Geology of the Biwabik Iron Formation, Dunka River area, Minnesota

Abstract

The Dunka River area is at the eastern end of Minnesota's Mesabi range where the Middle Precambrian Biwabik Iron Formation lies unconformably on Early Precambrian granitic rocks and is overlain conformably by the Middle Precambrian Virginia Formation. The iron-formation has been intruded and thermally metamorphosed to the pyroxene hornfels facies by mafic rocks of the Late Precambrian Duluth Complex. The formation dips 15 degrees to 35 degrees southeast beneath the Duluth Complex, and is 175 to 300 feet thick. It consists of four members--Lower Cherty, Lower Slaty, Upper Cherry, and Upper Slaty--which are recognized throughout the Mesabi range. The formation is considerably thinner in the Dunka River area than in other parts of the Mesabi range. The Upper Cherty and portions of adjacent members are mined for their magnetite content by Erie Mining Company. The Lower Cherty, which is the principal source of Fe ore throughout most of the Mesabi range, is only a few feet thick in the area and is not mined.Quartz and magnetite are the most abundant minerals in the iron-formation, but iron-rich silicates that formed during metamorphism are common. Orthopyroxene, Ca pyroxene, fayalite, cummingtonite, and hornblende are the most abundant silicates. The metamorphism was isochemical except for the loss of the original H 2 O and CO 2 . The original carbonates and phyllosilicates were converted principally to pyroxenes and fayalite. The amphiboles formed mainly during retrograde metamorphism from the pyroxenes and fayalite. The quantity of magnetite did not change appreciably during metamorphism, whereas part of the original quartz was consumed by reaction with the carbonates.Prograde devolatilization reactions took place in a rock atmosphere which varied locally from predominantly H 2 O to mainly CO 2 . Where rich in CO 2 , the original siderite and ankerite were converted directly to olivine and pyroxenes, whereas prograde amphiboles formed only in layers initially rich in H 2 O. Because the CO 2 was more completely removed from the formation than H 2 O, the retrograde reactions consisted principally of conversion of the fayalite and pyroxenes to amphiboles.