Copper, gold and silver can't just be found anywhere buried under the soil. It takes very specific conditions to produce deposits of mineral ores that can be mined. The conditions happen in what geologists call zones of cementation.
First, the water in the ground needs to contain dissolved metals. The dissolved metals need to react with other elements, like sulfur, to form solid mineral deposits.
Geologists think that chemical processes like this formed most of the mineral deposits on Earth.
However, there is one important factor that these traditional theories don't take into account.
Biologists now know that whole ecosystems of microbial life thrive deep underground. They are beginning to understand the role that this deep biosphere of microbes plays in subterranean chemistry. This might include a central role in the formation of mineral deposits.
In 2019 a team of Spanish and American researchers published a paper arguing that bacteria control the deposition of copper ore at a major deposit in Spain. The researchers discovered that the cementation zone of this deposit is full of microbial life. There isn't much oxygen underground, so many of these bacteria use compounds that contain sulfur instead in order to extract chemical energy from their food.
The researchers argue that this biological process produces the chemical conditions needed to from copper ore deposits, where purely chemical processes couldn't do so. Their microscope images of rock samples show that the bacterial communities are closely intertwined with deposits of copper in the cementation zone.
If this is true, it would tell us that life plays a much larger role in shaping the Earth's crust than we thought.
Sources and Further Reading
- Gavin, H. For Some Copper Deposits, Microbes Make Minable Materials. Eos, April 10, 2019.
- Tornos, F., et al. (2018). Do microbes control the formation of giant copper deposits? Geology. 47 (2), 143-146.