Earth’s most important iron ore deposits are found in sedimentary rocks. They formed from chemical reactions that combined iron and oxygen in marine and fresh waters. The two most important minerals in these deposits are iron oxides: hematite (Fe2O3) and magnetite (Fe3O4).
So, how does Iron Ore Form?
Nearly all of Earth’s major iron ore deposits are in rocks that formed over 1.8 billion years ago. At that time Earth’s oceans contained abundant dissolved iron and almost no dissolved oxygen. The iron ore deposits began forming when the first organisms capable of photosynthesis began releasing oxygen into the waters. This oxygen immediately combined with the abundant dissolved iron to produce hematite or magnetite. These minerals deposited on the sea floor in great abundance, forming what is now known as the “banded iron formations.” The rocks are “banded” because the iron minerals deposited in alternating bands with silica and sometimes shale. The banding might have resulted from seasonal changes in organism activity.
Read More: Types of Iron Ore
The mining involves moving tremendous amounts of ore and waste. The waste comes in two forms, non-ore bedrock in the mine (overburdenor interburden locally known as mullock), and unwanted minerals which are an intrinsic part of the ore rock itself (gangue). The mullock is mined and piled in waste dumps, and the gangue is separated during the beneficiation process and is removed as tailings. Taconite tailings are mostly the mineral quartz, which is chemically inert. This material is stored in large, regulated water settling ponds.