Bacteria forming high-grade copper

Materials World magazine
,
27 Feb 2019

The effect of bacteria on low-temperature ore may help miners detect higher grade deposits. Meka Beresford finds out how.

Microorganisms found at copper deposits indicate a new way to identify areas of high-grade ore, according to a recent paper. A group of Spanish geologists led by Dr Fernando Tornos at the Instituto de Geociencias (IGEO) studied south-west Iberia’s Las Cruces deposit, which is considered unusual due to the high-grade copper ore that has formed as thick veins of copper sulphides.

The geologists found a large and diverse ecosystem living in Las Cruces, and set out to prove if this was responsible for the unique characteristics of the formation. ‘These microbes are responsible of the current formation of the high-grade copper ore being extracted in a process that perhaps has lasted for more than eight million years but continues today,’ Tornos told Materials World.

Cultural exploration

Mining company First Quantum assisted the team in extracting pristine ore samples that had not been in contact with the atmosphere. Several microbiological analysis methods were used to study the samples, including metagenomics and CARD-FISH, a refined method that helps quantify and identify microbes. Both techniques use chemical analysis to highlight the presence of metabolically active microbes.

Analysis revealed that microbes were reducing the amount of sulphate and copper in acid and oxidised water that would usually filter into the copper sulphide veins. The microbes made the material form into covellite, a rare copper sulphide mineral. Microscope studies showed that the secondary sulphide formation was made up of nanometre-sized crystals of covellite. These small amounts of covellite had embedded and merged together in the polysaccharides, or ‘glue’ that protect the microbes.

This filtered and consolidated material is what makes Las Cruces unconventional because it forms into the thick veins of high-grade ore. Results obtained so far suggest that microbes have indeed had a direct impact on the deposit’s formation.

Researching the correlation between bacteria and ore deposits has been a restricted process in the past because deposits are often too close to the surface. This presents a whole host of setbacks, including biomarkers not always being definitive and microbes which are unlikely to have fossilised. With the Las Cruces sample, the most difficult issue was making sure the system was isolated to prevent contamination.

To ensure high quality and diverse research, Tornos recruited a team of specialists from the Spanish Astrobiology Center and the National Museum of Natural History of Spain – which are part of the Spanish National Research Council – as well as researchers from The Autonomous University of Madrid.

Proof of concept

This study adds to the long-standing debate on the impact of microbes on secondary sulphide formation, but more work is needed for geologists to understand if these processes happen outside of Las Cruces, the extent of the microbe effect, and if this can be controlled.

‘Our major goal should be to see if this process can be applicable elsewhere,’ Tornos said. Many of the microbes in the samples were then unknown so could not be classified. The more the team learns about them, the more this can inform future use in remediation, water purification, exploration and metal recovery.

Believing the research will spark further studies into microbes, Tornos says, ‘We feel we have opened a door to a field with a lot of possibilities’. He added that the study provides more and more evidence that microbes play a significant role in the formation of low-temperature ore deposits.

Although additional research is needed, the team believes that if they can identify which bacteria forms metal, they will be able to efficiently identify zones of potential high-grade ore, meaning less time on exploration and sample anaylsis. He explained that, in practice, miners would collect a sample from either marginal zones of a deposit or in active underground aquifers, and send it for testing.

‘Sampling is not difficult if you are trained to not contaminate it, such as not touching directly or breathing on it, store it in aluminium foil and put it under vacuum as soon as possible,’ he said. ‘Afterwards, the analysis must be done by microbiologists in the lab.’ While currently micro-scale, if proven at more sites, this technique could help miners save time, money and energy in the initial stages of exploration.

Read the paper, Do microbes control the formation of giant copper deposits?, published in Geology, here: bit.ly/2SjSdkN