Earth's mineral ores a lucky accident of ancient atmosphere

A team of researchers recently made a surprising discovery: much of the Earth’s most valuable mineral ores exist thanks to an ancient atmosphere that was oxygen-poor but higher in sulphur.

The scientists based their conclusions on geochemical clues from rocks nearly 3 billion years old.

Such ancient ores — iron-nickel sulphide deposits in particular — yield 10 per cent of the world’s annual nickel production.  They formed for the most part between 2 and 3 billion years ago when hot magmas erupted on the ocean floor.

Scientists have long puzzled over the origin of these ores, which require sulphur to form, because neither seawater nor the magmas hosting the ores were thought to be rich enough in sulphur for this to happen.

“These nickel deposits have sulphur in them arising from an atmospheric cycle in ancient times,” said Douglas Rumble, a researcher with the Carnegie Institution and co-author of the study published in the journal Science. “The isotopic signal is of an anoxic atmosphere.”

Rumble, with lead author Andrey Bekker — formerly a Carnegie Fellow and now at the University of Manitoba — and four other colleagues used advanced geochemical techniques to analyse rock samples from major ore deposits in Australia and Canada. They found that to help produce the ancient deposits, sulphur atoms made a complicated journey from volcanic eruptions, to the atmosphere, to seawater, to hot springs on the ocean floor, and finally to molten, ore-producing magmas.

The key evidence came from a form of sulphur known as sulphur-33, an isotope in which atoms contain one more neutron than “normal” sulphur (sulphur-32). Both isotopes act the same in most chemical reactions, but reactions in the atmosphere in which sulphur dioxide gas molecules are split by ultraviolet light (UV) rays cause the isotopes to be sorted or “fractionated” into different reaction products, creating isotopic anomalies.

“If there is too much oxygen in the atmosphere then not enough UV gets through and these reactions can’t happen,” said Rumble. “So if you find these sulphur isotope anomalies in rocks of a certain age, you have information about the oxygen level in the atmosphere.”

By linking the rich nickel ores with the ancient atmosphere, the anomalies in the rock samples also answer the long-standing question regarding the source of the sulphur in the ore minerals. Knowing this will help geologists track down new ore deposits, said Rumble, because the presence of sulphur and other chemical factors determine whether or not a deposit will form.

“Ore deposits are a tiny fraction of a percent of the Earth’s surface, yet economically they are incredibly important,” Rumble said. “Modern society cannot exist without specialised metals and alloys. But it’s all a matter of local geological circumstance whether you have a bonanza — or a bust.”