A paleomagnetic study on the Gunflint, Mesabi, and Cuyuna iron ranges in the Lake Superior region

Abstract

One hundred and eleven samples of Animikie iron formation, partially altered iron formation, and direct-shipping iron ore were collected from the Gunflint, Mesabi, and Cuyuna Ranges in the Lake Superior region of North America. Paleomagnetic measurements were made on the samples using an astatic magnetometer, and demagnetization by an alternating magnetic field was applied to test for remanence stability. The primary iron formations---the Gunflint, Biwabik, and Deerwood iron formations from the Gunflint, Mesabi, and Cuyuna Ranges, respectively--appear to be contemporaneous with stable, primary chemical remanent magnetization (CRM) which, as indicated by the Gunflint iron formation, yields an Animikie paleomagnetic pole position of 94 degrees W, 28 degrees N. The remanence characteristics of the Gunflint iron formation further suggest that the iron formation was deposited at a very slow rate in a polar geographic region, and that reversals in the earth9s magnetic field may have occurred during deposition. The samples from the Mesabi and Cuyuna Ranges yield a smeared distribution of remanence directions that start from Animikie--Lower Keweenawan directions for the primary iron formation samples, and end in Mesozoic--Cenozoic directions for the partially altered iron formation and soft iron ore samples. The hypothesis of ore genesis which best fits this distribution is that meteoric solutions weathered the primary Animikie iron formations during the Mesozoic--Cenozoic to form the soft, direct-shipping iron ore deposits. The oxidation of the primary iron minerals by the solutions resulted in a stable, secondary CRM in the ore samples. The partially altered iron formations probably have intermediate remanence directions produced by the combination of primary and secondary remanence components.