New explanation for gold's voyage from Earth's crust to surface

New research by University of Toronto geologists James Mungall and James Brenan, along with colleagues in Australia and France, suggests ore deposits of gold and other precious metals formed near Earth’s surface after floating upwards on vapour bubbles released from magma chambers deep inside the planet’s crust.

That same process also may have produced conditions ripe for affecting climate – the sulphur droplets to which the metals were bound that eventually attached themselves to the vapour bubbles, were frequently spewed into the atmosphere during volcanic eruptions.

“We learned years ago that these minerals bind to molten iron sulphide and create very dense droplets. It was widely assumed they therefore would sink to the bottom of magma chambers,” said Mungall, an economic geologist in U of T’s department of Earth sciences. “But we found a mechanism for moving sulphide liquids up instead of down.”

Mungall and colleagues combined laboratory experiments and theoretical models to analyse the behaviour of sulphide melts in magmas containing vapour bubbles.

“We’ve been melting sulphides and silicates in the lab for many years and only noticed in passing that sulphide droplets tend to stick to gas bubbles,” said Mungall of experiments done with Brenan.

Colleagues at Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) then measured the exact shape of the droplets and bubbles and determined that they could easily float upwards with the droplets suspended beneath the bubbles like tiny hot air balloon baskets.

“This mechanism explains a possible route for the transfer of metals into the shallow crust to form ore deposits,” said Mungall. “It also suggests a process for the release of large amounts of sulphur into the atmosphere.”

That is where the findings get even more interesting.

The same sticky magmas that can generate large deposits of precious metals can also produce massive eruptions, such as the eruption of Mount Pinatubo in the Philippines in 1991. Such eruptions are loaded with quantities of sulphur gas that dramatically exceed the amounts that could have been dissolved in the erupted magma. The sulphur forms microscopic droplets of sulphuric acid in the stratosphere which reflect sunlight and cause widespread cooling of temperatures over periods of months or even years.

“When emitted during volcanic eruptions, that sulphur can significantly affect climate,” said Mungall. “Our findings provide a way of moving all of that excess sulphur up from the sulphur-rich magma into the shallow layers below Earth’s surface. If those amounts break through to the surface rather than solidify in the crust, they spew it all into the atmosphere.”

The research is published this week in Nature Geoscience in a study titled “Transport of metals and sulphur in magmas by flotation of sulphide melt on vapour bubbles.”