Minnesota Iron (IMAGE) University of Wisconsin-Madison Caption Banded iron formations are the primary source of iron ore worldwide. These rocks, at Soudan Underground Mine State Park, Minnesota, shows banding caused by layers of different minerals in a sample 2.7 billion years old. The study by Johnson and Li at UW-Madison showed that half of the iron in such rocks was metabolized by microbes before being deposited in rocks. Credit Courtesy of Clark Johnson Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.