearance. Its lower contact may be disrupted, with multiple intercalations of relatively homogeneous 'Unit 2 like' rocks. This generally alternating sequence, or transition zone, is common in the southwestern part of the deposit, and can occur over many tens of metres in the transition to Unit 2. The transitional zone is interpreted to indicate that Unit 3 is disturbed and intruded by Unit 2 near the base of the former. Like Units 4 and 5 (see below), the thicknesses of Units 2 and 3 tend to be highly variable, whereas their combined depth is relatively consistent throughout the deposit, although not as consistent as Units 4 and 5. Locally Unit 3 contains both footwall Virginia Formation metasedimentary rocks and hanging wall basalt inclusions, which are interpreted as an indication of early emplacement within the intrusive sequence. Few sedimentary inclusions are found higher than Unit 3, and few basalt inclusions are found below it.
Unit 4 - averages ~75 m in thickness and is somewhat more mafic than Unit 5. It is characterised by homogeneous, coarse-grained, ophitic augite troctolite with some anorthosite troctolitic. Its basal section comprises a local discontinuous, <15 cm thick ultramafic layer or oxide-rich zone above a generally sharp contact with the underlying Unit 3. With the exception of the Magenta Zone (see below), sulphides only occur in trace amounts of finely disseminated chalcopyrite and pyrrhotite grains.
Unit 5 - averages 75 m in thickness and is composed primarily of homogeneous, equigranular-textured, coarse-grained anorthositic troctolite that locally grades into troctolite and augite troctolite towards the base of the unit. The lower contact of this unit is gradational and lacks any ultramafic sub-unit. As a result of the ambiguity of the location of the contact, the thicknesses of units 4 and 5 vary dramatically, although their combined thickness is relatively consistent across the deposit.
Unit 6 - averages 120 m in thickness, with a continuous 4 to 5 m basal ultramafic sub-unit. It is composed of homogeneous, fine to coarse-grained, troctolitic anorthosite and troctolite. Sulphide mineralisation is generally minimal, although significant copper sulphides and associated elevated platinum group elements occur in the southwestern portion of the deposit (Geerts 1991, 1994). Sulphides generally occur as disseminated chalcopyrite/cubanite with minimal pyrrhotite.
Unit 7 - is the uppermost unit encountered at the NorthMet Deposit, where it is truncated by the erosional surface and as a consequence has an unknown thickness. It is predominantly composed of homogeneous, coarse-grained, anorthositic troctolite and troctolitic anorthosite, similar to Unit 6. A continuous basal ultramafic sub-unit that averages 6 m in thickness, consists of fine to medium-grained melatroctolite to peridotite and minor dunite.

The principal metals at NorthMet are copper, nickel, cobalt, platinum, palladium, silver and gold, with minor rhodium and ruthenium. In general, except for cobalt and gold, the metals are positively correlated with copper mineralisation. Cobalt correlates well with nickel. Most of the metals are associated with chalcopyrite, cubanite, pentlandite and pyrrhotite, with platinum, palladium and gold also occurring as native elements and in bismuthides, tellurides and alloys. These sulphides are found throughout the troctolitic sequence, but are generally more prevalent near the basal contact with the underlying meta-sedimentary rocks. Mineralisation occurs in four broadly defined horizons or zones throughout the NorthMet deposit where they occur as 1 to 2% disseminated sulphides. Three of these horizons are within Unit 1, whilst the upper locally extends into the base of Unit 2. The thickness of each of the three Unit 1 enriched horizons varies from 1.5 to >60 m. Unit 1 mineralisation is found throughout the base of the NorthMet deposit. A less extensive mineralised zone, the copper-rich, sulphur-poor Magenta Zone, is found in Units 4, 5 and 6 in the western part of the deposit.

The NorthMet deposit has an open-pit minable resource of 808 Mt @ 0.432% Cu, 0.109% Ni, 0.116 g/t Pt, 0.437 g/t Pd, 0.061 g/t Au, and 1.5 g/t Ag at a 4:1 stripping ratio (PolyMet Mining Web site, 2005).

For detail see the reference(s) listed below.

This summary is largely drawn from Black, Z.J., Brown, J.J., Dempers, N., Drielick, T.L., Ibrado, A.S., Patterson, E.L., Radue, T.J., Ubl, J.S. and Welhener, H.E., 2018 - NorthMet Project, Minnesota, USA, an NI 43 -101Technical Report, prepared by M3 Engineering and Technology Corporation for Poly Met Mining, Inc., 273p.

The most recent source geological information used to prepare this summary was dated: 2018.     Record last updated: 22/4/2020
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 & Additional Information
   Selected References:
Chandler V W  1990 - Geologic interpretation of gravity and magnetic data over the central part of the Duluth Complex, northeastern Minnesota: in    Econ. Geol.   v85 pp 816-829
Chandler V W, Ferderer R J  1989 - Copper-Nickel mineralization of the Duluth complex, Minnesota - a gravity and magnetic perspective: in    Econ. Geol.   v84 pp 1690-1696
Green J C,  2002 - Volcanic and sedimentary rocks of the Keweenawan Supergroup in Northeastern Minnesota: in Miller J D, et al., (Eds.), 2002 Geology and Mineral Potential of the Duluth Complex and Related Rocks of Northeastern Minnesota Minnesota Geological Survey   Report of Investigations 58 pp 94-105
Miller J D and Green J C,  2002 - Geology of the Beaver Bay Complex and related hypabyssal intrusions: in Miller J D, et al., (Eds.), 2002 Geology and Mineral Potential of the Duluth Complex and Related Rocks of Northeastern Minnesota Minnesota Geological Survey   Report of Investigations 58 pp 144-163
Miller J D and Severson M J,  2002 - Geology of the Duluth Complex: in Miller J D, et al., (Eds.), 2002 Geology and Mineral Potential of the Duluth Complex and Related Rocks of Northeastern Minnesota Minnesota Geological Survey   Report of Investigations 58 pp 106-143
Peterson D M and Severson M J,  2002 - Archean and Paleoproterozoic rocks that form the footwall of the Duluth Complex: in Miller J D, et al., (Eds.), 2002 Geology and Mineral Potential of the Duluth Complex and Related Rocks of Northeastern Minnesota Minnesota Geological Survey   Report of Investigations 58 pp 76-93
Queffurus M and Barnes S-J,  2014 - Selenium and Sulfur Concentrations in Country Rocks From the Duluth Complex, Minnesota, USA: Implications for Formation of the Cu-Ni-PGE Sulfides : in    Econ. Geol.   v.109 pp. 785-794
Rao B V, Ripley E M  1983 - Petrochemical studies of the Dunka Road Cu-Ni Deposit, Duluth Complex, Minnesota: in    Econ. Geol.   v78 pp 1222-1238
Ripley E M  1981 - Sulfur isotopic studies of the Dunka Road Cu-Ni deposit, Duluth Complex, Minnesota: in    Econ. Geol.   v76 pp 610-620
Ripley E M, Taib N I, Li C and Moore C H,  2007 - Chemical and mineralogical heterogeneity in the basal zone of the Partridge River Intrusion: implications for the origin of Cu-Ni sulfide mineralization in the Duluth Complex, midcontinent rift system : in    Contrib. to Mineralogy & Petrology   v154 pp 35-54
Severson M J, Miller J D, Peterson D M, Green J C and Hauck S A,  2002 - Mineral potential of the Duluth Complex and related intrusions: in Miller J D, et al., (Eds.), 2002 Geology and Mineral Potential of the Duluth Complex and Related Rocks of Northeastern Minnesota Minnesota Geological Survey   Report of Investigations 58 pp 164-200
Theriault R D, Barnes S-J, Severson M J  2000 - Origin of Cu-Ni-PGE Sulfide mineralization in the Partridge River intrusion, Duluth Complex, Minnesota: in    Econ. Geol.   v95 pp 929-943

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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