April 15. Clinton Iron Ore

You remember the Taconic Mountains?  They were uplifted toward the end of the Ordovician by the collision between the North American continent and a probable volcanic island arc terrane. As with all mountains, they began to be eroded as soon as they were uplifted above sea level. We talked about some of the erosion products – the thick pile of sediments called the Queenston Delta.  

Some of the rocks in the uplifted Taconic Mountains, which originally extended from New England south into Georgia, must have contained a lot of iron. Dissolved in water, iron came into Silurian sediments in a long belt from Alabama to New York to eventually produce the only significant iron ore in the 48 United States apart from those in the Precambrian around Lake Superior. The rocks go by various names, but generally the iron-rich package is called the Clinton Iron Ore. Iron was necessary, of course, to early settlers, and even moreso as the nation entered the Civil War, so this resource was aggressively exploited in the 1700s and 1800s.

Oolitic hematite

Some of the richest deposits are near Birmingham, Alabama, where the rock is called the Red Mountain Formation because of its rusty color due to the iron content. The iron occurs right along with fossil crinoids and brachiopods, which together with other aspects of the rock indicates that the iron came in as the rock was solidifying. Don’t visualize an iron-rich sea—crinoids and such probably couldn’t survive in such water. The iron became part of the rock in a process called diagenesis, which is all of the action related to a rock solidifying from loose sediment to solid rock. That includes things like compaction, dewatering, and dissolution of some material and redepositing it as the cement that holds the rock grains together.

A good bit of the iron in the Clinton rocks is oolitic. Oolitic comes from a Greek word for eggs, and they are typically little round or oval grains, a millimeter or two across, that grow by precipitation of mineral matter in concentric layers. They usually start on some nucleus, a grain of sand or a fossil fragment, and oolites are often just calcium carbonate, calcite, the stuff many fossils are made of. But in the Clinton, the material is iron oxide, the mineral hematite, pretty much the same as rust on the underbelly of your car. This oolitic hematite is actually the state mineral of Alabama.

Industrialization after the Civil War really drove the iron and steel industry, and while most of it was centered in the Lake Superior area and the steel mills of Pittsburgh and elsewhere, Birmingham, Alabama, also became an important iron and steel center. In 1940, Birmingham produced 40% of America’s pig iron, high-carbon iron produced in smelters as an intermediate product that would ultimately become wrought iron or steel.

Today, 99% of U.S. iron ore production comes from Michigan and Minnesota, but there’s still a steel industry in Alabama. By some estimates, as much as 10% of U.S. iron ore came from the Silurian Clinton ores, with most of that from Alabama and most of it before 1950.

—Richard I. Gibson

Reference: http://geology.teacherfriendlyguide.org/index.php?option=com_content&view=article&id=196&Itemid=207

Photo by Dave Dyet via Wikipedia (public domain)