How did Earth’s early oxygen-free atmosphere help form ancient iron ore deposits?

Earth’s early oxygen-free atmosphere was essential for the formation of ancient iron ore deposits, specifically banded-iron formations (BIFs), because it allowed iron to remain mobile and soluble in the world's oceans.12

In the absence of free atmospheric oxygen, iron existed primarily in its ferrous state (${\text{Fe}}^{2+}$), which is highly soluble in water. This solubility allowed vast quantities of iron to be transported from the Earth's crust into oceans and large lakes. In contrast, the presence of oxygen today causes iron to oxidize into the highly insoluble ferric state (${\text{Fe}}^{3+}$), which prevents such widespread transport and deposition.132

The deposition of these iron-rich sediments was likely triggered when this dissolved iron encountered localized oxygen, potentially produced as a waste product by early cyanobacteria. As oxygen levels in the atmosphere and oceans rose, the geochemical cycle of iron was profoundly altered, leading to the disappearance of these massive sedimentary iron deposits from the geological record approximately 1.7 billion years ago.142