Bulletin No. 45. Progressive Contact Metamorphism of the Biwabik Iron-fomation, Mesabi Range, Minnesota

Abstract

The recent, spectacular growth of the taconite industry, and the expansion of taconite operations from the eastern to the central and western parts of the Mesabi range, emphasizes the importance of knowledge of the geology of the range. The earliest taconite plants were established in the Eastern Mesabi district, in areas in which the Biwabik Iron-formation was metamorphosed by the Duluth Gabbro Complex; most of the recent plants are in the Main and Western Mesabi districts, in areas of "unaltered" iron-formation. This report describes the changes in mineralogy and texture from "unaltered" taconite in the Main Mesabi district to highly metamorphosed taconite in the Eastern Mesabi district. It describes not only the silicate minerals, but also the opaque iron oxides, carbonate minerals, and carbonaceous material. Knowledge of the mineralogic changes is extremely important to the practical problems related to beneficiating characteristics of the magnetic taconites. The report is modified from a Ph.D. thesis submitted to the Graduate School at Johns Hopkins University by Bevan M. French.

The Biwabik Iron-formation, on the Mesabi range in northern Minnesota, extends for about 120 miles in a generally east-northeast direction, from west of Grand Rapids on the Mississippi River to Birch Lake, east of Babbitt. The formation is the middle unit of the Animikie Group of Middle Precambrian age. On the eastern end of the Mesabi range, the Animikie Group has been metamorphosed by the intrusive Duluth Gabbro Complex; mineralogical changes in the sediments, particularly in the iron-formation, appear to have been caused by the gabbro. From the data of the present study, four metamorphic zones may be distinguished within the Biwabik Iron-formation by changes in mineralogy along the strike of the formation toward the gabbro contact: