Rich iron ore deposits 'linked to ancient atmosphere'

Using geochemical clues from rocks nearly 3 billion years old, scientists have made the surprising discovery that the creation of economically important nickel ore deposits was linked to sulphur in the ancient oxygen-poor atmosphere.

These ancient ores -- specifically iron-nickel sulfide deposits -- yield 10 per cent of the world's annual nickel production. They formed the most part between two and three billion years ago when hot magmas erupted on the ocean floor.

"These nickel deposits have sulphur in them arising from an atmospheric cycle in ancient times. The isotopic signal is of an anoxic atmosphere," said Andrey Bekker of the Carnegie Institution.

The scientists 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, the 'Science' journal reported.

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," says 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.