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

Abstract

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-

The Role of Magmatic Sulfur in the Formation of Ore Deposits

“…Keith, Australia (e.g., Lesher and Keays 2002), and those of the Raglan area in Quebec (e.g., Lesher 2007) and Thompson belt of Manitoba (Paktune 1984). Flood basalt-associated deposits include those of Noril'sk, Siberia (e.g., Naldrett et al 1995;Czamanske et al 1995), and those in the Midcontinent Rift System of Minnesota (e.g., Duluth Complex) and Michigan (e.g., Eagle) (see Hauck et al 1997;Ripley et al 2007;Ding et al 2010). The ores of the Voisey's Bay deposit in Labrador (e.g., Ryan 2000;Li and Naldrett 2000) are the best examples of those in the anorthosite-related class.…”

Stable Isotopic Constraints on Fluid-Rock Interaction and Cu-PGE-S Redistribution in the Sonju Lake Intrusion, Minnesota

“…Our study used many of the samples described by Miller and Ripley (1996) and was designed to address two aspects of the mineralization. First, the PGE occurrence is distinct from more sulfide-rich Ni-Cu-(± PGE) occurrences within basal troctolitic-gabbroic units of the Partridge River and South Kawishiwi intrusions of the Duluth Complex (e.g., Ripley, 1990;Hauck et al, 1997). The importance of country-rock assimilation, and particularly externally derived sulfur, in the mineralization processes within these intrusions is well established (e.g., Ripley et al, 1999).…”

U–Pb age constraints on Variscan magmatism and Ni–Cu–PGE metallogeny in the Ossa–Morena Zone (SW Iberia)

“…Two contrasting hypotheses have been considered: a Cambrian-Ordovician age related to a rifting tectonic stage (Ortega et al 2004), a possibility consistent with the fact that many magmatic sulphide deposits elsewhere in the world occur in rift environments (Lesher 2003;e.g. Noril'sk, Distler & Kunilov 1994; the Duluth Complex, Hauck et al 1997;Pechenga, Melezhik et al 1994;Lesher & Keays 2002), or a Late Palaeozoic age related to the oblique collision during the Variscan orogeny . This latter interpretation linked the Aguablanca stock with the Santa Olalla stock, the age of which has been traditionally considered to be similar to that of a group of calc-alkaline Variscan intrusions cropping out in the region (Castro et al 2002).…”