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Duluth Complex Overview - Spruce Road, Maturi, Birch Lake, Dunka Pit, Serpentine, Minnamax (Babbitt), NorthMet (Dunka Road), Wetlegs, Wyman Creek.

Minnesota, USA

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A series of low grade Ni-Cu and PGE deposits are distributed over a 35 km interval on the lower margin of the late Mesoproterozoic (1.1 Ga) Duluth Mafic Complex in Minnesota, USA.   These include from NE to SW - Spruce Road, Maturi, Birch Lake, Dunka Pit, Serpentine, Minnamax (Babbitt), NorthMet (Dunka Road), Wetlegs and Wyman Creek.   Together these eight are said to contain 3.6 Gt @ 0.66% Cu, 0.2% Ni. A series of other prospects occur to the south of Wyman Creek, namely Sec, Skibo, Skibo South and Water Hen.

The Duluth Complex lies within the Precambrian shield rocks of the Superior Province of North America, and outcrops as a crescent shaped, NE-SW elongated, 280 km long by 45 km wide body on the north-western shores of Lake Superior. It is made up of up to 40 separate sheet like and cone shaped tholeiitic mafic intrusions ranging from anorthosite to troctolite and ferrogabbro in composition.   The complex was emplaced at the northern end of the 1500 km long, horseshoe-shaped, 1.1 Ga Mid Continental Rift zone beneath the 2 km thick Keweenawan suite of mafic lavas during a period of crustal scale tectonic extension within the older cratonic mass during the Mesoproterozoic.

The Duluth complex comprises a series of distinct mafic to felsic tholeiitic magmatic pulses that were intermittently intruded at the base of a comagmatic volcanic edifice during the formation of the Midcontinental rift system between 1108 and 1098 Ma. The Duluth Complex represent a relatively continuous mass that extends in an arcuate fashion from Duluth in the SW to the border between Minnesota and Canada in the NE near the town of Grand Portage.

Four general rock series have been recognised within the near continuous mass of intrusive rocks that form the Duluth Complex (Miller and Severson, 2002), namely:
Felsic Series - massive granophyric granite and lesser intermediate rock that occur as a semi continuous mass of intrusions strung along the eastern and central roof zone of the complex. These are volumetrically minor and cut the Anorthosite and Gabbro Series.
Early Gabbro Series - layered sequences of dominantly gabbroic cumulates that occur along the northeastern contact of the Duluth Complex, also emplaced during early stage magmatism at ~1108 Ma.
Anorthositic Series - a structurally complex suite of foliated, but rarely layered, plagioclase-rich gabbroic cumulates that was emplaced throughout the complex during main stage magmatism to ~1099 Ma.
Layered Series a suite of 'stratiform' troctolitic to ferrogabbroic cumulates that comprises at least 11 variably differentiated mafic layered sub-intrusions that occur mostly along the base of the Duluth Complex on its northwestern margin. These intrusions were emplaced during main stage magmatism, but generally ~1099 Ma. They are largely distributed along the northwestern edge of the Complex, and host the known copper-nickel deposits (some contain platinum group elements). The layered series comprises at least 11 discrete mafic layered intrusions. Field relations suggest the Anorthositic Series, which is in the upper part of the Duluth Complex, is older than the troctolitic Layered Series that forms the lower two thirds of the Complex, although identical age dates of 1099 Ma for both series suggests rapid intrusion.
  The Felsic Series and late stage basalts and aplite dykes, cut the Anorthosite and Troctolite Series. Inclusions of assimilated footwall rocks, including magnetite-rich ex-iron formation and hornfels, are found in the troctolites near the base of the complex.

Four distinct types of magmatic mineral deposit have been recognised within the Duluth Complex:
i). large, low-grade, disseminated Ni-Cu concentrations, some of which contain local zones enriched in platinum-group elements (PGEs);
ii). localised high-grade zones of massive Ni-Cu sulphides, some of which are moderately enriched in PGEs;
iii). stratabound PGE-enriched 'reefs' associated with specific types of phase-layer transitions; and
iv). oxide-rich ultramafic plugs with anomalous Ti and V.

Known significant Cu-Ni-PGE sulphide mineralisation of the first two styles is hosted very near the basal contact of the Complex in three of the Layered Series intrusions which were emplaced during the main stage of rift related magmatism (Miller and Severson, 2002). These mineralised intrusions are, from NE to SW, the extensive South Kawishiwi and Partridge River intrusions and the more localised Bathtub Intrusion within the footwall zone of the latter. The Spruce Road to Serpentine deposits are hosted by the South Kawishiwi Intrusion and the Babbitt deposit to Water Hen Prospect in the Partridge River Intrusion. Between Spruce Road and Wyman Creek, the basal contact of the Duluth Complex, and the South Kawishiwi and Partridge River intrusions, strikes near NE-SW. At Wyman Creek, the base of the Complex and corresponding Partridge River intrusion contact swings to north-south. The Partridge River intrusion persists for a further ~20 km along this trend before terminating. Further to the south and then SSW, the base of the Complex is occupied by the Western Margin and then the Boulder Lake intrusions which also host low grade Ni-Ci-PGE prospects which are more PGE rich as well as Ti and V mineralisation.

The 'stratigraphy' of the South Kawishiwi and Partridge River intrusions are described in the NorthMet and Birch Lake records respectively.

The emplacement of these intrusions produced intense mineralogical, textural and geochemical modification of the footwall country rocks. Ripley (1981), Andrews and Ripley (1989), Ripley and Al-Jassar (1987) and Mogessie and Stumpfl (1992) have shown that sulphur δ34S values in the basal zones of the Partridge River and the Bathtub intrusions range from 0 to 26‰ and the sulphur associated with the mineralisation has been derived from the metasedimentary footwall, particularly from the pyrite in the sulphidic black slates of the Virginia Formation.

Hanging wall rocks to the Duluth Complex are composed of mafic volcanic rocks and hypabyssal intrusions, and internally scattered bodies of strongly granoblastic mafic volcanic and sedimentary hornfels can be found.

The Duluth Complex has been tilted by 15 to 20° to the SE, and as a consequence, the immediate footwall units of all three intrusion crop out along the north-western margin of the complex. This footwall is predominantly composed of Archaean basement granites of the Giants Range Batholith from Spruce Lake to Birch Lake, and to the SW by a Palaeoproterozoic sequence comprising the Biwabik Iron Formation, and Virginia Formation siltstone and mudstone.   The basal contact zone of the three intrusions locally contain 1 to 5% disseminated chalcopyrite, cubanite, pyrrhotite and pentlandite over an interval of several tens of kilometres.

See also the more detailed records for Babbitt (Minnamax), Birch Lake and Dunka Road and NorthMet records.

For detail see the reference(s) listed below.

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:
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
Mansur, E.T., Barnes, S.-J. and Duran, C.J.,  2021 - An overview of chalcophile element contents of pyrrhotite, pentlandite, chalcopyrite, and pyrite from magmatic Ni-Cu-PGE sulfide deposits: in    Mineralium Deposita   v.56, pp. 179-204.
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
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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