Exploration of the Mesabi Iron Range

For over a century, the 1.85-billion year old iron-rich sedimentary rocks of the Mesabi Iron Range in Northern Minnesota have been an important resource of high-grade iron ore to the United States. Due to extensive 20th century mining, the range has been almost entirely depleted of known high-grade deposits. High-grade iron ore on the Mesabi Range resulted from fluids dissolving soluble minerals from the iron formation at some time in the past, before the rocks became exposed at the Earth‘s surface by erosion. If these fluids had flowed up from below, there could be more high-grade ore beneath what has already been mined. To determine the possibility of geothermal fluids responsible for the high-grade iron, fluid inclusion and geochemical analysis was performed on iron range samples, including high-grade iron ore. Fluid inclusions are microscopic bubbles of fluid trapped in minerals as they precipitated from hot water and when heated or frozen under the microscope, indicate the homogenization temperature and salinity of the fluid. Our data showed that fluids in the iron formation had an average temperature of 206° C and average salinities of 9.6% (seawater salinity is 3.5% for comparison), consistent with fluids flowing up from below. Research of rare earth elements in altered rocks show that these fluids flowed through large areas of the iron formation. An assessment can be made from the data derived from our analysis. From the information gathered, it appears that high-grade iron ore below the shallow mined surface is a distinct possibility