Iron ore is the main ingredient in steel production. One of the mainstays of the Australian economy, it contributes $124 billion to national income each year.
This is not surprising, given that Western Australia is home to some of the largest deposits of iron ore on Earth, and 96% of Australia’s iron ore comes from this state. However, despite the importance of the metal, we still do not know exactly how and when iron deposits formed within the continent.
In new research published in the Proceedings of the National Academy of Sciences, we answer some of these questions by directly measuring the radioactive elements in the iron oxide minerals that underlie these sources.
We found that some of Western Australia’s richest iron deposits – such as Mt Tom Price and Mt Whaleback – are up to 1 billion years younger than previously understood. This redefines the way we think about iron deposits at all scales: from mining sites to supercontinents. It also provides clues as to how we might be able to find more iron.
Where does iron ore come from?
Billions of years ago, Earth’s oceans were rich in iron. Then the early bacteria began photosynthesis and rapidly introduced large amounts of oxygen into the atmosphere and oceans. This oxygen combines with iron in the oceans, causing it to settle on the sea floor.
Today, these 2.45-billion-year-old sedimentary rock deposits are called banded iron formations. They represent a unique archive of the interactions between the Earth’s continents, oceans and atmosphere over time. And, of course, banded iron formations are what we mine for iron ore.
These sedimentary deposits have distinct, rhythmic bands of reddish iron and paler silica. They lay alternately on the bottom of the sea seasonally. Such magnificent rocks can be visited today in Karijini National Park, WA.
The iron content of these banded iron formations is generally less than 30%. For the rock to become economically valuable for mining, it must be naturally converted by subsequent processes to about 60% iron.
The nature of this rock conversion is still debated. In simpler terms, a fluid – such as water – will remove silica and bring in more iron during an “enhancement” process which transforms the original composition of the rock.
The geochronology (age dating) of this transformation and chemical enhancement is not well understood, largely because the tools needed to directly date iron ores have only recently become available.
Previous age estimates for the Pilbara iron deposits were indirect but suggested they were at least 2.2 billion years old.
What did we discover?
You can think of iron ore as a rusty, red powder. However, it is usually a hard, heavy, blue-steel material. When ground into a fine powder, iron ore turns red. So the red landscape we see across the Pilbara today is the result of iron ore being eroded from beneath our feet.
We extracted microscopic-scale “fresh” iron ore from drill core samples at some of the most important iron deposits in Western Australia.
Using recent advances in radiometric dating, we measured naturally occurring radioactive elements in rocks. In particular, the ratio of uranium to lead isotopes in a sample can reveal how long ago individual mineral grains crystallized.
Using newly generated iron ore age data, we constructed the first timeline of the formation of Western Australia’s major iron deposits.
We found that all major iron ore deposits in the region formed between 1.4 and 1.1 billion years ago, making them up to 1 billion years younger than previous estimates.
These deposits were formed in association with major tectonic events, particularly the breakup and reemergence of supercontinents. It shows how dynamic the history of our planet is and how complex are the processes that led to the formation of the iron ore we use today.
Now that we know that giant ore deposits are linked to changes in the supercontinent cycle, we can use this knowledge to better predict where we’re most likely to discover more iron ore.
Liam Courtney-Davies completed this research while at the John de Laeter Centre, Curtin University.
Liam Courtney-Davies received funding for this research through MRIWA Project 557 and the Australian Research Council.
/Courtesy of The Conversation. This material from the original organization/author(s) may be current in nature and edited for clarity, style and length. Mirage.News does not take institutional positions or sides and all views, opinions and conclusions expressed herein are solely those of the author(s).