Bacteria can play an important role in the formation of secondary gold grains, according to Dr Frank Reith, a geo-microbiologist at the Australian Commonwealth Science and Research Organisation (CSIRO). He believes this discovery could have a significant impact on the mineral exploration industry.
‘The origin of secondary gold grains is a controversial topic that is widely debated within the scientific community,’ explains Reith. One theory is that these high purity grains are detrital – that is, they are formed in fractures or as sedimentary particles through the process of weathering or erosion and are deposited in alluvial gravels. Another approach focuses on chemical accretion – precipitation of gold from ground and soil waters at moderate temperatures.
Reith says, ‘There is evidence for both mechanisms, and I am sure that under the appropriate environmental conditions either one applies.’ However, he believes a third biological theory has potential. Reith’s research builds on previous studies in the area. ‘I wanted to unravel more of the story by being able to integrate field and lab work with spectroscopic and microscopic techniques,’ he explains.
Having collected samples from two Australian gold mines – Tomakin Park in southern New South Wales and Hit or Miss in northern Queensland – Reith analysed them using confocal stereo laser microscopy and the 4’-6-Diamidino-2-phenylindole (DAPI) staining technique. This revealed the presence of thin films of micro-organisms (biofilms) on the gold grains. DNA profiling of this structure identified 30 bacterial species.
However, it was one particular type – Ralstonia metallidurans – that was observed on all of the DNA-positive grains, suggesting that it may contribute to their formation. ‘We have not found the Ralstonia species in the surrounding soil, indicating that it lives in a niche where gold concentrations are high,’ says Reith. By placing a culture of the bacteria in highly toxic gold chloride solution to discover if metallic pure gold did grow in this way, Reith said he ‘observed active gold precipitation.
A unique attribute of Ralstonia metallidurans is that it is able to survive in concentrations of gold that would kill most other micro-organisms.’ The next step will be an attempt to replicate the process in an in situ environment in sands and soils. Reith is convinced of the potential of this research for the gold exploration industry to save time and money.
‘If organisms are associated with gold, we may be able to use them as bio-indicators and create field kits for narrowing down target areas.’ With regards to minerals processing, he proposes the creation of a biological processing plant for gold ores. ‘We have organisms that decompose sulphides, making gold more leacheable. Then there are cyanide-producing organisms that could be used to bring gold into solution, and there are these organisms that precipitate and form pure gold grains.’
Moreover, he explains that although the discipline of geo-microbiology is relatively new, it has been found that many minerals are formed by microbes, including carbonates, ferrihydroxides, silicates and pyrite.
Reith concludes, ‘We need to obtain more data to be able to predict mineral deposits.’
Australian Commonwealth Science and Research Organisation
Frank Reith, email: Frank.Reith@csiro.au.