Scientists have identified why some of the richest ore deposits such as copper, zinc and uranium were formed during the middle period of the Earth’s history.
The team from the University of Tasmania and University of California, led by emeritus professor Ross Large from the ARC Centre of Excellence in Ore Deposits (CODES), attributed this to the rise in oxygen in the atmosphere at the time.
Oxygen in the atmosphere 2.3 to 1.8 billion years ago not only caused a change in the evolution of life, it also caused a shift from the formation of iron, gold, nickel ore deposits to zinc, silver, copper and uranium ore deposits.
Some of Australia’s biggest and richest ore deposits formed after this switch, such as those at Broken Hill (zinc-lead-silver), Olympic Dam (copper-uranium), Ranger (uranium) and Mt Isa (copper-zinc-lead-silver).
The team tracked the level of oxygen in the Earth’s ancient atmosphere using a laser based analytical technology developed at the University of Tasmania. It helped determine the changes in the chemistry of pyrite (fools gold) that grew in seafloor muds billions of years ago, tracking the ups and downs of oxygen in the atmosphere.
They found that massive deposits of gold, iron and nickel formed in the oldest (Archean) rocks when oxygen was very low in the atmosphere and oceans.
However, when oxygen drastically increased following the Great Oxygenation Event – around two billion years ago – the amount of sulfate and salt increased in crustal fluids, leading to a switch to giant deposits of copper, zinc, silver and uranium.
Professor Large said the basic reason for the switch is very simple.
“Copper, zinc, silver and uranium are readily mobile in oxygen-rich and salty crustal fluids, which were plentiful during middle Earth history, whereas gold and iron are only mobile in low oxygen crustal fluids, like the most ancient fluids, that preceded the Great Oxygenation Event,” he said.
“We now have a much better idea of how oxygen changed in the ancient atmosphere and how this relates to the generation of large and rich ore deposits in ancient rocks.
“Australia has extensive areas of rocks deposited after the Great Oxygenation Event and our research provides a new lead to assist geologists in the discovery of rich deposits of copper, zinc and uranium.”