Geologic interpretation of aeromagnetic and chemical data from the Oaks Belt, Wabigoon subprovince, Minnesota: implications for Au-rich VMS deposit exploration

Abstract

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.