Gold nuggets are often the creations of bacterial biofilms, say Australian researchers who have demonstrated the process and even identified the bacteria at work.
Layers of bacteria can actually dissolve gold into nanoparticles, which move through rocks and soils, and then deposit it in other places, sometimes creating purer 'secondary' gold deposits in cracks and crevices of rocks.
The process overturns the long-held belief by some scientists that gold ore is created only by 'primary' physical geological processes.
By looking at the DNA in biofilms that grow on gold grains collected from the Prophet gold mine in southeast Queensland, Australia , the University of Adelaide's Frank Reith and his colleagues discovered that 90% of the bacteria were of just two species Delftia acidovorans and Cupriavidus metallidurans. The bacteria share genes that make them resistant to the toxic effects of heavy metals.
"It's the first time we actually see the mechanism laying on top of the gold grain," said Joël Brugger of the South Australian Museum and University of Adelaide, a co-author on a report about the discovery which appears in this month's Geology.
"We tagged the DNA and saw the beautiful active biofilm (dissolving the gold)," says Brugger. "That was very interesting because gold in soluble form is very toxic." That dissolved gold can then be redeposited in other places in a much purer form.
The discovery is especially important because it could point to a new high-tech way to prospect for gold.
"[We've] been looking for gold in Australia for a hundred years," says Brugger. "It's getting more and more difficult." In fact most of the gold mining activity is just extensions of old discoveries made decades ago. "Finding something new is really, really rare."
One thing that makes it particularly hard to find new gold deposits in southeast Queensland is that the rocks there, and over most of Australia, are some of the oldest on Earth and have been largely ground down and buried by many metres of soil.
Finding gold deposits has often meant chancing upon the element on or near the surface soil, then digging down in search of ore-bearing rocks. Sometimes that has worked, sometimes not, says Brugger.
"At the moment we don't really understand how gold moves around in the environment," says Brugger. "I think that here we can see for the first time how it happens."
Microbes dissolve it and move it around with the groundwater flow — which can be pretty quick.
The presence of the bacteria could be a quick way to test if gold is present in the ground, Brugger suggests. Field geologists could even someday use biosensors that are tuned to detect the genes of these gold-specific microbes.
"It may have a direct application in understanding how gold is going to exist in these environments," agrees geochemist DC "Bear" McPhail of Australian National University.
McPhail is looking at how microbes might alter the concentrations of different uranium isotopes, also a toxic metal, in soils, which can affect radioisotope dating techniques.
"We still have some way to go until we have direct application," says McPhail. "But it may lead to looking in different parts of the soil. It gets more and more sophisticated all the time."