Duluth Complex - Birch Lake

Minnesota, USA

Main commodities: PGE PGM Cu Ni
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The Birch Lake PGE-Ni-Cu deposit is located along the western margin of the Duluth Complex, ~130 km NNE of Duluth in Minnesota, USA.

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

The Duluth Complex hosts three types of PGE, Cu-Ni magmatic mineralisation at or near its footwall, as follows:
i). Large, low grade disseminated Cu-Ni sulphide deposits that are locally enriched in PGEs;
ii). Localised high grade zones of massive Ni-Cu sulphides, some of which are moderately enriched in PGEs;
iii). Disseminated, PGE enriched, Cu-Ni sulphides associated with specific types of phase-layer transitions, and in this sense are stratabound deposits.

The Birch Lake deposit occurs on the northwestern margin of the Duluth Complex and is an example of type iii.  It occurs within the 40 x 7 km, NE-SW elongated South Kawashiwi Intrusion that is bordered on the southwest by the Partridge River intrusion and on the southeast by the Bald Eagle troctolite cumulate pluton.

The stratigraphic sequences of the South Kawashiwi and Partridge River intrusions are completely dissimilar. At least 17 correlative subhorizontal igneous units are defined within the layered South Kawishiwi Troctolite Series, although not all are equally developed across the intrusion. They are, from bottom to top (after Severson, 1994):
• BAN - Bottom Augite troctolite and Norite;
• BH - Basal Heterogeneous troctolites, which are sulphide bearing;
• U3 - Ultramafic Three, which is also sulphide bearing and the principal host at Birch Lake;
• PEG - Pegmatitic Unit;
• U2 - Ultramafic Two, which is sulphide bearing;
• U1 - Ultramafic One, which is sulphide bearing;
• AT-T - homogeneous Anorthositic Troctolite to Troctolite;
• UW = Updip Wedge, which is sulphide bearing;
• LOW AGT - homogeneous Lower Augite Troctolite zone;
• MAIN AGT - homogeneous Main Augite Troctolite zone;
• AT&T - homogeneous Anorthositic Troctolite and Troctolite;
• AT(T) - homogeneous Anorthositic Troctolite with lesser amounts of Troctolite;
• AN-G Group - intermixed Anorthositic and Gabbroic rocks;
• UPPER GABBRO - oxide-bearing gabbroic rocks;
• INCL - large shallow-dipping inclusion of magnetic basalt;
• UPPER PEG - Upper Pegmatitic zone;
• T-AGT - Troctolite to Augite Troctolite
The last five units are restricted to a small area known as the Highway 1 Corridor area where they overlie rocks interpreted to represents a large inclusion of older Anorthositic Series rocks that belong to the South Kawashiwi Intrusion magmas. Rock relationships are interpreted to suggest the Partridge River Intrusion was intruded before the South Kawashiwi Intrusion.

The lower units of the sequence are the most varied in texture, rock type, and sulphide content, and are very unevenly distributed along the strike length of the intrusion, being 'compartmentalised', suggesting a complicated intrusive history. The lowest units were introduced early into several restricted magma chambers as repeated and close-timed magmatic pulses. These units were most effected by contamination from assimilated and devolatilised country rocks. The three main ultramafic-bearing packages (U1, U2, and U3) occur within the lower portion of the intrusion and are characterised by alternating layers of troctolitic and olivine-rich ultramafic rock. These ultramafic-bearing units are interpreted to represent periods of rapid and continuous magma injection that crystallised more primitive ultramafic layers before mixing with the resident magma (Severson, 1994). Unit U3 is unique in that several massive oxide (magnetite-rich) pods are distributed along its strike length. Severson (1994) notes an empirical relationship between the U3 Unit and the Biwabik Iron-formation which suggests that the massive oxides were derived from intruded and assimilated BIF. The U3 Unit also contains the majority of high Platinum Group Elements (PGE) values encountered to date in the South Kawashiwi Intrusion.

The upper South Kawishiwi Troctolite Series has a significantly different intrusive history. Each unit is characterised by a monotonous sequence of texturally homogeneous and sulphide-free rocks with gradational contacts, and in contrast are generally distributed throughout the intrusion. Ultramafic members are found in only two thin horizons referred to as High Picrite 1 and 2. All features quiescent and open magmatic system. The upper South Kawishiwi Troctolite Series units appear to have been emplaced as widely-spaced pulses into a progressively developed, single magma chamber with little interaction with the country rocks (Severson, 1994).

The PGE and chromium mineralisation at Birch Lake is primarily restricted to the U3 ultramafic unit and locally in the basal portion of the overlying PEG Unit which is otherwise sulphide-barren. The base of the deposit within the U3 unit is gradational. The average PGE content of the U3 Unit is higher than all other units in the South Kawishiwi Intrusion (Severson, 1994). At Birch Lake, the U3 Unit comprises interlayered melatroctolite (picrite), oxide melatroctolite, massive oxide (magnetite > ilmenite), and minor troctolite. It also contains recognisable inclusions of Biwabik Iron-Formation. As mentioned above, Severson (1994) suggested that the more pyroxenitic units within the U3 Unit are actually recrystallised assimilated iron formation. The PEG Unit overlying U3 is predominantly composed of pegmatitic troctolitic rock intermixed with lesser amounts of medium- to coarse-grained troctolitic rock. Beneath U3, there are the troctolitic and noritic rocks of the Basal Augite Troctolite-Norite Unit (BAN). In the Birch Lake the footwall of the intrusion is composed of contact metamorphosed granitic rocks of the Archean Giants Range Batholith, which in the area of PGE-Cr mineralisation, also contain disseminated and massive sulphide chalcopyrite > pyrrhotite mineralisation (Severson, 1994).

The Birch Lake deposit comprises 1 to 5% disseminated Cu and Ni sulphides bearing significant Pd, Pt and Au values with lesser Ag, Co and Rh. Sulphide minerals are disseminated interstitially in the rock matrix. The sulphides are essentialy stratabound and are consistently found within the upper portion of the U3 ultramafic unit of the South Kawashiwi Intrusion. In the deposit area the U3 unit is 14.6 m to 114.3 thick. Late granitic and felsic dykes cut both the South Kawishiwi Intrusion and the U3 unit.

Pd and Pt enrichment and the Pt:Pd ratio is greater near the top of the U3 unit although Cu and Ni grades are variable, as is the Ni:Cu ratio. The PGEs occur as various fine-grained Pd tellurides. Approximately 90% of the PGMs are associated with copper sulphides. Strong PGE enrichment is associated with late stage Cu-Ni sulphides, particularly with chalcopyrite, talnakhite and bornite where they occur as replacement mineralisation. Weaker PGEs accompany chalcopyrite and pyrrhotite where they occur as primary sulphides. The PGEs may form halos around, or be included in, interstitial copper sulphides, pyroxenes, secondary amphiboles and biotite. PGEs are also remobilised in chlorite, serpentine or secondary magnetite.

The deposit has an inferred resource of approximately 29 Mt @ 3.9 g/t Pt equivalent for 115 t Pt Eq with an average thickness of 24 m and is potentially amenable to low cost, mechanised underground mining. In detail grades for the resource above are 0.694% Cu, 0.218% Ni, 0.25 g/t Au 1.13 g/t Pd, 0.57 g/t Pt, 0.011% Co, 2.90 g/t Ag.

Sources include: Franconia Minerals website, 2005.
Hauck, S., Severson, M., Ripley, E., Goldberg, S. and Alapieti, T., 1997 - Geology and Cr-PGE Mineralization of the Birch Lake area, South Kawishiwi Intrusion, Duluth Complex; Technical Report NRRI/TR-97/13; Minnesota Mining and Mineral Resources Research Institute, Univ. of Minnesota, Minneapolis, Minnesota, 38p.
Severson, M.J., 1994 - Igneous Stratigraphy of the South Kawishiwi Intrusion, Duluth Complex, Northeastern Minnesota, Natural Resources Research Institute Technical Report NRRI/TR-93/34, Natural Resources Research InstituteUniversity of Minnesota, Duluth, 227p.

The most recent source geological information used to prepare this summary was dated: 2005.     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:
Benko, Z., Mogessie, A., Molnar, F., Krenn, K., Poulson, S.R., Hauck, S., Severson, M. and Arehart, G.B.,  2015 - Hydrothermal alteration and Cu-Ni-PGE mobilization in the charnockitic rocks of the footwall of the South Kawishiwi intrusion, Duluth Complex, USA: in    Ore Geology Reviews   v.67, pp. 170-188.
Benko, Z., Mogessie, A., Molnar, F., Severson, M.J., Hauck, S.A. and Raic, S.,  2015 - Partial Melting Processes and Cu-Ni-PGE Mineralization in the Footwall of the South Kawishiwi Intrusion at the Spruce Road Deposit, Duluth Complex, Minnesota : in    Econ. Geol.   v.110 pp. 1269-1293
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
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

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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