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Iron Ore Deposits
Eventually, as with the development of photosynthesis along sulfur and methane pathways, where sulfur and methane products are produced, photosynthesis along the
oxygen pathway, where oxygen is produced, came into being.
One important thing to know about oxygen is that it is very reactive. It is one of the most reactive of all the elements in nature. This means that it will readily attack and attach itself to other elements. In particular, iron, sulfur, and manganese are readily attacked by oxygen.
As molecular oxygen began to be produced by early autotrophs, a curious phenomena occured. Large amounts of iron which had accumulated in the early ocean as both FeSO4- and a solid FeS2 (pyrite) from naturally occuring weathering and as a product of heterotrophic activity, were attacked by the accumulating O2. As O2 reacts with FeS2 (pyrite), Fe2O3 (limonite) is produced. When O2 reacts with FeSO4-, Fe2O3 (hematite) is produced. These rocks, along with magnetite (Fe3O4 - a magnetic rock) and siderite (FeCO3), are also called iron ores. Oxide rocks such as these are mined today, and the iron (Fe) they contain is extracted.
Over a period of a billion years, huge deposits of iron ores were laid at the bottom of the sea. This activity took place between 3.5 and 2.5 billion years ago. Iron ores mined today in the United States, Australia, and South Africa, are part of the huge deposits laid down at that time. Once the oceans were swept clean of iron, then the oxygen could begin to accumulate in the atmosphere, and respiration
by sophisticated life forms could begin in earnest. It took a billion years for this process to complete. When it was finished, it closed the period in the history of the Earth which we call the Archean
.
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