There are two types of gold. Primary gold forms when gold precipitates during chemical reactions between hot hydrothermal fluids and rocks in the Earth's crust. Secondary gold deposits form later, at the earth's surface, when gold is weathered, eroded and then reconcentrated. Some of this secondary gold is geologically very young, just 10-20,000 years old. “In the case of large nuggets, the gold just keeps on growing. If we [don’t] remove them from the environment [and interrupt them] they would keep on growing. Sort of like fish – you can throw them back to get bigger.”The reconcentration of gold from flake to nugget occurs in a number of ways. One is physical."Gold attracts gold,” says Dave. “Gold has a cubic structure when you look at it at the atom scale, and I think new gold likes to build on top of the old gold, just like salt does.”
Some geologists have also begun to think that biological processes, as well as chemical and physical processes, contribute to tiny flakes of secondary gold being reconcentrated into nuggets.
In a 2007 report on ‘The geomicrobiology of gold’, co-authored by Dave Craw, the authors write:
‘Microorganisms capable of actively solubilizing and precipitating gold appear to play a larger role in the biogeochemical cycling of gold than previously believed. Recent research suggests that bacteria and archaea are involved in every step of the biogeochemical cycle of gold, from the formation of primary mineralization in hydrothermal and deep subsurface systems to its solubilization, dispersion and re-concentration as secondary gold under surface conditions.’
Dave is using a scanning electron microscope to investigate samples of New Zealand gold at a nano scale, and what he is seeing in some cases has all the hallmark of bacterial activity. Patterns that resemble worm casts appear to be the work of bacteria which live on the gold, and excrete any gold they ingest from their cells. This process has been shown to happen in the lab, and Dave suspects it is also happening in streams and rivers.... he has seen the same processes that he sees in New Zealand happen in California, but not in the Yukon.
Gold nuggets are often the creations of bacterial biofilms, say Australian researchers who have demonstrated the process and even identified the bacteria at work.
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 percent 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 the September issue of the journal Geology.
Special ‘nugget-producing’ bacteria may hold the key to more efficient processing of gold ore, mine tailings and recycled electronics, as well as aid in exploration for new deposits, University of Adelaide research has shown.
For more than 10 years, University of Adelaide researchers have been investigating the role of microorganisms in gold transformation. In the Earth’s surface, gold can be dissolved, dispersed and reconcentrated into nuggets. This epic ‘journey’ is called the biogeochemical cycle of gold.
Now they have shown for the first time, just how long this biogeochemical cycle.“Primary gold is produced under high pressures and temperatures deep below the Earth’s surface and is mined, nowadays, from very large primary deposits, such as at the Superpit in Kalgoorlie,” says Dr Frank Reith, Australian Research Council Future Fellow in the University of Adelaide’s School of Biological Sciences, and Visiting Fellow at CSIRO Land and Water at Waite.
“In the natural environment, primary gold makes its way into soils, sediments and waterways through biogeochemical weathering and eventually ends up in the ocean. On the way bacteria can dissolve and re-concentrate gold – this process removes most of the silver and forms gold nuggets.
Working with John and Johno Parsons (Prophet Gold Mine, Queensland), Professor Gordon Southam (University of Queensland) and Dr Geert Cornelis (formerly of the CSIRO), Dr Reith and postdoctoral researcher Dr Jeremiah Shuster analysed numerous gold grains collected from West Coast Creek using high-resolution electron-microscopy.
Published in the journal Chemical Geology, they showed that five ‘episodes’ of gold biogeochemical cycling had occurred on each gold grain. Each episode was estimated to take between 3.5 and 11.7 years – a total of under 18 to almost 60 years to form the secondary gold.
The gold you see in the photo ... It was produced by a bacteria that, according to researchers at Michigan State University, can survive in extreme toxic environments and create 24-karat gold nuggets. Pure gold.
Kashefi and Brown are the ones who have created this compact laboratory that uses the bacteria Cupriavidus metallidurans to turn gold chlroride—a toxic chemical liquid you can find in nature—into 99.9% pure gold.
Accoding to Kashefi, they are doing "microbial alchemy" by "something that has no value into a solid [in fact, it the toxic material they use does cost money. Less than gold, but still plenty], precious metal that's valuable."The bacteria is incredibly resistant to this toxic element. In fact, it's 25 times stronger than previously thought. The researchers' compact factory—which they named The Great Work of the Metal Lover—holds the bacteria as they feed it the gold chloride. In about a week, the bacteria does its job, processing all that junk into the precious metal—a process they believe happens regularly in nature.
So yes, basically, Cupriavidus metallidurans can eat toxins and poop out gold nuggets.