These ancient ores – specifically iron-nickel sulphide deposits – yield 10% of the world’s annual nickel production.  They formed two to three billion years ago when hot magma erupted on the ocean floor.  The ore mineral required sulphur to form, but where it came from previously puzzled scientists.

“These nickel deposits have sulphur in them arising from an atmospheric cycle in ancient times.  The isotopic signal is of an anoxic atmosphere” said Doug Rumble from the Carnegie Institution’s Geophysical Laboratory, who co-authored the study paper.

Rumble, with lead author Andrey Bekker and four other colleagues, analysed rock samples from major ore deposits in Australia and Canada.  They discovered that sulphur atoms made a complex journey from volcanic eruptions to ore-producing magmas via the atmosphere, seawater and hot springs on the ocean floor.

Sulphur-33 is an isotope of sulphur which contains one more neutron that sulphur which is more commonly found.  In reactions in the atmosphere, sulphur dioxide gas molecules are split by ultraviolet (UV) light and the isotopes are fractionated into different reaction products creating isotopic anomalies – the presence of sulphur isotopes in the rock provided information about the low levels of atmospheric oxygen.

“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.”

This knowledge will help geologists track down new ore deposits because the presence of sulphur and other chemicals factors determine whether a deposit will form or not.  “Modern society cannot exist without specialised metals and alloys,” Rumble said, “But it’s all a matter of local geological circumstances whether you have a bonanza – or bust.”