An overview of the geology and oxide, sulfide, and platinum-group element mineralization along the western and northern contacts of the Duluth Complex

Sulfide and oxide mineralization occurs along the western and northern footwall contacts of the 1.1-Ga Duluth Complex (Complex) in northeastern Minnesota, which was emplaced during the formation of the Midcontinent Rift System. Platinum-group element mineralization is known to occur only along the western contact. The Duluth Complex is composed of troctolitic, gabbroic, and anorthositic rocks that form a series of individual intrusions that make up the Complex. These were emplaced into footwall rocks of Archean granite-greenstone terranes and Lower Proterozoic metasedimentary rocks (Biwabik and Gunflint Iron-Formations; Rove,Virginia, and Thomson Formations) and into the penecontemporaneous North Shore Volcanic Group basalts, which form the hanging wall, and, in some areas, the footwall. Development of riftrelated structures, both extensional and contractional, affected development of mineralization. Structural discontinuities provided conduits for emplacement of late-stage granophyric and pegmatitic iron-rich ultramafic rocks. Faults and folds were responsible for localizing massive sulfide mineralization, as well as, for providing conduits for later syn- to post-magmatic hydrothermal fluids. On the northern margin of the Complex, 81.6 million tons of low-grade titaniferous magnetite ore (approximately 12 to 14% TiO₂) occur in 14 bodies that range in size from 1 to 19 million tons. Oxide mineralization along the western contact contains 245 million tons of ≥10% TiO₂. These include the Wyman Creek area, Longnose, Longear, and Section 17 bodies of the Partridge River Intrusion, and the Water Hen Intrusion of the southern Duluth Complex. Overall three types of oxide mineralization occur in the Duluth Complex: (1) oxide-rich metasedimentary inclusions in mafic or ultramafic rocks that exhibit metasedimentary textures and/or can be traced laterally into footwall iron-formation; (2) banded or layered oxide segregations that include cumulus oxide-rich horizons; and (3) late discordant oxide-bearing ultra- mafic pipes and bodies (OUI). The latter type is often associated with inclusions of iron-formation or appears to be related to windows of iron-formation subcrop within the Complex. These bodies are remarkably similar to iron-rich, ultramafic pegmatites and pipes located within the Bushveld and Stillwater Complexes that have been interpreted to have a metasomatic origin. Along the western margin of the Complex, the copper-nickel sulfide resources are estimated at 4.4 billion tons of 0.66% copper with a Cu:Ni ratio of 3.3:1. There are five types of mineralization: (1) sedimentary (Cu-Fe-Zn) sulfides in the footwall, (2) disseminated Cu-Ni sulfides within basal troctolitic rocks, (3) semimassive to massive sulfides, (4) sulfides in OUI, and (5) late-stage secondary sulfides. The primary sulfide minerals in types 2 to 5 include pyrrhotite, chalcopyrite, cubanite, and pentlandite. Only types 2 and 5 are known to occur along the northern contact. Platinum-group minerals (PGMs) are only known to occur along the western margin of the Complex. PGMs found include: ferro-alloys and sulfides to arsenides, bismuthinides, stannides, tellurides, and antimony-based minerals. Platinum-group elements (PGEs) are found associated with disseminated, massive, and secondary sulfides, and with massive oxides. Anomalous PGE-enriched zones are also associated with structural discontinuities and alteration; however, remobilization appears to be locally confined. The PGE/base metal ratios in their igneous host rocks suggest that most of the PGEs were removed from the troctolitic magmas before emplacement into their present positions. Exceptions to this interpretation include the Birch Lake area of the South Kawishiwi Intrusion and stratabound Cu-Ni sulfides within the Dunka Road deposit. Sulfur isotope data indicate that sulfur contamination and homogenization in the magma occurred prior to emplacement. Carbon, oxygen, and hydrogen isotope data demonstrate that devolatization of the footwall sedimentary rocks contributed to synto post-magmatic fluids that altered the Complex rocks and remobilized the sulfide and PGE. Stable isotope data also show the presence of meteoric and magmatic components to these fluids. Limited fluid inclusion data indicate that as many as four fluid events affected the rocks along the western contact. Secondary chloride precipitates on drill core and in footwall rocks show that chlorine-rich fluids affected the entire western margin of the Complex. This event may also have been related to remobilization of Cu-Ni ± PGE mineralization.