NEW GEOPHYSICAL SURVEYING AND GEOCHRONOLOGY: REFINING GEOLOGIC MAPPING IN THE SUPERIOR PROVINCE OF NORTHWESTERN MINNESOTA, USA

The USGS Earth Mapping Resources Initiative (Earth MRI) program recently funded a high-resolution airborne geophysical survey and acquisition of new geochronologic, petrologic, and geochemical data in northwestern Minnesota that is prospective for numerous Archean critical-mineral-producing systems. These new data supported 1:100,000 scale re-mapping of a three-county area. Previous geologic mapping of the area was limited by an absence of outcrop, sparse drill hole data, and only one geochronologic age. The study area comprises three conterminous subprovinces of the Superior Province; the Quetico metasedimentary subprovince pinches down to 5 km of thickness in map view where it separates the Wabigoon and Wawa volcanoplutonic subprovinces on either side.

Newly obtained LA-ICP-MS U-Pb zircon ages from a biotite tonalite in the Snake River batholith (ca. 2758 Ma), a diorite in the Grygla pluton (ca. 2771 Ma), and a biotite-hornblende tonalite in the Red Lake Falls pluton (ca. 2701 Ma) define multiple Neoarchean episodes of intermediate to felsic intrusive activity near the southern margin of the Wabigoon subprovince. Near the northern margin of the Wawa subprovince, two additional new LA-ICP-MS U-Pb zircon ages confirm a Neoarchean age (ca. 2737 Ma) for the previously undated Mentor Anorthosite Intrusive Complex (MAIC). A final newly acquired LA-ICP-MS U-Pb zircon age from the Fertile pluton, a biotite-granodiorite which appears to intrude the MAIC, provides a minimum age (ca. 2702 Ma) of emplacement for the MAIC.

New geophysical interpretations reveal that the MAIC is up to twice as large and much more structurally complex than previously thought (Drenth et al., this session). These findings are consistent with what is known of other Archean anorthosites in the Superior Province. Interpretation of new aeromagnetic data also suggests a revised position of the Wabigoon-Quetico subprovince boundary at both the eastern and western edges of the map area, and modified depictions of numerous other supracrustal and intrusive units.

Forthcoming geochemical analyses, ID-TIMS geochronologic analyses, and mineral potential modeling will aid in constraining the depositional history, provenance, and timing of mineralization of volcanic strata in both the Wawa and Wabigoon subprovinces.