GEOPHYSICAL ARCHITECTURE OF THE ARCHEAN MENTOR ANORTHOSITE INTRUSIVE COMPLEX, NORTHWESTERN MINNESOTA

The Mentor anorthosite intrusive complex lies near the northern margin of the Wawa subprovince of the Archean Superior Province, in an area of northwestern Minnesota where the Wawa, Quetico, and Wabigoon subprovinces are juxtaposed in close proximity. The rocks of interest are entirely concealed by unconsolidated Quaternary sediments. New interpretations are based on an Earth MRI-sponsored aeromagnetic dataset and pre-existing ground gravity data, constrained by approximately ten boreholes in the immediate area. The Mentor complex comprises a large volume of megacrystic anorthosite, with a lesser volume of oxide-rich gabbros. The anorthosite is weakly magnetized and dense (mean measured density is 2940 kg/m³), producing a 10-60 mGal gravity high. The gabbros are strongly magnetized, producing aeromagnetic anomalies as large as 6000 nT, making them readily mappable across the complex. The gabbros are also known, from a single borehole intersection, to be enriched in vanadium, and have further theoretical potential for chromium and titanium mineralization. New geophysical interpretations show that the Mentor complex is significantly broader in extent than previously mapped and can be traced along strike for approximately double its originally interpreted length. The Mentor anorthosite and gabbros collectively cover an area of about 700 km² along a strike length of about 85 km, making the Mentor complex one of the world’s largest Archean megacrystic anorthosite intrusive complexes. No modern radiometric dates exist within the study area. However, the mafic complex is observed in drill core to intrude a package of basalt flows at its northwest boundary and is itself intruded by multiple low-density felsic plutons that produce 10-20 mGal, 4-20 km wide gravity lows. This tectonomagmatic setting is consistent with other anorthosite complexes of the Superior Province, such as the Bad Vermilion Lake Anorthosite Complex which also intrudes mafic volcanic flows and is intruded by felsic plutons. Disrupted trends and patterns of geophysical anomalies indicate that the Mentor complex was variably deformed, likely via both faulting and folding, in a complex fashion. Numerous NW-striking magnetic dikes cut all Archean rocks and are thought to be parts of the Paleoproterozoic Marathon or Fort Frances swarms.