The iron ores of southeastern Minnesota

Abstract

Iron ore deposits of greater or lesser extent are scattered over a wide area in southeastern Minnesota. They are limited to the area that is underlain by Paleozoic limestones and dolomites, where they occur as replacements and residual deposits over rocks of Cambrian, Ordovician, Devonian, and Upper Cretaceous age. In Fillmore County the ores attain such thickness that they are of economic value. In the vicinity of Etna, where the glacial drift is thin, test-pits show from 2 to 15 feet of high-grade limonitic ore. The ore lies on Cedar Valley limestone of Devonian age, which it penetrates along joints and bedding planes, and forms linings on solution cavities. Devonian fossils occur in the ore and in the limestone on which it rests.Most of the ore is soft and earthy and contains from 10 to 30 per cent of siliceous silt, most of which is removed by washing before it is shipped to the blast furnace. Locally the ore is in the form of hard, massive blocks of limonite and goethite. The dry, washed ore averages about 55 per cent iron. About a quarter of a million tons have been shipped and the known reserves are estimated at several million tons.