Sunlit sulfur breathed life into nickel
An international team of geologists have resolved a 30-year debate on how sulfur-rich ores that blanket Western Australia formed billions of years ago, when there was little sulfur around.
The research, led by geologists Professor Mark Barley of the University of Western Australia and Assistant Professor Andrey Bekker of the University of Manitoba, Canada, appears in the journal Science.
Western Australia hosts some of the world's most important nickel-iron deposits, used for steel manufacturing, among other industries.
Many of these nickel-iron ores appear in 2.9 to 1.8-billion year old rocks called komatiites, which formed from very hot magma ejected from deep within the earth.
While sulfur plays an important role in concentrating nickel, the nickel-iron ores formed at a time when the interior of the earth and the ocean was relatively sulfur-poor.
"The question is: where did the sulfur come from?" says Barley.
Previous theories had suggested that the erupting magma may have either brought the sulfur up from the mantle or mixed with sulfur-rich rocks on the sea floor, but it turns out the story is more complex.
Rays of sunshine
The team analysed the composition of different isotopes of the sulfur and iron within the ore and in adjacent 2.7 billion year old rocks from Australia and Canada.
They found that most of the sulfur in the ore came from volcanic gases that were lifted high into the atmosphere. This sulfur reacted with ultraviolet (UV) light from the sun, changing the relative concentration of the particles and then deposited on the deep ocean floor.
The UV-altered sulfur in the sediments eventually mixed with molten magma to produce the nickel-iron ore found today.
Barley says the results are significant for not only understanding early earth systems, but also for pinpointing new nickel-iron deposits.
"We have unambiguously worked out where the sulfur in the komatiites came from, solving a debate that has been going on for 30 years," he says.
"It shows that to get really big enrichments of metals you often need very complex interactions of different parts of the earth's systems."
Geologist Professor Ray Cas of Monash University in Melbourne says it is a significant finding because it clarifies the origin of the deposits, which he points out are the world's major source of nickel.
"It's giving us a better understanding of the dynamics and relationship between these very exotic old magma types, and clarifies that the sulfur comes from older volcanics with massive sulfide components, which makes a lot of sense," he says.
Cas says, in terms of exploration potential, the research highlights that komatiites associated with particular volcanic rocks are obviously good places to look for other nickel deposits.
"But you still need to know the location of the system in the first place."