GEOCHRONOLOGY OF THE WALLACE AND KALISPELL 1 X 2-DEGREE QUADRANGLES, IDAHO AND MONTANA, USA, AND ALBERTA AND BRITISH COLUMBIA, CANADA: INSIGHTS INTO THE GEOLOGIC EVOLUTION OF THE MESOPROTEROZOIC BELT SUPERGROUP

The Wallace and Kalispell 1° X 2° quadrangles cover approximately 33,000 square kilometers in ID and MT, USA, and AB and BC, Canada. The Mesoproterozoic Belt Supergroup is widespread throughout this area and hosts a variety of mineral deposits. The USGS is working to update previously published geologic maps of both quadrangles. This effort includes integration of geochronologic data to improve correlations of stratigraphic units across the Belt basin and to provide insight into the geologic evolution of the area. Sedimentary ages of Belt Supergroup rocks are best constrained by U-Pb geochronology of igneous zircon from sills, dikes, flows, and tuffs, and by U-Pb dates on detrital zircon. Published data indicate ages of 1469 Ma for the Prichard Formation of the lower Belt Supergroup, 1454 Ma for the Helena Formation of the Piegan Group, 1443 Ma for the Purcell Lava between the Shepherd and Snowslip formations of the Missoula Group, and 1401 Ma for a tuff between the Bonner and Libby Formations of the Missoula Group. Other data indicate that the Sullivan sediment-hosted massive sulfide deposit has an age of ~1470 Ma, and the Troy sediment-hosted copper deposit formed at 1409 Ma. The age of Coeur d’Alene district veins is controversial, but Pb isotopes, Sr isotopes, and K-Ar and ⁴⁰Ar/³⁹Ar ages are consistent with two ages of mineralization: Proterozoic formation of base metal veins and Mesozoic to Cenozoic formation of silver-rich veins. The western margin of the Belt basin was subject to localized metamorphism and plutonism during the Mesoproterozoic from 1379 to 1325 Ma. A Grenville-age metamorphism occurred between ~1200 and 1000 Ma, as shown by ages for metamorphic minerals. The Belt Supergroup was affected by Cretaceous compressional tectonism from 110-83 Ma. Numerous intrusive bodies were emplaced from 120 to 70 Ma, such as the ~94 Ma Rainy Creek alkaline-ultramafic igneous complex. Oligocene volcanic rocks host the Hog Heaven high sulfidation epithermal deposit. New radiometric dates from the Kalispell and Wallace quadrangles could provide additional information on the geologic evolution of this area. Careful consideration of geochronologic data facilitates geological interpretations and helps identify rock units with insufficient age data that may require further investigation.