On Earth, nearly all iron ore comes from specific rocks called banded iron formations, the vast majority of which originate around 2,000 million years ago (2,400-1,800 Mya to be more precise).  At this time, the world’s oceans were far more acidic, able to dissolve iron minerals.  The acidity arose due to the large amount of carbon dioxide in the atmosphere that dissolved in the oceans to form carbonic acid.  Around 2,000 million years ago, there was no breathable oxygen on Earth’s surface.  But then, a cellular mutation allowed certain bacteria to breathe in carbon dioxide and breathe out oxygen – the birth of photosynthesis.  The new bacteria thrived.  As they pumped oxygen into the oceans, it bonded with the iron minerals that then precipitated as solid sediments onto the ocean floor.  For reasons still being investigated by earth scientists, the available oxygen fluctuated through time.  When there was an abundance of oxygen, the ocean sediments became rich with iron-oxide minerals.  When there was a dearth of oxygen, the ocean sediments returned to their anoxic, muddy state (forming chert).  This cycle repeated for 100s of millions of years, creating alternating bands of rock, some with precipitated iron, some without: the banded iron formations.  If Westeros has followed a similar geologic evolution as Earth, then the Iron Islands record the birth of breathable air on the Game of Thrones planet.