Fluid Movement Through the Mesabi Iron Range, Minnesota

The Mesabi Iron Range in northern Minnesota has been an important contributor of iron necessary for products that we use every day. The iron-rich sedimentary rocks were initially deposited in a shallow sea about 1.8 billion years ago. Later, fluids flowed through the rocks dissolving everything but the iron oxides and concentrating them into high-grade ore. I have been studying the some of these fluids as well as their chemical effect on the rocks. Microscope examination of the iron ores revealed that the early-formed iron-rich mineral greenalite was replaced by other iron-rich minerals, minnesotaite and stilpnomelane. The source of fluids has been studied by analysis of fluid inclusions, which can give both the temperature and the salinity of fluids that are trapped in minerals. Fluids that ascended through the rocks would be expected to have a relatively high temperature and salinity, whereas meteoric fluids that descended from the surface would be expected to have much lower temperatures and salinity. One sample from a fault associated with high-grade iron ore in the Thunderbird mine near Eveleth showed high homogenization temperatures values (mean 125°C) and a high weight % NaCl equivalent (mean 3.98 weight %), suggesting the fluids associated with high-grade ore ascended from depth. Cathodoluminescence of quartz in veins and in the iron formation shows growth banding, produced by pulses of fluid moving through the rock and precipitating minerals episodically. All of the observations are consistent with fluids ascending through the iron formation, interacting with the rocks as they flowed