Do microbes control the formation of giant copper deposits?

January 25, 2019, Geological Society of America
Las Cruces Mine, Spain. Credit: Fernando Tornos.

One of the major issues when studying ore deposits formed in surficial or near-surface environments is the relationship between ore-forming processes and bacteria. At a first glance, these environments appear to be a preferred place for the growth of microbial ecosystems because they potentially have large amounts of nutrients. However, studies have been restricted because of the low likelihood of microbe fossilization and because biomarkers are not always definitive.

This contribution to Geology by Fernando Tornos and colleagues tries to solve the long-standing debate on the control of on secondary formation. They predict that future multidisciplinary studies will prove that microbes have a key control on the precipitation of metals in these shallow environments.

Their is based on the unusual Las Cruces deposit in southwest Iberia, where a significant part of the high-grade copper ore occurs as thick, massive veins of copper sulfides. Tornos and colleagues have direct evidence that the mineralization is currently being formed there in relationship with active aquifers and in an area isolated from the surface by a thick layer of marl. Thus, the place is ideal for tracking for anaerobic microbes.

With the help of the mining company, First Quantum, the team was able to extract pristine samples that had never been in contact with the atmosphere.

Different microbiological techniques and detailed electron microscope studies have shown that copper sulfides are precipitating today in relationship with colonies of sulfate-reducing microbes. The nanometer-sized crystals of covellite are embedded in the polymeric compounds that encapsulate bacteria. These crystals coalesce, later forming the big veins. However, much more work is needed in order to know to which extent these processes are global and if microbes control most of the formation of the secondary deposits.

Explore further: Scientists probe Canadian sulfide ore to confirm microbial activity in seawater 2.7 billion years ago

More information: Fernando Tornos et al. Do microbes control the formation of giant copper deposits?, Geology (2018). DOI: 10.1130/G45573.1

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Parsec
5 / 5 (3) Jan 25, 2019
It is very likely that most ore formation in geologic history is due to bacteria. After all, chemical processes sans biologics can occur only in limited areas, and the second major process, precipitation due to solute boundaries, can really only happen where there are sharp thermal gradients (so-called "smokers" and the like). It is really difficult to see how either of these processes can lead to the truly massive ore deposits around the globe simply because both chemical and thermal gradients are inherently localized.

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