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The Oaks Belt (OB) is a Neoarchean volcanic complex located in northwestern Minnesota, USA. It is part of the Wabigoon granite–greenstone terrane that hosts the world-class Rainy River gold deposit in nearby Ontario, Canada. Rocks in the OB form a north-dipping homocline in the fault-bounded pressure shadow of a sigma-shaped volcano-plutonic wedge that spans east–west for 220 km across the Minnesota, USA – Ontario, Canada border. Exploration drilling in the area delineated pyrrhotite–pyrite massive sulfide deposits, iron formation, chert, and semi-massive sphalerite mineralized zones. High-resolution aeromagnetic data indicate a large (∼60 km2) composite subvolcanic intrusion underlies these iron-rich strata in the OB. The position of this inferred intrusion elucidates the low base metal content of known massive sulfide deposits, as they were too far away (6–10 km) from a heat source to have been favorable sites for base metal deposition. The relative abundance of Au and Zn in the OB, alongside correlation coefficients between metals in massive sulfide deposits, iron formation, and chert, indicates the rocks were affected by a low-temperature hydrothermal system under relatively shallow water conditions (<1000 m). Negative correlation between Na2O and CaO in basalt, and their mutual moderate positive correlation with immobile corundum (Al2O3), implies alteration in the upper part of the volcanic pile did not result in substantial element mobility in most samples. Geochemical data from mafic and felsic volcanic rocks plot mainly in the calc-alkaline field. Thus, the OB is most prospective for hosting Au-rich VMS deposits and future exploration should focus on paleo-thermal corridors and favorable stratigraphic horizons near the newly inferred composite subvolcanic intrusion.

The McAra deposit is in eastern Ontario, Canada, and is hosted in an Archean inlier to the Paleoproterozoic Huronian basin. It is currently estimated to contain ∼2.4 million pounds of cobalt at an average grade of 1.25%. New drill data show the mineralized zone comprises glaucodot–cobaltite veins and breccias that transect a mafic–siliciclastic volcanogenic massive sulfide (VMS) deposit. The high cobalt grade and host stratigraphy at the McAra deposit contrast with five-element (Ag–Co–Ni–Bi–As) deposits at the Cobalt and Gowganda camps in the region that produced high-grade silver and by-product cobalt from veins spatially associated with Nipissing Gabbro intrusions. However, geochemical data from recent core samples alongside fluid inclusion and mineralogical data suggest the cobalt zone at McAra and the five-element veins share a similar metal assemblage and were deposited from similar fluids. The mafic–siliciclastic VMS deposit at McAra contains anomalous amounts of cobalt, suggesting the Archean host stratigraphy was the source for the high-grade cobalt zone. Basin brines in the Paleoproterozoic are interpreted to have leached cobalt from Archean rocks and then redeposited it through oxidation–reduction reactions along synvolcanic faults that controlled earlier VMS deposit formation. High-resolution aeromagnetic data show that McAra is immediately adjacent to a mafic dike that transects the Huronian basin along a northwest-striking, crustal-scale fault system. These data, alongside observations from field mapping, also suggest the deposit is on the margin of a sub-basin that contains an 80 km2 Nipissing sill that may have originally overlain the deposit area and been a hydrologic seal during mineralization. The new deposit- and regional-scale data and interpretations are used to create a model for the McAra deposit and provide evidence for why it is cobalt-rich relative to other five-element veins. The model and data can be used to guide exploration for additional cobalt-rich deposits in the region and similar settings globally.