The Dunka Road deposit is one of several Cu – Ni – platinum-group element (PGE) sulfide occurrences found along the northwestern margin of the Duluth Complex, where the host troctolitic rocks are in contact with metasedimentary rocks of the Animikie Group. Magma contamination through assimilation of sulfidic argillaceous country rocks is generally recognized as having played a key role in the genesis of the mineralization. Three main types of disseminated sulfide mineralization have been identified within the Dunka Road deposit: (i) norite-hosted sulfides, (ii) troctolite-hosted sulfides, and (iii) PGE-rich sulfide horizons. The norite-hosted sulfides are found either adjacent to country-rock xenoliths or near the basal contact. The troctolite-hosted sulfides form the bulk of the deposit, and occur throughout the lower 250 m of the intrusion. The PGE-rich sulfide horizons are typically localized directly beneath ultramafic layers. The composition of the different types of sulfide occurrences is modelled using Cu/Pd ratios. It is shown that each type results from the interplay of two main parameters, namely the degree of magma contamination and the silicate magma to sulfide melt ratio (R factor). The norite-hosted sulfides formed at low R factors and high degrees of contamination, as expressed by their PGE-depleted nature, low Se/S ratios, and elevated content in pyrrhotite and arsenide minerals. The troctolite-hosted sulfides formed at moderate R factors and small degrees of contamination, as shown by their moderate PGE content and mantle-like Se/S ratios. Finally, the PGE-rich sulfide horizons are modelled using elevated R factors from an uncontaminated parental magma, which is substantiated by their elevated noble metal content and Se/S ratios, and low pyrrhotite to precious metal sulfide ratio.

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The tectonic setting of intraplate magmas, typically a plume intersecting a rift, is ideal for the development of Ni -Cuplatinum-group element-bearing sulphides. The plume transports metal-rich magmas close to the mantle-crust boundary. The interaction of the rift and plume permits rapid transport of the magma into the crust, thus ensuring that no sulphides are lost from the magma en route to the crust. The rift may contain sediments which could provide the sulphur necessary to bring about sulphide saturation in the magmas. The plume provides large volumes of mafic magma; thus any sulphides that form can collect metals from a large volume of magma and consequently the sulphides will be metal rich. The large volume of magma provides sufficient heat to release large quantities of S from the crust, thus providing sufficient S to form a large sulphide deposit. The composition of the sulphides varies on a number of scales: (i) there is a variation between geographic areas, in which sulphides from the Noril'sk-Talnakh area are the richest in metals and those from the Muskox intrusion are poorest in metals; (ii) there is a variation between textural types of sulphides, in which disseminated sulphides are generally richer in metals than the associated massive and matrix sulphides; and (iii) the massive and matrix sulphides show a much wider range of compositions than the disseminated sulphides, and on the basis of their NiICu ratio the massive and matrix sulphides can be divided into Cu rich and Fe rich. The Cu-rich sulphides are also enriched in Pt, Pd, and Au; in contrast, the Fe-rich sulphides are enriched in Fe, Os, lr, Ru, and Rh. Nickel concentrations are similar in both. Differences in the composition between the sulphides from different areas may be attributed to a combination of differences in composition of the silicate magma from which the sulphides segregated and differences in the ratio of silicate to sulphide liquid (R factors). The higher metal content of the disseminated sulphides relative to the massive and matrix sulphides may be due to the fact that the disseminated sulphides equilibrated with a larger volume of magma than massive and matrix sulphides. The difference in composition between the Cu-and Fe-rich sulphides may be the result of the fractional crystallization of monosulphide solid solution from a sulphide liquid, with the Cu-rich sulphides representing the liquid and the Fe-rich sulphides representing the cumulate.RCsumC : Le contexte tectonique des magmas d'intraplaque, caracterise par un panache croisant un rift, fournit les conditions ideales pour le dkveloppement de sulfures de Ni-Cu et d'element du groupe du platine. Le panache transporte les magmas riches en metaux prks de la limite separant le manteau de la croQte. L'interaction du rift et du panache permet un transport rapide du magma dans la croQte, ce qui attinue la perte des sulfures du magma en route vers la croGte. Le rift peut contenir des sediments susceptibles de fournir le soufre necessaire pour atteindre le point de saturati...

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% TiO 2 ) 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% TiO 2 . 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-

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