Close-up photo of a rock
A covellite specimen housed at the Harvard Museum of Natural History, originally mined in Sardinia, Italy. Credit: Hannah Gavin

The Las Cruces copper deposit sits at the easternmost end of the Iberian Pyrite Belt, about 32 kilometers northwest of Seville, Spain. There, Fernando Tornos of Madrid’s Instituto de Geociencias and his team of scientists found the first known bacterial fossils of the mineral galena in 2014.

After discovering these ghosts of microbes past, researchers wondered whether there were live bacteria at Las Cruces and, if so, what they were doing, recalls John Slack. Slack is an emeritus scientist of the U.S. Geological Survey and senior member of the research team.

Tornos, Slack, and their colleagues did unearth live bacteria and, with them, evidence that defies prior assumptions about copper sulfide formation in ores. The scientists recently published their results in the journal Geology.

Mining for Knowledge

Minable mineral ores occur in a layer of rock called the cementation zone. In this stratum, water-dissolved metals react with other elements, like sulfur, to form solid minerals. Copper-rich cementation zones contain copper sulfides such as covellite (CuS) and chalcocite (Cu2S).

Laboratory scientists employed microbes to generate covellite way back in 1961, says Slack, referring to a study published in Economic Geology. Yet geologists long inferred that purely chemical processes, not organisms, create the majority of minerals in actual cementation zones. After all, research hadn’t turned up appreciable evidence for life in these underground sites.

These findings constitute “the strongest evidence to date” that there is direct involvement of microbial sulfur reduction in the mineralization of copper deposits.

That is, no evidence existed until Tornos’s team drilled into the cementation zone at the bottom of an open pit mine at Las Cruces. The Las Cruces samples host an average of 10 million live microbes per gram of rock. This concentration, which is 40,000 times higher than published in a previous study, constitutes the “first demonstration of abundant living bacteria” from a cementation zone, explains Slack.

Form and Function

Las Cruces’s subterranean community is diverse, home to microbes from both the domain Bacteria and the domain Archaea. Importantly, a subset of the creatures performs the process of sulfate reduction. This metabolic process generates the type of reaction-ready sulfur that precipitates with metals.

Illustratively, Las Cruces rock boasts crystals of covellite exclusively in pockets where microbes reside. Using microscopy, scientists witnessed a “close association between the microbial aggregates and covellite.” In sample areas distant from the living communities, however, no covellite crystals could be found.

Together these findings constitute “the strongest evidence to date” that there is direct involvement of microbial sulfur reduction in the mineralization of copper deposits, writes Columbia University’s Benjamin Bostick, a geochemist who did not participate in the research.

Here, There, and Everywhere?

If microbial sulfur reduction is controlling copper mineralization at Las Cruces, one of the richest cementation zones in the world, do microbes dictate copper availability globally?

“The geomicrobiological cycles involved in…ore genesis are probably not unique to Las Cruces,” concludes the Geology article, “strongly suggesting that similar processes may be responsible for forming many other copper deposits worldwide.” And, Slack suggests, the investigative approaches used at Las Cruces can be applied to study deposits of additional metals, like cobalt and nickel.

Today, minerals are extracted from cementation zone ore for use in jet engines, water desalination plants, and batteries, among other things. Continued studies will reveal just how many modern activities are linked, through metals, to generations of microbial metabolism.

—Hannah Gavin (; @micro_cultured), Science Writer


Gavin, H. (2019), For some copper deposits, microbes make minable minerals, Eos, 100, Published on 10 April 2019.

Text © 2019. The authors. CC BY-NC-ND 3.0
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