Overview and exploration − resource assessment of oxide ultramafic intrusions (OUIs) within the mesoproterozoic (1.1 Ga) midcontinent rift system, northeast Minnesota, USA

• Oxide ultramafic intrusions (OUIs) within the Mesoproterozoic (1.1 Ga) Midcontinent Rift System (MRS) are considered prime exploration targets for magmatic Cu-Ni-PGM sulfide resources. • Older composite differentiated mafic–ultramafic intrusions in the MRS have the capacity to host very large quantities of Cu-Ni sulfides relative to the size of the intrusion. • OUIs are younger equivalents to productive composite mafic–ultramafic intrusions. • Sulfide and oxide mineralization formed from two separate immiscible liquids by fractionation within dynamic magmatic OUI conduit systems. • Late magmatic − hydrothermal episodes have overprinted some OUI systems introducing and/or remobilizing Cu ± PGMs. The Mesoproterozoic (1.1 Ga) Midcontinent Rift System (MRS) is considered a prime exploration target for magmatic copper (Cu)-nickel (Ni)-platinum-group metals (PGM) resources. Current exploration is focused on smaller composite differentiated mafic–ultramafic intrusions within the MRS that have the capacity to host very large quantities of Cu-Ni sulfides relative to the size of the intrusion. These intrusions form dikes, sills, sheets, and funnel-shaped bodies that were emplaced during initiation and early (1115–1106 Ma) magmatic stages of the MRS. Prime examples include the Eagle, Eagle East and Tamarack intrusions. Within the north limb of the MRS, late-forming (<1099 Ma) oxide-bearing ultramafic intrusions (OUIs) occur in five structurally controlled clusters scattered along the southern and northwestern margin of the Duluth Complex. These small discordant bodies, traditionally known for hosting potentially economic iron (Fe)-titanium (Ti)-vanadium (V) resources have been ignored as possible magmatic Cu-Ni-PGM exploration targets by companies focused on the early-forming (older), rift-related intrusions. In this field-based, exploration-oriented investigation, evidence is presented that suggests OUIs are younger equivalents of the older mafic–ultramafic intrusions. Further, that these bodies are considered dynamic magmatic conduit feeder systems and the oxides-sulfides they host are best described by the confined conduit model. Published and unpublished geologic, geochemical and geophysical attributes for most identified OUIs have been compiled for this investigation. Based on new exploration data, four OUIs (Skibo OUI#1 & #2 and Titac North & South) are examined in more detail to gain some understanding of how these systems may operate. These new data have been used in developing a working geologic model that can guide future OUI exploration. Textural evidence suggests oxides and sulfides hosted by many OUIs formed from two coexisting immiscible melts produced by fractionation of fertile (metal-laden) ultramafic magmas. It is proposed that Fe-Ti-enriched oxide melts were generated first by vertical flow and entrained within an active feeder conduit(s). Oxide crystallization was followed by formation of immiscible, strongly fractionated sulfide melts that achieved iss and mss sulfide saturation. Sulfide liquid saturation (SCSS) resulted from early Fe-Ti oxide crystallization that removed FeO from the silicate magmas, causing a sharp drop in sulfur solubility. Both oxide and sulfide melts migrated independently through the partially crystallized (crystal mush) magmas by channelized peculation. Introduction of late fractionated sulfide melts resulted in crystallization of thick, pervasive, low-grade (≥0.1 %, Cu ± Ni) disseminated sulfide zones (halos) and in some instances basal massive-semimassive (net-texture) sulfide segregations as well as high-grade (Cu-PGM ± Ni) footwall stockwork sulfide zones that include sharp-wall veins. This magmatic process which involves three immiscible liquids is uncommon but appears to be characteristic of many OUI systems. The process is considered a mechanism for efficient sulfide localization that can potentially generate magmatic Cu-Ni-PGM sulfide ores within fertile OUI systems.