Panning for gold is tedious and time-consuming. Imagine a hand-held tool that, when placed in the sample, tells you yes or no—gold, no gold. A ‘pregnancy test’ for gold.
This is no far-fetched idea; the tool could be on the shelf within a few years. And with a recent discovery about how platinum nuggets can form, a similar test for the world’s rarest precious metal looks promising.
Like gold, platinum is on the move
“The origin of platinum nuggets has long been debated,” says Dr Frank Reith, a senior lecturer at the University of Adelaide’s School of Biological Sciences and a visiting researcher at CSIRO Land and Water.
“Traditionally it was thought that platinum only formed deep underground under high pressure and temperatures, and that when it was brought to the surface through weathering, it just sat there and nothing further happened to it.
“We’ve shown that is far from the case—nuggets of platinum and related metals can be transformed at the surface through bacterial processes.”
Dr Reith’s project is a collaboration with Monash University and Mineral Resources Tasmania. The University of Queensland, University of Western Australia, RMIT and the Federal Institute for Geosciences and Natural Resources, Germany, are also partners.
The team’s findings were published last month in Nature Geoscience.
Rarer than gold and highly prized
Platinum is one of the six metals that constitute the platinum-group metals—platinum, palladium, iridium, osmium, rhodium and ruthenium.
Extremely dense and highly resistant to corrosion, these elements are highly prized, not just for jewellery but for use in a range of industrial processes. Your car’s catalytic converter is likely to use platinum-group metals to treat emissions.
What makes platinum more precious than the other precious metals—gold, silver and palladium—is its extreme rarity.
Global production of platinum is dominated by South Africa, with Russia coming in a distant second. The rest of the world is struggling to secure adequate supplies.
A survival mechanism for bacteria
With gold, it has been known for some time that certain types of bacteria help to ‘dissolve’ gold, allowing it to enter the water cycle and be dispersed through the landscape, transported by rivers and floodwaters. Other types of bacteria can convert it back to its metallic state.
It has long been suspected that similar bacterial processes were at play with platinum, but up to now there was no evidence of this.
The bacteria live in a biofilm that can cover the grains of platinum. Finding live samples for lab experiments proved a challenge.
“We needed to find fresh grains of platinum group minerals and extract them from soils and sediments in a manner that preserves fragile biofilms and tell-tale DNA,” says Monash University Professor Joël Brugger. “These grains are incredibly rare, and the chase took us all over the world.”
Finally, using scanning electron microscopy, live bacterial biofilms were found at three sites—in Brazil, Colombia and Tasmania.
“A biofilm is like a village, where individuals have different roles and the common goal is survival,” explains Dr Reith.
“Gold is as toxic as mercury when in solution so, to protect themselves, some bacteria turn it back into metal. Over time, these bacteria become encrusted with gold and these nanoparticles can eventually become grains.
“We think the same might be true for platinum.”
Exploration companies stand to benefit
Understanding the role of bacteria in the formation of gold and platinum offers opportunities for exploration companies to save on costs.
Dr Reith is already well down the track in developing a device that will be able to be used in the field to test a sample for the presence of gold.
From the biofilms found on the surface of the gold, he has isolated a gold-binding protein from one of the organisms that is attached to the gold. This protein will form the core part of a ‘biosensor’—a device that uses a living organism or biological molecules to detect the presence of chemicals.
“Imagine you’re exploring in a remote area of the Kimberley. You fly your sample back to a lab in Perth; the data is sent back to site; the exploration team goes out again to take further samples. Exploration is a costly exercise.
“What we’re developing is like a pregnancy test for gold. You place it in the sample and it tells you yes or no. It doesn’t preclude lab analysis, but you might say this is a really interesting area so let’s collect more samples while we’re here.”
The gold ’pregnancy test’ could be on the shelf in 2.5 years, says Dr Reith, if he can secure funding for this project.
“For platinum, it’s maybe 10 more years of work. But we know what to do now based on our gold experience.”
A Brazilian twist
Examination of the platinum from Brazil has yielded another piece of the jigsaw puzzle of how the metal can be transformed in the landscape.
“In the matrix of the Brazilian platinum grains, we found traces of organic carbon, nitrogen and iodine—elements associated with life. We don’t know why but it was not just the surface that was transformed; it was the entire grain.”
Researchers first noticed these ‘bioorganic grains’ back in 1904, says Dr Reith. “Back then they thought, ‘That’s really weird’. But they didn’t have the analysis tools to be able to conclude that the grains were biological in origin. We have been able to bring our whole armoury of tools, which includes DNA sequencing and nanotechnology, to demonstrate that.”
Today, Australia produces a relatively minute amount of platinum. But in the 1880s and 1890s the biggest platinum field in the world was in Fifield, New South Wales, where platinum is still mined to this day.
Dr Reith has been working with mining company Rimfire Pty Ltd since 2010 and has found evidence of bacteria being involved in the platinum at Fifield.